Intelligent air compressor control system and control method thereof

Abstract

The present application relates to the technical field of intelligent control of air compressors, in particular to an intelligent air compressor control system and a control method thereof, which dynamically determines the rotor lubrication safety state and the lubricating oil emulsification risk by real-time collection of the outlet pressure of a lubricating oil pump coaxially connected with a screw host, the lubricating oil temperature in an oil-gas barrel, the current operating frequency of a frequency conversion host and the real-time pressure of a compressed air pipe network, dynamically generates the lowest allowable operating frequency under the current working condition in the frequency reduction process, and cooperates with non-impact constant pressure regulation, oil filter early warning and automatic warming protection, so that the present application replaces the traditional fixed minimum frequency control mode, expands the frequency conversion regulation range under the premise of ensuring the lubrication safety of the host and avoiding lubricating oil emulsification, reduces the operating energy consumption of the air compression station, does not need to additionally increase hardware, and is suitable for frequency conversion control and transformation of screw air compressors.

Description

Technical Field

[0001] This invention relates to the field of intelligent control technology for air compressors, and more specifically, to an intelligent air compressor control system and its control method. Background Technology

[0002] Variable frequency screw air compressors are widely used in industrial air supply due to their strong constant pressure regulation capability and good energy-saving effect under partial load conditions. Currently, most variable frequency air compressors use constant pressure PID control logic to match the main unit speed with changes in air consumption. To ensure that the lubricating oil pump, coaxially connected to the screw compressor, can build up sufficient oil pressure during low-speed operation to provide reliable lubrication for the rotor and bearings, a fixed minimum operating frequency limit is usually set. Most equipment sets the minimum operating frequency to around 25Hz. This fixed threshold control method can meet the basic operating requirements of the equipment under normal stable conditions and has been validated through long-term application in industrial fields. However, in actual operation, the equipment conditions will dynamically change with ambient temperature, lubricating oil usage time, and the degree of oil filter blockage. A fixed minimum frequency cannot adapt to the lubrication requirements under different operating conditions. In low-temperature environments or when the oil filter is partially blocked, the lubrication frequency may become insufficient. Higher lubricating oil viscosity or greater oil circuit resistance may prevent the fixed minimum frequency oil pump speed from establishing sufficient lubricating oil pressure, posing a risk of insufficient rotor lubrication during long-term operation. In high-temperature environments or when lubricating oil and oil filters are newly replaced, the lubricating oil has better fluidity and lower oil circuit resistance. Fixing the minimum frequency limits the frequency conversion adjustment range, preventing the frequency from being lowered further when air consumption is low, which can easily lead to frequent loading and unloading or frequent start-stop of multiple units, increasing additional operating energy consumption. At the same time, existing control logic mostly uses fixed temperature thresholds to judge the lubricating oil state, rarely combining the changes in dew point and oil viscosity under actual pressure. The accuracy of judging the risk of lubricating oil emulsification is limited, and it is difficult to dynamically adjust control parameters according to the equipment's operating status. The control effect varies throughout the equipment's entire life cycle. To solve this problem, we provide an intelligent air compressor control system and its control method. Summary of the Invention

[0003] The purpose of this invention is to provide an intelligent air compressor control system and its control method to solve the problems mentioned in the background art.

[0004] To achieve the above objectives, an intelligent air compressor control system is provided, comprising: The multi-source operating parameter acquisition unit is used to collect in real time the lubricating oil pump outlet pressure, lubricating oil temperature in the oil-gas tank, current operating frequency of the inverter host, and real-time pressure of the compressed air pipeline network, which are coaxially connected to the screw compressor host. The dynamic lubrication condition determination unit is used to determine the rotor lubrication safety status at the current inverter host speed based on the lubricating oil pump outlet pressure, determine the lubricating oil emulsification risk based on the lubricating oil temperature in the oil-gas separator and the oil viscosity-temperature characteristics, and identify abnormal oil filter pressure loss based on the changing trend of the lubricating oil pump outlet pressure. The dynamic minimum frequency generation unit is used to obtain the judgment result output by the dynamic judgment unit of lubrication condition on a cycle during the frequency reduction adjustment process of real-time pressure change of compressed air pipeline. When the rotor lubrication safety state reaches the critical boundary or there is a risk of lubricating oil emulsification, the current operating frequency of the inverter host collected at the current moment is determined as the minimum allowable operating frequency under the current condition. The minimum allowable operating frequency is dynamically updated with the lubricating oil temperature in the oil-gas tank and the oil filter pressure loss status. The frequency lock-up closed-loop control unit is used to perform constant pressure closed-loop regulation based on the real-time pressure of the compressed air pipeline network. During the regulation process, the output frequency of the inverter host is limited to no less than the minimum allowable operating frequency output by the dynamic minimum frequency generation unit. At the same time, it performs impact-free frequency transition control to avoid pressure fluctuations in the compressed air pipeline network caused by frequency abrupt changes. The status-linked protection unit is used to trigger an oil filter blockage warning based on the continuous downward trend of the lubricating oil pump outlet pressure and to trigger a main unit warm-up and frequency increase command based on the continuous downward trend of the lubricating oil temperature in the oil-gas separator during frequency-locked operation. A second objective of this invention is to provide a control method for implementing an intelligent air compressor control system, comprising the following steps: S1. Real-time data acquisition of lubricating oil pump outlet pressure, lubricating oil temperature in oil-gas tank, current operating frequency of variable frequency drive, and real-time pressure of compressed air pipeline, which are coaxially connected to the screw compressor, is obtained through the multi-source operating parameter acquisition unit. S2. Through the lubrication condition dynamic judgment unit, the rotor lubrication safety status under the current variable frequency host speed is determined based on the lubricating oil pump outlet pressure. The lubricating oil emulsification risk is determined based on the lubricating oil temperature in the oil-gas tank combined with the pre-stored oil viscosity-temperature characteristics. At the same time, the abnormal oil filter pressure loss is identified according to the changing trend of the lubricating oil pump outlet pressure. S3. When responding to real-time pressure changes in the compressed air pipeline network and performing frequency reduction regulation, if it is determined that the rotor lubrication safety status has reached the critical boundary or there is a risk of lubricating oil emulsification, the current operating frequency of the inverter host collected at this time will be determined as the minimum allowable operating frequency under the current working conditions. S4. The frequency lock-up closed-loop control unit performs constant pressure PID regulation based on the real-time pressure of the compressed air pipeline network. During the regulation process, the output frequency of the inverter host is limited to no less than the minimum allowable operating frequency, and the non-impact frequency transition control is executed synchronously to avoid pressure fluctuations caused by frequency changes. S5. After entering the frequency lock operation state, the status linkage protection unit triggers an oil filter blockage warning based on the continuous downward trend of the lubricating oil pump outlet pressure, and triggers a main unit warm-up and frequency increase command based on the continuous downward trend of the lubricating oil temperature in the oil-gas tank. After the protection action is completed, it returns to the frequency lock constant pressure regulation step for continuous operation.

[0005] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention replaces the traditional fixed minimum frequency control method by dynamically generating the minimum allowable operating frequency under different working conditions. It can automatically adjust the frequency lock threshold according to the ambient temperature, oil state, and oil filter blockage. While ensuring rotor lubrication pressure and avoiding lubricating oil emulsification, it can expand the frequency regulation range as much as possible. The entire control logic is implemented based on the existing pressure and temperature sensors of the air compressor, without the need for additional hardware costs. The control process adopts a shock-free transition to ensure stable air supply pressure. The online self-correction of oil viscosity-temperature characteristics and oil filter status monitoring functions can maintain control accuracy throughout the entire life cycle of the equipment, reduce the risk of wear and maintenance costs during long-term operation, reduce unnecessary loading and unloading and power frequency start-stop, and achieve a balance between safe operation and energy saving. Attached Figure Description

[0006] Figure 1 This is an overall block diagram of the present invention; Figure 2 This is the overall flowchart of the present invention. Detailed Implementation

[0007] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0008] This invention provides an intelligent air compressor control system. Please refer to [link / reference]. Figure 1 As shown, it includes: The multi-source operating parameter acquisition unit is used to collect in real time the lubricating oil pump outlet pressure, lubricating oil temperature in the oil-gas tank, current operating frequency of the inverter host, and real-time pressure of the compressed air pipeline network, which are coaxially connected to the screw compressor host. The dynamic lubrication condition determination unit is used to determine the rotor lubrication safety status at the current inverter host speed based on the lubricating oil pump outlet pressure, determine the lubricating oil emulsification risk based on the lubricating oil temperature in the oil-gas separator and the oil viscosity-temperature characteristics, and identify abnormal oil filter pressure loss based on the changing trend of the lubricating oil pump outlet pressure. The dynamic minimum frequency generation unit is used to obtain the judgment result output by the dynamic judgment unit of lubrication condition on a cycle during the frequency reduction adjustment process of real-time pressure change of compressed air pipeline. When the rotor lubrication safety state reaches the critical boundary or there is a risk of lubricating oil emulsification, the current operating frequency of the inverter host collected at the current moment is determined as the minimum allowable operating frequency under the current condition. The minimum allowable operating frequency is dynamically updated with the lubricating oil temperature in the oil-gas tank and the oil filter pressure loss status. The frequency lock-up closed-loop control unit is used to perform constant pressure closed-loop regulation based on the real-time pressure of the compressed air pipeline network. During the regulation process, the output frequency of the inverter host is limited to no less than the minimum allowable operating frequency output by the dynamic minimum frequency generation unit. At the same time, it performs impact-free frequency transition control to avoid pressure fluctuations in the compressed air pipeline network caused by frequency abrupt changes. The status-linked protection unit is used to trigger an oil filter blockage warning based on the continuous decreasing trend of the lubricating oil pump outlet pressure and to trigger a main unit warm-up and frequency increase command based on the continuous decreasing trend of the lubricating oil temperature in the oil-gas separator during frequency-locked operation. After real-time acquisition of the current operating frequency of the inverter main unit, the speed conversion of the lubricating oil pump coaxially connected to the screw main unit is first performed. The lubricating oil pump drive shaft and the screw main unit output shaft coaxially connected to the screw main unit are rigidly direct-connected, with no transmission belt or gear reduction structure between them. The speeds are kept in a fixed proportional synchronization, with a transmission ratio of 1. The speed conversion formula is: in This refers to the real-time rotational speed of the lubricating oil pump, which is coaxially connected to the screw compressor, in revolutions per minute (rpm). The real-time operating frequency of the inverter host, in Hertz. The rated operating frequency of the frequency converter is expressed in Hertz. The rated output speed of the screw compressor at its rated operating frequency is expressed in revolutions per minute (rpm). This is the transmission ratio between the screw compressor and the lubricating oil pump coaxially connected to the screw compressor; in a direct-drive configuration, this value is 1.

[0009] After completing the speed conversion, the real-time lubricating oil temperature inside the oil-gas separator is read. The oil viscosity-temperature characteristics stored in the lubrication condition dynamic judgment unit are then retrieved. These characteristics are the viscosity-temperature curves calibrated during the initial equipment startup or lubricating oil replacement. The actual kinematic viscosity of the lubricating oil corresponding to the current lubricating oil temperature inside the oil-gas separator is calculated through linear interpolation, with units of millimeters per second. The oil filter pressure loss correction factor is used to compensate for changes in oil circuit resistance caused by blockage during oil filter use. This factor is calculated using lubricating oil pump outlet pressure data from the most recent one hundred stable operating condition control cycles. Stable operating conditions refer to conditions where the frequency fluctuation of the inverter host is less than 0.1 Hz and the temperature fluctuation of the lubricating oil in the oil-gas separator is less than 0.5 degrees Celsius. The formula for calculating the oil filter pressure loss correction factor is as follows: in This is the oil filter pressure loss correction factor, with a value ranging from 0.8 to 1. A value of 1 is used when the oil filter is brand new and unclogging; the value decreases as the oil filter becomes more clogged. The reference value for the lubricating oil pump outlet pressure, based on the actual kinematic viscosity of the lubricating oil at the same operating frequency and with the same oil filter in its brand-new condition, is pre-stored in the dynamic lubrication condition determination unit. This represents the real-time lubricating oil pump outlet pressure under stable operating conditions, expressed in megapascals (MPA).

[0010] After calculating the oil filter pressure loss correction coefficient, the critical threshold of the lubricating oil pump outlet pressure that meets the rotor lubrication requirements under the current operating conditions is calculated. The critical threshold of the lubricating oil pump outlet pressure is the minimum pump inlet pressure that ensures continuous fluid lubrication between the screw compressor rotor and bearing. The calculation formula for the critical threshold of the lubricating oil pump outlet pressure is derived based on the one-dimensional Reynolds equation combined with the screw compressor clearance leakage model. The lubricating oil flow within the clearance between the screw compressor rotor and bearing is laminar. Under laminar flow conditions, the clearance leakage flow rate is directly proportional to the pressure difference across the clearance and inversely proportional to the actual kinematic viscosity of the lubricating oil. The lubricating oil pump coaxially connected to the screw compressor is a positive displacement pump, and its theoretical output flow rate is directly proportional to the rotational speed. When the lubricating oil pump output flow rate equals the total leakage flow rate of the rotor-bearing clearance, a continuous oil film of critical thickness can be maintained within the clearance. In the derivation process, lubricating oil density, clearance geometry, rotor eccentricity, meshing clearance leakage, and bearing clearance leakage are all integrated into an equivalent leakage coefficient, eliminating the need for on-site measurement of complex structural parameters. The calculation formula for the critical threshold of the lubricating oil pump outlet pressure is as follows: in This refers to the critical threshold outlet pressure of the lubricating oil pump that meets the rotor lubrication requirements under current operating conditions, expressed in megapascals (MPA). This is the oil filter pressure loss correction factor. This represents the actual kinematic viscosity of the lubricating oil at the current temperature, expressed in square millimeters per second. This refers to the real-time rotational speed of the lubricating oil pump, which is coaxially connected to the screw compressor, in revolutions per minute (rpm). This refers to the theoretical displacement per revolution of the lubricating oil pump, which is coaxially connected to the screw compressor. The unit is milliliters per revolution. This is an inherent structural parameter marked at the factory and is pre-stored in the dynamic lubrication condition determination unit. The equivalent leakage coefficient is the lubrication clearance of the screw compressor, expressed in milliliters per second per megapascal. 60 is a time unit conversion factor used to convert revolutions per minute to revolutions per second to ensure unit consistency. The equivalent leakage coefficient is obtained through full-load lubrication calibration tests before the screw compressor leaves the factory. Different models of screw compressors correspond to different equivalent leakage coefficients. During equipment production, each model of screw compressor is individually calibrated and the parameters are pre-stored in the dynamic lubrication condition judgment unit, enabling direct adaptation to different models without requiring users to modify complex structural parameters on-site.

[0011] The calibration test was conducted on a standard lubrication test bench. First, the screw compressor of the model to be calibrated was installed on the test bench, and new standard lubricating oil of the specified type was added. The external temperature control system was activated to stabilize the lubricating oil temperature in the oil-gas separator at 40 degrees Celsius. The external frequency converter was then activated to control the screw compressor to operate stably at its rated speed. Calibration began after the lubricating oil temperature and pressure fluctuations were below the set threshold for 10 consecutive minutes. During calibration, the lubricating oil pump outlet pressure was gradually reduced using an external throttle valve. Simultaneously, the oil film thickness was measured in real time using high-precision eddy current displacement sensors installed at the main bearing end and the rotor meshing end. The resolution of the eddy current displacement sensors was no less than 0.1 micrometers. When the measured minimum oil film thickness decreased to the design allowable level for the corresponding screw compressor model, calibration was initiated. When the critical oil film thickness is reached, throttling is stopped and the operating conditions are kept stable. The lubricating oil pump outlet pressure, the actual kinematic viscosity of the lubricating oil, and the speed of the lubricating oil pump coaxially connected to the screw compressor are continuously recorded for 30 seconds. The average value of the three parameters is substituted into the critical threshold formula for the lubricating oil pump outlet pressure to calculate the equivalent leakage coefficient. The above pressure reduction calibration process is repeated three times. If the deviation of the equivalent leakage coefficient obtained from the three calculations is less than 5%, the average value is taken as the final calibration value. If the deviation is greater than 5%, the test bench status is checked and recalibrated. When the equipment is overhauled and the rotor or bearing is replaced, or when the user replaces a different model of screw compressor, the equivalent leakage coefficient can be recalibrated through the controller's maintenance interface according to the same calibration process to match the new clearance characteristics.

[0012] The critical oil film thickness allowed by the screw compressor design is determined comprehensively based on the surface roughness, geometric tolerances, and thermal deformation allowance of the rotor and bearing. In this embodiment, the surface roughness of the rotor journal is Ra0.8, and the surface roughness of the bearing inner hole is Ra0.8. The combined surface roughness of the two surfaces is 1.13 micrometers. Considering the radial runout, thermal expansion deformation, and installation coaxiality error of the rotor during operation, a safety margin of 1.8 is taken, and the critical oil film thickness is finally determined to be 2 micrometers. When the oil film thickness is greater than 2 micrometers, the two friction surfaces are completely separated by the oil film and are in a fluid lubrication state. When the oil film thickness is less than 2 micrometers, the probability of contact between surface micro-protrusions exceeds 50%, and the surface will enter a boundary friction state. Long-term operation will lead to increased wear.

[0013] The above calculation process will be re-executed in each control cycle to dynamically update the critical threshold of the lubricating oil pump outlet pressure, and the factory-fixed value will not be used for judgment.

[0014] After calculating the critical threshold of the lubricating oil pump outlet pressure, the lubricating oil pump outlet pressure collected in real time is compared with the critical threshold of the lubricating oil pump outlet pressure on a cycle-by-cycle basis. When the lubricating oil pump outlet pressure is greater than or equal to the critical threshold of the lubricating oil pump outlet pressure, the rotor lubrication safety state is determined to be safe. At this time, the frequency converter is allowed to continue to respond to the real-time pressure change of the compressed air pipeline and perform frequency reduction regulation. At the same time, the lubricating oil pump outlet pressure data under the current stable operating condition is stored in the benchmark dataset to update the oil filter pressure loss correction coefficient.

[0015] When the lubricating oil pump outlet pressure is less than the critical threshold of the lubricating oil pump outlet pressure, it is determined that the rotor lubrication safety state has reached the critical boundary. At this time, a frequency lock trigger signal is immediately sent to the dynamic minimum frequency generation unit to determine the current operating frequency of the inverter host collected at the current moment as the minimum allowable operating frequency under the current working condition. The minimum allowable operating frequency is sent to the frequency lock-up closed-loop control unit. In the subsequent constant pressure PID closed-loop regulation process, the output frequency of the inverter host will not be lower than the minimum allowable operating frequency, ensuring that the speed of the lubricating oil pump coaxially connected to the screw host is always maintained in the range that can establish sufficient lubrication pressure, avoiding wear caused by insufficient lubrication of the rotor and bearings.

[0016] Even under frequency lock-in conditions, the critical threshold for lubricating oil pump outlet pressure will be recalculated cycle by cycle and compared with the real-time lubricating oil pump outlet pressure. When the warm-up process causes the lubricating oil temperature in the oil-gas tank to rise and the actual kinematic viscosity of the lubricating oil to decrease, or when the frequency increase adjustment causes the speed of the lubricating oil pump coaxially connected to the screw compressor to increase, so that the lubricating oil pump outlet pressure is again greater than or equal to the critical threshold for lubricating oil pump outlet pressure, the frequency lock-in restriction will be automatically released, allowing the inverter to continue to adjust the frequency according to the pressure demand.

[0017] After the air compressor is successfully commissioned for the first time or after the lubricating oil is replaced, it automatically enters the oil viscosity-temperature characteristic calibration process. First, the inverter host is controlled to run under no-load for warm-up. During no-load operation, the inverter host keeps the intake valve fully open and the exhaust pressure zero. The screw compressor has the minimum load and will not experience speed or temperature jumps due to load fluctuations. During the warm-up process, the temperature of the lubricating oil in the oil-gas separator is continuously collected until the temperature of the lubricating oil in the oil-gas separator rises to the preset calibration start temperature. After that, the inverter host is controlled to run stably at the rated frequency. At the rated frequency, the speed of the lubricating oil pump coaxially connected to the screw compressor remains stable at the rated value and will not cause fluctuations in the outlet pressure of the lubricating oil pump due to changes in speed, ensuring that the data collected at different temperature nodes are comparable.

[0018] During the operation of the variable frequency drive at its rated frequency, the temperature of the lubricating oil in the oil-gas tank will rise slowly and continuously as the equipment operates. Data acquisition is triggered once at fixed temperature intervals, covering a temperature range from the calibrated starting temperature to the upper limit of the normal operating temperature of the equipment. At each temperature node, the lubricating oil temperature in the oil-gas tank is kept stable within 0.5 degrees Celsius above or below the set value for 30 seconds. After confirming that there are no significant temperature fluctuations, three sets of operating parameters are simultaneously collected by the multi-source operating parameter acquisition unit: the lubricating oil temperature in the oil-gas tank at the current temperature node, the lubricating oil pump outlet pressure at the current temperature node, and the current operating frequency of the variable frequency drive at the current temperature node.

[0019] After completing data acquisition for all temperature nodes, the actual kinematic viscosity of the lubricating oil at each temperature node is calculated. The calculation process incorporates an oil filter pressure loss correction factor to eliminate the influence of oil filter resistance on the pressure value. The lubricating oil pump, coaxially connected to the screw compressor, is a positive displacement pump; its output flow rate is proportional to its rotational speed. The pressure loss of the lubricating oil in the lubrication circuit is proportional to its actual kinematic viscosity. The formula for calculating kinematic viscosity is as follows: in Let be the actual kinematic viscosity of the lubricating oil calculated at the i-th temperature node, in millimeters per second. The lubricating oil pump outlet pressure collected at the i-th temperature node is expressed in megapascals (MPa). The equivalent leakage coefficient of the screw compressor lubrication clearance, measured in milliliters per second and megapascals per second, is an inherent structural parameter obtained through full-load lubrication tests before the screw compressor leaves the factory. 60 is a time unit conversion factor used to convert revolutions per minute to revolutions per second to ensure dimensional consistency. This is the oil filter pressure loss correction factor calculated at the i-th temperature node. The calculation method is consistent with the oil filter pressure loss correction factor in the lubrication determination process. This refers to the rated output speed of the screw compressor at the rated frequency, expressed in revolutions per minute (rpm). This is the theoretical displacement per revolution of the lubricating oil pump that is coaxially connected to the screw compressor, expressed in milliliters per revolution. It is an inherent structural parameter of the lubricating oil pump as specified by the manufacturer and is pre-stored in the controller.

[0020] After calculating the actual kinematic viscosity of the lubricating oil at all temperature nodes, a set of discrete data points of the lubricating oil temperature and the corresponding actual kinematic viscosity of the lubricating oil in the oil-gas separator are obtained. Cubic spline continuous fitting is performed on the discrete data points to obtain a viscosity-temperature relationship curve covering the entire operating temperature range. This continuous curve is the oil viscosity-temperature characteristic, which is stored in the non-volatile memory of the lubrication condition dynamic judgment unit. When performing rotor lubrication safety status judgment and lubricating oil emulsification risk judgment, the real-time collected lubricating oil temperature in the oil-gas separator is directly read, and the actual kinematic viscosity of the lubricating oil at the corresponding temperature can be quickly obtained through linear interpolation without repeating the calibration process.

[0021] During normal operation of the equipment, the lubrication condition dynamic judgment unit will self-correct the viscosity-temperature characteristics of the oil to match the viscosity characteristics changes caused by oxidation and aging of the lubricating oil during long-term use. The self-correction process does not require equipment shutdown or manual intervention and includes two triggering methods: incremental update and periodic full correction. Incremental update is automatically executed after each stable operating condition is met. Periodic full correction is automatically triggered once every 100 hours of cumulative operation. A full correction is also automatically triggered after each oil filter replacement. After changing the lubricating oil, the data is reset through the maintenance interface, and the weight of all historical data is directly set to zero. The data is re-collected to fit the viscosity-temperature characteristics of the new lubricating oil.

[0022] During the self-correction process, only operating data under stable conditions is collected. The stable operating condition criteria are: the current operating frequency fluctuation of the inverter host is less than 0.1 Hz, the lubricating oil temperature fluctuation in the oil-gas separator is less than 0.5 degrees Celsius, and the duration reaches 30 seconds. After meeting the stable operating condition conditions, the current lubricating oil temperature in the oil-gas separator, the lubricating oil pump outlet pressure, and the current operating frequency of the inverter host are collected. The actual kinematic viscosity of the lubricating oil at the current temperature is calculated using a viscosity calculation formula completely consistent with the factory calibration process. The current oil filter pressure loss correction factor and equivalent leakage factor are simultaneously substituted into the calculation process to ensure consistency between the calculation results and the calibration process. Newly obtained data points are added to the sliding window dataset. The sliding window retains the most recent 1000 valid data points, and the oldest data points outside the window range are automatically discarded. For full correction, the exponentially weighted least squares method is used to fit the oil viscosity-temperature characteristic curve. The oil viscosity-temperature characteristic is described by the Vogel equation, which is a standard empirical formula for the viscosity-temperature characteristic of lubricating oil, in the form of... in denoted as , where is the actual kinematic viscosity of the lubricating oil at temperature T, in square millimeters per second; T is the temperature of the lubricating oil in the oil-gas separator, in degrees Celsius; 'a' is the viscosity coefficient, in square millimeters per second; 'b' is the viscosity-temperature coefficient, in degrees Celsius; and 'c' is the temperature correction constant, in degrees Celsius. All three parameters are to be fitted. The weight of each data point is calculated based on the time elapsed between the initial acquisition and the current correction time. The weight calculation formula is as follows: in Let be the fitting weight for the i-th data point, and be a dimensionless quantity. As a forgetting factor, it is pre-stored at 0.001 per hour. This refers to the time of this full correction, in hours. The data point is the collection time of the i-th data point, in hours. The more recent the data point is, the higher its weight. The weight of old data points with a collection time of more than 1000 hours is close to 0. The weight of all historical data points before the lubricating oil is changed is set to zero and they are not included in the fitting of the characteristics of the new lubricating oil.

[0023] The fitting process uses the Levenberg-Marquardt algorithm to iteratively solve for the optimal parameters. After each iteration, the parameter changes and the sum of squared residuals are calculated. The convergence condition is that the relative change of viscosity coefficient a obtained from two consecutive iterations is less than 0.1%, the relative change of viscosity-temperature characteristic coefficient b is less than 0.01%, the relative change of temperature correction constant c is less than 0.01%, and the sum of squared residuals of all data points is less than the pre-stored residual threshold of 0.5 mm² / s. After the convergence condition is met, the iteration stops, and the new three parameters are updated in the non-volatile memory of the lubrication condition dynamic judgment unit, replacing the old oil viscosity-temperature characteristic parameters. Subsequent rotor lubrication safety status judgment and lubricating oil emulsification risk judgment both call the updated oil viscosity-temperature characteristics to calculate the actual kinematic viscosity of the lubricating oil.

[0024] If the convergence condition is not met after 20 iterations, the fitting residual for each data point is automatically calculated. Outlier data points with residuals greater than 3 times the standard deviation of the residuals are removed, and the remaining data points are used for refitting until the convergence condition is met. If the number of valid data points is less than 10, parameter updates are not performed, and full correction is performed only after sufficient data points are collected. During the incremental update process, full fitting is not performed. New stable operating condition data points are added to the sliding window, and the three parameters are updated step by step using the recursive weighted least squares method. If the parameter change after a step update is less than the convergence threshold, it is not written to non-volatile memory, but only updated in memory. After 10 incremental updates, a convergence judgment is performed. If the convergence condition is met, it is written to non-volatile memory.

[0025] Taking a 75 kW screw air compressor as an example, after 100 hours of cumulative operation, a full correction is triggered. There are 246 valid stable operating condition data points within the sliding window, covering temperatures from 25°C to 65°C. The weight of each point is calculated using the exponential weighting formula. The weight of data points within the most recent 100 hours is above 0.9. After 7 iterations, the parameter changes meet the convergence condition. The new viscosity coefficient 'a' changes by 0.32% compared to the old parameter, the viscosity-temperature characteristic coefficient 'b' changes by 0.008%, the temperature correction constant 'c' changes by 0.005%, and the residual sum of squares is 0.32 mm² / s, meeting the convergence condition. The new parameters are written to memory, and the updated curve is used for subsequent lubrication determination. After changing the lubricating oil, a data reset is triggered, and all old data weights are reset to zero. After 3 hours of operation, 12 stable operating condition points are collected, and the first full correction is performed, fitting the viscosity-temperature characteristic parameters of the new lubricating oil.

[0026] The self-correction process includes anomaly removal logic. If the deviation between the newly calculated actual kinematic viscosity of the lubricating oil and the viscosity at the corresponding temperature of the current oil viscosity-temperature characteristics exceeds a preset threshold, it is determined to be abnormal data caused by load fluctuation or sensor interference. Abnormal data will not be added to the correction dataset to avoid erroneous data causing curve shift.

[0027] After each self-correction, the lubrication condition dynamic judgment unit will automatically update the stored oil viscosity-temperature characteristic curve parameters. All subsequent judgment processes involving lubricating oil viscosity calculation will use the updated curve parameters, including the calculation of the critical threshold of lubricating oil pump outlet pressure in the rotor lubrication safety condition judgment process, and the viscosity comparison process in the lubricating oil emulsification risk judgment process.

[0028] In each control cycle, the lubrication condition dynamic judgment unit first reads the real-time lubricating oil temperature in the oil-gas tank and the real-time pressure of the compressed air pipeline network, which are collected in real time by the multi-source operating parameter acquisition unit. It then calculates the pressure dew point temperature of the compressed air under the current system pressure. The pressure dew point temperature is the critical temperature at which condensate forms in the compressed air under the current pressure. The calculation process uses Magnus's saturated vapor pressure formula combined with Dalton's law of partial pressures. First, the saturated vapor pressure at the corresponding temperature is calculated using the formula for the partial pressure of saturated water vapor under standard atmospheric pressure. The formula is as follows: in The partial pressure of saturated water vapor at standard atmospheric pressure corresponding to temperature T is expressed in kilopascals (kPa), where T is the temperature in Celsius. The constant in the formula is an empirical coefficient of the Magnus formula, pre-existing within the dynamic lubrication condition judgment unit. According to Dalton's law of partial pressures, the partial pressure of water vapor in compressed air is proportional to the total pressure. Therefore, the partial pressure of saturated water vapor at the current real-time pressure of the compressed air pipeline is the saturated vapor pressure at standard atmospheric pressure multiplied by the pressure ratio. The formula for calculating the pressure dew point temperature at the current pressure is derived from this. in This refers to the pressure dew point temperature under the current real-time pressure of the compressed air pipeline network, in degrees Celsius. This refers to the real-time pressure of the compressed air pipeline network, measured in megapascals (MPA). The standard atmospheric pressure is 0.101325 MPa. The ambient temperature of the air compressor intake is measured in degrees Celsius. It is obtained by collecting data from a pre-stored ambient temperature sensor or by calculating the intake temperature.

[0029] After completing the pressure dew point temperature calculation, the lubrication condition dynamic judgment unit calls the pre-stored oil viscosity-temperature characteristics and, based on the real-time collected lubricating oil temperature in the oil-gas tank, obtains the actual kinematic viscosity of the lubricating oil at the current temperature through linear interpolation, with the unit being square millimeters per second.

[0030] The system reads the baseline critical viscosity for water separation corresponding to the currently used lubricating oil model from the pre-existing dynamic lubrication condition determination unit. The baseline critical viscosity for water separation is the critical viscosity value at which condensate cannot be effectively separated and precipitated under the condition of new lubricating oil. The baseline critical viscosity for water separation is obtained by testing according to the standard test for the water separation performance of petroleum and synthetic liquids. The baseline critical viscosity for water separation of different types of lubricating oil is calibrated in advance by testing and is pre-existing in the dynamic lubrication condition determination unit. When users change the lubricating oil model, they can directly select the corresponding model parameter through the maintenance interface, or they can recalibrate according to the standard procedure.

[0031] The calibration process is completed on a standard demulsification test bench. First, take 405 ml of the corresponding model of new lubricating oil, pour it into a special measuring cylinder, and keep it at the set temperature. Start the stirrer and stir at 1500 rpm for 5 minutes to fully mix the lubricating oil and the added distilled water to form an emulsion. After stopping the stirring, start timing and observe the separation of the emulsion. When the volume of the separated water layer reaches more than 36 ml, and the total volume of the water layer and the emulsion layer in the oil does not exceed 3 ml, record the time taken, which is the water separation time at that temperature. Repeat the test three times at each temperature point and take the average value. Adjust the test temperature to change the actual kinematic viscosity of the lubricating oil and test the water separation time at different viscosities. When the water separation time exceeds 30 minutes, it is considered that the lubricating oil can no longer effectively separate condensate naturally. The corresponding viscosity is the basic critical viscosity for water separation of the lubricating oil of that model. The basic critical viscosity for water separation and the model correspondence table of different models of lubricating oil are pre-stored in the dynamic judgment unit of lubrication conditions. During equipment debugging, the table can be directly called according to the model of lubricating oil used on site, without the need for on-site testing.

[0032] Based on this, the actual critical viscosity for water separation is obtained by dynamically correcting the lubricating oil aging coefficient. The lubricating oil aging coefficient is calculated as the ratio of the actual kinematic viscosity of the lubricating oil to the viscosity of new oil at the same temperature during the self-correction process of the oil's viscosity-temperature characteristics. The longer the lubricating oil is used, the more severe the oxidation and aging, the greater the deviation in the actual kinematic viscosity, the more anti-emulsifying additives are consumed, the worse the water separation capacity, and the lower the actual critical viscosity for water separation. The correspondence between the lubricating oil aging coefficient and the correction coefficient is obtained through a lubricating oil thermal oxidation aging test. New lubricating oil is continuously oxidized at 120 degrees Celsius for different durations, and the critical viscosity for water separation under different aging degrees is tested. The correspondence between the aging coefficient and the critical viscosity for water separation is fitted and pre-stored in the dynamic judgment unit of lubrication conditions. The formula for calculating the actual critical viscosity for water separation is as follows: in This is the corrected actual critical viscosity for water separation, expressed in square millimeters per second. The pre-stored critical viscosity of the new lubricating oil base for water separation. This is the lubricating oil aging coefficient, with a value ranging from 0 to 0.3. The value is 0 when the lubricating oil is new and 0.3 when the lubricating oil reaches its oil change cycle.

[0033] After calculating the actual critical viscosity for water separation, the cumulative condensate precipitation is calculated. This process uses the cumulative precipitation to determine the total amount of condensate actually entering the lubricating oil, avoiding misjudgments caused by short-term temperature fluctuations. The cumulative condensate precipitation is calculated iteratively according to the control cycle, using the following formula: in This represents the cumulative condensate precipitation in the current cycle, expressed in grams. This represents the cumulative condensate precipitation from the previous control cycle. The condensate precipitation coefficient is proportional to the compressed air volumetric flow rate, obtained based on the rated volumetric flow rate of the air compressor, and is pre-stored in the lubrication condition dynamic judgment unit. The unit is grams per degree Celsius per second. This indicates that condensate will only precipitate when the pressure dew point temperature is higher than the temperature of the lubricating oil in the oil-gas tank. A positive temperature difference is represented by the actual difference, while a negative difference is represented by 0, indicating no condensate precipitation. To control the cycle duration, the unit is seconds.

[0034] When the temperature of the lubricating oil in the oil-gas tank is higher than the pressure dew point temperature, the amount of accumulated condensate precipitation decreases according to a fixed time constant, simulating the process of lubricating oil naturally separating and evaporating condensate. When the amount of accumulated condensate precipitation exceeds the pre-stored condensate threshold, and the current actual kinematic viscosity of the lubricating oil is higher than the corrected actual water separation critical viscosity, if both conditions are met simultaneously and continue for a preset time, it is determined that there is a risk of lubricating oil emulsification. All other situations are determined to have no risk of lubricating oil emulsification.

[0035] After determining that there is a risk of lubricating oil emulsification, the lubrication condition dynamic judgment unit sets the lubricating oil emulsification risk flag to 1 and sends it to the dynamic minimum frequency generation unit. After receiving the flag, the dynamic minimum frequency generation unit stops the current frequency reduction process, determines the current operating frequency of the inverter host as the minimum allowable operating frequency under the current conditions, and sends it to the frequency lock-up closed-loop control unit. At the same time, the flag is forwarded to the status linkage protection unit to trigger the host warm-up frequency increase command. By increasing the current operating frequency of the inverter host, the temperature of the lubricating oil in the oil-gas tank is increased, the evaporation of condensate is accelerated, and lubricating oil emulsification is avoided.

[0036] When it is determined that there is no risk of lubricating oil emulsification, the lubrication condition dynamic judgment unit sets the lubricating oil emulsification risk flag to 0, allowing the inverter host to continue to perform frequency reduction adjustment according to the real-time pressure of the compressed air pipeline network, and the cumulative condensate precipitation amount is continuously updated iteratively.

[0037] During normal equipment operation, the lubrication condition dynamic judgment unit will determine whether the oil filter pressure loss anomaly identification trigger condition is met in each control cycle. The trigger condition includes two parallel judgment items. The first item is that the current operating frequency of the inverter host remains stable and the fluctuation range of the current operating frequency of the inverter host does not exceed the preset frequency fluctuation threshold within a continuous preset time. The second item is that the actual kinematic viscosity of the lubricating oil obtained by calling the viscosity-temperature characteristics of the lubricating oil based on the temperature of the lubricating oil in the oil-gas tank remains stable and the fluctuation range of the actual kinematic viscosity of the lubricating oil does not exceed the preset viscosity fluctuation threshold within a continuous preset time. Only when both judgment items are met simultaneously will the oil filter pressure loss anomaly identification process be triggered to avoid the normal fluctuation of the lubricating oil pump outlet pressure caused by frequency changes or temperature changes being mistakenly judged as oil filter pressure loss anomaly.

[0038] After triggering the oil filter pressure loss anomaly identification process, within a continuously preset judgment period, the lubricating oil pump outlet pressure data is continuously collected at a fixed sampling frequency through a multi-source operating parameter acquisition unit. Simultaneously, the current operating frequency of the inverter host and the lubricating oil temperature in the oil-gas separator are collected at each sampling point. The lubricating oil pump outlet pressure at each sampling point is normalized to eliminate the influence of different speeds and viscosities on the lubricating oil pump outlet pressure. The normalization process converts the lubricating oil pump outlet pressure collected under the current operating condition into the equivalent lubricating oil pump outlet pressure under rated speed and rated viscosity, making the pressure values ​​under different operating conditions comparable. The normalization calculation formula is as follows: in The equivalent lubricating oil pump outlet pressure at the current sampling point, in megapascals (MPa). This is the real-time outlet pressure of the lubricating oil pump collected at the current sampling point, in megapascals (MPa). This refers to the rated speed of the lubricating oil pump coaxially connected to the screw compressor at the rated frequency, expressed in revolutions per minute (rpm). This is the real-time rotational speed of the lubricating oil pump coaxially connected to the screw compressor, corresponding to the current operating frequency of the inverter at the current sampling point. The unit is revolutions per minute (rpm). The rotational speed is calculated by the ratio of the current operating frequency of the inverter to its rated speed. This refers to the actual kinematic viscosity of the lubricating oil at the equipment's rated operating temperature, expressed in square millimeters per second. This represents the actual kinematic viscosity of the lubricating oil corresponding to the current temperature of the lubricating oil in the oil-gas tank, expressed in square millimeters per second.

[0039] After completing the normalization calculation for all sampling points, the pre-existing lubrication condition dynamic judgment unit reads the lubrication pump outlet pressure reference value. This reference value is the historical calibrated normal pressure value corresponding to the rated operating condition under normal, unblocked oil filter conditions. After oil filter replacement, the lubrication pump outlet pressure reference value is reset to the new oil filter calibration value. During normal, unabated equipment operation, the lubrication pump outlet pressure reference value is slowly updated via a weighted sliding method to adapt to the slow pressure changes caused by normal equipment wear. The update formula is as follows: in This is the updated reference value for the lubricating oil pump outlet pressure, in megapascals (MPa). The weights are updated based on the baseline. A larger weight is assigned after a new oil filter replacement, and the weight gradually decreases as the oil filter's operating time increases. This prevents missed detections caused by the lubricating oil pump outlet pressure benchmark shifting synchronously with the blockage pressure during the slow clogging of the oil filter. This represents the average value of the equivalent lubricating oil pump outlet pressure at all sampling points within the current judgment window. The reference value for the lubricating oil pump outlet pressure in the previous period is used. Then, a least-squares linear fit is performed on the equivalent lubricating oil pump outlet pressure sequence within the preset judgment period to obtain the slope of the equivalent lubricating oil pump outlet pressure change over time. At the same time, the difference between the equivalent lubricating oil pump outlet pressure at the first and last sampling points within the window is calculated as the cumulative decrease. In addition, the variance of the fitting residual is calculated to determine whether the pressure change is a trend of decrease or random fluctuation.

[0040] When the cumulative decrease in the equivalent lubricating oil pump outlet pressure relative to the lubricating oil pump outlet pressure benchmark exceeds the preset pressure loss threshold, and the slope obtained by linear fitting is negative and the absolute value of the slope exceeds the preset slope threshold, and the variance of the fitting residual is less than the preset variance threshold, the oil filter pressure loss is judged to be abnormal when all three conditions are met. If any one of the conditions is not met, the oil filter pressure loss is judged to be abnormal.

[0041] After determining that the oil filter pressure loss is abnormal, the lubrication condition dynamic judgment unit sets the oil filter pressure loss abnormality flag to 1. At the same time, it updates the oil filter pressure loss correction coefficient according to the cumulative decrease. The updated oil filter pressure loss correction coefficient will be substituted into the calculation formula of the critical threshold of the lubricating oil pump outlet pressure in the rotor lubrication safety status judgment process to dynamically correct the critical pressure value and ensure the accuracy of lubrication safety judgment during oil filter blockage. The oil filter pressure loss abnormality flag is also sent to the status linkage protection unit. When the equipment enters the frequency lock operation state, the status linkage protection unit will continuously monitor the downward trend of the lubricating oil pump outlet pressure. When the decrease reaches the warning threshold, it will trigger the oil filter blockage warning.

[0042] When the oil filter pressure loss is determined to be abnormal, the lubrication condition dynamic judgment unit sets the oil filter pressure loss abnormality flag to 0, updates the lubrication pump outlet pressure reference value using the equivalent average value of the lubrication pump outlet pressure in the current window, and enters the next cycle of continuous monitoring.

[0043] This normalization and trend fitting identification method does not require additional differential pressure sensors at both ends of the oil filter. It can accurately identify pressure loss anomalies caused by oil filter blockage using only the existing data collected by the equipment. The normalization of operating conditions eliminates the influence of different operating conditions on the pressure value. Linear fitting and residual judgment can eliminate misjudgments caused by random pressure fluctuations. The weighted benchmark update can adapt to the slow pressure changes caused by normal wear and tear of the equipment, and will not miss the slow blockage of the oil filter after long-term use. The identification accuracy is much higher than that of the fixed threshold comparison method. It can detect anomalies in advance before the oil filter is completely blocked, avoiding insufficient lubricating oil supply caused by oil filter blockage and resulting wear of the main unit.

[0044] After the frequency reduction adjustment is initiated, the dynamic minimum frequency generation unit sends status query requests to the lubrication condition dynamic judgment unit periodically according to a pre-stored control cycle. The control cycle is pre-stored at 100 milliseconds, and a request is sent once per control cycle, without waiting for the lubrication condition dynamic judgment unit to actively report, thus avoiding frequency reduction overshoot caused by status delay. The dynamic minimum frequency generation unit receives the rotor lubrication safety status judgment result and lubricating oil emulsification risk judgment result returned by the lubrication condition dynamic judgment unit for the current cycle. Simultaneously, it obtains the current operating frequency of the inverter and the lubricating oil temperature in the oil-gas separator through the multi-source operating parameter acquisition unit, as well as the oil filter pressure loss anomaly judgment result output by the lubrication condition dynamic judgment unit. To avoid false triggering caused by instantaneous pressure fluctuations or sensor interference, the dynamic minimum frequency generation unit internally sets a latching anti-jitter counter. The frequency is latched only when multiple consecutive control cycles return to the critical state. The counting rule of the latching anti-jitter counter is as follows: in This is a latched debounce counter, measured in units, with an initial value of 0. This is the rotor lubrication safety status flag. A value of 1 indicates that the rotor lubrication safety status has reached the critical boundary, while a value of 0 indicates that the rotor lubrication safety status is safe. This is a flag indicating the risk of lubricating oil emulsification. A value of 1 indicates the presence of lubricating oil emulsification risk, while a value of 0 indicates the absence of lubricating oil emulsification risk.

[0045] When the latch anti-jitter counter count value is greater than or equal to 2, the dynamic minimum frequency generation unit stores the current operating frequency of the inverter host collected at the current moment as the minimum allowable operating frequency under the current operating conditions, and sends the minimum allowable operating frequency to the frequency lock-up closed-loop control unit in real time as the basis for frequency limiting. After the frequency lock-up closed-loop control unit receives the minimum allowable operating frequency, all subsequent frequency output values ​​will not be lower than the minimum allowable operating frequency.

[0046] During the frequency reduction process, the dynamic minimum frequency generation unit continuously monitors the lubricating oil temperature in the oil-gas separator and the abnormal oil filter pressure loss judgment result. When the lubricating oil temperature in the oil-gas separator changes by more than 2 degrees Celsius relative to the minimum allowable operating frequency, or when the abnormal oil filter pressure loss judgment result jumps, the dynamic minimum frequency generation unit automatically triggers an additional state judgment. Without waiting for the next fixed control cycle, it immediately sends a state query request to the lubrication condition dynamic judgment unit. Based on the new judgment result and the new current operating frequency of the inverter, the minimum allowable operating frequency is updated. If the new judgment result still shows that the rotor lubrication safety status has reached the critical boundary or there is a risk of lubricating oil emulsification, and the latched anti-jitter counter meets the counting requirements, the new current operating frequency of the inverter is overwritten as the new minimum allowable operating frequency and sent to the frequency latching closed-loop control unit. If the new judgment result shows that the rotor lubrication safety status is safe and there is no risk of lubricating oil emulsification, the minimum allowable operating frequency is not directly cleared; instead, the anti-jitter counter is cleared to avoid frequent frequency locking and unlocking caused by temporary fluctuations in operating conditions. The clearing rule for the anti-jitter counter is as follows: in To clear the anti-vibration counter, the unit is counted as a single counter. The initial value is 0, and the counter is incremented by 1 or cleared to zero each control cycle. When the count value of clearing the anti-vibration counter is greater than or equal to 300, the rotor lubrication safety status is safe for 30 consecutive seconds and there is no risk of lubricating oil emulsification. The dynamic minimum frequency generation unit automatically clears the stored minimum allowable operating frequency and sends a release limit signal to the frequency lock-up closed-loop control unit, allowing the inverter host to continue to perform frequency reduction adjustment according to the real-time pressure of the compressed air pipeline network.

[0047] Taking a screw air compressor with a rated power of 75 kW and a rated discharge pressure of 0.8 MPa as an example, the equipment initially operates at the rated frequency of 50 Hz. A decrease in on-site air consumption causes the real-time pressure of the compressed air pipeline to rise to 0.75 MPa, exceeding the pre-stored constant pressure regulation target value of 0.7 MPa. The frequency lock-up closed-loop control unit initiates frequency reduction regulation, and the inverter host's current operating frequency decreases from 50 Hz at a rate of 0.5 Hz per second. At this time, the lubricating oil temperature in the oil-gas separator is 25 degrees Celsius, the oil filter pressure loss anomaly judgment result is 0, and the lubrication condition dynamic judgment unit calculates the critical lubricating oil pump outlet pressure under the current operating condition. The threshold is 0.25 MPa. The real-time lubricating oil pump outlet pressure decreases synchronously with the decrease of the current operating frequency of the inverter host. When the current operating frequency of the inverter host drops to 32 Hz, the real-time lubricating oil pump outlet pressure is 0.248 MPa, which is lower than the critical threshold of the lubricating oil pump outlet pressure. The lubrication condition dynamic judgment unit returns the result that the rotor lubrication safety state has reached the critical boundary after two consecutive control cycles. The dynamic minimum frequency generation unit stores 32 Hz as the minimum allowable operating frequency and sends it to the frequency lock-up closed-loop control unit. The frequency lock-up closed-loop control unit stops the frequency reduction and keeps the current operating frequency of the inverter host at 32 Hz.

[0048] After 10 minutes of equipment operation, the on-site air consumption increases, and the current operating frequency of the inverter host rises to 40 Hz. The temperature of the lubricating oil in the oil-gas tank rises to 35 degrees Celsius with the increase in load. The temperature change reaches 10 degrees Celsius, exceeding the trigger threshold of 2 degrees Celsius. The dynamic minimum frequency generation unit immediately triggers an additional state judgment. At this time, the actual kinematic viscosity of the lubricating oil decreases with the increase in temperature. The dynamic judgment unit of lubrication condition calculates a new critical threshold of 0.22 MPa for the lubricating oil pump outlet pressure. The real-time lubricating oil pump outlet pressure is 0.28 MPa, which is higher than the critical threshold. The rotor lubrication safety state is safe and there is no risk of lubricating oil emulsification. The anti-vibration counter is cleared and starts counting. After 30 consecutive seconds of judgment results all being safe, the dynamic minimum frequency generation unit clears the minimum allowable operating frequency of 32 Hz. The frequency lock-up closed-loop control unit resumes normal frequency reduction adjustment. Subsequently, when the current operating frequency of the inverter host drops to 28 Hz, the lubrication critical threshold is triggered again. The dynamic minimum frequency generation unit stores 28 Hz as the new minimum allowable operating frequency.

[0049] If the frequency reduction process continues from 50 Hz until the real-time pressure of the compressed air pipeline returns to the constant pressure regulation target pressure value, and the critical boundary of the rotor lubrication safety state or the risk of lubricating oil emulsification is never triggered, the dynamic minimum frequency generation unit will not generate the minimum allowable operating frequency. The frequency lock-up closed-loop control unit will control the current operating frequency of the inverter host normally according to the constant pressure regulation requirements, without applying an additional frequency lower limit.

[0050] In each control cycle, the frequency lock-up closed-loop control unit acquires the real-time pressure of the compressed air pipeline through the multi-source operating parameter acquisition unit. The control cycle is pre-stored for 100 milliseconds, consistent with the query cycle of the dynamic minimum frequency generation unit. It reads the constant pressure regulation target pressure value pre-stored in the frequency lock-up closed-loop control unit, and simultaneously reads the minimum allowable operating frequency under the current operating conditions output by the dynamic minimum frequency generation unit. First, it calculates the pressure deviation, which is the current real-time pressure of the compressed air pipeline minus the constant pressure regulation target pressure value, in megapascals (MPA). Then, based on the real-time pressure of the compressed air pipeline for five consecutive control cycles, it calculates the pressure change rate using the sliding window least squares method, in megapascals per second (MPA). The pressure change rate is used as a feedforward input to the PID calculation module to adjust the frequency output in advance when the pressure changes rapidly, reducing pressure overshoot.

[0051] The frequency locking closed-loop control unit uses an incremental PID algorithm to calculate the frequency increment. Unlike existing fixed-parameter PID and hard-limiting schemes, this algorithm employs variable parameter adjustment and a frequency locking integral separation mechanism. It automatically adjusts the PID parameters under different pressure deviations and frequency locking conditions, avoiding response lag caused by integral saturation. The incremental PID calculation formula is as follows: in The frequency output increment for the current control cycle, in Hertz. This is the proportionality coefficient, measured in Hertz per megapascal. The integral coefficient is expressed in Hertz per megapascal per second. These are the differential coefficients, with units of Hertz-seconds per megapascal. To control the cycle duration, a value of 0.1 seconds is used. The pressure deviation for the current control cycle, This represents the pressure deviation from the previous control cycle. The pressure deviation from the previous control cycle is used as the unit for all parameters, and the dimensional calculation result is in Hertz.

[0052] When the absolute value of the pressure deviation is greater than 0.05 MPa, the integral term is disabled, and proportional-derivative control is used for rapid response to large deviations, with a large proportional coefficient. When the absolute value of the pressure deviation is less than or equal to 0.05 MPa, the integral term is engaged to eliminate steady-state error, with a small proportional coefficient. When in frequency-locked operation, if the pressure deviation is positive (i.e., the real-time pressure of the compressed air pipeline is higher than the constant pressure regulation target pressure value), the integral term is directly set to 0 and does not accumulate, avoiding saturation of the integral term due to prolonged frequency locking, which would cause a lag in frequency increase response when the pressure decreases subsequently. If the pressure deviation is negative (i.e., the real-time pressure of the compressed air pipeline is lower than the constant pressure regulation target pressure value), the integral term accumulates normally to ensure the frequency increase response speed. After calculating the frequency increment, it is added to the output frequency of the previous control cycle to obtain the target output frequency. The target output frequency is subject to bidirectional limiting, ensuring that the target output frequency does not exceed 50 Hz of the inverter host's rated frequency and does not fall below the minimum allowable operating frequency under the current conditions.

[0053] When the real-time pressure of the compressed air pipeline is higher than the constant pressure regulation target pressure value, the PID calculation outputs a negative frequency increment, i.e., a frequency reduction signal. If the calculated target output frequency is higher than the minimum allowable operating frequency, the current operating frequency of the variable frequency drive is reduced to decrease the compressor's discharge volume. If the calculated target output frequency is lower than or equal to the minimum allowable operating frequency, the minimum allowable operating frequency is directly output, the frequency lock operation status flag is set, the frequency lock operation status is entered, and the frequency reduction regulation channel is closed, leaving only the frequency increase regulation channel.

[0054] When the real-time pressure of the compressed air pipeline is lower than the constant pressure regulation target pressure value, the PID calculation outputs a positive frequency increment, i.e., a frequency increase signal, which controls the current operating frequency of the inverter host to increase, thereby increasing the compressor's discharge volume. If it was previously in frequency lock operation mode, when the target output frequency is higher than the minimum allowable operating frequency, the frequency lock operation status flag will be automatically cleared, and the bidirectional regulation function will be restored.

[0055] The entire frequency regulation process employs shock-free frequency transition control. The frequency change within a single cycle is not subject to a fixed limit, but rather dynamically adjusted based on the pressure deviation. The larger the pressure deviation, the greater the permissible single-cycle change; conversely, the closer the pressure deviation is to zero, the smaller the permissible change. This ensures a fast response time under large deviations while preventing overshoot in frequency adjustment under small deviations. The dynamic single-cycle frequency limit calculation formula is as follows: in The maximum allowable frequency change of the output during the current control cycle, in Hertz. The pre-stored reference single-cycle frequency variation limit is set to 0.5 Hz. The deviation calibration coefficient is set to 0.02 MPa. When the absolute value of the frequency increment calculated by the PID is less than or equal to the dynamic limit, the frequency increment is directly output to adjust the current operating frequency of the inverter host. When the absolute value of the frequency increment is greater than the dynamic limit, the dynamic limit is output according to the positive and negative directions of the frequency increment, and the excess part is accumulated to participate in the calculation in the next control cycle.

[0056] Taking a screw air compressor with a rated discharge pressure of 0.8 MPa as an example, the constant pressure regulation target pressure value is set to 0.7 MPa. Under the current operating conditions, the minimum allowable operating frequency output by the dynamic minimum frequency generation unit is 32 Hz. The equipment initially operates at 40 Hz. The current real-time pressure of the compressed air pipeline is 0.75 MPa, the pressure deviation is 0.05 MPa, and the pressure change rate is 0.01 MPa per second. The PID calculation yields a frequency increment of -0.4 Hz, and the dynamic limit calculation is 0.49 Hz. Since the frequency increment is less than the limit, the output is 39.6 Hz. The frequency is reduced cycle by cycle. When the current operating frequency of the inverter host drops to 32 Hz, the target output frequency calculated by the PID is 31.5 Hz, which is lower than the minimum allowable operating frequency. The frequency lock-up closed-loop control unit directly outputs 32 Hz and enters the frequency lock-up operation state. At this time, the real-time pressure of the compressed air pipeline is 0.71 MPa, the pressure deviation is positive, and the integral term stops accumulating to avoid integral saturation. As the on-site air consumption increased, the real-time pressure of the compressed air pipeline dropped to 0.69 MPa, with a pressure deviation of -0.01 MPa. The PID calculation showed a frequency increment of 0.1 Hz, and the target output frequency was 32.1 Hz, which was higher than the minimum allowable operating frequency. The frequency lock-up closed-loop control unit automatically released the frequency lock-up operation state, and the frequency increased cycle by cycle, increasing the exhaust volume to bring the real-time pressure of the compressed air pipeline back to 0.7 MPa.

[0057] If the dynamic minimum frequency generation unit updates the minimum allowable operating frequency, for example, adjusting it from 32 Hz to 28 Hz, the frequency lock-up closed-loop control unit will reduce the frequency cycle by cycle according to the dynamic limit, smoothly transitioning to 28 Hz after 8 control cycles without any frequency jump. After entering the frequency lock-up operation state, the frequency lock-up closed-loop control unit sends the frequency lock-up status signal to the status linkage protection unit. Upon receiving the signal, the status linkage protection unit initiates the oil filter blockage warning and main unit warm-up and frequency increase related monitoring processes.

[0058] In each control cycle, the frequency lock-up closed-loop control unit obtains the target output frequency of the constant pressure PID closed-loop regulation output, reads the current operating frequency of the inverter host collected by the current multi-source operating parameter acquisition unit, and calculates the frequency difference between the target output frequency and the current operating frequency of the inverter host. The unit of the frequency difference is Hertz. A positive frequency difference indicates that frequency increase regulation needs to be performed, and a negative frequency difference indicates that frequency decrease regulation needs to be performed.

[0059] The maximum frequency change limit for a single cycle is read from the pre-stored value in the frequency lock-up closed-loop control unit. Different maximum frequency change limits for single-cycle frequency change are used for frequency ramp-up and frequency ramp-down adjustments. The pre-stored maximum frequency change limit for single-cycle frequency change in frequency ramp-up is 0.5 Hz per second, and the pre-stored maximum frequency change limit for single-cycle frequency change in frequency ramp-down is 0.3 Hz per second. The ramp-up limit is greater than the ramp-down limit to balance the ramp-up response speed and the ramp-down adjustment smoothness. Unlike the fixed ramp-up rate transition method in existing technologies, this invention uses an S-curve to plan the frequency change process. The frequency change rate gradually increases from 0 to the maximum limit, and then gradually decreases back to 0 when approaching the target output frequency. The acceleration of the frequency change is continuous without abrupt changes, avoiding mechanical shock and pipeline pressure fluctuations caused by motor torque steps. The formula for calculating the single-cycle frequency increment of the S-curve is as follows: in This represents the actual frequency increment of the output during the k-th transition cycle, in Hertz. The maximum frequency change limit per cycle in the corresponding adjustment direction is multiplied by the control period of 0.1 seconds, in Hertz. The total number of transition cycles is calculated by dividing the absolute value of the frequency difference by the average change per cycle. This is a sign function; it takes the value 1 when the frequency difference is positive and -1 when the frequency difference is negative. All parameters have matching units, and the dimensional calculation result is in Hertz. When the absolute value of the frequency difference is less than or equal to the maximum frequency change limit per single cycle multiplied by the control cycle, the target output frequency is directly output to adjust the current operating frequency of the inverter host, without needing to perform an S-type transition.

[0060] When the absolute value of the frequency difference is greater than the maximum frequency change limit in a single cycle multiplied by the control cycle, the current operating frequency of the inverter host is adjusted cycle by cycle according to the S-shaped curve, and the calculated frequency increment is output in each control cycle until the current operating frequency of the inverter host reaches the target output frequency.

[0061] When the dynamic minimum frequency generation unit updates the minimum allowable operating frequency, the frequency lock-up closed-loop control unit calculates the difference between the new minimum allowable operating frequency and the current operating frequency of the inverter host. Based on this difference, it adaptively plans the total transition time; the larger the difference, the longer the total transition time, preventing pressure fluctuations caused by rapid frequency changes. The formula for calculating the total transition time is as follows: in The total transition time after updating to the minimum allowed operating frequency, in seconds. The pre-stored basic transition duration is set to 2 seconds. This is the transition duration coefficient, with a value of 0.2 seconds per hertz. The updated minimum allowed operating frequency, in Hertz. The current operating frequency of the inverter host is in Hertz, and the result is in seconds.

[0062] The transition process adjusts the frequency output cycle by cycle according to the S-shaped curve, smoothly transitioning to the new minimum allowable operating frequency within multiple control cycles without frequency jumps.

[0063] During the transition process, the real-time pressure of the compressed air pipeline network is continuously collected through the multi-source operating parameter acquisition unit. The absolute value of the pressure change rate over three consecutive control cycles is calculated. When the absolute value of the pressure change rate exceeds the pre-stored pressure fluctuation limit of 0.02 MPa per second, the current frequency change rate is automatically reduced to half of the original change rate. At the same time, the total transition time is extended until the absolute value of the real-time pressure change rate of the compressed air pipeline network over three consecutive control cycles is less than 0.01 MPa per second. After the pressure is determined to have returned to a stable range, the normal frequency change rate is restored and the transition continues.

[0064] Taking a 75 kW screw air compressor as an example, the equipment is in frequency-locked operation mode, and the current operating frequency of the variable frequency drive is 32 Hz. The air consumption on site suddenly increases, and the target output frequency of the constant pressure PID closed-loop regulation is 40 Hz. The frequency difference is 8 Hz, and the maximum change limit of the single cycle in the frequency increase direction is 0.5 Hz per second. The total number of transition cycles is calculated to be 160 control cycles. In the S-shaped transition process, the output frequency increment in the first control cycle is 0.005 Hz, and the current operating frequency of the variable frequency drive becomes 32.005 Hz. The frequency change rate gradually increases, and the single cycle increment reaches the maximum value of 0.05 Hz in the 80th control cycle. After that, the frequency change rate gradually decreases, and after 16 seconds, it smoothly transitions to 40 Hz. The motor torque does not jump during the whole process, and the real-time pressure fluctuation of the compressed air pipeline does not exceed 0.01 MPa. If the gas consumption decreases, the target output frequency changes from 40 Hz to 32 Hz, with a frequency difference of -8 Hz. The maximum change limit for the frequency reduction direction per cycle is 0.3 Hz per second, resulting in a total transition cycle of approximately 267 control cycles. The frequency smoothly decreases to 32 Hz in about 26.7 seconds, making the frequency reduction process more stable and preventing sudden pressure increases. If the dynamic minimum frequency generation unit updates the minimum allowable operating frequency from 32 Hz to 28 Hz, with a limit difference of -4 Hz, the calculated total transition time is 2.8 seconds. Adjusting cycle by cycle according to the frequency reduction S-curve, the smooth transition to 28 Hz avoids pressure fluctuations caused by sudden frequency drops.

[0065] If the dynamic minimum frequency generation unit updates the minimum allowable operating frequency from 28 Hz to 35 Hz, the limit difference is 7 Hz, and the calculated total transition time is 3.4 seconds. The frequency is increased cycle by cycle according to the frequency increase S-curve to smoothly transition to 35 Hz, ensuring that the lubrication pressure is established quickly.

[0066] If the real-time pressure change rate of the compressed air pipeline reaches 0.03 MPa per second during the transition process, the frequency change rate will be automatically halved. For example, during the frequency reduction process, the single-cycle increment changes from 0.03 Hz to 0.015 Hz, and the normal increment will be restored after the pressure stabilizes.

[0067] After the frequency transition is complete, the frequency lock-up closed-loop control unit enters the normal constant voltage regulation state and adjusts the frequency normally according to the constant voltage PID closed-loop regulation output.

[0068] This shock-free transition method, which combines an S-curve, adaptive transition time, and pressure feedback adjustment, can eliminate motor torque impact caused by frequency changes, reduce wear on mechanical transmission components, and avoid compressed air pipeline pressure fluctuations caused by frequency steps. The design of different limits for frequency increase and decrease ensures both the speed of frequency increase response under heavy loads and the pressure stability during frequency decrease. The pressure feedback adjustment mechanism can prioritize the stability of the air supply pressure during the transition process, avoiding the impact of the adjustment process on the end user's air consumption.

[0069] After entering the frequency lock operation state, the status linkage protection unit automatically starts two parallel monitoring processes. The first is oil filter blockage monitoring, and the second is lubricating oil temperature monitoring. The two monitoring processes operate independently without interference, and both acquire raw data through the multi-source operating parameter acquisition unit. The oil filter blockage monitoring process continuously collects the lubricating oil pump outlet pressure, using only pressure data under stable operating conditions for calculation. Stable operating conditions refer to a condition where the frequency fluctuation of the inverter host is less than 0.1 Hz and the lubricating oil temperature fluctuation in the oil-gas separator is less than 0.5 degrees Celsius. This avoids misinterpreting normal pressure fluctuations caused by frequency or temperature changes as blockage trends. The weighted least squares method is used to calculate the slope of the lubricating oil pump outlet pressure drop over 60 consecutive seconds, or 600 control cycles. Data closer to the current time has a higher weight, with the latest data having a weight three times that of the oldest data. This allows for earlier identification of accelerating oil filter blockage trends. The formula for calculating the weighted slope is as follows: in The slope of the lubricating oil pump outlet pressure drop is expressed in megapascals per second, and n is the number of data points within the sliding window, with a value of 600. The weight of the i-th data point increases linearly with time approaching the current time. The time for collecting the i-th data point is in seconds. The time is the weighted average of the data within the sliding window, in seconds. The value represents the lubricating oil pump outlet pressure at the i-th data point, in megapascals (MPa). This is the weighted average lubricating oil pump outlet pressure of the data within the sliding window, in megapascals (MPA), with the dimensional calculation result in MPA per second.

[0070] Simultaneously, the cumulative decrease value is calculated. The cumulative decrease value is the difference in lubricating oil pump outlet pressure between the first valid data point and the latest data point within the sliding window, in megapascals (MPa). When the lubrication condition dynamic judgment unit has determined that there is an abnormal oil filter pressure loss, and the cumulative decrease value of the lubricating oil pump outlet pressure exceeds the pre-stored warning pressure threshold of 0.05 MPa, and the decrease slope remains negative for 60 seconds, a first-level oil filter blockage warning is triggered. This is indicated by a local low-frequency audible and visual alert, and a warning signal is simultaneously sent to the remote operation and maintenance platform. The signal includes the current decrease slope and the cumulative decrease value. The remaining service life of the oil filter is also calculated and sent synchronously. The formula for calculating the remaining service life is... RUL represents the remaining service life of the oil filter, in seconds. This is the outlet pressure of the latest lubricating oil pump, in megapascals (MPa). The pre-stored emergency pressure threshold is set to 0.2 MPa. The absolute value of the descent slope is calculated in seconds and can be converted to hours for display, prompting maintenance personnel to schedule replacement within the remaining service life.

[0071] When the cumulative decrease exceeds 0.1 MPa, or the absolute value of the decrease slope is greater than 0.001 MPa per second, a secondary oil filter blockage warning is triggered. In addition to local high-frequency audible and visual prompts and remote emergency signals, a maximum frequency limit command is sent to the frequency lock-up closed-loop control unit to temporarily limit the current operating frequency of the inverter host to no more than 80% of the rated frequency, so as to avoid insufficient lubricating oil supply at high speeds, which could lead to wear of the host.

[0072] The lubricating oil temperature monitoring process continuously collects the lubricating oil temperature in the oil-gas tank through the multi-source operating parameter acquisition unit. The same weighted least squares method is used to calculate the slope of the lubricating oil temperature drop in the oil-gas tank over a continuous 120 seconds. At the same time, the cumulative temperature drop value at the beginning and end of the window is calculated. When the lubricating oil temperature in the oil-gas tank is lower than the pre-stored warm-up trigger temperature threshold of 50 degrees Celsius, and the temperature drop slope is negative for 120 seconds, or when the lubrication condition dynamic judgment unit determines that there is a risk of lubricating oil emulsification, the status linkage protection unit sends a host warm-up frequency increase command to the frequency lock-up closed-loop control unit.

[0073] This invention dynamically calculates the warm-up frequency and duration based on the current temperature and emulsification risk. The lower the temperature, the higher the emulsification risk, and the higher the warm-up frequency. The formula for calculating the warm-up frequency is as follows: in The target operating frequency for this warm-up is in Hertz. The pre-stored base warm-up frequency is set to 35 Hz. This is the warm-up frequency temperature coefficient, with a value of 0.5 Hz per degree Celsius. The pre-stored warm-up exit temperature threshold is set to 60 degrees Celsius. This refers to the current temperature of the lubricating oil inside the oil-gas tank, in degrees Celsius. The emulsification risk frequency coefficient is set to 5 Hz. This serves as a risk indicator for lubricating oil emulsification; a value of 1 indicates a risk, while a value of 0 indicates no risk. The dimensionless calculation is in Hertz. The estimated warm-up time is calculated based on the current temperature and historical temperature rise slope, using the following formula: in This is the estimated duration of the warm-up, in seconds. This represents the average slope of the temperature rise of the lubricating oil in the oil-gas separator over the past 10 minutes, expressed in degrees Celsius per second. The pre-stored basic warm-up time is set to 60 seconds, and the result is calculated in seconds.

[0074] After receiving the host warm-up frequency increase command, the frequency lock-up closed-loop control unit temporarily increases the minimum allowable operating frequency to the calculated warm-up operating frequency, controls the current operating frequency of the inverter host to increase in a shock-free S-shaped curve, and maintains normal operation of constant pressure PID closed-loop regulation during the frequency increase process. When the real-time pressure of the compressed air pipeline increases, the frequency increase is automatically stopped to avoid pressure overpressure.

[0075] After the inverter host's current operating frequency increases to the warm-up operating frequency, it maintains operation. During the warm-up process, the temperature of the lubricating oil in the oil-gas tank is continuously collected, and the actual temperature rise slope is calculated in real time. If the actual temperature rise slope is less than the historical average slope, the warm-up holding time is automatically extended. If the actual temperature rise slope is greater than the historical average slope, the warm-up is terminated in advance after the temperature reaches the warm-up termination temperature threshold.

[0076] When the temperature of the lubricating oil in the oil-gas separator rises to 60 degrees Celsius, or when the dynamic lubrication condition judgment unit determines that the risk of lubricating oil emulsification has disappeared, the warm-up process ends. The frequency lock-up closed-loop control unit controls the inverter host to reduce its current operating frequency to the original minimum allowable operating frequency in a shock-free S-curve manner, and re-enters the frequency lock-up operation state. If the warm-up operation lasts for 30 minutes and the temperature of the lubricating oil in the oil-gas separator still has not risen to 55 degrees Celsius, the status linkage protection unit triggers a temperature control fault warning, prompting maintenance personnel to check the temperature control valve or heating components.

[0077] Taking a 75 kW screw air compressor as an example, in winter, the ambient temperature is 5 degrees Celsius. The equipment operates at 32 Hz with frequency locking. The temperature of the lubricating oil in the oil-gas separator gradually decreases from 55 degrees Celsius to 48 degrees Celsius. The temperature drop slope is negative for 120 consecutive seconds, and there is no risk of lubricating oil emulsification. The warm-up command is triggered. The calculated warm-up frequency is 35 + 0.5 * (60 - 48) = 41 Hz. The expected warm-up time is (60 - 48) / 0.05 + 60 = 300 seconds. The frequency converter's current operating frequency increases to 41 Hz after an 8-second S-shaped transition and remains there. After 280 seconds of operation, the lubricating oil temperature in the oil-gas separator rises back to 60 degrees Celsius, ending the warm-up prematurely. After an 8-second S-shaped transition, the frequency drops back to 32 Hz and re-enters the frequency locking state. Throughout the entire process, the real-time pressure fluctuation of the compressed air pipeline does not exceed 0.01 MPa.

[0078] If, after 6 months of use, the lubricating oil pump outlet pressure drops from 0.3 MPa when the oil filter is new to 0.24 MPa under stable operating conditions, a cumulative drop of 0.06 MPa, with the rate of decrease remaining negative for 60 seconds, and the lubrication condition dynamic judgment unit has determined that the oil filter pressure loss is abnormal, triggering a first-level oil filter blockage warning, the remaining service life is calculated to be approximately 4000 seconds, prompting maintenance personnel to replace the oil filter in a timely manner.

[0079] Please see Figure 2 As shown, the second objective of this invention is to provide a control method for implementing an intelligent air compressor control system, comprising the following steps: S1. Real-time data acquisition of lubricating oil pump outlet pressure, lubricating oil temperature in oil-gas tank, current operating frequency of variable frequency drive, and real-time pressure of compressed air pipeline, which are coaxially connected to the screw compressor, is obtained through the multi-source operating parameter acquisition unit. S2. The lubrication condition dynamic judgment unit determines the rotor lubrication safety status at the current inverter host speed based on the lubricating oil pump outlet pressure, determines the lubricating oil emulsification risk based on the lubricating oil temperature in the oil-gas tank combined with the pre-stored oil viscosity-temperature characteristics, and identifies abnormal oil filter pressure loss based on the changing trend of the lubricating oil pump outlet pressure. S3. During the process of frequency reduction regulation triggered by real-time pressure change in compressed air pipeline, the judgment result output by the dynamic judgment unit of lubrication condition is obtained cycle by cycle. When the rotor lubrication safety state reaches the critical boundary or there is a risk of lubricating oil emulsification, the current operating frequency of the inverter host collected at the current moment is determined as the minimum allowable operating frequency under the current condition. The minimum allowable operating frequency is dynamically updated with the lubricating oil temperature in the oil-gas tank and the oil filter pressure loss status. S4. The frequency lock-up closed-loop control unit performs constant pressure PID closed-loop regulation based on the real-time pressure of the compressed air pipeline network. During the regulation process, the output frequency of the frequency converter is limited to no less than the minimum allowable operating frequency output by the dynamic minimum frequency generation unit. At the same time, impact-free frequency transition control is performed to avoid pressure fluctuations in the compressed air pipeline network caused by frequency changes. S5. After entering the frequency lock operation state, the status linkage protection unit triggers an oil filter blockage warning based on the continuous downward trend of the lubricating oil pump outlet pressure, and triggers a main unit warm-up frequency increase command based on the continuous downward trend of the lubricating oil temperature in the oil-gas tank or the risk of lubricating oil emulsification. After the protection action is completed, it returns to the constant pressure frequency lock adjustment step for continuous operation.

[0080] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. An intelligent air compressor control system, characterized in that, include: The multi-source operating parameter acquisition unit is used to collect in real time the lubricating oil pump outlet pressure, lubricating oil temperature in the oil-gas tank, current operating frequency of the inverter host, and real-time pressure of the compressed air pipeline network, which are coaxially connected to the screw compressor host. The dynamic lubrication condition determination unit is used to determine the rotor lubrication safety status at the current inverter host speed based on the lubricating oil pump outlet pressure, determine the lubricating oil emulsification risk based on the lubricating oil temperature in the oil-gas separator and the oil viscosity-temperature characteristics, and identify abnormal oil filter pressure loss based on the changing trend of the lubricating oil pump outlet pressure. The dynamic minimum frequency generation unit is used to obtain the judgment result output by the dynamic judgment unit of lubrication condition on a cycle during the frequency reduction adjustment process of real-time pressure change of compressed air pipeline. When the rotor lubrication safety state reaches the critical boundary or there is a risk of lubricating oil emulsification, the current operating frequency of the inverter host collected at the current moment is determined as the minimum allowable operating frequency under the current working condition. The minimum allowable operating frequency is dynamically updated with the lubricating oil temperature in the oil-gas tank and the oil filter pressure loss status. The frequency lock-up closed-loop control unit is used to perform constant pressure closed-loop regulation based on the real-time pressure of the compressed air pipeline network. During the regulation process, the output frequency of the inverter host is limited to no less than the minimum allowable operating frequency output by the dynamic minimum frequency generation unit. At the same time, it performs impact-free frequency transition control to avoid pressure fluctuations in the compressed air pipeline network caused by frequency abrupt changes. The status linkage protection unit is used to trigger an oil filter blockage warning based on the continuous downward trend of the lubricating oil pump outlet pressure and to trigger a main unit warm-up and frequency increase command based on the continuous downward trend of the lubricating oil temperature in the oil-gas tank during frequency lock operation.

2. The intelligent air compressor control system according to claim 1, characterized in that: The determination of the rotor lubrication safety status at the current inverter host speed based on the lubricating oil pump outlet pressure specifically includes: The real-time speed of the lubricating oil pump coaxially connected to the screw compressor is determined based on the real-time collected current operating frequency of the inverter host. By combining the temperature of the lubricating oil in the oil-gas tank with the pre-stored viscosity-temperature characteristics of the oil, the actual kinematic viscosity of the lubricating oil at the current temperature can be obtained; The critical threshold of lubricating oil pump outlet pressure that meets the rotor lubrication requirements under the current operating conditions is calculated based on the real-time rotation speed and the actual kinematic viscosity of the lubricating oil. The lubricating oil pump outlet pressure is compared with the critical threshold value of the lubricating oil pump outlet pressure: When the lubricating oil pump outlet pressure is greater than or equal to the critical threshold of the lubricating oil pump outlet pressure, the rotor lubrication safety state is determined to be safe. When the outlet pressure of the lubricating oil pump is less than the critical threshold of the lubricating oil pump outlet pressure, the rotor lubrication safety state is determined to have reached the critical boundary.

3. The intelligent air compressor control system according to claim 1, characterized in that: The oil viscosity-temperature characteristics are pre-existing within the dynamic lubrication condition determination unit, serving as a calibration characteristic curve characterizing the relationship between the actual kinematic viscosity of the lubricating oil and the temperature change of the lubricating oil in the oil-gas separator. These oil viscosity-temperature characteristics are obtained through calibration in the following manner: After the air compressor is first started and debugged or the lubricating oil is replaced, the inverter host is controlled to warm up under no-load to the preset state, and the inverter host is controlled to run at the rated frequency. When the temperature of the lubricating oil in the oil-gas separator is stable at different preset temperature nodes, the multi-source operating parameter acquisition unit synchronously collects the lubricating oil temperature in the oil-gas separator, the lubricating oil pump outlet pressure, and the current operating frequency of the inverter host at the corresponding temperature node. The real-time speed of the lubricating oil pump coaxially connected to the screw compressor is determined based on the current operating frequency of the inverter host at each temperature node. Combined with the pre-stored rated structural displacement parameters of the lubricating oil pump coaxially connected to the screw compressor, the actual kinematic viscosity of the lubricating oil at each temperature node is calculated. The lubricating oil temperature in the oil-gas separator at all temperature nodes is continuously fitted with the corresponding actual kinematic viscosity of the lubricating oil to obtain the viscosity-temperature relationship curve, which is the oil viscosity-temperature characteristic. During equipment operation, the lubrication condition dynamic judgment unit also periodically self-corrects the oil viscosity-temperature characteristic based on the lubricating oil temperature in the oil-gas separator, the lubricating oil pump outlet pressure, and the current operating frequency of the inverter host to match the viscosity characteristic changes caused by lubricating oil aging.

4. The intelligent air compressor control system according to claim 1, characterized in that: The method of determining the risk of lubricating oil emulsification based on the temperature of the lubricating oil in the oil-gas tank and the viscosity-temperature characteristics of the oil specifically includes: The lubricating oil temperature in the oil-gas tank and the real-time pressure of the compressed air pipeline are obtained. The table of saturated water vapor partial pressure under standard atmospheric pressure is called in the pre-existing dynamic judgment unit of lubrication conditions. The pressure dew point temperature of the condensate from the compressed air under the current system pressure is calculated based on the real-time pressure of the compressed air pipeline. The system calls up the pre-stored oil viscosity-temperature characteristics, obtains the actual kinematic viscosity of the lubricating oil at the current temperature based on the real-time collected temperature of the lubricating oil in the oil-gas tank, and calls up the water separation critical viscosity pre-stored in the lubrication condition dynamic judgment unit that corresponds to the current lubricating oil model. The water separation critical viscosity is the critical viscosity value at which the lubricating oil cannot effectively separate and precipitate condensate. When the temperature of the lubricating oil in the oil-gas tank is lower than the pressure dew point temperature and the actual kinematic viscosity of the lubricating oil is higher than the critical viscosity for water separation for a preset duration, it is determined that there is a risk of lubricating oil emulsification; otherwise, it is determined that there is no risk of lubricating oil emulsification.

5. The intelligent air compressor control system according to claim 1, characterized in that: The method of identifying abnormal oil filter pressure loss based on the changing trend of lubricating oil pump outlet pressure specifically includes: When the current operating frequency of the inverter host remains at the preset frequency and the actual kinematic viscosity of the lubricating oil obtained by calling the viscosity-temperature characteristics of the lubricating oil based on the temperature of the lubricating oil in the oil-gas tank remains at the preset kinematic viscosity, the oil filter pressure loss abnormality identification process is triggered. Within a continuously preset judgment period, the lubricating oil pump outlet pressure data is continuously collected by the multi-source operating parameter acquisition unit. The reference lubricating oil pump outlet pressure stored in the lubrication condition dynamic judgment unit is called. The reference lubricating oil pump outlet pressure is the historical calibrated normal pressure value corresponding to the current operating frequency of the same inverter host and the actual kinematic viscosity of the same lubricating oil under the condition that the oil filter is normal and unblocked. The changing trend and cumulative decrease of the continuously collected lubricating oil pump outlet pressure relative to the reference lubricating oil pump outlet pressure are calculated. When the cumulative decrease of the lubricating oil pump outlet pressure relative to the reference lubricating oil pump outlet pressure exceeds the preset pressure loss threshold and the lubricating oil pump outlet pressure maintains a continuous downward trend for the preset judgment period, it is judged that the oil filter pressure loss is abnormal.

6. The intelligent air compressor control system according to claim 1, characterized in that: After frequency reduction regulation is initiated, a status query request is sent to the lubrication condition dynamic judgment unit once every preset control cycle. The unit receives the rotor lubrication safety status judgment result and lubricating oil emulsification risk judgment result for the current cycle from the lubrication condition dynamic judgment unit. Simultaneously, the current operating frequency of the inverter and the lubricating oil temperature in the oil-gas separator are acquired through the multi-source operating parameter acquisition unit, along with the oil filter pressure loss anomaly judgment result output by the lubrication condition dynamic judgment unit. When the returned result indicates that the rotor lubrication safety status has reached the critical boundary or that there is a risk of lubricating oil emulsification, the current operating frequency of the inverter is immediately stored as the minimum allowable operating frequency under the current conditions. This minimum allowable operating frequency is then sent in real-time to the frequency lock-up closed-loop control unit as the basis for frequency limiting. During the frequency reduction process, if... When the temperature of the lubricating oil in the oil-gas separator changes or the oil filter pressure loss changes abnormally, the dynamic minimum frequency generation unit automatically triggers an additional state judgment. Based on the new judgment result and the current operating frequency of the frequency converter, the minimum allowable operating frequency is updated. When the rotor lubrication safety status is safe and there is no risk of lubricating oil emulsification within a continuous preset time, the dynamic minimum frequency generation unit automatically clears the stored minimum allowable operating frequency and sends a release limit signal to the frequency lock-up closed-loop control unit, allowing the frequency converter to continue to perform frequency reduction adjustment according to the real-time pressure of the compressed air pipeline. If the frequency reduction process does not trigger the critical boundary of rotor lubrication safety status or the risk of lubricating oil emulsification, no minimum allowable operating frequency is generated, and the frequency lock-up closed-loop control unit controls the current operating frequency of the frequency converter normally according to the constant pressure adjustment requirements.

7. The intelligent air compressor control system according to claim 1, characterized in that: Each control cycle acquires the real-time pressure of the compressed air pipeline network through a multi-source operating parameter acquisition unit, reads the constant pressure regulation target pressure value pre-stored in the frequency lock-up closed-loop control unit, and simultaneously reads the minimum allowable operating frequency under the current conditions output by the dynamic minimum frequency generation unit. The pressure deviation between the real-time compressed air pipeline network pressure and the constant pressure regulation target pressure value is calculated. Combining the real-time compressed air pipeline network pressure acquired from multiple consecutive control cycles, the pressure change rate is calculated. The pressure deviation and pressure change rate are input into the PID calculation module to obtain the target output frequency. Bidirectional limiting processing is applied to the target output frequency, ensuring that the target output frequency does not exceed the rated frequency of the inverter host and is not lower than the minimum allowable operating frequency under the current conditions. When the real-time compressed air pipeline network pressure is higher than the constant pressure regulation target pressure value, the PID calculation outputs a frequency reduction signal. If the calculated target output frequency is higher than the minimum allowable operating frequency, the PID calculation outputs a frequency reduction signal. If the frequency is controlled to decrease, the current operating frequency of the inverter host will decrease. If the calculated target output frequency is lower than or equal to the minimum allowable operating frequency, the minimum allowable operating frequency will be output directly, entering the frequency lock operation state and closing the frequency reduction adjustment channel, leaving only the frequency increase adjustment channel open. When the real-time pressure of the compressed air pipeline is lower than the constant pressure adjustment target pressure value, the PID calculation outputs the frequency increase signal to control the current operating frequency of the inverter host to increase the exhaust volume. When the target output frequency is higher than the minimum allowable operating frequency, the frequency lock operation state will be automatically released. The entire frequency adjustment process is performed with impact-free frequency transition control. The frequency change within a single control cycle does not exceed the pre-stored single-cycle frequency change limit to avoid pressure fluctuations in the compressed air pipeline caused by sudden changes in the current operating frequency of the inverter host. After entering the frequency lock operation state, the frequency lock-up closed-loop control unit sends the frequency lock status signal to the status linkage protection unit to trigger the corresponding protection process.

8. The intelligent air compressor control system according to claim 1, characterized in that: Each control cycle acquires the target output frequency of the constant-pressure PID closed-loop regulation, reads the current operating frequency of the inverter host acquired by the multi-source operating parameter acquisition unit, calculates the frequency difference between the target output frequency and the current operating frequency of the inverter host, and reads the single-cycle maximum frequency change limit pre-stored in the frequency lock-up closed-loop control unit. When the absolute value of the frequency difference is less than or equal to the single-cycle maximum frequency change limit, the target output frequency is directly output to adjust the current operating frequency of the inverter host. When the absolute value of the frequency difference is greater than the single-cycle maximum frequency change limit, the current operating frequency of the inverter host is adjusted cycle by cycle according to the positive and negative directions of the frequency difference and the single-cycle maximum frequency change limit until the current operating frequency of the inverter host reaches the target output frequency. When the dynamic minimum frequency generation unit updates the minimum allowable operating frequency, the frequency lock-up closed-loop control unit... The system calculates the difference between the new minimum allowable operating frequency and the current operating frequency of the inverter host. It then adjusts the frequency output cycle by cycle according to a pre-stored ramp transition rate, smoothly transitioning to the new minimum allowable operating frequency over multiple control cycles without frequency jumps. Different single-cycle maximum frequency change limits are used for frequency increase and decrease regulation, with the latter being greater than the former, balancing the speed of frequency increase response and the stability of frequency decrease regulation. During the transition, the system continuously collects the real-time pressure of the compressed air pipeline. When the real-time pressure fluctuation exceeds the pre-stored pressure fluctuation limit, the current frequency change rate is automatically reduced to half of the original rate until the real-time pressure returns to a stable range, at which point the normal rate of change is restored. After the frequency transition is complete, the system enters a normal constant pressure regulation state.

9. The intelligent air compressor control system according to claim 1, characterized in that: After entering the frequency lock operation state, the status linkage protection unit continuously collects the lubricating oil pump outlet pressure through the multi-source operating parameter acquisition unit, calculates the descent slope and cumulative descent value of the lubricating oil pump outlet pressure within a continuously preset warning duration, and triggers an oil filter blockage warning when the lubrication condition dynamic judgment unit has determined that there is an abnormal oil filter pressure loss, and the cumulative descent value of the lubricating oil pump outlet pressure exceeds the pre-stored warning pressure threshold and the descent slope remains negative for the preset warning duration, and outputs local prompts and remote operation and maintenance signals. At the same time, the status linkage protection unit continuously collects the lubricating oil temperature in the oil-gas separator through the multi-source operating parameter acquisition unit, calculates the temperature descent slope and cumulative temperature drop value of the lubricating oil temperature in the oil-gas separator within a continuously preset warm-up trigger duration, and triggers a warning when the lubricating oil temperature in the oil-gas separator is lower than the pre-stored warm-up trigger temperature. When the temperature drop slope remains negative and reaches the preset warm-up trigger time, or when the lubrication condition dynamic judgment unit determines that there is a risk of lubricating oil emulsification, a host warm-up frequency increase command is sent to the frequency lock-up closed-loop control unit. After receiving the host warm-up frequency increase command, the frequency lock-up closed-loop control unit temporarily increases the minimum allowable operating frequency to the pre-stored warm-up operating frequency, controls the current operating frequency of the inverter host to increase to the warm-up operating frequency in a non-impact frequency transition mode and maintains the pre-stored warm-up time. During the warm-up process, the temperature of the lubricating oil in the oil-gas tank is continuously collected. When the temperature of the lubricating oil in the oil-gas tank rises back to the pre-stored warm-up exit temperature threshold, the frequency lock-up closed-loop control unit controls the current operating frequency of the inverter host to decrease to the original minimum allowable operating frequency in a non-impact frequency transition mode and re-enters the frequency lock-up operation state.

10. A control method for implementing an intelligent air compressor control system according to any one of claims 1-9, characterized in that: Includes the following steps: S1. Real-time data acquisition of lubricating oil pump outlet pressure, lubricating oil temperature in oil-gas tank, current operating frequency of variable frequency drive, and real-time pressure of compressed air pipeline, which are coaxially connected to the screw compressor, is obtained through the multi-source operating parameter acquisition unit. S2. The lubrication condition dynamic judgment unit determines the rotor lubrication safety status at the current inverter host speed based on the lubricating oil pump outlet pressure, determines the lubricating oil emulsification risk based on the lubricating oil temperature in the oil-gas tank combined with the pre-stored oil viscosity-temperature characteristics, and identifies abnormal oil filter pressure loss based on the changing trend of the lubricating oil pump outlet pressure. S3. During the process of frequency reduction regulation triggered by real-time pressure change in compressed air pipeline, the judgment result output by the dynamic judgment unit of lubrication condition is obtained cycle by cycle. When the rotor lubrication safety state reaches the critical boundary or there is a risk of lubricating oil emulsification, the current operating frequency of the inverter host collected at the current moment is determined as the minimum allowable operating frequency under the current condition. The minimum allowable operating frequency is dynamically updated with the lubricating oil temperature in the oil-gas tank and the oil filter pressure loss status. S4. The frequency lock-up closed-loop control unit performs constant pressure PID closed-loop regulation based on the real-time pressure of the compressed air pipeline network. During the regulation process, the output frequency of the frequency converter is limited to no less than the minimum allowable operating frequency output by the dynamic minimum frequency generation unit. At the same time, impact-free frequency transition control is performed to avoid pressure fluctuations in the compressed air pipeline network caused by frequency changes. S5. After entering the frequency lock operation state, the status linkage protection unit triggers an oil filter blockage warning based on the continuous downward trend of the lubricating oil pump outlet pressure, and triggers a main unit warm-up frequency increase command based on the continuous downward trend of the lubricating oil temperature in the oil-gas tank or the risk of lubricating oil emulsification. After the protection action is completed, it returns to the constant pressure frequency lock adjustment step for continuous operation.

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