A variable frequency converter control method based on multi-section speed superposition and number selection

Abstract

This invention discloses a frequency converter control method based on multi-speed superposition and number selection, relating to the field of frequency converter drive control technology. It includes: pre-setting N digital input terminals in the frequency converter, configuring a base frequency value and a time-varying waveform function for each terminal; configuring the final output frequency as the sum of the products of the base frequency value and the waveform function of each activated terminal; determining the activation sequence of each terminal according to the target process curve; the frequency converter calculating and superimposing the instantaneous frequency contribution value of each activated terminal in real time to generate the final output frequency to drive the motor. This invention replaces traditional fixed frequency values ​​with waveform functions such as ramp functions, pulse functions, and exponential decay functions, allowing the synthesis of complex process speed curves using simple digital input terminals. While ensuring control performance, it significantly reduces hardware costs and can be widely applied in injection molding machines, die-casting machines, textile machinery, and other applications.

Description

Technical Field

[0001] This invention relates to the field of variable frequency drive control technology, and in particular to a variable frequency drive control method based on multi-speed superposition and number selection, which is especially suitable for process equipment that needs to generate complex speed curves, such as injection molding machines, die casting machines, material testing machines, textile machinery and printing machinery. Background Technology

[0002] As the core device for motor speed control, frequency converters are widely used in various industrial automation applications. Multi-speed control is one of the basic functions of frequency converters. Its principle is to use multiple digital input terminals to select a preset fixed frequency value, thereby realizing the switching of the motor between different speeds.

[0003] Existing multi-speed control methods can be mainly divided into the following two categories:

[0004] One method is binary encoding selection mode: The on / off states of multiple digital input terminals are treated as binary codes, with each code corresponding to a preset fixed frequency. This method allows for the implementation of multiple speed ranges with fewer terminals (N terminals can achieve a maximum of 2...). N (Segment speed), but when it is necessary to switch between two adjacent speeds, the state of multiple terminals needs to be changed simultaneously. Due to the difference in physical delay of switching devices or PLC output signals, an instantaneous illegal coding state may occur during the switching process, causing the inverter output frequency to jump drastically, impacting mechanical equipment, and even causing product quality problems.

[0005] The second method is direct selection of the superposition mode: each digital input terminal independently corresponds to a preset fixed frequency, and the final output frequency is the arithmetic sum of the frequencies corresponding to all active terminals. This method has a simple structure, but in existing technologies, each terminal corresponds to only one constant frequency value, making it impossible to generate complex speed curves that change over time. For processes such as injection molding machine injection and die casting machine impact, which require multiple complex waveform characteristics such as "slow start-up—fast impact-deceleration and pressure holding," traditional multi-segment speed methods cannot meet the requirements.

[0006] Currently, in industrial settings, analog output modules of PLCs or servo drives are commonly used to replace frequency converters to address the aforementioned issues. Using analog output modules of PLCs involves the PLC calculating and outputting continuously changing analog voltage or current signals to the frequency converter in real time. This requires the PLC to have analog output channels, resulting in high hardware costs and demanding high processing speed and real-time communication performance from the PLC. While servo drives can achieve precise speed curve control, they are even more expensive, and in some high-power applications (such as large injection molding machines), the power rating of servo drives may not be sufficient.

[0007] Therefore, there is an urgent need for a control method that can generate complex speed curves in a low-cost manner using the existing digital input terminals of the frequency converter. To this end, a frequency converter control method based on multi-speed superposition and number selection is proposed. Summary of the Invention

[0008] The present invention aims to provide a frequency converter control method based on multi-speed superposition and number selection, so as to solve the technical problems that existing frequency converter multi-speed control cannot generate complex speed curves, and that the cost of using analog output or servo drive schemes is too high.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] This invention provides a frequency converter control method based on multi-speed superposition and number selection, comprising the following steps:

[0011] Step S1: Parameter preset

[0012] The frequency converter has N preset digital input terminals (DI1, DI2, ..., DIN), and each digital input terminal is configured with a corresponding base frequency value. and the corresponding waveform functions , where i = 1, 2, ..., N, N ≥ 2.

[0013] The waveform function is selected from any one or more combinations of the following function groups:

[0014] ramp function Used to achieve linear acceleration or deceleration, represented as:

[0015] ,in Is it an upward or downward pattern?

[0016] impulse function Used to achieve instantaneous impact, represented as: ;

[0017] Exponential decay function Used to achieve asymptotic deceleration, represented as: ;

[0018] sinusoidal oscillation function Used to achieve damped oscillations, represented as:

[0019] ;

[0020] Custom table lookup function Used to implement arbitrary custom waveforms, represented as: .

[0021] Step S2: Output frequency synthesis

[0022] Configure the final output frequency of the frequency converter The sum of the products of the fundamental frequency value and the waveform function of each active terminal is expressed as: ,in This represents the switch state of the i-th digital input terminal, with a value of 0 or 1. This is a normalized waveform function with a range of [0,1]. This is the fundamental frequency value corresponding to the i-th terminal, in Hertz;

[0023] The fundamental frequency values ​​of the plurality of digital input terminals Configured according to binary weight incrementing method, that is ,in To achieve the minimum frequency step size, 2 is achieved through the switching combination of each terminal. N -1 discrete frequency outputs.

[0024] In a preferred embodiment, the final output frequency It also includes the fundamental frequency term. Fundamental frequency term Analog input signals or communication bus signals are given, represented as: Among them, the fundamental frequency term The continuously adjustable main speed signal is usually given by the PLC through a 0~10V analog output or a fieldbus (such as Profibus, Modbus, EtherCAT).

[0025] Step S3: Timing Control

[0026] Based on the requirements of the target process curve, determine the activation timing of each digital input terminal and generate timing control signals.

[0027] The activation timing adopts a "select M from N" combined activation strategy, that is, at the same time, M terminals are selected from N digital input terminals for activation, where 1≤M≤N, and different activation combinations correspond to different synthesized waveform characteristics.

[0028] Step S4: Real-time calculation

[0029] The timing control signal is applied to the corresponding digital input terminal, and the frequency converter calculates the instantaneous frequency contribution value of each activated terminal in real time and superimposes them to generate the final output frequency.

[0030] The frequency converter is equipped with a frequency calculation unit, which includes:

[0031] The waveform generator module is used to generate waveforms based on the waveform parameter vectors of each terminal. The corresponding normalized waveform function value is generated;

[0032] A multiplier array is used to calculate the instantaneous frequency contribution value of each active terminal. ,

[0033] An accumulator is used to sum all contribution values ​​to obtain the final output frequency. ;

[0034] A limiter is used to limit the final output frequency to a preset minimum frequency. and maximum frequency between.

[0035] In a preferred embodiment, the frequency converter further includes an output smoothing module for adjusting the final output frequency. A first-order low-pass filter is performed to eliminate the frequency jump caused by the instantaneous switching of the digital input terminal states. The filtered output frequency is: ,in , The sampling period is This is the filtering time constant.

[0036] Step S5: Drive Output

[0037] The frequency converter drives the motor to run according to the final output frequency, thereby synthesizing the target process curve.

[0038] In a preferred embodiment, the method further includes a waveform parameter self-tuning step:

[0039] In trial operation mode, the actual motor speed feedback signal is collected in real time. ;

[0040] Calculate the actual speed feedback signal and the target process curve. Error between ,in: ;

[0041] According to the error The waveform parameter vectors of each digital input terminal are automatically adjusted using gradient descent or particle swarm optimization algorithms. This continues until the root mean square value of the error is less than a preset threshold.

[0042] In a preferred embodiment, the method is applied to the injection process control of an injection molding machine, wherein the N digital input terminals include at least:

[0043] First terminal, configure ramp function and fundamental frequency value This is used to achieve a slow start-up curve;

[0044] The second terminal is configured with a pulse function. and fundamental frequency value , used to achieve a rapid impact curve;

[0045] The third terminal is configured with an exponential decay function. and fundamental frequency value This is used to achieve the deceleration and pressure holding curve;

[0046] The activation sequence of the three terminals is as follows: first activate the first terminal, then delay time. Then activate the second terminal, and then delay time. Then the second terminal is closed and the third terminal is activated, thereby synthesizing a three-stage injection speed curve of "slow start-fast impact-deceleration and pressure holding".

[0047] In a preferred embodiment, the method further includes a fault protection step:

[0048] Set a watchdog timer to monitor the update status of the timing control signals;

[0049] If within the preset timeout period If no timing control signal update is detected, a communication fault or a PLC fault is determined.

[0050] Triggering fault protection, the inverter's output frequency is switched to a preset safe frequency. It also outputs a fault alarm signal;

[0051] At the same time, the on / off status of all digital input terminals is checked. When reset to 0, the frequency converter decelerates according to the preset deceleration time inside the frequency converter. Reduce speed and stop the machine.

[0052] Beneficial effects

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] This solution replaces traditional fixed frequency values ​​with time-varying waveform functions, enabling the synthesis of various complex speed curves such as slow start, rapid impact, exponential decay, and damped oscillation using simple digital input terminals, thus meeting the process requirements of equipment such as injection molding machines and die casting machines.

[0055] This solution is based entirely on the existing digital input terminals of the frequency converter, without the need to add analog output modules, servo drives or dedicated motion controllers, which greatly reduces the system hardware cost.

[0056] Because digital signals have stronger anti-interference capabilities than analog signals, this solution is more stable and reliable than analog control solutions under harsh electromagnetic environments in industrial settings.

[0057] This solution can flexibly expand the complexity and frequency resolution of synthesizable curves by increasing the number of digital input terminals or by adopting a binary weight configuration method.

[0058] This solution can be implemented using the existing parameter system of the frequency converter without modifying the frequency converter hardware, and has good compatibility and ease of use. Attached Figure Description

[0059] Figure 1 This is an overall flowchart of the method of the present invention.

[0060] Figure 2 This is a schematic diagram of the calculation process of the method of the present invention;

[0061] Figure 3 Flowchart of the self-tuning optimization stage of the method of this invention;

[0062] Figure 4 This is a structural block diagram of the frequency calculation unit inside the inverter in this invention;

[0063] Figure 5 This is a flowchart of the fault protection logic of the present invention. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0065] Example 1:

[0066] Injection Molding Machine Injection Process Control

[0067] This embodiment applies the method of the present invention to the injection process control of an injection molding machine. The injection process of an injection molding machine typically needs to achieve a three-stage speed curve of "slow start-up - rapid impact-deceleration and pressure holding" to ensure that the plastic melt fills the mold cavity uniformly and avoids the generation of air bubbles or flash.

[0068] Step 1: Parameter Configuration

[0069] Three digital input terminals are preset in the frequency converter, and are configured as follows:

[0070]

[0071] Step 2: Timing Control

[0072] The activation sequence of each digital input terminal is as follows:

[0073] Phase 1 (0≤t<2.0s): Only DI1 is activated, while DI2 and DI3 are in the off state;

[0074] Phase 2 (2.0≤t<2.3s): DI1 and DI2 are activated simultaneously, while DI3 is turned off;

[0075] Phase 3 (2.3≤t≤4.0s): DI1 and DI3 are activated simultaneously, while DI2 is turned off.

[0076] Step 3: Frequency Synthesis

[0077] Based on the above configuration, the expression for the final output frequency is:

[0078] ;

[0079] The basic frequency term f was not set. base (t), meaning the main speed is generated entirely by the superposition of digital terminals.

[0080] The analytical expressions for each stage are as follows:

[0081] Phase 1: 0 ≤ t < 2.0s

[0082] S1(t) = 1 (DI1 activated), S2(t) = 0 (DI2 deactivated), S3(t) = 0 (DI3 deactivated);

[0083] but:

[0084] f out (t) = 1 × 5 × φ ramp (t) + 0 + 0

[0085] = 5 × φ ramp (t);

[0086] φ ramp (t) = t / 2 = 2 / 2 = 1 (linearly increases from 0 to 1 in 2 seconds);

[0087] Then f out (t) = 5 Hz, reaching 5 Hz at t=2.0s;

[0088] Phase 2: 2.0 ≤ t < 2.3s

[0089] S1(t) = 1 (DI1 activated), S2(t) = 1 (DI2 activated), S3(t) = 0 (DI3 deactivated);

[0090] but:

[0091] f out (t) = 1×5×φ ramp (t) +1×20×φ pulse (t) + 0

[0092] = 1×5×1 +1×20×1 + 0

[0093] =25;

[0094] Phase 3: 2.3 ≤ t ≤ 4.0s

[0095] S1(t) = 1 (DI1 activated), S2(t) = 0 (DI2 deactivated), S3(t) = 1 (DI3 activated);

[0096] but:

[0097] f out (t) = 1×5×φ ramp (t) + 0 + 1×(-8)×φ decay (t)

[0098] =5 - 8 × φ decay (t)

[0099] φ decay (t)= e -(t - 2.3) / 1.5 (Decays down from t=2.3s)

[0100] =e -1.7 / 1.5

[0101] = e -1.1333

[0102] ≈2.576;

[0103] Therefore: f out (t) = 5 - 2.576 = 2.424Hz, which drops to about 2.424Hz at t=4.0s.

[0104] Step 4: Running Results

[0105] When operated with the above configuration, the frequency converter outputs a three-segment speed curve, perfectly matching the injection molding machine's injection process requirements. Compared to traditional solutions, this embodiment eliminates the need for analog output modules or servo drives, achieving complex process curve synthesis using only three digital input terminals.

[0106] Example 2:

[0107] Binary weighted frequency configuration

[0108] This embodiment demonstrates how to implement the "coarse stepless speed regulation" function using the method of the present invention, which is suitable for manual multi-speed regulation or simple positioning scenarios.

[0109] Step 1: Parameter Configuration

[0110] Four digital input terminals are preset in the frequency converter, and their base frequency values ​​are configured in an incremental binary weighting manner:

[0111]

[0112] The waveform function of each terminal is configured as a unit step function (Φ(t) = 1), that is, each terminal directly contributes its fundamental frequency value when it is activated.

[0113] Step 2: Frequency Synthesis

[0114] The final output frequency is the arithmetic sum of the frequencies of all activated terminals: ;

[0115] Due to F i = 2 i-1 Hz, therefore, by different combinations of switches, a total of 16 discrete frequency outputs from 0 to 15Hz with a step size of 1Hz can be achieved.

[0116] Step 3: Running Results

[0117] This embodiment treats the four digital input terminals as a single 4-bit digital-to-analog converter (DAC), achieving the effect of simulating stepless speed regulation using digital signals. Compared to traditional analog speed regulation schemes, this embodiment has the advantages of strong anti-interference capability, low hardware cost, and simple implementation.

[0118] Example 3: Analog signal superposition with multi-speed signal

[0119] This embodiment is applicable to situations in continuous production lines where the main speed and fine-tuning offset need to be superimposed, such as synchronous control of textile machinery and tension control of coating machines.

[0120] Step 1: Parameter Configuration

[0121] base frequency fbase (t): The speed signal is given by the PLC through a 0~10V analog output and serves as the main speed signal. The continuously adjustable range is 0~50Hz.

[0122] Digital terminal configuration:

[0123]

[0124] Step 2: Frequency Synthesis

[0125] The expression for the final output frequency is: .

[0126] Step 3: Run the logic

[0127] When the production line needs to operate synchronously: DI1, DI2, and DI3 are all inactive. out = f base ;

[0128] When it is necessary to stretch the film (increase tension): Activate DI1, f out = f base +5 Hz;

[0129] When greater stretching is required: Activate DI1 and DI2 simultaneously, f out = f base +15 Hz;

[0130] When it is necessary to stack products (reduce speed): Activate DI3, f out = f base - 3 Hz.

[0131] Step 4: Running Results

[0132] In this embodiment, the main velocity f is changed. base At the same time, all superimposed offsets remain constant, ensuring that the synchronization ratio of the production line remains unchanged. Compared with the traditional solution that requires two analog output channels to control the main speed and offset respectively, this embodiment greatly simplifies the PLC hardware configuration.

[0133] Inverter internal structure description

[0134] like Figure 4 As shown, the frequency calculation unit inside the frequency converter used to implement the method of the present invention includes:

[0135] Waveform generator module 1: Receives status signals S from each digital input terminal. i (t) and the corresponding waveform parameter vector θ i Generate the corresponding normalized waveform function value Φ i (t,θ i This module contains multiple sub-modules, corresponding to the ramp function generator, pulse function generator, exponential decay function generator, sine oscillation function generator, and custom lookup table function memory, respectively;

[0136] Multiplier array 2: Contains N parallel multipliers, each multiplier calculating the instantaneous frequency contribution value S of the corresponding terminal. i (t)·F i ·Φ i (t,θ i );

[0137] Accumulator 3: Summes the outputs of all multipliers to obtain the original synthesized frequency f. raw (t);

[0138] Limiter 4: Limits the original synthesized frequency to a preset minimum frequency f. min and maximum frequency f max Between, we get f out，raw (t);

[0139] Output smoothing module 5: Performs a first-order low-pass filter on the limited frequency to eliminate frequency jumps that may occur during the switching of digital input terminal states. The filtered output frequency is:

[0140]

[0141] in , The sampling period is The filtering time constant;

[0142] Watchdog timer 6 and fault protection logic unit 7: used to monitor the update status of timing control signals and perform safety protection actions when a communication failure is detected.

[0143] Fault protection steps

[0144] The method of the present invention also includes a fault protection step, such as... Figure 5 As shown, the specific process is as follows:

[0145] A watchdog timer is configured to monitor the update status of timing control signals (i.e., the status of each digital input terminal). The watchdog timer automatically resets each time a change in the timing control signal is detected.

[0146] If no timing control signal update is detected within the preset timeout period (TWD), it is determined that the PLC or host computer has experienced a communication failure, program crash, or system freeze.

[0147] Trigger fault protection action: Switch the inverter's output frequency to the preset safe frequency F. safe Simultaneously, output a fault alarm signal to the alarm device (such as an alarm light, buzzer, or host computer monitoring system); and change the on / off status of all digital input terminals. i (t) is reset to 0, stopping the calculation of the waveform function.

[0148] The frequency converter decelerates according to its internally preset deceleration time T decel Reduce speed and stop the machine to avoid impact damage to the mechanical equipment caused by the sudden stop of the motor.

[0149] Waveform parameter self-tuning

[0150] In a preferred embodiment, the method of the present invention further includes a waveform parameter self-tuning step for automatically optimizing the waveform parameters θ of each digital input terminal. i This allows the actual output speed curve to more accurately match the target process curve.

[0151] The specific steps are as follows:

[0152] In trial operation mode, the actual motor speed feedback signal f is acquired in real time via an encoder installed on the motor shaft or a speed observer inside the frequency converter. fb (t).

[0153] Calculate the actual speed feedback signal and the target process curve f target The error between (t) and (t) is e(t) = f target (t) -f fb (t).

[0154] An optimization algorithm is used to automatically adjust the waveform parameter vector θ of each digital input terminal. i The process continues until the root mean square value of the error is less than a preset threshold. The optimization algorithm may employ, but is not limited to, gradient descent, particle swarm optimization, or genetic algorithms.

[0155] This self-tuning step can significantly reduce the workload of manual debugging and can adapt to the process optimization needs under different load characteristics.

[0156] Industrial applicability

[0157] The method of this invention can be widely applied in the following industrial applications:

[0158]

[0159] This invention is particularly suitable for high-power applications that are cost-sensitive, require complex curves, and are located in harsh electromagnetic environments.

[0160] 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 illustrative of the principles of 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 this invention is defined by the appended claims and their equivalents.

Claims

1. A frequency converter control method based on multi-speed superposition and number selection, characterized in that, Includes the following steps: S1: N digital input terminals are preset in the frequency converter, and a corresponding basic frequency value is configured for each digital input terminal. and the corresponding waveform functions , where i = 1, 2, ..., N, N ≥ 2; S2: Configure the final output frequency of the inverter. It is the sum of the products of the fundamental frequency value and the waveform function of each active terminal; S3: Determine the activation timing of each digital input terminal according to the requirements of the target process curve, and generate timing control signals; S4: The timing control signal is applied to the corresponding digital input terminal. The frequency converter calculates the instantaneous frequency contribution value of each activated terminal in real time and superimposes them to generate the final output frequency. S5: The frequency converter drives the motor to run according to the final output frequency, and synthesizes the target process curve.

2. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, The waveform function It can be any one or more combinations of ramp function, impulse function, exponential decay function, sinusoidal oscillation function, and user-defined lookup table function.

3. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, In step S2, the final output frequency It also includes the fundamental frequency term. The basic frequency term It is given by analog input signal or communication bus.

4. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, The activation timing adopts a "select M from N" combined activation strategy, that is, at the same time, M terminals are selected from N digital input terminals for activation, where 1≤M≤N, and different activation combinations correspond to different synthesized waveform characteristics.

5. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, The fundamental frequency values ​​of the plurality of digital input terminals Configured according to binary weight incrementing method, that is ,in To achieve the minimum frequency step size, 2 is achieved through switching combinations of each terminal. N -1 discrete frequency outputs.

6. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, In step S4, the frequency converter is equipped with a frequency calculation unit, which includes: The waveform generator module is used to generate waveforms based on the waveform parameter vectors of each terminal. Generate the corresponding normalized waveform function value; A multiplier array is used to calculate the instantaneous frequency contribution value of each active terminal. ,in This represents the switch state of the i-th digital input terminal, with a value of 0 or 1. This is a normalized waveform function; An accumulator is used to sum all contribution values ​​to obtain the final output frequency. ; A limiter is used to limit the final output frequency to a preset minimum frequency. and maximum frequency between.

7. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, In step S5, the frequency converter further includes an output smoothing module for smoothing the final output frequency. A first-order low-pass filter is performed to eliminate the frequency jump caused by the instantaneous switching of the digital input terminal states. The filtered output frequency is: ,in , The sampling period is This is the filtering time constant.

8. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, The method further includes a waveform parameter self-tuning step: In trial operation mode, the actual motor speed feedback signal is collected in real time. ; Calculate the actual speed feedback signal and the target process curve. Error between ,in: ; According to the error The waveform parameter vectors of each digital input terminal are automatically adjusted using gradient descent or particle swarm optimization algorithms. This continues until the root mean square value of the error is less than a preset threshold.

9. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, The method is applied to the injection process control of an injection molding machine, and the N digital input terminals include at least: First terminal, configure ramp function and fundamental frequency value This is used to achieve a slow start-up curve; The second terminal is configured with a pulse function. and fundamental frequency value , used to achieve a rapid impact curve; The third terminal is configured with an exponential decay function. and fundamental frequency value This is used to achieve the deceleration and pressure holding curve; The activation sequence of the three terminals is as follows: first activate the first terminal, then delay time. Then activate the second terminal, and then delay time. Then the second terminal is closed and the third terminal is activated, thereby synthesizing a three-stage injection speed curve of "slow start-fast impact-deceleration and pressure holding".

10. The inverter control method based on multi-speed superposition and number selection according to claim 1, characterized in that, The method also includes a fault protection step: Set a watchdog timer to monitor the update status of the timing control signals; If within the preset timeout period If no timing control signal update is detected, a communication fault or a PLC fault is determined. Triggering fault protection, the inverter's output frequency is switched to a preset safe frequency. It also outputs a fault alarm signal; At the same time, the on / off status of all digital input terminals is checked. When reset to 0, the frequency converter decelerates according to the preset deceleration time inside the frequency converter. Reduce speed and stop the machine.

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