METHOD AND SYSTEM FOR DIAGNOSTIC INLET AIR PRESSURE FLUCTUATIONS AND / OR OUTLET LEAKS IN A CONTROLLED DOUBLE-CONECONIC AIR POWER AMPLIFIER

Description

Field of the invention

[0001] The invention relates to a method for diagnosing supply-side pressure fluctuations and / or outlet-side leaks in a controlled double-cone air power amplifier, as well as corresponding systems and / or devices. The aforementioned air power amplifiers are frequently used for the controlled compressed air supply to pneumatic actuators of process control valves or other compressed air consumers.

[0002] Such control valves are subject to increasingly stringent demands regarding their ability to detect changes and, in particular, deteriorations in their operating conditions. This includes, in particular, indications of actuator irregularities, e.g., due to fluctuations in the compressed air supply. Unintentional leaks are of particular interest, as these can cause significant long-term costs and even compromise safety, for example, as certain valve positions may no longer be accessible. State of the art

[0003] An electropneumatic control system that includes a secondary pneumatic power stage is described, for example, in EP 1 769 159 B1. This system uses, for example, a double-cone air power amplifier. Diagnostic options relating to the compressed air system, particularly leaks, are not provided.

[0004] Publication WO 2018 / 035181 A1 discloses methods and devices for testing pneumatic signal amplifiers during operation. This allows, for example, statements to be made about the basic functionality of these devices. However, the methods and devices described therein cannot provide information about the compressed air supply and / or leaks in the compressed air system.

[0005] A leak detection system is described in US 4,157,656 A. However, this requires the use of additional devices, which in turn require a pneumatic amplifier to amplify the output signal so that it can be clearly distinguished from normal fluctuations.

[0006] US 4,653,523 A describes a fluid amplifier used in a pneumatic converter to provide a control output pressure in response to a control pressure provided as a function of a current signal. The fluid amplifier includes a valve that opens a pressure source to the control port and also controls the pressure supply through an outlet port. The valve is equipped with negative feedback when the exhaust back pressure increases to stabilize operation and make the amplifier less sensitive. The negative feedback makes the valve easier to control and ensures more stable operation.

[0007] US 4,825,198 A describes a method in which, for the leak test of two valves arranged one behind the other in a fluid line, the pressure state in the line section between the valves is recorded with the valves closed. After a waiting period following the valves' closure, it is determined whether the pressure in the line section is above or below a predetermined pressure threshold. This is used to determine the tightness of one or the other valve. A line path to the line section is then opened via the valve assessed as tight, and the line section is either emptied or filled. Using a differential pressure sensor, it is determined in a subsequent measuring period whether and how often the pressure in the line section reaches or exceeds the pressure threshold.The tested valve is considered to be tight if the pressure threshold is reached or passed once and is considered to be insufficiently tight if the pressure threshold is not reached or is reached twice within the measuring time. Task

[0008] The object of the invention is to provide a method and a system that improves the possibilities for diagnosing irregularities, in particular leaks, in the compressed air system of a double-cone air power amplifier. Solution

[0009] This problem is solved by the subject matter of the independent claim. Advantageous developments of the subject matter of the independent claim are characterized in the subclaims. The wording of all claims is hereby incorporated by reference into this description.

[0010] The use of the singular shall not exclude the plural, and this shall also apply in the reverse sense unless otherwise disclosed.

[0011] Individual method steps are described in more detail below. In a preferred variant of the invention, the steps are carried out in the order specified. However, the steps do not necessarily have to be carried out in the order specified, and the method to be described may also include additional, unmentioned steps.

[0012] To solve this problem, a method is proposed for diagnosing supply-air pressure fluctuations and / or outlet-side leaks in a controlled double-cone air power amplifier. The double-cone air power amplifier is controlled by a controller with a setpoint and an actual value of a controlled variable. The method comprises the following steps: Initial measurement of at least one downward step, wherein the measurement of a downward step comprises the following: 1. Waiting for a downward step in the setpoint of the controlled variable; 2. Ensuring exclusively proportional control; 3. Recording the time course of the actual value of the controlled variable, starting with the downward step in the setpoint and ending when no further change in the actual value is detected within a specified measurement accuracy, whereby a curve with two break points is obtained; 4. Determining a reversal amplitude and / or a reversal duration, wherein the reversal amplitude is the difference between the actual values ​​of the curve at the two break points and the reversal duration is the time interval between the two break points. Re-measurement of a downward step.Diagnosing a supply-air pressure drop if the reversal amplitude of the re-measurement is smaller than the reversal amplitude of the initial measurement by more than a specified tolerance; and / or diagnosing a supply-air pressure increase if the reversal amplitude of the re-measurement is larger than the reversal amplitude of the initial measurement by more than a specified tolerance; and / or diagnosing an increased outlet-side leakage if the reversal duration of the re-measurement is shorter than the reversal duration of the initial measurement by more than a specified tolerance.

[0013] The setpoint and actual value of the control can be related to the output pressure of the air power amplifier as the controlled variable, or for example to the position of a control valve or other consumer. The downward steps can result from the process requirements or can also be specifically requested by a user, a diagnostic program, or similar. The method takes advantage of the fact that when the setpoint drops downwards, the air power amplifier must first go into exhaust air mode and then, when the desired pressure in the compressed air system is reached, switch to supply air mode. This change is necessary because pneumatic systems always have a certain basic leakage that must be compensated for. A certain amount of time, the changeover time, is required for this change, depending on the leakage.This results in a further change in the actual value, which is caused by the pressure conditions in the air power amplifier differing between extract air and supply air mode. This is the reversal amplitude. To achieve the desired behavior during this reversal process, the air power amplifier's control must operate as a pure proportional control (P control). Therefore, any existing I or D components of the control must be deactivated beforehand.

[0014] The initial measurement of a downward jump can be usefully carried out as part of calibration measurements when commissioning the air power amplifier or the associated controller.

[0015] This method makes it possible to detect leaks in the consumer system connected to a double-cone air booster, such as a pneumatic control valve actuator, as well as changes in supply pressure. This provides early indications of any necessary maintenance, thus saving costs for maintenance needs that are detected too late. Additional sensors are not required for this method.

[0016] In a preferred development of the method, at least two downward jumps are initially measured, with both downward jumps occurring at different pressure conditions, preferably one at very high and one at very low pressure (e.g., 10% and 90% of the maximum pressure). This allows any existing dependencies of the reversal amplitude and / or the reversal duration on the target value of the controlled variable to be taken into account in the diagnosis. Values ​​for the reversal amplitude and / or reversal duration are thus known for the pressure conditions present during the at least two initial downward jumps, and they can be determined for other pressure conditions, for example, from the formulas mentioned below or by interpolation or extrapolation.

[0017] The proportional control works with a gain K P All intended diagnoses will be more accurate if the amplification K P the proportional control for measuring the downward jump is set to a lower value, in particular if it is reduced by at least 5%, preferably at least 10%, again preferably at least 25%, again preferably at least 50%, again preferably at least 75%. This leads to an increase in the reversal amplitude.

[0018] A frequent implementation of the procedure without significant impairment of the function of the system in which the air power amplifier is installed can be made possible if, after measuring the downward jump, the gain K P The proportional control is reset to the value existing before the downward step was measured and / or, if the control had non-proportional components before the downward step was measured, these are reactivated. In this case, a possibly worse control is only performed for the duration of the downward step for diagnostic purposes, which is then improved again.

[0019] For further evaluation of the results of the procedure, it is advantageous to create a data storage medium on which the obtained data and / or the diagnosis are stored.

[0020] The problem is further solved by a system for diagnosing supply-air pressure fluctuations and / or outlet-side leaks in a controlled double-cone air power amplifier. This system comprises a double-cone air power amplifier and a controller for controlling a controlled variable of the double-cone air power amplifier, wherein the controller is configured to operate as a pure proportional controller. The system further has means for recording a time profile of the actual value of the controlled variable, means for determining time intervals and intervals between actual values ​​of the controlled variable, and means for outputting a diagnosis, as well as means suitable for carrying out a method as described above.

[0021] The air booster has a lower diaphragm with an effective area and an air supply hole. Diagnosis is facilitated if the cross-sectional area of ​​the air supply hole is as large as possible relative to the effective area of ​​the lower diaphragm. This ratio is preferably between 0.09 and 0.27, more preferably between 0.16 and 0.26. This maximizes the difference between the pressure conditions in the air booster during supply air and exhaust air operation, allowing the measured effects to be detected as clearly as possible.

[0022] For the same reasons, diagnosis is further facilitated if the air booster has an auxiliary spring for the double cone, which pushes the double cone into the air supply hole, which is designed to be as strong as possible. This auxiliary spring ensures that the double cone remains in its correct position in the delivery state or in depressurized operating state.

[0023] Basic calibration is improved if the system has a throttle or flow regulator downstream of the air booster, ensuring a baseline leakage. This has a predefined value that can always be assumed.

[0024] The problem is further solved by a positioner for a pneumatically driven control valve with a system as described above.

[0025] The problem is also solved by a pneumatically driven control valve with an aforementioned positioner, and also by a process plant with at least one aforementioned pneumatically driven control valve.

[0026] The object is further achieved by a computer program comprising instructions which cause the above-mentioned device or the above-described system or the above-described position controller to carry out the method steps described above.

[0027] The task is finally solved by a computer-readable medium on which the aforementioned computer program is stored.

[0028] Further details and features will become apparent from the following description of preferred embodiments in conjunction with the figures. The respective features can be implemented individually or in combination with one another. The possibilities for solving the problem are not limited to the embodiments. For example, range specifications always include all intermediate values ​​(not mentioned) and all conceivable sub-intervals.

[0029] The exemplary embodiments are illustrated schematically in the figures. Identical reference numerals in the individual figures denote identical or functionally identical elements, or elements that correspond to one another in terms of their functions. In detail: Fig. 1: a schematic sectional view through a double-cone air power amplifier with an additional spring in holding mode (prior art); Fig. 2: a schematic sectional view through a double-cone air power amplifier without an additional spring in supply air mode (prior art); Fig. 3: a schematic sectional view through a double-cone air power amplifier without an additional spring in exhaust air mode (prior art); Fig. 4: characteristic curves of a double-cone air power amplifier; Fig. 5: a simplified schematic circuit diagram of the position control of a control valve; Fig. 6: an actual value curve for a downward jump in the setpoint value with a first, low leakage; Fig. 7: an actual value curve for a downward jump in the setpoint value with a second, higher leakage; and Fig. 8: two actual value curves for a downward jump in the setpoint value at different supply pressures.

[0030] First, based on the Figuren 1 bis 3 The functionality of a double-cone air booster is explained. This is prior art, which is not claimed, but must be explained for better understanding.

[0031] Fig. 1 shows the structure of a double-cone air power amplifier 100. This has a control input 105 for the control pressure P st , an air supply inlet 110, which is supplied with compressed air at the pressure P z a vent outlet 115 which is open to the environment and an outlet 120 for compressed air with the desired output pressure P a .

[0032] Between the control inlet 105 and the vent outlet 115 there is a first membrane 125 with the effective area A o , and between the vent outlet 115 and the outlet 120 there is a second membrane 130 with the effective area A u The effective area of ​​each diaphragm is approximately defined by the center of the diaphragm's bulges. These two diaphragms are connected by a control body 135, so that they move together. Control body 135 contains the exhaust seat 140 at its center, which is closed or opened by the exhaust cone 145.

[0033] Between the supply air inlet 110 and the outlet 120 is the supply air seat 150 with the cross-sectional area A z , which is closed or opened by the supply air cone 155. The supply air and exhaust air cones 155, 145 are firmly connected to each other so that they always move together, and together form the double cone 160. This can be preloaded by an additional spring 165, but this is not absolutely necessary.

[0034] In addition, there is an offset spring 170, which moves the control body 135 and the two membranes 125, 130 in Fig. 1 upwards and ensures that the air power amplifier only begins to operate above a certain minimum control pressure that overcomes the force of the offset spring 170. The offset spring 170 also supports reliable venting in the event that the pilot system, which supplies the control pressure, cannot completely vent (this is often the case with conventional nozzle-flapper IP converters, for example). Furthermore, the lower effective surface 175 of the double cone can be seen. Its significance is explained further below.

[0035] In Fig. 1 The air power amplifier is shown in holding mode. This means that the control pressure P st and outlet pressure P a are exactly large enough that the first and second diaphragms 125, 130 and the control body 135 connecting them are in a middle position in which both the supply air seat 150 and the exhaust air seat 140 are closed by the double cone 160. Therefore, there is no fluid connection between the outlet 120 and the supply air inlet 110 or the vent outlet 115, so that the outlet pressure P a In practice, this state (holding mode) is rarely important, since pneumatic consumers always have a certain basic leakage, and a controlled air power amplifier must therefore permanently switch to supply air mode to maintain a consumer's state.

[0036] In Fig. 2 a double cone air power amplifier 200 is shown, which is different from the Fig. 1 only differs by the absence of the additional spring 165. In Fig. 2 The air power amplifier is in supply air mode. The control pressure P st is increased, the output pressure P a but not yet, which is why the two membranes 125, 130 and the control body 135 are pushed downwards. Therefore, the exhaust air seat 140 remains closed by the exhaust air cone 145, but the supply air cone 155 is pushed downwards out of the supply air seat 150, so that compressed air can flow from the supply air inlet 110 and the output pressure P a increased at the output 120.

[0037] Fig. 3 shows the same double cone air power amplifier 200 as Fig. 2 , but in exhaust air mode. The control pressure P st is lowered so that the offset spring 170 and the output pressure P a , under which the compressed air is located in the consumer, push the two diaphragms 125, 130 and the control body 135 upwards. Therefore, the supply air seat 150 is closed by the supply air cone 155, but the exhaust air seat 140 is opened by the exhaust air cone 145, since the exhaust air seat 140 is also raised, but the exhaust air cone 145, as part of the double cone 160, cannot follow, since it is connected to the supply air cone 155. As a result, compressed air escapes from the consumer through the exhaust air outlet 115, so that the outlet pressure present at the outlet 120 P a sinks.

[0038] By varying the control pressure P st the output pressure P a to any value between the ambient pressure and the supply air pressure P z be brought.

[0039] For a better qualitative and quantitative understanding, the corresponding equations are derived below. Considering the controlled state, a force equilibrium applies to control body 135. Without considering other forces, the following applies: P st ⋅ A o = P a ⋅ A u

[0040] For a more precise description, the force F z the additional spring 165 (if present) and the effective force on the lower effective surface 175 of the double cone 160 A z , which corresponds to the cross-section of the supply air seat 150, must be taken into account, but only in supply air operation. In exhaust air operation, these forces are supported by the supply air seat 150 on the housing and are absorbed by it. The force F However, the offset of the offset spring 170 must always be taken into account. Consequently, different force balances result for supply air and exhaust air operation: Supply air operation: P a ⋅ A u + F offset + P z ⋅ A z + F z = P st ⋅ A o Exhaust air operation: P a ⋅ A u + F offset = P st ⋅ A o

[0041] The spring forces F z and F offset is assumed to be constant in a first approximation, since the double cone 160 and the control body 135 typically only move over short distances.

[0042] The following applies to the output pressure: in supply air mode: P a = P st ⋅ A o − P z ⋅ A z − F offset − F z A u in exhaust air mode: P a = P st ⋅ A o − F offset A u

[0043] At the same control pressure, the output pressure in supply air mode is therefore reduced compared to exhaust air mode. The reduction depends on the cross-section A z the supply air seat 150, the strength of the additional spring 165 and the supply air pressure P z Specifically, the reduction applies: P a , Abluft − P a , Zuluft = P z ⋅ A z + F z A u

[0044] In Fig. 4 simulated characteristics of a double-cone air power amplifier are shown. Real characteristics can be obtained, for example, by slowly increasing and decreasing the control pressure (without regulation). The control pressure P st is plotted on the x-axis and varies in this example between 0 and just over 1.2 bar. The outlet pressure P a of the air power amplifier is on the y -axis and lies between 0 and 3 bar. Due to the transitions from supply air to extract air operation (top) and vice versa from extract air to supply air operation (bottom), a hysteresis-like progression of the characteristic curves is obtained. The characteristic curve on the left in the diagram corresponds to extract air operation. The middle characteristic curve corresponds to supply air operation at a supply air pressure of 4 bar, the right characteristic curve corresponds to supply air operation at a supply air pressure of 6 bar. The width and, equivalently, the height of the hysteresis therefore depends on the supply pressure. This can be used, for example, when initializing an air performance amplifier to determine an unknown supply pressure on the supply air line. According to the above equation, the reduction is: Höhe der Hysterese = P z ⋅ A z + F z A u , so P z = Höhe der Hysterese ⋅ A u − F z A z .

[0045] Fig. 5 shows schematically the circuit of the position control loop of a pneumatically driven control valve 510, which represents a typical application for a double-cone air power amplifier 500. The setpoint W is specified by the user or by a process control system or similar. A linear correction to the corrected setpoint is applied via a pre-filter 520. W'This compensates for the constant control deviation that always occurs with the P-controller 530 used here, so that - at least in supply air mode - the specified setpoint VV actually corresponds to the position of the control valve in the regulated state. The - typically electrical - control signal Y of the controller 530 is converted by an IP converter (not shown for simplification) into a control pressure Y, which is applied to the control input of the air power amplifier 500. The air power amplifier is supplied with a supply air pressure P z supplies and delivers an output pressure in the manner already described P a to the actuator of control valve 510. The leakage 540 present in the compressed air system is indicated schematically between them. This can also be a preferably built-in throttle that specifies a fixed minimum leakage. The position sensor 550 on control valve 510 supplies the respective (corrected) setpoint W' the actual value X to controller 530.

[0046] In the following, the transfer functions and control parameters are examined in more detail to illustrate the reversal behavior in a pure proportional control. The transfer function of the P-controller 530 is Y = K P ⋅ W ′ − X , with K P as the gain factor of the P controller. The gain factor K P of the P-controller is determined during commissioning, whereby K P should be as high as possible to achieve the most effective control, but not so high that it causes overshoot. A typical normal, non-reduced value is approximately 30.

[0047] The transfer function of the guide prefilter is W ′ = a 1 ⋅ W + b 1 .

[0048] The parameters are set during commissioning, after setting K P , fixed so that in the controlled state, when the air power amplifier is in supply air mode, a setpoint W = 0 also actually a position X = 0 of the consumer, and also X = 100% is achieved when W = 100%. This setting is done either manually or automatically during an initialization run.

[0049] For diagnosis, a lower gain is preferred K P to increase the accuracy of the diagnosis. A typical value is K P = 10. The parameters a 1 and b 1 of the guide pre-filter can preferably be used for diagnosis matching this reduced K P be determined.

[0050] Three more transfer functions are still required: P a = a 2 ⋅ Y + b 2 Air power amplifier in supply air mode, P a = a 3 ⋅ Y + b 3 = a 2 ⋅ Y + b 3 Air performance amplifier in exhaust air mode and X = a 4 ⋅ P a + b 4 Consumer, e.g. control valve.

[0051] The parameters a 2 , b 2 , a 3 , b 3 of the air power amplifier are typically already known and result from the data of the air power amplifier, as in Fig. 4 As shown in Fig. 4 As can be seen, the characteristic curves for the supply air and the extract air operation are essentially parallel and only slightly shifted. a 2 = a 3 and the transfer functions differ only in the parameters b 2 and b 3 .

[0052] According to the above derivations, the following applies: b 3 − b 2 = P z ⋅ A z + F z A u

[0053] The consumer's parameters are also known from the consumer's specifications and are fixed. For a control valve, for example, the minimum pressure in the actuator at which the zero position is left and the pressure at which the maximum position is reached can be used for the determination.

[0054] For the actual value, e.g. the position of the consumer, the following is obtained by inserting P a and Y into the equation for X and dissolving after X in supply air mode: X Z = a 2 ⋅ a 4 ⋅ K P ⋅ W ′ + b 2 ⋅ a 4 + b 4 1 + a 2 ⋅ a 4 ⋅ K P .

[0055] In exhaust air operation, the following applies: X A = a 2 ⋅ a 4 ⋅ K P ⋅ W ′ + b 3 ⋅ a 4 + b 4 1 + a 2 ⋅ a 4 ⋅ K P .

[0056] The consumer X This means that in supply air and extract air mode, with the same setpoint (corrected for supply air mode) W' a different position. The difference between these positions is the reversal amplitudeU : U = X A − X Z = b 3 − b 2 ⋅ a 4 1 + a 2 ⋅ a 4 ⋅ K P .

[0057] Here it can be seen that this reversal amplitude U with a smaller gain factor K P of the controller becomes larger, since K P in the denominator. If one substitutes (b 3 - b 2 ) the result obtained above, we get: U = P z ⋅ A z + F z ⋅ a 4 A U ⋅ 1 + a 2 ⋅ a 4 ⋅ K P .

[0058] It can be seen that the reversing amplitude U - in addition to a reduction of K P - can also be increased by increasing the supply air pressure P z or the area ratio A z / A u be enlarged.

[0059] The Figuren 6 , 7 and 8 show simulated temporal courses of the actual value X when the target value drops W or W' the control of a double-cone air power amplifier with proportional control. The pressure in the consumer can serve as the variable to be controlled (i.e. the outlet pressure P a ), or, for example, the position, if the consumer is a valve. The almost vertical rise within the first second is due to the setup of the simulation. Shortly after the second second, the setpoint value drops W or W'. In Figs. 6 and 7The setpoint drops to approximately 30.5%. The air booster switches to extract air mode, causing the pressure in the consumer to drop rapidly, so that the actual value drops almost vertically to approximately 30.9%. However, the setpoint is not reached in extract air mode because the P control has a permanent control deviation and the pre-filter compensates for the control deviation only for supply air mode. Due to leakage, the pressure in the consumer continues to drop slowly until the air booster switches to supply air mode and prevents a further drop in pressure. If a pre-filter is present, which, as already described, compensates for the permanent control deviation of the proportional controller for supply air mode, the actual value that is ultimately reached corresponds to the set setpoint. This creates a curve with two break points, which are shown in the Figuren 6 , 7 and 8are clearly visible. The difference between the actual values ​​at the two break points corresponds to the reversal amplitude U. With higher leakage, the further decrease of the actual value occurs faster, the reversal duration, ie the time interval between the two break points, is therefore shorter. A comparison of Fig. 6 and Fig. 7 shows this clearly.

[0060] To detect an increased leakage in the compressed air system, you can proceed as follows: During commissioning, not only are all settings and parameter definitions made, as already described, but a downward step in the setpoint is also initially measured. For this purpose, any existing I and / or D components of the control must be switched off before measuring the downward step to ensure that a pure proportional control is present, since other control components could distort the reversal behavior due to their dynamic behavior. This is not a problem, since the corresponding control components can be switched back on afterwards, so that less precise control only takes place for a short time. Preferably, the gain factor is also used for the initial measurement of a downward step. K P temporarily set to a lower value, as this magnifies the effects to be measured, as already described. The temporal progression of the actual value during the downward jump is recorded, and from the resulting curve (smoothed if necessary), the differences between the two break points are determined, i.e., the reversal amplitude and the reversal duration. These two values, as well as the lower gain factor, are saved as reference values.

[0061] At any later point in time, a downward step in the setpoint can be measured again. This does not need to be explicitly triggered for diagnostic purposes; one can simply wait until, for example, operational requirements require such a downward step anyway. For this downward step, non-proportional components of the control are also switched off, and the same lower gain factor is applied. K P temporarily stopped. The time course of the actual value is resumed and the reversing amplitude U and the reversal duration are determined from the two inflection points of the curve. If the reversal duration is shorter than the reference value, the leakage in the compressed air system has increased. A threshold can be specified for this, above which a warning message is issued, for example, to trigger maintenance.

[0062] In Fig. 8 Simulated time courses of the actual value during a downward jump of the setpoint value for two different supply pressures (4 bar and 6 bar) are shown. It is clearly visible that the reversing amplitude U from supply pressure P z depends on the formula U = P z ⋅ A z + F z ⋅ a 4 A U ⋅ 1 + a 2 ⋅ a 4 ⋅ K P . This allows any supply pressure fluctuations to be monitored and reported back. glossary Permanent control deviation

[0063] See proportional control. flow regulator

[0064] A flow regulator is a combination of a pressure reducer and a throttle. A flow regulator is thus able to ensure a defined flow rate. This prevents (within certain limits) the flow through the throttle from becoming dependent on pressure. Breakpoints of a curve

[0065] These are discontinuities in the first derivative of the curve. These are clearly evident as jumps in the gradient of the curve. For experimentally determined curves, the curve must be smoothed beforehand to ensure that fluctuations, measurement inaccuracies, etc., do not simulate such inflection points. Proportional control, proportional controller, P controller

[0066] In a P-controller, the manipulated variable Y is always proportional to the detected control deviation e (the difference between the setpoint and actual value). This means that a P-controller reacts to a control deviation without delay and generates a manipulated variable when such a deviation occurs. The amplitude of the manipulated variable depends on the control deviation e and the magnitude of the proportional coefficient. K P , which is also called reinforcement: Y = K P ⋅ e

[0067] A controller compensates for the effect of disturbances by generating a correspondingly opposite manipulated variable. However, a P-controller can only generate this manipulated variable if a control deviation is present. Therefore, persistent disturbances can never be completely compensated with a P-controller; a permanent control deviation always remains. A large K P leads to smaller control deviations due to a stronger control intervention. Too large K PHowever, values ​​increase the tendency of the control loop to oscillate. Reversal amplitude

[0068] This is the actual value change resulting from the transition from extract air mode to supply air mode. Only then, when the setpoint drops downward, is the setpoint actually reached, as the remaining control deviation for supply air mode is compensated for by presettings. A slow "creep" is measured, which stops once supply air mode has been reached. Reference symbol

[0069] 100, 200, 500 Double-cone air power amplifier 105 Control inlet 110 Supply air inlet 115 Exhaust outlet 120 Outlet 125 First diaphragm 130 Second diaphragm 135 Control body 140 Exhaust air seat 145 Exhaust air cone 150 Supply air seat 155 Supply air cone 160 Double cone 165 Additional spring 170 Offset spring 175 Lower effective area of ​​the double cone 510 Pneumatically driven control valve 520 Lead prefilter 530 P-controller 540 Leakage or throttle 550 Position sensor cited literature cited patent literature

[0070] EP 1 769 159 B1 US 4,157,656 A US 4,653,523 A US 4,825,198 A WO 2018 / 035181 A1

Claims

1. Method for diagnosing supply-air-side pressure fluctuations and / or output-side leaks (540) in a regulated double-cone air power amplifier (100; 200; 500), 1.1 the double-cone air power amplifier (100; 200; 500) being regulated by a closed-loop controller (530) with a setpoint value and an actual value of a regulated variable; the method comprising the following steps: 1.2 initial measurement of at least one downward step, the measurement of a downward step comprising the following: 1.2.1 waiting for a downward step of the setpoint of the regulated variable; 1.2.2 ensuring an exclusively proportional closed-loop controller (530); 1.2.3 recording the time characteristic of the actual value of the regulated variable, 1.2.3.1 starting with the downward step of the setpoint and 1.2.3.2 ending when no further change in the actual value is detected within a specified measuring accuracy; 1.2.3.3 whereby a curve with two inflection points is obtained; 1.2.4 determining a reversal amplitude and / or a reversal duration, 1.2.4.1 wherein the reversal amplitude is the difference between the actual values of the curve at the two inflection points; 1.2.4.2 wherein the reversal period is the time interval between the two inflection points; 1.3 re-measuring a downward step; 1.4 diagnosing a supply-air-side pressure drop if the reversal amplitude of the re-measuring is smaller than the reversal amplitude of the initial measuring by more than a specified tolerance; and / or 1.5 diagnosing an increase in supply-air-side pressure if the reversal amplitude of the re-measurement is greater than the reversal amplitude of the initial measurement by more than a predetermined tolerance; and / or 1.6 diagnosing increased leakage on the output side (540) if the reversal duration of the re-measurement is shorter than the reversal duration of the initial measurement by more than a predetermined tolerance.

2. Method according to the previous claim, characterised in that at least two downward steps are initially measured, with both downward steps occurring under different pressure conditions.

3. Method according to one of the preceding claims, with the proportional closed-loop controller (530) operating with a gain KP, characterised in that the amplification KP of the proportional closed-loop controller (530) is reduced by at least 5% for measuring the downward step.

4. Method according to the preceding claim, characterised in that after measuring the downward step 4.1 the amplification KP of the proportional closed-loop controller (530) is set to the value present before measuring the downward step and / or 4.2 if the close-loop controller had non-proportional components before the downward step was measured, these are re-enabled.

5. Method according to one of the preceding claims, comprising the further step of creating a data carrier on which the acquired data and / or the diagnosis are stored.

6. System for diagnosing supply-air-side pressure fluctuations and / or output-side leaks (540) in a regulated double-cone air power amplifier (100; 200; 500), comprising 6.1 a double-cone air power amplifier (100; 200; 500) and 6.2 a closed-loop controller (530) for controlling a regulated variable of the double-cone air power amplifier (100; 200; 500); 6.2.1 the closed-loop controller being configured in such a way that it can operate purely as a proportional controller (530); and having 6.3 means for recording a time characteristic of the actual value of the regulated variable; 6.4 means for determining time intervals and intervals between actual values of the regulated variable; 6.5 means for outputting a diagnosis; and with 6.6 means which are suitable for carrying out the steps of the method according to one of the method claims.

7. System according to the immediately preceding claim, 7.1 wherein the air power amplifier (100; 200; 500) has a lower membrane (130) with an effective diameter and an air supply bore (150), characterised in that 7.2 the ratio of the cross-sectional area of the air supply bore to the effective area of the lower diaphragm (130) is between 0.09 and 0.27, preferably between 0.16 and 0.26.

8. System according to one of the two immediately preceding claims, 8.1 wherein the air power amplifier (100; 200; 500) has a air supply bore (150), characterised in that 8.2 the air power amplifier (100; 200; 500) has an additional spring (165) for the double cone (160), which spring presses the double cone (160) into the air supply bore (150).

9. System according to one of the three immediately preceding claims, characterised in that a throttle (540) or a flow controller is provided on the output side of the air power amplifier (100; 200; 500), whereby a basic leakage is ensured.

10. Positioner for a pneumatically driven control valve (510) with a system according to one of claims 6 to 8.

11. Pneumatically driven control valve (510) with a positioner according to the immediately preceding claim.

12. Process plant with at least one pneumatically driven control valve (510) according to the immediately preceding claim.

13. Computer program comprising instructions that cause the system of claim 6 or the positioner of claim 10 to carry out the method steps according to one of claims 1 to 5.

14. Computer-readable medium on which the computer program according to the immediately preceding claim is stored.