VALVE CONTROL DEVICE, PROCESS ENGINEERING PLANT HAVING A VALVE CONTROL DEVICE, DIAGNOSTIC METHOD AND USE OF A VALVE CONTROL DEVICE

Abstract

In a position closed-loop controller (31) for a valve control device (1) of a process engineering plant (100), comprising a first signal input (11) for a pilot signal, such as a process control signal (pg) from a process closed-loop controller (120) of the process engineering plant (100), a second signal input for a position actual signal (i) with regard to a control valve (35), wherein the position control-loop controller (31) is designed to generate a more particularly pneumatic control variable (g) for an actuator (33) to actuate the control valve on the basis of the pilot signal and the position actual signal (i) and comprises a more particularly pneumatic control output for the control variable (g). According to the invention, the position closed-loop controller (31) is configured to calculate an approximation signal (ap), by means of a configurable closed-loop controller model based on a particular closed-loop controller, more particularly the process closed-loop controller (120), proceeding from the pilot signal, wherein the closed-loop controller model is configured such that a signal generated by the particular closed-loop controller proceeding from the approximation signal (ap) corresponds to the pilot signal.

Claims

1. A positioner (31) for a valve control device (1) of a process plant (100), comprising: a first signal input (11) for a command signal, such as a process control signal (p.sub.g) from a process controller (120) of the process plant (100), a second signal input for an actual value position signal (i) relating to a control valve (35), wherein the positioner (31) is configured to generate an in particular pneumatic control variable (g) for an actuator (33) for actuating the control valve based on the command signal and the actual value position signal (i) and comprises an in particular pneumatic control output for the control variable (g), characterised in that the positioner (31) is configured to calculate an approximation signal (a.sub.p) based on the command signal by means of a configurable controller model relating to a specific controller, in particular to the process controller (120), wherein the controller model is configured in such a manner that a signal generated based on the approximation signal (a.sub.p) by the specific controller corresponds to the command signal.

2. The positioner (31) according to claim 1, configured to carry out at least one diagnostic routine while taking into account the approximation signal (a.sub.p).

3. The positioner (31) according to claim 1 or 2, further comprising a memory (9) containing process context data (k), and wherein the positioner (31) is configured to determine the approximation signal (a.sub.p) while taking into account the process context data (k) and/or is configured to carry out the at least one diagnostic routine while taking into account the process context data (k).

4. The positioner (31) according to one of the preceding claims, wherein the positioner (31) is further configured to carry out the at least one diagnostic routine while taking into account at least one control device signal from the list comprising control variable (g), actual value position signal (i) and process control signal (p.sub.g).

5. The positioner (31) according to one of the preceding claims, wherein the positioner (31) is configured to determine an approximation signal (a.sub.p) corresponding to a process control difference signal (p.sub.g) and/or actual value process signal (p.sub.i) of the process controller (120) that is not available to the valve control device (1).

6. A valve control device (1) for a process plant (100), comprising a control valve (35) for adjusting a process fluid flow, an in particular pneumatic or electric actuator (33) for actuating the control valve (35), and a positioner (31) according to one of claims 1 to 5.

7. A diagnostic method for a valve control device (1) in particular according to claim 6, in a process plant (100) with a process controller (120) that controls the valve control device (1), wherein a process control signal (p.sub.g) is provided to the valve control device (1) by the process controller (120), and wherein a controller model relating to a specific controller, in particular to the process controller (120), is configured, and wherein an approximation signal (a.sub.p) is determined by the valve control device (1) based on the process control signal (p.sub.g) and the controller model relating to the controller, and wherein at least one diagnostic routine is carried out by the valve control device (1) while taking into account the approximation signal (a.sub.p).

8. The diagnostic method according to claim 7, wherein the determination of the approximation signal (a.sub.p) and/or the diagnostic routine is further carried out based on process context data (k) which characterise the process setpoint signal (p.sub.w), in particular a constant process setpoint signal.

9. The diagnostic method according to claim 8, wherein a time interval is determined to which the process context data (k) relate and/or wherein the process context data (k) comprise a delay line and/or a signal form definition of the process setpoint signal (p.sub.w).

10. The diagnostic method according to one of claims 7 to 9, wherein the process control signal (p.sub.g), in particular its time curve, is saved.

11. The diagnostic method according to one of claims 7 to 10, wherein the controller model, in particular the control inversion, is determined based on a predetermined continuous-time or time-discrete controller structure.

12. The diagnostic method according to claim 11, wherein the control inversion is formed by an inverse in the z-domain over the variable z by $G_{\mathrm{approx}}^{-1}\left(z\right):=\frac{\text{?}}{\text{?}}.$ $\text{?}\text{indicates text missing or illegible when filed}$

13. The method according to one of claims 8 to 12, wherein a comparison of the diagnosis result with a predetermined setpoint behaviour of the valve control device (1) is carried out.

14. The diagnostic method according to claim 13, wherein a diagnostic code is generated if a deviation between the diagnostic result and the predetermined setpoint behaviour is established when the comparison is carried out, and/or wherein a diagnostic code is suppressed and/or deleted if no deviation between the diagnostic result and the predetermined setpoint behaviour is established when the comparison is carried out.

Description

[0041] Further properties, advantages and features will become clear from the following description of preferred embodiments with reference to the accompanying drawings, wherein:

[0042] FIG. 1 is a schematic illustration of a process plant with a valve control device;

[0043] FIG. 2 is a schematic block diagram of a digital positioner,

[0044] FIG. 3 is a schematic illustration of a first method for operating the process plant with the valve control device; and

[0045] FIG. 4 is a schematic illustration of a second method for operating the process plant with the valve control device.

[0046] In the following description of preferred embodiments, identical or similar components are furnished with identical or similar reference signs to facilitate readability.

[0047] The embodiments can illustrate in particular how a positioner in a control device can gain additional information for the diagnosis of the control device through the approximate reconstruction of the command or control variable of the master controller and how the approximate reconstruction can be understood in each case mathematically as an inversion of the transfer function of a sufficiently accurate model of the master controller.

[0048] FIG. 1 shows a schematic illustration of a process plant 100 comprising one or more cascaded control circuits. For the sake of simplicity, merely a single control cascade is illustrated in the schematic illustration according to FIG. 1. The control cascade comprises a master process controller 120 and a slave valve control device 1. Although not illustrated in detail here, it is possible for exactly one field device, two, three or more field devices, in particular control devices, to be slaves to the master process controller 120.

[0049] The master process controller 120 can be configured to manage a plant process 111. To this end, the process controller 120 can receive as an input signal an actual value process signal p.sub.i or a plurality of actual value process signals, for example, from a process sensor 105 of a process fluid user 110. A process setpoint signal p.sub.w is specified for the process controller 120 in view of a desired process behaviour. The process setpoint signal p.sub.w can be specified for the process controller 120, for example, by means of a user interface, such as a control computer 101 in a control room of a process plant. The process controller 120 is configured to carry out a comparison of the process setpoint signal p.sub.w and the actual value process signal p.sub.i based on which a process control difference p.sub.e is determined. Based on the process control difference p.sub.e, a process control routine is implemented by the process controller 120. As the result of the process control routine, the process controller 120 outputs a process control signal p.sub.g, which is delivered to a slave valve control device 1 so that the valve control device 1 acts on a process fluid in a desired manner, with the aim of causing the actual value process signal p.sub.i to converge with the process setpoint signal p.sub.w.

[0050] The valve control device 1 comprises a positioner 31, an actuator 33 and a control valve 35. The control valve 35 acts on the process fluid which is provided to the process fluid user 110. Alternatively, it is also possible for control valves to act on the outgoing flow of a process fluid from a process fluid user 110 (not illustrated). The control valve 35 can influence a process fluid pressure, a flow velocity, or the like. It is evident that a process fluid user 110 can comprise a plurality of incoming process fluid flows and/or a plurality of outgoing process fluid flows, wherein valve control devices can be associated with a single or a plurality of the incoming process fluid flows and/or outgoing flows of the process fluid user 110.

[0051] The valve control device 1 comprises a positioner 31 with an analogue or digital positioner electronics assembly 400, provided at least in part as a computer-implemented control module 401 as described in detail below with reference to FIG. 2. The positioner electronics assembly 400 has a first signal input 420 for a process control signal p.sub.g and a second signal input 436 for an actual position value i. Depending on the actual position value i and the process control signal p.sub.g, a comparison is carried out with a control routine in order to calculate a positioner control difference and determine a control signal g. The positioner electronics assembly 400 outputs the control signal g for actuating the actuator 33 at an output 433.

[0052] The positioner electronics assembly 400 further comprises an inversion module 405 to which the process control signal p.sub.g is specified in the form of a signal input and which can receive process context data k describing the behaviour of the master process controller 120 from a memory 404. The inversion module 405 is configured to determine, based on the process control signal p.sub.g and by means of a control inversion, an approximation signal a.sub.p that describes the master process controller 120. The approximation signal a.sub.p can correspond, for example, to an actual process controller value or to a process controller control difference p.sub.g. An actual value process signal generally depends on the time curve of an actual process controller value, for example a series of discrete actual process controller values or a continuous-time progression of the actual process controller value. Suitable embodiments and functions of the inversion module 405 are described in detail in the following.

[0053] The valve control device 1 can further include a diagnostic module 407, for example a diagnostic electronics assembly, which can carry out diagnostic routines relating to the valve control device 1. The diagnostic module 407 can be configured to carry out known diagnostic routines for valve control devices. The diagnostic module 407 can be configured to generate at least one diagnostic code 408 as a function of at least one diagnostic routine. The diagnostic code can be shown on a visual display of the valve control device 1 for a user. The diagnostic code 408 can be transmitted to the process control room, to the process controller 120 or to a portable computer such as a tablet computer for processing. The diagnostic module 407 can be configured to carry out at least one diagnostic routine based on the approximation signal a.sub.p. The diagnostic module 407 can take into account other signals available in the valve control device 1, in particular in its positioner electronics assembly 31, in the diagnostic routine based on the approximation signal. For example, the diagnostic module 407 can take into account the process control signal p.sub.g, the control signal g, an actual value position signal i, a positioner control difference or the like when carrying out the diagnostic routine based on the approximation signal a.sub.p. Possible variants of the positioner 400 with configurable electronic computing/data storage device with a computer-implemented diagnostic module 407 are described in the following.

[0054] FIG. 2 shows a block diagram of a digital positioner 31 formed with a configurable electronic computing/data storage device 400. The schematic block diagram illustrates a digitised positioner electronics assembly 400 with a configurable electronic computing/data storage device. However, the functions disclosed in this connection could be provided in part or entirely by means of analogue positioner electronics components.

[0055] The digital positioner electronics assembly 400 comprises a processor 403, for example in the form of a microprocessor, which is configured to carry out various calculations. The processor 403 of the digital positioner electronics assembly 400 is linked to a memory 404. Different data and/or routines can be saved on the memory 404 for use by the processor 403. A first calculation module 401 can be saved on the memory 404 for a control routine of the valve control device 1. The first calculation module 401 can be described as a control module. A digital position control can be implemented with the control module. The control module 401 can be implemented with the processor 403 in order to provide a control signal g at the output 433 of the digital positioner electronics assembly 400 for actuating the actuator 35. The processor 403 can be configured by means of the control module 401 to calculate a control routine based on an actual value position signal i provided by a position sensor 36 and a process control signal p.sub.g provided by the master process controller 120. The control routine of the first calculation module 401 can be, for example, a digital PID control routine.

[0056] The positioner electronics assembly 400 can have two or more signal inputs 420, 436 for signals that are to be processed in the processor 403 according to a routine. The digital positioner electronics assembly 400 comprises a first signal input 420 for receiving a process control signal p.sub.g. In embodiments, this first signal input 420 can comprise an analogue-to-digital converter in order to generate a digital signal for use in the digital positioner electronics assembly 400 based on, for example, simple analogue signals, such as an analogue 4.20 mA process control signal. The digital positioner electronics assembly 400 further comprises a second signal input 436 for receiving an actual value position signal i. The digital positioner electronics assembly 400 further comprises a control signal output 433. The processor 403 can be configured to execute the control module 401 with the control routine in order to carry out a position control based on the signals i, p.sub.g received at the inputs 420, 436 and to provide as its result a control signal g at the control signal output 433 for actuating an actuator 33. In embodiments, the control signal output 433 can comprise a digital-to-analogue converter or an electropneumatic converter in order to provide a control signal g adapted to the actuator 33. The digital positioner electronics assembly 400 can include further signal inputs or outputs (not illustrated in detail). The positioner can further include an interface for the manual input of data.

[0057] The actuator 33 can be equipped with a signal amplifier in order to amplify a control signal g using auxiliary electrical and/or pneumatic power from an auxiliary power source.

[0058] The memory 404 of the digital positioner electronics assembly 400 can contain one or more diagnostic routines in order to provide a diagnostic module 407. The diagnostic routines are configured to be carried out by the processor 403. The processor 403 can carry out a diagnostic routine, for example, relating to the control signal g and the actual value position signal. For example, a diagnostic routine can initiate the execution of a partial stroke test and evaluate its result.

[0059] The memory 404 can contain input data 402, wherein the input data 402 saved in the memory 404 are expediently associated with a specific time or time interval. In one embodiment, input data 402 consist exclusively of control device signals. The processor 403 can be configured to carry out a diagnostic routine using the input data 402 relating to a predetermined time interval, for example in order to compare current input data 402 of a current time interval with historical input data 402 of another time interval or determined reference interval. A deviation from historical diagnostic results of a reference interval can indicate, for example, wear on the control valve 35. The diagnostic routine can be designed to establish whether there is a conspicuous signal curve such as a trend in one direction.

[0060] As set out above, the valve control device 1 can comprise a positioner 31 which is configured to carry out a diagnostic routine, the positioner 31 being equipped with a digital positioner electronics assembly 400 comprising a memory 404 with a diagnostic routine 407 saved on it and a processor 403 for carrying out the diagnostic routine 407. The positioner 31 can generate a control variable g for an actuator 33 by means of the digital positioner electronics assembly 400.

[0061] In the present disclosure, the positioner is further configured to carry out a control inversion relating to a master process controller 120. With the control inversion, an approximation signal a.sub.p can be determined based on the process control signal p.sub.g. The approximation signal a.sub.p can serve as the basis for a diagnostic routine 407. To this end, the memory 404 of the configurable electronic computing/data storage device 400 can be equipped with a second calculation module 405, which can be called a modelling module or inversion module.

[0062] The second calculation module comprises configuration data 406 for defining a controller model for a particular controller. The configuration data 406 for adapting the model are used to adapt the controller model to a specific controller, for example to the master process controller of the valve control device. It is optionally possible to additionally save process context data 409 that characterise the behaviour of the master process controller 120 of the valve control device 1 on the memory 404. The processor 403 can be configured to execute the second calculation module 405 and/or at least one diagnostic routine 407 while taking into account process context data 409. A schematic illustration of the position control occurring in the positioner 31 as well as of the control inversion and, if applicable, diagnostics occurring in parallel is illustrated in a first embodiment in FIG. 3 and in a second embodiment in FIG. 4.

[0063] With reference to FIG. 1, it is evident that, in the process control carried out with the process controller 120, an actual value process signal p.sub.i and a process setpoint signal p.sub.w are necessary as known process variables based on which a process control signal p.sub.g is determined as the unknown process variable to be calculated by means of the predetermined process control routine. This process control routine can be represented by a model controller. The process control signal p.sub.g is transmitted to the slave positioner 1. The process setpoint signal p.sub.w, actual value process signal p.sub.i and other process signals are generally not known to the valve positioner 1.

[0064] FIG. 3 shows an embodiment of an operating method in which a control and, in parallel, a diagnostic method including a control inversion and, where appropriate, a diagnostic routine are carried out. The second calculation module 405 can comprise configuration data 406 for adjusting the model. The second calculation module or inversion module 405 can in particular be configured to replicate the inverse of a model controller with which the process control routine of the master process controller 120 of the valve control device 1 is approximated. The second calculation module 405 is configured to invert a transfer function relating to the master process controller 120 in a spectral domain such as a Laplace or z-space. The implementation of the control inversion carried out with the second calculation module 405 in the processor 403 of the digital positioner electronics assembly 400 serves to reconstruct approximately the input data (p.sub.i, p.sub.w) of the process controller 120 based on the process control variable p.sub.g output by the process controller 120 and received by the control device 1. The second calculation module 405 is designed to carry out a calculation by means of the processor 403 based on the process control signal p.sub.g as the known variable with the help of process context data k in order to calculate an approximation signal a.sub.p, which is considered the unknown variable. The approximation signal a.sub.p corresponds to a process signal, in particular to the process control difference p.sub.e or to the actual value process signal p.sub.i.

[0065] The process context data k or 409 can be relate to the process setpoint signal. Using the process context data k, it is possible to assume an at least occasionally constant process setpoint signal p.sub.w during the control modelling, in particular inversion, with the second calculation module 405. By means of the process context data k, the control inversion can be based on a plurality of different, in particular constant, process setpoint signals p.sub.w associated with different time intervals. Alternatively or additionally, the second calculation module 405 can use process context data k that replicate a time curve of a non-constant process setpoint signal p.sub.w. According to a further alternative option, the process context data k can provide the second calculation module 405 with information regarding signal curve characteristics of the process setpoint signal p.sub.w. For example, process context data k can provide the second calculation module 405 with information regarding a, for example linear, ramp-like curve of the process setpoint signal p.sub.w. The process context data k can provide the second calculation module with information regarding a sinusoidal curve of the process setpoint signal, for example its frequency, amplitude, offset in the amplitude direction or offset in the time dimension. Alternatively or additionally, the second calculation module can take into account process context data describing a jumpy behaviour of the process setpoint signal, for example its amplitude, frequency, jump time or the like.

[0066] The operating method illustrated in FIG. 3 relates to a positioner 31 which processes exclusively the process control signal p.sub.g of the master process controller 120. The positioner 31 according to the embodiment illustrated in FIG. 3 does not process any signals received directly from the master controller cascade other than the process control signal p.sub.g. In particular, the positioner 31 does not receive any signals from the master process control cascade other than the process control signal p.sub.g. In the method illustrated in FIG. 3, the positioner is provided with predetermined process context data k(t) relating in particular to time intervals t.sub.2-t.sub.1. For example, process context data k can be entered in the form of empirical values manually via a user interface. Alternatively or additionally, process context data k can be saved in a factory setting on the memory 404 of the positioner electronics assembly 400 before the valve control device 1 is put into operation for the first time.

[0067] FIG. 4 shows an alternative embodiment of an operating method in which a control and, in parallel, a diagnostic method including a control inversion and, where appropriate, a diagnostic routine are carried out. The operating method illustrated schematically in FIG. 4 differs from the method according to FIG. 3 essentially only in the provision/receipt of process context data k. In the embodiment according to FIG. 4, the valve control device 3 includes a further input for receiving at least one process setpoint signal p.sub.w of the process plant 100. The process setpoint signal p.sub.w can be received, for example, as a series of digital process setpoint signals. For example, an analogue or digital signal input of the valve control device 1 can receive the process setpoint signal p.sub.w as a further input value, which is simultaneously fed in the process plant 100 to the process controller 120 for the process control. Alternatively, it is conceivable for the process controller 120 to be configured and connected to the valve control device 1 according to the signal transmission in order to additionally transmit the process setpoint signal p.sub.w to the further input of the valve control device 1. The process setpoint signal p.sub.g can be received by the valve control device 1, for example, in digital form in the form of a plurality of process setpoint signals p.sub.w to be processed in sequence. The time series q of process setpoint signals can be saved in a retrievable manner by the digital positioner electronics assembly 31 in the memory 404 as process context data k(q). According to an advantageous embodiment, a specific time or time interval t is respectively associated with the received process setpoint data of the time series q of process context data. Process setpoint signals p.sub.w received as a time series q can be saved correlated to a delay line or to a temporal offset in the form of process context data 409. A temporal offset or delay line 411 allows a correlation of temporally upstream process setpoint signals p.sub.w and temporally downstream process control signals p.sub.g that are to be linked in the in particular inverted control routine, which can be adapted with configuration data 406, according to the second calculation module 405. For example, the control inversion can be based on a process control signal p.sub.g in combination with the process setpoint signal p.sub.w shifted by a cycle time of the digital positioner electronics assembly 400.

[0068] In a digital process control or digital control device control, there is usually a time delay of in particular exactly one (process) controller cycle time between the reception of a (process) setpoint value and the output of the (process) control signal assigned by means of the (process) control routine to this (process) setpoint value. In order for the control inversion to be able to determine a more precise approximation of an actual process value or a process command value based on the process control value, it can be helpful to include a process control value and in particular a process setpoint value that is older by exactly one process controller cycle time for the control inversion. Alternatively, it is possible to carry out a sufficiently accurate approximation by control inversion without a delay line, in particular in cases of slow processes with a gradually, in particular continuously, variable process setpoint signal.

[0069] The second calculation module 405 can preferably replicate the inverse of a process controller 120 in the form of a model controller configured according to a PID controller structure. For example, the second calculation module 405 can implement a control inversion in particular with respect to a process controller 120 modelled as a PID controller. In particular, the inversion module 405 can carry out the control inversion in relation to a process controller 120 modelled as a P controller, I controller, PD controller, PI controller or PID controller. A process controller 120 modelled in a PID controller structure can be described by a mathematical function G.sub.PID which determines, based on the known variables process setpoint signal p.sub.w, actual value process signal p.sub.i and/or process control deviation p.sub.e, a process control signal p.sub.g as the unknown variable and output value. In the modelling module or inversion module 405, it is possible for the known mathematical control function to serve as the basis for an inverted calculation in such a manner that the process control signal p.sub.g is taken into account as a known variable together with a process setpoint signal p.sub.w, which is deemed to be an at least approximately known variable, in order to determine an approximation signal a.sub.p that corresponds in particular to an actual process value p.sub.i or to a process control deviation p.sub.g. The process control routine of a process controller 120, for example in a PID controller structure outlined in FIG. 5, can be represented mathematically in Laplace space L by means of the equations:

X(s)=W(s)G.sub.PID.sup.1(s).Math.Y.sub.V(s)(4)

x(t)=L.sup.1{X(s)}(5)

where
W(s) is the Laplace transform of the real-value function p.sub.w(t): [0070] describing the time curve of the pro .fwdarw. setpoint value p.sub.w; [0071] X is an actual process value of the discrete or continuous-time actual value process signal p.sub.i; [0072] Y.sub.V is a process control value of the discrete or continuous-time process control signal p.sub.g; and [0073] G.sub.PID describes the transfer function of the PID process controller and [0074] s represents a dependence on the Laplace variable.

[0075] The following holds true for the transfer function of a continuous-time PID controller in a parallel structure with a real D-part time constant in the Laplace domain with the proportional gain K.sub.P, the reset time T.sub.N and the lead time T.sub.V:

[00004]$\begin{array}{cc}{G}_{\mathrm{PID}}\left(s\right)=K_p.Math.\left(1+\frac{1}{\text{?}}.Math.\frac{1}{\text{?}}+\text{?}.Math.\frac{\text{?}}{\text{?}}\right)& \left(6\right)\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

[0076] It can be advantageous for a valve positioner if the equation (5) is rephrased as a differential equation, inserted into the following equation (6) and considered with regard to a discrete scanning step k:

[00005]$\begin{array}{cc}x\left(k\right)=w\left(k\right)-e\left(k\right)=w\left(k\right)-Z^{-1}\text{?}\frac{\text{?}}{\text{?}}\text{?}& \left(7\right)\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

[0077] For a discrete-time controller, the transfer function can be determined using the Tustin transformation according to equation (7), wherein T.sub.C is chosen according to a cycle time of the process controller:

[00006]$\begin{array}{cc}s=\frac{2}{\text{?}}.Math.\frac{\text{?}}{\text{?}}& \left(8\right)\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

This yields

[00007]$\begin{array}{cc}{G}_{\mathrm{PID}}^{-1}=\frac{1}{\text{?}}.Math.\frac{\text{?}+a_{1}z+a_0}{\text{?}+b_{1}z+b_0}& \left(9\right)\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

where

a.sub.2=2T.sub.N(T.sub.C+2)

a.sub.1=8T.sub.N

a.sub.0=2T.sub.N(T.sub.C2)

b.sub.2=2T.sub.N(T.sub.C+2)+T.sub.C(T.sub.C+2)+4T.sub.NT.sub.V

b.sub.1=8T.sub.N+2T.sub.C.sup.28T.sub.NT.sub.V

b.sub.0=2T.sub.N(T.sub.C2)+T.sub.C(T.sub.C2)+4T.sub.NT.sub.V

[0078] Using the positioner electronics assembly 400, an approximation signal a.sub.p can be calculated approximately with the modelling or inversion module 405 based on the process control signal p.sub.g, which correlates to the process control difference p.sub.e or to the actual value process signal p.sub.i.

[0079] Alternatively, for some controller structures, a different calculation can be performed directly in the time interval without using a Laplace space. For example, an approximation signal corresponding to a signal response of a PI controller can be determined. The inversion of a time(t)-dependent transfer function G.sub.PI(t) of a PI process controller is known. The controller model can be configured with a proportional backcalculation factor with respect to the P factor of the transfer function G.sub.PI(t) and with a differentiation along with a proportional differentiation factor with respect to the I factor (not illustrated in detail). With such an inverted transfer function of a continuous-time PI controller G.sub.PI(t), an approximation signal a.sub.p can be determined based on a command signal provided by the process controller.

[0080] The diagnostic routine 409 can be configured to analyse one or more approximation signals a.sub.p replicating process control differences, for example a set of approximated control differences p.sub.e established in a chronological sequence q. Using this set, the diagnostic routine 407 can determine a diagnostic code 408 relating to the steady-state accuracy, overshoot, stability, etc. of the master process control.

[0081] For example, the stability of the overall control circuit including process and position control can be evaluated.

[0082] For example, the number of overshoots can be taken into account for diagnostic purposes. Based on the number and amplitude of overshoots of the diagnostic routine 407, a diagnostic code 408 can be determined regarding the quality of process control and position control. For example, it can be determined with a diagnostic routine 407 as a diagnostic result that there is an efficient process control and position control if there are few overshoots. If the number of overshoots exceeds a threshold value, it can be determined with the diagnostic routine 407 that an improvement of process control and position control through an adjustment of the control parameters of the positioner 1 and/or process controller 120 is recommended.

[0083] The features disclosed in the foregoing description, figures and claims can be significant both on their own as well as in any combination for the provision of the invention in the different variants.

LIST OF REFERENCE SIGNS

[0084] 1 Control device [0085] 31 Positioner [0086] 33 Actuator [0087] 35 Control valve [0088] 100 Process plant [0089] 105 Process sensor [0090] 110 Process fluid user [0091] 111 Plant process [0092] 120 Process controller [0093] 400 Positioner electronics assembly [0094] 401 Control module [0095] 402 Input data [0096] 403 Processor [0097] 404 Memory [0098] 405 Calculation module [0099] 406 Configuration data [0100] 407 Diagnostic module [0101] 408 Diagnostic code [0102] 409 Process context data [0103] 411 Delay line [0104] 420 First signal input [0105] 433 Output [0106] 436 Second signal input [0107] a.sub.p Approximation signal [0108] g Control signal [0109] i Actual position value [0110] k Process context data [0111] p.sub.d Process control difference [0112] p.sub.e Process control difference [0113] p.sub.g Process setpoint signal [0114] p.sub.i Actual value process signal [0115] p.sub.w Process setpoint signal [0116] q Time series