CN111930010A - LSTM network-based general MFA controller design method - Google Patents
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Abstract
The invention discloses a method for designing a general Model Free Adaptive (MFA) controller based on an LSTM network. And (3) constructing a self-adaptive network output value by considering the influence of the historical error sequence information on the output of the controller, and finishing updating the weight of the network by a back propagation algorithm over time through inputting and outputting data. And the error value at the current time may be added to the output of the controller to react faster to sudden changes. The LSTM-MFA controller can effectively control a single-variable industrial process, and has the advantages of simple structural design, small calculated amount, no dependence on an accurate mathematical model and parameters of a controlled system, good performance effect of the controller and the like.
Description
Technical Field
The present invention relates to an adaptive control method based on an LSTM network, and more particularly, to a general MFA controller design method.
Background
In recent years, the increasing complexity of industrial processes has made the demand for intelligence on automation controllers, which need to be adaptive to different processes in order to cope with unforeseen events that affect the economy and safety of the process. However, in actual process production, PID control still plays a dominant role.
While PID controllers can provide satisfactory control performance for many single-input single-output systems with relatively small dynamic ranges, significant difficulties remain with complex control systems. Many advanced control theories and methods have been developed to deal with these complex systems, such as model predictive control, robust control, and adaptive control. But these advanced techniques all rely on an accurate and relatively simple dynamic model of the process. These accurate models are often difficult to acquire in practice, and even if a model is obtained, model uncertainty can severely impact control performance.
Accordingly, there is a need to provide a universal controller that can easily and efficiently control a variety of complex systems, which can accommodate different process configurations. The MFA control method does not need to depend on the knowledge of the system dynamics with accurate process, only needs the real-time input and output data of the controller and the qualitative knowledge of the system behavior, and depends on the strong self-learning and self-adaptive capacity of the controller to deal with the uncertainty and the change in the system environment to complete the MFA control of the process.
Disclosure of Invention
The invention mainly aims to provide a method for designing a universal MFA controller based on an LSTM network, which is applied to a process control system.
The technical scheme for realizing the purpose of the invention is as follows: an LSTM-MFA universal control system for process control, comprising the steps of:
step 1: generating a set value signal of a desired value or a desired trajectory output by the process by a set value generating device;
step 2: generating an error signal between the set value signal and the measured variable signal as an input signal to the controller by the comparator means;
and step 3: the LSTM-MFA controller generates a control signal according to an input error signal and acts on the actual process;
and 4, step 4: a measured variable of the actual process is obtained. In each sampling period, the LSTM-MFA controller constructs a group of input and output data according to historical error sequence information and real-time output of the controller, and updates the weight of an LSTM recurrent neural network in the controller by using a time back propagation algorithm;
and 5: and (5) continuing to execute the steps 1-4 until the process reaches a set value and keeps stable, and finishing the iteration.
Further, the specific implementation process of the control system is as follows:
first, consider the form of the industrial process transfer function as follows:
whereinIs a Laplace transform operator;a Laplace transform which is a process transfer function;for process output or measured variablesThe Ralsberg transform of (1);as process inputs or controller outputsThe Ralsberg transform of (1).
Determining a sampling timeUsually one tenth of the process-dominant time constant is taken. In practical applications, an approximation may be obtained by artificially estimating the process-dominant time constant, since the LSTM-MFA controller is not sensitive to this parameter.
For process control systems, the control objective is to make the measured variableTracking set pointIs given as a track signal.
That is, the task of the LSTM-MFA controller is to make the error signalAt a minimum, the objective function is formulated as follows:
then, the general LSTM-MFA controller of the invention is designed by the following steps:
step 1: current time of dayError signal ofBy a normalizing unitGenerating a normalized error signalAs input to the controller at the current time;
wherein, the normalization unit is a tanh function, and the expression is as follows:
step 2: the normalized error signal is input into two LSTM hidden layers, and the network output at the current moment is obtained through the feedforward calculation process of the LSTM。
Wherein, the feedforward calculation process of the single LSTM is as follows:
wherein,representing the multiplication of corresponding elements in the operation matrix;then representing the matrix addition operation;for memory-losing gates, for deciding the state of the last cellWhat information is thrown away;for a candidate vector, this value is added to the cell state;is an input gate for updating the cell state;Determining which portions of the cell state are output for the output gate; a total of 8 sets of weight vectors to be adjusted are contained in each LSTMUnlike a normal neural network, all LSTM units in a single recursive hidden layer share this set of weights and do not increase the computational effort.
The activation function adopted by the gate control unit is a sigmoid function, and the expression is as follows:
and step 3: at the current momentThe output of the LSTM-MFA controller is output by the networkAnd error signalThe formula is as follows:
And 4, step 4: finally, at each sampling period, the weights of the network need to be iteratively updated using a back-propagation over time algorithm based on a set of input and output data.
Wherein, the input data is the error of the current sampling periodThe output data is historical error sequence information and current sampling period network outputConstructed function valueThe expression is as follows:
wherein,the adaptive rate for the output adjustment of the controller can effectively adjust the effect of the controller, and the value range is 0-1.Is the length of the historical error sequence information under consideration.
Too large a value of (c) will result in too much oscillation of the closed loop system response;too small a value will result in process drift or slow response speed.
The invention applies the general MFA controller design method based on the LSTM network to the process control system. Compared with the prior control technology, the invention has the advantages that:
(1) the LSTM network-based general MFA controller design method can be applied to linear or nonlinear systems in process control, and the controller structure does not need to be redesigned.
(2) The general MFA controller design method based on the LSTM network does not need the accurate mathematical model of the process and the quantitative knowledge of the parameters, and solves the difficulty that the process system is difficult to model.
(3) The general MFA controller design method based on the LSTM network can deal with the situation that the model structure changes, namely the model parameter or the structure changes in the actual process, the parameter of the controller does not need to be adjusted, the performance of the controller can not be influenced, the prior PID control and other technologies need to adjust the corresponding parameter, and the parameter adjusting process is very complicated.
(4) The design method of the general MFA controller based on the LSTM network is relatively simple in structure and can be easily applied to the actual process.
Drawings
FIG. 1 is a single input single output LSTM-MFA universal control system of the present invention.
Fig. 2 is a schematic structural diagram of the LSTM-MFA controller of the present invention.
FIG. 3 is a schematic diagram of the LSTM structure of the present invention.
FIG. 4 is a diagram of an LSTM-MFA controller simulation for a setpoint change process of the present invention.
FIG. 5 is a graph of an LSTM-MFA controller simulation of the present invention for the presence of white noise in the measured variables.
FIG. 6 is a comparative simulation diagram of the LSTM-MFA and PID controllers for the texture transformation process of the present invention.
FIG. 7 is a comparative simulation diagram of the LSTM-MFA and PID controllers of the present invention for a hysteresis process.
Detailed Description
The invention will be further explained with reference to the drawings.
The invention relates to a method for designing a general MFA controller based on an LSTM network, which is applied to a process control system and is implemented specifically as follows:
considering the simplest application structure of the invention, the LSTM-MFA general control system with single input and single output as shown in FIG. 1 is composed of a single input and single output process, an LSTM-MFA controller, two signal summers and a back propagation module with time.
The signals in fig. 1 are defined as follows:
-a set value, representing a set value signal of a target value or a process trajectory of the process.
The control objective being to make the measured variableTracking set pointGiven track signals, i.e. error signalsTending towards zero.
Step 1: the setpoint signal of the desired value or desired trajectory of the process output is generated by the setpoint generating device.
Step 2: an error signal between the set-point signal and the measured variable signal is generated by the comparator means as an input signal for the controller.
And step 3: the LSTM-MFA controller generates a control signal according to an input error signal and acts on an actual process.
And 4, step 4: a measured variable of the actual process is obtained. In each sampling period, the LSTM-MFA controller constructs a group of input and output data according to historical error sequence information and real-time output of the controller, and updates the weight of the LSTM recurrent neural network in the controller by using a time-reversal propagation algorithm.
And 5: and (5) continuing to execute the steps 1-4 until the process reaches a set value and keeps stable, and finishing the iteration.
The structure of the general LSTM-MFA controller is shown in FIG. 2, and the specific implementation steps are as follows:
it can be seen that the controller is structured in the form of an LSTM recurrent neural network, which can be expanded along the sampling time axis.
WhereinInputting a signal for the error at each sampling instant;outputting a signal for the network at each sampling instant;for the controller output at each sampling instant.
Sampling timeIs generally constant with the dominant time of the processCorrelation is usually taken to be one tenth of the dominant time constant.
Wherein in the actual processIs usually unknown and can be estimated manuallyTo an approximation that the LSTM-MFA controller is not very sensitive to this parameter and therefore does not significantly affect the performance of the controller.
Step 1: current time of dayError signal ofBy a normalizing unitGenerating a normalized error signalAs the output of the controller at the current momentAnd (6) adding.
Wherein, the normalization unit is a tanh function, and the expression is as follows:
step 2: the normalized error signal is input into two LSTM hidden layers, and the network output at the current moment is obtained through the feedforward calculation process of the LSTMWherein the complete structure of the LSTM is shown in FIG. 3.
And step 3: at the current momentOutput of LSTM-MFA controllerOutput from the networkAnd error signalThe formula is as follows:
And 4, step 4: finally, at each sampling period, the weights of the network need to be iteratively updated using a back-propagation over time algorithm based on a set of input and output data.
Wherein, the input data is the error of the current sampling periodThe output data is historical error sequence information and current sampling period network outputConstructed function valueThe expression is as follows:
wherein,the adaptive rate for the output adjustment of the controller can effectively adjust the effect of the controller, and the value range is 0-1.Is the length of the historical error sequence information under consideration.
The effect of using the specific implementation of the invention and the proposed control method is illustrated by simulations as follows:
consider several single-input single-output processes as follows:
for the case where the model set value is changed, model 1 is taken as an example. In the simulation process, the set values of the process are constantly changed.
In the simulation process, the parameters controlled by the LSTM-MFA are default values, and the self-adaptive rate。
All controller tuning parameters remain unchanged despite the changing process set points. The simulation results are shown in fig. 4.
It can be seen that the LSTM-MFA controller can adapt well to the set value changing process, and has good control effect.
For the case of model interference, model 2 is taken as an example. In the simulation process, white noise interference is added to the process output.
In the simulation process, the parameters controlled by the LSTM-MFA are default values, and the self-adaptive rate。
All controller tuning parameters remain unchanged despite the presence of white noise interference, and the simulation results are shown in fig. 5.
It can be seen that the LSTM-MFA controller can adapt well to the white noise interference process, with good control effect.
For the control of the model switching process, in the simulation process, the process model is changed from model 1 to model 2 on-line to create the model switching process, and the set value is changed from 10 to 20.
In the simulation process, the parameters controlled by the LSTM-MFA are default values, and the self-adaptive rate. And the PID parameters adjusted for model 1 are. The sampling time is taken as。
All controller tuning parameters remain unchanged despite process variations. The simulation result is shown in fig. 6.
It can be seen that the LSTM-MFA controller can adapt well to changes in process configuration, whereas the PID controller cannot.
For the process with model lag, in the simulation process, the process model is switched from model 1 to model 3 lag, and the set value is changed from 10 to 20.
The controller parameter settings are as described above.
Although the process switches to the model with hysteresis, all controller tuning parameters remain unchanged. The simulation results are shown in fig. 7.
It can be seen that in the presence of hysteresis, the LSTM-MFA controller is still well adapted and has better performance than the PID controller.
Claims (9)
1. An LSTM-MFA universal control system for process control, comprising the steps of:
step 1: generating a set value signal of a desired value or a desired trajectory output by the process by a set value generating device;
step 2: generating an error signal between the set value signal and the measured variable signal as an input signal to the controller by the comparator means;
and step 3: the LSTM-MFA controller generates a control signal according to an input error signal and acts on the actual process;
and 4, step 4: acquiring a measurement variable of an actual process, constructing a group of input and output data by the LSTM-MFA controller according to historical error sequence information and real-time output of the controller in each sampling period, and updating the weight of an LSTM recurrent neural network in the controller by using a time-based back propagation algorithm;
and 5: and (5) continuing to execute the steps 1-4 until the process reaches a set value and keeps stable, and finishing the iteration.
2. The LSTM-MFA universal control system according to claim 1, wherein said universal LSTM-MFA controller comprises:
1) an input layer: taking an error signal of a controlled process as the input of a controller, and then performing normalization processing to be used as the input of an LSTM network;
2) hiding the layer: the first layer of LSTM input not only includes the normalization error signal of the current time, but also includes the hidden state information transmitted by the first layer of LSTM at the previous time; the input of the second layer LSTM comprises the output of the last layer LSTM and the hidden state transmitted by the second layer LSTM at the last moment;
3) an output layer: at the current sampling instant, the output of the controller consists of the network output and the current error signal.
3. The LSTM-MFA universal controller of claim 2, wherein the controller is configured to include a recurrent neural network, such that the controller can be spread out along the sampling time, and at each sampling instant, the controller outputs correspond to the network outputs one-to-one.
4. The LSTM-MFA generic controller of claim 2, wherein the normalization of the input-layer error signal is by transforming the error signal into the interval-1 to 1 by a normalization function, wherein the normalization error function is a tanh function.
5. The LSTM-MFA controller of claim 2, where the sampling time is chosen in relation to the dominant time constant of the process, typically taken to be one tenth of the dominant time constant, and in practice, to be approximated, since the LSTM-MFA controller is not very sensitive to this parameter.
6. The LSTM-MFA generic controller of claim 2, wherein the controller employs a network architecture that is an LSTM recurrent neural network that models a length of historical error sequence information to derive a continuous function of the network output asTo obtain the output of the controllerThe continuous function expression of (a) is:
8. The LSTM-MFA universal controller of claim 2, wherein at each sampling instant, LSTM-MFThe learning process of the controller A comprises constructing proper input and output data as training data for updating network weight, and completing the adaptive learning process of the controller by using a time-dependent back propagation algorithm, wherein the input data is the current momentError value ofThe output data is historical error sequence information and current sampling period network outputThe function value of the structure can be expressed by the following formula:
9. The LSTM-MFA universal controller as claimed in claim 8, wherein said adaptive parameters are adapted to control the operation of said MFA controllerThe performance of the controller can be effectively adjusted, the value range of the controller is 0-1, and the larger the value is, the more historical error sequence information components are considered.
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