Abstract
The manufacturers of Programmable Logic Controllers (PLC) usually equip their products with extremely simple control algorithms, such as PID and on-off regulators. However, modern PLCs have much more efficient processors and extensive memory, which enables implementing more sophisticated controllers. The paper discusses issues related to the implementation of matrix operations, time limitations for code execution within one PLC cycle, and memory requirements. The adaptive Model Predictive Controller (aMPC) algorithm is selected for predictive control with on-line adaptation of model parameters. The combination of predictive and adaptive properties in the regulator enables control of many industrial objects for which PID control is ineffective, e.g. nonstationary plants with time-varying delays. The presented generic approach consists in developing a C++ application for desktop PC that generates, based on user provided parameters - such as MPC horizons, the code in Structured Text (ST) language compliant with the IEC-61131 standard for PLCs. Despite the enhanced capabilities of programmable controllers, there are limitations to this platform that cannot be overcome. The implementation of optimization-based control algorithms requires cooperation with an external computing server. In the article, the PLC/external computer cooperation is used to implement the control with constraints taken into account. The robust control using the Linear Matrix Inequalities (LMI) for a multivariable plant is also presented. A number of tests were carried out to verify the correctness of implementation of this control in software-in-the-loop and hardware-in-the-loop structures.
Highlights
Programmable Logic Controllers are a popular industrial platform for the implementation of control algorithms
STATE OF THE ART This section presents, based on literature and Internet references, the most important aspects of the article, including: what we call advanced control, what industrial equipment enables the implementation of advanced control algorithms, and which algorithms are implemented in their devices by leading manufacturers
COOPERATION WITH EXTERNAL COMPUTATIONAL SERVER It is assumed that due to Programmable Logic Controllers (PLC) drawbacks and limitations, the algorithm used in the control system cannot be implemented in the PLC, even using the guidelines given in the previous section
Summary
Programmable Logic Controllers are a popular industrial platform for the implementation of control algorithms. The assumption is that the user enters the controller parameters, and the code generator working on PC prepares the program in the form of a text file in ST language for the PLC. This approach is very flexible, it provides the information on the amount of memory used and the time needed to execute such a code in the PLC. These algorithms use different forms of the model, but are functionally similar to each other They preserve all advantages of predictive control related to: extension of the horizon, entering the parameters as trajectories, taking into account multidimensionality, the ability to control non-minimal phase objects, and natural handling of delays, including time-varying delays. A large part of the present achievements comes from master’s theses [3] and [4]
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