Asynchronous motor variable frequency speed regulation system based on active disturbance rejection controller

1 Introduction

With the continuous development of power electronics technology, microelectronics technology and microprocessors, the speed regulation performance of asynchronous motor variable frequency speed regulation system has been greatly improved. Compared with the traditional DC motor speed regulation system, it has the characteristics of simple structure, wide speed regulation range, high efficiency, good characteristics, stable operation, safety and reliability, etc., and has been widely used in production practice. The variable frequency speed regulation system composed of inverter and asynchronous motor has a great development trend of replacing the DC speed regulation system.

Programmable logic controller (PLC) is recognized as one of the three pillars of modern industrial automation. Its control system is stable and reliable, and its communication network is flexible. It can be easily integrated into the fieldbus control system to meet the requirements of the increasing degree of automation. The PLC variable frequency speed regulation system has gained more and more attention due to its superior performance. However, for asynchronous motors with multi-variable nonlinear strong coupling, the conventional fixed parameter PID control method has poor adaptability to load changes, weak anti-interference ability, and is greatly affected by changes in system parameters. Therefore, on the basis of existing hardware equipment, how to further improve the control performance of the variable frequency speed regulation system is an urgent problem to be solved.

Here, an anti-disturbance controller method is adopted in the asynchronous motor variable frequency speed regulation system. The internal and external disturbances of the system are regarded as the total disturbance of the system, which are uniformly observed and compensated by the extended state observer, so that the control object is approximately linearized and deterministic, thereby realizing the nonlinear control of the system, and the effectiveness of the control scheme is verified through experiments.

2 Mathematical model of asynchronous motor variable frequency speed regulation system

The state equation of the asynchronous motor variable frequency speed regulation system powered by a current tracking SPWM inverter in the d, q two-phase rotating coordinate system can be described by a fifth-order nonlinear model. When the inverter time lag is ignored, the system model can be described by a reduced second-order nonlinear model:

In the formula: ω1 is the electrical synchronous angular velocity; ωr is the rotor speed; isd, isq are the stator currents of the d and q axes respectively; ψrd, ψrq are the rotor flux of the d and g axes respectively; np is the number of pole pairs; Lm is the mutual inductance; Lr is the rotor inductance; J is the moment of inertia; Tr is the motor rotor time constant; TL is the load torque.

It can be seen from the literature that the inverter system is reversible in vector operation mode, and the entire system can be simplified into a single-input and single-output system for speed.

3. Asynchronous Motor Variable Frequency Speed ​​Regulation Self-disturbance Control System

3.1 Design of First-Order Active Disturbance Rejection Controller

Figure 1 shows the structure of the ADRC. The controller is a nonlinear controller based on a tracking differentiator (TD) to arrange the transient process, an extended state observer (ESO) to estimate the system state, model and disturbance, and a nonlinear error feedback (NLSEF) to give a control signal.

[page] For vector-controlled asynchronous motor drive systems, a first-order model controller is used, and a second-order ESO structure is used accordingly. In vector control, the rotor flux is generally kept constant. By utilizing the characteristics of ADRC, the system model error caused by the change in the moment of inertia and the influence of load disturbance are attributed to the expansion state z2 for unified observation and compensation. The principle block diagram of the speed controller based on the first-order ADRC is shown in Figure 2.

3.2 ADRC Optimization

In the first-order ADRC structure, the ESO outputs the observed values ​​of the controlled object and the unknown disturbance. There is no differential output of the controlled object. The controller does not need to track the output of the differentiator, so the tracking differentiator link is omitted in the ADRC structure. For the first-order object, linear proportional regulation is used instead of NLSEF. Under the premise of ensuring the performance of the controller, the model can be effectively simplified and the amount of calculation can be reduced, thereby obtaining a first-order ADRC model with optimized structure. Figure 3 is a block diagram of the first-order ADRC speed control with optimized structure. The complete algorithm of the optimized speed controller is:

Where: is the motor speed given value; is the tracking signal of the motor speed feedback ωr; is the observed value of the total disturbance W(t); u is the control quantity; β01, β02 are the ADRC output error correction gains; h is the sampling period; kp is the proportional coefficient; b0 is the compensation factor. The control performance of the system can be adjusted by adjusting kp and b0.

4 Experiments and results analysis

4.1 System Hardware Connection

The whole system includes the host computer and monitoring software (WinCC), S7-300PLC, Micro Master Vector (MMV) inverter, asynchronous motor and photoelectric encoder, as shown in Figure 4.

[page]4.2 System software design

4.2.1 System communication design

The system communication consists of three parts: ① PROFIBUS-DP fieldbus communication between PLC and inverter, realizing on-site remote control of inverter by PLC; ② MPI communication between industrial computer and PLC, on the one hand, realizing communication between STEP7 and PLC, completing program upload, download, debugging, fault diagnosis and online monitoring, etc.; on the other hand, realizing communication between WinCC and PLC, completing process data transmission and real-time monitoring of system status; ③ OPC communication between WinCC and Excel, archiving motor speed process data through software and enabling OPC communication service, exporting process data to Excel for fitting system response curve and analyzing various dynamic and static performance indicators.

4.2.2 System control software design

In the experiment, statement list STL is used to program in industrial software STEP7V5.2. The whole system adopts structured programming, and the system program structure is shown in Figure 5.

4.3 ADRC parameter setting

Research shows that β01 and β02 are mainly determined by the discrete control cycle of the controller, which is generally: β01=1/h, β02=1/(5h2). The speed sampling period in the experiment is h=100 ms, so β01=10, β02=20. For the controller parameters kp and b0 that need to be adjusted, the trial and error method from small to large is used in the experiment. Through on-site debugging and parameter modification, a set of relatively ideal controller parameters is determined when better dynamic and static effects are obtained. Parameter adjustment is relatively easy.

4.4 Comparative study of experimental results

The inverter is set to vector control mode. The initial speed setting is 200 r·min-1. After 40 s, the speed setting is a triangle wave with a period of 60 s and n changes from 200 to 500 r·min-1 to obtain the closed-loop response of the system. As shown in Figure 6, the following performance under ADRC control mode is significantly better than that of conventional PID control.

Figure 7a shows the system response curves under the two control modes. It can be seen from the figure that the robustness and anti-interference performance of the system under ADRC control are better than those under PID control. Figure 7b shows a partial enlarged view of the first 40 seconds of Figure 6.

It can be seen from the figure that the dynamic and static performance under ADRC control mode is significantly better than that under conventional PID control.

5 Conclusion

In view of the problem that the control performance of the PLC variable frequency speed regulation system needs to be further improved, the mathematical model of the variable frequency speed regulation system is briefly introduced, and an asynchronous motor variable frequency speed regulation system based on the active disturbance rejection controller is designed. Compared with the traditional linear PID control method, the operating performance of the variable frequency speed regulation system using the active disturbance rejection controller has been significantly improved. Under the premise of ensuring a faster dynamic response, the ADRC is optimized, the controller parameters are reduced, the algorithm calculation amount is reduced, and the engineering practicality of the controller is enhanced.