AC Servo System Based on Integral Separation PID Control

In view of the characteristics of PID control, this paper designs a control method of integral separation, that is, when the system error is large, the integral link is cancelled to avoid large overshoot of the system due to integral accumulation; when the system error is small, the integral link is introduced to eliminate the error and improve the control accuracy. This integral separation PID control is applied to the real-time position control of the AC servo system, so that the static and dynamic performance indicators of the control process are more ideal.

2 System Structure Design

The structure of the integral separation PID control AC servo system is shown in Figure 1. In the figure, θd is the given angular displacement, θ is the actual angular displacement of the motor shaft, and e is the deviation obtained by comparing θd and θ. Then:

In Figure 1, u is the expected speed value of PID control; ωd is the expected motor speed; ω is the actual motor speed; the deviation between ωd and ω generates the expected motor electromagnetic torque Td through the speed regulator. Since the deficiency of the inner loop can be compensated by the outer loop control, the speed regulator can use a general PI regulator, and the electromagnetic torque control of the motor adopts the direct torque control method.

3 Integral separation PID controller

PID control is a mature and widely used control method with a simple structure and good control effect on most processes. The discrete PID control law is:

Where u(k) is the output of the controller at time k; KP, KI, and KD are the proportional coefficient, integral coefficient, and differential coefficient, respectively; e(K) is the difference between the position of the AC servo system at the current moment and the expected value; and e(k-1) is the difference between the position of the AC servo system at the last sampling moment and the expected value.

From formula (2), we can get the increment between the control quantity u(k) at the kth cycle and the control quantity u(k-1) at the k-1th cycle:

In PID control, the role of the integral link is to eliminate static errors and improve the control accuracy of the system. If the integral link is introduced in the initial stage when the error is large, it will cause the integral accumulation of PID, thereby causing a large overshoot of the system. Therefore, this paper designs a control method of integral separation based on the characteristics of PID control. The flowchart of the integral separation PID control algorithm is shown in Figure 2.

When the system error is large, the integral link is cancelled and PD control is adopted to avoid large overshoot of the system due to integral accumulation; when the system error is small, the integral link is introduced and PID control is adopted to eliminate the error and improve the control accuracy. That is:

In the formula, ε>0 is an artificially set threshold.

The integral separation control algorithm can be expressed as:

Where T is the sampling time and a is the switching coefficient of the integral term, namely:

4 Experimental studies

The parameters of the AC motor used in the experiment are Pn=2.2 kW, Un=220 V, In=5 A, nn=1 440 r/min, r1=2.91 Ω, r2=3.04 Ω, Is=0.456 94 H, Ir=0.456 94 H, Im=O.444 27 H, Ten=14 N.

The experiment shows that the integral separation PID control gives full play to the advantages of high regulation accuracy of PID control and improves the control accuracy of the system.

5 Conclusion

This paper proposes an AC servo system based on integral separation PID control. When the system error is large, the integral link is cancelled; when the error is small, the integral link is introduced, so that the static and dynamic performance indicators of the system are more ideal. This control method improves the accuracy of the system. The simulation experimental results show that the controller has good dynamic and static performance and is an effective controller.