A laser tracing measurement system is a type of portable three-dimensional coordinate measurement system. To achieve high-precision tracing and measurement of a spatially dynamic target, it is necessary for the laser tracing measurement system to detect the relative change of target position accurately in real time. It is important to study the high-speed servo control method of the driving motor, which controls the pitch axis and rotation axis motion in the tracing measurement system. The proportional-integral (PI) control is the most common method for controlling servo motors, but the dynamic control accuracy effect of PI control is poor. The PI control method is more suitable for a static or stable model parameter system. Although the laser tracing measurement system is a follow-up system, the traditional PI control method has big challenges in meeting the system control requirements, especially in dynamic response speed, speed steady-state error, and anti-interference ability. A current predictive control (CPC) method based on a nonlinear extended state observer (NESO) is proposed herein. The CPC method is used to improve the dynamic response speed of the system. The improved NESO is used to eliminate the interference of nonlinear disturbances and improve the stability and robustness of the laser tracing control system. It can realize high-precision tracing control of a laser tracing system.
A laser tracing measurement system uses a permanent magnet synchronous motor to drive the movement of a two-degree-of-freedom rotary mechanism. The control system adopts a current-speed-position three-closed-loop system. To improve the dynamic response speed of the laser tracing system, a CPC algorithm that has an advantage in terms of the dynamic response speed is introduced into the current loop. However, the accuracy of the mathematical model of the system affects the accuracy of the CPC algorithm. When the model is mismatched, the predictive control performance is affected. To eliminate the influence of nonlinear disturbance on the CPC control algorithm during motor operation, an NESO is introduced into the current loop to observe the disturbance and then compensate for it back to CPC to improve the robustness of the system. To verify the control performance of the proposed CPC-NESO, MATLAB/Simulink is used for simulation experiments, and a multimotor control platform based on semiphysical simulation is used for real experiments (Fig. 7). The anti-interference ability of the control method is verified by adding an external load to the system. The control effect of the proposed method is evaluated by comparing the PI control method and the system control standard.
Laser tracing control systems must track 1-m/s linear speed motion with a cat's eye reflector within 1 m, and the output stability of the motor must reach ±0.01 r/min. In the simulation results (Figs. 4-6), when the improved NESO control method is compared with the PI control method, the steady-state error is reduced by 50%, improving the stability of the motor. The response capability of the laser tracing measurement system is improved. In the case of external interference, the speed is less affected. The real experimental results agree well with the simulation results (Fig. 8), demonstrating that, under the same speed overshoot, the improved NESO control method has a smaller steady-state error, faster response, and more stable speed response than the PI method. The anti-interference ability of the system is improved, and the control accuracy better meets the control requirements of a laser control system.
A permanent magnet synchronous motor control system in a laser tracing measurement system requires a fast response, high precision, and high stability. Therefore, a laser tracing measurement control model based on an improved NESO is established. The experimental results show that, when the motor speed is 955 r/min, the steady-state error of speed is 1.7 r/min, and the speed decreases by 1.85% when the 0.1 N·m load is added to the motor after the speed becomes stable. Experimental results show that, compared with the traditional PI control method, under the same speed overshoot, the method proposed in this study results in a smaller steady-state error, a faster response speed, a more stable speed response, and an improved anti-interference capability of the system. The proposed method can satisfy the fast response, high steady-state accuracy, and high robustness control requirements of permanent magnet synchronous motors in laser tracing control systems.
