Abstract:

Objective: The lifting system of the continuous flat press has such characteristics as multiple controlled objects, nonlinear, strong coupling. In order to reduce the synchronous motion error of multiple hydraulic cylinders, ensure the hot platen be kept at the same level during the lifting and descending period respectively, and reduce the damage to hot platen and related components caused by non-synchronization, a ring coupling synchronization control method based on generalized predictive control(GPC)was proposed in this paper. Methods: Firstly, the synchronous control model of the hot platen lifting system was established, and the generalized predictive control algorithm was used to optimize the control parameters of each cylinder. The optimal performance index function influenced by the synchronization error of the hot platen lifting system was established, and the ring coupling synchronization control law based on predictive control was derived for each subsystem. To reduce computational complexity, the optimal index function was decomposed, the synchronization error between the hydraulic cylinder and its adjacent one was added in each hydraulic cylinder's optimal control index function, and the disturbance and random noise were suppressed by on-line rolling optimization of control parameters, then a ring coupling control method was adopted to achieve the synchronization control for all hydraulic cylinders. Moreover, based on the established system control model, a simulation experiment was carried out using Matlab. The position tracking, synchronization error and anti-disturbance ability were compared between the PID based master-slave control method and our proposed method. Results: The ring coupling synchronization control method proposed in this paper could ensure the tracking accuracy of each cylinder. Meanwhile, the synchronization error between the adjacent subsystems could converge to zero, hence the synchronization control of multi-channel hydraulic cylinder was achieved. The results of the Matlab simulation showed that the ring coupling synchronization control based on predictive control method resulted in a better position tracking accuracy, synchronization performance, anti-disturbance ability and response speed compared with the conventional master-slave control method. For the system disturbance and sudden changes of parameters during the lifting processes, the system could recover stability quickly, which reflected the robustness and adaptability of the control algorithm proposed in this paper. Conclusion: Ring coupling control mode was much more suitable for the multi-objects controlled system, and the system complexity would not increase along with the number of the controlled objects. By combining the prediction control algorithm with ring coupling synchronization control strategy, the desirable position tracking precision and synchronization accuracy of the hot platen in continuous flat press could be obtained. Meanwhile, the damage to the equipment caused by non-synchronous hydraulic cylinders could be reduced.

Key words: continuous flat press, hydraulic lifting system, synchronization control, generalized predictive control, ring coupling control strategy