Control of Hydraulic Injection Moulding Machine With Electro-Hydraulic Variable-Speed Drive

In this work an industrial hydraulic injection moulding machine, is retrofitted with a electro-hydraulic variable-speed drive. The focus is placed on the injection cylinder, where the design of the hydraulic system is realized with standard components. The design is based on two permanent magnet synchronous motors connected to two fixed displacement pumps, allowing the control of the injection cylinder without the need for valves in the flow path, removing the dynamic influence of a valve and reducing throttle losses. The controller structure is designed from a model-based approach, where the electric motors and associated drives are operated in speed control modes. A sum and load pressure decoupling control structure is proposed, enabling the use of SISO control design for the pressure level and the load pressure separately. For the latter, a cascaded hydraulic drive controller structure is proposed, where the inner loop consists of a pressure controller, and the second and third loop consists of a velocity and position controller respectively. A method to smoothly switch from velocity to pressure control is furthermore proposed. Experimental results comparing injection cycles of an industrial injection moulding machine and the retrofitted injection drive and proposed control are presented.