Session: Controls 3

Paper Number: 111868

111868 - Practical Implementation of Secondary Control Principles in an Electro-Hydraulic Speed-Variable Drive Applied to an Injection Moulding Machine 

Previous published research suggests using electro-hydraulic speed-variable drives to improve the bandwidth of hydraulic systems. A hydraulic system driven by speed variable electric drives and fixed displacement pumps are considered in this work. The focus is on improving the obtainable bandwidth of the injection cylinder through secondary control principles, directly providing the torque references for the electric machines.

Previous publications have theoretically shown the advantages of applying secondary control principles, when the electric motors and associated drives is in torque control modes instead of speed control modes, enabling a considerable increase in control bandwidth. In this work a cascaded control structure is proposed based on such principles. The inner loop consists of two electric motors and drives in torque control mode, which essentially directly realizes pressure control in the hydraulic circuit. The second loop is pressure control based on decoupling of the piston load pressure and sum pressure enabling the possibility to utilize two SISO controllers. One controlling the pressure level via the sum pressure and one controlling the load pressure. The last loop is a piston velocity control loop. The implementation of secondary control principles and the search for a high bandwidth however requires large loop gains, amplifying for example signal noise. Furthermore, the use of axial piston pumps induces pressure pulsations, which are also amplified in high gain control loops. For an axial piston pump with 9 pistons, 18 pressure pulses per revolution are introduced. This limits the possible physically obtainable bandwidth, and to be able to realize the theoretical proven bandwidths the pressure pulsations needs to be handled in some proper way.

Experimental results for the proposed secondary control structure are presented and difficulties of implementation due to pressure pulsations are documented. Lastly a discussion of possible methods to minimize the influence of the pulsations in the control loops is given.

Presenting Author: Rasmus Aagaard Hertz LEGO System A/S

Presenting Author Biography: Rasmus Aagaard Hertz received his M.Sc. in Electro Mechanical System Design from Aalborg University, Denmark, in 2017. With special interest in dynamic modelling and control of hydraulic systems. From 2017 to 2020 he has been working in industry with R&D Moulding, LEGO System A/S. Focusing on data and development of moulding platforms for the internal moulding factories. Since 2020 he has been an Industrial PhD student in cooperation with LEGO System A/S and Aalborg University, Denmark. Topics of interests include injection moulding, with focus on in-line material characterization, dynamic process modelling and control of the injection moulding process.