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

In this work, a hydraulic state of the art injection molding machine is retrofitted with a dual pump drive. The dual pump drive is an electro-hydraulic speed-variable drive, with focus on high bandwidth control through secondary control principles. Secondary control is based on providing the torque references for the electric machines directly, instead of a speed reference. The pressure control loop is based on decoupling of the piston load pressure and sum pressure enabling the possibility to utilize two SISO controllers. The application is the injection cylinder of an industrial injection molding machine, retrofitted with the designed dual pump drive. Experimental results for the proposed secondary control structure are presented and difficulties of implementation are documented, and a large gap between the theoretical achievable bandwidth and the experimental achievable bandwidth is shown. Three possible reasons for this gap are identified, namely amplification of high frequency components from e.g. signal noise on sensors, the use of axial piston pumps which induce pressure pulsations and the influence of the discrete sample time of the digital controller hardware. Lastly future work within the field is considered, including possible methods to minimize the influence of the identified issues.