The Speed-variable Differential Pump (SvDP)-concept, designed by Bertelsen and Madsen (2013) in collaboration with Bosch Rexroth A/S, is intended to be an alternative to the classic hydraulic setup, which utilizes a valve to direct the oil flow. The valve is replaced with two pumps mounted on the same servo drive, resulting in a higher energy efficiency of the hydraulic system.

Initially, a nonlinear simulation model of the system is established. The simulation model is verified using experimental measurements, obtained in the test facility provided by Bosch Rexroth A/S. To highlight the properties of the SvDP-concept, a thorough steady state analysis of the system is performed. The analysis of the system reveals that the system pressure levels rise when the direction of rotation in the pumps is negative, while the pressure levels always decrease when the direction of rotation is positive. This means that the pressures, in the current configuration of the SvDP-concept, can only be controlled during negative rotation of the pumps.

Two types of trajectories are designed to evaluate the performance of the designed controllers. Two trajectories based on quintic functions are designed, ensuring realizable demands to acceleration and velocity of the piston. In addition to the quintic trajectories, two ramp based trajectories are designed, which require step in velocity.

The model is linearized, and the critical operating points are identified, based on the results of the steady state analysis and the designed trajectories. The linear system, in the critical operating points, results in two linear systems depending on the direction of rotation. A decentralized control strategy is designed for the two linear systems, such that SISO control can be applied. Two sets of controllers are designed, due to limitations in the test facility, where a poor velocity estimate is available. A set of conservative controllers are designed, which can be implemented and tested despite the limitations, and a set of high performance controllers are designed to prove the potential of the SvDP-concept. The high performance controllers utilize pressure feedback to improve the damping of the system, but the proportional gain of the PI-controllers regulating the velocity, prevents implementation, as a noisy position signal results in chattering of the control signal.

The conservative controllers are implemented and tested, resulting in a proof of concept. The rms velocity error, obtained from the experimental results, deviate from the simulations by 23-55 %. Assuming this deviation applies to the simulation results, for the high performance controllers, an rms velocity error of 2-3 mm/s can be expected, when a decent velocity feedback is available. It is therefore concluded that the SvDP-concept can be an alternative to the traditional valve-cylinder configuration.