Displacement control of positive displacement machines has been a part of fluid power since the early days. Some of the early hydraulic presses already used two different displacement settings, though this was realised by using two different pumps rather than changing the displacement. Later, radial motors with variable stroke length appeared, followed by other designs of variable machines, such as swashplate machines, bent-axis machines, and variable vane machines. All these solutions control the displacement by varying the volume difference of the displacement element – but there are other ways of achieving this. Most have not passed the research state, but some are commercially available. In this paper, different ways of varying the displacement are presented and classified. The classification divides concepts into either control of displaced fluid or control of usage of displaced fluid. In turn, these concepts can be either on system level or displacement element level. This results in four main classes, which to some extent can describe the characteristics of the control.

Imagine a system with a pump driven by a speed-controlled electric motor. What and how much can be gained by using a pump with discretely variable displacement instead of a conventional fixed pump in such a system? This question is the focus in this paper, in which a simulation study based on a drive cycle for a loader crane is presented. The results indicate that the system efficiency from inverter input to pump output can increase by a few percentages. This might be considered small in relation to the increasing complexity that comes with discrete displacement. However, the results also show that a system with discrete displacement substantially reduces torque and cooling requirements on the electric motor. The required maximum torque can be reduced by 30 to 50 % and the motor can generate up to 40 % less heat since it can work in more efficient conditions. These potential benefits will be obtained with only a few discrete displacement settings available.

The interest in speed-control of hydraulic pumps is increasing with the electrification of mobile machinery. With speed-control at hand, it is tempting to use fixed pumps where variable pumps earlier have been used. However, it can be beneficial to use variable pumps in combination with variable speed since it can allow downsizing of electric machines and reduce losses. But there are downsides with conventional variable pumps, such as increased complexity, cost, and low efficiency. Digital pumps (i.e., pumps with a discrete number of displacement settings) can address these problems. This paper is focused on the performance of a digital pump, both efficiency-wise and from a dynamic perspective. Shunt-based concepts that can improve the dynamics during switching are proposed. Simulation results show that the concepts can reduce the disruption in output flow during switchings.

There is growing interest in using electric motors as prime movers in mobile hydraulic systems. This increases the interest in so-called pump-controlled systems, where each actuator has its own drive unit. Such architectures are primarily appealing in applications where energy efficiency is important and electric recuperation is relevant. An issue with pump-controlled systems is, however, mode-switch oscillations which can appear when the pressure levels in the system are close to the switching condition. In this paper, the mode-switching behavior of different generalized closed and open circuit configurations is investigated. The results show that the choice of where to sense the pressures has a huge impact on the behavior. They also show that, if the pressure sensing components are properly placed, closed and open circuits can perform very similarly, but that mode-switch oscillations still can occur in all circuits. Active hysteresis control is suggested as a solution and its effectiveness is analyzed. The outcome from the analysis shows that active hysteresis control can reduce the risk for mode-switch oscillations significantly.