With the electrification of mobile machinery comes the demand for new, efficient hydraulic systems. One system type of interest is the multi-pump architecture, which uses multiple pumps that can be connected to different actuators via on/off valves. This is a modular system that can perform efficiently, and the required installed power can be kept low compared to other similar approaches. It requires many valves, but offers many possible modes of operation. However, switching between modes is non-trivial and can cause disturbances and losses. In this paper, different controllers for a multi-pump system with two pumps and one actuator are investigated. Controllers that keep the pressure side of the pumps fixed are compared to controllers that allow varying pressure sides (meaning they can work in two or four quadrants, respectively). The ideal operating mode for each operating point was found using a genetic algorithm. The controllers were tested for different dynamics of the valves and pumps. It was found that the dynamics of the components have a similar impact regardless of the control strategy, assuming the dynamics are sufficiently fast. However, the controllers with fixed pressure sides generally performed marginally better.

 Hybrid and fully electric heavy machinery introduce new possibilities for hydraulic system design. Due to their lower energy density compared to conventional combustion engine systems, improving hydraulic efficiency is crucial. Electro-hydraulic actuators can achieve this but often require high installed power, as each actuator must be sized for maximum demand. The multi-pump system (MPS) in this paper addresses this by allowing all hydraulic machines to serve any actuator via a network of on/off valves, reducing losses and installed power. However, its multiple degrees of freedom make optimal operation non-trivial. This paper proposes a filtering strategy using a genetic algorithm to identify efficient operating points for the MPS. Although applicable for larger systems, the results here focus on an MPS with two pumps and one actuator as an example. A quasi-static system model is introduced, which the GA uses to determine steady-state control signals that minimise power consumption. The results highlight ideal operating conditions, significantly narrowing the range of viable valve combinations and pump/motor speeds. Finally, the paper discusses the limitations of the approach and its potential extension to more complex multi-pump systems for the development of dynamic control strategies.

In search of more efficient hydraulic systems, new system architectures are explored. These system architectures are often electrically driven and include energy recuperation. This requires hydraulic machines to function both as pumps, converting mechanical power into hydraulic power, and as motors, converting hydraulic power back into mechanical power. However, the availability of machines that can operate in all desired modes is limited. This indicates that operation in multiple modes comes with performance penalties. This paper highlights the challenges for multi-quadrant operation of hydraulic piston pump/motors, with a particular focus on commutation, i.e., the transition between high- and low-pressure level for each chamber. Various commutation strategies for piston machines are examined. Furthermore, other important aspects for pump/motor operation such as hydrostatic compensation ratios, design of inlet channels, low-speed capability, and flow control through speed or displacement control are discussed. The article shows that the design of multi-quadrant machines is challenging, and this has to be considered when choosing the system architecture.

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.

Pump-controlled asymmetric cylinders need some kind ofcompensation for the difference between the in- and outlet flow.This can be done by using valves or additional pump/motors.An advantage with using valves is that only one pump/motor isrequired, but problems with mode-switch oscillations are morelikely to occur. On the other hand, cylinder areas should bewell matched with the pump/motor displacements for pump/motorcompensation if the pump/motors are fixed and connected to thesame shaft. However, this problem is avoided if the pump/motorsare individually controlled. Furthermore, with individually controlledpump/motors, it is possible to control the pressure in one ofthe cylinder chambers and the piston speed simultaneously. Thispaper is focusing on such architectures. There are several possibleconfigurations for architectures and control strategies for individuallycontrolled pump/motors, and some are compared here.Results show that one of the configurations is more efficient thanthe others, but that it is less efficient than valve-compensation (assuminglarge valves). A controller design where one pump/motoris used to control the speed and another to control the pressureis suggested. Results show that the controller should be adaptedbased on the mode of operation.

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.

High power density in combination with flexible power distribution possibilities and extreme robustness are reasons why fluid power has been the preferred technology in mobile machinery, such as excavators and cranes, since the mid-20th century. In principle, the machines have been powered by a combustion engine which powers a pump, with the output from the pump being distributed to different functions via valves. However, a transformation is currently underway. Combustion engines are being replaced by electric motors, and batteries able to store energy corresponding to several hours of operation are often desired. Since batteries tend to be heavy and expensive, reducing the energy consumption is getting higher priority than ever before. There are applications where electrification means that hydraulic components are replaced by electric counterparts, but fluid power has characteristics that are highly desirable in mobile machinery. Therefore, many hydraulic actuators will remain. Conventional hydraulic systems, which are known for their inefficiency, should, however, be adapted to the new conditions brought about by electrification. The question, and the overall subject of this thesis, is: how? The research has focused on two main topics: pump-controlled systems, which are systems where each actuator has its own supply unit, and the use of variable displacement pumps in electrified systems.

A large proportion of the losses in many conventional hydraulic systems is due to the simultaneous operation of functions that require different pressure levels. One way to avoid these losses is to use pump-controlled systems. How these systems should be designed is, however, far from obvious. In this thesis, different types of pump-controlled systems are compared, both statically and dynamically.

Regarding variable displacement pumps, they have had a natural place in many conventional systems, but electrification may change this, since speed-control can now also be used for flow- and pressure control. However, there are still aspects relating to energy consumption and component dimensioning, among other things, that makes variable pumps relevant. These aspects are investigated here, and different types of variable pumps are reviewed.

Positive displacement pumps have been around for thousands of years, but it was first in the beginning of the 19th century they started to be used for power transmission purposes. At that time, the fluid was water, and the applications were primarily presses. During the century, the technology developed and towards its end, fluid power systems were used to transmit power to hundreds or even thousands of consumers within several cities. However, in the 20th century, these large-scale fluid power transmission systems were outcompeted by the electric grid. But at the same time, the focus for fluid power was shifted towards self-contained, oil-based systems, which were suitable in many mobile applications powered by combustion engines. Once again, fluid power systems are now undergoing a transition. This especially apply to mobile applications, where combustion engines are being replaced by electric motors. This puts new requirements on the hydraulic systems as well as the pumps and motors that are to be used. Electrification means increased focus on energy efficiency, and speed-control becomes more relevant than before. New system designs are therefore highly relevant. Depending on the architecture that is chosen, different requirements will be set on the pumps and motors. Aspects such as multi-mode operation, high- and low-speed performance, and displacement control will be discussed in this paper.