Interest in the control of variable fluid power pumps/motors has increased in recent years. The actuators used are inefficient and expensive and this reduces the variable units' usability. This paper introduces displacement control of pumps/motors by means of a rotating valve plate. By changing the angle of the valve plate, the effective use of the stroke is changed. The rotating valve plate is experimentally verified by a modified in-line pump. In the prototype, the valve plate is controlled with a worm gear connected to an electric motor. The results show potential for this kind of displacement control. However, the rotating valve plate creates pressure pulsations at part-displacement due to the commutation being performed at high piston speeds. If the piston speed and hence the flow from each piston is low, the pressure pulsation is acceptable.
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.
More and more vehicles are being electrified. Mobile working machines and heavy trucks are not excluded, and these machines are often hydraulically intense. Electrification entails new requirements for the hydraulic system and its components, and these requirements must be taken into consideration.Hydraulic systems have looked similar for a long time, but now there is an opportunity to advance. Many things change when a diesel engine is replaced with an electric motor. For example, variable-speed control becomes more relevant, electric regeneration becomes possible, and the use of multiple prime movers becomes an attractive alternative. The noise from the hydraulic system will also be more noticeable when the diesel engine is gone. Furthermore, the introduction of batteries to the system makes the energy more valuable, since batteries are heavy and costly compared to a diesel tank. Therefore, it is commercially viable to invest in the hydraulic system.This thesis revolves around the heart of the hydraulic system, that also is the root of all evil. That is the pump. Traditionally, a pump has had either a fixed displacement or a continuously variable displacement. Here, the focus is on something in between, namely a pump with discrete displacement. The idea of discrete displacement is far from unique, but has not been investigated in detail in combination with variable speed before. In this thesis, a novel design for a quiet pump with discrete displacement is presented and analysed. The results show that discrete displacement is relevant from an energy perspective for machines working extensively at high pressure levels and with low flow rates, and that a few discrete values are enough to make a significant difference. However, for other cycles, the possible energy gains are very limited, but the discrete displacement can be a valuable feature if downsizing the electric machine is of interest. i This work has been conducted within the STEALTH -Sustainable Electrified Load Handling project. The project is a collaboration between Hiab, Sunfab, Tube Control, Huddig, OilQuick and the division of Fluid and Mechatronic Systems (Flumes) at Linköping University, all of which are members of the Hudiksvalls Hydraulikkluster. I would like to take the opportunity to thank everyone who has been involved in the project, and especially Amy Rankka, Alessandro Dell'Amico and my supervisors Liselott Ericson and Petter Krus. I would also like to thank my other colleagues at Flumes. Furthermore, I am grateful to the Swedish Energy Agency, which has contributed funding.Tack! Linköping, May 2020 Samuel Kärnell
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 speedcontrol 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.Linköping, August 2022 Samuel Kärnell v vi PapersThe following publications are included in the thesis and can be regarded as its foundation. They will be referred to by their Roman numerals. Apart from formatting changes, they are reproduced in their original form. Notice that they are not appended in chronological order. Instead, they are sorted based on the topics; the first three focus on displacement control and the last two on pump-controlled systems.[I] S. Kärnell and L. Ericson, "Classification and review of variable displacement fluid power pumps and motors," International Journal of Fluid Power (submitted), 2022.[II] S. Kärnell, A. Rankka, A. Dell'Amico, and L. Ericson, "Digital pumps in speed-controlled systems -an energy study for a loader crane application," in
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