Pump controlled and self-contained electro-hydraulic cylinder drives may improve energy efficiency and reduce installation space compared to conventional valve solutions, while being in line with the trend of electrification. The topic has gained increasing interest in industry as well as in academia in recent years. However, this technology has failed to break through in industry on a broad scale, with the reason assumed to be lack of meeting industry requirements. These requirements include high drive stiffness enabling a large application range, and the ability to maintain cooling and filtration in required ranges, enabling proper reliability and durability. Furthermore, at this point the cost of realization of such drives is comparable only to high end valve drive solutions, while not providing dynamics on a similar level. An initiative to improve this technology in terms of a class of drives evolving around a hydraulic cylinder locking mechanism is proposed. The resulting class of drives generally rely on separate cylinder forward and return flow paths, allowing for fluid cooling and filtration as well as control of the drive stiffness. The proposed class of drives is analyzed regarding energy loss and recovery potential, a basic model based control design is realized, and the industrial feasibility of the drive class is considered. It is found that the proposed class of drives may be realized with standard components maintained in their design ranges at competitive costs compared to conventional valve solutions. Furthermore, it is found that pressure levels may be controlled in a proper way, allowing to produce either highly efficient operation or a high drive stiffness.
Traditional valve controlled hydraulic drives have an inherent power loss, due to the throttling over the valves, which limits the maximum system efficiency. Pump controlled direct drives do not have this inherent limitation, but are limited when it comes to controlling asymmetric cylinders, why most solutions that have tried to overcome this problem have incorporated some kind of accumulator. In the present paper a new concept is presented, modelled and analyzed, and it is shown that the concept overcomes the problem with asymmetric cylinders, without the use of an accumulator. The paper first presents an analysis of the general concept, showing that both cavitation and excessive pressure build up needs to be handled by the system, after which the system is presented and modelled. Finally both simulation and experimental results are presented showing the validity of the concept.
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