A pump is a principal element in hydraulic systems and converts mechanical energy into hydraulic energy. Among the many types of pumps, axial piston pumps have wide applications in the fluid power industry because of their high efficiency and easiness in control. Recently, variable displacement pumps gain a growing recognition in the development of energy effective and productive systems, particularly in mobile hydraulics.
The controllers for variable displacement pumps, which are available on the market today, are nearly exclusively mechanical-hydraulic systems. Equipped with a mechanical-hydraulic controller, a conventional variable displacement axial piston pump can realize control features such as constant pressure, constant flow, load sensing, and power limiting by varying its displacement in an appropriate manner. But those controllers have to be developed according to the pump specifications and the operation conditions respectively by manufacturer's own know-how.
Compared to the mechanical-hydraulic controller, electro-hydraulic controller reduces the construction effort and diversity of parts because the pump can be built up in the same way as the electro-hydraulic control concept. Its displacement equipment consists of a displacement cylinder and a flow control valve. In variety of types of controllers and their combinations can be expressed by the employed sensors and the implemented control algorithms. Moreover, the electro-hydraulic pump control offers a large amount of flexibility in the adaptation of the controller.
Recently, the cost of the sensors and the electronic components are remarkably reduced according to the progress in electronics. Moreover, the demanded functions of the pump controllers become more and more complex because multiple pump control functions are needed at the same time. Therefore, in recent years, an electro-hydraulic pump controller is getting much attention in fluid power industry.
Although the vast amount of researches on the variable displacement pumps has been reported in the past ten years, most of the researches have focused on the mechanical-hydraulic controller. However, there have been few researches on the electro-hydraulic pump controller.
The objective of this thesis is to provide the basis for the development of the practical electro-hydraulic pump controller.
In this thesis, following researches are carried out. Firstly, a mathematical model of an axial piston pump with a proportional valve was developed and the simulation results using this model showed good agreement with experimental results. Secondly, A swash plate angle controller is designed using a PD control as a pump discharge flow controller. The experimental results show that fast control response without overshoot and steady state error can be obtained by using designed controller. Thirdly, It is shown that an internal swash plate angle feedback loop increases the damping of a pressure control system by the root locus method with fifth order-linearized model. Fourthly, (1) PD control, (2) double loop control with internal swash plate angle control loop, and (3) state feedback control with reduced order model are designed respectably for a practical pressure controller and tested. Experimental results show that, (1) and (2) give good pressure control performance while the large steady state errors are observed in (3). Fifthly, by using genetic algorithms, it was possible to optimize the control gains of the proposed controllers within reasonable number of experiments. Optimized gains are confirmed by plotting the fitness function in a given gain space. Sixthly, an observer/compensator for the load flow which acts as a disturbance in pressure control system is proposed to reduce the effects of load flow. The observer/compensator observes the load flow and generates a desired swash plate angle so that the effects of flow could be compensated. Controllers (2) and (3) are modified by adopting the proposed observer/compensator. It is found in experiment that modified controllers show excellent pressure control performance. Finally, load-sensing and power limiting control are carried out satisfactorily by applying the flow controller and the pressure controller proposed in this study
