Development and implementation of a control concept for a hydraulic load unit
A very common method to apply passive loads on hydraulic cylinders is a down-stream throttling valve, e.g. a proportional pressure relief valve (PPRV). The increased outlet pressure caused by the PPRV results in a force acting on the piston, which has to be compensated by a force applied by the inlet pressure. For example, this effect is also used by lowering break valves, which help to avoid cavitation during lowering /11/. The advantages of this method are an easy handling and implementation.
... and implementation. By using a bypass valve and a PPRV in both cylinder ports, passive loads can be applied in both directions. As a disadvantage, no active loads can be applied this way. Active loads can be applied on cylinders e.g. by using test masses /12/.With masses, however, active loads can only be applied in one movement directionsince the force of the test mass is caused by gravity, the load in the opposing direction is passive. Applying exchangeable active and passive loads in both movement direction without changing the setup of the test rig is often not easy and very complex /9/. Another, more flexible method of applying loads is using a load cylinder and a test cylinder. In this configuration, the test cylinder simulates the motion while the load cylinder applies the intended load, preferably independent of the direction of motion. /8/ In order to improve the test capabilities, the Chair of Mobile Machines (Mobima) of the Karlsruhe Institute of Technology (KIT) developed a linear hydraulic load unit which can apply a load duty cycle, both variable in force direction and magnitude and thus interchangeable between active and passive loads. The main focus of the development was on the design of the load unit and its control concept, which is required to make possible a force application independent of the motion direction of the cylinders. Also, the control concept should prevent cavitation due to load magnitude and direction changes and allow for a high reproducibility and accuracy of applied load cycles. More detailed information about the control concept can also be found in /13/. In this paper, the developed load unit and the implemented control concept will be introduced and discussed. The results of functionality verification by simulation will be shown and discussed. At the end of paper completion, the load unit was set up on the test rig, but was not put into operation yet.