Hydraulic Switching Type Position Control Of A Large Cylinder Drive

Hydraulic drives are well known for their outstanding force and power density and drive stiffness. They are indispensable when heavy load applications have to meet strict demands on fast response and high precision. A typical example is the steel rolling mill where the majority of material forming operations is hydraulically actuated. Before long the only available control unit for these drives were servo valves. The latter have numerous disadvantages intrinsic to their concept and design: extreme sensitivity to oil cleanliness, vast leakages and high prices. The resulting high maintenance and installation costs, low efficiency, and reliability motivate to find a replacement for the servo valves, which do not have these disadvantages and provide similar or better performance. One of the possible solutions is employing one of the many digital hydraulic concepts, in this paper an elementary switching concept using fast switching valves. Basically, such valves do much better than the servos in terms of robustness to oil contamination and leakage, and they have also a reasonable potential for significantly lower price provided sufficient production quantities are reached. The main challenges of switching control are oscillations excited by fast switching and cavitation caused by fast valve closure. Oscillations have negative influence on the tracking performance and can be a source of unwanted noise. The problem is likely to be worsened by a transmission line between the cylinder and the valve control unit if the latter has to be placed some distance away from the cylinder or if the cylinder wall is thick and the connecting channel length cannot be neglected even by a directly mounted valve block. This paper presents prototypal realizations of an elementary hydraulic switching control drive concept for heavy load actuation. To this end a comprehensive analytical model in frequency domain is derived, which describes the plant- cylinder with transmission line - and the specially designed hydraulic compensator (RC-Filter). This model gives direct insight into the parameter influence on the system response. Series of simulations in MatLab Simulink are performed to study the features neglected in the analytical model like, e.g., valve dynamics or nonlinearities and to test and optimize the switching control algorithm. Finally, experimental work is reported which verifies the analytical and numerical models and evaluates the switching control position tracking performance for a number of different scenarios including steps, ramps and sinusoidal trajectories. The effect of control strategy is studied. The promising results lead to the conclusion that such type of switching control can be applied in heavy load industrial drive applications with high demands on response dynamics.