A comparison of some control strategies for a hydraulic manipulator

A simulation and experimental study of the control of a two-axis servo-hydraulic manipulator has been undertaken. The objective was to control each axis independently with the aim of achieving good tracking performance for a wide range of payloads and for different configurations. For hydraulic systems, which comprise many components with nonlinear characteristics, a controller which is robust against changes in dynamic characteristics and disturbances is required. In this study a disturbance observer scheme and an adaptive control scheme were compared with a classical controller. A simulation study was undertaken to assist in controller design and the selection of controller parameters. This was done using the Bathfp simulation package. Each control method was also tested experimentally. For one particular configuration the classic controller with fixed gains could not achieve the required tracking accuracy target of ±2 [mm] for the largest payload of 90 [kg]. However, the disturbance observer and the adaptive controller met this target and in general the accuracy of these schemes over a wide range of manipulator configurations and payloads was much better than with the classic controller. In particular, with the disturbance observer, the accuracy improved remarkably. The adaptive controller exhibited larger tracking errors at start up due to the difficulty in setting initial values of the parameter estimates