Adaptive open-loop control method for a hydraulically driven flexible manipulator

A novel method applicable for the open-loop control of hydraulic driven flexible manipulators is presented. By using a new kind of adaptive control function instead of the traditional open-loop control strategies the dynamic behaviour of such systems can be significantly improved. The adaptive control function is developed for the open-loop control of a log crane equipped with an electrically controlled proportional directional valves and digital control system. The proposed method utilizes the existing hydraulic components and control hardware of commercially used log crane. In addition to this, the cylinder position and pressure transducers are used. The dynamics of manipulator can be determined by studying the natural frequencies. The lowest natural frequencies were used in selecting and tuning the adaptive algorithm. By using piston positions and mass load as input variables, the adaptive algorithm calculates the lowest natural frequency of crane and uses a particular part of this frequency to filter the higher frequencies from the input signal of control valves. With a second order filter it is theoretically possible to affect the jerk of the manipulator (derivative of acceleration), that has a strong coupling with vibrational behaviour. By applying the proposed adaptive control function a significant reduction in vibration amplitudes in the test simulations was achieved. The major benefits of proposed control method were achieved when loading small masses with long reach and when driving the manipulator slowly by giving small control increments into the valves.