The influence of friction in the hydraulic cylinders on the behaviour of a manipulator

In mobile units such as construction machines that handle high loads, hydraulic cylinders are often used to actuate the manipulators. Such machinery is often manually operated, with each cylinder operating separately. However, the increased use of microcomputers opens up the possibility of computer control of the motions of all types of manipulator. Such control would facilitate the implementation of end-point-controlled hydraulic-driven manipulators. It could also reduce the learning time for operators and prolong the life of machines. However, the nonlinear effects of friction in the hydraulic cylinders may be a problem if they disturb the motion of the manipulator arms. There is thus a need to investigate the influence of friction in hydraulic cylinders on manipulator motion. This paper reports on the use of a computer model of a hydraulic-powered manipulator arm to simulate the effects of friction, as represented in several different friction models, on the dynamics of a manipulator. The model used was modular and included two hydraulic cylinders, whose motion was influenced by friction in the cylinders. This model and the work reported in this paper are part of a project to investigate the possibility of implementing end-point control. A simulation with realistic data revealed that in general the friction in the hydraulic cylinders has only a minor effect on the motion of the manipulator arms. However, very high friction can have a noticeable effect, particularly if the static friction is much higher than the dynamic friction.