Friction Compensation for an Industrial Hydraulic Robot

oint friction is one of the major limitations in performing high J precision manipulation tasks. It affects both static and dynamic performances, and may cause instability when coupled to position or force feedback control. Thus, compensating for joint friction has been one of the main research issues in robot design and control over the years. The aim of this paper is to show how friction compensation based on the LuGre (for Lund and Grenoble) dynamic model, [3], which was applied to an electric actuator in [4], can be successfully used for an hydraulically actuated manipulator. Fig. 1 is a photograph of the Schilling Titan I1 manipulator used in this research. Friction compensation is particularly important for hydraulic manipulators. First, due to high supply pressure, tight sealing is required to prevent the actuators from significant internal leaks. This generates very high joint friction. As an example, the joint friction of the Schilling Titan I1 manipulator can reach 30% of the nominal actuator torque. Secondly, nonlinear Stribeck friction, a well known source of stick-and-slip oscillations, has a particular importance in hydraulic systems [ 151. (Fig. 4 illustrates the Stribeck effect for the first joint of the Titan I1 robot. A 25% drop between the static friction torque and the minimum torque is observed.) While compensating for friction is specially important for hydraulic devices, it is also particularly difficult. There are several ways to compensate for friction, basically divided into nonmodel based and model based compensation [ 11. Among the first type there is, for example, classical dither noise, which consists in adding a high frequency signal to the control signal. Another possibility is to design a joint torque feedback, requiring extra torque sensors to be mounted on the robot or on its base; see for example [9]. In most cases (including our industrial application), none of those sensors are available. Model-based friction compensation uses on-line friction torque estimation [2]. The estimated friction torque is added to the torque reference generated by the position controller and gravity compensation. This kind of compensation assumes that the actuator has a fast and accurate torque response. This is generally verified with electric actuators. Nevertheless, most servovalves, which are the control devices of the hydraulic actuators, do not provide a sufficiently fast and efficient torque response. Then an inner torque loop has to be designed.