Dynamic control of a manipulator with passive joints-position control experiments by a prototype manipulator

The authors propose a method of controlling the position of a manipulator with passive joints which have holding brakes instead of actuators. In this method, the coupling characteristics of manipulator dynamics are used, and no additional mechanisms are required. In this paper, the effectiveness of the method is verified by experiments using a prototype manipulator. The prototype is a two degree of freedom, horizontally articulated manipulator. The first axis is an active joint and the second axis is a passive joint. While the brake of the passive joint is released, the passive joint is indirectly controlled to point control of the manipulator was also presented. The feasibility of the method was demonstrated by computer simulation of a manipulator with two degrees of freedom. In this paper, a 2-d.0.f. horizontally articulated manipulator with a passive joint has been developed. The efficiency of the proposed method was experimentally verified by this prototype manipulator The control algorithm and the design of the control system is also described. 2 . Prototype Manipulator by the motion of the active joint, through the use of dynamic coupling. While the brake is engaged, the active joint is controlled. By combining these two control modes. total position of the manipulator is controlled. The experiments show that the precise positioning of the passive joint is feasible by use of the proposed method. 2.1 Hardware Structure of the Manipulator The view of the prototype manipulator with two degrees of freedom is shown in Fig.1. The first joint is an active joint with an actuator, and the second joint is a passive joint which has a holding brake instead of an actuator. The lengths of both link 1 and link 2 are 300mm. and the mass distribution of link 2 1 . Introduction is uniform. The actuator of the active joint is a 35W The number of degrees of freedom that a manipulator possesses is commonly equal to the number of joint actuators. In order to reduce weight. cost and energy consumption of a manipulator, various approaches have been proposed for controlling a manipulator which has more degrees of freedom than actuators[ll . However, they require special mechanisms in addition to the basic links and joints. On the other hand, the dynamics of a manipulator are non-linear and highly coupled. When each joint is controlled by a local linear feedback loop, these factors result in disturbance. The elimination of such disturbances has been one of the major problems in the control of a manipulatorC2-43. However, the force of this disturbance is available t o drive a joint which in itself does not have an actuator. These factors are utilized in several control methodsC51. As a means of controlling a manipulator which has more joints than actuators without additional mechanisms, the authors have proposed a method of controlling passive joints which have holding brakes instead of actuators by using dynamic coupling. In a previous paperC61, the principle of this method and the condition that ensures controllability of the passive joints was presented. An algorithm for point DC motor with a 1/50 harmonic-drive gear. The motor torque is controlled by motor current. A PWM current amplifier is used. The maximum power of the amplifier is + 7 5 V , + 4 A and 300W. The holding brake of the passive joint is an electro-magnetic type with a single friction disk. The static friction torque of the brake is 12". In order to achieve high-speed operation, the brake is driven by an over-excitation controller. An optical encoder is used to detect the angle of each joint. The encoder of the active joint is ZOO,OOOP/R at the output shaft of the actuator. The encoder of the passive joint is 24,00OP/R Fig. 1 The prototype manipulator