As is well known, a robot arm of an industrial robot comprises at least one articulated mechanism such as a pivot joint or a universal joint in order to obtain a desired movement in three dimensional space. Additionally, an end effector, such as a tool, an implement or a hand to grasp a workpiece, is attached to a wrist provided at the distal end of the robot arm. The articulated mechanism and the end effector have considerable weight. Thus, a considerable moment is generated by gravity on the articulated mechanism, the end effector, and the work on the robot arm. Therefore, considerable torque is necessary in order to move the robot arm against the moment generated by gravity, which results in a considerable load on the driving system, such as a servo-motor, for the robot arm. In order to reduce the load on the driving system, the conventional industrial robot comprises a balancer device applying a counter torque which cancels the moment, generated by gravity, on the robot arm.
FIG. 4 illustrates a conventional balancer device, as one example, which uses the moment generated by gravity on a balance weight. The balancer device comprises a balance weight positioned on the opposite side of the robot arm to generate counter torque.
FIG. 5 illustrates another conventional balancer device using force of a spring. The balancer device comprises a spring means S, such as a coil spring, attached to a point 14 positioned at a distance from the rotational center of the robot arm and to a point 15 positioned on a robot arm. The balancer device uses the force of the spring to generate a counter torque.
The conventional balancer devices are widely used because of their simple construction and the low cost. However, they have disadvantages as described below.
The balancer device of FIG. 4 uses the balance weight having considerable weight in order to obtain the enough counter torque. Thus, the balancer device adds weight to the robot arm and impairs the maneuverability, the response characteristics, and the positioning accuracy of the robot arm.
On the other hand, when the balancer device of FIG. 5 is mounted on a ceiling at the site where the robot is used, as shown in FIG. 6, the balancer device cannot obtain the counter torque since the direction of the torque by the spring corresponds to that of the moment by gravity. However, if the balancer device comprises a mechanism, as shown in FIG. 7, using a wire W and the spring means S, the balancer device can generate a counter torque by the tension in the wire W. In FIG. 7, the tension in the wire W is equal to the spring force of the spring means S. Thus, the relation of the tension in the wire W when the robot arm is at the lowest point to that of the wire W when the robot arm is at another position is represented as follow EQU .DELTA.X 1.multidot.k&gt;.DELTA.X 2.multidot.k (1)
where
k: spring constant PA1 .DELTA.X 1: elongation of the spring when the robot arm is at the lowest point PA1 .DELTA.X 2: elongation of the spring when the robot arm is at a position other than the lowest point PA1 a fluid-pressure operated spring means having a working element which can advance and retract along a linear axis; PA1 an attachment portion provided on the robot arm near the articulation; PA1 a tension transmitting member for applying tension to the attachment portion by cooperation with the fluid-pressure operated spring means; PA1 a plurality of rotational guide means for extending and guiding the tension transmitting member; and PA1 the tension transmitting member being extended so as to pass the plurality of guide means and the attachment portion, and to apply the tension to the attachment portion in two different directions, whereby the balancer device balances the moment generated by gravity on the robot arm.
From FIG. 7 and the inequality (1), it may be understood that the counter torque is minimum when the rotational angle .theta. of the robot arm is maximum, that is, when the moment by gravity applied to the robot arm is maximum. Therefore, the balancer device of FIG. 7 cannot act suitably as a balancer device.