The present invention relates to a robot system having a function to hold an object and to move the object to a predetermined position, and in particular, to a robot system of a type in which a force (torque) sensor is employed to effect a control determining a location of the object to be moved.
In the case where an assembling work is achieved by means of a robot system, the positioning accuracy of an operation to hold an object to be moved and to move the object to a target position becomes to be important. Particularly, in the case of a work where a shaft is inserted into a hole with a strict fitting tolerance, the assembling success rate is considerably lowered if there exists unevenness or variations in the positioning accuracy.
Conventionally, as one of the means for controlling the positioning with a high precision or accuracy, there has been used a position control method in which a force (torque) sensor is disposed on a wrist of a robot hand or gripper so as to sense the magnitude and the direction of the force and torque applied in the proximity of a target position on the object to be moved (the force and torque appearing when an interference occurs between the shaft and the hole in the example above), thereby moving the object to a direction in which the force and the torque are reduced. The control method of this kind has been described, for example, in Sugimoto, et al U.S. Pat. No. 4,621,332 and in "Virtual Compliance Control of Multiple Degree of Freedom Robot", Digests of the Society of Instrument and Control Engineers, Japanese issue Vol 22, No. 3 (March, 1986), pp. 343-349.
As represented by Equation (1) in page 344 of the paper above, the movement of the robot gripper is expressed as follows. ##EQU1## where, q: External force applied on gripper (f F.sub.r)
v: Velocity of gripper (v) PA1 .DELTA.x: Deviation of gripper from target position (x-x.sub.r) PA1 [M]: Virtual mass PA1 [K]: Virtual spring constant PA1 [C]: Virtual coefficient of viscosity
In the example of a fitting operation between the shaft and the hole, v, dvdt and fr are respectively almost 0 and hence, when these factors are neglected, the Equation (1) can be reduced as follows. EQU q=[K].DELTA.x
Consequently, when the value of [K] is appropriately selected and the external force q is sensed by use of a force (torque) sensor, the deviation .DELTA.x of the position can be computed. Using the result of the computation, the fitting operation by the robot is successfully accomplished.
In the prior art technology, the value sensed by the force (torque) sensor is assumed to be equal to the external force applied to the robot q in Equation (1) when the positional deviation is computed.
However, it is quite difficult to sense the force q purely applied to the robot gripper. This is because the sense value Q obtained by the force (torque) sensor is expressed as follows; ##EQU2## where [m] is the total mass of the object and the gripper. Namely, the resultant value Q includes the inertia force (acceleration) applied to the gripper.
In ordinary cases, as the mass [m] is small, the inertia force exerted on the robot gripper can be neglected as in the above mentioned literature. However, in the case where the value of [m] is quite large or in the case of operation in an outer space, the inertia force exerted on the gripper cannot be neglected and hence the accurate positioning control cannot be accomplished.