A closed-chain Jacobian-based hybrid control for two cooperating arms with a passive joint: an application to sawing task

This work deals with a sawing task performed by two cooperating arms. For the sawing task to follow a line in a horizontal plane, three directional motions have to be controlled (i. e., two translational motions and one rotational motion). A [so, a certain level of force has to be controlled toward the vertical direction not to loose the contact with the object to be sawn. The two-arm system under our hand consists of a four degree-of-freedom SCA RA robot and a jive degree-ofjieedom PT200V robot. When the two arms are rigidly grasping a saw, the mobility of the system is 3, which is not enough for sawing tasks. Therefore, we deliberately insert a passive joint at the end of the SCARA robot to increase the mobili~ up to 4. A )ybrid control method to regu[ate the force and position by the two arms is proposed in this work. The proposed scheme has three @pical features ; jirst, the two arms are treated as one arm in a kinematic viewpoint. Secondly, our approach is dl~erent from other acceleration-based approach, in the sense that our hybrid control method is based on a Jacobian and an internal kinematics for a single closedkinematic chain of the two arms to rejlect the nature of the position-controlled industrial manipulator. Thirdly, the proposed scheme is not only able to operate the system even If a passive joint exists, but also is able to utilize the internal loaak for useful applications such as pitch motion control. We experimentally show that the performance of the position and force response are satisfactory, and that one additional passive joint not on[y prevents the system from unwanted roll motion in the sawing task, but also alIows an unwanted pitch motion to be notably reduced by an internal load control.