In a compliane control of a robot manipulator, an independent joint control approach is quite important because its features include: (1) simple SISO controllers, (2) collocated sensors and actuators. In a previous paper, we have proposed a method named “Stable Negative Compliance Control (SNCC)” based on such an independent control approach, whereby arbitrary end-point compliance can be achieved in arbitrary arm configurations.
In this paper, stability analysis for the SNCC are first discussed. When the SNCC is realized by an ideal torque feedback and nonlinear dynamic terms of the arm can be neglected, the control system remains always stable in a free space. Comparing conventional Direct Compliance Control (DCC) and Salisbury's Active Stiffness Control (ASC), it is shown that SNCC only can realize arbitrary symmetric end-point compliance matrix. Next, the control schemes for the Stable Negative Compliance (SNC), which is based on the characteristics of an inverse response of non-minimum phase shift system, have been experimentally confirmed by a 1 DOF actuator system. Although the controller has one unstable pole, the overall closed loop can be easily stabilized and provide an arbitrary SNC value. Finally, the SNCC is implemented on a real planar 3-link direct drive arm. Featuring the decoupled end-point compliance, the validity of the method is demonstrated by a real task.