Impedance force control is very practical in the field of robotic compliance control and the main concept is based on the impedance equation which is the relationship between force and position/velocity error [1].Many researchers have improved the performance of the impedance control and expanded the application range since it was primarily proposed by Hogan [2�C5]. However, the classical impedance control is unsatisfying when the environment parameters are not exactly known. To overcome this problem, Lasky et al. [6] proposed a two-loop control system that the inner-loop is a classical impedance controller and the outer-loop is a trajectory modified for force-tracking. This algorithm uses the outer-loop to automatically modify the reference position by a simple force-feedback scheme when the environment is not exactly known.
Jung et al. [1] proposed an adaptive impedance control. The main idea of this algorithm is to minimize the force error directly by using a simple adaptive gain when the environment is changed. Seraji [7] proposed an adaptive admittance control based on the concept of mechanical admittance, which relates the contact force to the resulting velocity perturbation. Two adaptive PID and PI force compensators are designed in Seraji’s paper.In this paper an adaptive impedance control is proposed that uses an adaptive PID force compensator as an offset to adjust the output of the impedance controller when the environment position or stiffness is changed. It is a way that adjusts the impedance parameters indirectly, which is different from Jung’s.
In order to validate the algorithm, a joint simulation with MATLAB and ADAMS is presented. Firstly, the model of the tendon-driven dexterous hand is built in ADAMS referring to Cilengitide the robot hand of Robonaut-2, which is the first humanoid robot in space and has the typical tendon-driven dexterous hands [8]. A three-DOF finger of the robot hand is chosen as the research object. Then a control module of the robot finger is generated in ADAMS. Finally, the control system is built in MATLAB using the control module. The results of the joint simulation demonstrate that the proposed algorithm is robust. In addition, the position controller and inverse kinematics solver are designed for the tendon-driven finger.2.
?Features of the Robot Hand and Dynamic ModelThe model of the tendon-driven dexterous hand in ADAMS consists of four three-DOF fingers, a four-DOF thumb and a palm, as shown in Figure 1. For the three-DOF fingers, the fingertip’s motion depends on the coupled link, as shown in Figure 1. The actuation system of the robot hand is remotely packaged in the forearm, which makes the size of the robot hand as large as a man’s hand. Each unit of the actuation system consists of a brushless motor and a lead screw. The lead screw can convert rotary motion to linear motion. Each of the tendons connects the finger joint and the lead screw.