Possibilities of Force based Interaction with Robot Manipulators

Abstract

One way of interaction between a human and a robot manipulator is the interaction via forces and torques. We will call it also force guidance. For this purpose the human acts on the robot arm or on the robot end-effector. From the interaction forces and torques than a suitable motion of the robot is generated. This kind of human robot interaction may be useful e.g. for the comfortable teach-in process. Commonly, positions and orientations of the robot tool are taught by the operator using the manual control pendant. With the keys on this device he or she moves the robot in joint or in task space. To improve the usability of the robot, some manual control pendants are additionally equipped with a more intuitive teach in device. It is called 6D mouse or space mouse (Hirzinger & Heindl, 1986). For further optimization of the teach-in process another way to move the robot would be force guidance. It will be shown that it is possible, with some differences, both in joint or in task space. Force based human robot interaction can be seen as a special kind of active robot force control (Zeng & Hemami, 1997). To perform this, the robot has to be equipped with a force/torque sensor (Gorinevsky et al., 1997). Usually this sensor is mounted in the robot wrist and it measures forces and torques in all Cartesian directions. The cost of such a 6D F/T sensor can exceed 10% of the price of a low payload six axes articulated robot. For that reason it should be searched for an alternative possibility of force/torque measurement. One idea is to estimate the interaction forces and torques from the joint motor currents. For this purpose an algorithm is presented and verified with experiments. Besides the kinematics of the robot motion during human robot interaction also its dynamics is important. For its representation the so called target or desired impedance behaviour will be defined as the relationship between interaction forces/torques and the velocity components of the robot motion. The simplest desired impedance behaviour is the behaviour of the mass damper system. Moreover, there are some more variants and additional features, e.g. the intuitive collision avoidance which will be described in this article. Apart from the desired impedance behaviour selected and parameterized by the operator the dynamics of the robot system has been respected. It depends on the access level of motion generation. Commonly, the robot motion is controlled by the trajectory generator. However, some robot systems permit the direct access to the position or velocity control loops which is favourable in all kinds of robot force control.