FORCE AND POSITION SCALING LIMITS FOR STABILITY IN FORCE REFLECTING TELEOPERATION

Many telerobotic systems require the use of a slave robot with large inertial and frictional properties. Using a force sensor on the end effector can hide the slave’s inertia and friction from the user providing a more accurate sense of the environment, but introduces dangers of system instability. Both the position and force scale directly affect the system loop gain and hence stability. This opens up the possibility of trading off between them based on the environment and task. In this paper we derive explicit limits for their product. In particular we consider varying environment stiffnesses, as well as distinguishing impact and contact phases. The theoretical limits closely align with experimental results using a large slave telerobotic system interacting with very soft to nearly rigid environments. INTRODUCTION Telerobotics seeks to combine the strength, precision, and robustness of a robot with the intelligence and real-time decision making skills of a human. This cooperation enables robots to perform more complex tasks in unstructured environments. In particular, since its first appearance, telerobotics has been proposed and used for a variety of tasks including robotic space repair, [1, 2], deep-sea and nuclear environments [3], and medical robotic surgery [4, 5]. Some of these tasks require the use of a strong and stiff slave robot. These robots are excellent for completing tasks requiring strength and precision surpassing that of humans, but they also possess relatively high inertial and frictional properties. If traditional passivity based controllers are ∗Address all correspondence to this author. used [6, 7], the slave robot’s inertia and friction will be transmitted to the human operator making telemanipulation cumbersome and awkward. The slave’s high friction can further cause the robot to become non-backdrivable severely limiting the user’s sense of haptic feeling of the environment. Force sensors can be used on the slave’s end-effector to deal with the negative effects of high friction and high inertia. By feeding back environment forces directly, the slave’s inertia and friction are hidden from the user by providing a direct sense of touch of the environment. This force reflecting system can provide the user with a more accurate sense of transparency [8] but can also tend toward instability. Local force feedback has been proposed as a method of hiding the slave’s large inertial and frictional properties and increasing the slave’s environment sensitivity while simultaneously maintaining overall system stability [9,10]. It is also possible to use the measured and estimated environment force for stable force loop control on environments of varying stiffnesses [11]. All force reflecting teleoperation systems utilizing an end effector force sensor are prone to closed-loop instabilities and thus must adhere to certain overall loop gain limitations. The maximum closed-loop gain is dependent on a variety of system properties including master/slave inertia and damping, overall loop time delay, force/position scaling, and environment stiffness. The environment stiffness acts as a multiplicative gain potentially driving the closed-loop system unstable. For example if the user telerobotically moves the slave robot from contact with a soft environment to contact with a stiff environment, the increase in environment stiffness will likely cause system instability. The product of position and force scaling also acts as a multiplicative gain and can be varied to maintain stability with changing envi-