Our group have been working on a new teleoperation scheme that utilizes local force control while exchanging only position information between the master and slave station. This scheme provides much better stability characteristics, while offering the user a realistic feeling of teleprescence, through the use of a virtual spring. This proposed architecture is especially appropriate in the case where force sensing is limited only to the slave robot and the master device is relatively light and frictionless. Specifically, the use of a Phantom device and PUMA in our setup meets this criteria.

Technical Approach

The Operational Space Formulation decouples the dynamics of a mobile manipulator into end-effector dynamics and posture dynamics.Furthermore, each end-effector DOF can be independently controlled. The control of the base can be separately synthesized since its dynamics is decoupled from that of the end-effector.

Based on this formulation, a new teleoperation scheme is applied for each end-effector DOF. The scheme connects two master/slave systems with a virtual spring which generates a desired contact force based on position errors. This desired force is sent to the haptic device (master side) and to the remote compliant motion controller. The master device uses feed-forward control of the desired contact force whereas the slave system uses feedback of the contact force measurement. As a consequence, the human operator feels the desired contact force.

To deal with uncertainties and time-varying parameters (e.g. dynamic environments), the force control on the slave robot uses Active Observers (AOBs), which modify the Kalman estimation structure to achieve model-reference adaptive control. In this case, the stiffness parameter has to be identified on-line to improve robustness and telepresence. The AOB is designed to cover a medium range of stiffness values. However, for large changes, on-line stiffness estimation is necessary.

Experimental Setup

In our setup, the slave station consists of a PUMA560 manipulator mounted on an XR 4000 mobile base. The master station is a Phantom haptic device. The end-effector is controlled by teleoperation and the operator is provided with force feedback through the PHANTOM haptic device. The XR4000 mobile base is controlled with moderately slow dynamics to maintain a specified distance and

orientation with respect to the end-effector. The communication delay associated with the wireless LAN network is about 26ms.

Last update: 03 - 2005 by F.Conti