Tao Ming Lim

Compliant motion using a mobile manipulator: an operational space formulation approach to aircraft canopy polishing

The operational space formulation provides a framework for the analysis and control of robotic systems with respect to interactions with their environments. In this paper, we discuss its implementation on a mobile manipulator programmed to polish an aircraft canopy with a curved surface of unknown geometry. The polishing task requires the robot to apply a specified normal force on the canopy surface while simultaneously performing a compliant motion keeping the surface of the grinding tool tangentially in contact with the workpiece. A human operator controls the mobile base via a joystick to guide the polishing tool to desired areas on the canopy surface, effectively increasing the mobile manipulators reachable workspace. The results demonstrate the efficacy of compliant motion and force regulation based on the operational space formulation for robots performing tasks in unknown environments with robustness towards base motion disturbances. The mobile manipulator consists of a PUMA 560 arm mounted on top of a Nomad XR4000 mobile base. Implementation issues are discussed and experimental results are shown.

Adaptive joint friction compensation using a model-based operational space velocity observer

An operational space controller that employs a velocity observer and a friction adaptation law to achieve higher tracking accuracy is presented. Without velocity measurements, the overall observer-controller system can achieve a semi-global asymptotic stability for the position and velocity tracking errors, and position and velocity estimation errors. The estimated friction coefficients can also approach the actual coefficients asymptotically. Experimental results indicate that the proposed adaptive observer-controller is able to achieve higher tracking accuracy than the observer-controller without friction compensation.

The Operational Space Formulation implementation to aircraft canopy polishing using a mobile manipulator

The Operational Space Formulation provides a framework for the analysis and control of manipulator systems with respect to the behavior of their end-effectors. Its application to aircraft canopy polishing is shown using a mobile manipulator. The mobile manipulator end-effector maintains a desired force normal to the canopy surface of unknown geometry in doing a compliant polishing motion, while, at the same time, its mobile base moves around the shop floor, effectively increasing the mobile manipulators workspace. The mobile manipulator consists of a PUMA 560 mounted on top of a Nomad XR4000. Implementation issues are discussed and simultaneous motion and force regulation results are shown.

Singularity robust manipulator control using virtual joints

A singularity handling method is proposed in this paper. It is done by introducing virtual redundant joints into the Jacobian matrix to maintain the rank of the Jacobian matrix when singularity occurs. These additional joints do not exist physically. Therefore, although mathematically stable, the manipulator still cannot perform tasks in the degenerate direction(s). This method is comparatively straight forward to implement and it does not have a singular subspace defined within which a special and different control algorithm is performed, thus it avoids the problem associated with discontinuous control or switching of control. The method was tested on simulation and implemented in real-time on the PUMA 560 robot.

An analysis of the operational space control of robots

Theoretically, the operational space control framework [1] can be regarded to be the most advanced control framework for redundant robots. However, in practice, the control performance of this framework is significantly degraded in the presence of model uncertainties and discretizing effects. Using the singular perturbation theory, this paper shows that the same model uncertainties can create different effects on the task space and joint space control performance. From the analysis, a multi-rate operational space control was proposed to minimize the effects of model uncertainties on the control performance and while maintaining the advantages of the original operational space framework [2]. In this paper, we present a stability analysis of the multi-rate operational space control framework using the Lyapunovs direct method.

Implementation of an output feedback controller in operational space

This paper presents an operational space output feedback controller for non-redundant robot manipulators to achieve trajectory tracking without velocity measurements. The overall system can achieve a semi-global exponential stability (SGES) result for the position, orientation and velocity tracking errors as well as velocity observation errors. Experimental results of the proposed controller indicate good position and orientation tracking performance under parametric uncertainty and payload variations.

A feasible work-piece placement method for contact-type operations

A method to find a feasible work-piece location for a given desired task-space trajectory is presented in this paper. By making use of the operational space control framework [1], this paper proposes a method to assist the search of the work-piece location with respect to the robot in such a way that the path reachability is always guaranteed. The method proposed in this work does not require the solutions of the inverse kinematics problem, thus, this method is expected to have more flexibility and generality in practice. In addition, since the internal robot posture can be controlled through the null-space controller, optimization of the work-piece placement could be achieved by imposing artificial constrains at the robot joints/links. Simulation is provided to show the feasibility of the proposed algorithm.

Unified force and motion control using an open system real-time architecture on a 7 DOF PA-10 robot

The operational space formulation provides a framework for the analysis and control of robotic systems with respect to interactions with their environments. Using the modified PA-10, we implemented unified force and motion control to achieve force and position tracking. Impact control algorithm has also been implemented to remove oscillation when the end-effector comes into contact with stiff environment. The control algorithm uses a PC to perform the required complex realtime computation. Coupled with the modification to the PA-10 controller, we are able to achieve high sampling rates with minimum communication latency which is crucial for our application. A software architecture was also developed which uses the concept of device drivers to achieve modularity in a realtime subsystem environment

Robust observer-based controller and its application in robot control

In this paper, we present a robust observer-based controller (ROC) for robot manipulators to achieve robust velocity estimation and better operational space tracking performance. Without link velocity measurements, the overall ROC system can achieve a semi-global asymptotical stability result for the position and velocity tracking errors, and position and velocity estimation errors. Experimental results using PUMA 560 indicate that the proposed ROC is able to obtain more accurate and less ripple velocity estimation than that obtained from an observer-controller, hence higher tracking performance can be achieved.

Parallel force and motion control using adaptive observer-controller

A parallel force and motion control algorithm using observed velocity is presented in this paper. Experimental results show improved control performance in both force and motion subspaces compared with the same controller using filtered velocity.