Sensing and Control within a Robotic End Effector

Abstract

This research programme investigates aspects of end effector design and control, to carry out grasping operations in a range of unstructured environments. A conceptual three fingered end effector design has been developed. The articulated finger is operated by a novel mechanism which provides all the finger motions. Detailed force and kinematic analyses have been carried out which establish mechanical integrity of the system and help size the various finger components. A vectorial method of link representation has been used to derive finger kinematics. This representation has been used for position control in the controller. A numerical technique based on the Newton-Raphson method has been derived to undertake the finger's inverse kinematics in realtime. To validate the theoretical operation of the finger drive, a mechanism has been built with the necessary electronic interface, and programmed for position control.

A photoelasticity based sensor has been developed which is capable of detecting applied force as well as slip and is largely immune to external disturbances. The sensor has a small size allowing it to be easily incorporated into a robotic finger. Mechanics of slip has been investigated to develop a theoretical model of the slip sensor. This allows modelling of various material and geometrical parameters involved in its design.

In order to control the end effector, grasping strategies have been planned and a controller structure defined. The top level of the controller uses the kinematic relation to move the finger to a goal position. When fingers make contact with an object, the controller switches over to an inner fuzzy logic algorithm. The rule base of the fuzzy logic ensures that a stable grasp has been acquired with minimum fingertip force. The implementation of the fuzzy logic has been validated on an experimental test-rig. It has been found that the controller applies different minimum fingertip force to objects of different mass and it responds very quickly to the external disturbances by applying extra force to the object. The fingertip force comes back to its previous level as soon as the disturbance vanishes. The important feature exhibited by the controller is that it forms optimal grasp of objects without knowing their mass and frictional properties. This offers a very useful capability to an end effector controller operating in unstructured environments.

A complete model of the end effector has been developed which ensures equilibrium and stability of the grasped object taking dynamic conditions of grasp into account. The model estimates unbalances in position, force and moment of the grasped object and tries to minimise these unbalances. The simulated results have shown that for every grasp situation, the algorithm is capable of minimising the unbalances and the operation of the algorithm is fast enough for real-time applications.

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