Sensing and Control within a Robotic End Effector

Abstract

This research programme investigates aspects of end effector design and control, to carry
\nout grasping operations in a range of unstructured environments.
\n
\nA conceptual three fingered end effector design has been developed. The articulated
\nfinger is operated by a novel mechanism which provides all the finger motions. Detailed
\nforce and kinematic analyses have been carried out which establish mechanical integrity
\nof the system and help size the various finger components. A vectorial method of link
\nrepresentation has been used to derive finger kinematics. This representation has been
\nused for position control in the controller. A numerical technique based on the Newton-
\nRaphson method has been derived to undertake the finger's inverse kinematics in realtime.
\nTo validate the theoretical operation of the finger drive, a mechanism has been built
\nwith the necessary electronic interface, and programmed for position control.
\n
\nA photoelasticity based sensor has been developed which is capable of detecting applied
\nforce as well as slip and is largely immune to external disturbances. The sensor has a
\nsmall size allowing it to be easily incorporated into a robotic finger. Mechanics of slip has
\nbeen investigated to develop a theoretical model of the slip sensor. This allows modelling
\nof various material and geometrical parameters involved in its design.
\n
\nIn order to control the end effector, grasping strategies have been planned and a
\ncontroller structure defined. The top level of the controller uses the kinematic relation to
\nmove the finger to a goal position. When fingers make contact with an object, the
\ncontroller switches over to an inner fiizzy logic algorithm. The rule base of the fiizzy
\nlogic ensures that a stable grasp has been acquired with minimum fingertip force. The
\nimplementation of the fuzzy logic has been validated on an experimental test-rig. It has
\nbeen found that the controller applies different minimum fingertip force to objects of
\ndifferent mass and it responds very quickly to the external disturbances by applying extra
\nforce to the object. The fingertip force comes back to its previous level as soon as the
\ndisturbance vanishes. The important feature exhibited by the controller is that it forms
\noptimal grasp of objects without knowing their mass and frictional properties. This offers
\na very useful capability to an end effector controller operating in unstructured
\nenvironments.
\n
\nA complete model of the end effector has been developed which ensures equilibrium and
\nstability of the grasped object taking dynamic conditions of grasp into account. The
\nmodel estimates unbalances in position, force and moment of the grasped object and tries
\nto minimise these unbalances. The simulated results have shown that for every grasp
\nsituation, the algorithm is capable of minimising the unbalances and the operation of the
\nalgorithm is fast enough for real-time applications.