Dubey, Venketeshwar Nath
Sensing and Control within a Robotic End Effector.
University of Southampton, Department of Electrical Engineering,
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 fiizzy logic algorithm. The rule base of the fiizzy
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
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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