Despite the fact that the main advantage of robot manipulators was always meant to
be their flexibility, they have not been applied widely to the assembly of industrial
components in situations other than those where hard automation might be used. We
identify the two main reasons for this as the 'fragility' of robot operation during tasks
that involve contact, and the lack of an appropriate user interface. This thesis describes
an attempt to address these problems.We survey the techniques that have been proposed to bring the performance of cur¬
rent industrial robot manipulators in line with expectations, and conclude that the
main obstacle in realising a flexible assembly robot that exhibits robust and reliable
behaviour is the problem of spatial uncertainty.Based on observations of the performance of position-controlled robot manipulators and
what is involved during rigid-body part mating, we propose a model of assembly tasks
that exploits the shape invariance of the part geometry across instances of a task. This
allows us to escape from the problem of spatial uncertainty because we are 110 longer
working in spatial terms. In addition, because the descriptions of assembly tasks that
we derive are task-invariant, i.e. they are not dependent on part size or location, they
lend themselves naturally to a task-level programming interface, thereby simplifying
the process of programming an assembly robot.the process of programming an assembly robot.
However, to test this approach empirically requires a manipulator that is able to control
the force that it applies, as well as being sensitive to environmental constraints. The
inertial properties of standard industrial manipulators preclude them from exhibiting
this kind of behaviour. In order to solve this problem we designed and constructed a
three degree of freedom, planar, direct-drive arm that is open-loop force-controllable
(with respect to its end-point), and inherently accommodating during contact.In order to demonstrate the forgiving nature of operation of our robot arm we imple¬
mented a generic crank turning program that is independent of the geometry of the
crank involved, i.e. no knowledge is required of the location or length of the crank.
I11 order to demonstrate the viability of our proposed approach to assembly we pro¬
grammed our robot system to perform some representative tasks; the insertion of a peg
into a hole, and the rotation of a block into a corner. These programs were tested on
parts of various size and material, and in various locations in order to illustrate their
invariant nature.We conclude that the problem of spatial uncertainty is in fact an artefact of the fact
that current industrial manipulators are designed to be position controlled. The work
described in this thesis shows that assembly robots, when appropriately designed,
controlled and programmed, can be the reliable and flexible devices they were always
meant to be
