Accomplishing task-invariant assembly strategies by means of an inherently accommodating robot arm
Deacon, Graham Edwin
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.