Control of Flexible Load Deformations and Environment Contact Forces in Dual-arm Manipulation

A pseudo-inverse-Jacobian scheme to control deformations of a exible load while attaining contact force goals with an external environment is discussed in this paper. The exible load is manipulated by two arms. A kinematic description of the exible load is obtained from a lumped-parameter model. Coordinated motion of the arms is achieved through kinematic redundancy resolution at the diierential kinematic level. The strategy proposed here uses load exibility to accommodate for position errors that would result in the build-up of internal forces in a rigid-load case. Simulation results are presented that illustrate the success of the method. A variety of assembly operations involve the manipulation of exible parts. Sometimes, demanding operations like part mating of exible componenents are required. For part mating to take place, it might be desirable to have a surface-following control that guarantees contact until parts are matched. Examples of such tasks include the plastic or rubber molding of car interiors, and manufacturing of cooling units for refrigerators where a ex-ible copper pipe is to be inserted in a anged oriice. The latter application is the central motivation for the work presented here. The problem is formulated as a general environment-following task, being performed by two manipulators on a long, exible beam. Two arms are used to perform the task so that the behaviour of the beam is better controlled, allowing for an enhanced dexterous manipulation of the exible load. Dual-arm manipulation of exible loads has only very recently become part of the robotics literature, despite the fact that there are many industrial tasks that involve assembly of exible parts. Whereas some authors acknowledge the fact that there are possible advantages to be exploited when dealing with exible loads, there has been no work that does it systematically. This paper presents a method that systematically exploits exibility 1 Supported by grant 3392/92, CAPES, Minist erio da Educaa c~ ao, Brazil, and Cooperative Research Centre for Robust and Adaptive Systems (CR) 2 ASys], Australia. to achieve task goals. Load deformations in the elastic region are used so that a purely kinematic approach can be used to control the interaction force between the load and a static environment. Maintaining contact in surface following tasks is a dif-cult problem for rigid robotic manipulators. Many strategies have been proposed, most of them looking at controlling contact forces. For instance, hybrid posi-tion/force control (Raibert and Craig 1981, Mills and Goldenberg 1989) tries …