• Part Orientation with One or Two Stable Equilibria Using Programmable Force Fields. K. Böhringer, B. R. Donald, F. Lamiraux, and L. Kavraki. IEEE Transactions on Robotics and Automation, 16(2), April 2000, pp. 157-170.


    Programmable force fields are an abstraction to represent a new class of devices for distributed, non­ prehensile manipulation for applications in parts feeding, sorting, positioning, and assembly. Unlike robot grippers, conveyor belts, or vibratory bowl feeders, these devices generate force vector fields in which the parts move until they may reach a stable equilibrium pose.

    Recent research in the theory of programmable force fields has yielded open­-loop strategies to uniquely position, orient, and sort parts. These strategies typically consist of several fields that have to be employed in sequence to achieve a desired final pose. The length of the sequence depends on the complexity of the part.

    In this paper, we show that unique part poses can be achieved with just one field. First, we exhibit a single field that positions and orients any part (with the exception of certain symmetric parts) into two stable equilibrium poses. Then we show that for any part there exists a field in which the part reaches a unique stable equilibrium pose (again with the exception of symmetric parts). Besides giving an optimal upper bound for unique parts positioning and orientation, our work gives further evidence that programmable force fields are a powerful tool for parts manipulation.

    Our second result also leads to the design of ``universal parts feeders'', proving an earlier conjecture about their existence. We argue that universal parts feeders are relatively easy to build, and we report on extensive simulation results which indicate that these devices may work very well in practice. We believe that the results in this paper could be the basis for a new generation of efficient, open­-loop, parallel parts feeders.