In typical robotic assembly, a single robot sequentially performs a number of tasks to assemble complex objects. However, if the number of parts required for assembly is very large, or if the size of the parts is very small, this method becomes cumbersome or even impossible. A solution is to endow the parts themselves with simple sensory and actuating capabilities. An example is the design of magnetized parts that fit together like puzzle pieces when properly oriented. To induce self-assembly or pattern formation, parts can be powered by controllable external energy from the environment in the form of heating, shaking, stirring, etc. This method of assembly is advantageous because it enables massive parallel manipulation of multiple parts.
Our research is focused on controlling the motion of parts placed on a flat rigid plate using only a small number of sensors and actuators. To this end, we have built a prototype device called the PPOD (Programmable Part-feeding Oscillatory Device). The PPOD consists of a rigid plate attached to six speakers through six linkages. Each linkage has two flexures made of compliant tubing that serve as joints. Thus, the PPOD is a flexure-based Stewart platform that allows six-DOF plate motion. Four dual-axis accelerometers are mounted around the perimeter of the plate to provide information about the plate's motion. By controlling the voltages to the six speakers, we are able to make the plate to move with a desired small-amplitude periodic motion.
The PPOD:
Parts placed on the plate move because of the frictional forces between them and the plate. For any small-amplitude periodic plate motion, we have shown that point parts move as if they are immersed in a position-dependent velocity field. Specifically, there is a mapping from every small-amplitude periodic plate motion to a corresponding velocity field on the plate surface. We call this the asymptotic velocity field.
The PPOD is novel for its ability to vibrate out of the horizontal plane. This capability allows control to be gained over the effective gravitational force (and therefore the frictional force) that the part experiences as a function of its location on the plate. Previously studied systems based on vibratory plates have been limited in the types of fields they can generate because the plate motion has been restricted in some manner. For example, Reznik and Canny's three degree-of-freedom system (the Universal Planar Manipulator) cannot create fields with sinks or sources because it cannot vibrate out of the horizontal plane. Because the PPOD can move with all six-DOF, it can generate a larger class of fields than previous vibratory devices. In particular, this class includes fields with sink and source characteristics (i.e., fields with nonzero divergence).
Example Fields:
Because the PPOD can create fields with sink behavior, it can potentially be used to sensorlessly position and orient parts. There are some non-vibratory parts handling devices that are capable of doing this as well, but they usually do so using pixelated arrays of numerous actuators. The PPOD is the first device capable of creating a continuous field on a rigid plate with sources and sinks. Another advantage of the PPOD is its flexibility. Because it is software driven, the plate motion can be easily reprogrammed to deal with new parts or new manipulation tasks without the need to make any mechanical alterations. Most traditional manipulation devices, such as bowl feeders, require hardware modification every time a new part is introduced.
We have written a graphical user interface (GUI) in MATLAB to study our system. The GUI allows us to create and animate periodic plate motions, simulate the motion of parts, and generate asymptotic velocity fields.
MATLAB GUI Screenshot:
