Independent control of two millimeter-scale soft-bodied magnetic robotic swimmers

We present a method to independently control two millimeter-scale soft-bodied magnetic swimmers, with nominal dimensions of 1.5 × 4.9 × 0.06 mm. A swimmer's speed depends on its relative angle with the actuation magnetic field. The two swimmers under control have different directions of net magnetic moments, and assume distinct orientations in one global magnetic field. With this fixed heading difference between two swimmers, the global actuation field forms different relative angles with the two swimmers. By manipulating these two relative angles, the two swimmers can exhibit different speeds. Theoretically, any speed ratios can be achieved between the two swimmers. In practice, a relatively accurate speed ratio can be obtained when both swimmers have nonzero speeds and one swimmer is no more than twice as fast as the other. Adding the control over the strength of actuation field, two swimmers can obtain independent speeds within a certain range. Two feedback controllers are proposed to control two such swimmers to arrive at independent global points (positioning) and move along paths (path following). Type I Sequential Controller manipulates two swimmers to move to their respective goals in sequence, while Type II Parallel Controller moves both swimmers simultaneously. Experiments are presented in which two swimmers are controlled to pass a series of points to form the letters UT.