Harnessing magnetic fields to control micro-scale entities
Goal
To employ external magnetic fields to controllably position and orient a magnetic micro-robot. We demonstrate this approach in the 2007 and 2008 RoboCup Nanogram Demonstrations.
Approach
Five electromagnetic coils surround a working volume, wherein the magnetic micro-robot resides. Four of the coils are in-plane with the micro-robot, and one coil provides an orthoganal clamping force. Large DC magnetic field gradients are developed using the coils, which employs a force onto the micro-robot. The robot also experiences a magnetic torque; combined with a pulsed magnetic field, the micro-robot experiences a continuously rocking motion. This, in effect, induces stick-slip behavior in the robot resulting in translation. By varying the pulsing frequency, control of micro-robot velocity is achieved. Maximum velocities observed are about 13 mm/s, or about 60 body lengths per second.
The micro-robot is a Neodymium-Iron-Boron permanent magnet. It is machined using a laser micro-machining system to a desired shape, typically a rectangular solid with approximate dimensions 200x100x50 microns.
Visual servoing is possible using computer vision to track the micro-robot. Motion tasks can be planned and executed using path-planning techniques from a computer. Control strategies will be implemented to ensure stability of manipulation, and will be implemented for task-based guided nanomanipulation.
Advantages and Benefits
Externally controlled magnetic actuation for micro-scale robots benefits from the ability of operation on arbitrary surfaces. Most surfaces are feasible, provided that they are not magnetically active and not overly sticky. Examples of valid surfaces are glass, silicon, hard plastics, and machined aluminum; in observation, the micro-robot operates better on slightly rough surfaces. In comparison, approaches such as electrostatic actuation for micro-robots requires a specialized surface with electrodes. Without these constraints, micro-robot motion on arbitrary surfaces can be realized.
In addition the micro-robot can operate in a fluid environment, provided the fluid is not too viscous such that it impedes the micro-robot's motion. In experiment, water is used, where the micro-robot experiences slight velocity reduction due to fluid damping. Fluid environments are advantageous as micro-scale stiction forces are reduced, which can improve the reliability of micro-scale manipulation tasks.
As the micro-robot is simply a permanent magnet, it is not fragile and is physically robust, capable of being handled in harsh environments. Susceptibility to dirt, contamination, and humidity fluctuations are minimal with this design, when compared to electrostatic micro-robot approaches.
Videos
Magnetic micro-robot moving on a glass slide in air with a US dime for scale reference (2008) [YouTube video] [WMV video]
Magnetic micro-robot moving on a dime underwater, traversing across the 50 to 100 micron features on the coin (2008) [YouTube video] [WMV video]
Magnetic micro-robot pushing 50-micron polystyrene beads underwater (2008) [YouTube video] [WMV video]
High-speed video of the side-view of a micro-robot in translation, displaying stick-slip motion, taken at 200 fps (2008) [YouTube video] [WMV video]
Interview of magnetic micro-robot system at EngineeringTV.com [Link]
Members
Steven Floyd,
Chytra Pawashe,
Metin Sitti
Former Members
Brad Camburn
Publications
Pawashe, C., Floyd, S., Sitti, M., "Dynamic Modeling of an Untethered Magnetic Micro-Robot," Robotics: Science and Systems, to appear, 2008.
Floyd, S., Pawashe, C., Sitti, M., "An Untethered Magnetically Actuated Micro-Robot Capable of Motion on Arbitrary Surfaces," IEEE Int. Conf. on Robotics and Automation, to appear, 2008.
