Local Magnetic Field System Design and Control For Independent Control of Multiple Mobile Microrobots

2020-04-30T16:18:51Z (GMT) by Benjamin V Johnson
This dissertation describes the evolution of the different local magnetic field generating systems for independent actuation of multiple microrobots. A description of the developed hardware, system characterization tests, and experimental results are presented. The system is designed for automated control of multiple microrobots. Finally, sample micromanipulation tasks are demonstrated using the new microrobot design, showcasing its improved manipulation capabilities.
First, a mm-scale local magnetic field generating system designed for single layer coils is used to control 3.175 mm size N52 magnets as robots independently in the workspace. The controller used a set of local equilibrium points that were generated from a sequence of coil currents around the robots from one state to the next. The robots moved along paths computed through optimal control synthesis approach to solve complex micromanipulation tasks captured by global LTL formulas. However, the use of local equilibrium points as the states limited the motion of the robot in the workspace to simple tasks. Also, the interaction between the robots limited the robots to stay within far distances with each other. Hence a larger workspace based coil is designed to actuate up to four mm-scale robots in the workspace.

To improve the resolution of motion of these robots in the workspace, the mm-scale coils are modeled extensively. The forces generated by various coil combinations of the array are modeled and solutions for different actuation force directions are discovered for different locations in the coil. A path planning problem is formulated as a Markov decision process that solves a policy to reach a goal from any location in the workspace. The MDP formulation is also expanded to work when other robots are present in the workspace. The formulation considers the interaction force between the robots and changes the policy to reach the goal location which reduces in the uncertainty of motion of the robot in the presence of interactions from other robots in the workspace.

The mm-scale coils are difficult to scale down for microrobotic applications and hence a new microscale local magnetic field system was designed. A new microscale local magnetic field system which consisted of two 8 × 8 array of coils aligned in two axes in two layers of a PCB was designed which could actuate robots as small as 1 mm in the plane. The microcoils in the second layer are also able to generate sufficient magnetic field gradients in the workspace, while the traces below it are spaced adequately to eliminate their influence in the workspace. A new microrobot design also enabled the orientation control of the microrobot for performing micromanipulation tasks. However, only two robots could be independently actuated in this workspace due to interaction between the robots.

In pursuit of actuation smaller and multiple robots in a small workspace, a serpentine coil based local magnetic field generating system was designed to control of the motion of magnets as small as 250 µm. The net size of the robot is 750 µm to enable orientation control and prevent tipping during motion. This system is capable of simultaneous independent closed loop control of up to 4 microrobots. The motion of the robot using the coils resembled that of a stepper motor which enabled the use of sine-cosine functions to specify currents in the coils for smooth motion of the
microrobot in the workspace. The experiments demonstrated the capability of the microrobot and platform to simultaneously actuate up to four robots independently and successfully perform manipulation tasks. The ability to control the orientation of the magnet is finally demonstrated that has improved ability to perform manipulation tasks.