A micro-robotic system with integrated force sensing capability for manipulation of magentic particles in three dimensions

Abstract

Micro-robotic systems are used in various fields of science and technology for the manipulation of objects of size less than a millimeter. Magnetic tweezers can be considered as micro-robotic systems due to their ability to manipulate samples of size in the range of few micrometers. In magnetic tweezers, a magnetic microparticle is manipulated by applying magnetic fields near the particle. Magnetic tweezers are popular for manipulating biological samples due to their high specificity, bio-compatibility and having an untethered end effector that enables them to manipulate inside the samples. Despite these benefits, magnetic tweezers suffer from limitations such as non-linearity in actuation, poor actuation bandwidth, the measurement strategy demanding the particle to be clearly visible and finally, the necessity of sophisticated control strategies for controlling the position of magnetic particles. This thesis investigates the design and development of a micro-robotic system with force sensing capability that addresses the actuation, measurement and control limitations of magnetic tweezers systems.

In order to address the actuation and measurement limitations of magnetic tweezers, a current carrying micro-actuator is proposed to apply magnetic forces to the magnetic particles while an integrated force sensor measures the applied force. A simple analytical model for the force of interaction between the micro-actuator and magnetic particle is proposed and is employed to show that the force is proportional to the actuation current and position of the magnetic particle in three-dimensions (3-D). Further, simple models for mechanical stiffness and rise in temperature due to ohmic heating of the micro-actuator with force sensing capability are proposed. Subsequently, systematic guidelines are proposed for the design of the micro-actuator with force sensing capability. The designed micro-actuator with force sensing capability is fabricated and evaluated. The micro-actuator has an electrical bandwidth of about 1 MHz. The ability of the micro-actuator to apply 3-D forces to a magnetic particle is demonstrated by actuating permanent-magnet microparticles attached to micro-cantilever beams. The force sensing capability of the micro-actuator is demonstrated by measuring the deflection of the micro-actuator while it is actuating permanent-magnet microparticles.

The applicability of the micro-actuator with force sensing capability is shown by employing it for the development of a magnetometer to estimate the magnetic moment of micrometer-scale magnetic particles in 3-D. The developed magnetometer is evaluated by measuring magnetic moments of both hard and soft ferromagnetic particles and of untethered magnetic particles. The measured magnetic moments agree with their theoretical counterpart with an average error of 18%.

Finally, an open loop control strategy is proposed for controlling the position of magnetic particles in 2-D and 3-D respectively by applying appropriate actuation currents to the micro-actuator. Also, the force sensing capability is utilized to estimate the position of the magnetic particle along the out-of-plane axis of the micro-actuator. The estimated position of the magnetic particle is used to develop a novel scanning probe microscope (SPM) with an untethered probe. The motion of the magnetic particle in 3-D due to actuation current to the micro-actuator is estimated numerically and analyzed by using Mathieu equation with Dehmelt’s approximation. Control of the position of a magnetic particle is demonstrated by moving the particle in pre-defined trajectories in 2-D and 3-D. Further, a new strategy is developed to push samples of dimensions much smaller than the size of the magnetic particle. Finally, the imaging capability of the developed SPM is shown by imaging artificially generated topographies using a tethered probe.