Control of Hexapole Electromagnetic Actuator Enabling Ultra-Precise High-Speed 3-D Untethered Manipulation of Magnetic Microprobe and 3-D Scanning of Biological Sample in Aqueous Solutions

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Scott Lab E339
201 W. 19th Avenue Columbus, OH 43210
Columbus, OH 43210
United States

Speaker Name: Ta Min Meng

Abstract: This research presents a hexapole electromagnetic actuator system that enables ultra-precise high-speed 3-D untethered manipulation of a magnetic microprobe and 3-D scanning of living biological samples by controlling the probe-sample interaction force at the piconewton scale in aqueous solution under a microscope. The hexapole actuator uses the six input currents to produce magnetic fluxes at the six pole tips to control the magnetic field in the 3-D workspace and the 3-D magnetic gradient force exerted on the magnetic probe. It is an over-actuated, nonlinear, and position-dependent system with six inputs and three outputs. Magnetic remanence and hysteresis could lead to uncertainties in current-based magnetic flux generation. These uncertainties increase when greater forces need to be generated, leading to greater force errors, and reducing the performance of the actuator. In addition, the dynamics of magnetic flux generation is important for advanced applications that require high-bandwidth precision force generation. A 6-input-6-output model-based digital control law along with a disturbance estimator has been designed and implemented to control the magnetic fluxes at the six pole tips of the actuator to control the 3-D magnetic force exerted on the microprobe. The closed-loop magnetic flux control significantly suppresses the uncertainty due to remanence and hysteresis and greatly increases the bandwidth of the flux generation. Together with optimal flux allocation, the developed magnetic flux control system has been used to render accurate magnetic force generation. It enables the 6-input hexapole system to behave like a decoupled 3-axis force producer with high bandwidth. An integrated control system has been designed and implemented to render ultra-precise and high-speed untethered manipulation of a single magnetic scanning probe in aqueous solutions under a microscope. The control system uses the hexapole actuator to control the 3-D magnetic force exerted on the scanning probe and uses a vision-based 3-D particle tracking system to enable visual servo control. When the scanning probe moves in aqueous solutions, the drag coefficient varies with its distance from the surface in the 3-D workspace of the actuator. A position-dependent discrete-time motion control law has been derived and implemented to achieve high-speed manipulation of the magnetic microprobe in aqueous solutions, wherein the motion error is zero-mean random error. In addition, the variance of motion error can be analytically determined by random thermal forces and random measurement noise. In an unstructured environment, e.g., probing samples having 3-D solid surfaces or viscoelastic mechanical properties, no theoretical model can be used to determine the position dependence of the drag coefficient due to wall effects. An augmented estimator using Kalman filtering has been designed to estimate the disturbance, drag-coefficient-dependent input gain, and the surface orientation simultaneously. Specifically, the covariance matrix of the motion error is calculated in real-time and used to estimate the surface normal of the sample through real-time Principal Component Analysis. Two scanning modes, namely non-contact scanning mode and tapping mode, have been designed and realized, and the rapid scanning of 3-D samples has been experimentally demonstrated. The non-contact scanning mode uses the real-time estimated variables to move the probe along the surface tangential direction while regulating the estimated drag-coefficient-dependent input gain in the surface normal direction. The tapping mode moves the probe along the surface tangential direction while regulating the probe-sample interaction force in the surface normal direction at the sub-piconewton scale. Finally, a force controller has been designed and implemented to control the probe/sample interaction force with high accuracy and bandwidth. The force controller has then been integrated with automatic scanning to create a new scanning-indentation mode for live-cell applications, wherein the microprobe is controlled to undertake normal indentation and tangential scanning alternately and automatically to produce the surface geometry of the cell and the spatial distribution of the mechanical properties of the cell surface. The integrated system was applied to scan three Hela cells at distinct phases of the cell cycle. The experimental results provide quantitative measures of the temporal variation and spatial distribution of the mechanical properties of the cell surface as well as cell geometry during the cell cycle.

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Committee Members
Ralph W. Kurtz Chair in Mechanical Engineering Chia-Hsiang Menq
Associate Professor David Hoelzle
Associate Professor Hanna Cho

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