Near-Wall Ultra-precise Motion Control of a Magnetically Driven Scanning Microprobe in Aqueous Solutions

As an untethered scanning probe approaches a wall in aqueous solutions, the drag coefficient increases and approaches infinity, which challenges the precise motion control of the scanning probe near the wall. This article presents a real-time estimation of space-dependent drag coefficient and unknown disturbances for a magnetically driven scanning microprobe close to a wall in aqueous solutions. The estimated drag coefficient is used to update the motion gain with a time-varying discrete-time control law to achieve a prespecified control objective for ultraprecise motion control of the scanning probe near the wall. The control law and the estimator have been integrated into a previously developed visual servo control system to achieve ultraprecise motion control of the scanning probe close to wall surfaces. The motion control system has been realized using a high-speed field programmable gate array (FPGA) system. Experiments have confirmed that near-wall ultraprecise motion control of the scanning probe has been achieved. In experiments, the tracking error of the microprobe was close to a random error with zero mean due only to random thermal forces and measurement noise. In addition, the accuracy of real-time estimation of motion gain/drag coefficient was experimentally verified.