Objective’s High-bandwidth Nanometer Positioning System Driven By Voice Coil Motor

Confocal microscopy, which works to detect the micromorphology of samplesurface, plays as a more and more important role in modern manufacturing andbiomedical science. Confocal microscope objective sweeps back and forthfrequently during operation, thus requiring a higher objective’s positioning systembandwidth, and in order to obtain the fine structure of the DUT, requires objective’spositioning system has a high displacement resolution. In order to improve theefficiency of confocal microscopy measurements and resolution, we designed andbuilt an objective’s positioning system based on voice coil motor to realize theobjective of high-bandwidth nanometer positioning.First, we designed the mechanical structure of objective’s positioning systemand established the mathematical model. Among them, the system uses hollowvoice coil motor as actuator; support mechanism is designed to have a goodfrequency response; dual ball linear guide as guiding mechanism, dynamichigh-resolution grating ruler as displacement measuring unit and laserinterferometer to facilitate the system. On this basis, we completed the processingand assembly of the system, and so did the system electromechanical model.Secondly, in order to close the voice coil motor current loop and to achievehigh-bandwidth voice coil motor current control, we have designed ahigh-bandwidth analog drive circuit with the help of MATLAB/SIMULINKsimulation. To ensure the reliability of the design, we used Multisim to make circuitsimulation, and the results showed that the current loop’s bandwidth is3.252kHz.On this basis, we have completed the analog drive circuit welding production.Thirdly, combining with the objective’s positioning system mathematicalmodel and the current loop model, we established the position loop model inMATLAB/SIMULINK, and simple PID algorithm and differential forward PIDalgorithm were both tested. The results showed that the controller using differentialforward PID algorithm can make the system’s bandwidth up to188.5Hz, meetingthe target.Finally, we experimented with the current loop unit and objective’s positioningsystem respectively. Experimental results showed that the current loop unit has astep response stability time of0.15ms, overshoot of about2%, bandwidth of about6kHz, linearity of0.65%and current ripple of±7.74%, under load of20.9and4.5mH; objective’s positioning system has a step response stability time of17ms,overshoot of1.8%, positioning noise of±15nm, displacement resolution of less than15nm and system bandwidth of195Hz, can meet the confocal microscopeobjective positioning system’s high bandwidth nanometer indicator.