A 3D position controller for a charged particle suspended in a modified electrodynamic chamber

A three-dimensional (3D) position control system for a charged particle suspended in an electrodynamic chamber (EDC) was developed. The new controller is based on two major developments: (i) modification of the EDC that allowed the adjustment of the particle position at all three dimensions, by three dc electrodes and (ii) a special hardware that enabled the reading of the correct position of an arbitrarily shaped particle at high speed from its shadow, measured by a photodiode array (PDA). Three 128-pixel linear PDAs were used to measure the trace of the particle at the three dimensions. The hardware extracted directly the digital value of the particle position at each dimension by determining the center of the particle shadow. The three coordinates of the particle position were fed into a computer that served as a proportional-integral derivative and a storage device. The hardware allowed sampling of the particle position at frequency (7.8 kHz) and was able to handle particles at a wide range of sizes and various shapes (for example, with holes). The 3D position controller can maintain a particle at the center of the EDC, with a short characteristic time, under a wide range of disturbances, acting from arbitrary directions. The controller can also maintain the particle without an ac field. The results can be summarized as follows: (i) for a step disturbance, acting on a particle from an arbitrary direction, the controller can fully follow a force value as much as 10% of particle weight with a characteristic time of 100 ms; (ii) for a gradual disturbance, acting on the particle from an arbitrary direction, the controller can correct the disturbance with a changing force rate of 1 g/s with a delay time of less than 25 ms.