A 3D POSITION CONTROLLER FOR A CHARGED-PARTICLE SUSPENDED IN A MODIFIED ELECTRODYNAMIC CHAMBER

A three-dimensional (3D) position control system for a charged particl e 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 de electrodes and (ii) a special hardware that e nabled the reading of the correct position of an arbitrarily shaped pa rticle at high speed from its shadow, measured by a photodiode array ( PDA). Three 128-pixel linear PDAs were used to measure the trace of th e particle at the three dimensions. The hardware extracted directly th e digital value of the particle position at each dimension by determin ing the center of the particle shadow. The three coordinates of the pa rticle position were fed into a computer that served as a proportional -integral derivative and a storage device. The hardware allowed sampli ng of the particle position at frequency (7.8 kHz) and was able to han dle particles at a wide range of sizes and various shapes (for example , with holes). The 3D position controller can maintain a particle at t he center of the EDC, with a short characteristic time, under a wide r ange of disturbances, acting from arbitrary directions. The controller can also maintain the particle without an ac held. The results can be summarized as follows: (i) for a step disturbance, acting on a partic le from an arbitrary direction, the controller can fully follow a forc e value as much as 10% of particle weight with a characteristic time o f 100 ms; (ii) for a gradual disturbance, acting on the particle from an arbitrary direction, the controller can correct the disturbance wit h a changing force rate of 1 g/s with a delay time of less than 25 ms. (C) 1996 American Institute of Physics.