Xf. Zhang et E. Barziv, A 3D POSITION CONTROLLER FOR A CHARGED-PARTICLE SUSPENDED IN A MODIFIED ELECTRODYNAMIC CHAMBER, Review of scientific instruments, 67(10), 1996, pp. 3483-3490
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.