PM motors for direct driving optical telescope

This article details the task of developing the drive systems for VLT (Very Large Telescope) control. The project entails the realization and the coordinated operation of four 8 m diameter optical telescopes in Chile for the period 1995-1998. A direct drive system has been chosen as the optimal drive solution from a performance standpoint. It consists of a segmental dual axial air gap brushless motor which is an unusually large torque motor, which must be manufactured piecewise and assembled on-site. Furthermore, the torque motor must couple in the stiffest possible way to the relatively light telescope assembly, and the thermal perturbations to the environment must be minimized. Finally, in spite of their very large size, the torque motors must be controlled with a very high control bandwidth; the torque slew rate must be extremely steep. The authors describe the design of the drive system including motor torque ripple. Finite element analysis of the motor is also discussed.