Two Drives Yield Precise, Dual-Speed Motion

Jan. 9, 2009
Highly precise linear positioning systems, such as those used to focus and scan in measuring and inspection equipment often need two different motion modes

Edited by Stephen J. Mraz

Highly precise linear positioning systems, such as those used to focus and scan in measuring and inspection equipment often need two different motion modes: a rapid one (100 mm/sec) followed by a slower one (20 nm/sec). The fast mode reduces move time, while the slower mode ensures precision. Until now, a common design used separate stages, one driven by a ball screw or linear motor and the second, mounted atop the first, driven by a motorized micrometer or piezoelectric motor.

Engineers at Steinmeyer FMD, Dresden, Germany (steinmeyer.com), however, have developed an alternative approach: an economical single stage with two independent drive systems. Two rotary motors, a standard dc servomotor and a PiezoLeg motor, mount on either end of a Steinmeyer high-precision ball screw. An electromagnetic clutch controls the connection between the PiezoLeg motor and ball screw. The dc servo is always connected, but only powered for rapid moves.

In high-speed mode, the clutch energizes and disengages the PiezoLeg’s motor. A conventional dc motor with rotary encoder takes over the driving task. Because high-speed movements can be executed quickly, the heat introduced by the dc motor is very low. Depending on the ball screw’s pitch, the usable velocity ranges from 0.1 to 100 mm/sec.

With clutch power off, the PiezoLeg’s motor connects to the ball screw. A high-resolution linear measuring subsystem provides position information to the motion controller. With clutch power off, heat effects from electromagnets are minimized. And in the rest position the piezomotor works as a passive brake — preventing unwanted stage movement. However, after switching to the PiezoLeg motor, a velocity range of 0.15 to about 0.00002 mm/sec (20 nm/sec) is possible. The stability of speeds at the lower velocity range depends on the resolution of the linear scale used.

The ratio of maximum to minimum speed is 1 million to 1, or higher, and it is possible to switch from fast to high-precision slow speed and vice versa. Movement in both modes is limited only by the travel range of the positioning system.

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