Machine Design

Direct drive leaves gears behind

ULT Series brushless, direct-drive motors feature heights as low as 10 mm and diameters up to 165 mm. Torque range is from 30 to 600 oz-in. With high-resolution feedback, the motors can run at speeds up to 250 rpm and position to submicron accuracy.

Many applications in semiconductor processing, medical diagnostics, imaging, test, and inspection require rotary motion to move lightweight parts. The traditional solution was a mechanical rotary stage, but more of these applications are giving way to direct-drive rotary motion.

A direct-drive system improves accuracy and reliability by eliminating gearing and mechanical connections which add compliance. Brushless, direct-drive motors are the key ingredient. The motors have a machine-wound stator with a single-piece, NdFeB, multipole rotor. Poles can be magnetized according to specific design requirements. This allows magnetic field domains to be optimized for smooth operation and low cogging torque, which eliminates the need for skewed stators.

Advances in encoder technology, such as high-resolution, ring-type encoders, have led the way for thinner, larger diameter motors. Many of the applications for these motors require a large throughhole in the center. Until recently, encoders with large throughholes were relatively expensive. This has changed in the last few years with the advent of affordable ring-type, 360° encoders.

Direct-drive technology has been used in clean rooms and high-reliability and aerospace applications for years. For instance, in semiconductor manufacturing, table height is critical. As a result of inspection operations moving in-tool as opposed to being in a separate machine, a low-profile motor is especially useful.

A common misconception is that switching to a direct-drive system is more expensive than a rotary stage with a gearhead. ULT motors from Applimotion Inc., Roseville, Calif., are actually less expensive than brushless motors. A comparable rotary stage using ULT motors with direct-drive technology is competitively priced to a precision mechanical rotary stage driven by a traditional servomotor. Depending on the resolution and positioning control required, direct-drive motors used in a rotary stage can also be competitively priced to stepmotor driven mechanical rotary stages.

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