March 1, 2010
Do small-scale designs have typical speed specifications? No. Because of the short distances covered, the key factor in terms of small-scale positioning

Do small-scale designs have typical speed specifications?

No. Because of the short distances covered, the key factor in terms of small-scale positioning is not maximum speed (as in normal drives) but rather:

  • The overall setup's mechanical reach, plus
  • Acceleration and deceleration time.

To deliver reach, two-drive positioners typically use a ballscrew, with one motor coupled to each end. Theoretically, ballscrews have no feed-length limit, so the spindle can be as long as required. This allows larger objects to be measured at several key areas — without requiring that an operator repeatedly remove and reclamp the object to place key areas in the machine's active sensing area. In contrast, conventional (stacked) piezo drives are restricted to just a few millimeters of positioning width.

To deliver acceleration, dual-drive actuators switch powerlessly — with no heat — to the rotary piezo motor at a speed of 0.5 mm/sec by way of a permanent magnetic coupling. At rest, this drive then works as a passive spindle brake, damping oscillation and reducing settling time. The maximum-to-minimum speed ratio also exceeds 1,000,000:1 with the dual-actuator setup.

What kind of dc motor drives the big moves?

For the faster initial positioning, a conventional graphite-commutation brush motor with a rotary encoder connected to the shaft by a bellows coupling is adequate. The motor runs for a relatively short time in this application, so motor heat input is negligible. Depending on the spindle pitch used, speed from 0.5 to 100 mm/sec is possible — suitable for standard positioning requirements. 7,000 rpm and 16 mNm are typical outputs. Gear output ratios vary; magnetic two-channel incremental encoders are typically used with up to 512 pulses per revolution.

What are capabilities of the smaller motor that drives the final positioning?

The piezo rotary motors are smaller — about one cubic inch and 70 grams or so. They work with control voltages from 0 to 3,000 Hz. Holding torque typically reaches 90 mNm; incremental stepwidth reaches 0.35 mrad. A small shaft mechanically links the motor to a permanent-magnet coupling that then connects to the ballscrew.

A high-resolution linear measuring system continuously records the movement and transmits the information to the motor controller. In this way, the drive moves the linear positioner in high-precision mode at a speed of 0.00002 to 0.15 mm/sec — 20 nm per second. The speed constancy at the bottom end of the range is only depends only on the resolution of the linear scale. Repetition accuracy is better than 100 nm.

This month's handy tips provided by MICROMO of the Faulhaber Group, Clearwater, Fla., PiezoMotor AB in Sweden, and Feinmess Dresden GmbH. For more information, call (800) 807-9166 or visit micromo.com and www.piezomotor.se.

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