DN Staff

November 18, 2002

3 Min Read
'Smallest' servo actuator positions precisely

"World's smallest backlash-free servo actuator." That is the claim from Micromotion GmbH regarding its new hollow-shaft micro servo actuator. A joint development between Harmonic Drive AG (Limburg, Germany), the Institute for Microtechnology in Mainz, and Maxon Motor AG (Sachseln, Switzerland), the micro drive system offers new positioning possibilities to the semiconductor, medical, and optical fields, as well as industry in general.

Miniature positioning drive unit comprises a Micro Harmonic Drive, maxon EC motor and digital MR encoder.

Three components comprise the system: a Harmonic Drive(R) micro-gear system; a hollow-shaft, 1.2W brushless maxon EC motor; and a high-resolution micro encoder. Here's how they work.

Backlash-free micro gearhead. Measuring only 9 mm long by 8 mm in diameter, the Micro Harmonic Drive achieves a 160:1 reduction ratio in a single stage. Output torque equals 10 mNm with friction torque loss below 16 muNm. Repeatability is less than 10 arc seconds.

Individual gear components are manufactured from NiFe alloy using standard LIGA (German letters for lithography, electroforming, and molding) techniques. These parts include an elliptical wave generator comprising a planetary gear stage with sun wheel and radially deformable planet wheels, an elastically deformable gear, the flexspline and two solid ring gears, the circular spline, and dynamic spline.

During operation, the wave generator deflects the elastically deformable flex-spline. As a result, the flexspline simultaneously engages the circular spline at either end of the major elliptical axis. When the wave generator sun wheel, fixed to the motor shaft, rotates, tooth engagement of the flexspline travels with the planet wheels. Because the circular spline has two more teeth than the flexspline, there is relative movement between the gear wheels. The dynamic spline, as output element, has the same number of teeth as the flexspline and serves as a toothed coupling.

Hollow-shaft mo-tor plus encoder. Use of a motor with a central hollow shaft enables system engineers to pass an optical fiber laser beam, or media such as fluid, compressed air, and vacuum along the servo actuator axis. Drill hole diameter measures 0.65 mm. The maxon EC6 (electronically commutated) dc motor not only matches the micro gearhead's tiny diameter, it-with the encoder-adds just 22 mm to the unit's overall length.

The two-channel magneto resistance (MR) encoder features 100 encoder pulses for a resolution of 0.0225 degrees at a 160:1 gearhead reduction ratio. Output signals are TTL compatible.

Because of its small dimensions, low mass, low inertia, and low power consumption-with no sacrifice in high positioning accuracy and high dynamic performance-the micro-drive system provides precise positioning for a variety of applications:

  • Lens and mirror adjustment for laser systems.

  • Surgical instrument drive systems.

  • Adjustment of semiconductor components.

  • Adjustment of non-contacting sensors.

  • Micro robotics.

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