Machine Design

Small motors position big glass

Optical assemblies are precisely positioned using a miniature dc motor, encoder, and a low-backlash planetary gearhead.

The LBT will be part of the Mt. Graham International Observatory near Safford, Ariz. The two 8.4-m mirrors are mounted 14.4-m apart. Swing arms rotate the secondary and tertiary mirrors, and their supports, switching the telescope between binocular and single observation mode. Total cost: nearly $88 million.
A three-axis positioning system moves the appropriate optical system on the two reflectors on the large binocular telescope (LBT) into position.
Coreless dc motor-encoder units from Faulhaber align the optical system in the new large binocular telescope at Mt. Graham in Ariz.

Associate Editor

The most powerful stand-alone telescope in the world goes into operation this year on Mt. Graham near Safford, Ariz. Astronomers are particularly interested in setting their sights on distant galactic systems, young double stars, and newborn suns.

In principle, this large binocular telescope (LBT), over 20-m high and weighing over 600 tons, is an oversized pair of binoculars. Together, the two 8.4-m reflectors make up an approximately 100-m2 dish for collecting light from weakly illuminated objects at the limits of the universe. The interaction of the two reflectors, mounted 14.4-m apart, provides the telescope with a resolution that would correspond to a pair of binoculars having a diameter of 23 m. Each reflector resembles a giant honeycomb made from borosilicate glass and weighs 15.6 tons.

Interference is key to higher-definition images. The telescope gives scientists added flexibility when making observations. They can use each of the reflectors independently of one another to view the same object, or study different objects by tilting the viewing axes slightly, or use both reflectors to observe the same object at maximum resolution. A physical trick helps out. To get sharp, high-definition images, rays of light reflected by each reflector are superimposed, or brought to a state of interference. Consequently, resolution is nearly 10 times better than with conventional stand-alone telescopes. However, individual components made in three partner countries -- the U.S., Italy, and Germany -- have to interact smoothly without problems. Furthermore, they have to operate under adverse conditions. Mt. Graham is approximately 3,300-m high and the climate is characterized by below-freezing temperatures, humidity to 90%, and sizable temperature fluctuations.

To obtain a high-resolution image, the drive assemblies attached to the two reflectors have to position with an accuracy of 5 µm. The Feinmess Co. of Dresden, Germany, developed a three-axis positioning system. Horizontally, distances of up to 200 mm have to be covered (longitudinal positioning), and vertically, for focusing purposes, distances of up to 50 mm. At the same time, the optical assembly has to rotate through an angle of 36°. For accurate positioning, the system has to operate with as little play as possible.

The spindle drives have to be accurate. Conventional drive technology is large and too cumbersome for many applications. But small, powerful electric motors from Faulhaber did the job. The traditional bell-type armature motors with coreless rotor coils and associated dc drives operate reliably under hostile ambient conditions. They handle ambient temperatures between -30 and 125°C and are unaffected by humidity to 98%.

The key criterion for motor selection was instant, high-torque starting for the dc motor after applying voltage. This ensures a direct response to control signals. The motors have lower inductance and hence short electrical time constants because the wires are not wound on a piece of steel in lamination slots. So a voltage to current rise in the coils is extremely rapid and makes for fast system response. The coreless copper coil yields motor efficiencies of 80%. Motors on all three spindles of the positioning system have a diameter of 26 mm and are only 42-mm long. At speeds of 6,000 rpm, they provide 23.2-W output.

Another advantage is that motors can be overdriven for short periods. Doubling the current doubles the torque. Exceeding the rated current on conventional motors end the linear torque-current relationship so the torque doesn't double.

The motors were combined with two-stage planetary gearheads with a 16:1 ratio. The compact gearheads are flanged to the motor ends. Gearhead backlash was optimized for use on the positioning system. Instead of the values of about one degree customary on standard gearheads, these planetary gearheads have a backlash of only 12 angular min, measured at the output shaft.

An optical-pulse encoder generating 500 pulses/rev provides sufficient feedback to position motors precisely. Using a metal disk, a transmitted-light system generates two-phase quadrature output signals. The index pulse is synchronized with output B. Each of three channels has inverted complementary signals. The pulse encoder attaches to the free end of the motor shaft with three screws. A ribbon cable and a 10-pin socket connect supply voltage for the pulse encoder, the miniature dc motor, and output signals.

Because the drive units, comprised of the motor, gearhead, and pulse encoder are extremely compact, they are easy to integrate into the three-axis positioning system.

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