Miniaturized Motion Control Technology for the World's Largest Pair of "Binoculars"

If you want to set things in motion, you will always require some form of drive. However, conventional technology employs relatively large designs and is too cumbersome for many applications. The trend toward miniaturization, though, has definitely made its mark on motion control engineering. Small, powerful electric motors with a diameter of only a few millimeters guarantee pioneering innovations in a wide variety of fields. It is not only industrial automation that benefits but, to an increasing extent, other sectors as well. Nowadays modern miniature drives are even enhancing science, as demonstrated by the following example of an application in the field of astronomy. A combination of miniature DC motor, encoder, and a low-backlash planetary gearhead ensures that optical assemblies are positioned with precision.

The most powerful standalone telescope in the world will be put into operation on Mount Graham in the US state of Arizona in 2004.
The astronomers are particularly interested in setting sights on distant galactic systems, young double stars and newborn suns. In principle, this Large Binocular Telescope (LBT), which has a height of over 20 m and weighs over 600 t, is an outsized pair of binoculars. Its two reflectors each have a diameter of 8.4 m and together they make up an approx. 100 sq. m dish for collecting light. In this way it can even collect the radiation from weakly illuminated objects at the limits of the universe being observed. The interaction of the two reflectors mounted 14.4 m apart provides the telescope with a resolution that would correspond to that of a pair of binoculars having a diameter of 23 meters. Each reflector resembles a giant "honeycomb" made from borosilicate glass and weighs 15.6 t.

Interference is the key to higher-definition images

The design of the telescope and its integrated optical systems provides scientists with a high level of flexibility when making their observations. That way they can use each of the reflectors independently of one another to view the same object, but also study different objects by tilting the viewing axes slightly, or use both reflectors to observe the same object at maximum resolution. They are assisted by a physical trick:
In order to achieve the unusually high definition, the rays of light reflected by each reflector are superimposed, i.e. brought to a state of interference. Consequently, the resolution is nearly ten times better than with conventional standalone telescopes. However, the requirement that has to be met to ensure the LBT works smoothly is that individual components made in the three partner countries - the US, Italy, and Germany - interact without any problems. Furthermore, they have to operate properly at the site of use under adverse conditions. After all, Mount Graham is approx. 3,300 m high. The climate at that altitude is characterized by temperatures below freezing, a very high humidity of up to 90%, as well as extreme temperature fluctuations.

Positioning unit for interference generation

If a high-resolution image is to be created by the generation of interference, the optical assemblies that are attached to the two reflectors for bundling and superimposing the reflected light have to be positioned with an accuracy of 5 µm. For this purpose, the Feinmess company in Dresden (Germany) developed a three-axis positioning system that moves the appropriate optical system on the two reflectors of the LBT into the correct position. Horizontally, distances of up to 200 mm have to be covered (longitudinal positioning), and vertically - for focusing purposes - there are distances of up to 50 mm. At the same time the optical assembly has to be rotated through an angle of up to 36 degrees. In order to ensure the required positioning accuracy, the system has to operate with as little play as possible. That is why great importance is accorded to the drives on the spindles.
In this case, drive solutions from FAULHABER were selected. The traditional bell-type armature motor with coreless rotor coil provides an excellent basis for such fields of application. The small DC drives operate reliably even under hostile ambient conditions. They can cope with ambient temperatures between -30°C and +125°C and are not even affected by a high level of humidity (up to 98%) when specified appropriately. An important basic criterion for motor selection was also instant, high-torque starting for the DC motor after application of voltage. That ensures a direct response to control signals. The coreless copper coil allows an extremely lightweight motor design with a high efficiency of up to 80%. The motors used on all three spindles of the positioning system have a diameter of 26 mm and are only 42 mm long; at speeds of up to 6,000 rpm they provide a power output of 23.2 W.

Motor, gearhead, and pulse encoder – a compact unit

In the application described above the motors were combined with two-stage planetary gearheads that have a ratio of 16:1. Flanged to the end of the motor, their performance is extremely impressive, not only due to their compact design but also because of their steady running and durability. Gearhead backlash was optimized for use on the positioning system. Instead of the values of about 1 degree, customary on standard gearheads, these planetary gearheads have a backlash of only 12 angular minutes, measured at the output shaft.
Knowing the actual position of the motors is an essential prerequisite for precision positioning. With the positioning systems employed on the LBT it is detected at each motor by an optical pulse encoder that generates 500 pulses per revolution. Using a metal disk, a transmitted-light system generates two phase quadrature output signals. The index pulse is synchronized with output B For each of the three channels there are inverted complementary signals. The pulse encoder is fitted to the free end of the motor shaft and fixed with three screws. Supply voltage for the pulse encoder and the miniature DC motor and the output signals are connected via a ribbon cable and a 10-pin connector. Since the drive units, comprised of the motor, gearhead, and pulse encoder, are extremely compact, they are easy to integrate into three-axis positioning systems. Miniaturized motion control technology thus plays an instrumental role in enabling the LBT to open up a new dimension in astronomical research.