Telescope Drive



Traditional gear drives are commonly used with step motors but the fine resolution of a microstepping motor can make gearing unnecessary in many applications. However, they are useful when very large inertias have to be moved. In this application, an astronomer building a telescope needs to track celestial events at a slow speed (15 degrees/hour) and also slew quickly (15 degrees in 1 second).
Traditional Gear Drives
Traditional gear drives are more commonly used with step motors. The fine resolution of a microstepping motor can make gearing unnecessary in many applications. Gears generally have undesirable efficiency, wear characteristics, backlash, and can be noisy.
Gears are useful, however, when very large inertias must be moved because the inertia of the load reflected back to the motor through the gearing is divided by the square of the gear ratio.
In this manner large inertial loads can be moved while maintaining a good load inertia-to-rotor inertia ratio (less than 10:1).
Machine Objectives:
• Smooth, slow speed is required -- microstepping
• High data-intensive application – USB “UMX” product
• Future capabilities to control at least 2 axes of motion
• Visual C++ interface
Application Solution:
A 30:1 gearbox is selected so that 30 revolutions of the motor result in 1 revolution (360 degree) of the telescope. A tracking velocity of 15 degree sign/hour corresponds to a motor speed of 1.25 revs/hour or about 9 steps/sec on a 25,000 steps/rev. Moving 15 degree sign (1.25 revolutions) in 1 second requires a velocity of 1.25 rps.
The inverse square law causes the motor to see 1/900 of the telescope's rotary inertia. The equations are solved and the torque required to accelerate the telescope is 455-oz-in. The step pulses required to drive the motor are obtained from a laboratory oscillator under the operator's control.
Application Solution:
A 30:1 gearbox is selected so that 30 revolutions of the motor result in 1 revolution (360 degree) of the telescope. The inverse square law causes the motor to see 1/900 of the telescope's rotary inertia. The equations are solved and the torque calculated. The step pulses required to drive the motor are obtained from a laboratory oscillator.