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Star tracking mount - Stepper Motors and Stepper Drives

BarnDoor stepper drive

What is a 'Stepper Motor' ?

A Stepper Motor is like a normal ('servo') motor with a built-in 'stop/start' circuit. This allows the motor to turn in small increment's (or 'steps') 'on command'. Because they are constructed with 'permanent' magnets, a stepper Motor holds it's position at each 'step' (so power has to be applied to move from one position to the next, but not to hold position).

Since each 'step' is fractional part of a complete turn, multiple steps are required to achieve each complete revolution. A typical Stepper Motor may require 400 steps to complete one revolution !

This means extremely accurate low speed control can be achieved (1/400 th of a turn) so they can be directly connected to the Barn Door screw bolt (in the 'Basic Tangent' style design) or directly (typically via 1:1 belt drive or gearing) turn the drive Nut (in the Arc design).

Furthermore, Stepper Motors are largely insensitive to changes in the drive voltage. So there is no need for highly accurate voltage regulators etc. = they can be run direct from a battery supply (although the associated digital control electronics may well need a regulated supply).

Finally they are inherently 'digital' - power is only applied when a 'step' is needed and can be removed as soon as the motor is 'committed' to moving onto the next step position.

The drive chip applies a step voltage and measures the current drawn. Initially the current is high as it has to overcome the permanent magnetic force holding the motor at it's current position. However once the rotor starts to move, the current drawn drops drastically and the voltage can be removed)

All this convenience comes at the price of complexity. Stepper Motors are either 2 phase (or 'bi-polar', requiring 2 pairs (4 wires) for bi-directional power drive) or 4 phase (requiring 5 wires, being 4 power 'sink' wires plus a common power 'supply' wire). The order in which power is applied via the drive wires determines the direction of rotation.

How do I drive a stepper motor ?

If 1 turn = 60 seconds and requires 400 steps (= 100 cycles of a 4 phase drive circuit), then each complete '4 phase cycle' will take 6/10th of a second (600 mS).

If we wish to achieve 10% per phase cycle timing accuracy this means control circuits with a response of better than 15 mS (1/10 x 1/4 x 600mS)).

Whilst it may be possible to find Relays with response (activate/release) times down to 10 mS, it would be a very brave decision indeed to try driving a stepper motor using a simple Relay system !

We thus move into the area of electronic / transistor drive systems. It is possible to design your own, but a far better solution would be to simply purchase a pre-built circuit or at least a special purpose stepper drive chip.

Building (or buying) a stand-alone speed control & Stepper Drive is possible (for example, see here - although Maplin UK no longer stocks the SAA1027 chip**)

** Maplin currently (2008) suggests 2008 the L293D chip for stepper drives. However this chip will only drive a 2 phase (bi-polar) motor see circuit diagram and specification

A good place to find a 'second hand' stepper motor (and maybe even a drive chip) is inside an old scrap ink-jet printer - most use stepper motors for the paper feed (the ink head is usually driven by a servo motor).

How is a basic Tangent drive BarnDoor driven by a stepper with computer control ?

A stepper motor is ideal for computer control - individual motor 'phases' can be controlled with high accuracy by timing signals from the Parallel Port (and driven by simple Darlington Pair (or MOSFET) transistors).

Such direct control allows software to vary the drive speed with high accuracy and thus provide compensation for any mechanical errors in the build / drive screw accuracy.

It is even possible to program out the mechanical tracking the errors introduced by a 'Basic Tangent' design. This allows a much simpler mechanical design, but at the expense of complex speed control.

Clicking Next>> (on the navigation bar, left) takes you to my 'Astro-imaging with a barn door' page

Next page :- Astroimaging - (with the Barndoor)