Steve's 'Barn Door' - Stepper Motor Drive

Stepper Motor

Stepper Motors are like normal Motors with built in fractional turn 'stop/start' circuits ('steps'). Because they are constructed with 'permanent' magnets, a stepper Motor also holds it's position after each 'step'.

Each 'step' is fractional part of a complete turn - so multiple steps are required to achieve each complete revolution. A typical Stepper Motor may have 400 steps per 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 design) or (typically via 1:1 belt drive or gearing) to turn the drive Nut (in the Arc design).

Furthermore, Stepper Motors are largely insensitive to (smallish) 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 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 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 motor starts to move the current 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.

Motor Drive system

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. 

Building (or buying) a stand-alone speed control & Stepper Drive is possible (eg here - although Maplin UK no longer stocks the SAA1027 chip**), however with the accuracy available from Stepper Motors, the phase drive would typically be linked to a laptop computer (usually the Parallel Port is used) to allow software drive control and thus programmed compensation for any mechanical errors in the build / drive screw.

Basic Tangent drive with computer stepper control

Computer control means that it is possible to vary the drive speed with high accuracy.

This means the errors introduced by the Basic Tangent design can be 'programmed out', thus leading to simpler mechanical design (at the expense of complex speed control).

Notes.

** Maplin currently (2008) stocks the L293D chip (here) - however this chip will only drive 2 phase (bi-polar) motors - for circuit see here, for specifications see here.