Stepper motor advice for absolute novice

[djc]

...so with this in mind I plan on using some 42mm 0.9deg stepper motors available from RS components. I will also be using absolute rotary encoders to obtain accurate position measurement.

I don't know how cost comes into your project, but there are cheaper places than RS for steppers (e.g. ArcEuroTrade). Similarly, you probably don't need an absolute encoder, just an incremental one (after all, once it has gone round 360 degrees, it's indistinguishable from its starting position). The encoder you link to has sine-cosine output; this is quite difficult to interface. Simple quadrature output is probably adequate.

...do any of you have any particularly useful links to something along the lines of "Stepper Motors for Dummies"? A lot of the stuff I've seen online doesn't explain the practical steps of getting the motor to turn by some specified number steps and what equipment I'll need to drive them (do I need a computer, do I just connect the motors to some kind of signal generator; I just don't know!).

Google for 'Jones on stepper motors' for the most-cited reference. At it's most basic, you will need two inputs: the first to create the signal that tells the motor 'move one step'; the second to tell it in which direction to move. These can be as simple as a couple of switches. The state of this switch feeds into a stepper motor driver which for all intents and purposes is a black box that converts the input signal into the correct form, direction and level of electricity to turn the motor.

Making the stepper motor move is the easy part of your project: a driver, a motor and a power supply. These are plug and play. It is the stuff upstream of that you need to think of (i.e. how many steps do I need; in what direction; how fast (max speed); how fast should the speed change - acceleration and deceleration?).

Generally, steppers are not used with encoders - one tells the stepper how much to move and assumes it does what it's told (i.e. open loop control). If the motor size, speed and acceleration is designed correctly, they should not lose position (lose steps). If you want to close the loop with the encoders, you have a further set of calculations to perform to determine what to do if the current position does not match the commanded position (e.g. E-stop, panic, spontaneous combustion, try to recover position - push harder). You then move into the territory of servo motors, a whole different game.