Stepper Motors and Drivers

The box of everything that moves on command. Stepper motors for precise positioning, servos for turning to an angle, drivers for the steppers and a little mechanical hardware to go with them.

Stepper or servo

The difference is fundamental, and the choice shapes the whole design:

• A stepper motor turns exactly the number of steps you command and holds its position while current flows. It does not know where it is: if a step is missed, nobody finds out until you run into an endstop. It needs a driver and usually its own supply, higher than logic voltage.
• A servo is given an angle and reaches it with its own internal feedback. No driver needed — three wires, one of them control. The travel is usually limited, though, and the accuracy depends on how cheap the gearbox inside is.

For a feed, a machine axis, a rotary table — a stepper. For “turn this flap thirty degrees” — a servo.

About drivers and current

A stepper driver is set up for the particular motor, and the main parameter is the current limit. Setting it higher than the winding can take means heating the motor until it smells, and possibly burning the driver.

• Look up the rated current on the motor itself or on its bag.
• Drivers get hot in use, properly hot. The heatsink on a driver chip is not decoration.
• Do not disconnect a motor from a running driver. Breaking a winding under current kills the output stage: it is the most common cause of death for cheap drivers.

Power

A motor does not run off your board. A microcontroller's 5 V rail is good for an unloaded servo at best, and even that will sag the supply enough to reset the controller. Use a separate source: the power supplies are on the same bench.

How to use

1. Take the motor together with its bag: the connector for it is usually in there.
2. Find the rated current and voltage and set the driver to them before first power-up.
3. Test it on the bench, with no mechanics attached, before building it into anything.
4. Put everything back in the box and in the same bag.

Access

The container is in the hackspace room, so a plan that includes hackspace access is required. No induction is needed.