Experiments with Stepper Motors

Deal Extreme has a large number of electronic modules for very low prices and, in most cases, shipping is free. Among these modules is the break out board for an A4988 Stepper Motor Controller. This tiny device can switch up to 2A at 35v, according it its data sheet.

From other sources (inoperable photocopiers) I have several stepper motors. My initial experiment is to see whether I can mount two of these motors back to back, stick some wheels on them and make a small, steerable robot. This has been done many times and the Internet is full of write ups and how tos so it shouldn't be too difficult to achieve.

Step 1 - Stepper Motor Assessment
The model I have is a 14PM-M247. This is a hybrid stepper motor, performing 1.8 degrees per step and rated at 24v. The product page lists 2 versions of this model. One is uni-polar and other is bi-polar. My motor doesn't specify which one it is.

Unipolar vs Bipolar are quite different motors, needing different signal patterns to drive the motor. It turns out that 6 pin motors (such as this one) are universal, or can be used in either a unipolar or bipolar mode.

While the motor has 6 pins, the plug it came with has only 4 wires.The two common pins (2 and 5) are missing. This means it was being used in a bipolar mode.

Pin numbers start at the right hand side, when looking down on the motor with the shaft up. 1 and 3 are the B coil and 4 and 6 are the A coil (A -> B is left to right and 1 -> 6 is right to left...confused?)

Step 2 - Driver Assessment
The driver has two electrical sides to it. A low voltage (logic level) side of 3 - 5.5v and high voltage side (motor level) of 8 to 35.

In the minimal configuration one logic level connection, to signal when to step, is all that is needed on the low side and 4 connections to the motor on the high side. It should be noted that the direction pin should be connected to ground or power, if not connected to a logic pin, otherwise results will be unpredictable.

The drive also supports microstepping, which allows a greater degree of control, although accuracy may suffer. I'm not interested in microstepping at this point so I'll ignore these inputs.

Current limiting is an important aspect that is well covered in this video. The basic message is to not skip the step of adjusting the current limiter to match the motor.


Summary

Feature	Motor	Driver	Compatible?
Voltage	24v	8v - 35v	Yes
Current	0.8A	2A	Yes
Style	Universal	Bipolar	Yes

Overall, it seems that these devices should work together. We'll see...