Linux Stepper Motor Control

Introduction

Stepper Motors are unlike standard electrical motors, in that they offer fine-grained control over rotation angle by dividing 360° rotation into a number of "steps". The motor's centre rod is affixed to a magnetic gear, which is surrounded by a number of electomagnets. These electromagnets are energised in phase, causing the rod to rotate. Wikipedia has a good article on the workings of a stepper motor. This interface allows stepping instructions (such as "rotate 3 steps clockwise") to be sent to the motor from the GNU/Linux command line using the computer's parallel interface.

Hardware

The components required for this project can be bought cheaply from electronics retailers such as CPC, Farnell or Maplin.

BitsBox is a brilliant UK-based electronics shop with a great range of components; speedy, cheap delivery and they accept Paypal.
My new favorite!

Schematic
Data output pins 2 through 5 of the computer's parallel port are connected to the input lines of the SN7407 hex buffer chip. As the input lines of the 7407 are taken high by the parallel interface, the corresponding outputs are supplied with current from the 5V supply. Four TIP122 transistors are driven by the outputs of the 7404. As the transistor's base is energised, a 5V pulse is fed to the connected phase of the stepper motor. Each transistor has a 3.3kΩ pull-up resistor. I've included five LEDs, one for power and one connected to the collector of each TIP122. These were originally 100% novelty but proved very useful for debugging both motor phase-order and software.
Construction & Mounting
The prototype of the stepper control interface has been built onto veroboard. Despite various jokes being made about the quality of my construction, (I work in an office full of electronics engineers) I'm pleased with the performance of the board. The next job will be to package the unit in a casing which protects the circuits and is easily mounted on my Magnetic Loop aerial. At present, I'm looking at plastic enclosures so as not to interfere will loop resonance.

Software

There are several ways to drive this parallel interface from GNU/Linux but by far the easiest method is to use Linux's support for userspace parallel port drivers.

The first step is to ensure your kernel has correct parameters set for userspace parallel port access:

Device Drivers --->
   <M> Parallel port support  --->
      <M>   PC-style hardware
   Character Devices --->
      <M> Support for user-space parallel port device drivers

My Motor Control Utility is provided for download as C source code. Provided your Linux distribution includes a C compiler, building the application should be trivial. Compile the code with the simple command:

gcc -Wall -O2 -o stepper ./stepper.c

Usage

Valid syntax for the newly-compiled stepper application is viewed with the -h option:

bash#: $ ./stepper -h
Usage: stepper [options] [byte-value, ...]
    -h, --help			show this help and exit
    -p, --port 			select parallel port device
    -l, --loop-delay 		set loop delay time [us].
    -c, --loop-count 		repeat the sequence  times (-1 = unlimited).
    -F, --full-step		use sequence to drive a stepper motor with full steps.
    -H, --half-step		use sequence to drive a stepper motor with half steps.
    -R, --reverse		stepper motor turns in reverse direction.
    -i, --interactive		interactive fine-tune mode.
    -r, --reset-on-exit		reset the data lines on exiting the program.
    -d, --debug			print debugging information.

Use of the -r option is recommended. Without it, the motor is left on "hold-step" and can become hot after a period of time.

Interactive Mode

When run with the -i option, stepper will listen for key presses from the user. Keys F, R, G & D are used to move the motor in fine steps.

bash#: $ ./stepper -i
Interactive Fine-Tuning Mode.
WARNING - Prolonged use of this mode may heat up your motor!
Controls:
        F --> Half-Step Clockwise.
        R --> Half-Step Anti-Clockwise.
        G --> Full-Step Clockwise
        D --> Full-Step Anti-Clokwise

Press CTRL-D to exit.

Examples

bash#: $ ./stepper -F

bash#: $ ./stepper -c3 -H -R

bash#: $ ./stepper -p /dev/parport1 -r -F