The BeagleBone Black (BBB) is a $45 credit-card-sized computer that runs embedded Linux. I recently purchased a BBB along with an $8 4-phase stepper motor and driver to start playing around with slightly “closer-to-the-metal” motion control.
Next, I wrote a small, simple python module,
BBpystepper, with a
class for creating and working with a stepper control object. Currently the
control uses full-step drive logic, with wave drive available by changing
Stepper.drivemode. I may add half-stepping in the future, but for now
full-stepping gets the job done.
After installing Adafruit_BBIO and BBpystepper on the BeagleBone, the module can be imported into a Python script or run from a Python interpreter. For example:
>>> from bbpystepper import Stepper >>> mystepper = Stepper() >>> mystepper.rotate(180, 10) # Rotates motor 180 degrees at 10 RPM >>> mystepper.rotate(-180, 5) # Rotates motor back 180 degrees at 5 RPM >>> mystepper.angle 0.0
By default the GPIO pins used are P8_13, P8_14, P8_15, and P8_16. These can be changed by modifying the
By default the
Stepper.steps_per_revparameter is set to 2048 to match my motor (it has a built-in gearbox).
The code doesn’t keep track of where it ends in the sequence of pins. It simply sets all pins low after a move. This means there could be some additional error in the
Stepper.anglevariable if the amount of steps moved is not divisible by 4.
As mentioned previously, half-stepping would be a nice future add-on, along with a more accurate way of keeping track of the motor shaft angle. Another logical next step would be to use one of the BBB’s Programmable Real-Time Units (PRUs) to control the timing more precisely, therefore improving speed accuracy and allowing synchronization with other processes, e.g. data acquisition. However, for now this simple method gets the job done.