Example of Beaglebone Black driving a DC motor using only the command line
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README.md

Drive a DC motor from the Beaglebone command line

Summary

In this project the Beaglebone Black drives a DC motor with pulse-width modulation through a motor driver, and measures the motor's shaft angle with a rotary encoder that uses quadrature-encoded pulses.

It does this entirely from the command line; there is no C or Python code needed. This is possible because you can do I/O through special files called "sysfs entries". For example, after loading the PWM device-tree overlay (see below), you can set the PWM duty cycle and period with

echo 100000 > /sys/devices/ocp.3/pwm_test_P8_34.12/period  # nanosec
echo  10000 > /sys/devices/ocp.3/pwm_test_P8_34.12/duty    # nanosec, this is actually 90% duty because of a bug in how the BBB's PWM polarity is set up.

See the Background section for more details on sysfs entries.

The script run-lil-dc-motor.sh shows how to configure the Beaglebone Black to drive the DC motor and read its position with a rotary encoder. The script does this:

  • loads the PWM and EQEP device-tree overlays
  • loads the GPIO sysfs entries
  • runs the motor clockwise
  • runs the motor counter-clockwise
  • sets PWM's "run" to 0 and motor driver's "standby" GPIO pin to low (standby)

Install

Here is the hardware setup:

The motor setup.

  • $3 DC motor from Sparkfun
    • 0-5V
    • Solder capacitors btwn terminals and case to reduce inductive kickback into BBB
      • Without them, sometimes BBB will hang when driving motor & reading ADCs
  • $30 Dual H-bridge motor driver from Sparkfun (model: TB6612FNG)
  • $100 rotary encoder from US Digital (model: S1-1024-250-NE-B-D)
    • Outputs 5V EQEP signal, 4096 EQEP ticks per rev
    • Use bone_eqep1 device-tree overlay
  • transistors for 5V-to-3.3V conversion between rotary encoder and BBB

There is no software setup; just run run-lil-dc-motor.sh.

After the script runs, it will leave the PWM, EQEP, and GPIO sysfs entries enabled.

Background

You can control various peripherals on the Beaglebone Black from the command line. For example, to configure pin P9_31 as an output and set it high (3.3V), do this:

echo 110   > /sys/class/gpio/export   
echo "out" > /sys/class/gpio/gpio110/direction
echo 1     > /sys/class/gpio/gpio110/value

The number 110 is the number of the GPIO that's wired up to header pin P9_31. The mapping between BBB header pins (P9_31) and GPIO numbers (110) is weird, see Derek Molloy's Exploring Beaglebone Figures (pdf 1, pdf 2)

Line 1 creates /sys/class/gpio/gpio110/ with various files inside like direction and value. They are not "real" files on disk; the kernel catches reads/writes to the files inside gpio110/ and invokes a special kernel module to interact with the GPIO hardware. This method of interfacing with hardware is called "sysfs".

Line 2 writes the string "out" to the direction sysfs file. This causes the GPIO kernel module to configure the GPIO as an output.

Line 3 write the string "1" to the value sysfs file. This causes the GPIO KM to set the GPIO to 3.3V (high).

Notes

  • I should probably run both H-bridges in parallel. The driver is rated for 1A continuous, 3A peak, but with 5V across the 2.5-Ohm motor coils we're pushing it.

  • Beaglebone software version:

    uname -a

    Linux beaglebone 3.8.13-bone80 #1 SMP Wed Jun 15 17:03:55 UTC 2016 armv7l GNU/Linux

Credits

Justin Pearson, Jan 2017