Motors

One of the many fun things about embedded computers is that you can move physical things with motors. But there are so many different kinds of motors (servo, stepper, DC), so how do you select the right one?

The type of motor you use depends on the type of motion you want:

• R/C or hobby servo motor

  Can be quickly positioned at various absolute angles, but some don’t spin. In fact, many can turn only about 180{deg}.

• Stepper motor

  Spins and can also rotate in precise relative angles, such as turning 45°. Stepper motors come in two types: bipolar (which has four wires) and unipolar (which has five or six wires).

• DC motor

  Spins either clockwise or counter-clockwise and can have the greatest speed of the three. But a DC motor can’t easily be made to turn to a given angle.

When you know which type of motor to use, interfacing is easy. This chapter shows how to interface with each of these motors.

Note

Motors come in many sizes and types. This chapter presents some of the more popular types and shows how they can interface easily to the Bone. If you need to turn on and off a 120 V motor, consider using something like the PowerSwitch presented in Toggling a High-Voltage External Device.

Note

The Bone has built-in 3.3 V and 5 V supplies, which can supply enough current to drive some small motors. Many motors, however, draw enough current that an external power supply is needed. Therefore, an external 5 V power supply is listed as optional in many of the recipes.

Note

All the examples in the book assume you have cloned the Cookbook repository on git.beagleboard.org. Go here Cloning the Cookbook Repository for instructions.

Controlling a Servo Motor

Problem

You want to use BeagleBone to control the absolute position of a servo motor.

Solution

We’ll use the pulse width modulation (PWM) hardware of the Bone to control a servo motor.

To make the recipe, you will need:

• Servo motor.

• Breadboard and jumper wires.

• 1 kΩ resistor (optional)

• 5 V power supply (optional)

The 1 kΩ resistor isn’t required, but it provides some protection to the general-purpose input/output (GPIO) pin in case the servo fails and draws a large current.

Wire up your servo, as shown in Driving a servo motor with the 3.3 V power supply.

Note

There is no standard for how servo motor wires are colored. One of my servos is wired like Driving a servo motor with the 3.3 V power supply red is 3.3 V, black is ground, and yellow is the control line. I have another servo that has red as 3.3 V and ground is brown, with the control line being orange. Generally, though, the 3.3 V is in the middle. Check the datasheet for your servo before wiring.

Fig. 527 Driving a servo motor with the 3.3 V power supply

The code for controlling the servo motor is in servoMotor.py, shown in Code for driving a servo motor (servoMotor.py). You need to configure the pin for PWM.

bone$ cd ~/beaglebone-cookbook-code/04motors
bone$ config-pin P9_16 pwm
bone$ ./servoMotor.py

PythonJavaScript

Listing 39 Code for driving a servo motor (servoMotor.py)

#!/usr/bin/env python
# ////////////////////////////////////////
# //	servoMotor.py
# //	Drive a simple servo motor back and forth on P9_16 pin
# //	Wiring:
# //	Setup:  config-pin P9_16 pwm
# //	See:
# ////////////////////////////////////////
import time
import signal
import sys

pwmPeriod = '20000000'    # Period in ns, (20 ms)
pwm =     '1' # pwm to use
channel = 'b' # channel to use
PWMPATH='/dev/bone/pwm/'+pwm+'/'+channel
low  = 0.8 # Smallest angle (in ms)
hi   = 2.4 # Largest angle (in ms)
ms   = 250 # How often to change position, in ms
pos  = 1.5 # Current position, about middle ms)
step = 0.1 # Step size to next position

def signal_handler(sig, frame):
    print('Got SIGINT, turning motor off')
    f = open(PWMPATH+'/enable', 'w')
    f.write('0')
    f.close()
    sys.exit(0)
signal.signal(signal.SIGINT, signal_handler)
print('Hit ^C to stop')

f = open(PWMPATH+'/period', 'w')
f.write(pwmPeriod)
f.close()
f = open(PWMPATH+'/enable', 'w')
f.write('1')
f.close()

f = open(PWMPATH+'/duty_cycle', 'w')
while True:
    pos += step    # Take a step
    if(pos > hi or pos < low):
        step *= -1
    duty_cycle = str(round(pos*1000000))    # Convert ms to ns
    # print('pos = ' + str(pos) + ' duty_cycle = ' + duty_cycle)
    f.seek(0)
    f.write(duty_cycle)
    time.sleep(ms/1000)

# | Pin   | pwm | channel
# | P9_31 | 0   | a
# | P9_29 | 0   | b
# | P9_14 | 1   | a
# | P9_16 | 1   | b
# | P8_19 | 2   | a
# | P8_13 | 2   | b

servoMotor.py

Running the code causes the motor to move back and forth, progressing to successive positions between the two extremes. You will need to press ^C (Ctrl-C) to stop the script.

Controlling a Servo with an Rotary Encoder

Problem

You have a rotary encoder from from chapter 2 rotary encoder example that you want to use to control a servo motor.

Solution

Combine the code from rotaryEncoder.js and servoMotor.js.

bone$ config-pin P9_16 pwm
bone$ config-pin P8_11 eqep
bone$ config-pin P8_12 eqep
bone$ ./servoEncoder.py

Listing 41 Code for driving a servo motor with a rotary encorder(servoEncoder.py)

#!/usr/bin/env python
# ////////////////////////////////////////
# //	servoEncoder.py
# //	Drive a simple servo motor using rotary encoder viq eQEP 
# //	Wiring: Servo on P9_16, rotary encoder on P8_11 and P8_12
# //	Setup:  config-pin P9_16 pwm
# //			config-pin P8_11 eqep
# //			config-pin P8_12 eqep
# //	See:
# ////////////////////////////////////////
import time
import signal
import sys

# Set up encoder
eQEP = '2'
COUNTERPATH = '/dev/bone/counter/counter'+eQEP+'/count0'
maxCount = '180'

ms = 100 	# Time between samples in ms

# Set the eEQP maximum count
fQEP = open(COUNTERPATH+'/ceiling', 'w')
fQEP.write(maxCount)
fQEP.close()

# Enable
fQEP = open(COUNTERPATH+'/enable', 'w')
fQEP.write('1')
fQEP.close()

fQEP = open(COUNTERPATH+'/count', 'r')

# Set up servo
pwmPeriod = '20000000'    # Period in ns, (20 ms)
pwm     = '1'  # pwm to use
channel = 'b'  # channel to use
PWMPATH='/dev/bone/pwm/'+pwm+'/'+channel
low  = 0.6 # Smallest angle (in ms)
hi   = 2.5 # Largest angle (in ms)
ms   = 250 # How often to change position, in ms
pos  = 1.5 # Current position, about middle ms)
step = 0.1 # Step size to next position

def signal_handler(sig, frame):
    print('Got SIGINT, turning motor off')
    f = open(PWMPATH+'/enable', 'w')
    f.write('0')
    f.close()
    sys.exit(0)
signal.signal(signal.SIGINT, signal_handler)

f = open(PWMPATH+'/period', 'w')
f.write(pwmPeriod)
f.close()
f = open(PWMPATH+'/duty_cycle', 'w')
f.write(str(round(int(pwmPeriod)/2)))
f.close()
f = open(PWMPATH+'/enable', 'w')
f.write('1')
f.close()

print('Hit ^C to stop')

olddata = -1
while True:
	fQEP.seek(0)
	data = fQEP.read()[:-1]
	# Print only if data changes
	if data != olddata:
		olddata = data
		# print("data = " + data)
		# # map 0-180  to low-hi
		duty_cycle = -1*int(data)*(hi-low)/180.0 + hi
		duty_cycle = str(int(duty_cycle*1000000))	# Convert from ms to ns
		# print('duty_cycle = ' + duty_cycle)
		f = open(PWMPATH+'/duty_cycle', 'w')
		f.write(duty_cycle)
		f.close()
	time.sleep(ms/1000)

# Black OR Pocket
# eQEP0:	P9.27 and P9.42 OR P1_33 and P2_34
# eQEP1:	P9.33 and P9.35
# eQEP2:	P8.11 and P8.12 OR P2_24 and P2_33

# AI
# eQEP1:	P8.33 and P8.35
# eQEP2:	P8.11 and P8.12 or P9.19 and P9.41
# eQEP3:	P8.24 and P8.25 or P9.27 and P9.42

# | Pin   | pwm | channel
# | P9_31 | 0   | a
# | P9_29 | 0   | b
# | P9_14 | 1   | a
# | P9_16 | 1   | b
# | P8_19 | 2   | a
# | P8_13 | 2   | b

servoEncoder.py

Controlling the Speed of a DC Motor

Problem

You have a DC motor (or a solenoid) and want a simple way to control its speed, but not the direction.

Solution

It would be nice if you could just wire the DC motor to BeagleBone Black and have it work, but it won’t. Most motors require more current than the GPIO ports on the Bone can supply. Our solution is to use a transistor to control the current to the bone.

Here we configure the encoder to returns value between 0 and 180 inclusive. This value is then mapped to a value between min (0.6 ms) and max (2.5 ms). This number is converted from milliseconds and nanoseconds (time 1000000) and sent to the servo motor via the pwm.

Here’s what you will need:

• 3 V to 5 V DC motor

• Breadboard and jumper wires.

• 1 kΩ resistor.

• Transistor 2N3904.

• Diode 1N4001.

• Power supply for the motor (optional)

If you are using a larger motor (more current), you will need to use a larger transistor.

Wire your breadboard as shown in Wiring a DC motor to spin one direction.

Fig. 528 Wiring a DC motor to spin one direction

Use the code in Driving a DC motor in one direction (dcMotor.py) to run the motor.

PythonJavaScript

Listing 42 Driving a DC motor in one direction (dcMotor.py)

#!/usr/bin/env python
# ////////////////////////////////////////
# //	dcMotor.js
# //	This is an example of driving a DC motor
# //	Wiring:
# //	Setup:  config-pin P9_16 pwm
# //	See:
# ////////////////////////////////////////
import time
import signal
import sys

def signal_handler(sig, frame):
    print('Got SIGINT, turning motor off')
    f = open(PWMPATH+'/enable', 'w')
    f.write('0')
    f.close()
    sys.exit(0)
signal.signal(signal.SIGINT, signal_handler)

pwmPeriod = '1000000'    # Period in ns
pwm     = '1'  # pwm to use
channel = 'b'  # channel to use
PWMPATH='/dev/bone/pwm/'+pwm+'/'+channel

low = 0.05     # Slowest speed (duty cycle)
hi  = 1        # Fastest (always on)
ms = 100       # How often to change speed, in ms
speed = 0.5    # Current speed
step = 0.05    # Change in speed

f = open(PWMPATH+'/duty_cycle', 'w')
f.write('0')
f.close()
f = open(PWMPATH+'/period', 'w')
f.write(pwmPeriod)
f.close()
f = open(PWMPATH+'/enable', 'w')
f.write('1')
f.close()

f = open(PWMPATH+'/duty_cycle', 'w')
while True:
    speed += step
    if(speed > hi or speed < low):
        step *= -1
    duty_cycle = str(round(speed*1000000))    # Convert ms to ns
    f.seek(0)
    f.write(duty_cycle)
    time.sleep(ms/1000)

dcMotor.py

See Also

How do you change the direction of the motor? See Controlling the Speed and Direction of a DC Motor.

Controlling the Speed and Direction of a DC Motor

Problem

You would like your DC motor to go forward and backward.

Solution

Use an H-bridge to switch the terminals on the motor so that it will run both backward and forward. We’ll use the L293D a common, single-chip H-bridge.

Here’s what you will need:

• 3 V to 5 V motor.

• Breadboard and jumper wires.

• L293D H-Bridge IC.

• Power supply for the motor (optional)

Lay out your breadboard as shown in Driving a DC motor with an H-bridge. Ensure that the L293D is positioned correctly. There is a notch on one end that should be pointed up.

Fig. 529 Driving a DC motor with an H-bridge

The code in Code for driving a DC motor with an H-bridge (h-bridgeMotor.js) (h-bridgeMotor.js) looks much like the code for driving the DC motor with a transistor (Driving a DC motor in one direction (dcMotor.js)). The additional code specifies which direction to spin the motor.

Listing 44 Code for driving a DC motor with an H-bridge (h-bridgeMotor.js)

#!/usr/bin/env node

// This example uses an H-bridge to drive a DC motor in two directions

var b = require('bonescript');

var enable = 'P9_21';    // Pin to use for PWM speed control
    in1    = 'P9_15',
    in2    = 'P9_16',
    step = 0.05,    // Change in speed
    min  = 0.05,    // Min duty cycle
    max  = 1.0,     // Max duty cycle
    ms   = 100,     // Update time, in ms
    speed = min;    // Current speed;

b.pinMode(enable, b.ANALOG_OUTPUT, 6, 0, 0, doInterval);
b.pinMode(in1, b.OUTPUT);
b.pinMode(in2, b.OUTPUT);

function doInterval(x) {
    if(x.err) {
        console.log('x.err = ' + x.err);
        return;
    }
    timer = setInterval(sweep, ms);
}

clockwise();        // Start by going clockwise

function sweep() {
    speed += step;
    if(speed > max || speed < min) {
        step *= -1;
        step>0 ? clockwise() : counterClockwise();
    }
    b.analogWrite(enable, speed);
    console.log('speed = ' + speed);
}

function clockwise() {
    b.digitalWrite(in1, b.HIGH);
    b.digitalWrite(in2, b.LOW);
}

function counterClockwise() {
    b.digitalWrite(in1, b.LOW);
    b.digitalWrite(in2, b.HIGH);
}

process.on('SIGINT', function() {
    console.log('Got SIGINT, turning motor off');
    clearInterval(timer);         // Stop the timer
    b.analogWrite(enable, 0);     // Turn motor off
});

h-bridgeMotor.js

Driving a Bipolar Stepper Motor

Problem

You want to drive a stepper motor that has four wires.

Solution

Use an L293D H-bridge. The bipolar stepper motor requires us to reverse the coils, so we need to use an H-bridge.

Here’s what you will need:

• Breadboard and jumper wires.

• 3 V to 5 V bipolar stepper motor.

• L293D H-Bridge IC.

Wire as shown in Bipolar stepper motor wiring.

Fig. 530 Bipolar stepper motor wiring

Use the code in Driving a bipolar stepper motor (bipolarStepperMotor.py) to drive the motor.

Listing 45 Driving a bipolar stepper motor (bipolarStepperMotor.py)

#!/usr/bin/env python
import time
import os
import signal
import sys

# Motor is attached here
# controller = ["P9_11", "P9_13", "P9_15", "P9_17"]; 
# controller = ["30", "31", "48", "5"]
# controller = ["P9_14", "P9_16", "P9_18", "P9_22"]; 
controller = ["50", "51", "4", "2"]
states = [[1,0,0,0], [0,1,0,0], [0,0,1,0], [0,0,0,1]]
statesHiTorque = [[1,1,0,0], [0,1,1,0], [0,0,1,1], [1,0,0,1]]
statesHalfStep = [[1,0,0,0], [1,1,0,0], [0,1,0,0], [0,1,1,0],
                      [0,0,1,0], [0,0,1,1], [0,0,0,1], [1,0,0,1]]

curState = 0    # Current state
ms = 100        # Time between steps, in ms
maxStep = 22    # Number of steps to turn before turning around
minStep = 0     # minimum step to turn back around on

CW  =  1       # Clockwise
CCW = -1
pos =  0       # current position and direction
direction = CW
GPIOPATH="/sys/class/gpio"

def signal_handler(sig, frame):
    print('Got SIGINT, turning motor off')
    for i in range(len(controller)) :
        f = open(GPIOPATH+"/gpio"+controller[i]+"/value", "w")
        f.write('0')
        f.close()
    sys.exit(0)
signal.signal(signal.SIGINT, signal_handler)
print('Hit ^C to stop')

def move():
    global pos
    global direction
    global minStep
    global maxStep
    pos += direction
    print("pos: " + str(pos))
    # Switch directions if at end.
    if (pos >= maxStep or pos <= minStep) :
        direction *= -1
    rotate(direction)

# This is the general rotate
def rotate(direction) :
    global curState
    global states
	# print("rotate(%d)", direction);
    # Rotate the state according to the direction of rotation
    curState +=  direction
    if(curState >= len(states)) :
        curState = 0;
    elif(curState<0) :
        curState = len(states)-1
    updateState(states[curState])

# Write the current input state to the controller
def updateState(state) :
    global controller
    print(state)
    for i in range(len(controller)) :
        f = open(GPIOPATH+"/gpio"+controller[i]+"/value", "w")
        f.write(str(state[i]))
        f.close()

# Initialize motor control pins to be OUTPUTs
for i in range(len(controller)) :
    # Make sure pin is exported
    if (not os.path.exists(GPIOPATH+"/gpio"+controller[i])):
        f = open(GPIOPATH+"/export", "w")
        f.write(pin)
        f.close()
    # Make it an output pin
    f = open(GPIOPATH+"/gpio"+controller[i]+"/direction", "w")
    f.write("out")
    f.close()

# Put the motor into a known state
updateState(states[0])
rotate(direction)

# Rotate
while True:
    move()
    time.sleep(ms/1000)

bipolarStepperMotor.py

When you run the code, the stepper motor will rotate back and forth.

Driving a Unipolar Stepper Motor

Problem

You want to drive a stepper motor that has five or six wires.

Solution

If your stepper motor has five or six wires, it’s a unipolar stepper and is wired differently than the bipolar. Here, we’ll use a ULN2003 Darlington Transistor Array IC to drive the motor.

Here’s what you will need:

• Breadboard and jumper wires.

• 3 V to 5 V unipolar stepper motor.

• ULN2003 Darlington Transistor Array IC.

Wire, as shown in Unipolar stepper motor wiring.

Note

The IC in Unipolar stepper motor wiring is illustrated upside down from the way it is usually displayed.

That is, the notch for pin 1 is on the bottom. This made drawing the diagram much cleaner.

Also, notice the banded wire running the P9_7 (5 V) to the UL2003A. The stepper motor I’m using runs better at 5 V, so I’m using the Bone’s 5 V power supply. The signal coming from the GPIO pins is 3.3 V, but the U2003A will step them up to 5 V to drive the motor.

Fig. 531 Unipolar stepper motor wiring

The code for driving the motor is in unipolarStepperMotor.js however, it is almost identical to the bipolar stepper code (Driving a bipolar stepper motor (bipolarStepperMotor.py)), so Changes to bipolar code to drive a unipolar stepper motor (unipolarStepperMotor.js.diff) shows only the lines that you need to change.

Listing 46 Changes to bipolar code to drive a unipolar stepper motor (unipolarStepperMotor.py.diff)

# controller = ["P9_11", "P9_13", "P9_15", "P9_17"]
controller = ["30", "31", "48", "5"]
states = [[1,1,0,0], [0,1,1,0], [0,0,1,1], [1,0,0,1]]
curState = 0   // Current state
ms = 100       // Time between steps, in ms
max = 200      // Number of steps to turn before turning around

unipolarStepperMotor.py.diff

Listing 47 Changes to bipolar code to drive a unipolar stepper motor (unipolarStepperMotor.js.diff)

# var controller = ["P9_11", "P9_13", "P9_15", "P9_17"];
controller = ["30", "31", "48", "5"]
var states = [[1,1,0,0], [0,1,1,0], [0,0,1,1], [1,0,0,1]];
var curState = 0;   // Current state
var ms = 100,       // Time between steps, in ms
    max = 200,      // Number of steps to turn before turning around

unipolarStepperMotor.js.diff

The code in this example makes the following changes:

• The states are different. Here, we have two pins high at a time.

• The time between steps (ms) is shorter, and the number of steps per direction (max) is bigger. The unipolar stepper I’m using has many more steps per rotation, so I need more steps to make it go around.