1. Arduino Application: Speed control of small DC Motors
ME 120
Mechanical and Materials Engineering
Portland State University
Fall 2017

2. Learning Objectives
Be able to describe the use of PWM for motor speed control
Be able to explain the role of a snubber diode
Be able to implement PWM speed control of a DC motor
Be able to use the H-Bridge with the Sparkfun Tinker Kit to control a small DC Motor
Additional references:
Experiment 9 in the Sparkfun Tinker Kit experiment guide
ME 120 course notes on PWM
motors/overview

3. Using a transistor as a high speed switch

4. Transistor as a switching device
Each Arduino output channel has a 40 mA limit
Only current to power a very small DC motor
Arduino is not designed as a power supply
Maximum current draw for an Arduino is 200 mA
Use the Arduino as the brain
Let another switching element be the brawn

5. Use an NPN Transistor as a switch
This device is designed for use as a medium power amplifier and switch requiring collector currents up to 500 mA

6. Use Digital I/O pin to switch LED on/off
Digital I/O pin → LED

7. Code to control brightness of an LED
int LED_pin = 11; // PWM pin LED or motor control
void setup() {
pinMode( LED_pin, OUTPUT ); }
void loop() {
int duty, pot_pin=0, reading;
reading = analogRead(pot_pin); // read potentiometer
duty = map(reading,0,1023,0,255); // rescale to 8-bit
duty = constrain(duty,0,255); // be safe
analogWrite(LED_pin,duty); // set duty cycle
In the following examples, the Arduino code does not need to change when the electrical circuit is changed. The Arduino code only needs to used a single digital output pin, which in this code is LED_pin.

8. Use a Transistor to switch LED on/off
Digital I/O pin → Transistor → LED

9. NPN Transistors as Switches
Transistors can be used as switches: By applying relatively small voltage to the base, electrical current will flow from the collector to the emitter.
C is the collector
B is the base
E is the emitter

10. NPN Transistors as Switches
When used as a switch, ICE, the current from the collector to the emitter is large compare to IBE, the current from the base to the emitter.
C is the collector
B is the base
E is the emitter

11. What is a snubber diode, and why should I care?

12. Simplest DC Motor Circuit
Connect the motor to a DC power supply

13. Current continues after the switch is opened
Opening the switch does not immediately stop current from flowing in the motor windings

14. Reverse current
Charge build-up can cause damage

15. Motor Model Simple model of a DC motor:
Windings have inductance and resistance
Electrical energy is stored in the windings – the inductance effect
We need a way to safely dissipate electrical energy when the switch is opened after the motor has been running

16. Flyback diode or snubber diode
Adding a diode in parallel with the motor provides a path for the dissipation of stored energy when the switch is opened

17. DC motor speed control circuit
The circuit for DC motor speed control uses the idea from the LED brightness control circuit. Replace the LED circuit with the DC motor and snubber diode

18. Motor control with an H-Bridge

19. H-Bridges simplifies motor control and enable features not possible with a single transistor
An H-bridge
Uses logic-level signals to switch higher current power to the motor – just like a transistor
Allows polarity of motor power, and hence direction of rotation, to be reversed,
Includes fly-back diodes (snubbers)
Is available in a single IC package
Image of L293DNE quadruple half-H
motor driver from Sparkfun.com

20. Basic H-Bridge has four switches
Note that there are many ways to implement this concept. For example, 24 different
designs are shown at

21. Closing two switches supplies power that causes motor to turn one way

22. Closing two other switches supplies power that causes motor to turn the opposite way

23. Using the H-bridge from the Tinker kit
Motor +
Motor –

24. Using the H-bridge from the Tinker kit

25. Using the H-bridge from the Tinker kit
//define the two direction logic pins and the speed / PWM pin
const int DIR_A = 5;
const int DIR_B = 4;
const int PWM_pin = 6;
void setup(){
// -- set all motor control pins as output
pinMode(DIR_A, OUTPUT);
pinMode(DIR_B, OUTPUT);
pinMode(PWM_pin, OUTPUT);
Serial.begin(9600); // Used to display motor speed values }
// Code continued on next slide

26. Using the H-bridge from the Tinker kit
// Code continued from previous slide
void loop(){
int motorSpeed, potpin=3, potval;
// -- Set control lines to drive forward
digitalWrite(DIR_A, HIGH);
digitalWrite(DIR_B, LOW);
// -- Read potentiometer to set PWM for motor speed
potval = analogRead(potpin);
motorSpeed = map( potval, 0, 1023, 0, 255);
motorSpeed = constrain( motorSpeed, 0, 255);
analogWrite(PWM_pin, motorSpeed);
Serial.print(potval);
Serial.print(" ");
Serial.println(motorSpeed); }

27. Next Steps After getting the basic code to work
Find the minimum PWM setting that will make the motor spin
Use the minimum PWM setting in the constrain() command
Make a servo sweep back and forth while the DC motor is running
Add a button to change direction of the motor