EMS1EP Lecture 8 – Pulse Width Modulation (PWM)

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EMS1EP Lecture 8
Pulse Width Modulation (PWM)
Dr. Robert Ross
Overview
(what you should learn today)
•
•
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Analog v’s digital
Theory behind PWM
PWM sample circuit/Demo
Worked examples
Servo Control
Digital Revision – Analog/Digital
• Analog Voltages: Continuous in time and value
• Digital Voltages: Discrete in time and value
Analog/Digital
• The real world is analog
• Most things we measure or control can be
described my continuous values
– Pressure, temperature, acceleration, sound, light
• Computers are digital
• Need to convert these analog voltages as
digital values
Analog/Digital
Digital
Voltages
Physical
Variable
(Sound,
light,
pressure,
temp, ect)
DAC
ADC
Transducer
(Sensor)
Analog
to
Digital
Converter
Microcontroller
(LArduino)
Analog
Voltages
Digital
to
Analog
Converter
Actuator
Control
Physical
Variable
(Sound,
light,
pressure,
temp, ect)
ADC and DAC
• Analog to Digital Converters (ADCs) are used to
convert the analog voltages into a digital value
• Digital to Analog Converters (DACs) are used to
generate analog voltages using digital values
• The LArduino board has internal ADCs and has pins
which can be configured as analog outputs using
Pulse Width Modulation
Quantisation
• The analog values produced by a DAC look analog
but are actually quantised values (not continuous)
• These values are in small steps
• The LArduino has 256 steps (0-255) over the range 05V
• 19.6mV Steps (5V/255=0.0196)
• To improve resolution:
– Lower the voltage range (0-3V)
– Increase the number of bits:
• 8Bits = 255 Steps
• 16Bits = 65535 Steps
PWM Characteristics
• Create a square-wave
• Constant period
• Varying duty cycle (proportional to required
analog voltage)
5V
0V
Generating Analog Voltages
5V
analogWrite(pin, 0);
0% Duty cycle - Average Voltage: 0V
0V
5V
analogWrite(pin, 63);
25% Duty cycle - Average Voltage: 1.25V
0V
5V
analogWrite(pin, 127);
50% Duty cycle - Average Voltage: 2.5V
0V
5V
analogWrite(pin, 191);
75% Duty cycle - Average Voltage: 3.75V
0V
5V
0V
analogWrite(pin, 255);
100% Duty cycle - Average Voltage: 5V
Filtering PWM Signal
Without filtering circuitry
With filtering circuitry:
Pulse Width Modulation (PWM)
• Use:
– analogWrite(pin, <AnalogVoltage>);
– AnalogVoltage can vary from 0-255
• Will toggle the pin really fast high and low – to
produce something more like an analog
voltage need to smooth this out
• Use a Low Pass Filter to smooth the voltage
out to approximate an analog voltage
PWM Applications
• Communication – length of pulse specifies 1’s
and 0’s
• Voltage Regulation
• LED brightness control
• Controlling the speed of motors
• Servo control
PWM on the LArduino
• PWM can be performed
on any of the pins
marked with a ‘P’
• Use pins 5, 6, 9, 10, 11
• Pins used for PWM
don’t need to be setup –
analogWrite does this
automatically
Class Challenge
• Write some code to do the following:
– A switch is connected to Pin 8
– A LED is connected to Pin 6
• If the button is pressed the LED should be at
75% brightness
• If the button is not pressed the LED should be
at 25% brightness
Worked example – Sawtooth signal
• Create a sawtooth signal on Pin 5
• When connected to an LED the LED should
successively fade brighter and dimmer
• When connected to a scope should give
output (when connected to filter):
Sawtooth - Code
int PWMOUT1 = 5; // PIN ASSIGNMENTS
void setup()
{ //PWM pins don't require initialisation
}
void loop()
{
int onTime = 0;
while(onTime < 255){
onTime++;
analogWrite(PWMOUT1, onTime);
delay(10);
}
while(onTime > 0){
onTime--;
analogWrite(PWMOUT1, onTime);
delay(10);
}
}
Sawtooth - Results
No Filtering (Value increased
and then decreased)
LED connected – goes brighter
then dimmer
Servo Control
• Remote control aircraft (and some robotics) are
operated using servos
Servo Control
• Servos need to be attached to pin 9 or 10 on the
LArduino board
• Servos are controlled using a PWM signal
– Period: 20ms
– On time: 1.25 to 1.75ms
– On time sets the position of the servo arm
Servo Control: Hardware connections
Red – Power (4-6V)
Black (or Brown) – GND
Yellow (or White or Orange) – PWM signal pin
Arduino
5V
Servo Worked Example
• If button on Pin 5 is pressed servo to go to 30O
• If button is not pressed servo to go to 120O
• Servo connected to Pin 9
• Using Arduino Servo library
– Allows desired angle of servo to be specified
Servo Worked Example
#include <Servo.h>
Servo servo1; // create servo object to control a servo
int button1 = 5;
void setup()
{
servo1.attach(9); // Servo on Pin 9
pinMode(button1, INPUT);
}
void loop()
{
if (digitalRead(button1) == LOW){ //Button pressed
servo1.write(30);
}
else{ //Button not pressed
servo1.write(120);
}
}
Continuous Rotation Servos
• By default servo motors only have about 180O of
motion that they can travel over
• Servos can be modified for continuous rotation (they
can go right around)
• PWM signal controls speed not position of
continuous rotation servos
• Involves:
– Removing feedback potentiometer (variable resistor)
– Soldering in resistors
– Cutting notch out of gears
Summary
(What you learnt in this session)
• The real world is analog, but our
microcontrollers are digital
• PWM is one type of DAC to generate analog
like voltages from a microcontroller
• Use analogWrite() to set PWM value
• Servos use PWM to set the position of the
servo arm – can use the servo library

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