EMS1EP Lecture 8 – Pulse Width Modulation (PWM)

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Transcript EMS1EP Lecture 8 – Pulse Width Modulation (PWM)

EMS1EP Lecture 8 Pulse Width Modulation (PWM) Dr. Robert Ross

Overview

(what you should learn today) • • • • • 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 Transducer (Sensor) (Sound, light, pressure, temp, ect)

ADC

Analog to Digital Converter

Microcontroller

(LArduino)

DAC

Digital to Analog Converter Actuator Control Physical Variable (Sound, light, pressure, temp, ect) Analog Voltages

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 0 5V 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

5V 0V 5V 0V 5V 0V 5V 0V 5V 0V

Generating Analog Voltages

analogWrite(pin, 0);

0% Duty cycle - Average Voltage: 0V

analogWrite(pin, 63);

25% Duty cycle - Average Voltage: 1.25V

analogWrite(pin, 127); 5

0% Duty cycle - Average Voltage: 2.5V

analogWrite(pin, 191);

75% Duty cycle - Average Voltage: 3.75V

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 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 30 O If button is not pressed servo to go to 120 O Servo connected to Pin 9 • Using Arduino Servo library – Allows desired angle of servo to be specified

Servo Worked Example

#include 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 180 O motion that they can travel over of 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