Distributed Real- Time Control Systems. Lecture 3 Embedded Systems Interfacing the OuterWorld

Transcription

1 Distributed Real- Time Control Systems Lecture 3 Embedded Systems Interfacing the OuterWorld 1

2 Bibliography ATMEGA 328 Datasheet. arduino.cc Book: Arduino Cookbook, 2nd Ed. Michael Margolis O Reilly,

3 Interfaces I/O Digital I/O Analog Input PWM Serial (USART) SPI TWI 3

4 Digital I/O Port function (input/output) is programmablebyenablingand disabling some switches. 4

5 Output Modes Output High Output Low 5

6 Input Modes Input floating (high impedance) Input with Internal Pull Up 6

7 Digital I/O Registers 7

8 Basic Digital Output - Driving LED s Caution! Source/Sink limits 40mA at5vcc How much current passes through the LED s? const int led1 = 7; // LED1 connected to digital pin 7 const int led2 = 8; // LED2 connected to digital pin 8 const int rate = 500; // Delay in ms to commute leds int counter = 0; // loop counter void setup() // enable output on the led1 pin pinmode(led1, OUTPUT); // enable output on the led2 pin pinmode(led2, OUTPUT); // initialize serial comms to host Serial.begin(9600); void loop() Serial.println(++counter); // print rate to serial monitor digitalwrite(led1, HIGH); // set the LED1 on digitalwrite(led2, HIGH); // set the LED2 off delay(rate); // wait duration digitalwrite(led1, LOW); // set the LED1 off digitalwrite(led2, LOW); // set the LED2 on delay(rate); 8

9 Basic Digital Input Reading Switches Must use pull- ups. int inputpins[] = 2,3,4,5; // create an array of switch inputs int ledpins[] = 10,11,12,13; // create array of output LEDs void setup() for(int index = 0; index < 4; index++) pinmode(ledpins[index], OUTPUT); // LED as output pinmode(inputpins[index], INPUT); // pushbutton as input digitalwrite(inputpins[index],high); // enable pull- ups void loop() for(int index = 0; index < 4; index++) int val = digitalread(inputpins[index]); // read input value // check if the switch is pressed if (val == LOW) // turn LED on if switch is pressed digitalwrite(ledpins[index], HIGH); else // turn LED off digitalwrite(ledpins[index], LOW); 9

10 Digital Inputs Switch Debouncing // debounce returns true if the switch in the given pin // is closed and stable boolean debounce(int pin) boolean state; boolean previousstate; // store switch state previousstate = digitalread(pin); for(int counter=0; counter < debouncedelay; counter++) // wait for 1 millisecond delay(1); // read the pin state = digitalread(pin); if( state!= previousstate) // reset the counter if the state changes counter = 0; // and save the current state previousstate = state; // here when the switch state has been stable // longer than the debounce period return state; 10

11 Reading Incremental Encoders Incremental RotaryEncoders have two quadrature channels. Direction of rotation can be measured by detectingthe raising/falling edge of one channel and checking the value of the other. 11

12 Reading Incremental Encoders Polling based solution const int encpina = 4; const int encpinb = 2; const int encspr=16; //Steps Per Revolution : Depends on HW int angle = 0; int val; int encpos = 0; boolean encalast = LOW; // remembers the previous state void setup() pinmode(encpina, INPUT); pinmode(encpinb, INPUT); digitalwrite(encpina, HIGH); //enable pull ups digitalwrite(encpinb, HIGH); Serial.begin(9600); void loop() boolean enca = digitalread(encpina); if ((encalast == HIGH) && (enca == LOW)) (digitalread(encpinb) == LOW? encpos- - : encpos++); angle=(encpos%encspr)*360/encspr; Serial.print(encoderPos); Serial.print(" "); Serial.println(angle); encoderalast = encodera; 12

13 Reading Incremental Encoders Interrupt Based Solution An Interrupt Service Routine (ISR) can associated to a falling edge on encoder channel A. Notes: Only one ISR can run at a time. ISR cannot have paramenters nor return values. Variables that are changed in ISRs must be marked volatile. Code in the ISR should be as fast as possible. Serial data received while in the ISR may be lost. In Arduino UNO you can attachinterrupts to Pins 2 and 3. Function millis() do not increment while in an ISR. Function delay() does not work inside and ISR. Function delaymicroseconds() does not use conters so it works. const int encoderpina = 2; const int encoderpinb = 4; int Pos, oldpos; volatile int encoderpos = 0; //maychange at any time void doencoder() //ISR for FALLING edge on channel A if (digitalread(encoderpina) == digitalread(encoderpinb)) encoderpos++; // count up if both encoder pins are the same else encoderpos- - ; // count down if pins are different void setup() pinmode(encoderpina, INPUT); pinmode(encoderpinb, INPUT); digitalwrite(encoderpina, HIGH); digitalwrite(encoderpinb, HIGH); Serial.begin(9600); attachinterrupt( digitalpintointerrupt( encoderpina ), doencoder, FALLING); void loop() nointerrupts(); //disable interrupts Pos = encoderpos; //critical section interrupts(); //enable interrupts if(pos!= oldpos) Serial.println(Pos,DEC); oldpos = Pos; delay(1000); 13

14 Reading Pulse Width Signal Some sensorscommunicate output via Pulse Width Duration. Arduino has a function (pulsein()) to read pulse durations. Example: Parallax PING))) Sonar. const int pingpin = 5; void setup() Serial.begin(9600); void loop() int cm = ping(pingpin) ; Serial.println(cm); delay(500); int ping(int pingpin) long duration, cm; // The PING))) is triggered by a HIGH pulse of 2 or more usec. pinmode(pingpin, OUTPUT); digitalwrite(pingpin, LOW); delaymicroseconds(2); digitalwrite(pingpin, HIGH); delaymicroseconds(5); digitalwrite(pingpin, LOW); pinmode(pingpin, INPUT); duration = pulsein(pingpin, HIGH); // speed of sound 29 usec per centimeter. cm = duration/29/2; return cm ; 14

15 Analog Input ARDUINO UNO ATMEL 328 ADC Characteristics 10- bit Resolution (0x000 0x3FF) 0.5 LSB Integral Non- linearity ± 2 LSB Absolute Accuracy μs Conversion Time Free Running or Single Conversion Mode Interrupt on ADC Conversion Complete Sleep Mode Noise Canceler The ADC is optimized for analog signals with an output impedance of approximately 10 kω or less. Up to 76.9 ksps (Up to 15 ksps at Maximum Resolution) 6 Multiplexed Single Ended Input Channels Temperature Sensor Input Channel 0 - VCC ADC Input Voltage Range Selectable 1.1V ADC ReferenceVoltage Voltage Range Externally Selectable 15

16 Analog Input Example Light depedent LED blinking. Analog Read returns values between 0 (min) and 1024 (max). const int ledpin = 13; // LED connected to digital pin 13 const int sensorpin = 0; // connect sensor to analog input 0 // the next two lines set the min and max delay between blinks const int minduration = 100; // minimum wait between blinks const int maxduration= 1000; // maximum wait between blinks void setup() pinmode(ledpin, OUTPUT); // enable output on the led pin Serial.begin(9600); // initialize Serial void loop() int rate = analogread(sensorpin); // read the analog input //scales the blink rate between the min and max values rate = map(rate, 200, 800, minduration, maxduration); rate = constrain(rate, minduration,maxduration); // saturate Serial.println(rate); // print rate to serial monitor digitalwrite(ledpin, HIGH); // set the LED on delay(rate); // wait duration dependent on light level digitalwrite(ledpin, LOW); // set the LED off delay(rate); 16

17 Measuring Voltages If voltages are in the range [0 5]V, they can bemeasured directly. Otherwise we need voltage dividers. Question: How to measure negative voltages? 17

18 Analog Output via PWM Analog Output can be emulated via Pulse Width Modulation. Arduino function analogwrite(pin, value) generates a PWM signal with frequency ~500Hz and duty cycle proportionalto value. 0 <= value <= 255 pin = 3,5,6,9,10,11 18

19 Modulate LED Brightness via PWM Fade LED brightness in and out. const int firstled= 3; const int secondled = 5; const int thirdled = 6; int brightness = 0; int increment = 1; void setup() // pins driven by analogwrite do not need to be declared as outputs void loop() if(brightness > 255) increment = - 1; // count down after reaching 255 else if(brightness < 1) increment = 1; // count up after dropping back down to 0 brightness = brightness + increment; // write the brightness value to the LEDs analogwrite(firstled, brightness); analogwrite(secondled, brightness); analogwrite(thirdled, brightness ); delay(10); // mean 2.55 secs to fade up or down 19

20 Generating Frequencies Example: Playing Tones const int speakerpin = 9; // connect speaker to pin 9 const int pitchpin = 0; // frequency of the tone const int durationpin = 1; // duration of the tone void setup() void loop() int sensor0reading = analogread(pitchpin); // frequency int sensor1reading = analogread(durationpin); // duration // map the analog readings to a meaningful range int frequency = map(sensor0reading, 0, 1023, 100,5000); int duration = map(sensor1reading, 0, 1023, 100,1000); tone(speakerpin, frequency, duration); // play the tone delay(duration); //wait for the tone to finish 20

21 Driving High Power Devices Arduino PINs can source/sink 40 ma at most. To drive more than 40mA: Use amplification (transistors, opamps, etc). Check datasheets. Use multiple pins (limited to 400mA if USB power). 21

22 Drive a DC Motor in a Single Direction When the motor does not need to reverse direction, a simple PWM drive is as follows: 22

23 Drive a DC motor in both directions To drive a DC motor in both directions we need what is typically called a H- Bridge. Solutions: make a H- bridge with discrete components. Buy an IC with H- bridges. Buy a motor shield for arduino. 23

24 Drive a DC motor in both directions Example with the L239 H- Bridge. const int enpin = 5; // H- Bridge enable pin const int in1pin = 7; // H- Bridge input pins const int in2pin = 4; void setup() Serial.begin(9600); pinmode(in1pin, OUTPUT); pinmode(in2pin, OUTPUT); Serial.println("Speed (0-9) or + - to set direction"); void loop() if ( Serial.available()) char ch = Serial.read(); if(isdigit(ch)) // is ch a number? int speed = map(ch, '0', '9', 0, 255); analogwrite(enpin, speed); else if (ch == '+') //Rotate CW digitalwrite(in1pin,low); digitalwrite(in2pin,high); else if (ch == '- ') //Rotate CCW digitalwrite(in1pin,high); digitalwrite(in2pin,low); else Serial.print("Unexpectedcharacter "); Serial.println(ch); 24

25 Hobbyist Servo Motors HobbyistServo Motors use a potentiometer attached to a DC motor shaft to perform position control. The position reference is commanded bypulse Position Modulation PPM and is not the same as the analogwrite() PWM signal. Onecan use the Servo library to implement the PPM signals. 25

26 Drive a Hobby Servo Externalpower may be required. Do not forget to connect the Grounds. The Servo library uses Timer 1, thus disables PWM on pins 9 and 10. #include <Servo.h> Servo myservo; // create servo object to control a servo int angle = 0; // variable to store the servo position void setup() myservo.attach(9,1000,2000); //Servo on pin 9. //Minimal position corresponds to 1000microsec. //Maximal position corresponds to 2000microsec. //These values may need to be adjusted, // depending on the servo model. void loop() for(angle = 0; angle < 180; angle += 1) myservo.write(angle); delay(20); for(angle = 180; angle >= 1; angle - = 1) myservo.write(angle); delay(20); 26

27 Devices with Different Voltage Levels Do not connect the ARDUINO to devices with other voltage levels. This may damage the I/O ports. One way to prevent damage is to use opto- couplers. Example: control a TRS camera shutter. //TAKE 20 PICTURES AND EXIT int focus = 4; //optocoupler attached to focus int shutter = 3; //optocoupler attached to shutter long exposure = 250; //exposure time in milliseconds long interval = 10000; //time between shots, in milliseconds void setup() pinmode(focus, OUTPUT); pinmode(shutter, OUTPUT); for (int i=0; i<20; i++) //camera will take 20 pictures takepicture(exposure); //takes picture delay(interval); //wait to take the next picture void loop() //NO LOOP void takepicture(long exposuretime) int wakeup = 10; //camera take time to wake up and focus //adjust this to suit your camera digitalwrite(focus, HIGH); //wake the camera and focus delay(wakeup); //wait for it to wake up and focus digitalwrite(shutter, HIGH); //open the shutter delay(exposuretime); //wait for the exposure time digitalwrite(shutter, LOW); //release shutter digitalwrite(focus, LOW); //release the focus 27

28 Final Note Always read the documentation (datasheets) of the devices to interface! 28