main.cpp

00001 /** A demo application to show how to mangage and control a heater element 
00002  * through a PID loop using a Thermistor input and PWM output
00003  * for the @NXP (@freescale) FRDM-K64F demo board.
00004  *
00005  * This particular example drives the heater element for a 3d Printer Extruder.
00006  *
00007  * For more information on PID control check out Brett Beauregard's Arduino PID library:
00008  *
00009  *  https://github.com/br3ttb/Arduino-PID-Library
00010  *
00011  * The wikipedia article on PID controllers is a good place to start on
00012  * understanding how they work:
00013  *
00014  *  http://en.wikipedia.org/wiki/PID_controller
00015  *
00016  * The Thermistor value to Temerature routine uses the Steinhart-Hart equation.
00017  This is a Thermistor to Temerature conversion demo 
00018 
00019 Much thanks to @Adafruit for this tutorial:
00020 https://learn.adafruit.com/thermistor/using-a-thermistor
00021 
00022 The 100K Thermistor is configured with a 4.7k series resistor 
00023 tied to vcc (3.3v)  like this:
00024 
00025     +3.3v
00026       |
00027       \
00028       /  4.7k series resistor
00029       \
00030       /
00031       |
00032       .-----------O To Anlog pin on FRDM board
00033       |
00034       \
00035       /
00036   Thermistor  100k Nominal
00037       \
00038       /
00039       |
00040      ---
00041      GND
00042  *
00043  * Author(s): Michael Ball  unix_guru@hotmail.com
00044  *
00045  */
00046 
00047 #include "mbed.h"
00048 #include "millis.h"
00049 #include "PID.h"                                       
00050                                         
00051 float getTemperature();
00052                                         
00053 Serial pc(USBTX, USBRX);
00054 Ticker PrintTicker;                     // Send process results to Console once per second       
00055 Ticker ticker;                          // Set up the millis() ticker.
00056                                        
00057 #define  DEFAULT_Kp 1
00058 #define  DEFAULT_Ki 0.002
00059 #define  DEFAULT_Kd 20
00060 
00061 #define AUTOMATIC 1
00062 #define MANUAL    0
00063 #define DIRECT  0
00064 #define REVERSE  1
00065 #define thermistor A3                       // FRDM-K64F Analog input pin A3   - Adjust to your particular board
00066 #define driver PTC3                         // FRDM-K64F PWM output pin PTC3   - Adjust to your particular board
00067 
00068 AnalogIn Thermistor(thermistor);            // Read temperature value from thermistor on A3
00069 PwmOut Driver(driver);                      // PWM drive FET heater on PTC3  values are 0-1.0 
00070                                             // For 0-100% 
00071              
00072 float Input, Output, Setpoint; 
00073 PID controller(&Input, &Output, &Setpoint, DEFAULT_Kp , DEFAULT_Ki , DEFAULT_Kd , DIRECT);
00074 
00075 #define RATE 1.0                            // Print rate  Once per second
00076   
00077 void PrintValues() {                        // Routine to print out results to console
00078     pc.printf("Input      Output     Setpoint   Kp        Ki        Kd        time\r\n");
00079     pc.printf("%f, %f, %f, %f, %f, %f, %d \r\n", 
00080             Input, Output, Setpoint, controller.GetKp() , controller.GetKi() , controller.GetKd() , millis() );
00081 
00082 }    
00083 
00084 
00085 int main(){
00086  
00087   startMillis();                            // Initialize timer.
00088  
00089   pc.baud(115200);    
00090   pc.printf("\r\nThermistor PID Test - Build " __DATE__ " " __TIME__ "\r\n");
00091   
00092   PrintTicker.attach(&PrintValues,RATE);    // Start PID process running at 100ms rate.
00093 
00094   Setpoint = 80;                            // Set target temperature in degrees Celcius.
00095   controller.SetMode(AUTOMATIC);            // Turn PID controller on.
00096   
00097 
00098   while(1){
00099    
00100      Input = getTemperature();              // Actual temperature in Degrees Celcius
00101 
00102      controller.Compute();                  // Process PID loop. 
00103 
00104      Driver = Output/1000;                  // Sent PWM value scaled 0 - 1.0 as mbed requires 
00105 
00106   }
00107  
00108 }
00109 
00110 
00111 // This is the workhorse routine that calculates the temperature
00112 // using the Steinhart-Hart equation for thermistors
00113 // https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
00114 
00115 float getTemperature() {
00116 #define THERMISTORNOMINAL 100000      // 100k 
00117 // temp. for nominal resistance (almost always 25 C)
00118 #define TEMPERATURENOMINAL 25   
00119 // The beta coefficient of the thermistor (usually 3000-4000)
00120 #define BCOEFFICIENT 3950
00121 // the value of the 'other' resistor
00122 #define SERIESRESISTOR 4700    
00123 
00124 // This is the workhorse routine that calculates the temperature
00125 // using the Steinhart-Hart equation for thermistors
00126 // https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
00127 
00128     float temperature, resistance;
00129     float steinhart;
00130     int a;
00131     
00132     a = Thermistor.read_u16();       // Read 16bit Analog value
00133 //    pc.printf("Raw Analog Value for Thermistor = %d\r\n",a);
00134   
00135     /* Calculate the resistance of the thermistor from analog votage read. */
00136     resistance = (float) SERIESRESISTOR / ((65536.0 / a) - 1);
00137 //    pc.printf("Resistance for Thermistor = %f\r\n",resistance);
00138    
00139     steinhart = resistance / THERMISTORNOMINAL;         // (R/Ro)
00140     steinhart = log(steinhart);                         // ln(R/Ro)
00141     steinhart /= BCOEFFICIENT;                          // 1/B * ln(R/Ro)
00142     steinhart += 1.0 / (TEMPERATURENOMINAL + 273.15);   // + (1/To)
00143     steinhart = 1.0 / steinhart;                        // Invert
00144     temperature = steinhart - 273.15;                   // convert to C
00145 
00146 //    pc.printf("Extruder Temperature is %f\r\n", temperature);
00147  
00148     return temperature;    
00149 
00150 }
00151