Faster temperature sampling and moving average

crepemaker.ino

@@ -6,19 +6,26 @@
 // libraries to be installed from the library manager
 #include "max6675.h"
 
-// other compile directives
-#define LOOPTIME 500 // cycle time, in ms
-
 // global variables and objects
 MAX6675 thermocouple1(CLKPIN, CS1PIN, DOPIN);
 MAX6675 thermocouple2(CLKPIN, CS2PIN, DOPIN);
 
+// variables for the heating control
 float previoustemp1 = 0;
 float previoustemp2 = 0;
-unsigned long timestamp = 0;
-int counter = 0;
-
-void seriallogger(float temp1, float temp2, float ror1, float ror2, float projectedtemp1, float projectedtemp2) {
+unsigned long lastControlUpdate = 0; // last time controls were updated
+
+// variables for the moving averages on temperature readings
+float tempReadings1[NUMREADINGS]; // the temperature readings from the analog input
+float tempReadings2[NUMREADINGS];
+int readIndex = 0; // the index of the current reading
+unsigned long lastRead = 0; // last time readings were updated
+float tempReadingsTotal1 = 0; // the running total
+float tempReadingsTotal2 = 0;
+float temp1; // the average
+float temp2;
+
+void seriallogger(float temp1, float temp2, float ror1, float ror2, float projectedtemp1, float projectedtemp2, byte control) {
   Serial.print(temp1);
   Serial.print(",");
   Serial.print(temp2);
@@ -29,42 +36,92 @@ void seriallogger(float temp1, float temp2, float ror1, float ror2, float projec
   Serial.print(",");
   Serial.print(projectedtemp1);
   Serial.print(",");
-  Serial.println(projectedtemp2);
+  Serial.print(projectedtemp2);
+  Serial.print(",");
+  Serial.println(control);
 }
 
 void setup() {
   pinMode(SSR1PIN, OUTPUT);
   pinMode(SSR2PIN, OUTPUT);
 
   Serial.begin(BAUD);
-  
+
   // wait for MAX chips to stabilize
   delay(500);
+
+  // initialize the readings array to zero
+  for (int i = 0; i < NUMREADINGS; i++) {
+    tempReadings1[i] = 0;
+    tempReadings2[i] = 0;
+  }
+
+  // initialize the temperature readings and averages with actual values
+  while (readIndex < NUMREADINGS - 1) {
+    updateTemperatureReadings();
+    delay(LOOP_PERIOD_MS);
+  }
+}
+
+void loop() {
+  updateTemperatureReadings();
+  updateControls();
+  delay(LOOP_PERIOD_MS);
+}
+
+void updateTemperatureReadings() {
+  if (millis() - lastRead < READING_PERIOD_MS) {
+    return;
+  }
+
+  lastRead += READING_PERIOD_MS;
+
+  // subtract the last reading
+  tempReadingsTotal1 = tempReadingsTotal1 - tempReadings1[readIndex];
+  tempReadingsTotal2 = tempReadingsTotal2 - tempReadings2[readIndex];
+
+  // read from the sensor
+  tempReadings1[readIndex] = thermocouple1.readCelsius();
+  tempReadings2[readIndex] = thermocouple2.readCelsius();
+
+  // add the reading to the total
+  tempReadingsTotal1 = tempReadingsTotal1 + tempReadings1[readIndex];
+  tempReadingsTotal2 = tempReadingsTotal2 + tempReadings2[readIndex];
+
+  // advance to the next position in the array
+  readIndex = readIndex + 1;
+
+  // if we are at the end of the array...
+  if (readIndex >= NUMREADINGS) {
+    // ...wrap around to the beginning
+    readIndex = 0;
+  }
+
+  // calculate the average
+  temp1 = tempReadingsTotal1 / NUMREADINGS;
+  temp2 = tempReadingsTotal2 / NUMREADINGS;
 }
 
-void loop() {  
+void updateControls()
+{
   unsigned long timeelapsed = 0;
-  float temp1;
-  float temp2;
   float ror1;
   float ror2;
   float projectedtemp1;
   float projectedtemp2;
 
-  temp1 = thermocouple1.readCelsius();
-  temp2 = thermocouple2.readCelsius();
-
-  if (counter >= SAMPLING) {
-    timeelapsed = (millis() - timestamp);
-    ror1 = (((temp1 - previoustemp1) / timeelapsed) * 1000 * 60); //Raise of rise in C/min  
-    ror2 = (((temp2 - previoustemp2) / timeelapsed) * 1000 * 60); //Raise of rise in C/min  
-    timestamp = millis();
-    counter = 0;
-    previoustemp1 = temp1; // We should really calculate an average rather...
-    previoustemp2 = temp2;    
+  timeelapsed = (millis() - lastControlUpdate);
+
+  if (timeelapsed < CONTROL_UPDATE_PERIOD_MS) {
+    return;
   }
-
-  counter++;
+
+  lastControlUpdate += CONTROL_UPDATE_PERIOD_MS; // do not use timeelapsed here to avoid drift
+
+  ror1 = (((temp1 - previoustemp1) / timeelapsed) * 1000 * 60); //Raise of rise in C/min
+  ror2 = (((temp2 - previoustemp2) / timeelapsed) * 1000 * 60); //Raise of rise in C/min
+  previoustemp1 = temp1;
+  previoustemp2 = temp2;
 
   if (ror1 >= 0) {
     projectedtemp1 = (temp1 + (ror1 * RISEINERTIA));
@@ -80,21 +137,24 @@ void loop() {
     projectedtemp2 = (temp2 + (ror2 * FALLINERTIA));
   }
 
-  seriallogger(temp1,temp2,ror1,ror2,projectedtemp1,projectedtemp2);
+  byte control = 0;
 
-  if (projectedtemp1  <= SV1) {
-   digitalWrite(SSR2PIN, LOW); 
-   digitalWrite(SSR1PIN, HIGH); 
-  } 
+  if (projectedtemp1 <= SV1) {
+    digitalWrite(SSR2PIN, LOW);
+    digitalWrite(SSR1PIN, HIGH);
+    control = 1;
+  }
   else {
-     digitalWrite(SSR1PIN, LOW); 
+    digitalWrite(SSR1PIN, LOW);
 
     if (projectedtemp2 <= SV2) {
-     digitalWrite(SSR2PIN, HIGH); 
+      digitalWrite(SSR2PIN, HIGH);
+      control = 2;
     }
     else {
-      digitalWrite(SSR2PIN, LOW); 
+      digitalWrite(SSR2PIN, LOW);
     }
   }
-  delay(LOOPTIME);
+
+  seriallogger(temp1, temp2, ror1, ror2, projectedtemp1, projectedtemp2, control);
 }

user.h

@@ -14,14 +14,22 @@
 #define RISEINERTIA 3 //how long it takes the system to stop rising (in min)
 #define FALLINERTIA 1 //how long it takes the system to stop falling (in min)
 
-#define SAMPLING 5 //loops to calculate ror
+// cycle time, in ms
+#define LOOP_PERIOD_MS 1
 
 // Delay time between temperature readings
 // from the temperature sensor (ms).
-#define DELAY_TIME 20
+// (must be larger than LOOP_PERIOD_MS)
+// MAX6675 takes about 200 ms to convert
+#define READING_PERIOD_MS 200
 
 // How many readings are taken to determine a mean temperature.
-#define READINGS 10
+// accounts for the thermocouple noise
+#define NUMREADINGS 10
+
+// delay between 2 control updates
+// (must be larger than LOOP_PERIOD_MS)
+#define CONTROL_UPDATE_PERIOD_MS 2000
 
 // Pin mapping 
 // Common SPI pins