Add example of back-caclulation in userspace using outputSum

Author: David R Forrest

examples/PID_Basic_Backcalculation/PID_Basic_Backcalculation.ino

@@ -0,0 +1,41 @@
+/********************************************************
+ * PID Basic Example
+ * Reading analog input 0 to control analog PWM output 3
+ ********************************************************/
+
+#include <PID_v1.h>
+
+#define PIN_INPUT 0
+#define PIN_OUTPUT 3
+
+//Define Variables we'll be connecting to
+double Setpoint, Input, Output;
+
+//Specify the links and initial tuning parameters
+double Kp=2, Ki=5, Kd=1;
+PID myPID(&Input, &Output, &Setpoint, Kp, Ki, Kd, DIRECT);
+
+void setup()
+{
+  //initialize the variables we're linked to
+  Input = analogRead(PIN_INPUT);
+  Setpoint = 100;
+
+  //turn the PID on
+  myPID.SetMode(AUTOMATIC);
+}
+
+void loop()
+{
+  Input = analogRead(PIN_INPUT);
+  if(myPID.Compute()){
+    // check if wound-up beyond output limits:
+    if(myPID.outputSum + Kp * (Setpoint - Input) > 255){
+       // wound up too high, backcalculate to feasible:
+       myPID.outputSum = 255-Kp * (Setpoint - Input); 
+    }
+  }
+  analogWrite(PIN_OUTPUT, Output);
+}
+
+