PID question again

Projects Programming
1

First off thanks for all the answers to my previous questions. Ok here's the scenario. I have my Mega connected to a DC motor via an H bridge. The motor is connected to a 10 turn pot via a drive belt. The pot is connected back to the Mega's analog 'A0' port. My program lets you input a setpoint via the serial monitor and the system is supposed to drive to the new setpoint. if I manually set the pot to a low value (input<100) and then input a high setpoint (setpoint=1000) ,it drives to the new set point (+- about 40).
if I then input a low setpoint nothing happens. Long story short the whole program is wildly unpredictable. Anyone have any ideas to whats going on? I am a PID newbie. Thanks for any help!

/********************************************************
 * Arduino PID Library
 * by Brett Beauregard
 * contact: br3ttb@gmail.com
 * PID Adaptive Tuning Example
 * One of the benefits of the PID library is that you can
 * change the tuning parameters at any time.  this can be
 * helpful if we want the controller to be agressive at some
 * times, and conservative at others.   in the example below
 * we set the controller to use Conservative Tuning Parameters
 * when we're near setpoint and more agressive Tuning
 * Parameters when we're farther away.
 ********************************************************/
#include <PID_v1.h>

//Define Variables we'll be connecting to
double Input, Output;
double Setpoint;
int pin=2;

//Define the aggressive and conservative Tuning Parameters
double aggKp=4, aggKi=0.2, aggKd=1;
double consKp=1, consKi=0.05, consKd=0.25;

//Specify the links and initial tuning parameters
PID myPID(&Input, &Output, &Setpoint, consKp, consKi, consKd ,DIRECT);

void setup()
{
  //initialize the variables we're linked to
  Serial.begin(9600);
  Serial.setTimeout(10);
  Input = analogRead(0);
  Setpoint = 10;

  //turn the PID on
  myPID.SetMode(AUTOMATIC);
  myPID.SetSampleTime(200);
}

void loop()
{
  Input = analogRead(0);
  if (Serial.available()>0)    
  {                            
    Setpoint=Serial.parseInt();   
  }
    if (Setpoint >= Input)
  {
    pin=2;
    myPID.SetControllerDirection(DIRECT);
  }
  else
  {
    pin=3;
    myPID.SetControllerDirection(REVERSE);
  }
  
  double gap = abs(Setpoint-Input); //distance away from setpoint
  if(gap<10)
  {  //we're close to setpoint, use conservative tuning parameters
    myPID.SetTunings(consKp, consKi, consKd);
  }
  else
  {
     //we're far from setpoint, use aggressive tuning parameters
     myPID.SetTunings(aggKp, aggKi, aggKd);
  }

  myPID.Compute();
  analogWrite(pin, Output);
  Serial.print(Setpoint);
  Serial.print("    ");
  Serial.print(pin);
  Serial.print("    ");
  Serial.print(Output);
  Serial.print("    ");
  Serial.println(Input);
}

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2

You seem to have logic to work out which direction you need to move based on the raw sensor input, separately from the PID algorithm. That's unexpected. You should be able to subtract the actual and required positions to determine the positional error; the PID should be able to handle positive and negative errors and give you a positive or negative output command as appropriate. It is the output command which you should be looking at to decide which direction and power to apply to the motor.