Servo Plus Potentiometer

I had to do one more of these before I took it apart. 

Not too long ago I saw a link on Make: regarding using potentiometers and servos. To be honest, I never got around to watching the video. But after the last blog entry I made, adding in the potentiometer didn't seem all that hard.

The potentiometer takes 5v on one side, ground on the other. In the middle I hooked to one of the analog pins. 

Analog read on the Arduino ranges from 0 to 1023. The servo however took a range of values from 375 to 2400. The leaves me needing to convert from 1023 to 2025. This is a fairly simple ratio, which I originally coded something like this:

pulse value = ((potentiometer value * 2025) / 1023) + 375

Using this formula introduced me to an interesting detail that I had not seen for a while. Not since a project I had done for the State of Minnesota. At the time I was working on a project for the Department of Education. I don't recall the specifics, but I had been doing some calculations with large numbers in Java. Every so often the numbers would turn negative. This is exactly what I started seeing here when multiplying the potentiometer value with 2025.

What's going on is something I learned about much further back in college. The type value I was using was too small for the value I was putting into it. Every bit of the value - or perhaps just one particular one - was overwriting the slot that holds the negative/positive indicator. Very annoying.

To solve the problem on the DOE project I ended up using a larger type. I tried a few different types with the Arduino (doubles, long ints, floats) but to no avail. Instead of digging into a solution further in that direction, a different solution occurred to me - change the calculation:

pulse value = ((2025 / 1023)  * potentiometer value) + 375

This forces the division to occur first, which winds up with some value like 1.979 and so forth. This is a much smaller value that when multiplied by the pot value and added to 375 is still managable.

Anyway, here's the code:

/*
 * Servo plus potentiometer - a match made in... 
 * well, a match made on an Arduion at least.
 */

int potPin = 1;
int potVal = 0;
int servoPin = 3;
int baseChar = 48;
int maxCycles = 40;
int minPulse = 375;
int maxPulse = 2400;
int maxAnalog = 1023;
int distance = maxPulse - minPulse;
int list[4] = {-1, -1, -1, -1};
int input = 0;
int pulse = minPulse;
int lastPulse = maxPulse;

void setup()
{
  // setup serial schtuff
  Serial.begin(9600);
  pinMode(servoPin, OUTPUT);
  
  // say hello
  Serial.println("Servo with potentiometer control - start");
  
  // get first potentiometer value
  potVal = analogRead(potPin);
  
  // calculate pulse from it
  calcPulse();
    
  // then go there
  specific();
}

void loop()
{
  // read from potentiometer 
  int tmpVal = analogRead(potPin);
  
  // if this is a new value
  if (tmpVal != potVal)
  {
    // keep it
    potVal = tmpVal;

    // calculate pulse from it
    calcPulse();
    
    // then go there
    specific();
  }
}

void calcPulse()
{
  double ratio = distance / maxAnalog;
  
  if (potVal == 0) 
    pulse = minPulse;
  else if (potVal == maxAnalog)
    pulse = maxPulse;
  else
    pulse = ratio * potVal + minPulse;
    
  Serial.print("potVal: ");
  Serial.print(potVal, DEC);
  Serial.print(", ratio: ");
  Serial.print(ratio, DEC);
  Serial.print(", pulse: ");
  Serial.println(pulse, DEC);
}

void specific()
{
  if (pulse >= minPulse && pulse <= maxPulse)
  {
    //Serial.print("Last pulse ");  
    //Serial.println(lastPulse);  

    //Serial.print("Current pulse ");  
    //Serial.println(pulse);  

    int travel = max(lastPulse, pulse) - min(lastPulse, pulse);
    //Serial.print("Travel ");  
    //Serial.println(travel);  
    //Serial.print("Distance ");  
    //Serial.println(distance);  
    
    float cut = (float)travel / (float)distance;
    //Serial.print("Cut ");  
    //Serial.println(cut, DEC);  
    
    unsigned int cycles = (maxCycles * cut) + 1;
    cycles = max(cycles, 5);
    //Serial.print("Cycles ");  
    //Serial.println(cycles);  
    
    for (int i = 0; i <= cycles; i++)
    {
      digitalWrite(servoPin, HIGH);
      delayMicroseconds(pulse);
      digitalWrite(servoPin, LOW);
      delay(20);
    }
    
    lastPulse = pulse;
  }
  else
  {
    Serial.print("Pulse given ");  
    Serial.print(pulse);  
    Serial.print(" is outside of range: ");  
    Serial.print(minPulse);  
    Serial.print("-");  
    Serial.println(maxPulse);  
  }
  
  pulse = 0;
}