Arduino Uno W/Parallax Continuous Rotation Servo "Kicking Back" problem

Other Hardware Motors, Mechanics, Power and CNC
1

I'm having a minor novice problem here, but I can't seem to do the right combination of keyword searches to quite find the answer if it's already out there.

I am setting up a very simple exercise: wiring a Parallax Continuous Rotation Servo to an Arduino Uno board. The servo is powered by an external AA battery case, no problem there. The Arduino is being powered by its USB for now. The servo is connected via the usual 3-wire arrangement to power, ground and pin 13 on the Arduino.

The servo is a PWM job, and rotates clockwise at 1.3, coming to a stop at 1.5. I have already done the pot calibration so that it absolutely stops at 1.5.

I have been able to successfully get the servo to rotate 1 full 360 turn, then stop. The idea is that every time someone pushes a button, the wheel attached to the servo will rotate 1 time and stop (it is not intended to ever go in the other direction).

The problem I'm having is, with everything else working fine: every time I re-set the board or re-upload the program to test, or simply turn the power on to everything to "start," the servo "kicks" backwards a few degrees (in other words, I want it to rotate cleanly from a stop point and then stop again, each time...I don't want it always keep 'kicking back' a quarter-turn before operating.

Do I have to just keep the thing continually powered-on once I have it all assembled? Once it's up and running, it responds fine and doesn't do this kick-back, it only does it upon a cold power-up or re-set of the board or re-upload of the code. Or is it possibly that there still isn't sufficient power from the 4xAA battery case to the servo, and this "kick back" is some symptom of low power or a surge during the initial load?

I've searched the various boards here and on Parallax and other hobbyist sites but couldn't find this same issue. I'm not sure if it's the servo, the power source, or maybe the code I'm using.

I'm just using a very basic code setup:

#include <Servo.h>
Servo myServo;        // Create Servo object to control the servo 

void setup() { 

  myServo.attach(13);  // Servo is connected to digital pin 13 

  myServo.writeMicroseconds(1300);  //Rotate clockwise for 1 full rotation
  delay(1264);

  myServo.writeMicroseconds(1500);  //Then stop

} 

void loop() { 
 
}
2

Make sure the external power supply ground is connected to the arduino ground. Below is some servo code you can try. The default startup command position is 1500, so you might test to see if the servo jumps. Make sure your servo is calibrated to have no rotation at 1500us.

// zoomkat 3-28-14 serial servo incremental test code
// using serial monitor type a character (s to increase or a 
// to decrease) and enter to change servo position 
// (two hands required, one for letter entry and one for enter key)
// use strings like 90x or 1500x for new servo position 
// for IDE 1.0.5 and later
// Powering a servo from the arduino usually *DOES NOT WORK*.

#include<Servo.h>
String readString;
Servo myservo;
int pos=1500; //~neutral value for continous rotation servo
//int pos=90;

void setup()
{
  myservo.attach(7, 400, 2600); //servo control pin, and range if desired
  Serial.begin(9600);
  Serial.println("serial servo incremental test code");
  Serial.println("type a character (s to increase or a to decrease)");
  Serial.println("and enter to change servo position");
  Serial.println("use strings like 90x or 1500x for new servo position");
  Serial.println();
}

void loop()
{
  while (Serial.available()) {
    char c = Serial.read();  //gets one byte from serial buffer
    readString += c; //makes the string readString
    delay(2);  //slow looping to allow buffer to fill with next character
  }
  if (readString.length() >0) {
    if(readString.indexOf('x') >0) { 
      pos = readString.toInt();
    }

    if(readString =="a"){
      (pos=pos-1); //use larger numbers for larger increments
      if(pos<0) (pos=0); //prevent negative number
    }
    if (readString =="s"){
      (pos=pos+1);
    }

    if(pos >= 400) //determine servo write method
    {
      Serial.println(pos);
      myservo.writeMicroseconds(pos);
    }
    else
    {   
      Serial.println(pos);
      myservo.write(pos); 
    }
  }
  readString=""; //empty for next input
}