I am having trouble interfacing Pololu’s BCM servo to my ATMega328P, using the Arduino development environment.
The Arduino standard Servo Library caused the servo to jarr right up against one end of its range. The motion also seemed jittery.
Thinking the servo might be non standard I hunted for other control software that might offer more control. I found some code that didn’t cause the servo to hit one end of its range, however the results were still terrible…please see the attached video. In the video, the ATMega is transmitting a constant, mid-range pulse on a digital pin i.e. the servo arm should be held still.
Here is the code:
/*
* NewSerialServo
* --------------
* Servo control from the Serial port
*
* Alteration of the control interface to use < and > keys
* to slew the servo horn left and right. Works best with
* the Linux/Mac terminal "screen" program.
*
* Created 10 December 2007
* copyleft 2007 Brian D. Wendt
* http://principialabs.com/
*
* Adapted from code by Tom Igoe
* http://itp.nyu.edu/physcomp/Labs/Servo
*/
/** Adjust these values for your servo and setup, if necessary **/
int servoPin = 8; // control pin for servo motor
int minPulse = 1000; // minimum servo position
int maxPulse = 2000; // maximum servo position
int turnRate = 100; // servo turn rate increment (larger value, faster rate)
int refreshTime = 20; // time (ms) between pulses (50Hz)
/** The Arduino will calculate these values for you **/
int centerServo; // center servo position
int pulseWidth; // servo pulse width
int moveServo; // raw user input
long lastPulse = 0; // recorded time (ms) of the last pulse
void setup() {
pinMode(servoPin, OUTPUT); // Set servo pin as an output pin
centerServo = maxPulse - ((maxPulse - minPulse)/2);
pulseWidth = centerServo; // Give the servo a starting point (or it floats)
Serial.begin(9600);
Serial.println(" Arduino Serial Servo Control");
Serial.println("Press < or > to move, spacebar to center");
Serial.println();
}
void loop() {
// wait for serial input
if (Serial.available() > 0) {
// read the incoming byte:
moveServo = Serial.read();
// ASCII '<' is 44, ASCII '>' is 46 (comma and period, really)
if (moveServo == 44) { pulseWidth = pulseWidth - turnRate; }
if (moveServo == 46) { pulseWidth = pulseWidth + turnRate; }
if (moveServo == 32) { pulseWidth = centerServo; }
// stop servo pulse at min and max
if (pulseWidth > maxPulse) { pulseWidth = maxPulse; }
if (pulseWidth < minPulse) { pulseWidth = minPulse; }
// print pulseWidth back to the Serial Monitor (uncomment to debug)
// Serial.print("Pulse Width: ");
// Serial.print(pulseWidth);
// Serial.println("us"); // microseconds
}
// pulse the servo every 20 ms (refreshTime) with current pulseWidth
// this will hold the servo's position if unchanged, or move it if changed
if (millis() - lastPulse >= refreshTime) {
digitalWrite(servoPin, HIGH); // start the pulse
delayMicroseconds(pulseWidth); // pulse width
digitalWrite(servoPin, LOW); // stop the pulse
lastPulse = millis(); // save the time of the last pulse
}
}
I have tried various pulse widths including quite out-of-range ones, but none seem to have any effect at all.
My best guess is that this servo uses a non standard refresh rate so it is getting completely garbled pulses… head of one, tail end of another etc. Has anyone seem this behaviour before or can shed any light? I have tried to hunt for a data-sheet for this servo but neither Pololu nor Joinmax seem to offer one.
The Pololu website says this isn’t a digital servo, but I have read that this doesn’t affect the PWM control method. Note to Pololu: I have seen another post on this forum stating that they had trouble with controlling a Joinmax Servo, so I have to ask; has anyone at Pololu successfully controlled one of these servos?
Of course, the problem may be obvious to someone else as I’m a newbie to servos. Preparing to take the full force of someones clue bat 
Thanks,
Chris
BCM Servo Behaviour.zip (749 KB)