RC Servo

Servo motors are often used in radio-controlled (RC) models, and are very useful in many kinds of small robotics applications because they are compact and inexpensive. An RC servo motor includes a built-in DC motor, gearbox, position feedback sensor (usually potentiometer), and drive electronics. RC servo motors can be controlled by an external pulse-width modulation (PWM) signal. If a signal meets RC servo timing requirements, it usually “describes” a (target) position input for the servo drive electronics. Servo motor electronics compare the shaft position with the inputted position, trying to find a shaft position where they match. The position control signal is a continuous square-wave signal, as depicted in figure.

Servo motors have three wires:

• Red = Power supply Vcc (4.8–6V)
• Black = Ground
• White/yellow = PWM input (may vary type-by-type)

The PWM signal has the following shape:

1.0ms = farthest left position

1.5 ms = center position

2.0 ms = farthest right position

Table of motor drive signals from ATmega128 to the actuator board.

/*-------------------------------------------------------------
Title:		RC Servo motor with Actuator Board
Date:		080327
Ver.:		1.1
Compiler:	AVR-GCC
Target:		ATmega128
Hardware:	ATmega128 controller board, Actuator board, RC servo motor
Author: 	Raivo Sell 2008
 
Notes:	Description: Moves slowly RC motor shaft from one edge to another.
---------------------------------------------------------------*/
#define F_CPU 14745600UL //CPU Frequency (influences delay function)
 
#include <avr/io.h>
#include <util/delay.h>
 
///////////////// Main function //////////////////////////////////
int main (void){
 
	// Set timer control registers
	TCCR1A = _BV(COM1A1) | _BV(COM1B1) | _BV(WGM11);
	TCCR1B = _BV(CS11) | _BV(WGM13) | _BV(WGM12); 
 
    	DDRB 	 = 0x30;  // PB5 & PB6 output
	ICR1   = 36000; // Sets the upper limit to Timer1 
    			//(creates 50 Hz signal) XTAL/8/256*50   14745600
	OCR1A  = 2700;  // Sets when the PWM signal should toggle.
 
	while(1){
		for(OCR1A=500;OCR1A!=5000;OCR1A++)  {_delay_ms(3);}
		for(OCR1A=5000;OCR1A!=500;OCR1A--)  {_delay_ms(3);}
	}
}