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.

Electrical connections

1. Make sure all power is off.
2. Connect the actuator board and main board with a 26-pin cable.
3. Connect the RC servo motor to the actuator board, as shown in Figure 2.

Note: The signal wire (usually yellow or white) is towards the outer edge of the board.

1. Connect the JTAG converter with the ATmega128 main board and the PC.
2. Finally connect the power to the main and actuator boards.

/*-------------------------------------------------------------
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);}
	}
}