• Objectives:
  To control the position of Servo Motor using variable PWM generated through AVR

Introduction:
Servo refers to an error sensing feedback control, which is used to correct the performance of a system. Servo Motors are DC motors equipped with a servo mechanism for precise control of angular  position.  The  RC  servo  motors  usually  have  a  rotation  limit  from  0°  to  180°.  Some servos also have rotation limit of 360°. But servos do not rotate continuously. Their rotation is restricted in between the fixed angles.
A  servo  motor  consists  of  several  main  parts,  the  motor  and  gearbox,  a  position  sensor (potentiometer), PWM  to  voltage  converter, error  amplifier  and  motor  driver. Following  figure shows the block diagram of a typical servo motor.

      

PWM to Voltage Converter:
The PWM control input is feed to a PWM to voltage converter. This circuit charges a capacitor at a constant rate while the pulse is high. When the pulse goes low the charge on the capacitor is fed to the output via a suitable buffer amplifier. This produces a voltage related to the length of the applied pulse.
The circuit is tuned to produce a useful voltage over a 1ms to 2ms period. The output voltage is buffered and so does not decay significantly between control pulses.
Position Sensor (Potentiometer):
The current rotational position of the servo motor output shaft is read by a potentiometer which produces a voltage that is related to the absolute angle of the output shaft. The potentiometer then feeds its value into the Error Signal Amplifier which compares the current position with the required position from the PWM to voltage converter.

Error Signal Amplifier:
The error signal amplifier is an operational amplifier with negative feedback. It will always try to minimize the difference between the inverting and non-inverting inputs by driving its output in the  correct  direction.  The  output  of the  error  amplifier  is  either  a  negative  or  positive voltage representing  the  difference  between  its  inputs. Large  error  signal  will  produce  higher  output voltages from error signal amplifier.
The error amplifier output is used to drive the motor. If it is positive the motor will turn in one direction, otherwise  in  other  direction.  This  allows  the  error  amplifier to  reduce  the  difference between its inputs (thus closing the negative feedback loop) and so make the servo go to the commanded position.
A typical value of the pulse width is somewhere in the range of 1.0 to 2.0ms. For a standard servo,  a pulse  width  between  1.0ms  to  2.0ms  makes  the  servo  to  turn between  0 to  180.
However,  these  values  could  vary  depending  on  the  brand  and  make  of  the  motor. A  servo motor has three wires; two for supply voltages (Vcc and Ground) and third wire is used to supply the control PWM pulses.
Following figures show the different angle of rotation for the PWM of different duty cycles. Note that  for  1ms  duty  cycle  PWM,  servo  does  not  rotate  and  the  maximum  rotation  i.e.  180 is achieved with the PWM having duty cycle of 10% (2ms).

C Code to control and position of servo motor using variable PWM :
/*
This programs generates the PWM of variable size to control the direction of a
servo motor having 0 to 180 rotation. Min. control pulse is 1ms and max. control
pulse is 2ms */

#include <avr/io.h>
#include <util/delay.h>
/*
CALCULATION:
We  will  use mode  14  (as  per  datasheet  of  ATmega16)  to  generate  variab le  top  PWM
with ICR1. We need to calculate the top value for 50Hz PWM Freq
Formula:
Freq PWM = Freq cpu /N(1+Top) so
Top = (Freq CPU / (Freq PWM x N))-1
where N is prescaler
Freq CPU = 1MHz
Prescaler = 16
Freq PWM = 50Hz
Top = (1MHz / (50Hz x 8))-1
Top = 2499
Position of Servo Motor against different pulse widths
5% of 20ms = 1ms = 0 degree (min. rotation)
10% of 20ms = 2ms = 180 degrees (max. rotation)
5% Duty Cycle Calculation for non-inverting mode:
Duty Cycle (%) = ((OCR1x + 1) / Top + 1) * 100
5 / 100 = (OCR1x + 1) / (2499 + 1)
0.05 * 2500 = OCR1x + 1
OCR1x = 124
10% Duty Cycle Calculation for non-inverting mode:
Duty Cycle (%) = ((OCR1x + 1) / Top + 1) * 100
10 / 100 = (OCR1x + 1) / (2499 + 1)
0.1 * 2500 = OCR1x + 1
OCR1x = 249
*/
void main(void)
{
unsigned int x;
//Configure Timer1 registers
TCCR1A |= (1<<COM1A1)|(1<<WGM11); //Non-Inverted PWM
TCCR1B |= (1<<WGM13)|(1<<WGM12)|(1<<CS11); //Prescaler=16 Mode 14(Fast PWM)
ICR1 = 2499; //Freq PWM=50Hz, Time Period = 20ms
DDRD = 0xFF; //PWM generation on PortD5
while(1)
{
x = 124; //1ms = 5% of 20ms pulse
OCR1A = x; //0 degree rotation initially
_delay_ms(1000);
while(x <= 249) // continue till 2ms = 10% of 20ms Pulse
{
OCR1A=x;
_delay_ms(50);
x++;
}
}
}

Simulation: