How to control a servomotor without arduino servo library

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In the next project i intend to control a servomotor (a cheap one) directly from timers because with the library(servo library) i couldn’t obtained a real 180 degrees angle.

  1. The theory about those servos:

A servomotor is actually an actuator with a dc motor, a gear box and some electronics to control the motor.

Basically, the electronics receive a repetitive signal and turn it into a position angle.

The most used frequency on those cheap servos is 50Hz(PWM) and 3 different duty cycles, 1ms, 1.5ms and 2ms – 1.5ms is for 0 degree (middle position), 1ms is 90 degree to the right and 2ms is for 90 degree to the left.

servo pwm

Now, my problem is that if i put the pwm in those duty cycles, the servo movement is smaller than 180 degree, so i am controling it directly with timers .

2. Controlling the servo without library

To see exactly what happens and how it reacts at the duty cycle i used a timer – more exactly, a timer 1 with phase correct pwm mode controlling the frequency from ICR1 register and duty cycle from OCR1A register (mode 10). To learn more about timer1 and how to control signals:

PWM modes timer1

With ICR1, we set the frequency and with OCR1A, we set the duty cycle on pin 9 (for pin 10 we could use OCR1B).

The formula for frequency is : f=fclk/(2*N*TOP), where TOP is defined by ICR1.

Now, to set 50 Hz, we need to know the TOP value so from the formula above TOP=20000, where N is 8 the prescaler.

Because the frequency of the signal is 50Hz, the time period is 20ms and ICR1=20000, so to have a 1ms duty cycle we must make the OCR1A=1000 for 1.5ms OCR1A=1500 and for 2000ms OCR1A=2000.

But, at those values my servo look something like this:

standard duty cycle for pwm

So, if you want a more precise control of the servo, those values are not good enough. With the program below you can set the OCR1A value to make your servo more precise.

void setup() {
pinMode(9, OUTPUT);
TCCR1A=0;//reset the register
TCCR1B=0;//reset the register
TCCR1A=0b10100010;//COM1A0,COM1B0 are 0, COM1A1, COM1B1 are 0 0
//also WGM10 are 0 and WGM11=1
TCCR1B=0b00010010;//WGM13 is 1 and WGM12 is 0 with prescaler equal 8
OCR1A=2500;//here you set the values for duty cycle and also angle
//OCR1B=1000;//you can connect another servo here and control it
void loop() {
// put your main code here, to run repeatedly

For my servo, i used values for OCR1A=580, 1565 and 2550.

modified duty cycle for pwm

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