Servomotor

RC servo motor

The relationship between the width of the signal and the position of the Servo PWM.

RC (radio-controlled) servo-motors are very common actuator devices in robotics and model building. RC servo motors consist of a small DC motor, a reduction gear, and a control logic device. Usually, the rotor of the servo motor moves to a certain position and tries to maintain that position. The position of the rotor depends on the control signal received by the servo motor. Depending on the type of motor, the maximum revolving angle of the motor may differ. Servo motors that revolve constantly are rare. In this case, the control signal determines not the revolving angle but the speed of revolving.

The controlling signal of the servo motor is a specific pulse with a modulated signal (PWM), where the width of the pulse determines the position of the rotor. The period of the signal is 20 ms (50 Hz), and the width of the high period is 1 ms – 2 ms. 1 ms marks one extreme position, and 2 ms marks the second one. 1,5 ms marks the middle position of the servo motor’s rotor.

A traditional RC servo motor is also known as an analogue servo motor. It is because in the last decade, so-called digital servo motors have become common. The difference between those two is that in an analogue servo motor, the motor is controlled by the same 50 Hz PWM input signal. In a digital servo motor, the motor is controlled by a microcontroller with a much higher frequency signal. The input signal is the same in the digital servo motor, but a higher modulation frequency of the motor enables much more precise and faster position determination.

On the HomeLab module, there are two plugs for connecting RC servo motors. The PWM ends of the plugs are connected to the pins of the microcontroller, whose alternative functions are outputs of comparing units of the timer. The timer is capable of producing a PWM signal, and due to that, the control of motors is very simple in the program. The only difficulty is the setup of the timer.

The timer must be set up in PWM production mode, where the maximum value of the timer is determined with the ICR register. With the maximum value changed in the program and in the pace divider of the timer, the precise PWM frequency for controlling the servo motor can be determined. With the comparison register of the timer, the lengths of both high semi-periods of the PWM signal can be determined. The timers have special comparing units that monitor the value of the counter, and in case it remains equal to the value of the comparison register, they change the output value of the comparing units. The following is the program code of the servo motor control library of the HomeLab. For the purpose of functionality, it uses parameters for timers, which are determined with macro functions. For example, the period is found using the F_CPU constant, which marks the clock rate of the microcontroller. When using macros, there is no need to calculate the parameters of the timer for different clock rates, and the compiler converts the operations with macros to constants anyway, so the program memory does not grow and does not demand more time. The following example of a library is for HomeLab II (ATmega2561).

// The value of the timer (20 ms)for achieving the full period of PWM
// F_CPU is the clock rate of the microcontroller, which is divided by 
// 50 Hz and 8
#define PWM_PERIOD      (F_CPU / 8 / 50)
 
// Middle position of PWM servo (5 ms / 20 ms)
// Middle position is 15/200 of the full period
#define PWM_MIDDLE_POS  (PWM_PERIOD * 15 / 200)
 
// Factor for converting the percents (-100% to 100%)to periods
// +1 is added to ensure that semi periods would reach to the boundaries 
// of 1 ms and 2 ms or // a little over
#define PWM_RATIO       (PWM_PERIOD / 20 / 2 / 100 + 1)
 
// Set-up of the pins
static pin servo_pins[2] =
{
	PIN(B, 5), PIN(B, 6)
};
 
// Preparing the servo motor for working
void servomotor_init(unsigned char index)
{
	// The pin of PWM signal for output
	pin_setup_output(servo_pins[index]); 
 
	// Setup of timer 1
	// Prescaler = 8
	// Fast PWM mode, where TOP = ICR
	// OUTA and OUTB to low in comparison
	timer1_init_fast_pwm(
		TIMER1_PRESCALE_8,
		TIMER1_FAST_PWM_TOP_ICR,
		TIMER1_FAST_PWM_OUTPUT_CLEAR_ON_MATCH,
		TIMER1_FAST_PWM_OUTPUT_CLEAR_ON_MATCH,
		TIMER1_FAST_PWM_OUTPUT_DISABLE);
 
	// Determining the period by the maximum value
	timer1_set_input_capture_value(PWM_PERIOD);	
}
 
// Determining the position of the servo motor
// The parameter of the position is from -100% to +100%.
void servomotor_position(unsigned char index, signed short position)
{	
	switch (index)
	{
		case 0:			
			timer1_set_compare_match_unitA_value(
				PWM_MIDDLE_POS + position * PWM_RATIO);
			break;
 
		case 1:
			timer1_set_compare_match_unitB_value(
				PWM_MIDDLE_POS + position * PWM_RATIO);
			break;
	}
}

The example program uses the described functions of the library of the HomeLab. At the beginning of the program, the servo motor’s PWM signal generator is started with the servomotor_init function. The value of the position is defined.

// Testing program of the servo motor
#include <homelab/module/motors.h>
 
// Main program
int main(void)
{
 
	// Set-up of the motor
	servomotor_init(1);
 
	// Determining the position of the servo motor
	servomotor_position(1, 50);
 
}

1. Use a servo motor to block and unblock the distance sensor range and present the measured distance on the LCD (use previous scenarios and instructions for the sensor and LCD)