Servomotor

Necessary knowledge: [HW] User Interface Module, [HW] Combo module, [AVR] Counters/Timers, [AVR] Analog-to-digital Converter, [LIB] Motors, [LIB] Analog to Digital Converter

RC servo motor

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

RC (radio-controlled) servo-motors are very common actuator devices in robotics and model building. RC servo motors are consisting of small DC motor, reduction gear and 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 of the control signal received by the servo motor. Depending on the type of the 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. Servo motor “hack” is also quite common. This makes the position determining servo motor to constantly revolving servo. In this case the feedback potentiometer is replaced by two fixed resistors and the mechanical resistor that prevents full rotations is removed from the gear. A very important characteristic of servo motors is its power-weight ratio.

The controlling signal of servo motor is specific pulse with modulated signal (PWM), where 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.

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

On the board of module of motors of the HomeLab are two or four plugs for connecting RC servo motors. The PWM ends of the plugs are connected to the pins of the microcontroller, which alternative functions are outputs of comparing units of the timer. Timer is capable of producing PWM signal and due to that the control of motors is very simple in the program. Only difficulty is set-up of the timer.

The timer must be set up in PWM production mode, where the maximum value of the timer is determined with 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, lengths of both high semi periods of PWM signal can be determined. The timers have special comparing units which are monitoring the value of the counter and in case it remains equal with the value of the comparison register they change the output value of 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 F_CPU constant, which marks the clock rate of the microcontroller. When using macros, there is no need to calculate the parameters of timer for different clock rates and the compiler converts the operations with macros to constants anyway, so the program memory is not growing and does not demand more time. The following example of 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 with 
// 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 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 comparisson
	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 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 described functions of the library of the HomeLab. In the beginning of the program the first servo motor’s PWM signal generator is started with the servomotor_init function. The value of the position of the servo motor is obtained from the channel of the analogue-digital converter, where a potentiometer on the board of sensors is connected. To get the range -100 % - +100 % necessary for controlling the servo motor, half of the maximum (512) is subtracted of the ADC value and the result is divided with 5. The result is +/- 102, but small inaccuracy does not count because servo motors also differ by the relation of the PWM signal and revolving angle. Final PWM‘s semi period’s width in applications has to be determined using test-and-error method. Also the remote controls of RC models have corresponding opportunities for precise setup (trim function). When the program is started the rotors position of the servomotor is changed according to the position of the potentiometer.

// Testing program of the motors module of the HomeLab kit
#include <homelab/adc.h>
#include <homelab/module/motors.h>
 
// Main program
int main(void)
{
	short position;
 
	// Set-up of the ADC
	adc_init(ADC_REF_AVCC, ADC_PRESCALE_8);
 
	// Set-up of the motor
	servomotor_init(1);
 
	// Endless loop
	while (1)
	{
		// Reading the position of the potentiometer and 
		// converting the range of
		// the servo motor
		// For HomeLab II ADC must be read for the corresponding channel, 
		// and use the following formula:
		// position = ((short)adc_get_value(3) - (short)512) / (short)5;
		position = ((short)adc_get_value(15) / 10) - 102 ;
 
		// Determining the position of the servo motor
		servomotor_position(1, position);
	}
}