TinyFanControl-Firmware/src/fan_controller.c

#include "fan_controller.h"

#include "eeprom_config.h"

/*
 * The control loop is based on a temperature corridor concept. While the
 * temperature is inside the corridor, the PWM duty cycle will not be changed.
 *
 * If it is above the corridor’s maximum temperature, the duty cycle increases
 * as defined by a PI controller, i.e. there is a dynamic/temporary part (P)
 * that depends on how far outside the corridor the temperature is, and there
 * is a persistent part (I) that is continuously increased while the
 * temperature is outside the corridor.
 *
 * If the temperature is below the corridor’s minimum temperature, similar
 * rules are applied to decrease the fan speed. However, the P part makes
 * little sense here so its gain is usually set to 0.
 *
 * The fan starts in a stopped state and stays there until the temperature
 * rises above the startup temperature. Once that happens, the PWM is set
 * to a specific duty cycle (high enough to reliably start the fan) and
 * regulation begins.
 */

/* Note:
 * - all temperatures are in °C represented in fxp_t.
 * - all duty cycles are in percent represented in fxp_t.
 */

// corridor definition
#define CORRIDOR_MIN_TEMPERATURE     EEPROM_CONFIG_CORRIDOR_MIN_TEMPERATURE
#define CORRIDOR_MAX_TEMPERATURE     EEPROM_CONFIG_CORRIDOR_MAX_TEMPERATURE

// The fan is started when temperature rises above this temperature.
#define FAN_START_TEMPERATURE        EEPROM_CONFIG_FAN_START_TEMPERATURE

// The fan is stopped when the temperature falls below this point.
#define FAN_OFF_TEMPERATURE          EEPROM_CONFIG_FAN_OFF_TEMPERATURE

// Emergency temperature level. If this is reached, the duty cycle is directly
// set to the maximum to skip a long ramp-up phase.
#define EMERGENCY_TEMPERATURE        EEPROM_CONFIG_EMERGENCY_TEMPERATURE

// The fan PWM duty cycle cannot fall below this limit. It is held until
// temperature falls below the off-temperature.
#define DUTY_CYCLE_MIN               EEPROM_CONFIG_DUTY_CYCLE_MIN

// The fan PWM duty cycle cannot go above this limit. Should be set to 100.
#define DUTY_CYCLE_MAX               EEPROM_CONFIG_DUTY_CYCLE_MAX

// The fan is always started with this duty cycle. Do not set too low.
#define DUTY_CYCLE_START             EEPROM_CONFIG_DUTY_CYCLE_START

// gain unit: percent duty cycle change per °C deviation per update cycle
#define GAIN_T_HIGH_P                EEPROM_CONFIG_GAIN_T_HIGH_P
#define GAIN_T_HIGH_I                EEPROM_CONFIG_GAIN_T_HIGH_I
#define GAIN_T_LOW_P                 EEPROM_CONFIG_GAIN_T_LOW_P
#define GAIN_T_LOW_I                 EEPROM_CONFIG_GAIN_T_LOW_I

static fxp_t m_duty;

void fan_controller_init(void)
{
	m_duty = 0; // fan off by default
}


uint8_t fan_controller_update(fxp_t temperature)
{
	fxp_t duty_out = m_duty;

	// check emergency case. Directly starts the fan, if necessary.
	if(temperature > EMERGENCY_TEMPERATURE) {
		m_duty = duty_out = DUTY_CYCLE_MAX;
	}

	if(m_duty == 0) {
		// we are currently in off state. Check whether we need to start.
		if(temperature > FAN_START_TEMPERATURE) {
			m_duty = duty_out = DUTY_CYCLE_START;
		}
	} else {
		// fan is currently on

		// corridor handling: too warm
		if(temperature > CORRIDOR_MAX_TEMPERATURE) {
			fxp_t delta_above = fxp_sub(temperature, CORRIDOR_MAX_TEMPERATURE);

			fxp_t duty_p = fxp_mult(delta_above, GAIN_T_HIGH_P);
			fxp_t duty_i = fxp_mult(delta_above, GAIN_T_HIGH_I);

			m_duty = fxp_add(m_duty, duty_i);

			if(m_duty > DUTY_CYCLE_MAX) {
				m_duty = DUTY_CYCLE_MAX;
			}

			duty_out = fxp_add(m_duty, duty_p);

			if(duty_out > DUTY_CYCLE_MAX) {
				duty_out = DUTY_CYCLE_MAX;
			}
		}

		// corridor handling: too cold
		if(temperature < CORRIDOR_MIN_TEMPERATURE) {
			fxp_t delta_below = fxp_sub(temperature, CORRIDOR_MIN_TEMPERATURE);

			fxp_t duty_p = fxp_mult(delta_below, GAIN_T_LOW_P);
			fxp_t duty_i = fxp_mult(delta_below, GAIN_T_LOW_I);

			m_duty = fxp_add(m_duty, duty_i);

			if(m_duty < DUTY_CYCLE_MIN) {
				m_duty = DUTY_CYCLE_MIN;
			}

			duty_out = fxp_add(m_duty, duty_p);

			if(duty_out < DUTY_CYCLE_MIN) {
				duty_out = DUTY_CYCLE_MIN;
			}
		}

		// fan stop handling
		if(temperature < FAN_OFF_TEMPERATURE) {
			m_duty = duty_out = 0;
		}
	}

	return FXP_TO_INT(duty_out);
}