pid.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * pid.c PID controller for testing cooling devices
 *
 * Copyright (C) 2012 Intel Corporation. All rights reserved.
 *
 * Author Name Jacob Pan <jacob.jun.pan@linux.intel.com>
 */

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <sys/types.h>
#include <dirent.h>
#include <libintl.h>
#include <ctype.h>
#include <assert.h>
#include <time.h>
#include <limits.h>
#include <math.h>
#include <sys/stat.h>
#include <syslog.h>

#include "tmon.h"

/**************************************************************************
 * PID (Proportional-Integral-Derivative) controller is commonly used in
 * linear control system, consider the the process.
 * G(s) = U(s)/E(s)
 * kp = proportional gain
 * ki = integral gain
 * kd = derivative gain
 * Ts
 * We use type C Alan Bradley equation which takes set point off the
 * output dependency in P and D term.
 *
 *   y[k] = y[k-1] - kp*(x[k] - x[k-1]) + Ki*Ts*e[k] - Kd*(x[k]
 *          - 2*x[k-1]+x[k-2])/Ts
 *
 *
 ***********************************************************************/
struct pid_params p_param;
/* cached data from previous loop */
static double xk_1, xk_2; /* input temperature x[k-#] */

/*
 * TODO: make PID parameters tuned automatically,
 * 1. use CPU burn to produce open loop unit step response
 * 2. calculate PID based on Ziegler-Nichols rule
 *
 * add a flag for tuning PID
 */
int init_thermal_controller(void)
{
	int ret = 0;

	/* init pid params */
	p_param.ts = ticktime;
	/* TODO: get it from TUI tuning tab */
	p_param.kp = .36;
	p_param.ki = 5.0;
	p_param.kd = 0.19;

	p_param.t_target = target_temp_user;

	return ret;
}

void controller_reset(void)
{
	/* TODO: relax control data when not over thermal limit */
	syslog(LOG_DEBUG, "TC inactive, relax p-state\n");
	p_param.y_k = 0.0;
	xk_1 = 0.0;
	xk_2 = 0.0;
	set_ctrl_state(0);
}

/* To be called at time interval Ts. Type C PID controller.
 *    y[k] = y[k-1] - kp*(x[k] - x[k-1]) + Ki*Ts*e[k] - Kd*(x[k]
 *          - 2*x[k-1]+x[k-2])/Ts
 * TODO: add low pass filter for D term
 */
#define GUARD_BAND (2)
void controller_handler(const double xk, double *yk)
{
	double ek;
	double p_term, i_term, d_term;

	ek = p_param.t_target - xk; /* error */
	if (ek >= 3.0) {
		syslog(LOG_DEBUG, "PID: %3.1f Below set point %3.1f, stop\n",
			xk, p_param.t_target);
		controller_reset();
		*yk = 0.0;
		return;
	}
	/* compute intermediate PID terms */
	p_term = -p_param.kp * (xk - xk_1);
	i_term = p_param.kp * p_param.ki * p_param.ts * ek;
	d_term = -p_param.kp * p_param.kd * (xk - 2 * xk_1 + xk_2) / p_param.ts;
	/* compute output */
	*yk += p_term + i_term + d_term;
	/* update sample data */
	xk_1 = xk;
	xk_2 = xk_1;

	/* clamp output adjustment range */
	if (*yk < -LIMIT_HIGH)
		*yk = -LIMIT_HIGH;
	else if (*yk > -LIMIT_LOW)
		*yk = -LIMIT_LOW;

	p_param.y_k = *yk;

	set_ctrl_state(lround(fabs(p_param.y_k)));

}