en:lesson07 [ОрбиКрафт 3D]
en:lesson07

Lesson 07. OrbiCraft positioning with magnetometer

Program for turning by magnetometer

Create the program listed below. Factors for mag_calibrated function must be taken from Magneto software.

С code.

orientation.c
#include <stdio.h>
#include <stdint.h>
#include "libschsat.h"
#define LSS_OK 0 
#define LSS_ERROR 1 
#define LSS_BREAK 2
#include <math.h>
 
const int kd = 200; /* Differential feedback coefficient. If the satellite’s angular velocity is positive, than satellite must be spun clock-wise (i.e. the flywheel must rotated clock-wise)*/
 
const int kp = -50; /* Proportional feedback coefficient. If the current angle is more than target angle, than satellite must be spun counter-clock-wise (i.e. the flywheel must rotated counter-clock-wise).*/
const float time_step = 0.05; 			// working time-step
const uint16_t mtr_num = 1;			// flywheel number
const uint16_t hyr_num = 1; 		        // angular velocity sensor number
const uint16_t mag_num = 1;			// magnetometer number
// Maximum allowable flywheel speed, rpm
const int16_t mtr_max_speed = 5000;
int16_t mtr_new_speed;
 
int mag_calibrated(int16_t *magx, int16_t *magy, int16_t *magz ){
//mag_calibrated function will adjust the magnetometer’s readings considering the calibration factors
	float magx_cal;
	magx_cal = 1.06*(*magx + -7.49) + -0.01*(*magy + -23.59) + 0.07*(*magz + -108.24);
	float magy_cal;
	magy_cal = -0.01*(*magx + -7.49) + 1.11*(*magy + -23.59) + 0.09*(*magz + -108.24);
	float magz_cal;
	magz_cal = 0.07*(*magx + -7.49) + 0.09*(*magy + -23.59) + 1.00*(*magz + -108.24);
	*magx = (int16_t) magx_cal;
	*magy = (int16_t) magy_cal;
	*magz = (int16_t) magz_cal;	
	return 0;
}
 
int motor_new_speed_PD(int mtr_speed, int omega, int omega_goal){	
	/* Function for calculating the new flywheel speed*/
 
	mtr_new_speed = (int)(mtr_speed + kd * (omega - omega_goal));	
	if (mtr_new_speed > mtr_max_speed)
	{
		mtr_new_speed = mtr_max_speed;
	}
	else if (mtr_new_speed < -mtr_max_speed)
	{
		mtr_new_speed = -mtr_max_speed;
	}
	return mtr_new_speed;
}
 
void initialize_all(void){
/*	 All systems’ initialization function*/
	printf ("Enable motor №%d\n", mtr_num);
	motor_turn_on(mtr_num);
	Sleep(1);
	printf("Enable angular velocity sensor №%d\n", hyr_num); 
	hyro_turn_on(hyr_num); // Включаем ДУС
	Sleep(1);
	printf("Enable magnetometer %d\n", mag_num);
	magnetometer_turn_on(mag_num);
	Sleep(1);
}
 
int switch_off_all(){
/*	 All systems’ switching off function*/
	printf("Finishing...");
	int16_t new_speed = mtr_new_speed; 
	printf("\nDisable angular velocity sensor №%d\n", hyr_num);
	hyro_turn_off(hyr_num);
	printf("Disable magnetometer %d\n", mag_num);
	magnetometer_turn_off(mag_num);
	motor_set_speed(mtr_num, 0, &new_speed);
	Sleep (1);
	motor_turn_off(mtr_num);
	printf("Finish program\n");
	return 0;
}
 
int control(){
	float mtr_new_speed;
	//magnetometer data
	int16_t mgx_cal=0;
	int16_t mgy_cal=0;
	int16_t mgz_cal=0;
	int16_t *magx_raw = &mgx_cal;
	int16_t *magy_raw = &mgy_cal;
	int16_t *magz_raw = &mgz_cal;
	//angular velocity sensor data
	int16_t gx_raw;
	int16_t gy_raw;
	int16_t gz_raw;
	int16_t *hyrox_raw=&gx_raw;
	int16_t *hyroy_raw= &gy_raw;
	int16_t *hyroz_raw = &gz_raw;
	int16_t mtr_speed;
	int16_t omega;
	int16_t omega_goal=0;  // Target angular speed
 
	initialize_all();
 
	int mtr_state = 0;		// Initialize flywheel status
	int hyro_state = 0; 		// Initialize angular velocity sensor status
	int mag_state = 0; 		// Initialize magnetometer status
	int alpha_goal = 0;		// Target angle	
	double mag_alpha = 0;
	int i;
	for (i = 0; i < 500; i++)	{
		mag_state = magnetometer_request_raw(mag_num, magx_raw, magy_raw, magz_raw);
		hyro_state = hyro_request_raw(hyr_num,hyrox_raw,hyroy_raw,hyroz_raw); 
		gx_raw = *hyrox_raw; 
		gy_raw = *hyroy_raw; 
		gz_raw = *hyroz_raw; 
		mtr_state = motor_request_speed(mtr_num, &mtr_speed);
		if (!mag_state){
			mag_calibrated(magx_raw,magy_raw,magz_raw);
			*magy_raw = - *magy_raw;/* transition from the left coordinate system which is plotted on the magnetometer, to the right coordinate system to position the positive angle direction counter clock-wise*/
			mag_alpha = atan2(mgy_cal, mgx_cal)/M_PI*180;
			printf("magx_cal = %d", mgx_cal);
			printf(" magy_cal = %d", mgy_cal);
			printf(" magz_cal = %d", mgz_cal);
			printf(" mag_alpha atan2 = %f\n", mag_alpha);
		}
		else if (mag_state == 1){
			printf("Fail because of access error, check the connection\n");
		   }
		else if (mag_state == 2){
			printf("Fail because of interface error, check your code\n");
		  }
 
	if (!hyro_state){
			float gx_degs = gx_raw * 0.00875;
			float gy_degs = gy_raw * 0.00875;
			float gz_degs = gz_raw * 0.00875;
			/* if the angular velocity sensor sensor arranged via z-axis up, then satellite’s angular velocity matches to sensors’ readings of z-axis */
			omega = gz_degs;
			printf("\ngx_degs = %f", gx_degs);
			printf(" gy_degs = %f", gy_degs);
			printf(" gz_degs = %f\n", gz_degs);
		}
		else if (hyro_state == 1){
			printf("Fail because of access error, check the connection\n");
		}
		else if (hyro_state == 2){
			printf("Fail because of interface error, check your code\n");
		}
 
		if (!mtr_state)	{
			// if the flywheel returned error code 0 (no error)
			int16_t mtr_speed=0;
			motor_request_speed(mtr_num, &mtr_speed);
			printf("\nMotor_speed: %d\n", mtr_speed);	
			// setting a new flywheel speed
			mtr_new_speed = motor_new_speed_PD(mtr_speed,omega,omega_goal);
			motor_set_speed(mtr_num, mtr_new_speed, &omega);
		}	
		Sleep(time_step);	
	}
	switch_off_all();
	return 0;
}

Program performance analysis

To work with OrbiCraft the program uses such functions:

magnetometer_turn_on(num)

– magnetometer switching’on function, where num is the number of the magnetometer.

magnetometer_turn_off(num)

– magnetometer switching’off function, where num is the number of the magnetometer.

magnetometer_request_raw(num)

– the function that returns raw data measured with magnetometer with num number presented as list with 4 digit values.

hyro_turn_on(hyr_num)

– angular velocity sensor’s switching’on function, where hyr_num is the sensor’s number.

hyro_turn_off(hyr_num)

– angular velocity sensor’s switching’off function, where hyr_num is the sensor’s number.

hyro_request_raw(hyr_num)

– the function that returns raw data measured with sensor with hyr_num number presented as list with 4 digit values.

motor_turn_on(num)

– flywheel’s switching’on function, where num is flywheel number.

motor_turn_off(num)

– flywheel’s switching’off function, where num is flywheel number.

motor_set_speed(mtr_num, speed)

– the function that sets flywheel’s (with mtr_num, number) speed to the speed value.

Program’s test

Test the program at such values of alpha_goal target angle: 0, 90, -90, 180.

At the turning at 180 degree target angle the program will perform incorrectly.

To fix the functional bug add to the program angle_transformation function.

To define the steering command for the flywheel it is critical to know not the angular orientation of the satellite as related to magnetic field but the angular error (difference between the current angle and target angle alpha_goal). By default the angle will be defined in range from -180 to 180 degrees via atan2 funtion.

angle_transformation function will change the angle measurement range. For the task of positioning the angle must be measured in the range from (alpha_goal-180) to (alpha_goal+180), where alpha_goal is the target angle between magnetometer’s x-axis and projection of magnetic field vector to horizontal plane.

def angle_transformation(alpha, alpha_goal):
	if alpha<=(alpha_goal - 180):
		alpha = alpha + 360
	elif alpha>(alpha_goal +180):
		alpha = alpha - 360
	return alpha

Corrected program’s test

Test the program at such values of alpha_goal target angle: 0, 90, -90, 180. Now at the turning at 180 degree target angle the program will perform correct.

en/lesson07.txt · Last modified: 2020/08/14 15:36 by golikov