Webots notes day 2

webots note the second day

1. Actuator（ Actuator ） Use

• The actuator is similar to the sensor , The label used is also Wb_Device_Tag, So is the calling function wb_robot_get_device() , But there is no need to call the open function wb_*_enable().
• For motion control , The action is broken down into several steps for control .

wb_motor_set_position() Request the corresponding position of the motor .

#include <webots/robot.h>
#include <webots/motor.h>
#include <math.h>

#define TIME_STEP 32

int main() {
    
wb_robot_init();

WbDeviceTag motor = wb_robot_get_device("my_motor");

double F = 2.0; // frequency 2 Hz
double t = 0.0; // elapsed simulation time

while (1) {
    
double pos = sin(t * 2.0 * M_PI * F);
wb_motor_set_position(motor, pos);
wb_robot_step(TIME_STEP);
t += (double)TIME_STEP / 1000.0;
}

return 0;
}

• webots/motor.h Speed of motor in , The acceleration , Definition of force and various parameters .
• wb_motor_set_position() Specify only the desired target value , Similar to a real robot , The rotation angle of the motor can not reach the expected value , Because the torque of the motor is not enough to resist gravity . If you want to control the rotation angle of multiple motors at the same time , It is necessary to set the expected value for the rotation angle of each motor .

2. wb_robot_step() Use

Webots There are two different uses , Control delay and analog delay .

• Control delay （wb_robot_step() Use ）
• Analog delay （ Definition in the scene tree ：WorldInfo.basicTimeStep）

Each controller needs to call wb_robot_step(), Otherwise, the sensor and actuator will not update the data and get stuck （ Only in synchronous mode ）

3. Use sensors and actuators together

Webots And each robot controller runs in a separate process . such as , The simulation includes several robots , There are three processes in total ： One is Webots Of , The other two are robotic . Each controller procedure calls wb_robot_step() Exchange sensor and actuator data . such as ,wb_motor_set_position You can't send data to Webots.

Look at the code below ：

while(1){
    
    double d1 = wb_distance_sensor_get_value(ds1);
    double d2 = wb_distance_sensor_get_value(ds2);
    if(d2<d1)                //WRONG: d2 will always equal to d1 here
        avoidCollision();
    wb_robot_step(40);
}

Because there is no... Between the two sensor data reading functions wb_robot_step() function , So two values cannot be changed at the same time .

This is the right code ：

while(1){
    
    double d1 = wb_distance_sensor_get_value(ds1);
    wb_robot_step();
    double d2 = wb_distance_sensor_get_value(ds2);
    if (d2<d1)
        avoidCollision();
    wb_robot_step();
}

However, it is generally recommended to include only one in the main control loop |wb_robot_step() Function to update all sensor and actuator data at the same time . as follows ：

while(1){
    
    readSensors();
    actuateMotors();
    wb_robot_step(TIME_STEP);
}

Here we can put wb_robot_step() Function at the beginning of the loop . This allows the sensor to have valid data before reading the sensor data . otherwise , The first iteration may be performed without the sensor being defined .

while(1){
    
    wb_robot_step(TIME_STEP);
    readSensors();
    actuateMotors();
}

example ：

#include <webots/robot.h>
#include <webots/differential_wheels.h>
#include <webots/distance_sensor.h>

#define TIME_STEP 32

int main() {
    
wb_robot_init();

WbDeviceTag left_sensor = wb_robot_get_device("left_sensor");
WbDeviceTag right_sensor = wb_robot_get_device("right_sensor");
wb_distance_sensor_enable(left_sensor, TIME_STEP);
wb_distance_sensor_enable(right_sensor, TIME_STEP);

while (1) {
    
	wb_robot_step(TIME_STEP);
    
    // read sensors
	double left_dist = wb_distance_sensor_get_value(left_sensor);
	double right_dist = wb_distance_sensor_get_value(right_sensor);
	
    // compute behavior
    double left = compute_left_speed(left_dist, right_dist);
    double right = compute_right_speed(left_dist, right_dist);
    
    // actuate wheel motors
    wb_differential_wheels_set_speed(left, right);
}
    
return 0;
}