Notes on how to use Webots, especially how to make a robot fly in the air

How to create a new project

Wizard - New project directory

 

Scene Tree

Scene tree is a representation of the current world

 

How to create a controller

Wizards - New Robot Controller

 

How to link a controller with a robot

 

Scene tree - “Robot” - Controller - Choose

 

Sensors and Actuators in Robot(class):

 

#include <webots/Robot.hpp> 

 

  Accelerometer *Robot::getAccelerometer(const std::string &name); 

  Camera *Robot::getCamera(const std::string &name); 

  Compass *Robot::getCompass(const std::string &name); 

  Connector *Robot::getConnector(const std::string &name); 

  Display *Robot::getDisplay(const std::string &name); 

  DistanceSensor *Robot::getDistanceSensor(const std::string &name); 

  Emitter *Robot::getEmitter(const std::string &name); 

  GPS *Robot::getGPS(const std::string &name); 

  Gyro *Robot::getGyro(const std::string &name); 

  InertialUnit *Robot::getInertialUnit(const std::string &name); 

  LightSensor *Robot::getLightSensor(const std::string &name); 

  Motor *Robot::getMotor(const std::string &name); 

  Pen *Robot::getPen(const std::string &name); 

  PositionSensor *Robot::getPositionSensor(const std::string &name); 

  Receiver *Robot::getReceiver(const std::string &name); 

  Servo *Robot::getServo(const std::string &name); 

  TouchSensor *Robot::getTouchSensor(const std::string &name);

 

DESCRIPTION

These functions return a reference to an object corresponding to a specified name. Depending on the called function, this object can be an instance of a Device subclass. For example, if a robot contains a DistanceSensor node whose name field is "ds1", the function getDistanceSensor will return a reference to a DistanceSensor object. If the specified device is not found, the function returns NULL in C++, null in Java or thenone in Python.

 

DARwIn-OP model

The DARwIn-OP model contains principally:

- the 20 DOF

- LEDs

- accelerometer

- gyro

- camera

- realistic physics

 

There is no emitter or receiver in this model, so if we want to use emitter and receiver, we have to use other robots or define robots our own.

 

 

Supervisor

Supervisor  is a special kind of robot. They can call functions of other robots and can also take a screen shot or a video of the simulation, restart or terminate the simulation, etc. It can read or modify the value of every fields in the Scene Tree, e.g. read or change the position of robots, the colors of objects, or switch on or off the light sources, and do many other useful things.

 

 

 

How to make a robot hover in the air

The key idea here is to give the robot some force to overcome gravity.

Give the robot a name:

Scene Tree - Aircraft - DEF: AIRCRAFT

2.   Give the robot a initial position:

      Scene tree - “aircraft robot” - translation - 0, 0.5, 0.5

3.   Create a physics plugin:

          Wizard - New Physics Plugin

4.    After that, we will get a physics file in C or C++

5.    In the physics file:

        Define a dBodyID which will refer to the aircraft: 

        static dBodyID body1;

6.    In the function void webots_physics_init(dWorldID world, dSpaceID space, dJointGroupID contactJointGroup)  of the physics file, initialize body1:

        body1 = dWebotsGetBodyFromDEF("AIRCRAFT");

7.    In the function void webots_physics_step() of the physics file, add the force which would overcome gravity:

        if(body1)  dBodyAddForce(body1, 0, 9.81, 0);

8.     Add the physics plugin into the project:

        Scene tree - WorldInfo - physics - Select - “my-physics”.

 

 

How to output to console

Include the file stdio.h and use printf(“hello\n”).

The tricky thing here is that the command in printf must end up with a ‘\n’. Else it will not be output.

 

How to make a robot move in the air

The basic idea is to make the robot communicate with the physics plugin with radio and add force in the physics plugin to help it move.

Add an emitter in the robot to communicate with physics plugin:

Scene tree - “robot” - child - add emitter

2.   Set Emitter range to -1, channel to 0, baudRate to -1. This is because the physics can only receive radio in channel 0.

3.   Send the signal where you want the robot move with function wb_emitter_send.

4.    In the physics plugin, receive the signal sent by the robot with function dWebotsReceive.

5.    Use the function dBodyAddForce to give force to the robot and make it move.

PS.

If we only want the robot to move for several steps, we should remove the force given before by adding the opposite force.

If we want to make the robot move with keyboard instructions, we should enable keyboard in the robot controller and listen to the keyboard.

If we want to use one robot to send instruction to another robot to make it move, we can just sent radio signal in channel 0 with the designed data format.