Getting started with OpenRAVE

You can acquire knowledge from Wikipedia, books, articles, manuals, conversations with knowledgeable people, etc. You will then learn when, by experience, you "wire" this knowledge into your brain. If trials are hard and time consuming, your wiring will basically be a long (painful) line. But when trials are cheap and fast, the wiring will spread wider as you are free to explore many possibilities.

In motion planning, your first-hand experiences will mainly come from software, so you should get a good development environment. There are several options available today; the one I have been using during my PhD is OpenRAVE. In my opinion, it is very good for learning as the scripting level is in Python, which allows you to explore your objects interactively. (Core functionalities themselves are implemented in C++ for better performances.) For instance, in IPython:

  • robot.<TAB><TAB> gives the list of methods available for robot;
  • env.SetViewer? provides documentation on the SetViewer() function of env.

In this post, we will set up an initial environment, create our first robot and see how to manipulate it via the GUI. If you don't have it installed on your system, see the installation instructions for Ubuntu 14.04, or the official documentation if you run another system.

Environment and Robot

A basic Python script for OpenRAVE looks like this:

#!/usr/bin/env python

import openravepy

if __name__ == "__main__":
    env = openravepy.Environment()
    env.Load('env.xml')
    env.SetViewer('qtcoin')
    viewer = env.GetViewer()
    viewer.SetBkgndColor([.8, .85, .9])  # RGB tuple
    robot = env.GetRobots()[0]

To execute the script, run ipython -i myscript.py. The -i flag tells IPython to spaw an interactive shell at the end. To get a shorter command, you can also add the following two lines at the end of the Python script:

import IPython
IPython.embed()

And then make the script executable by chmod +x myscript.py. This way, the interactive shell will always spawn at the end, and you can call your script directly by:

./myscript.py

In this script, we define three objects: the environment env, the viewer and the robot. The environment object is your main interface with OpenRAVE, from which you will access robots, viewer, drawing primitives, etc. It is defined in an XML file.

env.xml

In this example, our environment env.xml contains a single robot loaded from a separate file:

<?xml version="1.0" encoding="utf-8"?>
<Environment>
    <Robot file="double-pendulum.xml" />
</Environment>

double-pendulum.xml

We define the double pendulum in double-pendulum.xml using OpenRAVE's XML format. For more complex robots, you can also import COLLADA (1.5) models. The robots consists of three bodies (Base, Arm0 and Arm1) connected by two circular joints. We also add some mass information for inverse dynamics, as well as some colors for readability:

<?xml version="1.0" encoding="utf-8"?>
<Robot name="Pendulum">
    <RotationAxis>0 1 0 90</RotationAxis> <!-- makes the pendulum vertical -->
    <KinBody>
        <Mass type="mimicgeom"><density>100000</density></Mass>
        <Body name="Base" type="dynamic">
            <Translation>0.0  0.0  0.0</Translation>
            <Geom type="cylinder">
                <rotationaxis>1 0 0 90</rotationaxis>
                <radius>0.03</radius>
                <height>0.02</height>
                <ambientColor>1. 0. 0.</ambientColor>
                <diffuseColor>1. 0. 0.</diffuseColor>
            </Geom>
        </Body>
        <Body name="Arm0" type="dynamic">
            <offsetfrom>Base</offsetfrom>
            <!-- translation and rotation  will be relative to Base -->
            <Translation>0 0 0</Translation>
            <Geom type="box">
                <Translation>0.1 0 0</Translation>
                <Extents>0.1 0.01 0.01</Extents>
                <ambientColor>1. 0. 0.</ambientColor>
                <diffuseColor>1. 0. 0.</diffuseColor>
            </Geom>
        </Body>
        <Joint circular="true" name="Joint0" type="hinge">
            <Body>Base</Body>
            <Body>Arm0</Body>
            <offsetfrom>Arm0</offsetfrom>
            <weight>4</weight>
            <axis>0 0 1</axis>
            <maxvel>3.42</maxvel>
            <resolution>1</resolution>
        </Joint>
        <Body name="Arm1" type="dynamic">
            <offsetfrom>Arm0</offsetfrom>
            <Translation>0.2 0 0</Translation>
            <Geom type="box">
                <Translation>0.1 0 0</Translation>
                <Extents>0.1 0.01 0.01</Extents>
                <ambientColor>0. 0. 1.</ambientColor>
                <diffuseColor>0. 0. 1.</diffuseColor>
            </Geom>
        </Body>
        <Joint circular="true" name="Joint1" type="hinge">
            <Body>Arm0</Body>
            <Body>Arm1</Body>
            <offsetfrom>Arm1</offsetfrom>
            <weight>3</weight>
            <axis>0 0 1</axis>
            <maxvel>5.42</maxvel>
            <resolution>1</resolution>
        </Joint>
    </KinBody>
</Robot>

After writing the two XML files and running the Python script, the GUI should pop up and display the pendulum, yet from above. To move the camera in front of it, do:

viewer.SetCamera([
    [0.,  0., -1., 1.],
    [1.,  0.,  0., 0.],
    [0., -1.,  0., 0.],
    [0.,  0.,  0., 1.]])

Now, you should see something like this:

OpenRAVE viewer at startup

Using the GUI

The user interface has two modes: camera mode and object interaction mode. Press Esc to switch between the two.

Camera mode

Camera mode is activated by default. If not, click on the left-hand icon (second icon from top) in the right-hand panel of the GUI. In camera mode, your mouse behaves as follows:

  • Left click: rotate the camera (you can also use the RotX and RotY wheels at the bottom-left corner of the window)
  • Shift + left click or Middle button: translate the camera
  • Left + middle button, or Mouse wheel: zoom in or out (you can also use the Dolly wheel at the bottom-right of the window)
  • S + left click: zoom in on the point clicked. After this, the camera will stay centered on the object when rotating.

Interaction mode

You can switch between camera and interaction by pressing the Escape key of your keyboard. Alternatively, click on the red-arrow icon (first icon from top) in the right-hand panel to activate interaction mode.

In interaction mode, left-click on an object to select it. A cube appears around it, which you can use to perform two operations:

  • Translation: click on a face of the cube and drag it around. It will translate the object in the two directions corresponding to the face of the cube.
  • Rotation: click on an edge of the cube and drag it around. It will rotate the object along the rotation axis parallel to the selected edge and passing through the center of the cube.

The control cube will vanish if you click again on the object.

Interaction mode is also used to manipulate the joints of a robot directly. Doing Ctrl + left click on a robot's link selects the parent joint of the link (that is to say, the joint connecting the link to its parent in the kinematic chain). A cylinder will then appear at the joint, which you can turn around to rotate the joint. You can also vary the size of the cylinder by ctrl-cliking it. Here is what you should see by selecting the link Arm1 of the pendulum:

OpenRAVE viewer at startup

The text area at the top-left corner of the 3D viewer displays relevant information on the selected object. Here, it says that the pointer is on the link Arm1 at the world-frame coordinates (x, y, z) = (0.10, 0.07, 0.27), with the surface normal at this point n = (1., 0., 0.). The second line tells us that we selected the robot Pendulum, and the third gives us information on the selected joint: name (Joint1), index (1) and angle in radians and degrees. The joint index tells you the position of the joint coordinate in the DOF vectors, which we will use later on to manipulate the robot's configuration.

Summary of controls

Here is the summary of the controls we have seen so far. There are other buttons in the right-side panel, but honestly I never use them.

  • Esc: switch between camera and interaction modes
  • S + left click: center the camera on a surface point
  • Ctrl + left click: (interaction mode) select a robot joint
  • Left click: (camera mode) rotate the camera
  • Left click: (interaction mode) select an object
  • Shift + left click or Middle button: (camera mode) translate the camera
  • Wheel: (camera mode) zoom in or out

Saving and setting the camera

You will notice that the camera's position and orientation (its transform) are lost when you exit and run the script again. You can get the current camera transform via the viewer object:

In [1]: viewer.GetCameraTransform()
Out[1]:
array([[-0.67725463,  0.23636616, -0.69674759,  0.40368351],
       [ 0.73512462,  0.25638653, -0.62758086,  0.43931767],
       [ 0.03029782, -0.93722835, -0.34739755,  0.42627937],
       [ 0.        ,  0.        ,  0.        ,  1.        ]])

To save and restore your camera settings between sessions, set the camera transform at the beginning of your script via viewer.SetCamera(). In the case above:

viewer.SetCamera([
    [-0.67725463,  0.23636616, -0.69674759,  0.40368351],
    [ 0.73512462,  0.25638653, -0.62758086,  0.43931767],
    [ 0.03029782, -0.93722835, -0.34739755,  0.42627937],
    [ 0.        ,  0.        ,  0.        ,  1.        ]])

Forward kinematics

Let us go back to our Python script. All functions to manipulate the robot model are accessible via the robot object. First, to check its degree of freedom, try:

In [1]: robot.GetDOF()
Out[1]: 2

OpenRAVE also calls "DOF" the generalized coordinates of the system, that is to say here the joint angles of the pendulum. The vector \({\bf q}\) of generalized coordinates is called "DOF values", the vector \(\dot{\bf q}\) of generalized velocities is called "DOF velocities", etc. Initially, the two joint angles are zero, so:

In [2]: robot.GetDOFValues()
Out[2]: array([ 0.,  0.])

To set the pendulum to a different configuration, for instance \({\bf q} = (\pi/4, \pi/4)\), do:

In [3]: robot.SetDOFValues([3.14 / 4, 3.14 / 4])

You will see the robot updated to a different pose in the GUI. This operation, the geometric update of all links of the robot from the joint-angle vector, is called forward kinematics.

Playing a trajectory

Let us interpolate and play trajectories on the pendulum. First, define a simple Trajectory class containing the duration \(T\) and joint-angle function \({\bf q}(t)\) of the trajectory:

class Trajectory(object):

    def __init__(self, T, q_ftn):
        self.T = T  # duration, in seconds
        self.q = q_ftn

We generate a simple linear interpolation between the current state of the robot (which may be anything: random, set by the user in the GUI, etc.) The default duration will be \(T = 1\) s.

from numpy import array, pi

def get_swing_trajectory(robot, T=1.):
    q0 = robot.GetDOFValues()
    q1 = array([pi, 0.])

    def q(t):
        x = 0. if t < 0. else 1. if t > T else (t / T)
        return (1. - x) * q0 + x * q1

    return Trajectory(T, q)

Finally, we play the trajectory by setting the DOF values at regular time intervals:

from numpy import arange
import time

def play_trajectory(robot, traj):
    dt = 1e-2  # play at 100 Hz
    for t in arange(0., traj.T + dt, dt):
        robot.SetDOFValues(traj.q(t))
        time.sleep(dt)

You can check out the result with:

traj = get_swing_trajectory(robot)
play_trajectory(robot, traj)

Whatever the current configuration of the robot is, you should see it swing up to its upward vertical configuration.

Conclusion

You now have your first environment set up and you know how to visualize and update your robot from a joint-angle vector by forward kinematics. Next, you will want to compute your joint-angle vectors to achieve a particular goal, for instance: how to put the tip of the robot at a specific location in space? The answer to this question is called inverse kinematics.

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