V-REP Introduction

From Mech
(Difference between revisions)
Jump to: navigation, search
(Scene 4: CSV Animation KUKA youBot)
(Scene 4: CSV Animation KUKA youBot)

Revision as of 15:53, 8 July 2017


V-REP Introduction

On V-REP's homepage the program is described as

  ...the Swiss army knife among robot simulators: you won't find a simulator with more functions, features, or more elaborate APIs.

V-REP's strength comes from several features:

  1. V-REP provides a unified framework combining many powerful internal and external libraries that are often useful for robotics simulations. This includes dynamic simulation engines, forward/inverse kinematics tools, collision detection libraries, vision sensor simulations, path planning, GUI development tools, and built-in models of many common robots.
  2. V-REP is highly extensible. V-REP developers provide an API that allows one to write custom plugins that add new features. You can embed Lua scripts directly into a simulation scene that, for example, process simulated sensor data, run control algorithms, implement user interfaces, or even send data to a physical robot. They also provide a remote API that allows one to develop standalone applications in many programming languages that are able to pass data in and out of a running V-REP simulation.
  3. V-REP is cross-platform, mostly open-source, and provides a free educational license.

The purpose of this page is not to teach you how to use V-REP. Rather it is to describe demonstration scenes that have been developed to help visualize robot arm kinematics. If you are interested in learning more about V-REP, check out the Useful Resources section below.

Demonstration V-REP Scenes

To run either of the scenes below, the first step will be to download V-REP for your operating system. You should download the latest Non-limited EDUCATIONAL version. Next you will have to install V-REP. On Windows, you simply have an EXE that installs V-REP. On a Mac, you first need to unzip the download. The directory that is produced by unzipping the download contains a vrep.app directory that should allow you start V-REP through normal mechanisms, e.g., Finder/Spotlight/Launchpad. On Linux, you will need to extract the compressed tar archive (e.g., using a command like tar xvf V-REP_PRO_EDU_V3_3_2_64_Linux.tar.gz). Then you need to change directories into the V-REP source directory and run the vrep.sh shell script.

Once V-REP is open you will want to run one of the scenes below. To run either of them, you first run V-REP, then you click File->Open scene... and open one of the ttt files that are linked below. Then click either the Play button from the top toolbar or click Simulation->Start simulation and a GUI should pop up. Clicking the Stop button or Simulation->Stop simulation will close the GUI and stop the simulation.

The scenes below are implemented similarly. They both feature a simulation of a kinematically-controlled, non-respondable UR5 robot from Universal Robots. Kinematically controlled means that all dynamics (inertias, torques, friction, etc.) of the system are neglected. We can specify a set of joint angles and the simulation is capable of instantaneously "teleporting" the robot to be at the new set of joint angles. Non-respondable means that the links of the robot are not capable of interacting with the world or each other through collisions. In other words, we can put the robot in configurations that result in self-collisions, but the simulation will ignore the collisions. The physical UR5 robot features 6 joints each with 720 degrees of rotation, and this simulated robot has the same joints and limits. The model of the UR5 in each of the scenes was created by importing a URDF from the ROS-Industrial ur5_description package.

In each scene, there is a single Lua script called a Non-threaded child script. When the scene is first run, there is a function that is called that sets up the GUI and creates variables that are going to be needed later on in the simulation. Then during every step of the simulation the main script, which is part of every V-REP scene, runs an "actuation" function from the child script. This actuation function is responsible for processing all of the changes to the GUI since the last time it was called (buttons clicked, label updates, etc.), and for sending joint commands to the simulated robot. Technically, there is also a "sensing" function in the child script that gets called by the main script, but in each of these demos, the sensing function is empty. The GUIs are both built with V-REP's Qt-based custom UI framework.

Scene 1: Interactive UR5

In this demo, the GUI features two tabs. One tab lets you drag sliders to modify the joint angles of each joint, and the other tab allows you to specify comma-separated angles for all 6 joints in an editable text box and ask for the SE(3) transformation from the base frame to the end-effector frame. The frames attached to the base and end-effector are persistently displayed (x-axis in red, y-axis in green, z-axis in blue). Note that all angles are specified in radians.

  • Download the ttt scene file here.

Scene 2: CSV Animation UR5

This demo allows you to specify a CSV file containing a trajectory of joint angles, and then animate this trajectory. Each column of the CSV file is the joint angle through time for one of the joints (first column is joint 1, last column is joint 6). In other words, a single row of the CSV file represents a complete configuration of the robot at a particular time. The assumed timestep between rows is equal to the timestep that V-REP uses for simulation; the default is 0.05 seconds. Note that will need to provide a complete path to your CSV file. Alternatively, you may be able to provide a relative path, but you'll need to figure out what V-REP uses as its base directory on your OS. On Linux, this directory is the root directory of the V-REP source code. If you figure out this directory, you could also move your CSV files to be in this directory and just reference them by name without specifying the path at all. A typical path on the Mac is something like /Users/kevin/yourdirectoryname/example.csv.

  • Download the ttt scene file here.
  • Download an example csv file here.

Scene 3: Interactive KUKA youBot

The KUKA youBot is a mobile manipulator consisting of a mecanum-wheel omnidirectional base and a 5R robot arm. This scene is similar to the UR5 interactive scene. You can interactively control the configuration of the mobile base and the arm.

  • Download the ttt scene file here.

Scene 4: CSV Animation KUKA youBot

This scene is similar to the UR5 animation scene, except now the csv file consists of 12 configuration variables per row: 3 for the mobile base (ordered as x, y, phi), 5 for the arm (joints 1 to 5), and 4 for the wheels. If you don't want to show the wheels moving, or if you have not correctly solved the kinematics relating the wheel motions to the mobile base motion, you can just set all 4 wheels angles to zero.

  • Download the ttt scene file here.
  • Download an example csv file here.

Switching Between Scenes

Press the Stop button to stop the simulation of the current scene, then choose File>Open scene.... You can also use File>Open recent scene to switch to a scene you previously loaded. Then you press the Play button to run the scene. Alternatively, stop the simulation and then press the Scenes button in the top toolbar to see which scenes are currently open and select one to be in the foreground. The scene selector toolbar button may also be used to switch between opened scenes. Read more here.

Recording a Movie

V-REP comes with a video recorder. Go to Tools>Video recorder. You may need to stop the current scene to be able to configure the video recorder. You can find more information on recording V-REP movies here: http://www.coppeliarobotics.com/helpFiles/en/aviRecorder.htm.

A simpler option may be to just use your computer's screen recording software. On the Mac, you can use Quicktime. On Linux, you can use SimpleScreenRecorder or recordMyDesktop. On Windows, you can use Screen Recorder. Or you may have your own solution.

Exploring Other Scenes

You are encouraged to explore some of the (quite impressive) scenes that come pre-loaded with V-REP. You can find these scenes in the scenes directory under the V-REP directory. Running and studying these can be a great way to learn more about the V-REP capabilities and to understand how to put together more complex scenes.

Useful Resources

Personal tools