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2014-04-14 11:39:26 -0600 asked a question Physics engine for jumping simulation and multi-legged robots

My objective is to simulate the jumping behaviour of a radially symmetrical hexapod robot. I used Gazebo before and I have a ready .sdf model, and that justifies my initial choice for this simulator. But I'm afraid that ODE is not the most suitable engine for legged robotics simulations (user comments, scientific papers...). Moreover, my real robot is equipped with series elastic actuation, meaning that the legs are not directly driven by electrical motors but those are coupled with the joints through a torsional spring element, to store elastic energy to be exploited in the jump. It is crucial for me to reproduce this in the transmission specification.

Even if I manage to, (a custom spring transmission was developed 2 years ago on request, working in Gazebo 1.0.2, within the pr2 mechanism package - I guess it's possible to port it, but using now the ros_control package) I'm afraid this would not be enough to have a quasi-realistic jumping behaviour.

I'm aware of the existence of the Bullet engine, that offers a better performance in terms of friction cone and constraint handling, sliding legs and compound collision shapes, but I really don't want to lose time modelling in a framework that will not grant me a quite accurate dynamics simulation.

I would like to have some feedback from users that have used / are using Gazebo for multi-legged robotic systems.

2014-04-14 11:17:42 -0600 commented answer gzfactory spawning issues

Thank you, I solved my problem by avoiding the gzfactory command, and by just using the <pose> offset in the model.sdf file.

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2014-04-11 08:33:48 -0600 asked a question gzfactory spawning issues

Hello, whenever I try to spawn my robot model in the gazebo world from command line, typing

gzfactory spawn -f model.sdf -z 0.19

I get

Error [parser_urdf.cc:2608] Unable to call parseURDF on robot model
Error [parser.cc:273] parse as old deprecated model file failed.
Error: SDF parsing the xml failed

At first, I thought that this was caused by the fact that I had in the same folder my model.sdf and my model.urdf, so I moved the urdf file into another folder and retried. Same result. If I load the robot.urdf model, it successfully spawns with the requested offset, just to collapse into the ground after a few instants (might be a problem of collision?). If I load my robot.sdf file with a world plugin, everything is fine again, except that I would like to spawn with a global z offset and that's why I'm using gzfactory. (Of course the error occurs even if don't specify the offset). What's happening?

EDIT: I'm working with Gazebo 2.2

2014-04-10 10:37:51 -0600 commented answer How to change the origin in Gazebo?

If I want to spawn a multi link robot with an offset on the z direction, do I have to specify the offset for each link or there's a more general command for this that moves the robot all at once?

2014-04-10 08:51:29 -0600 answered a question Model names and hierarchy

I suddenly realized that I made a dumb mistake. The GAZEBOMODELPATH was pointing to hexapodsim folder (the directory containing the model files) and not to the parent directory, hexapod_models.

2014-04-10 08:43:10 -0600 asked a question Model names and hierarchy

Hello. I'm trying to load my hexapod robot model in Gazebo 2.2, following this tutorial. I had an old hexapodsim.urdf.xacro file, that I converted to hexapodsim.urdf and then to hexapodsim.sdf. So far so good. Whenever I try to launch gazebo, I get the following:

Error [ModelDatabase.cc:407] Unable to download model[model://hexapodsim]
Error [parser_urdf.cc:2608] Unable to call parseURDF on robot model
Error [parser.cc:273] parse as old deprecated model file failed.
Error [World.cc:1544] Unable to read sdf file.

I followed carefully the tutorial, and I don't understand what's going on. My folder tree is like this:

  • hexapod_model

    • hexapodsim

      • hexapodsim.cc
      • CMakeLists.txt
      • model.sdf
      • model.urdf
      • model.config
      • meshes
      • build
      • hexapod.world

My hexapodsim.cc file looks like this:

#include "gazebo/physics/physics.hh"
#include "gazebo/common/common.hh"
#include "gazebo/gazebo.hh"

namespace gazebo
{
class Hexapodsim : public WorldPlugin
{
    public: void Load(physics::WorldPtr _parent, sdf::ElementPtr /*_sdf*/)
    {
        // Option 1: Insert model from file via function call.
        // The filename must be in the GAZEBO_MODEL_PATH environment variable.
        _parent->InsertModelFile("model://hexapodsim");  
    }
};

// Register this plugin with the simulator
GZ_REGISTER_WORLD_PLUGIN(Hexapodsim)
}

I made sure that the GAZEBOMODELPATH is set correctly to the hexapodsim folder; my GAZEBOPLUGINPATH is set correctly to the build folder where my libhexapodsim.so lies. I used the same name (hexapodsim) for the model.config, model.sdf and hexapod.world file. I thought it might be a filename convention problem, or a directories hierarchy issue, because I tried to rearrange the folder configuration and I was using different names for the files, and Gazebo managed to parse the .sdf file (even though I had some errors due to the lack of a couple of plugins I was trying to load). What is wrong now? Thanks for the help.

2014-04-04 07:02:07 -0600 commented answer Gazebo 1.9 on ros fuerte

I'm in a similar situation, working with fuerte after a migration from electric and having an ancient gazebo 1.0.2 installed, that is not working properly after this. I don't know whether to update ROS and Gazebo or not (I don't want to lose my work)

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2014-04-03 10:15:52 -0600 asked a question Gazebo / OROCOS integration

Hello. I'm developing some software within the OROCOS framework for an hexapod robot. I took over the work of another guy, and he was working with a really old version of Gazebo (1.0.2) and ROS Electric. In this version, the XML robot model is implemented and loaded, but some kind of interface between OROCOS and Gazebo is missing. I'd like to have a working simulator, possibly updated to a newer version, for testing purposes. I have some newbie questions. Taking into account that I'm working with Ubuntu 12.04 and ROS Fuerte:

1) I guess using ROS as intermediate interface to communicate with Gazebo is a must. A ROS node publishes the joint states and reads the control signal from topics. An OROCOS component publishes the control signal and reads the joint states from topics. Am I right?

2) Do you recommend to update the Gazebo version I'm supposed to work with? If yes, to which one?

3) I found a github repository with some apparently useful files to cope with the OROCOS integration (https://github.com/jhu-lcsr/rtt_gazebo). Has anyone had experience with this?

4) My model is in URDF. Should I convert it to SDF, to use it with a newer version of Gazebo?

Thanks.