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applying a force to a rigid body

asked 2011-08-29 01:23:09 -0500

erpa gravatar image

updated 2011-08-30 00:20:37 -0500

Hi all, I'm trying to write a controller for a robot that is modelled as a rigid body. In particular the robot is made by several link connected through fixed joints. I would like to apply a simple force along one of the axis to the main link of the robot. I tried to look at the erratic controller but i just found a way to apply velocity to the joints connected to the wheels. Is there a way to apply a force to a link?

thanks for your help!

EDIT: I'm working with gazebo and I would like to apply a force when a certain key is pushed (I already have implemented the teleoperation module). Is it possible to apply both an impulsive force and a continuos force?

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Are you talking about Gazebo? If yes, you could add the Gazebo tag to your question.
Martin Günther gravatar imageMartin Günther ( 2011-08-29 21:37:12 -0500 )edit

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answered 2011-08-29 21:38:52 -0500

updated 2011-08-30 00:44:03 -0500

Assuming you are talking about Gazebo, you can apply a force to a body using this command on the command line:

rosservice call /gazebo/apply_body_wrench '{body_name: "object_1::object_1_base_link", reference_frame: "object_1::object_1_base_link", wrench: { force: { x: 0, y: 0, z: -20.0 } }, start_time: 0, duration: -1 }'

Of course, you can also call the apply_body_wrench service from a program.

Edit: Another alternative is accessing the body from a Gazebo plugin. I haven't done that yet, but I think it should run along these lines:

  • you get a Gazebo Model object in the constructor of your controller

  • use Model->GetBody(const std::string &name) to get the body representing your link by name

  • use Body->SetForce(const Vector3 &force) to apply the force

I also found looking at turtlebot_gazebo_plugins as an example for some of these steps very helpful.

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hi, thanks for answering! yes i work with gazebo (i edited both the tags and the question for future reference) but rather then calling rosservice through a new terminal i would like to apply it in my controller so i was looking for the cpp function. i'm sorry if my question was missleading.
erpa gravatar imageerpa ( 2011-08-30 00:24:04 -0500 )edit
Okay, I've edited my answer to that effect.
Martin Günther gravatar imageMartin Günther ( 2011-08-30 00:44:27 -0500 )edit
thanks, it's exactly what i was looking for! Just one last question, trying to run my simulation it appears that the force is applied using world reference frame instead then body frame, am i mistaken? I will also take a look at turtlebot_gazebo_plugins as soon as it finish the download :D
erpa gravatar imageerpa ( 2011-08-30 01:29:06 -0500 )edit
That could be true. I don't know if there's a better way, but perhaps you have to rotate the force vector yourself (grep for "rotate" in gazebo_plugins to see some examples).
Martin Günther gravatar imageMartin Günther ( 2011-08-30 21:28:08 -0500 )edit
1
Solved! To apply the force in the object reference frame first you have to get the pose of the object itself with GetWorldPose() and rotate the force vector accordingly ( i used RotateVector() ), last you can apply the force to the object. thanks again for help.
erpa gravatar imageerpa ( 2011-08-31 03:13:13 -0500 )edit
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answered 2018-08-09 12:27:45 -0500

Requrements:

from gazebo_msgs.srv import ApplyBodyWrench from geometry_msgs.msg import Wrench

Gazebo service call for applying a force on a body

def applyForce(): rospy.wait_for_service('/gazebo/apply_body_wrench') force = rospy.ServiceProxy('/gazebo/apply_body_wrench',ApplyBodyWrench)

wrench          = Wrench()
wrench.force.x  = 0
wrench.force.y  = 0
wrench.force.z  = 0
wrench.torque.x = 0
wrench.torque.y = 0
wrench.torque.z = 0
# You can also define the start time if necessary... 
force(body_name = "base_link",wrench = wrench, duration = rospy.Duration(10))
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answered 2018-10-16 07:08:25 -0500

littleghost gravatar image

Firstly, in your URDF file, you need to use its native plugin called "libgazebo_ros_force.so". Secondly, you need to publish "geometry_msgs::Wrench" type data in a node. When you publish the force data you want, you can see the rigid body moves.

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Asked: 2011-08-29 01:23:09 -0500

Seen: 2,938 times

Last updated: Oct 16 '18