Where effort joint values come from

asked 2018-10-29 01:44:46 -0500

JonathanAI gravatar image

Dear all,

When I subscribe to the topic /joint_states I get the name of the joints, the angular positions of these joints and the current effort values. I assume that the effort values are in M.m, but my question is, where these effort values come from? The effort values should depend on the force and torque values of the parent and the child link of a joint. Torque and forces are denoted by 3D vectors (x,y,z), in N.m as well. So how can I get these effort values if I have the 3D force and torque vectors of the parent and child links?

Thank you!!!

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It doesn't matter for the more fundamental question, but can you please give a bit of context? Are you referring to a specific /joint_states topic (ie: published by a specific driver)?

gvdhoorn gravatar image gvdhoorn  ( 2018-10-29 03:12:29 -0500 )edit

Yes exactly. For example: rostopic echo /joint_states or for any specific group like rostopic echo /left_arm/joint_states.

Thank you

JonathanAI gravatar image JonathanAI  ( 2018-10-30 22:49:20 -0500 )edit

left_arm? Are you using a baxter?

Please provide a bit more context.

gvdhoorn gravatar image gvdhoorn  ( 2018-10-31 03:27:14 -0500 )edit

No, I am using a Kinova Jaco arm attached to a Kinova Movo base

JonathanAI gravatar image JonathanAI  ( 2018-10-31 17:57:02 -0500 )edit

I also want to know this question! I use UR5, which has no torque sensor on each joint/link. So the values are from motor currents or a computed torques?

Yummy_Ou gravatar image Yummy_Ou  ( 2021-04-28 03:50:10 -0500 )edit

ok, I got the result of UR5. /joint_states/effort is motor current, see this issue.

About the unit of effort, you could see this link. The effort is Nm for revolute.

Yummy_Ou gravatar image Yummy_Ou  ( 2021-04-29 02:07:34 -0500 )edit

@Yummy_Ou: all messages are documented. For JointState: sensor_msgs/JointState:

# This is a message that holds data to describe the state of a set of torque controlled joints. 
# The state of each joint (revolute or prismatic) is defined by:
#  * the position of the joint (rad or m),
#  * the velocity of the joint (rad/s or m/s) and 
#  * the effort that is applied in the joint (Nm or N).

which all makes sense, as those are the appropriate (derived) SI units, which adheres to REP 103.

gvdhoorn gravatar image gvdhoorn  ( 2021-04-29 02:26:28 -0500 )edit