UR Joint Positions

dannyhef 2020-05-22

I'm wondering if it is possible the get the joint positions (x,y,z) and not just the angles.

jelms 2020-05-24

I think what you are asking for is Cartesian coordinates for each joint. There is a way to get the TCP in xyz. I don't think there is a built in way to request them. Computing the forward kinematic transform for each intermediate joints position in base coordinates using the specific robot parameters. You can probably use pose_trans() if you need to compute it in a script.

I am curious why you would need that. Care to share?

matthewd92 2020-05-24

I know that the elbow position is available on the RTDE interface but I cannot find any other location where it would be available via script. I would submit your use case as a feature request to UR. They may be able to add that to the script manual in future releases if you have a compelling use case or may be able to help you calculcate it.

dannyhef 2020-05-26

Thanks guys for your anwsers. I will forward my use use case to UR and maybe I can figure something with the rtde interface or to calculate the positions via manual rotation matrices. I need the positions because I'm working right now on my master thesis and I'm implementing dynamic safety zones depending on the nearest position of any joint or TCP to a human.

I'll keep you updated

matthewd92 2020-05-26

Since the Elbow position and the TCP are being broadcast on the RTDE channel you could add the tool flange position as well to the RTDE interface, it's a bit clumsy as you would need to use 6 float registers but the position of the tool flange is available from URScript so you could have a function that is running in a background thread that is populating those registers at all time. This would allow you from an external source to know where the 3 largest sources of collision are located at any time, elbow, wrist group and tcp. Just a thought and this is available today without needing UR to do anything. Having worked around the robots for over 5 years I do not see any reason to need to calculate the cartesian position of the base or shoulder joints and the wrists are closely coupled enough that you could simply place a bounding box around the tool flange and know where all 3 wrists are with a pretty high level of confidence based simply on the pose of the tool flange. Their geometric relationship to each other are defined after all.

Would that give you everything that you need for the dynamic safety zones? My understanding was this is the reason that UR started broadcasting the elbow position as it was the greatest risk to safety as you could not know where it was prior without doing all the kinematics to calculate it.

Also, have you seen this post on the UR Support site? It may be helpful, I have only briefly read it as I am not overly interested in the underlying kinematics (it was after all one of my least favorite classes in college).

dannyhef 2020-05-28

Hey Matthew, you are totally right. That's all what I need, I have the Base, I have the Elbow via RTDE and I have the TCP. So now I have to work with the RTDE interface. Thanks a lot for your help.
I tried the control loop example of the rtde from UR but it tells me that a variable is already in use and I have no idea why. The problem was also stated on zacobria but with no solution. Any ideas?

matthewd92 2020-05-28

Not sure, do you have any URCaps installed on the robot? Try taking out the input map, just monitor the RTDE outputs. That is the example i used originally to build our first RTDE listener off of. We did not inject anything into the robot through RTDE so we only listened to what it was outputting.