UR10 CB3 series - Meaning and source of outputs received through RTDE

Aashish 2020-05-21

I am working on the robot (Universal Robots - UR10 CB3 series) data received through RTDE for quite a few months now. My objective is to understand the source of the values for the outputs received through RTDE as well as the meaning and relationship between the outputs.

For example, the values under "joint_temperatures_0" are received from the temperature sensor present in the "joint_0" of the robot.

I already have found out, that the output "joint_control_ouput_0" is the target current for the motor in "joint_0". This is true because the values under "actual_current_0" and "joint_control_output_0" are same. Whereas there is another output called "target_current_0", for which there is not much information provided and therefore I have the following questions:

1. What is the meaning for "target_current_0"?

2. What is the theoretical relationship between "target_current_0", "actual_current_0" and "target_control_output_0" (obviously w.r.t the joint motor)?

Thank you in advance :)

matthewd92 2020-05-26

The targets on RTDE are the theoretical values that the joint should achieve based on the motion that the controller is telling the robot to make. The actual sand the targets should be very close unless there is an issue such as hitting something or picking up more payload than it expects. My understanding is the controller is monitoring the actual versus the target and when the difference becomes too large that causes protective stops and faults.

If the motion model says it should take 1A at joint 0 to make a move and in reality it takes 3A then the robot can predict that either it’s hitting something or the payload exceeds greatly what it was told it’s picking up.

Aashish 2020-05-26

Hello,

thank you for your reply.

I have noticed that target_current and actual_current are not same, instead actual_current and joint_control_output are same.

Is there any documentation available how the motion model is designed for UR robots?

To my understanding so far, based on the target joint positions, target joint velocities and target acceleration, the target torques are estimated using inverse dynamics and further the joint controller estimates the actual joint voltages required to move to the joints to desired target positions.

I am still trying to understand the following:

Are the values under “actual_TCP_force_0” calculated or estimated from integrated sensors (for CB3 series)?
Are the values under “actual_tool_accelerometer” from an accelerometer at TCP?
Are the values under “tool_temperature” coming from a separate temperature sensor at the tool head?

matthewd92 2020-05-26

1. They are calculated values based on the torques each motor is seeing, no sensor in CB3

2. There is an accelerometer in the tool flange, I would assume these values are from there

3. This should be the temperature of the tool flange itself as opposed to wrist 3

I do not know of anything that is published regarding the motion model. I do know you can get all the motion parameters, DH parameters, etc for the arm. I believe there is a file on the controller that contains the actual specific parameters for your arm, check the calibration.config file under .urcontrol folder.