Force-Torque Sensor Calibration/Certification

What is the procedure for calibrating or certifying the robot force-torque sensor in the tool wrist? Is this a load cell, and/or what is the calibration specification for the sensor? Furthermore, how is the same achieved for validating the current sensing in the joints (e.g. how do I know when I set a limit of 80N in REDUCED MODE in the installation that over time wear/tear on the joints/motors haven’t caused this to drift)?

I have checked the Service Manual e-Series - Service Manual 4.0.0 but only found flags for no calibration or other issues with sensor, but very vague corrective actions like ‘reboot’, ‘update software’, or ‘contact Universal Robotics and/or distributor’.

If certain SAFETY FEATURES or FORCEMODE() rely on this sensor, how do we make sure it doesn’t drift over time, is this a semi-annual or annual process? And if it does, how do we re-calibrate and re-certify? Is this something UR or a distributor has to do? Is there training or specific equipment I can get to do this onsite (excluding buying an additional robot and a special mounting system to tether the two robots together)?

I’ve been searching for an answer to this (even contacted our distributor) but haven’t found an answer yet.

** If someone from Universal Robotics could answer this question or reach out that would be great, thanks **

Recalibration of the FT sensor is not a field service activity and requires the robot to be sent into the service center.

FWIW, the FT sensor is not a part of the safety system. The FT sensor was added to the robot to enable various applications that would benefit from it. If the sensor did experience drift and/or loss of function over time, the integrity of the safety control system is fully maintained.

As for certain types of wear and tear having an effect on safety system performance, the limits set in the installation are fixed. Let’s say there’s added friction in the system due to a bad gearbox. More current may be fed to that joint to maintain position and velocity, but once it crosses that fixed limit, a protective stop, fault, or violation is still going to occur.

Thanks @miwa, how often does the FT sensor need to be calibrated/certified at a service center? Is there documentation for this somewhere?

In addition, is this confirmed that the current sensing and FT sensor are independent systems? How is ‘tool force’ and ‘elbow force’ checked? In installation safety are these both dependent on current sensing or does tool force use the FT sensor?

This makes sense however how is this addressed when using the Force mode feature? How is drift avoided or accounted for?

Say the FT sensor (and/or current sensing is probably being monitored too) is used 24/7 for sanding and has been tuned for optimal force/speed (I know there is a +/-10N and +/-0.5Nm accuracy). Can vibration and/or wear cause drift in the readings over time causing the robot to exert more or less force?

I’m asking these questions as we have integrated a UR using the forcemode feature with promising results in our R&D lab. The equipment is currently going through our quality/testing review board for approval and I would like to have concrete answers for them before we start building a production ready system.

Thank you for the help.

Just recording as much information here as possible so someone potentially asking in the future doesn’t run down the same road.

Current answer as of 6/1/22: Basically I was told that the Force/Torque sensor is calibrated/certified for the lifetime of robot.
However, the lifetime of the robot is dependent on your application, making it different for each and every case so this isn’t very assuring. Again, how do we assure the FT sensor doesn’t drift or drastically change values after a hard collision or pinched up against something if left in E-Stop?

The only way I can see to add certification/calibration to the robot for now is create a program that going over to an externally calibrated load cell, tare the FT sensor, apply a force (say 13 Newtons) and a torque (say 0.7 Nm) to the load cell, record the values recorded in the robot and load cell then compare to a threshold we determine acceptable.

I’m currently waiting for more information from UR on the FT sensor and will share in this thread once I have more.