Universal Robots Forum

Internal Force Sensor properties

We are very interested in an application using the force sensor, right now we compare the Robotiq FT 300 and the internal sensor of the UR.
There are some interesting measurement results and I really like to dicuss these with other people, because they are not easily understandable for me.

Questions are e.g. Why is the red line descending after some time? Why is there an additional offset between both sensors, already considering the 680g of the tool and the FT300?

The setup is:
The UR 5 robot is lifting a defined weight in the Z- Axis. The payload of the UR is assigned to the weight including a small hook for attaching the weight to the TCP. Additionaly, the FT 300 is attached to the TCP.
The weighing process starts by zeroing the FT 300 when the weight is just lying on a table.
After that the UR lifts the weight and the measurement starts after a waiting time of 1s.
The measurement takes 30seconds.

The results look like this:

Measurement 1:

Measurement 2:

Measurement 3 with 680g weight:

The blue line is the FT 300 external sensor, the red line is the internal sensor calling values with get_tcp_force()
The FT 300 should weigh around 4680g minus the tool and sensor weight of 680g, that is 4000g. This corresponds to a force of around 4kg * 9,81N/ms^2 = 39,2N

Another measurement was to repeat the weighing process ten times, in the following with 3680g respectively 3000g for the FT300 sensor (blue line).

I used a statistic software to calculate mean value and standard deviation as to check if the data is normally / gaus distributed. You can´t read the results in the screenshots so here in letters:
Internal sensor mean value: 35,5 N
Standard deviation: 3,6 N

FT300 mean value: 28,0 N
Standard deviation: 0,6 N

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The precision of the internal force sensor is:

Force: +/- 10 N
Torque: +/- 5 Nm
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How does the internal force sensor work? Is there documentation to calibrate the force-torque sensors on a manipulator (CB3 or e-series)? Lastly, do the CB-series manipulators have force sensing capabilities?@jbm

On e-Series robots, there is a Force Torque sensor built in to the tool mount (after the last wrist) of the robot. This measures the forces and torques applied to the end of the robot arm. The software reports back the forces and torques computed to be at the Tool Center Point and is reported in base coordinates.

On CB3 robots, there is no Force Torque sensor built into the robot. The forces and torques are determined from the currents being applied to the joints to react the force.