How to feed the F/T sensor data into UR'S force control-loops (Force mode).

conghui

Hi All,

I just noted that in the release note software version 3.3.x.x from Univeral Robots states that:

External Force/Torque sensors can feed their measurements (directly) into UR's force control-loops (Force mode). - See more at: http://www.universal-robots.com/how-tos-and-faqs/faq/ur-faq/release-note-software-version-33xx/#sthash.bw2xRUgc.dpuf

Anyone has experiences on that? For example how to connect the FT300 sensor and feed the data into UR force_mode()?
Thank you!

Kaleb_Rodes

@conghui Software 3.3 and newer it is possible to feed external force/torque measurements into the robot controller, and use this input as an alternative to the internal measurements in e.g. force mode. The feature is enabled and disabled with a special script command and the force/torque measurements can be streamed into the robot controller using the Real-Time Data Exchange (RTDE) interface, see this document. The functionality can be integrated in a URCap for easy installation of external F/T sensors on Universal Robots.

UR Script Function: 
enable_external_ft_sensor(enable, sensor_mass = 0.0, sensor_measuring_offset = [0.0, 0.0, 0.0], sensor_cog = [0.0, 0.0, 0.0])

This function is used for enabling and disabling the use of external F/T measurements in the controller.

If no other RTDE watchdog has been configured (using script function rtde_set_watchdog, a default watchdog will be set to a 10Hz minimum update frequency when the external F/T sensor functionality is enabled. If the update frequency is not met the robot program will pause.

Parameters:

enable: enable or disable feature (bool).

sensor_mass: mass of the sensor in kilograms (float).

sensor_measuring_offset: [x, y, z] measuring offset of the sensor in meters relative to the tool flange frame.

sensor_cog: [x, y, z] center of gravity of the sensor in meters relative to the tool flange frame.

@Samuel_Bouchard is Robotiq working on a URCap for this? Could be a great addition to the FT sensors. I know customers would be more comfortable using the sensor data in force mode rather than the +/- 10N force readings generated by the robot.

Samuel_Bouchard

@Kaleb_Rodes thanks for the answer. It would indeed make sense to add some of these functionalities in our URCaps. We'll see how it ranks in the priorities after the Camera launch this week. 

conghui

@Kaleb_Rodes thank you for sharing the information. Get an idea of how to implement this function. 
More info page to share on UR website:
1. http://www.universal-robots.com/how-tos-and-faqs/how-to/ur-how-tos/external-ft-sensor-urcap-22231/
2. http://www.universal-robots.com/how-tos-and-faqs/how-to/ur-how-tos/real-time-data-exchange-rtde-guide-22229/

@Samuel_Bouchard
I think there is a strong demand over there to feed the F/T data into UR controller!!! We did some test by using the force_mode() function in UR script. There is no accuracy promise and sometime it is difficult to implement own compliance or hybrid control algorithm, CANNOT WAITING FOR ADDING THAT FUNCTION IN YOUR URCAPS

Alexandre_Pare

@mdauser  this is also something you may want to look at in your application. Can you share some information on how you are using force data and how you are programming your application. I am sure that some DOF pros can help.

heingab

Hi, I'm currently trying to implement this on our system. I'm fairly new to the UR ecosystem so this may be a dumb question but I'm not seeing how to actually pipe the FT sensor readings into the system. I'm currently using the RTDE interface so I know the basics of how that works. I understand that "enable_external_ft_sensor" enables using an external sensor but where do you actually override the internal FT estimates with the sensor readings for use in force control mode?

Sebastian

heingab said:

Hi, I'm currently trying to implement this on our system. I'm fairly new to the UR ecosystem so this may be a dumb question but I'm not seeing how to actually pipe the FT sensor readings into the system. I'm currently using the RTDE interface so I know the basics of how that works. I understand that "enable_external_ft_sensor" enables using an external sensor but where do you actually override the internal FT estimates with the sensor readings for use in force control mode?

I am having the same troubles and I could not find any instruction for this. Right now I am looking through  the rtde_client_3.5.zip files and reading https://www.universal-robots.com/how-tos-and-faqs/how-to/ur-how-tos/real-time-data-exchange-rtde-guide-22229/ but I do not see how to actually implement it.