How to Control the Force Around Specific Waypoints

Tyler_Berryman

Here’s another question from @NicoHanstein who is experimenting with the path recording for deburring with their spindles.

Rigid/Stiff Spindle

Until now, we did not test the system with a rigid spindle without deflection. With those spindles it will be necessary to adjust the pressure precisely.

(The exerted pressure against the workpiece influences the deburring result very much.)

How much is the normal/standard force in Newton which is applied when the force sensor will be used with those spindles?

We also would like to know if it is possible to use the standard waypoints of the UR program and combine it with the force sensor.

That means if programming with path is difficult due to different orientations and movement of the joints of the robot, it might help to use or even combine path with the standard waypoints. In case it is possible to preset a certain Force to the Torque sensor will it also continue to apply the same force with the standard waypoints?

Alexandre_Pare

@NicoHanstein the best way to do this in your application would be to use the force command available on the UR Polyscope. When setting up this option you need to select the force mode type. In your case I would recommend using MOTION type. This allows the robot to apply force in the direction based on the tcp motion direction. However, at this time it won't be possible to use this with a path recording option from Robotiq. @MarcAntoine_Lacasse you have done some testing on the force fonction of the UR, do you have any tips and tricks for Nico!

MarcAntoine_Lacasse

Without force sensor, the force mode of UR is not precise enougth to do deburring without a compliant spindle.
It is possible to insert force sensor data into controler to improve force mode and we are working on the URCap Sensor to do so but I have still not test it on deburring task. Once the force sensor data will be send to the UR controler, you should be able to mix recorded path and standard way point into a UR force node to do complex deburring task.

Ryan_Weaver

@MarcAntoine_Lacasse, you speak about using the FT sensor to improve force mode as if it is currently possible - is that true?  I understand there isn't a URCaps available yet, but how would one go about doing this manually? 

I have a customer who is very interested in using the path URCaps in a deburring operation, but when you use the built-in Force mode with the path recording, the results are poor.  We found we end up with better results simply using the path recording tool with a relative offset, so that the robot "pushes" trying to maintain commanded position.

matthewd92

@Ryan_Weaver do you know what the settings were that your customer was using inside the force node?  We had pretty good results using the URCaps path inside a force node when we did the sanding demo, we set it up pretty quick so could feel like we could have gotten better had we taken more time in developing the path but the results we achieved were pretty uniform across the surface considering the setup we were using.  I have found that the settings in the force mode are pretty sensitive to how well it works, too much velocity and it goes haywire, not enough and it doesn't respond very well and you get poor results

Ryan_Weaver

@matthewd92 there's a good chance we were using the frame force mode with the default speed settings.  The response seemed a bit anemic, so there's a good chance that we weren't running it fast enough.  The other challenge was that we were deburring a large ring from the outside (maybe a foot diameter), so the force direction was w.r.t. the tool and it didn't seem to work quite right.  May need to revisit it.

Thanks!

MarcAntoine_Lacasse

@Ryan_Weaver With RTDE it is possible to feed the UR controler with the FT sensor data but we found that it do not really improve the force mode. The reason is that we can't modify the force control loop parameters. For low speed the force is much more precise but the benefit decrease with speed. So, for deburring  operation, I think we should use the path URCap with a compliant deburring tool.

matthewd92

@MarcAntoine_Lacasse which inputs in the RTDE would you need to send the data into from the force sensor?

MarcAntoine_Lacasse

@matthewd92  

In RTDE, a special input variable should be used for this purpose:
Name: external_force_torque
Type: VECTOR6D

Script functions to enable an external F/T sensor in the robot controller :
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])

There is a guide in the universal forum

matthewd92

@MarcAntoine_Lacasse thanks for that, I thought it was possible to integrate the force into the robot, I just couldn't find that article again on UR's website.