What is a good sampling rate for polishing?

Tim_Smith 2016-11-03

Hi DoF,

I was wondering if anyone has used a force sensor for a grinding/polishing? Does anyone know what a good sampling rate would be for a polishing application? Some force sensors have data output rates of 100Hz, while other sensors have data output rates of 1000Hz. What is the advantage of having a higher output rate for a 6 axis force sensor?

matthewd92 2016-11-03

Is this with a UR robot?

Tim_Smith 2016-11-04

Ideally this would be with a UR robot. But since I am only thinking about this application, I am open to other robot brands.

matthewd92 2016-11-05

The reason I ask is that the robot processes on a 125 Hz cycle, at least for controlling motion. If in a thread you use the sync command to avoid the fatal error of "command didn't use enough time" you will flip through the thread every 8ms. The reason I bring this up is the faster sampling rate may not even matter with the UR as I'm not sure it will process threads any faster than 125 Hz but maybe @Stefan_Stubgaard has some more information on this.

Stefan_Stubgaard 2016-11-07

@matthewd92 you are absolutely right, UR robot processes at a fixed sampling rate of 125 Hz, so there is no advantage using a sensor with higher sampling rate.

matthewd92 2016-11-07

Thanks @Stefan_Stubgaard. Just out of curiosity, what happens if you tell a loop to have a 1 ms wait, will it actually loop every 1 ms or does it wait for 8?

Stefan_Stubgaard 2016-11-07

It will wait for 8 ms

Tim_Smith 2016-11-07

@Stefan_Stubgaard @matthewd92 Thanks you so much for the info. Just out of curiosity, do you have an idea of the processing time for other robot manufacturers? And in what situation would I need a processing time that is faster than 8ms?

Tim_Smith 2016-11-15

@Stefan_Stubgaard I have another question :
What would be the mechanical time constant for the UR? At what rate can I send an input to the robot so that the robot will actually be able to act on this input (movement)?

matthewd92 2016-11-15

Tim,

The robot processes commands at 125 Hz, 8 ms. Is that what you are asking?

Matt

Tim_Smith 2016-11-15

Not quite what I was asking. I understand that the controller processes the information at a rate of 125Hz, but if you send a command to the robot every 8ms, I am guessing that the motors in the joints will not be able to react to such a fast change of command.

matthewd92 2016-11-15

The motors are receiving their motion commands at that frequency currently. If you are trying to control motion from off the robot you have to send them a command every 8ms if you want . To give you an idea of what this time means relative to the motion, when I worked in a company that makes brushless DC motors, our motors were receiving commands at 20 kHz.

Stefan_Stubgaard 2016-11-16

@Tim_Smith, the answer from @matthewd92 is correct. The joints will react on new position/speed commands every 8 ms

Tim_Smith 2016-11-17

@matthewd92 @Stefan_Stubgaard Thank you very much for the feedback!

matthewd92 2016-11-17

You're welcome. That's what this is for, hopefully we all learn from each other.