What is a good sampling rate for polishing?

Tim_Smith

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

Is this with a UR robot? 

Tim_Smith

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

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

@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

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

It will wait for 8 ms

Tim_Smith

@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

@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

Tim,

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

Matt

Tim_Smith

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

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

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

Tim_Smith

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

matthewd92

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