paper: New Control Functions - Drive System Testing
 

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New Control Functions - Drive System Testing by Chris Fultz

Re: paper: New Control Functions - Drive System Testing
 

As part of our beta testing, we used the roboRIO for some drive system evaluation testing, capturing data not available through the 2014 control. We were testing 2, 4 and 6 CIM and 4 CIM+2mini-CIM drives and capturing data, including amperage and voltage.

Through this testing, we identified an operational effect of one of the control features that we felt would be good to share.

The new control disables output to the PWMs when the system voltage drops below 7V. This is a good feature as it helps keep the control and radio on-line.

However, operationally, this can cause the PWMs to cycle off and on as voltage recovers and then reduces if no change is made by the operator. Depending on the situation - pushing, lifting, etc. - the effect on the robot may be a non-event or may create a response that needs to be managed with software or by operator actions.

The attached paper describes the specific testing we were doing, and the results. It includes 2 charts that show graphically what was occurring.

As teams prepare to use the new control for 2015, we thought this was an interesting and different control response that would be worth sharing.

Re: paper: New Control Functions - Drive System Testing
 

Good information to know. Thanks.

Re: paper: New Control Functions - Drive System Testing
 

Thank you for sharing this. I noticed that the paper only refers to the two CIM configuration. What were your results when testing the various other combinations you mentioned above?

Re: paper: New Control Functions - Drive System Testing
 

Thanks a lot for sharing this. This is pretty interesting, and could potentially cause a lot of headaches for teams that run 6-cim drivetrains in high gear. I only have theoretical calculations, but it appears that if you have a fully loaded robot (weight-wise), and run in high (fast) gear, you could potentially not even start. I'm using my drivetrain calculator (available as a white paper), and looking at a gear ratio of 4.63:1, 4 inch wheels, and a battery voltage of 12.8V (typical of a mid-match voltage) (Note: this is the default setting for my drivetrain simulator). From a stop, it looks like the battery voltage could drop down to 6.67V if the robot were commanded to go full forward. Again, this is all just a simulation that has absolutely no guarantee of correctness, but if possible, I'd love to see if this causes problems in reality.

Re: paper: New Control Functions - Drive System Testing
 

We only did this specific test with the 4 CIM configuration because with 6 CIMs or 4+2 we just spun the wheels.

The full paper has data on 2, 4, 6 and 4+2 configurations for basic acceleration and for pushing another robot (130 pounds). We are still a few days away from publishing that.

Re: paper: New Control Functions - Drive System Testing
 

Out of curiosity, does this affect CAN controlled controllers? Or just PWM?

Re: paper: New Control Functions - Drive System Testing
 

My understanding from last night's GameSense is that the load shedding that occurs at 7v impacts all speed controllers, no matter how interfaced.

Re: paper: New Control Functions - Drive System Testing
 

It would also be interesting to see if SPI cut out as well.

Re: paper: New Control Functions - Drive System Testing
 

This worries me. I know during hard acceleration at IRI with our 6 CIM Drive we got below 7 volts. I'll have to look at our logs and see how often we actually dropped blow.

Re: paper: New Control Functions - Drive System Testing
 

SPI should function until the battery voltage drops too low for the roboRIO's 5v supply to be maintained. The brownout "load shedding" is a software function that explicitly turns off pneumatic solenoid valves and motor speed controllers when the battery voltage falls below a certain threshhold. It is done in order to try to prevent the voltage from dropping further and causing a complete loss of the control system.

Re: paper: New Control Functions - Drive System Testing
 

How would that be implemented with a CAN connection without affecting other non-motor CAN devices?

(I could guess, but if anyone knows where this is documented would you please post a link)

Re: paper: New Control Functions - Drive System Testing
 

Consider this an opportunity to do dynamic power management. You can monitor battery voltage and motor currents, and you should be able to come up with a way to keep the voltage from sagging low enough to trigger the automatic brownout.

Re: paper: New Control Functions - Drive System Testing
 

Yeah were probably going to look into doing something like this. I'm glad they added current monitoring this year.

Re: paper: New Control Functions - Drive System Testing
 

Here's the graphic from last night's Behind The Lines showing the key battery voltage points and what happens when they're reached.

NOTE: This is different from what was shown during the episode, and is more accurate.



The BTL episode can be viewed here: https://www.youtube.com/watch?v=uUYlS2Vkyuo