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Unread 04-01-2016, 10:56
BoilerMentor BoilerMentor is offline
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Re: 6 CIM robo-rio brown out

Someone asked for real world testing with a 6 cim drive. I have some things to report.

The robot tested uses the cRio based control system, but we logged voltage to look from situations where we saw voltage drop below 7.5V. We consider this to be an adequate safety factor to ensure we would be in the clear.

Testing completed on slightly lower pile carpet than normal field carpet, but similar in construction.

The drive train is configured as follows:

Mechanical

2 VexPro 3 Cim Ball Shifters at 18.75:1 and 7.08:1 ratios.
6 VexPro traction wheels 6" diameter 2" width with blue nitrile rough top tread
1/8" center drop
Center wheel direct driven.
#35 roller chain to drive front and rear wheel on each side
Outer axles are bolt-through-tube style dead shafts with bearings in the wheel/sprocket
No tensioners used, but chain run has worn in and is not putting un-due load on bearings/gear boxes.
Plate and standoff style construction bolted to a box channel core frame
****Robot weight: Roughly 100 lbs****

Electrical

6 Standard Cim motors, all new at the beginning of 2014 competition season
10 awg wire between PD Board -- motor controller -- motors
Power pole connector between PD Board and each speed controller as well as between each speed controller and motor
6 Black Jaguar speed controllers on CanBus through a 2Can driving motors
Battery was changed 2 times during testing.

Programming

Smoothing on inputs was implemented in code for this drive train, but still responds sharply and operates precisely
Automated shifting used in competition.
I need to verify the operational mode used, but I believe 1 speed controller on each side was implemented in speed control mode and the other 2 controllers were slaved to that controller based on current
In testing condition shifters were set manually to high or low gear and shifting code was disabled.

Results

High gear, No compressor, from standing, maximum achievable acceleration
-- Would have browned out; voltage dropped significantly below 7.5V

Low gear, No compressor, from standing, maximum achievable acceleration
-- Would not have browned out; voltage did not drop below 7.5V

High gear, Compressor running, from standing, maximum achievable acceleration
-- Would have browned out; As expected based on first indicated result

Low gear, Compressor running, from standing, maximum achievable acceleration
--Not conclusive; Voltage drop below 7.5V, but not below stated brown out condition voltages

High gear, no compressor, from standing against wall, ramp to traction limit
--Would have browned out; Motor stall achieved critical voltage drop

Low gear, no compressor, from standing against wall, ramp to traction limit
--Would not have browned out; traction broke before 7.5V limit was reached
****driver modulation to stall condition could force brown out.

As much as it pains me to say it, I don't have actual data to share. This was a quick and dirty test and I thought our experience might be useful.
I'm convinced automated shifting is of critical importance with the consideration of brown out. I recall a number of teams losing 120amp breakers with 6 cim shifting drives at competition in 2014, which is where this drive base originated, and we never had that problem.

Please let me know if you have questions. I'm going to try and set this up and collect actual data early in build season if it appears it might be useful.
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2006-2008 FIRST Team 1741 Red Alert-Founding Student
2008-2011 FIRST Team 1747 Harrison Boiler Robotics-College Mentor
2012 FIRST Team 4272 Maverick Boiler Robotics-Founding College Mentor
2013-Present FIRST Team 1747 Harrison Boiler Robotics-Engineering Mentor
2015-Present Ri3D Team Indiana - Mechanical and Fabrication
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