
08-04-2015, 14:46
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Registered User
 FRC #6054 (Dukes)
Team Role: Mentor
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Join Date: Jan 2012
Rookie Year: 2012
Location: United States
Posts: 310
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Re: 4165 Offseason swerve
Is it a heat issue with running 20A through the motors? Do you think that normal match play, with periodic travel in strait lines and pauses periodically for 2.5 mins would be a problem?
Quote:
Originally Posted by Joe Johnson
I fear your steering motors will be very unhappy.
You have a 48:1 ratio from the BB to the swerve axle. Let's say that each stage is 90% efficient, two stages, that is an effective ratio of 39:1. The BB has a stall torque at 12V of 0.8N-m so you've got 31N-m at the swerve axles.
BUT, your wheels look to be perhaps 4-5" on center, let's call it 110mm and be done, that means you're 31N-m of max steering torque can put about 280N as its dragging those grippy wheels over the carpet. That's about 60lbs. No problem you think to yourself. Even if the robot full weight in on one swerve module and the CoF of the wheels is 1.0, your BB motors can do the job. And you'd be right, they can turn the wheels, bearly. But they are not going to be happy about doing it for long periods of time.
Let's do the less drastic case: 120lbs robot, 1/4 on each wheel. That is a normal force per wheel of 15 lbs (67N). With a CoF of 1.0, it takes over 7N-m just to drag the tires over the carpet (which you have to do if you want to turn the wheels) 7N-m is about 1/4 of the stall torque you have available and will take 20A for each swerve module. Yes, the BB can to do this but again, not for long periods of time.
I am happy that you are trying this, I just want to put some things out the for you to consider.
Dr. Joe J.
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