View Single Post
  #27   Spotlight this post!  
Unread 09-12-2013, 18:33
Oblarg Oblarg is offline
Registered User
AKA: Eli Barnett
FRC #0449 (The Blair Robot Project)
Team Role: Mentor
 
Join Date: Mar 2009
Rookie Year: 2008
Location: Philadelphia, PA
Posts: 1,116
Oblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond reputeOblarg has a reputation beyond repute
Re: Omni vs Mecanum CoF ?

Quote:
Originally Posted by Ether View Post

For one-CIM-per wheel:

amps=(5760*Istall*mu*V*W)/(pi*eff*Sfree*Tstall)

or

V=(pi*amps*eff*Sfree*Tstall)/(5760*Istall*mu*W)



where:
amps = current required to slip the wheel

Istall = CIM spec stall amps

Tstall = CIM spec stall oz_in

Sfree = CIM spec free RPM

mu = static coefficient

V = robot ft/sec speed at CIM free RPM

W = weight in lbs on the wheel

eff = drivetrain torque efficiency fraction


So, plugging in Oblarg's numbers:

amps=(5760*133*1.0*25*40)/(pi*1.0*5310*343.4) = 133.7

You're not going to get 133.7 amps in each of 4 motors, so if you gear for 25 ft/sec you definitely will not slip.


Let's try some more reasonable numbers and try again:

mu=1.0 V=12 W=37.5 eff=0.8

amps=(5760*133*1.0*12*37.5)/(pi*0.8*5310*343.4) = 75.2

Will the wheels slip? Well, 75 amps per motor times 4 motors is 300 amps. Unless your battery and wiring are in top shape, you're not going to get 75 amps per motor.


Let's try V=10:

amps=(5760*133*1.0*10*37.5)/(pi*0.8*5310*343.4) = 62.7

OK, maybe that will slip.

Thanks for this. The calculation I did (obviously) placed no constraints on current draw on the motor.
__________________
"Mmmmm, chain grease and aluminum shavings..."
"The breakfast of champions!"

Member, FRC Team 449: 2007-2010
Drive Mechanics Lead, FRC Team 449: 2009-2010
Alumnus/Technical Mentor, FRC Team 449: 2010-Present
Lead Technical Mentor, FRC Team 4464: 2012-2015
Technical Mentor, FRC Team 5830: 2015-2016