In this wonderful presentation by Ken Stafford at 1:06:50, which we just presented to our kids as a training session, he mentions that you should be able to “spin out” your wheels on your drive train when driving into a wall, and if not, then you are too high geared.
So as an exercise, we tried this. We are running andymark 6" plaction wheels with blue nitrile tread, standard toughbox mini (AM14U4 in KOP) with the 10.75:1 gear ratio, standard 1:1 timing belt power distribution to the six wheels(AM14U4 in KOP), and powered by two falcon 500s on each side (four total).
The wheels did not spin. We got an annoying whining sound. We saw no slippage of the timing belt sprockets or any other part. I am assuming we probably browned out through the roboRIO. I will confirm this at our next meeting.
So, I decided to run through some calcs. A quick search of chief delphi showed that the static friction factor for nitrile on carpet was measured to be 1.3 (wow!).
I don’t have any data to show what the torque at stall of a falcon 500 would be at voltages less than 12V during brownout condition, so I used the standard curve for amps vs torque, as I had nothing else to go off of. I’m sure there is a better way to do this, please correct me if you know what it is. Is there a graph of stall torque vs input voltage that is available or can be generated from known data?
With a friction factor of 1.3 and a robot weight of 150lbf, it should take around 195 lbf of lateral force to slip the wheels.
At 60 Amps per motor (momentary power before brownout?)
1.08 n∙m/motor X 4 motors = 4.3 n∙m
4.3 n∙m x 10.75 (transmission) X 0.90 (efficiency) = 46.4 n∙m
46.4 n∙m x 8.85 (in∙lbf)/(n∙m)= 411 in∙lbf
6" wheels = 3" radius, force = 411 in∙lbf / 3 in = 137 lbf
Far less than the 195 lbf required to spin the wheels.
My same quick calcs show it would take about 100 amps per motor (based, im sure incorrectly, on the provided 12v spec curves) to produce enough torque to spin the wheels. Can an FRC robot supply 400 amps for a very short amount of time to get the wheels spinning?
I have two big unknowns, the first is how much amperage can the motors instantaneously draw before the roboRIO starts browning out. When brownout occurs, wplib states “PWM outputs will be disabled. For PWM outputs which have set their neutral value (all speed controllers in WPILib) a single neutral pulse will be sent before the output is disabled.” I assume this might be what is causing our whining sound as the PWMs are pulsed between on and off? Not sure what the pulse rate would be (roboRIO software loop rate?, CAN network rate?)
The second is, how do you figure out stall torques at reduced voltages during brownout conditions?
And finally, is the best solution at this point to drop the 10.75:1 gear ratio in the AM toughbox mini to try to achieve spin-out?