Ken Stafford's RSN Spring Conference Presentation and Spinning the Chassis Wheels

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?

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This is an indication that you hit the software configured current limit on your Falcon 500.

EDIT: I misremembered, this actually is an indicator that the motor is stalling. It’s an intentionally designed-in feature of the Falcon 500 specifically, other FRC motors do not do this.

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All of the motor constants (at least the four FRC common ones) can be multiplied by the voltage ratio to get adjusted values. Where the voltage ratio is the applied voltage divided by the voltage at which the specs were measured. So if you were applying 9V to the motor then your stall torque would be 9/12=75% times the spec’d voltage.

This depends on your robot’s wiring and battery health. You can model the battery as a non-ideal voltage source (that is a voltage source in series with a resistor), where the internal resistance is usually somewhere around 15-25 mOhm. And the wires can all be modeled as resistors, where the resistance is determined by the gauge size and length. If you know all of the resistances of the system and the maximum allowable voltage drop (the resting voltage minus the minimum allowable voltage) then you can use Ohm’s Law to calculate the maximum current draw.

Brownouts look like the robot jittering. The jitter rate is based on the impedance of the system and is a lot harder to calculate, but you should be able to visibly see the robot moving and stopping a few times a second, as well as the RSL blinking and dimming.

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A rough calculation with some conservative estimates:
Charged battery voltage: 12.6V
RoboRio brownout: 6.3V
Battery internal resistance 0.02ohm
Max current draw before brownout: (12.6-6.3)/0.02 = 315A

This is definitely good practice in a way, but the end goal here is breaker protection; by ensuring you have enough torque to slip your wheels instead of stall, you drastically reduce your current draw in this scenario which gives you a bit more time to react before the motor breaker pops.

While you can solve it mechanically by reducing your gear ratio, it’s also something that can be solved in software by limiting the maximum current draw of the motors. With many of the modern FRC motor controllers, this is a built-in feature and just requires you to configure what the limit should be on the controller.

This is a screenshot from the data sheet of the 40A breaker, showing the trip times as a function of current draw, which might help you in picking a good current limit for the drivetrain:

image

You can also use a drivetrain simulator to look at the effects current limiting will have in not-pushing-against-a-wall scenarios:

Anecdotally, I think the default Toughbox ratio is a great choice for the 2020-2021 game (we went with a a pretty similar speed despite doing a custom drivetrain). Upgrading the KoP chassis to nitrile wheels and brushless motors is a good choice as well; I’d say you have a great performing drivetrain with your current setup.

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Thanks all for the replies, very insightful. Based on the replies, and after playing with the calculator linked by Knufire, I think we are going to play with current limits in the falcon’s speed controller and do some testing to make sure we don’t pop breakers or brownout, and stick with the hardware we have.

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