Torque Calculation/Determination-Urgent

[JVN]

Also...
Think about your robot pushing against a stationary object (like a wall).

In this case, if your robot has more traction than torque at the wheel, the wheel will stall, the motor will stall, and it will draw a lot of current. Probably bad.

In this case, if your robot has more torque than traction, the wheel will be spinning, and the floor will be acting as a brake on the wheel. You know exactly how much this load is, because you know the force of friction between your wheel and the ground. The motor will be loaded at some amount, depending on your gear ratio, and will draw some current depending on this load.

In FRC, if we're designing robots to get into pushing matches with other robots, we design them such that when they're pushing against a wall -- the motor draws about 40-amps continuously.

This way, we know that even when we're pushing -- we won't pop circuit breakers.

This is another reason to design for increased torque.

-John

[JVN]

A few other notes:
If you have multiple motors driving the same wheel... the load will be divided evenly among those motors.

If you have multiple wheels, the weight of the robot is spread over those wheels. The weight resting on each wheel will obviously determine the friction force between that wheel and the ground.

Multiple wheels connected to the same motor/gearbox will contribute their loads together.

Non driven wheels, contribute no load, but represent traction which isn't being used to help accelerate the robot, or push obstacles.

-John

[EricH]

As a point of comparison, a couple years back my Lunabotics team used BaneBots RS-775-18 motors (running at 12V, and 1 for each of our 4 wheels), all running through a BaneBots P60 gearbox (I want to say 64:1 or 256:1)--and had one of the faster Lunabots if we kept our speed controllers alive.

Which brings up John's 2nd "extra" post. We had something like 20A (or 10A, in some cases!) fuses on the robot, one per wheel, and never blew one. OTOH, we rarely moved on the regolith because our speed controllers couldn't take the current, and shut down to prevent frying--and didn't restart until the robot did. A change in speed controllers--to something we could change the safety settings on () resulted in a lot of motion until we put ourselves in a sticky situation and couldn't get out... without frying speed controllers.

Plan for pushing, as John points out--you'll need it.