Power, speed, and torque... AGH

[patrickrd]

Ken hit most of the key points, but I think some simple equations would be useful.

Electrically: Power = Voltage * Current

Mechanically: Power = Torque * Rotational Velocity

Thus:

Voltage * Current = Torque * Rotational Velocity
This is only true if motors are 100% efficient at turning electrical into mechanical energy. In reality, motors are not. DC motors are extremely inefficient when they are stalled (NO mechanical energy is produced but TONS of current is being drawn), but pretty good when the load on the motor is less. That is why the motors heat up so much if the robot is in a pushing match.

Really:

Voltage * Current = Heat + Torque * Rotational Velocity

Heat = Voltage * Current - Torque * Rotational Velocity

Efficiency = (Torque * Rotational Velocity) / (Voltage * Current)

I think Ken's link is likely to provide graphs of most of these.

Now, how can we not waste this energy to heat? The answer is a transmission (continuously variable is nice, eh team 190?). If we are going to be pushing against another robot and likely to be close to stall torque, we change our gear ratio on the fly such that we provide a lot more torque and a lot less rotational velocity. This way we can push what we want to push at the expense of speed.

- Patrick