Quote:
Originally Posted by Bruceb
Don Don Don... I have the motor curves and I have the JVN design calculator.
I know the free speed is 5500 rpm. I am not an engineer. I don't know how to figure what I can realistically expect the CIMs to be turning at at nominal load at speed. Sorry, my calculus is nonexistent, I never had physics and I am just trying to help some kids build a robot. Now, if you are willing to help, great.
Bruce
|
Bruce, I find it better to think about how fast the robot SHOULD go than trying to target a specific speed and working out the gear ratio. What is often not mentioned is the fact that all drive motors run through circuit breakers that limit current to 40 amps. If you look at the CIM motor you will see that it's stall current is more like 130 amps. If you stall the motors, you will trip the breakers. If you trip the breakers you are not moving until they reset.
I encourage my team therefore to design the robot so the wheels will slip before the motor current exceeds 40 amps. To do this requires knowledge, or reasonable estimates, of both robot's weight and the coefficient of friction from the wheels.
It will take too long to describe the process in detail here. I have given a presentation on this subject and send it if you PM me with contact information.
The short version (which will likely not help without a background in engineering or physics) is that you need to determine the torque required at the wheel to cause slippage as well as the available torque from the motor at or below 40 amps. The ratio of the two gives you the desired maximum gear ratio. Each stage of gear reduction (gear, sprocket or pulley) could be roughly estimated to reduce the maximum motor speed by roughly 95%. Most gear boxes have 2 stages of gears and then chain to drive the wheels (0.95^3 is about .85). So the speed of the robot SHOULD be something like (motor free speed)*(~0.85 efficiency)*((motor torque at 40 amps)/(wheel torque for slippage))*(wheel circumference). Of course it gets more complicated with multiple motors, multiple wheels and such. Generally safe to assume symmetry and that one or two motors drive half the robot weight for tank or skid steer drive trains.
Now I've probably gone and scared anyone from asking for the 'long' version.
As I said, contact me and I can break it down into manageable size bits of information.