Quote:
|
Originally Posted by Ken Leung
It takes 130 lbs to lift up a robot. The stall torque is 20 in-lbs. If you have a winch, and the diameter of the winch is 5 inch, your motor can excert 4 lbs upward force at stall torque. 50% of that is 2 lbs upward force. 130 lbs / 2 lbs is 65:1 a reduction ratio. You should note that this is moving at 50% free speed, so you should check and see if this is too fast or not.
|
Alrighty, I think that this is an important enough issue that I'd like to make a number of quick corrections. One of the most difficult problems
I've had in the past was properly picking gear ratios... so I'll make a quick lesson here.
First and foremost... let's take a look at the CIM motor.
It has a stall current of 114 Amps, and a stall torque of 346.9 oz-in, which we'll switch over to 21.68 in-lbs since it's probably more applicable in this situation.
There's been talk of designing around the max power of motors, or 40% of the motor stall to optimize power, but this entire idea can lead to big trouble!
Designing the gear ratio around the max power is not the ideal situation in FIRST, since we're limited to the amount of current that the motors can pull without tripping the circuit breakers. Designing a gear ratio around the max power in this example is 114 * 50% = 57 Amps. Since I believe we're still on 40 amp breakers for the CIM motor, this would be a very poor choice, resulting in your robot being unable to lift itself!
Instead, design your components around a safe current draw, such as 35 Amps.
Solving:
114 * X = 35 A
X = 30.7% of max (stall) torque, which is 6.66 in-lbs.
Now, to look back at the problem, if the winch has a diameter of 5 inches, the radius will be 2.5 inches, which is the moment arm. However, I have a hunch that Ken was just typing a bit too quick, so I'll assume he meant a 5 inch radius / moment arm.
With this said, we need a torque equal to:
5 inches * 130 lbs = 650 in-lbs.
Since we know that we want to run at 6.66 in-lbs, we'll need to create an arm gearbox that has 650 / 6.66 ratio, or
97.65 : 1.
Including a moderate factor of safty of 1.3 or so.. I'd say you'd like at least a reduction of 125 : 1.
To reduce this massive reduction, simply reduce the size of the moment arm. Half size moment arm = half gear ratio.
I hope this helps! Good luck!
Matt