Needed Servo Torque
 

Now that my team's season is over, we were thinking of doing some sort of T-shirt cannon attachment to our robot. Our plan is to make an assembly very similar to the camera assembly in the rookie kit using 2 hobby-like servos. Each servo has a torque of 125oz/in, and I am wondering about how much weight could each of these servos support, or how i would figure this out mathematically. Our turret my guess will weigh about 2-3 pounds. Also, is it safe to power these 2 servos off of the Digital Sidecar?

Re: Needed Servo Torque
 

A torque of 125 oz-in means that you can push with 125 ounces of force at a distance of an inch radially from the center of the servo 'axle', or 1 ounce at 125 inches away. (Or 62.5 ounces at 2 inches, etc.)

3 pounds is 48 ounces, the math estimates that it must be about 2.5 inches away at most to 'lift' it. But that's lifting it against the force of gravity. You probably only need to Move that weight, and so you must find out exactly how much force is required to move it in the way you want. A fish scale is a good way to determine that experimentally, ask the Physics or Technology teacher to borrow a small spring scale to measure force.

Without any details of your mechanism - exact details - we can't even guess.

But weight, friction and distance are all your enemies. 125 oz-in is 'best case' - plan on 2/3 to 3/4 that for a factor of safety.

Re: Needed Servo Torque
 

Okay. Ill be sure to post some design pics within the next few days.

Re: Needed Servo Torque
 

There are 16 ounces to the pound, and 12 inches to the foot, so 125 oz/in torque is equivalent to about 0.65 ft/lbs. You can use a free body diagram to help you figure out if a given arm (with given mass and center of gravity) can be moved, however this means (at stall torque) the servo could maintain the position of an arm that weights 1 pound if the center of gravity of said arm is .65 feet from center of rotation.

However, it's unlikely these servos will work unless you plan on making a twelve inch long air cannon that only shoots confetti. And even if much larger servos that could handle the torque necessary to support a full-size air cannon were available, they would still constantly need power to maintain their position, otherwise they would just back drive.

A much simpler solution for a FRC team would be to utilize the KoP window motors to achieve the pan/tilt drive for an air cannon. These motors have internal worm gear drives to prevent back-driving, and thus can keep their position without having to constantly power them. You can then use a potentiometer to measure the angle the arm is at.