Throwing Problems/Motors

[stretch851]

We are having problems getting the throwing arms to move with much speed at all. It seems to be like the Motors aren't strong enough even though we know other teams have similiar setups. We have 2CIMs and 2 MiniCIMs hooked up, and have tried testing with out the arms and the no load speed of the shaft seems to look more than strong enough. Are motors attached on each side are attached with pulleys to 3 feet up, with a hex shaft across the hold and spin our thrower arms. Our arms weigh approx 5 lbs. Also, we tried running the arms without the ball on only two CIM's and it flipped with a lot of speed. However when we hooked all 4 motors up, it could hardly raise the ball 6 inches. We have already checked to make sure all the motors are driving in the same direction and labeled the wires, so it must not be a wiring issue. We think it may be a gearbox ratio( 2 Toughbox mini's with 8:45:1 ratio) issue, however we are not sure. Any ideas?

[Jibri Wright]

I would try getting a larger gear reduction or try to make the catapult shorter. I believe the problem you have is the amount of torque your system is creating with a 3' catapult. Not only that, but the ball being near the end of the catapult also creates quite a bit of torque.

[nathannfm]

Originally Posted by Jibri Wright

I would try getting a larger gear reduction or try to make the catapult shorter. I believe the problem you have is the amount of torque your system is creating with a 3' catapult. Not only that, but the ball being near the end of the catapult also creates quite a bit of torque.

I think he is saying that the pivot is 3 feet above the location that their gearboxes are mounted, not their arm is 3 feet long, but I could be wrong.
Question: is the ratio between your tough boxes and the hex shaft of the arm 1:1? If so I think this is your problem, try using a much larger pulley on the hex shaft. Or a smaller one on the tough box, or both.

[stretch851]

Yes our pivot is three foot off the ground. our arm is curved so that it's height/length(frame to pivot, dy) is about two feet. Oh what does the 1:1 ratio do? We figured a 1:1 ratio was good.

[Jibri Wright]

Originally Posted by stretch851

Yes our pivot is three foot off the ground. our arm is curved so that it's height/length(frame to pivot, dy) is about two feet. Oh what does the 1:1 ratio do? We figured a 1:1 ratio was good.

The ratio isn't strong enough. Changing the pivot pulley will give you a larger gear reduction. You probably don't even need to change it that much because the gear ratio of the gearbox will be multiplied by the gear ratio of the pulley.

[Chris Fultz]

do you have a sketch or picture of you set up? You can probably get some specific help if you can show the set up, the weight of you mechanism, how much it travels (angular movement) and how fast you need it to move.

[stretch851]

Sorry I just figured out how to attach the file. We know that the hex shaft will turn 120 degrees before releasing the ball and that our plate attachement+throw arms weigh 5 lbs. We were expecting 24 m/s to obtain a 7 ft sweet spot.

[BBray_T1296]

Originally Posted by stretch851

Yes our pivot is three foot off the ground. our arm is curved so that it's height/length(frame to pivot, dy) is about two feet. Oh what does the 1:1 ratio do? We figured a 1:1 ratio was good.

You are already running a net reduction of 8.45:1, from CIMs to arm, which is lacking torque needed to throw the ball reasonably.
If you changed one of the pulleys in the setup by factor of 2 (doubling the highest pulley, or halving the lowest pulley, you will change the ratio to a 16.90:1, halving the "top speed" but doubling the torque. You are never making it to the mathematical top speed anyways, because the arm is unable to accelerate to that number in the 120 degrees.

What you want to do is optimize the ratio of top speed to torque; having enough torque to reach the top speed, while having enough speed to throw the ball effectively.

Let's get into the math: You want to shoot for roughly the 4000RPM range on the CIM side of the gearbox. With your current setup, that would (in physics land) get the arm moving at 475rpm, with the tip of the arm (2' long) moving at 100 feet per second. Now obviously that would never happen and is basically completely impossible. Now the ball is not at the arm tip but is probably about 18 inches from the fulcrum (let's assume this). I don't know what speeds you were looking for, but the 15-20 feet per second range would definitely make it a competitive shooter (depending on the launch angle).

To achieve this speed on the arm, you will likely want to adjust the ratio on your pulleys from 1:1 to between 3:1 and 5:1. Of course I am working from a mathematics perspective so it may be best to buy 2 or 3 different ratios worth of pulleys and swap them in and out to get the best ratio.

[stretch851]

So besides changing the pulley ratio, how much would the numbers change if we changed our gearbox ratio to 5:95? I'm sorry but I couldn't follow how you produced all your calculations.

[EricH]

Originally Posted by stretch851

So besides changing the pulley ratio, how much would the numbers change if we changed our gearbox ratio to 5:95? I'm sorry but I couldn't follow how you produced all your calculations.

5:95 is 1:19 gearbox, which is an overdrive gearbox (increase speed, decrease torque) and will not help anything.

I think you meant to say 95:5, or 19:1.

Let's walk through the math (I'll try to keep it simple).

You have 8.45:1 and 1:1 ratios. These combine by multiplication to a 8.45:1 overall ratio. Note that a 1:1 ratio just means that you're moving the same power to a different place (ignoring efficiency losses). Take the CIM free speed (without load), and divide that by the ratio (8.45) to get the current theoretical RPM of the shooter. (Torque gets multiplied by the same ratio.) Using circle math, you can compute a linear speed for the tip, as well as force on the arm (and ball, by Newton's First Law) and the torque applied.

But, we know that an 8.45:1 reduction just isn't enough. What BBray suggested was to change the 1:1 reduction to a 3:1 or a 5:1, something like that. Using a 3:1 reduction (3x the teeth on the pulleys where the arm flips) results in (8.45*3): (1*1), or 25.35:1. Run through the numbers again for that reduction.

I'm sure someone can go more in depth at another time with gearing theory, but this should get you started.

[chigskonkwo]

Our team also had our catapult shooter powered by motors and it seemed to be powerful enough, but with the ball it didn't have enough power to get it high or far enough. The team decided to change the entire design to pneumatics (with one week left ) but got everything on and working in 3 days, and it works great , better than the motors.

[John]

A 24 m/s shot at 45 degrees would travel over 96 feet (neglecting air resistance), or almost twice the length of the field. You don't need anywhere near that speed.

To do a quick check for how reasonable your ratio is:
The free speed of the CIM motors is 5310 RPM. The toughbox mini at 8.45:1 reduces this to 628.4 rpm. This is equivalent to 10.4 revolutions per second. This means that your catapult, if it moves 120 degrees, would try to flip in 0.032 seconds, which is probably too fast. The system will not come close to this speed due to insufficient torque. This leads to inefficient operation, and in this case, no operation at all. The speed exiting your catapult (that you are geared for) would be about 10.4/s * 2 * pi * 2 ft * .3048 m/ft = 39.8 m/s.

[Ether]

Originally Posted by John

A 24 m/s shot at 45 degrees would travel over 96 feet (neglecting air resistance), or almost twice the length of the field. You don't need anywhere near that speed.

96 feet to the apex, which is 48 feet above the launch point

183 feet to 10 feet above the launch point.

[stretch851]

So because of physical limitations with the perimeter, we haven't been able to change our pulley size and ratio. However we have played with our gear ratios to see if that makes our shooter effective. We actually had started at the 5:95 ratio so we went to 8, and then 12. an 8.45 ratio allowed us to be able to clear the truss and high goal, but very terribly(a foot and a half away max, only cleared high goal once, truss multiple times). We are now considering changing the pulley ratio during the pause break, but wondering what numbers we must crunch to solve which gear ratio and pulley ratio we need. do we need a 5.95 gear ratio with a 5:1 pulley ratio, or a 8.45 gear with a 1.8 pulley ratio. My question is, aren't we just lacking enough energy to throw the ball, as torque and speed are inverses for gear reduction ratios. Or does changing the pulley ratio change the amount of energy we are throwing the ball with? Please help as we are frustrated and significantly confused as what to do and what is our problem