Clean Sweep Arm-Motor Problem

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1

Hi.

My kids have built a robot that uses an arm (driven by a motor) to lift a football.

The problem we have is, it can lift the ball just fine - as long as they do not stop the motion.

If they lift the ball a bit (but not all the way up and over) the motor allows the arm to slowly drop back down.

If no football is on the arm, the motor can raise it and keep it elevated.

Obviously the ball’s weight is more than the motor can handle (again, unless we continue the motion of lifting the ball up and over).

Thus, is the problem our gear ratio or maybe the need of a clutch?

We’re kind of at our wits end on what is wrong and ideas to fix it.

Thanks!
Dave

2

Do you have a picture? It might help understand what you’re dealing with. Otherwise I would say one motor will not be able to hold, but in the programming you should be able to have the motor hold it with power but I am not a programmer so i dont know how to program that

3

Your problem is the current gear ratio does not have enough torque to fully lift the arm. The clutches actually do not add any torque to the motors. (I’m assuming you are using the old 3 wire motors) Clutches are designed to break before the internal motor gears break. I would recommend that you either use a gear ratio more in favor of torque, (such as a twelve tooth gear driving an 84 tooth gear). So your problem is that the current alignment of gears, if you are using a gear ratio, is not powerful enough to lift the arm and the ball. If you are already at a 1:7 gear ratio (12 driving 84) then i would suggest compounding the gear ratio (so now a 12 is driving an 84 which next to it on the same shaft is a 12 tooth gear, which is then driving a larger gear, such as a 36 or a 60. If you have that second gear ratio be another 1:7 you will have a 1:49 gear ratio, which might drive it a little bit slower than you want) Hope this helps

4

You can either change the gearing or add elastic (latex tubing or rubber bands) to assist the motors.

5

I don’t think I was clear enough in my original post.

The motor is strong enough to lift the football up and over the robot (like it is designed). This would be great if you didn’t have to pick the ball up, move it, and then dump it over the wall (its the Clean Sweep competition).

The problem is the motor is NOT strong enough to lift and carry the football.

If you stop the lift action mid-way (so that you can carry the ball) the arm slowly drops back down.

Without the ball, the arm stays up in the air as expected.

In other words, there’s enough torque to lift - just not enough strength(?) to hold the ball up while you move the robot into position to complete the lifting action.

-Dave

P.S. We are using a 2-wire motor with a converter wire since the base needs 3 wires.

6

There is not enough torque to hold the arm up with the football. Elastics and rubber bands can help the motor, like previous post stated.

7

I am not sure our kits came with rubber bands or elastic. I’ll need to check.

We currently have the smallest gear on the axle off the motor and the largest gear on the axle through the arm. I’ll check into adding another set for a different ratio.

-Dave

8

No. Bringing it closer to a 1:1 reduces torque. You probably already have the most torque favored gear ratio possible without compounding. If you have additional gears, the quickest fix without ordering elastic would be to compound the gear ratios, as seen here
http://www.yenka.com/activities/Simple_Gear_Ratios_II/attachments/Simple%20Gear%20Ratios%20II%20-%20Model%201.jpg
Note that on the shaft with the big gear, you will add an addition gear ratio of a small gear driving a big gear. So its small gear drives big gear (with another smaller gear on the same shaft) which then drives big gear and arm. Hope this helps out
Here is a second view of a compound gear ratio.

9

Note that on the shaft with the big gear, you will add an addition gear ratio of a small gear driving a big gear. So its small gear drives big gear (with another smaller gear on the same shaft) which then drives big gear and arm. Hope this helps out
Here is a second view of a compound gear ratio.

This is very helpful - thanks. We’ll give this a shot.

-Dave

10

The added weight of the football is dragging your arm down, correct? Many teams with arms have this problem. There are several solutions to this:

  1. Add elastic to the arm to hold it up when the motor is not running.
  2. Make a gear ratio with more torque, so that the small resistance from the motor is multiplied enough to hold the arm up without power.
  3. Hold the motor at partial power (somewhere between 127 and 0) when you want to carry the football. This way, it won’t lift up, but it also won’t drop back down.

Hope this helps!

11

well we had this issue in clean sweep also (at first). If you are using a 1:3 ratio on the lift that might be no enough torque. I would perferable switch to a 1:5 or 1:7, this would help with your issue.

12

You are not putting any power into the motor when you are trying to carry the ball, correct?

In which case you need to put a small amount of upward power into the motor when it is not lifting. Not enough to cause it to lift when it is empty, but enough that it doesn’t fall down when it has a ball. About 10 or 20 probably, but you will need to work that out by experimenting.

If you use a much lower gear ratio then the weight of the ball will be unable to backdrive the idle motor, but that will slow your lifting speed down and it is not the best way to fix the problem. The posts saying that this is the only solution are not correct.