Bouncy Claw Arm

The kids made a 4 bar for their claw arm. They put the rubber bands on so a motor could lift the claw. It works, but the claw is bouncy. Also, sometimes when you lift up the claw, the rubber bands fall off.

Do you have any suggestions for how to assist the motor, but lose the bounce?


My first thought is to lower the motor speed. Try putting in it port 1 so you can use the analog stick to move the claw. A video would help.

What software commands are you using to move the motor?

Here’s a link to a youtube video. This is pretty exaggerated, because she is pulsing the button, but it still does it when the buttons are pushed more smoothly.
Code is ROBOT C
motor[motor name] = motor speed;
For example, if you are trying to move a motor named “claw” using the left bumper buttons,the code would be


 motor[claw] = (vexRT[BtnLUp] - vexRT[BtnLDown]) * 100;


A couple things you can do. Instead of just applying 100, you can place a delay of 20ms at the bottom of the loop and then add say 4 each time through the loop depending upon which button is pressed. So you are making a slope of the power instead of just applying full tilt.

The best thing to do is add a control position loop. This is done for you in the graphical RobotC commands depending upon which ones you use, I’ll have to look up the RobotC discrete commands.

The backs of your 4-bar links extend past the mounting towers. The ends of those arms are much better places for rubber bands than the middle of the links. This picture might help:

Good rubber band practice.png

Good rubber band practice.png

I’d also suggest running a lower gear ratio and less bands.It’s hard to see exactly what your gear ratio is but looks like 1:3 which will be very fast.
With a lower gear ratio, the bounce through to the motors will be reduced and you’ll also be able to have less rubber bands whilst maintaining an acceptable speed. On a short arm like that, you probably don’t need any rubber bands. Consider the basic Clawbot - 1:5 ratio is actually quite fast.

How many motors are on the lift? If you can spare one that would help as well… Might not need rubber bands at all!

So the clever solution is to use the rubber bands and not the motor. That saves the motor for another mechanism or saves the weight and battery for the robot. Look at it as engineering for the future, VEX metal robots use the rubber tubing all the time to help with lifts.