arm help

Ok i’m build ing a robot from the starter kits for a comp that says you can only use the vex parts for the robot. the robot needs to pick up a tennis ball and put it on top of a goal. i’ve got the 4wd base built and i just need sugestions for an arm if you could include pics for exlpenation it would be much appreciated!:slight_smile:

r there any size limits
also wat is teh goal height

yes there are it has to fit in a 16X16X16 box and i only have three simi-new starter kits to use and it only needs one pivit point in the arm

wat r teh goal heights
depending on teh height u mite need to have 2 pivots in teh arm

The goal is about a foot and a half high

You need to reach an 18" goal and still fit inside a 16" box. You can approach this in a couple of ways.

Your first option is to build a straight arm that has its base at either the very front corner or the very back corner of your robot and its tip when inside the parameters is at the top opposite corner. this will give you a long enough arm to reach your goal, but it might present some balance problems.

The second option is to make a jointed arm. This will give you more than enough length to reach your goal. Jointed arms require more motors and parts though, and you will most likely have to modify parts to get a good arm. It also uses more axes on your transmitter.

Teh only problem with the 2 option is I’m only alowed 8 motors/servos an it’s 4 wheel drive

The animal grabber project in this document has a claw and simple lift, using only 2 motors. The claw is slippery and would need modification, but it might give you some ideas.

thank that gives me an idea but now i have a problem with keeping the arm from lowering when i dont want to i’m useing the larger gear and the really smal gear that are included in the starter kit the small on the motor the large on the arm its self. and it always lowers when ime driving a round

A simple way is to use some rubber bands or – even better – the Vex latex tubing fastened to the arm to hold some of the weight of the arm. We find it doesn’t usually take much to keep the motor from “back-driving” and let the arm sag.

The latest method we use on our team is to combine a little latex tubing with a feedback mechanism. We put a Vex potentiometer on the axle shaft of the arm, and then watch it in code. If the motor speed is set to “0” and the pot is moving, we assume that it is sagging and give the motor a little bump up until the new pot value is the same as the original value. What we are doing is turning the arm motor into a servo with the arm as the servo shaft. By using a regular motor instead of a Vex servo for this job, we can gear the motor way down and still get the automatic set/hold feature of a servo.

On our current generation of Vex robots, you can’t move the arm by hand when the robot is powered up because the pot-motor-feedback mechanism won’t let you. It can be really strong.

It’s important to have most of the weight of the arm either counterbalanced or spring-latex loaded, though, because you can burn out a motor by constantly feeding power to it. It’s all part of a balanced design. When you get the weights and forces figured out, though, it’s miraculous how well it works.

you cold do something like this were the gripper is long and when completely closed will be short but when open is long. you could gear the part labeled arm to move up and down and using gears you could use one motor to open and close the gripper. i hope this makes sense if it doesn’t just ask.

To do this on the last robot I built for competition, I just turned up the trim on the arm’s respective axis. It’s not a perfect solution, but it’s simple and doesn’t require any modification.

This strikes me as a good teachable moment, or the start of a lesson for students.

Given two arms, one balanced and one unbalanced, running off of a servo, which one is going to drain the batteries quicker? For younger students the principle can be demonstrated by simply holding an object straight out to their sides and seeing how soon their arms gets tired. For high school students, this is a good time to pull out the ammeter and see how much a change in design could mean to battery life.

although this may work you are constantly running the motor and you could damage the motor doing this. it is good for last minute fixes but not a great way to plan on fixing this issue :wink:

Best used in a situation where it won’t be needed for a long time. The competition I used that particular bot for didn’t require me to lift anything heavy enough to unbalance the arm for more than a few seconds. You’d probably be better off going with the tubing, but I figured it couldn’t do any harm to provide another option.

right i wasn’t saying it was a bad idea. :slight_smile: