Robot arm and scooper

Please tell me if this is in the wrong spot.

I am very new to Vex, and have a starter kit, with a few extra parts. I want to build a bot, which is completely autonomous, based off the square bot (I already have the squarebot built). I have a line tracker kit, EasyC programming kit, an extra servo and motor. My goal is to create a bot that can scoop a pile of something up, follow a line, and then dump it. Follow the line back and repeat. I am looking for guidance on building the “arm” with a small box scooper on the end of it. Right now I have 2 motors and 2 servos, as well as a small spare gear kit. Being a newbie, I was hoping someone with experience could help me figure out how to do an arm. I have drawn out some ideas on a notepad, but I really dont know what would be necessary to create it. My design ideas have a servo right before the scooper (maybe 4" before it or so), used to straighten the arm. Also, a servo at the scooper, used to rotate it for dumping.

I really apologize for such a long post, but any help would be greatly appreciated.

Thank You in advance,

Hi Jason,

It sounds like your design probably would work for this job. I’m not sure what you mean by “a servo…used to straighten the arm”, perhaps a lever?

Personally if I were doing this project I’d first consider the weigh of the object I will be picking up. In this case, I’m not sure how heavy the pile is, but you may need to gear the servo for extra torque in order to have the strength to lift it. Then you run into a problem with the servo only being able to move 120 degrees.

I guess my only suggestion is to try and build the arm first. Experiment with it, and change your design accordingly.

Thank you, I am awaiting some more metal parts so that I can begin experimenting. Basically I have an idea drawn out on paper, but it is a matter of putting the parts together and seeing if it works. That is where I hoped I could get some help from some experienced vex builders.

I will try to get a drawing to attach.

Here is a drawing of what i am thinking. Hopefully some others will see this and give some input. Also, the scooper i am using is like a light metal box you find at office stores to hold pencils/pens, etc.
I have an angled bracket I am planning to mount it to. I really need help figuring out how to orientate the gears and such. I have looked at the gallery and kind of see how, but find it difficult from pictures with limited angles.

BTW, in the picture above, the opening of the scooper is facing the left.
Thanks in advance for any help

Servos may not have enough torque for your applications. You may find you need a Vex Continuous Rotation Motor, which you can then gear-down for added torque.


What other ways/designs would be good to scoop something up off the ground?

Depends on what you are trying to scoop. Trying to move tennis balls is going to yield a different design than trying to move sand. If you give us a bit more details, we might be able to give better suggestions.


  • Dean

You might want to consider a motor driven 4 bar linkage. If you search the gallery here you should be able to find some examples. And if you make the bar lengths different, you can get it to tilt the bucket as it lifts.

I was thinking of a backhoe like design.

I will be having it scoop a dry and loose substance, like the cereal cheerios, or dry dog food. Most likely cheerios because it is lighter. What do you guys think? I have limited Vex parts. What I have is: A starter kit, EasyC programming kit, Line Tracker Kit, Extra Servo and Motor, and more metal parts on the way (the advanced metal pack $69.99).
EDIT: Also a spare pack of gears, which has 4 12 tooth and 4 36 tooth gears.

Anybody have any suggestions?

Well let me see here look at:
This one is over kill for what you need, however it will give you a general idea of what you need to which is make you motor more powerful. Remember small gears driving bigger gears will make the output slower but more powerful. Big gears driving smaller gears makes the output faster but less powerful.

This one is over kill for what you need, however it will give you a general idea of what you need to which is make you motor more powerful. Remember small gears driving bigger gears will make the output slower but more powerful. Big gears driving smaller gears makes the output faster but less powerful.

Sadly I cant even tell what is going on in there, or what that is used for… It says gearbox, so is that used for a drivetrain that has multiple gear levels?

The main question I have is how can I set up gears to bend the arm where the two servos are in this picture:

Someone said there may be a problem with torque, but what if all I am scooping initially is cheerios cereal. It would be light and not need much power to lift.

Please help Vex community… Thank You all who have contributed so far.](

I think you just need to start experimenting and see how far you get. I never end up with the exact design I start with. Just break the design into several small problems and experiment with each until you are happy and then move on. First, design your scooper and scooper joint and get it working. Then work on the arm and arm joint - you’ll pretty quickly discover if gearing is needed. Then figure out how to mount the arm assembly on the squarebot.

The servos are pretty strong, so you might be able to directly drive the scooper from a servo. You can also add springs to help offset the fixed weight of the bucket - we did this with the forklift robot and it nearly doubled its lifting capacity.

The upper joint servo is lifting the payload and scooper servo, and it is doing it at a distance so mechanical advantage is working against you. If you use a 12t gear on the servo driving a 36t gear on the joint, that’ll triple your power but limit your range of motion to about 60 degrees. Unless you are trying to dump the scooper onto a higher surface, 60 degrees should be enough for the scooping action and to clear the ground for driving.

Another option you might want to consider is using motors, but add potentiometers on each joint to track the scooper and arm positions. That way, you can get the exact positioning control a servo gets you, but you can gear the motors down as much as you need for your application. Use of a potentiometer also eliminates the need for limit switches, which simplifies things a bit.

Oh, and make sure you have sufficient counter-weight on your robot. As drawn, I think it’ll tip forward if there is any weight in the scooper. You might consider adding an Omni-wheel on the front of the squarebot base. Mount it flat against the front rail on an axle so it turns freely.


  • Dean

Thanks Quazar, I will begin experimenting tomorrow afternoon, but I could really use some help on how you can get the two pieces to bend using a motor and gears. I do not have any potentiometers, nor do I have the money to get one now.
How could I mount the scooper to rotate using a server on the scooper joint? I am in need of an experienced vexxer to help me figure out how things are oriented in a system like I the one I am trying to create.

Surf through some pictures here.
Perhaps search for ARM.
Read your manual, in particular, the motion section.
Now take some gears in your hand and imagine.
Fasten a bearing here or there on a peice of metal, according to your sketch.

Thank you, will be doing exactly that tomorrow afternoon.

I have started trying to get my line tracker to work, will continue that tonight after work. Given the picture I drew up, do you think this plan is feasible? I am just wondering what some opinions are.

I’m pretty sure you will be able to do what you want. It will take several tries and some experimentation to get it to work the way you want it to. Good luck, post pictures here when you’re done.

I recently helped a team build a line-tracker starting with a squarebot, so I’ll share a few problems we hit:

*]The squarebot drivetrain is geared up a bit for speed (5:3), but the line-tracker needs to be slowed down a bit so that it doesn’t completely loose the line when it curves sharply.
*]To fix #1, we used slower settings for the motors, but this robbed them of power and so the squarebot had trouble turning smoothly on our whiteboard surface.
*]To fix #1 AND #2, we wound up changing the gearing so that it was naturally slower (3:5). We also changed out the back wheels for omni wheels - this made turning smoother, and made the robot’s spin axis closer to the line sensors which helped with line tracking.
*]The instructions for the line sensors shows to mount them on the front (exterior) face of the front bumper. We found we got slightly more predictable results if we mounted them on the back face (interior) of the front bumper, so that they are just under the robot instead of hanging out in front.
We spent a great deal of time with the source code (using EasyC 2.0). The code is posted in the “VexCode” section of the forum under the name “Line Tracker Code”, and discussed a bit on this thread.

One interesting result of our project was that the robot gracefully makes a U-turn if it gets to the end of the line it is tracking, and then picks up the line going the other direction. This isn’t something we specifically designed for, but it was emergent behavior of our “hunt” mode. In hunt mode, if it looses the line completely (all three sensors see white) then it rotates in the last direction it saw black until it picks up the line again.

You mentioned that you wanted your robot to follow a line to a dead-end, and then dump its payload. You might want to trigger the end-of-the-line code specifically by putting a “T” at the end - that would cause all three sensors to see black. This is not a condition you would see during normal line-tracking, so it would be easy for the code to do something special (like dump the load of yummy cheerios) and then do the U-turn.


  • Dean