Arm Difficulties :eek:

My classmate and I are seniors in highschool. We have been assigned the task of construction a VEX robot capable of obtaining soil samples. Our school is on the shore so we plan to use the robot to replace the need for human presence on sand dunes. Unfortunately, it is my partner and my first stab at VEX robotics so we are at a loss as to how to attach the arm so that we are able to manuever it enough so that we can obtain the sediment and transfer it to a soil sampling tray. If you could throw us any tips we’d appreciate it greatly!

Preliminary questions:
What materials do you have available to use?
Could you provide a diagram/rough sketch? This would help us understand what types of joints your robot requires.


For class KISS is very important. You may only want a 1 DOF arm, building an arm that looks like it came from the packbot or car assembly line will be very hard to build and control. Personally I think 3 DOF should be absolute max for a beginning project.

What have you built so far? A protobot?
What does a soil sample tray look like?
What is the difference between soil-samples, sand dunes, and sediment,
and what does a sampler have to do?

The very process of writing up your problem description in enough detail to get good help,
will help you figure out what it is you want to do, and how to do it.

For lots of ideas, try Google Images and search on “vex robot arm”

wow thank you guys for getting back so fastt.

RoboDesigners: For materials (relevant to the arm) we are allotted the general VEX Robotics starter kit. I believe we have an additional claw kit and two bumper switch kits.<–That’s an AutoCAD drawing of what our robot(Big Rhonda) resembles somewhat. We’re in class right now but we have taken pictures of our robot in its current state and will upload them when we get home.

to DiodeTech: I am so sorry but we really could not be any newer to VEX Robotics so we are a bit confused as to what a DOF arm is, but if you are referring to resembling the arms found in assembly lines we are really open to any type of claw, so long as it is able to pick up a soil sample.

jgraber: Our robot resembles the protobot in the sense that we do have a hull and wheels attached. The sample tray will be in the picture we will upload but it will basically just be a small rectangle attached to the hull of the back of the box so the robot does not have to go back and fourth. Our robot will primarily be picking up sand samples but when i say soil I refer to more densely packed dirt found on our campus.

TomW: thanks bro! haha

DOF = Degrees of Freedom = how many joints.
1 DOF arm goes up and down
2 DOF arm goes up and down, plus wrist turns?
2 DOF arm goes up and down, also side to side.

If a “sample tray” was just a basket on the top of your robot hull,
and if “taking a soil sample” was just “pick up a 1” square wood block with a claw"
then all you need is

  • an arm with a claw,
    – that can reach the ground in front of robot,
    – and also lift up high and over head to drop the block upside down in the basket.
    Thats just 1 DOF needed, from down in front to overhead on back.

Compare/contrast your problem with the above description.

Sorry for using a acronym and not explaining it. Looking at your picture, the arm goes up and down and can spin in a circle?

Here are some pictures of what we have so far!

Why not make a scoop? just pick up a whole lot of sample and dump it all into the rear tray.
If you want to make it complex, have some kind of carousel in the back that spins when you want to dump the sample into a different compartment.

I’m her partner for this project, here’s the robot:

I always rant about this to the kids in our club: bearing flats!!! They should be on the two axles for the arm. Also the arm most likely should be on shorter axles to prevent the axle from bending or there could be a support in the middle. Put 2 lock collars on the axle on the insides or outsides of the supports to keep the axles from sliding around. Also, can you make a sketch of what you want your arm to look like? It will make it easier to offer tips on the arm with a plan in mind.

I agree that a scoop should be used. You would probably get the best results if you used a two-jointed arm with a swinging scoop like on a bulldozer. It could pivot in a circular motion at its base and deposit the sample in the storage bin. I believe this is the type of soil scoop that NASA used on its Phoenix lander on Mars.

Ok so to create a basic arm, secure the piece of metal to the top gear in the video. If you look at the top gear, there are two holes that you can put screws through and then use a nut to secure the metal to the gear That’s the most basic arm and then attach a scooper or something to that piece of metal and you should be fine. Btw use a smaller gear for the bottom, preferable the smallest type because that gear ratio you have right now is probably not strong enough for your purposes. Finally use delrin bearings if you have them to make sure your axles don’t bend. Good luck.

Thank you guys so much, we really appreciate it.

Hello againn :), since we last spoke my partner and I have made a rather large amount of changes to our robot(who has taken on the nickname of Big Rhonda). Now that we have changed the structure of the robot and the claw to our liking, the only challenge left to tackle is the soil tray. Since the objective is to obtain a soil sample, the claw is going to hold a beaker with which we will scoop up sand, and then extend back with the beaker to drop the soil into some type of soil tray. We have been brainstorming what type of tray to use, such as a small rectangular box or a funnel leading to a beaker, but could not decide on which would be best. Alterate ideas would be much appreciated. Additionally, we could not seem to agree on a medium to construct the soil tray from. I personally believe cardboard would be a fine lightweight option, seeing as the conditions are not going to be bad, the sand will not be wet, and it is very easy to cut and construct a tray from. My partner tihnks it would be best to use some sort of plexiglass material or perhaps wood or plastic. I am worried this will make our robot too heavy and thus make it sink into the sand deeper, rendering it less maneuverable. Thank you for taking the time to read :wink:

If you only want one sample, a preformed plastic left-over or deli container would be good.
If you want a series of samples, maybe an egg carton would work, if you had a way to index it underneath the scoop.

I dont see the point in holding a scoop in a claw, if you never need to let go.
Better to have the claw motor power another joint in the arm to control the scoop angle.
A backhoe prototype has 3-5 joints.
A bucket scoop can get by with 2: bucket tilt and arm lift. Its simplest if you can lift the arm completely overhead to be over the tray while holding the bucket level, then dump the bucket in the tray.
good luck

Cut the bottom off of a milk carton. Light weight and deep, it will make a good tray.

Styrofoam meat trays or half of a styro take-out box would be light and water resistant.