A few mentors and I began working on a new robot cart for my team during the off-season this year, and it is still being worked on currently. I am looking for some advice or suggestions from other teams that have experimented with this sort of robot-lifting cart.
For some background:
Our current robot cart is just a wooden box bolted onto an old pneumatic lift, and we need a more robust and functional cart. I was the only CAD team member at the time, and so I was put in charge of designing the new cart. I drew inspiration from Team 1787’s robot cart post from 2017, which uses a winch and a motor to lift the robot on 2 steel square tubes.
Our design considerations / parameters
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The robot needed to sit lower than our current cart, so it’s easier to be put on and taken off. Our current cart has the robot resting at a height of approx 2.5 ft. It would be beneficial for our drive team and pit crew if it rested closer to the ground, at approx. 12-15in.
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The roller assembly would be made of welded steel, which we could have done by a local college, and would need bearings and shoulder bolts that could handle the static load of a robot and bumpers (~150-175lb), plus some extra headroom in terms of weight.
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The ability to lift the robot up in the air is essential for us, since we currently use Rev’s maxswerve 3in swerve modules, and we constantly need to be switching wheels. The ability to raise the robot is helpful for when our pit crew needs to swap out the wheels or swerve modules.
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We would also like the frame to be bolted together, to make assembly and disassembly (if necessary) easier. This also reduces the amount that we would need to weld, if it were difficult to get certain parts fabricated.
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The new cart needs to have proper storage for game pieces and some batteries so the robot battery can be swapped right before it goes on to the field. This is important since our current cart has barely enough space under it to fit a few crescendo notes, and the tools that we need.
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A method of locking the robot in place at certain heights, without relying on electrical power to hold it up, to allow pit crew to safely work on and under the robot. (Our solution to this is found in V3.1).
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The size constraints that we have are: 28in wide max (to fit through most doorways), ~44in height (40in max robot lift height due to safety concerns), and < 50 in length, so that the cart isn’t unwieldy in the pits area.
Initial concept design - V1.0
Here is what I came up with as a rough draft, initially just going off of the images that 1787 shared on their website:
The V1.0 just included approximate design details, but was the first step for me designing this robot cart.
Breaking it down - V2.0-V2.2
This is my second iteration, where I began sorting out the individual pieces that I would need to make, as well as beginning to figure out how it will actually be put together.
This version (V2.2) is where I began actually figuring out how 1787 designed their cart, and the pieces that I would need to fabricate to match their design. The main centerpiece is the all-welded steel “rolling assembly”, which is the primary piece that I spent a lot of time working on the best way to make.
Pro of this design:
- It is simple and fairly representative of 1787’s robot cart
Issues with this design:
- The steel tubing for the arms is to large, we’re going with 1.5x1.5in over 2x2in
- The upright parts of the frame were not stable enough, since they were only being held on by metal plates and bolts at the bottom
- The rolling assembly, which was a copy from 1787’s design, was too complex and we needed to simplify it
Iteration Iteration Iteration. - V3.0-V3.1
This is where a lot of work on the frame took place. We decided to make the jump to a box frame, and we also considered using 2 pieces of angle steel welded together on each side to act as the rails that the rolling assembly would move up and down on.
Around this point is when one of our team mentors tossed in the idea of using #35 chains to support the rolling assembly, and be powered by an old Cim motor. There was a big problem with this solution however, which was the lack of a physical brake or fail safe if the battery for the motor died (We did consider having a very high gear ratio to keep the assembly from rolling down, but it was deemed not safe enough).
Introducing the Winch. - V3.1
Several mentors and I worked out that a brake winch would be a best fit for this scenario. The brake winch can be driven by a motor (or drill/ screw gun) and has a ratcheting mechanism that allows it to ratchet up and down, which provides the safest lift mechanism for our use.
For this version, we were hopeful that we could get most of the frame welded, so there was no need for a large amount of metal plates to hold it together. However, there is some uncertainty about our source for the welding of this project, so I had to design in metal plates for the next iteration, and just rely on the rolling assembly to be majority welded.
We also wanted more storage, and this is the cabinet that I found that would fit within the inner frame (24x16x8in): 16 Gauge IP65 Carbon Steel Electrical Enclosure 24 Ă— 16 Ă— 8 In..
Several major design decisions were made for this version, one of which was moving away from the 2 angle steel pieces welded back to back. This decision was made because it would be easier for us to use a 1x1in aluminum tube that is incorporated into the frame, like so:
But we decided to go with this option, which uses a 2x1 to reinforce the 1x1 steel tubes that act as the rails:
Current Version V3.2.3A
This is the latest iteration that includes all the design input from earlier revisions. This version culminates weeks of work designing, figuring out the most practical ways to put together this cart, and then going back and reinforcing the entire frame with metal brackets so that it can retain the easy assembly that we wanted. Note: I started working on this cart before West Coast released their super cart, so I didn’t think to incorporate any angle aluminum into the frame.
It definitely looks complex, but it’s mostly 1x1in aluminum tubing and metal brackets so I hope that it holds up to other similar robot cart designs. The lift mechanism is intended to be driven by a drill, or manually cranked up and down, utilizing this brake winch: https://www.dutton-lainson.com/proddetail.php?prod=15941, along with the necessary parts to connect a drill to it.
Here’s the link to the public version of the CAD: Onshape | Team 3181 Robot Cart Design V3.2.3
We have not yet built this cart, but we are getting close to ordering the materials, fabricating the parts and assembling it. Any questions or suggestions are welcome, since I am not an expert in robot cart design and would like to know how I can better improve the design of this cart.
