We have been a little behind on getting blogpost material out so this is a bit of a catch-up update.
We have decided to go for a two-wheeled centripetal shooter that can shoot at different angles for upper and lower hub shots.
Lower Hub Shot: Fender, topspin, 20 degrees from vertical
More testing videos
Upper Hub Close Shot: 3 ft away, backspin, 20 degrees from vertical
Upper Hub Far Shot: Max 25 ft away, 40 degrees from vertical
We have been testing up to this point with 4 7/8" non-compliant wheels.
We finished testing the 4-inch compliant wheels and decided against using them for two main reasons. First, to get the same arch and distance as the 5-inch wheels, we had to use 4/5ths the amount of power. Additionally, when we attempted to feed two balls in close succession, the shots were inconsistent with each other.
Next, we tested the “marshmallow” wheels and found them convincingly inadequate. We worked on installing “stilts” to allow the shooter to merge with our indexing prototype, and this is nearly complete. We had to widen the shooter’s outside profile to allow the mounting, and the front legs will be long enough to allow a range of angles. Next meeting, we will fine-tune the adjustments for the shooter to indexer mount and then begin testing. I believe this will be a fairly involved process, as we have to replicate a higher degree of precision than the prototypes may initially allow.
Finally, due to the possibility of missing Friday and Saturday’s meetings due to snow, we are planning to meet (as is the rest of the team) on Wednesday.
We built a simple hopper prototype using 3D printed pulleys and polycord belts. In addition, we cut two pieces of wood in an arc for our “flup” design. We were able to test it and it worked well! However, we are now considering prototyping a different design without the flup wheel. If we have conveyor belts on the side feeding directly into the shooter, the balls would not have backspin and might shoot more reliably.
This is our progress on the upper hub so far.
We tested the intake prototype with surgical tubing attached and we found that 2.5" initial compression worked well enough and gave us good geometry for the rest of the intake. We also found that we didn’t need a third set of rollers so we are going to CAD a final prototyping plate to test the new geometry with 2" wheels and surgical tubing both with and without mecanum wheels because we aren’t sure if we need them since the hopper might be enough to center the balls. We are also going to start prototyping different materials to work as a slapper roller to help intake bouncing balls.
This is our climber CAD so far. As described in an earlier update, we are using a pair of pivoting arms and single-stage telescoping elevators to climb to the traversal rung. The goal is to first get a mid rung climber and add on the traversal rung capability if we have time. We plan to have a thriftybot telescoping kit for the top slider but to build a custom slider for the bottom of the 1x1 tube. We wanted to have bearings on the front/back faces as the robot will have some rotational forces.
We have spent a lot of time worrying about our climber’s robustness. In previous years, our climbers worked adequately under normal conditions, but unexpected forces at competitions have caused failures. This year, we want to be prepared for when things don’t go as planned.
The scenarios we have anticipated include:
- Climbing to mid rung with only one hook engaged
- Getting bumped from behind while climbing
- Getting bumped from the side while climbing to mid
- Transitory loads on arm from sudden impacts
The arm is 32" long and the pivot point is 8" away from the expected robot center of gravity (see earlier post for why we chose these values).
Some ideas we have thought of for mitigating these issues are:
“Cush drive” sprockets on the arm
These are used in dirt bikes. The idea is that the sprocket is mechanically linked through polyurethane rubber so that sudden impacts are absorbed. We could mold the polyurethane used in polycord if we wanted to make a custom sprocket.
J plate telescope + arm support
1/16th aluminum tube riveted to drivebase and to the outer telescope using gussets. The gusset on the telescope is a larger one, kind of J shaped, and attached to that (maybe offset with some spacers) is the arc-shaped aluminum plate, which lives spatially between the two climber arms. The rotating arm can use a shoulder bolt to constrain itself into the curve of the arc. It may be tricky to make the arc plate stiff enough to be helpful.
This would strengthen the telescope and arm, which could be helpful if we are bumped while climbing.
Throw the chain under the bus
Driving the arm with a #25 plate sprocket, perhaps with tensioners to make the chain longer, would make it more likely that the chain would break before shafts or planetary teeth.
Because we don’t want to risk breaking shafts, we decided to gear our Neos with a 25:1 maxplanetary followed by a 4.5:1 chain reduction. We would run the Neo at low power for controllability and to reduce the load on the gearbox.