End of Week 1 Update
Realistically, this will just be a weekly update barring needing to ask for help to fix issues or something very cool happening.
This week was interesting, as we were not able to meet in our normal build shop in school. We expect to get back to our normal build schedule mid-week 2, but this hiccup did slow down our fabrication team’s progress with prototypes a little. Even still, a lot got done this week, starting with design:
Yes, this looks like a snake to us, too.
The first couple of work days we had were used to work on the full system sketch. This is the first year we’ve done this in great detail, and it’s paid off incredibly well. Now, when the students are doing their detailed mechanism design, they start their part studios with a derived sketch from this master sketch and they have to think about very few dimensions, since they figured them out earlier in this sketch. The only downside is that making this sketch isn’t really effectively parallelizable, as far as I’ve seen, so the first couple of days had 9 kids huddled around one computer and they rotated who was actually drawing things in.
We drew a lot of inspiration from FIRST Capital’s design. Going into Monday, our intention was to build a robot that just shot for the OUTER PORT from the TARGET ZONE. However, based on the accuracy FIRST Capital was able to show shooting from the INITIATION LINE into the INNER PORT, we decided to try for inner port shots by modifying the robot concept to shoot from the INITIATION LINE.
This also simplified our concept greatly – initially, since one of our design constraints was to intake from the opposite side as we shot from, we had a double bend in our feeder which we were not confident in our ability to get working. This was because shooting from the TARGET ZONE required the shooter to be at least one robot length away from the port so the angle was not literally vertical. Therefore, since we had to shoot across our robot, the intake would have ended up on the same side of the bot as the shooter, forcing the double bend.
The downsides we brought up for this switch were that it’s easier to be defended on the INITIATION LINE because it’s not a protected zone, and shots need to be more accurate because it’s a longer range shot. Additionally, we’re not confident that we will hit INNER PORT shots with great consistency, so we may not see great benefit from this shift. However, because the point potential is higher, and it simplifies the design of the robot, we chose to stick with this concept.
The only major thing that should change from this sketch is the positioning of the intake pivot point – we noticed that as is, there is a 5" gap between the intake roller and the top of the feeder when we’re intaking. We are worried that the ball will sometimes shoot out of this gap, so will likely move the pivot point of the intake to be on that feeder roller and add something to stop the ball from flying out of the top.
We’re planning on running chain in tube this year, because our offseason build of that style of drivetrain was successful and we wanted to build a small and narrow robot this year. The chassis will be 24" wide and 30" long with 6" wheels. Currently, our intention is to use corner omnis and center Colsons, but we have a 1/8" center drop and will be testing all traction wheels to see if the resistance to being pushed is worth the loss in maneuverability.
Our hope is the narrow chassis will make it more likely for us to be able to fit on the rung with wider robots while also making it easier to put our CG closer to the center of the rung when we go up. Additionally, for the first time, we will now be able to easily fit our full bellypan on our OMIO X8 router, making it far easier to machine (last year all the holes fit but we had to manually cut out the outside because it was just over the size of the bed).
The intake is modeled in many ways after 971 in 2016. We’re using 2" mecanums vs. their 4" ones, and the shape of the intake is different, but the concept is the same. We’re using a 1" OD aluminum tube at the front of the robot to prevent direct hits to the 3D printed Thrifty Bot mecanums when intaking from near walls. The side plates are going to be 1/8" polycarb, also to deflect impacts. While this may change because the intake pivot may change, the current plan is to drive the roller with a NEO 550 on a 4:1 reduction (similar RPM to what Ryan from Thrifty Bot showed in his example intake video). We want both the intake roller and the intake arm motors to be inside the frame perimeter at all times, so the roller is belted (that’s the far side of the image). The intake arm gearbox has some interesting packaging constraints where it is right now – we spent a decent amount of time figuring out how we would fit that in. With our narrow robot, this has to fit into the 3.5" of space between the 8" wide feeder (in the middle of the robot) and the side rail. With this design, the plan is to use a NEO 550 with some gear reduction to a VersaPlanetary 180 degree drive kit and a custom mounting plate, then use a 1-stage VersaPlanetary to drive the hex hub mounted to the intake plate. This would get the intake to move at a semi-reasonable speed (currently we have 6:60 into 1:10 for a stupid quick 614.8 deg/s). This will be a pain to get right for the programming subteam, which is one of the reasons we’re considering moving the intake arm up (more space for more reduction there).
We had most of this design done well in advance of the release of the Greyt elevator, but we modified the hood to be polycarb with standoffs once that came out. It used to have rails, but that was mostly because we had no idea how teams fastened polycarb to the standoffs with these types of designs (for those that don’t know, apparently it’s just zipties).
They’re super tiny and super cheap, and for teams (like ours) that only have access to routers and no 3D CNC machining techniques, a great way to precisely line up things in 3D.
Our entire feeder assembly seems to have appeared between our last meeting and now, even though today was supposed to be a work-free day . But the general idea is very similar to FIRST Capital’s: the intake roller centers the ball and pulls it into the drivetrain, where the feeder rollers grab the ball and push it up. There will be 2" wide urethane belt running between the rollers. The reason you don’t see the bottom feeder roller is that one is going to go in the drivetrain, since it only mounts to the drivetrain.
We plan on having beam break or distance sensors along the ball path to detect how many balls we have, so signaling LEDs can show the drivers how many balls they still need to grab.
Initially, around the bend in this feeder, we planned on only having one roller. However, we realized that if we did this, one section of the feeder would have its polybelts off-center, changing the compression. Rather than worrying about what the new off-center compression should be, we decided to just go to two rollers at the bend, so the whole ball path will have the polybelt along the center.
The climber is, in my opinion, the most interesting mechanism on this robot, just because of design decisions we made due to not using pneumatics.
This design was also fairly heavily inspired by FIRST Capital. We’re using 1x1x0.125" tubing for the climber rails. We’re powering it with 2 NEOs geared at 10:72 on 12T #35 sprockets. This gives us ~1s actuation on the climber.
The interesting part of this climb is that the actual climb is heavily assisted by CF springs. The initial intention behind this design was to find a way to hold ourselves up after the buzzer without using a pneumatic brake, since the rest of this robot was done without pneumatics and we didn’t want to add them into this robot. One of our team members suggested that CF springs would work if they were balanced while holding the robot up. We realized that the heaviest CF springs sold by Vulcan (using the KOP voucher) were 40.9lbs, so we decided to use 3 of those per robot. We hope that even if we’re at max weight, the springs plus the gearbox friction and brake mode on the motors will prevent us from falling down.
More interestingly, though, if (as we expect to be) we are below max weight, and the robot is under 120lbs with battery and bumpers, we get an after the buzzer climb. This is a double edged sword, as if we are trying to line up a double/triple climb when the buzzer goes off, we may ruin the level state of the existing climb when the climber slams down. However, in matches where we are the only climbing bot, it would be quite helpful to be able to cycle till the last second and rush over to get the climb. This was not the initial design intention, but may end up being a nice bonus.
Unfortunately, it seems that the CG of this subsystem is quite high (25" off the ground!!) We hope that the other subsystems will drag that number down way lower. To that end, we plan on running belt up to the shooter wheel to move those motors down, in addition to the motor for the final stage of the feeder.
Other Subteams Update (far less detailed)
The mechanical subteam was able to prototype an intake and a feeder. However, due to some oversights in pre-season ordering, we did not have any long hex shafts for them to be able to test a full width intake with. They were therefore able to figure out ball compression for both the intake and feeder (2" seemed optimal for both), but not spacing from the bumper for the mecanum wheels. We will do more testing this week, especially after Tuesday when our new hex shafts will come in.
Programming wrote most of the basic subsystem code for all of the subsystems on the robot, and are currently working on signaling LED code, as well as state machines for multi-subsystem interaction.
One of the tools that made this way faster to do is our new
ServoMotorSubsystem class, pretty much copied from 254’s 2019 code. What I think they realized was that almost all the code in subsystems like turrets, elevators, arms, etc is the same, because fundamentally all these subsystems do is move to a target position based on encoder values. The only real differences are the encoder values, units, and PID values. For our purposes, we’re able to use this wherever there is a servoed subsystem (intake arm and climber this year).
Things I’m Proud Of This Week
- The whole team handled being out of our normal workspace incredibly well. The level of productivity stayed as high as I could reasonably expect given that our meeting location made it physically impossible to precisely cut prototype parts and some of our parts were just not accessible (stuck in the shop room).
- Our design subteam is almost entirely made up of new members – the only returning members are the two design leads and the design mentor (me). Even still, design is ahead of the ambitious schedule we set at the beginning of the year. Almost all of the mechanisms are being worked on by new members exclusively (climber has both design leads). I’m very proud of the job the design leads have done in teaching CAD over the offseason, and even more proud of the job the whole subteam have done themselves, successfully taking on (and already nearly finishing) full mechanisms.
Hiccups and Mistakes
I think one of the most important things about these build threads is when teams talk about mistakes they’ve made and how they’ve overcome them. We’re not quite far enough yet to have made massive mistakes with the robot yet, but we did make an administrative mistake a couple of days ago.
We try to avoid ordering parts for a mechanism until we have done a full design review of that mechanism. However, on Thursday, when placing one of our Vex orders for prototyping parts, we thought it would save some money to add in our drivetrain gearbox gears for both robots. We were fairly confident this would not result in an issue, because we had checked the gearing and were sure we would stick with it. However, while we had sketched the gearbox, we hadn’t done an assembly of it yet.
It turns out that the gears we ordered made it physically impossible to assemble the gearbox (the first stage output gear clipped the output shaft by a significant margin). It was an easy CAD fix, but it will result in a great deal of effort to now have to return the gears in question to The Robot Space (and a loss of at least $30 in shipping and restocking fees). If you’re reading this, please don’t rush orders this early in build season – it may end up costing you. We learned this lesson the hard way – no matter how confident you are, review before ordering parts.
Still To Do
- Add climber support beams
- Figure out feeder motor locations
- Potentially move shooter motors down
- Finish drivetrain