Why I didn’t post tonight…



Please take any time you would to read my blog to read JVN’s incredibly eloquent post on the subject of college mentoring. I tried to add something, but it was pretty rambly, so you can ignore that…

Shaker 2010 Drive Sizing

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So I was tasked with the selection of Shaker’s drive gearing. I initially did it very incorrectly, so we fixed it week 5 but I’ll explain the final drive gearing as if it were well thought out.

Anyway, we had some design criteria for the drivetrain this year in addition to the game specific stuff:

  • Must be COTS or fabricated off site
  • Must not cost too much (read this as shifters cost too much)
  • Preferred to use some AndyMark products (we already have a ton of them and they’re great)

Game specific criteria

  • Must resist pushing (high traction)
  • Should push a kit wheel drivetrain without tripping a circuit breaker
  • Should push a traction wheel drivetrain for a few seconds
  • Must turn on a dime
  • Must switch zones
  • Should go over the bump
  • Must be controllable and agile
  • Must go pretty fast

No one spelled this out that formally or at all on the team but it’s what we were going for.

With the constraints, we were limited to single speed drivetrains (DeWalts were voted down, and dog shifters cost upward of $300). We figured a max speed of about 8 FPS would be adequate (with acceleration getting from bump to bump in under 2 seconds, wall to wall in 3.5 or so). Anything faster would be a bit too hard to control for a driver that had never left regolith before. However, we also wanted to push. We decided the midfield would be the most important part of the field, and that we would need to fight for position under the return to pick up balls and kick them. Things like mecanum drive, omni drivetrains, etc. were just not acceptable as we could too easily be knocked out of alignment after grabbing a ball.

The main choice was between AndyMark Toughboxes and a 6 motor drivetrain using custom fabricated plates (thanks to RC from 1323). The 6 motor drive could be geared for 9 FPS and still basically be traction limited at 55 amps or so, which was perfectly adequate. Toughboxes taking only 2 CIMs each would be geared for a good clip but would not be traction limited so pushing against tougher robots would be limited. Ultimately because the latter drive could not be fabricated, and because we used the FPs for a vacuum system, we opted out of a custom drivetrain and used a standard Toughbox with no chain reduction to 6 inch plactions. 12.75:1

After the season I took the time to evaluate the robot from a drivetrain perspective. For the elims of our first regional onward we played the “front bot” so a few of our design considerations no longer really applied. We didn’t do nearly as much pushing, but we needed to resist being pushed a bit more. This made the single speed drivetrain less of a drawback. Our top speed in front was just fine as we could match most defensive robots; either they were slower because they wanted more traction, the same speed because they were a kitbot, or faster and we wouldn’t really get pushed by them. Driving skill helped here. We also resisted spinning and turning just fine. Overall, I’d say we lucked ourselves into a successful drive this year. If I did it again and knew we’d be in front the entire event, I’d have used 6 motors and geared it for around 9.5 FPS, but we did just fine where we were at.

Shaker 2011 Kickoff Schedule

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Or rather, what I want it to be. I still need to do things like “pester team leaders into going along with it”. Any comments appreciated.

Kickoff Saturday

12:00 PM – Webcast ends, lunch break – college “mentor” gets to make copies of the Manual for everyone!

1:00 PM – 4:00 PM – Read important sections of rules together (i.e. The Game, The Robot, The Tournament, etc).

4:00 PM – 5:00 PM – Legality questions, basic design and geometry concerns

Kickoff Sunday

10:00 AM – 1:00 PM – Match strategy brainstorming – list different tasks robots can do, group tasks together into “roles” robots can play, solidify general strategies and figure out the least defendable routes [this year as an example: a frontbot would kick and side hang, midbot would kick and hang in whatever way]

2:00 PM – 4:00 PM – Figure out information we need to know before making final design criteria (“how easy is it to hold a ball”, “how fast do we need to go”, “how quickly can we reasonably hang”), break information finding into prototype groups for next weekdays

2:00 – 4:00 PM (concurrently) – Brainstorming of robot and mechanism concepts

4:00 PM – 5:00 PM – Make a preliminary list of design criteria / priority. no one leaves until a strategy is decided on

Monday – Friday: Prototyping based on questions generated on Sunday, carefully taken down information and stuff

Next Saturday: Decide on mechanism designs to prototype and pursue based on information gathered, decide on final drivetrain, decide on final design criteria

(Short post cause I’m on the road!)

Analysing the Shaker 2010 Drive Train

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So my last few posts have been very Vex centric, so I figure my next one should focus a bit more on FRC. With 2011 slowly approaching, now’s a good a time as any to review and analyze the design of the 2010 drivetrain. I was tasked with picking out the gearing for it, but it was designed before I got there so I basically got to say what to do with the Toughboxes and the sprockets. What I’m trying to say: It wasn’t my fault. 😛 That being said, we had a nearly problem free drive all year, with the only failure being when a wheel got stuck in one of the chains of the goals. Our speed and acceleration were good and we had no structural problems.

However, I still think there’s room for improvement.

So here’s the drivetrain…

Shaker drivetrain

Drivetrain with some electronics thrown on it to make it go.

Drivetrain Printed Render

Nicely shows how the sides were connected.

Problem 1: Weight. While weight wasn’t much of a concern for Shaker this year, there were numerous ways we could have made it lighter. First, we had no chain reduction whatsoever, so we wasted about a pound overall not direct driving a wheel. Secondly, we could have very easily used 4 inch wheels instead of 6 inch wheels. This saves something like a pound and a half just in wheels, plus with a different gear reduction you cut weight just a bit as well. (If you wanted to stretch it, we could have used a single gear reduction and chain for the second stage, but we would have needed a custom transmission). Also weighty was the sideplates. Unpocketed 1/4″ AL didn’t weigh THAT much, but we could have done 1/8″ and just made some crappy flanges on our hand brake. (They didn’t need to be precise!) Overall, we could have easily knocked off 5 pounds from this drive before any serious lightening efforts (25 chain, lightened gears…)

Problem 2: The cross section. All of the support for the drivetrain was on top because we wanted to leave as much freedom for the kicker team as possible, but the side effect of that is that the drivetrain had a terrible shape. Essentially it was an upside down U. Bottom supports would have made it a lot more rigid and better able to stand up to side loads. I imagine we could have made a 1/2 standoff with a 1/4″ bolt through hole in it, and tapped a piece of 80 / 20 between two axle bolts to use as a cross support. Or we could have just put one somewhere… anywhere.

Overall, it was a pretty great drivetrain that taught Shaker a lot of lessons. We survived our first custom high traction drivetrain. And we know how to do it differently next year!

Next up in the CAD directory: A 4 inch wheel drivetrain in a similar style to this one. It’d be up by now if my hard drive was intact.

Swept Away: My Vex at IRI design process


So normally this would be the cool part of the blog where I post up renders of my latest CAD work and concepts. Unfortunately, my CAD folder bit the dust at the same time as my old hard drive, so I’ll be analyzing the latest robot I made for a Swept Away scrimmage at the IRI.

I don’t have my design notebook, but with all designs I started with a thorough analysis of the game.

Anyhow, the rules are simple enough that I can summarize the main points here. You’re constrained to a small number of Vex pieces, and must start no higher than 12 inches. No other size requirements exist. You are allowed 5 motors and 1 servo. The point swings are 6 points for a football, 2 points for a softball under the goal, and 4 locked points for a softball in the high goal.

I initially began this as a collaboration with Nick Lawrence of 1503, so the original idea was that I would design the drivetrain and that he would design the manipulator. Unfortunately he enrolled in summer classes and was unable to help build or drive the final robot. I was able to borrow some parts from 1114 in order to finish the robot, though.

The first very big tradeoff was in drive gearing. With four vex motors in the drivetrain, and a fairly light robot, one could gear the drive for around 3.2 feet per second while preserving reasonable acceleration and drawing reasonable amounts of current. This gearing would give my robot a large speed advantage, mainly because it would take only a second and a half to go from wall-to-wall and 3 seconds to “switch sides” of the goals. The drawback is that only one motor would be available for the entire manipulator. An analysis of the game field showed a few “scoring locations”; the wall on the left, the wall on the right, and the goals in the middle. A mechanism that could only score footballs on the 12 inch sections of the field would need this speed to switch from “side to side” to avoid defense, but a goal scorer would not. So if goal scoring became the primary strategy, this could be ruled out almost immediately.

The next decision to make was what game pieces to score. Due to the very limited build time after Nick Lawrence backed out, it was determined that “doing everything” would be foolish, and priorities had to be made. The obvious decision here was footballs or locked softballs. I decided because the goals couldn’t hold more than 8 or so softballs, and that not having a football mechanism would put me in a hard to challenge points deficit off the bat, that footballs over the wall was the way to go. Because of this, the fast drivetrain option was still in play.

In the brainstorming phase, multiple complex mechanisms were thought up that could complete the game objectives. One early design called for a unique four bar linkage that tilted a spatula-like plate up to carry balls, and down to dump them over the wall. This linkage required two different crossed “bars” secured at different points on the robot, so it was deemed far too complex to be built in a day. A simpler mechanism used a four bar linkage with a roller claw to suck up and dump balls over the wall. The main concern with the design is that it would be very heavy, require many parts, and would require 2 motors, limiting the drivetrain.

I still needed that light and simple solution, that ideally used only one motor for the fast drivetrain. I finally finalized it on the plane to the event. The dimensions of the robot mandate a 12 inch maximum height, and a single jointed mechanism would get much higher than that away from the rotary joint (duh). But the arm would stay right at that 12 inch point, just with the manipulator raised in the air. The arm pointed up would allow things to roll down itself off the back of the robot, so a design that could “reverse dunk” over the wall would do just fine with only one motor. With a simple little deflector plate and footballs oriented in the right direction, game pieces could roll right off the arm and over the wall, with some velocity. So at this point I settled on an arm design with a passive scoop on the bottom to pick up footballs, that could “reverse dunk” over the wall. I was concerned that the robot wouldn’t be capable enough, but I didn’t have much of a choice so I reminded myself that all teams overestimate how much scoring happens in a game.

I got to the event and got to work on the robot. I didn’t finish in time for the first round of matches, which kind of sucked, but in the end it turned out to be just fine. For the first matches, my drive team (Basel A from 2337 and I as coach) decided just to “scrape” softballs in the holes in the field’s wall. This pushing of balls under actually scored a respectable amount of points and won one match, tying one, and losing the third. After that I finished the arm and ended up with this:

The vex robot between rounds 1 and 2

The “cradle” at the bottom held one football (not very well). Back up into the wall and drive the arm up and the football rolls down and over if you’re relatively straight into the wall and there aren’t balls underneath you. The arm was far from problem free, though. Most notably, because I did not program any trim into the motors, and because the arm was not balanced, the arm easily backdrove and could not hold itself in any stationary position. With a bit of latex and design work I could have balanced the arm but I had neither, so I made do by making the cradle more secure over time.

After two more rounds of qualifying, the robot was second and advanced to the finals, where it was promptly beaten by 1640. In both matches, the cradle bent completely out of shape by the end of the match. I don’t recall changing the design nor do I know why it suddenly started bending, but it did. 1640 scored more efficiently than my team did and was able to use their long arm to play defense despite our faster drive base. Overall, the robot performed very well for being built in a morning.

Here’s a Vimeo video I can’t embed for some reason.

If I were to redo the design with more time, I would only make a few changes. I would replace the collector made of bent single pieces with a design made with the 45 degree angle pieces and several standoffs, lengthwise. The football is a lot more rigidly held in between the gaps of said standoffs, and ramming the much more rigid design into a wall wouldn’t bend it. I would also add some surgical tubing to the back of the arm in order to attempt to “balance” it, fixing the rest of it with trim. With those changes the design would have scored faster than 1640 fixing the slow acquisition problem.

COTS Challenges

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One of the things I really admire about Shaker Robotics is their unique use of COTS parts in many applications. Sometimes, we don’t have the equipment or machinists needed to hold tolerances needed for stuff like gearboxes. Other times, the off the self part used in a way no one ever intended is a fast and light solution to the problem at hand. Either way, the team members always find a way to use off the shelf components in creative applications.

Vex 5 inch wheels on the robot's orbit ball shooter.

In 2009, before I joined the team, 2791 used some Vex wheels on the robot’s turreted orbit ball shooter. They looked weird but they were really the obvious choice. Dirt cheap, already fairly perimeter-weighed, much lighter than an AndyMark wheel and hub. The wheels needed set screws, but a ball shooter is a low load application. Plus they had a cool factor that gathered a bit of attention at competition events. The shooter assembly was problem free all year, so I’m told.

In 2010 there were a few different situations where we needed to accomplish something that we couldn’t quite do ourselves. Our hanging mechanism was to winch our robot up in the air with a reduction of around 45:1, but we didn’t have the skills necessary to make our own gearbox. We also had to stop the winch from backdriving. Banebots planetaries were a no-go for our team, and unfortunately my team’s leader had an aversion toward the Dewalt gearboxes which would be perfect for the application. So the team got creative again.

Shaker's winch, with servo shifting wrench and modified AM parts

We figured out that you could feed a CIM through an AM Planetary with some modifications to the sun gear and the CIM output shaft. Since we already had an AM Planetary, all we needed to buy was the relatively inexpensive Toughbox Nano in order to achieve the reduction the design called for. While the design wasn’t free of problems (the Nano’s AL shaft failed under load, so we made one out of steel), it worked just fine for the amount of effort it took us in design. For a ratchet, we just took a wrench and put a 1/2 hex socket on the end. It eased the cantilever on the shaft and stopped the device from back driving. The servo was intended to “shift” the ratchet in the direction the motor spun the gearbox, but we changed our design to run in only one direction in order to have one less thing to debug.

I’m rather proud of these little solutions using off the shelf parts. Next year we’ll have a few more.

Conserve Steelers

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Recently I had the pleasure of finding this lovely little article on the first competition I ever entered, which included a nice section about my own robotics team at the time.

Enthusiastic Perseverance

Conserve School’s “Steelers” from Land O Lakes, WI

From the moment this team walked into the morning interviews, their enthusiasm and work ethic caught the judges’ attention. Their robot did not pass inspection (bigger than the 18” cube restraint), their scoring mechanism was the wrong size for grabbing the softballs, and their autonomous mode wasn’t functional. After losing each of their first two matches, they went back to work with a smile. Their modifications, including the ability to score in autonomous mode, paid off as they won their next three matches. They were defeated in their last qualifying match and lost their robot’s arm in the process. Their response – high fiving their alliance partners, picking up their robot pieces, and going back to work.

Their perseverance paid off as they steadily rose in the rankings throughout the day, ending the qualifying matches as the captain for the 8th seeded alliance. Although they lost in the quarterfinals to the number 1 seeded alliance, the judges recognized their efforts with the “Amaze Award.” Conserve School is a small boarding school in a rural Northwoods Wisconsin community.

Well, that certainly brings back some memories. Here’s some pictures of the robot from that year:

3305 Robot Hanging

The robot, scoring the 15 point bonus for being off the ground.

The team in 2007 with our robot in starting configuration

The game was the first FVC game “Hangin-a-Round”. Robots were tasked with scoring softballs in high 24 inch goals for 3 points each. A large bonus ball doubled your score of the balls in these goals if it was on your half of the field. Being on a spinning platform in the middle at the end of the match got you 5 points. Hanging got you 15. Winning autonomous was 10.

The robot had a double jointed arm for scoring softballs. It wasn’t exactly structurally stable, and it put a LOT of force on the motors. We had two motors coupled for each joint, and we routinely rounded out the holes in the gears, or twisted the steel shafts. Case hardening these shafts was our design solution that got us through the tournament with the arm snapping maybe once or twice. The end effector was a little metal cage with a roller to suck balls in. I built that! I stole it from someone else, but I rebuilt it with lighter metal to make it a lot easier on our arm.

Our design process is a bit of a blur for me, but we initially had a huge problem figuring out how to get our arm to extend outward at the beginning of the match so that it could pick up balls. That’s where the second joint came from. Our initial solution was to use the linear Vex slides combined with a one way latch made of epoxy, but epoxy isn’t legal and we couldn’t figure out a way to make it latch into place. (The next year, Vex would release their rack and pinion set and using the linear slides would be easy…)

I was the sole programmer of the robot, and I didn’t know what I was doing. I got it to drive around, but I didn’t know how to use sensors on the bot to make an autonomous mode. For some strange reason I refused them or something. I ended up getting the bot for the entire week before competition and making a dead reckoning autonomous… that controlled a 2 joint arm, without sensors. This sucked.

What the article said at competition about our robot not passing inspection was an understatement of the difficulty we had. Our robot was designed to pick up women’s softballs. Those are a lot bigger than men’s softballs. We also were about an inch out of the size box vertically and horizontally (the top of the arm could rotate up to be taller than vertical or down to be bigger than horizontal). We skipped all our practice matches and worked to get the robot rebuilt. Changed wheels for a height drop. Shortened the end effector. Fixed some stuff. I networked with all the other teams and told them how great our bot totally was going to be. We passed inspection 30 seconds before our first match, and being incredibly intelligent I left the teleop jumper in the bot. Whoops.

Oh, but the autonomous doesn’t work now that the arm’s shorter. So I had to redo that and got it to drop the balls correctly sometimes. We finally get it going and score points slowly… Then in one key match our alliance partner doesn’t have a moving robot. We all rushed to their pit and built it with them so we would at least have an annoyer in that match. The judges seemed to think it was cool that we did that, but really it just seemed like the kind of thing to do. Turns out, that would be the most important act of gracious professionalism I’d ever do. More about that one in some other blog post.

Anyway, we finally had a match where we scored in auto, won in auto, and hung from the bar. After that… Arm snaps. Oh, and it turns out through picking and picking that we’re an alliance captain. I grab a few random team numbers and shout them at the rep and we get knocked out fast in the quarterfinals. Ending on a happy note because we won the Amaze award, we go home ready for next year. We soon learn that we’re on the Champs waitlist, and we almost make it off… but then we didn’t.

I learned a lot that year about robotics. I was inspired to compete again after being dismissed from Conserve School. I wanted to build an out of the box design like the one that won the event there. I wanted to beat my old school at the same tournament. Little did I know that this would lead where it did… but the rest of the story will have to wait a little while.

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