Raptor SS – Behind the Design – Drivetrain

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2014 Competition Drivetrain.

2014 Competition Drivetrain.

The 2791 drivetrain has been carefully iterated over the past four years. We began using this style of drivetrain in 2011. A single 9mm HTD belt drove an entire side of the drivetrain, all contained within a 3×1.5 tube. The drive also featured a weird gearbox that finished with a belt reduction. This drivetrain was not issue free. The single belt was not strong enough for what we tried to do. Tensioning the system was extremely difficult. The belt reduction in the gearbox failed, replaced with 25 chain. The drive ratcheted going full forward to full reverse. All in all, a decent start but a long way from competitive. Considering our constraints in 2011, we should have built the kit frame.

Our 2012 drivetrain was a big improvement. Integrated single speed gearboxes laid flat with the profile of the tube. A pair of 15mm belts were used instead of a single 9mm belt. Instead of tensioning, exact center distances were machined. The result was an issue free drivetrain that led us to a Regional Finalist performance. Our 2013 drive was similar, but with a single belt and 4 wheels instead of two belts and 6 wheels. We took a bit of a step backward in 2013 due to some gearbox design problems and the challenges of repairing and iterating a gearbox integrated into the robot, but ultimately the 2013 drive was successful enough for offensive and defensive play.

We planned a few changes for 2014 even before the season began. We moved from an integrated gearbox to a self-contained unit to allow for upgrades, on-bench repairs, and replacement. Counterbores were added for all standoffs to ensure rigidity and help with concentricity between plates. We also switched to exclusively Vex gears for maximum efficiency and ratio flexibility. The result was a very efficient gearbox that could take 3 CIMs as input and output a variety of competitive speed ratios. As Kickoff neared, the design was further improved to match the shape and lightening style presented in the 973 RAMP series.

One preseason change we experimented with was switching to 4″ wheels and 2.5″ tube. We really wanted to use smaller wheels to reduce weight and gearing demands, and our previous year belts could fit inside a smaller tube with some effort. Ultimately we decided against this once the game came out, but it’s an option for the future.

Once the game was released, we began working on the drive train immediately We knew that the game used a fully open, completely flat field. The nature of single game piece, zone play indicated to us that quick acceleration and mobility across medium distances was of the utmost importance. Additionally, defense was clearly going to be a large part of the game and our drive needed be able to play some defense / resist pushing. Given our constraints and experience, we picked a single speed, 6 CIM drive train , geared about 6.1:1. This gave us some ability to push, very quick acceleration, and a top speed of 13 feet per second.

Due to the lack of obstacles in the game, it was decided there was no reason not to use 3″ drive tubes with 4″ wheels. We quickly designed a simple “west coast” style frame for this configuration using exact center distances. The drive was designed to accommodate 4″ wheels, but also to allow 5″ wheels in case of an unforeseen ground clearance issue. Custom gussets designed by students were used to hold it together in addition to a solid belly pan for rigidity. All parts could be CNC milled, except the belly pan which was water jet by RPI. Rivet nuts and hole patterns were placed throughout the structure of the chassis to allow mounting of whatever manipulator we felt necessary. The frame was laid out such that bumper supports would stick out about every 7.75 inches. We have gone back and forth between using a square tube “upper frame” for bumpers and just having supporting members stick out of the frame. This year we decided on these members to facilitate whatever kind of superstructure we ended up with.

The biggest decision to make with the frame was dimensions. Not wanting to compromise our ability to go with any particular design, we designed our frame to be 28×27.5 inches. In retrospect, if we knew we would be using an arm I would have designed a 30×25.5 inch frame instead to allow more room for the manipulator. If we went with this shape and changed our design, however, we would not be able to grab the ball using a 16-style intake.

Ultimately, our drivetrain this season was issue free, efficient, powerful, and generally well done. We had no issues with current draw, as I initially feared, nor were we ever in a situation where we lost a head to head pushing match. About the only change I would make is the addition of some kind of aid to spin out of T-bone pins. In addition, if we knew in advance that we could get a waterjet belly pan, we likely would have used a thicker pan and heavily lightened it.

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Raptor SS – Behind The Design – Introduction

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So it’s been a good solid two years since I’ve posted here. Considering I like to pretend anything I said more than six months ago never happened, I was just considering starting a new blog, hiding all the old posts, etc. but I’m too lazy and they could be of value to someone. Oh well.

In any case, I’ve been inspired by my newfound free time and procrastination to work on a series of blog posts about the design of Shaker Robotics’s 2014 robot, Raptor SS. I think it is the best robot we’ve ever fielded and at its prime it was one of the top truss scorers and pickups at our events. It’s got some cool successful features and a few notable design failures. Overall, I’m happy with how our season went and I’m convinced that if we keep this up we’ll be on our way to regional wins in no time.

Raptor SS

Raptor SS, as shipped. A few revisions between this and the final robot.

Currently here’s the plan for posts:

  • Drivetrain
  • Design Strategy
  • Claw / Intake
  • Shooter
  • Arm / Shoulder
  • FLR Iteration

As with any big plan I make, there’s a solid chance I won’t actually follow through and make all these posts, but hopefully starting with this one will help pressure me into finishing these. Any excuse to procrastinate, right?