Pi Wars has many challenges where fast acceleration and cornering are important for the fastest times. Certainly last year Piradigm, whilst it wasn’t the most powerful robot, when running well it was still traction limited in the Minimal Maze and Over The Rainbow Challenges. This year the Straight-ish Line speed test may also require good cornering and the Pi Noon challenge always favours good drivers (or code!) even more if the chassis has good handling. We’re hopeful that our software and hardware this year will be capable enough that extra traction would increase performance. To that end, we’ve started testing a novel ‘vortex generator’ style of generating downforce:

Before we explain the weird design,  some background:

Formula one cars corner much faster than normal cars due to their aerodynamics: as they move along,  they use wings to deflect the air upwards, pushing them into the ground, increasing grip without increasing weight. This works great if you have the power and speed to do that. unfortunately most Pi Wars challenges are completed at less than 5mph, so the wings would need to be huge to have any effect.

In another, more closely related analogy, Micromouse robots need to accelerate and corner quickly to solve their mazes in the fastest time. As designs have developed, the winning teams in this competition now all use fans to generate downforce. The mice have a flexible skirt under their chassis, much like hovercraft have, but these are arranged so the fan sucks the air out, creating a low pressure even when the mouse isn’t moving, sucking the robots to the ground. This works well since their course is very flat and smooth, so the skirt has almost no leaks. Check out their incredible performance in this video from a competition this year:

Inspired by those, and the small toys that can run on ceilings, I first included a downforce generator in one of my projects in my entry for power tool drag racing:

This design was a little different to the above mechanisms, it used a vortex suction generator, which is a vaned, high speed spinning bowl that spins the air rather than sucking it, to generate the required low pressure with lower power consumption and less reliance on a good seal with the ground. The theory is that because the air is spinning, there must be a pressure gradient to keep the air going in a circle. Since the outside of the bowl is at atmospheric pressure, the centre must be at a much lower pressure, sucking the bowl down.

For Pi Wars we’re hoping to use a similar design but on a smaller scale, and only if the rest of the system is fast enough to benefit from it. So far we have 3d printed the above CAD:

And done some spin up tests in a test rig:

In this test, we held the rotor above a metal toolbox, that was supported by some scales. We were hoping to both test if the rotor could survive the very high speeds required and,  if it did, what level of downforce we could generate (measured by the lift or reduction in weight of the toolbox). For the test we were stood well back, with a full face shield on in case the worst happened.

From the test video, you can see it was a successful test: the rotor survived spinning up to ~14000rpm and generated over 900grams of downforce, despite having over 5mm of ground clearance! For comparison, from the latest CAD model we have of the overall robot design, we’re expecting the all up weight to be about 800grams. Which means we should be able to corner at up to 2g, if we have sufficient control.

 

On the software side, we’ve been further researching kalman filters and how we might be able to fuse encoder data with the data from the IMU to give us the best possible positional information, and we’ve also had a few more components arrive:

 

• Posted to Pi Wars, Pi Wars 2019, Tauradigm on Tuesday, October 16th, 2018 at 9:17 pm
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