Sunday, December 6, 2009


In case you were wondering what this was all about, this might make more sense:

Introducing Failbot.

Recently, many of the projects I've been working on have been for credit, or for a class, or continuations of my never-ending quest to build a better motor controller. Speaking of which, the newest implementation is showing promising results. Except for the fact that I ordered some incorrect components, it went together perfectly and, without giving away the big reveal, is quietly proving that sine wave control of standard hall effect sensor brushless motors is ... easy.

Right, Failbot. I've been thinking that my projects have become a bit too large in scope and slow in execution. I say that being a grad student has basically slowed my pace down by a factor of two, since for every hour I spend actually doing something, I now have to spend another hour wondering if it's the right way to do it, what the potential problems are, and how to do it more efficiently in the future. So I am taking on this mini-project as an admission of my declining ability to just do something without thinking it through from beginning to end. In fact, every time I stop to think about Failbot, I realize how likely it is to not work at all. Solution: stop thinking about it!

Anyway, the idea is simple: linear tread motors. It's something I've had in mind since about the time when I realized you could actually build a motor. The operating principle and basic construction is the same as the 12-slot, 14-pole "LRK" scooter motors. Except, it is unwrapped into a linear motor that pulls magnets attached to a tank tread. I have no doubt in my mind that, conceptually, this works. The difference here is that I don't have the time or money to invest in a properly-designed linear bearing system. So bad idea #1: The Teflon Gap. It's like an air gap, except with Teflon. This is the part where I go back to high-school physics:

That is a Teflon-coated wooden stick, a nickel-plated NdFeB magnet, and a 1kg block of steel. This isn't the magnet size being used on the treads. But for the experiment, I just wanted a nice big surface. The objective here is to find the coefficient of friction between nickel-plated magnet and Teflon. So, an inclined plane is in order:

This dirt-simple experiment shows the static friction coefficient to be about 0.11 and the kinetic coefficient to be about 0.08. Which is good, because there will be approximately 75lbs of normal force holding the magnets to the Teflon! The good news is that, unlike my 2.007 robot, the treads will stay on. The bad news is that they may not move at all. This means that the linear motor needs to develop at least 10lbs of force to do anything, let alone move smoothly. The rear scooter motor puts out about 30lbf at the air gap...but it's 1" wide. These are only 3/8" wide. In order to work, it will need similar or higher amp-turns, which is no easy task given the small slots. And I guess the coefficient of friction might change a little when the Teflon gets scratched up and little iron filings stick to the tread magnets... Oh, did I mention that there is no commercially-available reversing brushless motor controller that runs off an RC signal? Well, except maybe this one. However, ordering it on eBay might be a problem:

So assuming I don't get arrested or have my account suspended, I get a $60 controller that might be capable of reversible speed control. If you wonder why I make controllers from scratch, this is why. But in this case I would rather try my luck with the RC stuff. If it works, it will save me a lot of time and will be 2.007 hardware-compatible. Like my last little robot...minus the high-speed acrylic-shattering collisions. Maybe some MechEs will look at it and be use wheels instead.

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