Tuesday, March 26, 2013

Motor Dave is Back! (And so am I...mostly.)

I took a bit of a break from blogging to move across the country.

Loading up my cube during the coldest day of the year in Boston.
I escaped the Northeast right before the string of major snow storms and am now based in sunny Seattle at a company called Freefly Systems where I will be...doing pretty much the same thing I did in academia, over-engineer motor and control systems. I may occasionally re-post some cool videos from the job, but for the most part this blog will remain dedicated to my personal projects.

Some brief updates:

My custom-built glass table survived the trip, somehow:

The right angle joints rotate up to protect the corners of the glass. I totally designed it this way...
Pro-level ratchet strapping.
And I finally have enough room to put all four chairs around it!
If you ever find yourself needing to move a 100lb slab of glass across the country, let me know and I'll give you some tips.

I now own a car:

It's a bit wet in Seattle, actually....
If you don't believe me, figure out what this is a picture of.
Well, I own some fraction of a car...maybe 25%? More than I used to, anyway. Unlike Boston, It's pretty much a requirement for getting around. I haven't owned a car since high school and the car I had back then was worth less than my computer, so I'm glad to finally have something (hopefully) reliable and fun.

My Tesla coil in now only 3-4 orders of magnitude less impressive than a oneTesla:

I intentionally did not ship my coil with the rest of my possessions because I wanted to work on it a bit more before I left. (I promised Clare Fairchild that it would make some form of sparks before I left.) All systems seem to be working as expected up to this test which was at 48V DC input, so still low power. I carefully wrapped and shipped the coil separately so that I can continue to work on it here, when I get the chance, tuning it and turning the power up so that it can break out on its own. Once I get some respectable arcs, I will take another stab at writing a MIDI parser so it can play music.

tinyKart is a wreck, but it's getting a complete overhaul:

Pretty much every part is covered two years worth of dirt deposited by snow and rain, including the most recent stretch of endurance laps run during a severe downpour at Maker Faire NY. Besides that, the damage report consists of: both pulleys and motor shafts (destroyed), one drive belt (destroyed), two spindles (bent), one brake mount (bent), both chassis plates (bent), and at least one full set of tires. Basically, the whole thing needs to be taken apart, cleaned, and fixed. However, that gives me the chance to do something I've wanted to do for a while: paint it.

Multirotor-assisted primer drying. Very effective.
It's done a lot more than we originally thought it would, so a bit of maintenance is definitely in order. It might take a bit of time to get it all back together properly, but when it is, it will look much better than it ever has, I think.

Lastly, Motor Dave is back! Dave Wilson, "Motion Products Evangelist" at Texas Instruments, has returned to the bloggernet as well with a new series on how to tune your PI controller, starting with Part I here. It's specific to the typical nested PI controllers involved in motor control (current, speed, position). If you haven't read his previous series, The Ten Commandments of Digital Control, you should. It starts here. I went to a TI workshop co-hosted by Dave about a year ago when TI's BLDC solution, InstaSPIN, was relatively new. At the time he hinted that there might be a future version: InstaSPIN FOC. Well, it's here now!

If you've been following my blog for a while, you probably have seen my many many posts of Field-Oriented Control, sensored and later sensorless implementations thereof. One thing I've not been very good with is putting out hardware that other people can easily access and play around with to try out this powerful motor control technique, without messing around with a lot of low-level code. (I like messing around with low-level code, but I can totally understand why most people hate that part.)

Anyway, you can now buy a kit for $300 that has enough power to drive a small electric vehicle motor and is loaded with a thorough FOC code library. Alternatively, you can just buy the control card with FOC library and develop your own high-power inverter stage. Or you can just buy the TMS320F28069 microcontroller with built-in FOC library and use it in your own hardware.

Though I haven't dug too deeply into the code, the basis for InstaSPIN FOC, and particularly the FAST observer, seems somewhat similar to the flux estimating structure I've used in my own sensorless routine. Flux is a nice quantity to observe, since it isn't speed-dependent like back EMF. The tricky part is keeping the observer happy at low speeds where not much rotor position information is available. InstaSPIN FOC seems to handle this situation using a "forced-angle" startup routine but then can also transition back through zero speed seemlessly when properly tuned.

I'm not sure it would cause me to deviate from making my own FOC software, but it's definitely something worth checking out. 

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