Wednesday, January 26, 2011

MechE Quails: More Field-Oriented Control

What is a quail? It's like a mixture of quals and fail.

What is a qual? It's like a mixture of...I'm not sure. It's the Mechanical Engineering (Course II) department's bi-annual doctoral qualifying examination. It's a three-day test consisting of:
  • Day 1: Up to three 1-hour written examinations in three subjects of your choosing. Examples: System Dynamics and Control, Machine Elements and System Design, Manufacturing, Thermodynamics and Heat Transfer, Optics. I chose the first three.
  • Day 2: Three 40-minute oral examinations in the same subjects. (20 minutes of reading, 20 minutes of Q&A.) The audience is 3-7 professors in that field.
  • Day 3: A 45 minute presentation on past or present research. (20 minutes of presenting, 25 minutes of Q&A.) Also 3-7 professors in the field of your choosing.
To pass, you must satisfy the professors on at least two of the three subjects, and the research presentation. And you win...well I'm not sure what you win. It's not a dissertation; no degree is awarded at the end of the process. It's more like a mid-term. If you lose, you get a second chance in a few months. If you lose twice, you get kicked out.

This is Try #1 for me. I figure now is the best time to give an honest reaction since I'm done with the testing but haven't yet gotten the results. One reason I thought I would burn a blog post on this is that people tend to Freak The Fuck Out (FTFO) about this test and maybe I can persuade future grad students that it's not really advantageous to do so. The other reason is to try to excuse myself for not doing any work on the snow scooter in the past few weeks. (Not that there hasn't been snow. Cambridge is expecting 8-12" more tonight...)

Anyway, my advice to future quals takers (besides don't FTFO) is to use Try #1 as a test probe. Part of the stress is the uncertainty and mystery of the quals process, which, by the way, is intentionally hyped by both the department and by past test-takers. But they do give you two chances, so you can map the process for yourself and that alone is worth the effort even if you don't pass on the first try. You might, for example, learn that you instantly become stupid on a topic you know well just by walking into a room full of seven controls profs, like I did on the orals. Now if I fail, I know I need to work on that. (Either not becoming instantly stupider, or having the transition still leave me enough phase margin to answer simple questions.)

According to the G.A.M.E. surveys (which were the only things that made me FTFO, briefly) from the past five years, survey responders spent on average 300 hours studying and had, on average, an 80% success rate on their first try. I've never studied 300 hours for anything in my life (including Quals Try #1...and my odds of passing are much lower than 80%) so I can't even really relate to what that means. I would much rather be doing something else. If necessary, would I suck it up and put in the hours on Try #2? Probably. But for Try #1...I don't think so. (Well, as experience shows, clearly not...) Too much other stuff to be working on. Like 2.007, which will have the best game in a loooong time, I promise.

One thing I was mildly excited about was the research presentation. I've been wanting to get feedback from professors (or experts) on my field-oriented control stuff. (Yes, I'm writing about motor control again.) So, against the advice of pretty much everybody, I presented to the controls group. These profs are infamous for tearing people apart, which they did on my oral subject test, and most would not be familiar with me or my work. Kind-of a high-risk strategy, for quals, but I really wanted feedback from that group, even if the feedback is: "Go away." 

So, we'll see how that went. My initial reaction is positive - I didn't get torn apart (that I was aware prof insisted that this has all been done already in the 80's) and I nailed some Technical Triva. In some sense, though, I got very lucky with who was on my panel, but I won't name any specific names. I would say 75% odds of passing, which is much better than my subject tests. I'd put those at about 40%. Any takers? (Is gambling on quals unethical?)

Anyway, the other outcome of all this is that you get yet another exceedingly technical PowerPoint presentation on the modified Synchronous Current Regulator!

Yaaaaay. 70 more where that came from.

(I assume there is no harm in posting a quals presentation...)

This one supersedes my previous PowerPoint on the topic, which is apparently a big hit on Google. It's got more data, this time from Pneu Scooter, better analysis and simulation, and most importantly, much shinier figures. It's all theory, though. If you're interested more in the implementation details, including the controller hardware development, this massive, unedited, neglected, out-dated document is the best I've managed. I promise I will bring it up to date with v3.1 some day soon.

If you're like I was not long ago and have some idea about motor control but not quite enough to jump right in to Field Oriented Control, then you should first check out James Mevey's Awesome Thesis of Awesomeness. Pretty much everything I know about modeling brushless motors and controllers came from that. It's also a review of basically every reference on the topic.

Anyway, that's it for quals...for now. I'll post the results in a comment tomorrow when I find out, for anyone who took me up on those odds... In the meantime I'm going to go design a motor controller.


  1. i bet one fankart that you win by a small margin

  2. I think you nailed it. If not, just give me the names of the profs who scored you badly and I'll make them an offer they can't refuse ;)

  3. And the results are in. On the subject tests:

    17/20 on Machine Design
    11/20 on Manufacturing
    8/20 on Controls

    As I found out, you need 13.5 magic professor points to pass, so I did not get the required 2/3. Since I lost by a small margin, Charles, you owe me one fankart.

    On the research presentation, I got 14/20 (and I earned every single one of those 14). The fact that I got the same group of controls profs who tore me apart the day before to give me a passing score on research they've never seen before (on the stength of an epic Q&A battle) is definitely going in my win category.

    Unfortunately, there are un-modeled dynamics in the quals taking process and/or a new rule that says I must re-take both parts even though I passed the research presentation. So I'll be back in May for Try #2. Try #1 was relatively low-stress for me, so I don't feel bad about the results.

    Now I've seen you play, and I know how to beat you.

  4. Man, and I thought I was going to just be able to copy the Atmel reference implementation of SVPWM control to build my own motor controller without really understanding it ... I better rethink that plan if I intend for it to work ...

  5. Fortunately, motors are very forgiving. There are many ways to make them spin.