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2016 May 22

Disappointing class-D amplifier reports

Filed under: Circuits course — gasstationwithoutpumps @ 15:59
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I have a huge stack of grading to do this weekend (about 20 new design reports and about 30 redone reports from earlier labs). I was rather disappointed with the reports for the class-D amplifiers, not because the amplifiers didn’t work (they mostly did), but because about 80% of the class is getting a REDO for errors in their schematics, which means I’ll be having to grade the reports again.  I consider 20% REDO an acceptable level, but not 80%.

The most common error in the schematics was one that I had seen a fair amount in previous years: getting the source and drain of the pFETs swapped.  It is a fairly serious error, as putting the transistors in backwards would cause a lot of shoot-through current from the body diode conducting.  Students were wiring their transistors correctly (after some false starts), but documenting their designs wrong.  A number of students also used the depletion-mode instead of enhancement-mode FET symbol, but I consider this a much less serious error (as long as they included the right part number), and would not have triggered an automatic redo for that mistake.

I warned the students about the source and drain orientation repeatedly, both as a class and (in many cases) individually.  I was very careful to point out the convention for the source and drain notation in class and in the book, and they had it on their data sheets as well.  I don’t know what else I can do, other than instituting in-class quizzes, which I may have to do next year.

There were a number of other documentation problems in the reports this week:

  • Using their loudspeaker models, but not including the model in the report.  In many cases, it was clear from their plots that they had screwed up the model somehow, but without any formulas or parameter values, it was impossible to figure out what they had screwed up.
  • Oscilloscope pictures that did not say what the probes were connected to, or had incorrect labeling of the probes. This was mainly really bad lab technique, where they failed to write down what they were doing, and couldn’t reconstruct it from their memories.  That is one aspect of the labs that I’ve not put much emphasis on this year—writing down what they are doing as they do it.  I may have to emphasize that more next year, especially since the labs will be broken up into 4 95-minute sections instead of 2 3-hour sections, which will make memory even more unreliable.
  • Not reporting the PWM frequency.
  • Not remembering to include their bypass capacitors in their schematics. Some students may not have had bypass capacitors, though it was very difficult to get the amplifiers to work without them, as the H-bridge dumps a lot of high-frequency energy into the power lines, which gets coupled back to the comparator and the preamp.
  • Generally bad copy editing.  The spelling and grammar problems in some reports are just common non-native problems with articles, plurals, and verb tense, but a lot of the reports had huge numbers of spelling errors, duplicate words, missing words, comma problems, and incomplete sentences.  I’ll be addressing this problem next year by giving students a bit more time to complete the reports (though that didn’t seem to help on the one report that students had more time on this year).

2014 May 21

Establishing the habit of writing

Filed under: Circuits course — gasstationwithoutpumps @ 09:19
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In Preparing for AP Physics 1: establishing the habit of writing Greg Jacobs writes

I’m in the infant stages of planning my AP Physics 1 course. The big trick is going to be establishing my students’ ability and willingness to write their reasoning, to get them to focus on communication rather than on getting a correct numerical answer. Once it’s clear that they are not taking a math course—once they see that the solution to a problem looks much more like what they’ve done in biology or economics than in calculus—I think the students will be able to move along quickly and enthusiastically through the material.

Students must get comfortable with calculation. However—as was correctly pointed out to me at the AP consultant meeting in April—if we start the course with lots of pure calculation, students will think that getting the answer is the holy grail of physics problems. If instead we begin the course demanding description, explanation, and all sorts of prose, students may become accepting of the idea that a numerical answer is merely the result of careful reasoning.

If this change in AP Physics actually works (something I’m always skeptical about in any curriculum reform, particularly at the high school level), it may help engineering students in college. Engineers do far more writing than most professions, with far less training at doing it.

I don’t think that a prompt that just says “In a clear, coherent, paragraph-length explanation, describe how you would figure out …” is going to do the trick, though. If they could already write clear, coherent paragraphs about how they would figure something out, then they would not need the curriculum change—they might not even need a physics class at the level of Physics 1.

I’m struggling with this problem in my applied circuits course, in which I require weekly design reports for the circuits they design and build. The students are staying in lab until they finish the designs and demo them, so they are clearly capable of doing the work (though not always as quickly as they should). But only a few students can explain their computations for the design parameters (like gain, corner frequency, and component values) clearly—others put down any nonsense that has a few of the right buzzwords in it.

The top students have gotten better at their explanations as a result of feedback, but the bottom students are still often producing word salad. Although there is some indication of a general writing problem (lack of topic sentences, poor grammar, and misused vocabulary), the problem is most pronounced when they are trying to explain how they selected component values. The more steps that there are in the underlying math, the more jumbled their explanations, even if the problem is just a chain of multiplications.

From time to time, I’ve suspected that the students don’t produce coherent sentences about how they computed something may not have actually done the computation, but “borrowed” the result.  This is not an explanation I believe in strongly, though, as the students have been (mostly) coming up with different solutions to the design tasks, so there isn’t simple copying going on. I’ve also seen the design process the students use, as they have been doing their pre-lab work in lab (instead of at home), so I hear them discussing the problems.  They do ask each other not just what answer they got, but how to get the answers, so they are trying to learn the method.

In looking at the pre-lab homeworks that were turned in on Monday I realized what part of the problem is—the students keep absolutely awful design notes. What the students turned in on Monday (even the top students) was mostly incomprehensible scribbling of numbers, with no indication where the numbers come from or what they were attempting to compute.  Half an hour after writing down the notes, I’m pretty sure that they could not reconstruct their reasoning—hence the often magical methods in their design reports, where they copy numbers out of their notes (some of which are correct), but can’t put together a coherent chain of reasoning that leads to those numbers. On the long multi-step computations needed to figure out what gain an amplifier needs, they can usually do each step (though often needing coaching on one or two of the steps, either by me or by one of the better students in the course), but they don’t record the meaning of each step or even what the sequence of steps is, and the “answer-getting” mentality causes them to flush the process from their minds as soon as they have a number.

I’ve seen a lot of lab exercises for other courses that try to scaffold the process by providing worksheets that give the step-by-step process and have the students fill it out as they go along. I don’t think that this is helpful though, as it encourages students to solve one step at a time and then forget about it—the scaffold prevents the students from exercising the very skill that I most need them to learn. Showing them worked examples, as I have done in class, doesn’t seem to help much either—they can follow along as I break the problem down with them, and think they understand, but then not be able to do the same thing themselves.  Again, the scaffolding prevents them from exercising the skill I most need them to learn—identifying problems and them into subproblems.

For next year, I’m probably going to have to come up with some exercises which get students to organize their thoughts external to their heads. So far, the only thing I’ve thought of is to have them create a fill-in-the-blank worksheet for each lab (like an income tax form), and turn in the blank worksheet and try filling out each other’s worksheets.  If they get in the habit of writing down the steps as steps, it may help them be able to reconstruct their work when they convert it into full sentences for the final reports. It may be too late for me to do anything formal this year (only 2.5 weeks left), but I’ll suggest it to the students anyway.

The advice I’d give to Greg Jacobs is to leave the “clear, coherent paragraph” until later in the quarter—get them to create worksheets first.

I’d welcome any suggestions from my blog readers on ways that I can get students to learn to organize their thoughts in a way that they can present them coherently to others. Block diagrams alone don’t seem to be enough, and vague things like “mind maps” are likely to do more harm than good.

2014 April 7

Feedback on first lab report

Filed under: Circuits course,Printed Circuit Boards — gasstationwithoutpumps @ 17:11
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Most of today’s class was taken up with feedback on the design reports that students turned in by e-mail on Saturday.  Overall the reports were not bad (better than the first reports last year), but I think that the students could do better.  Here are the main points:

  • Anyone can redo the report to get it re-evaluated (and probably get a higher grade).
  • No one attempted theV1 &V2 problem, so I reassigned it for Wednesday.

    The circuit I had given as an exercise, asking them to determine the output voltage V_out.

    The circuit I had given as an exercise, asking them to determine the output voltage V_out.

  • A lot of reports mixed together two different problems:the 1kΩ–3.3kΩ problem and the optimization to maximize sensitivity of the thermistor temperature sensor.  I encouraged students to use more section headers and avoid mixing different problems together.
  • Figures should be numbered and have paragraph-long captions below each figure.  I reminded students that most engineering reports are not read in detail—readers flip through looking at the pictures and reading the picture captions. If the pictures and captions don’t have most of the content, then most readers will miss it. I also pointed out that many faculty, when creating new journal articles, don’t ask for an outline, but ask for the figures.  Once the figures tell the right story, the rest of the writing is fairly straightforward.
  • A lot of the students misused  “would” in their writing, treating as some formal form of “to be”. The main use in technical writing is for contrary-to-fact statements: “the temperature would go down, if dissipating power cooled things instead of heating them”.  Whenever I see “would” in technical writing, I want to know why whatever is being talked about didn’t happen.
  • A number of the students had the correct answer for the optimization problem, but had not set up or explained the optimization. Right answers are not enough—there must be a rational justification for them. In some cases, the math was incomprehensible, with things that weren’t even well-formed equations. I suspect that in many cases, the students had copied down the answer without really understanding how it was derived and without copying down the intermediate steps in their lab notebooks, so they could not redo the derivation for the report.
  • A number of the plots showed incomplete understanding of gnuplot: improperly labeled axes, improperly scaled axes, plots that only included data and not the models that the data was supposed to match, and so forth. I pointed out the importance of sanity checks—there was no way that anyone ran their recording for 1E10 seconds! I was particularly bothered that no one had plotted the theoretical temperature vs. voltage calibration based on the parameters from their temperature vs. resistance measurements, so I could not tell whether the voltage divider was doing what they expected it to.
  • No one really got the solution for the 1kΩ–3.3kΩ problem perfectly. A number of them set up the equations right and solved for R (getting 2.538kΩ), but then not figuring out what Vin had to be.  It turns out that Vin depends strongly on R, so rounding R to 2.2kΩ or 2.7kΩ results in different good values for Vin, and the 2.2kΩ choice gives a more desirable voltage (around 3.3v, which we have available from the KL25Z boards, as it is a standard power-supply voltage).

I also showed the students how I had expected them to setup and explain the optimization to maximize sensitivity at a particular operating temperature.

After that feedback, I started on new material, getting the explanation of amplitude, peak-to-peak, and RMS voltage. I think that the RMS voltage explanation was a bit rough.  I was deriving it from the explanation that we wanted a measurement that represented the same power dissipation in a resistor as the DC voltage, and I got everything set up with the appropriate integrals, but I forgot the trig identity (cos(\omega t))^2 =\frac{1}{2} (1-cos(2 \omega t)), and ran out of time before I could get it right.  I did suggest that they look up the trig identity and finish the integration.

I had hoped to get at least partway into Euler’s formula, complex sinusoids, and phasors, but the feedback took longer than I had expected. Those topics will have to wait until Wednesday or even Friday, since Wednesday we’ll want to do the modeling of the DC characteristics of the electret mic, and talk about how the mic works.

 

2014 March 21

Exam day for freshman design seminar

Filed under: freshman design seminar — gasstationwithoutpumps @ 16:28
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Thursday the students turned in their final design reports (which I have not read), and showed me their prototypes.

None of the groups had working prototypes, because all of them started too late on acquiring parts and building. I will have to create and enforce a schedule next year that gets them started on prototyping much earlier. A lot ran into the problem that electronics parts need to be mail ordered, and that introduces a few days delay. Others ran into problems they had not expected (like that styrofoam coolers can’t be sold in Santa Cruz—a problem for the incubator group).

I went with two of the groups over to an electronics lab to help them debug the prototypes that they had started.

The incubator group had a thermistor temperature measurement circuit and Arduino program that was always reporting freezing temperatures. I walked them through measuring the thermistor resistance (seemed ok, and changed if the thermistor was warmed by holding it), measuring the voltage from the voltage divider (seemed ok, and went up if thermistor was warmed), looking at the raw reading from the Arduino (again, looked ok), and looking at the reading-to-temperature conversion routine (did not have a return statement, so was always returning 0).

The PCR machine group had two Peltier devices, one of which seems to have been damaged (open circuit with multimeter). I helped them debug their H-bridge circuit—they had it working when there was no load (though they had not verified that it did so), but the Peltier device was too big a load, and the H-bridge had a short-circuit protection circuit that turned off the H-bridge after 50μsec.  The Peltier device needed about 10A, and the H-bridge shut down at 8A.  Their power supply was only capable of delivering 1.5A anyway, so the Peltier device would not have cooled or heated the block fast enough anyway.

The centrifuge group had demoed their project earlier in the day.  Their design was primarily mechanical, and they were still having troubles with it not being stable. Based on the vibrations of their motor, I think that they were only getting up to about 600 RPM, not the 3000–4000 RPM that they wanted. The limitation may have come from an unbalanced rotor.

I think that next year I want to discourage mechanical design projects, because we don’t have facilities to do much construction and debugging of mechanical designs (and I have less expertise to help them). Electronic design and programming are fields that I can help them more with and which we have better facilities for.

The biggest changes for next year are

  • more programming exercises with Arduino (including thermistor or phototransistor)
  • start project sooner
  • weekly lab time
  • deadlines for parts ordering
  • maybe enforcing one design project for entire class (with different teams trying different approaches)

2014 February 26

Fourteenth day of freshman design seminar

Filed under: freshman design seminar — gasstationwithoutpumps @ 21:39
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Today’s class went fairly well—I started by returning the students’ drafts of their design reports and taking general questions.  Most of the questions were about the format I was expecting for the final report (which is due 3 weeks from tomorrow).

We then broke into groups and and the group tutor and I circulated answering questions.  I had some time with each group.  With the incubator group, which had mentioned using transistors to control power to the heater, I discussed how to use nFETs and how to read some of the critical specifications.

With the centrifuge group, I talked with them about the need to keep the motor case from turning—the torque on the motor is the same as the torque on the rotor.  We also talked about how to measure the rotor speed, and the difficulty of printing large objects with a filament-style 3D printer (I suggested they look into buying a used cooking pot at the thrift store to make the case, rather than trying to 3D print it).

I only talked for a short while with PCR group, mainly about relay-based control vs. FET-based control (relays are simpler to design with, but can only do on/off control, not PWM for proportional control).

The students seemed to pretty excited about their projects, and stayed on topic for the full 70 minutes (80 minutes, actually, since we ran over by 10 minutes before anyone noticed).

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