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2014 April 15

Hysteresis lab too long

After re-reading my notes on last year’s hysteresis lab, I realized that my schedule for this week in the Revised plan for circuits labs, with both the hysteresis lab and the sampling lab in the same week was too ambitious. There was a chance that the students could do the hysteresis lab in 3 hours, but only if they already understood everything in the pre-lab assignment and worked efficiently. A lot of the students, however, only learn by repeatedly bumping into a brick wall, and don’t really have any notion of solving general problems before they encounter them in the lab, so I expected a lot of students to waste time today doing the pre-lab assignment in lab.  My expectation there was amply fulfilled.

I decided to cancel (or at least postpone) the sampling and aliasing lab, and spend both Tuesday and Thursday on the hysteresis lab. I don’t think we’ll be able to double up the labs next week, but the week after may be a little thinner, and we may be able to squeeze in the sampling and aliasing then.

Everyone got the two input thresholds for the 74HC14N Schmitt trigger (with 3.3v inputs) measured, and they all got essentially the same values.  Some of them took a long time getting there, because I did not hand them a test circuit, but asked them to come up with one themselves. One group used an adjustable bench power supply for Vin, but the rest (eventually) came up with using a potentiometer as a voltage divider and recording the input and output with the PteroDAQ software. For some, I had to do more guidance than I really liked, getting them to decompose the problem into having the Schmitt trigger as one component with a variable input and the pot as another component with a variable output. Since they had done a very similar setup for the mic lab last week, and I had explained the pot as a variable voltage divider at that time, I had expected them to instantly see how to apply it, but most did not. Still, everyone eventually got it, and I think that the ones who struggled the most now have a much solider understanding of voltage dividers and potentiometers than if I had just given them a circuit to copy.

I did get to show the PteroDAQ users a useful feature of the program—by connecting the output to PTD4 (or one of the other digital pins of port A or port D), PteroDAQ can be set to trigger whenever the output changes values.  A few sweeps of the pot past the threshold values reveals quite repeatable voltages at which the transition occurs, without having to page through a long trace of uninteresting info.

The groups then struggled with coming up with the right RC time constant for their oscillators. I’m probably going go over the calculation in class tomorrow, since I think everyone got a reasonable result, but not everyone was clear enough about their method to write it up well. I want to see clear explanations in the lab report, so I’ll go over it to help them smooth out the bumps in their explanations.

Some other things I want to do tomorrow:

  • Talk about Carol Dweck’s work on mindset, as one of the students frequently wonders aloud whether the class is too difficult for her, and some of the other students may be thinking that they “don’t have the ability”. So far as I can tell, everyone in the class has the ability to master all the material in the class—but I need to get them out of “fixed mindset” into “growth mindset” and recognize that they can do more than they credit themselves with, if they are willing to work for it.
  • Have them go over their computations of the finger-touch capacitive sensor and compare answers with each other. I want to make sure that they express their answers in standard units (like pF) and that they are careful about units (mixing mils, cm, and F/m probably confused a lot of students).
    During the lab time, I had each group come up to use my micrometer to measure a double-thickness of packing tape. I must be using a different roll of tape than in previous years, because we consistently got about 1.7mil (0.043mm) with my Imperial units micrometer (that is we measured 3.4–3.5 mil for the double thickness), while last year I had 2.2mil.  I should probably get a metric one, but I may be too cheap to spend $14 on a tool I use once a year in this class. Besides, this gave me an opportunity to tell students the difference between mil and mm, which most of them did not know. Since a lot of materials still come with thickness specifications in mil, they should at least be aware of the existence of the unit and the potential for confusion. (Several had done the prelab homework assuming 2.2mm, which would be very thick packing tape.)
  • Assign one of the voltage-divider do-now problems from last year. Perhaps this one?
    • What is the output voltage for a 3-resistor voltage divider? (I’ll draw the circuit)
    • You have sensor whose resistance varies from 1kΩ to 4kΩ with the property it measures and a 5v power supply.  Design a circuit whose output voltage varies from 1v (at 1kΩ) to 2v (at 4kΩ).

Two or three of the groups managed to get their relaxation oscillators to oscillate and measured the frequency on the digital scopes. One group got as far as adjusting the R and C values to get the frequency within the spec given in the homework (10kHz to 100kHz), and started the next step (making the capacitance touch sensor out of aluminum foil and packing tape). Lab on Thursday will consist of everyone getting the oscillators working in spec, testing the change in frequency for a finger touch (which may need some capacitor changes, as I think some are using a small R and large C, which won’t have enough frequency change with the small capacitance of a finger touch), testing the oscillator with the KL25Z boards (with my new code), and soldering up the circuits on PC boards.

Students are beginning to get the  message that when they ask me whether some result is right, my answer will be what my father taught me: “Try it and see!” When they ask me for help using the equipment or debugging when they get too frustrated, I’m more helpful, but I’m not going to check their work for them when the real world can do that so much better.  Besides, the simple models we are using are not all that accurate—even if they do a perfect job of the computation, the real-world behavior will be enough different that they’ll need to tweak the component values anyway. This is another lesson I want them to get—the real world is not as simple as the spherical-cow models used in physics classes and intro EE, but the spherical-cow models are nonetheless useful.

 

 

 

 

2014 April 14

Hysteresis lecture

Filed under: Circuits course — gasstationwithoutpumps @ 23:16
Tags: , , ,

Today’s class started with feedback on their second design reports (the electret mic lab). Everyone in the class got a “redo” on this assignment.  Some of them actually had pretty good write-ups, but I had warned them that errors on schematics or with units would trigger automatic redos, and every report had at least one serious error (like 200A, instead of 200µA, or short-circuiting the mic). I’m going to hold them to getting their schematics and units right—details matter in engineering, and they have got to develop a habit of double-checking what they write.

After a little more feedback (on how to improve their plots, for example, and little details like capitalizing “Figure 1″ or using the prepositions with voltage and current), I switched to new material on hysteresis that they’ll need for tomorrow’s lab.  I actually gave them a fairly detailed description of hysteresis in the lab handout (I wonder if anyone has read it yet?), but I covered it again anyway. I also talked about DIP vs. SMD parts (the 74HC14N chip they’ll use is in a DIP), and introduced them to a simple relaxation oscillator.  We worked through how it functioned to produce a triangle wave on the input and square wave on the output, but I did not mention the capacitive coupling from the output to the input that changes the triangle wave rather dramatically when the capacitor in the RC circuit is small.

Input and output of a Schmitt-trigger relaxation oscillator (approx 67kHz). Note that the large output step is capacitively coupled to the input, causing a small step in addition to the expected triangle wave.

Input (yellow) and output (green) of a Schmitt-trigger relaxation oscillator (approx 67kHz). Note that the large output step is capacitively coupled to the input, causing a small step in addition to the expected triangle wave.  Note, the two traces are separate sweeps and the frequency modulation by 60Hz noise is big enough that the periods are not exactly the same on the two sweeps.  (click to embiggen)

The funny step in the input is not visible if large capacitors are used, but accounts for a big part of the charge transfer for small capacitors (throwing off the RC calculations that determine period).

With a 680kΩ resistor and a 10pF capacitor, attaching a BitScope probe to the input changes the period from about 4.5µs to about 14µs. With the same resistor and a 30pF capacitor, attaching the probe changes the period from 17.5µs to 28.5µs—the change due to the input impedance of the scope makes a big difference in the behavior of the circuit. I’ll have to make sure that the students observe the effect that a scope probe has on their circuit—they’re probably still thinking of the measurements as being non-disruptive.  (They may get even bigger changes in period with standard oscilloscope probes—with the 30pF capacitor I get periods of 220µs for a 1× probe and 30µs with a 10× probe on my Kikusui oscilloscope—the BitScope input is similar to the 10× probe.)

Last year’s hysteresis oscillator lab ran quite long, but I’m hoping for better time tomorrow. I went through the behavior of the oscillator a bit more thoroughly, and I think I impressed on them the importance of doing the algebra and calculations before lab time. I also suggested how they could find the input threshold voltages using PteroDAQ at home (triggering on both rising and falling edges).

2014 April 13

Designing courses to teach design—draft 3

The talk I was scheduled to give last quarter (2014 Feb 24) was rescheduled, because two of the four speakers were unable to make that date.  It is now scheduled for Wed 2014 Apr 23  at 3:30 in the Merrill Cultural Center, which used to be the Merrill Dining Hall, before they consolidated dining halls in the east colleges. There are now 6 speakers in 90 minutes, which means 15 minutes each (maybe 10 minutes speaking, 5 minutes for questions).  I’ll have to run over from my class which ends at 3:10 on the opposite side of campus (0.6 miles, 13 minutes according to Google Maps), though running may be difficult along the crowded sidewalks between classes.

The talk needs to be updated from last quarter, as I have now taught prototype runs for both the applied circuits class and the freshman design class, and am in the second run of the applied circuits class.

Here is my current draft of the text—please give me some suggestions in the comments for improvement.  The ending seems particularly awkward to me, but I’m having trouble fixing it.

Designing Courses to Teach Design

I believe that the main value of a University education does not come from MOOCable mega-lecture courses, but from students working in their field and getting detailed feedback on that work. I’ll talk today about some courses I’ve created ths year and last to teach students to do engineering design. These courses are high-contact, hands-on courses—the antithesis of MOOC courses.

Design starts from goals and constraints: “what problem are you trying to solve?” and “what resources are available?” So what were my goals and constraints?

The two problems I was trying to solve were in the bioengineering curriculum:

  1. students weren’t getting enough engineering design practice (and what they were getting was mostly in the senior year, which is much too late) and
  2. too many students were selecting the biomolecular concentration, where we were exceeding our capacity for senior capstone and senior thesis projects.  The other concentrations were under-enrolled.

The main constraints were that

  1. there was no room in the curriculum for adding required courses,
  2. there were no resources for new lab space or equipment, and
  3. all relevant engineering courses had huge prerequisite chains.

Furthermore, I would have to teach any new course myself, so the content had to be something I already knew or could learn quickly. Those constraints meant the new course would not have wet labs (though I have encouraged wet-lab faculty to add design exercises to their existing courses).

My first partial solution was to replace the required EE circuits course with a new Applied Circuits course. The existing EE101 course is a theory class (mostly applied math) that prepares students to do design in later courses—but most bioengineering students never take those later courses, so were getting prepared for something they didn’t do. Due to “creeping prerequistism” in the 8 or 9 departments providing courses for the major, the bioengineering students were already taking far too many preparatory courses and far too few courses where they actually did things.

The goal of the new course is to have students design and build simple amplifiers to interface biosensors to computers. I chose a range of sensors from easy ones like thermistors, microphones, and phototransistors to ones more difficult to interface like EKG electrodes and strain-gauge pressure sensors. I’m not interested in cookbook, fill-in-the-blank labs—I want students to experience doing design in every lab, even the first one where they knew almost no electronics—and I want them to write detailed design reports on each lab, not fill-in-the-blank worksheets, like they get in chem and physics labs, and even intro EE labs.

The course was designed around the weekly design projects, not around preset topics that must be covered. Themes emerged only after the design projects were selected—the class comes back again and again to variations on voltage dividers, complex impedance, and op amps with negative feedback.

Students used a free online textbook rather than buying one, but bought about $90 of tools and parts. I tried out every potential design exercise at home—rejecting some as too hard, some as too easy, and tweaking others until they seemed feasible. I designed and had fabricated three different printed circuit boards for the course (not counting two boards which I redesigned after testing the lab at home).  One of the PC boards is a prototyping board for students to solder their own amplifier designs for the pressure-sensor and EKG labs. (Pass boards around.)

Developing a hands-on course like this is not a trivial exercise. I spent about 6 months almost full time working on the course design (without course relief). I made over 100 blog posts about the design of the course before class even started, and I now have over 230 posts (the URL is on the quarter-page handout, along with the URL for the course syllabus and lab assignments).  Since the posts average a couple of pages, this is more writing than a textbook (though not nearly as organized).

The course was prototyped last year as BME 194+194F “Group Tutorial” before being submitted to CEP for approval, and I wrote up notes after each class or lab (another 60 or so blog posts). Last year’s prototyping lead me to increase the lab time from 3 hours to 6 hours a week, which means I’m spending a lot of time this quarter rewriting all the lab handouts—splitting the material between the lab times and adding at-home or in-class design exercises between the two parts. Some of the fixes have worked well (students got comfortable plotting their data with gnuplot weeks earlier this year), but we’ve still run over time in some labs, even with 6 hours a week of lab, so more tweaking is needed.

This course is expensive in terms of professor time: I’m spending over 10 hours a week of direct classroom and lab time (not counting office hours, grading, prep time, or rewriting the lab handouts). Just providing feedback on the 5–10-page weekly design reports takes about 15 minutes per student per week (half an hour per report).

The students taking Applied Circuits last year were mostly seniors who had been avoiding EE 101, rather than the sophomores I’d intended the class for. This year, I have juniors and seniors, but still no sophomores. So the course still does not provide early exposure to engineering design, nor does it direct more students to the bioelectronics concentration rather than the biomolecular one (those there’s still hope for the latter).

My second partial solution was to create a new freshman design seminar in conjunction with the student Biomedical Engineering Society. This course has no prereqs, is only 2 units, and does not count towards any major or campus requirements.

Unlike the Applied Circuits course, I didn’t choose the design projects for this course ahead of time, because I had no idea what skills and interests the students would bring to the class—I’d not taught a freshman class in over a decade, having taught mainly seniors and grad students. I did try out 3 or 4 design projects on my own to gauge the skills needed to do them, but those projects all assumed some computer programming skills.

I prototyped the freshman design course last quarter as BME 94F and have submitted course forms to CEP for approval. Once again, I blogged notes after each class meeting (only about 39 posts, though—this was a less intensive effort on my part).

With no prereqs, I couldn’t assume that students had any relevant skills, though it turned out that all this year’s students had had biology, chemistry, and at least conceptual physics in high school. Only one student had ever done any computer programming, though—a big hole in California high school education—and only a few had any experience building anything. (AP physics classes were the most common exposure to building something.) On the first-day survey the students indicated an interest in learning some programming and electronics, so we did a little programming with an Arduino microcontroller board—I’ll try to up that content next year.

I started out teaching generic design concepts using a photospectrometer as an example. The concepts include specifying design goals and constraints, dividing a problem into subproblems, interface specification, and iterative design.The photospectrometer turned out to be too complex, and I’ll probably start with a simpler colorimeter next year, and have the students design, build, and program it before they start on their own projects.

My third partial solution has been a complete overhaul of the bioengineering curriculum, which is currently before CEP for approval. No new courses were created for this overhaul, but all the concentrations were changed. For example, half the chemistry courses were removed from concentrations other than biomolecular, to make room for more courses in electronics, robotics, psychology, or computer science. And some the orphan math courses were removed from the biomolecular concentration to make room for more advanced biology. Long-term, I’m hoping to convince some of the other departments to remove excessive prerequisites, so that students can take more interesting and useful courses before their senior year.

I could go on all afternoon about these courses and curriculum design, but I’m running out of  time, so I’ll leave you with these take-away messages:

  1. The value of University education is in detailed feedback from professors in labs and on written reports, not in the lectures.
  2. Students should be solving real problems with multiple solutions, not fill-in-the-blank or multiple-choice toy exercises.
  3. These courses require a lot of time from the professors, and so they are expensive to offer.
  4. Failure to teach such courses, though, makes the University education no longer worthy of the name.

For those of you not present—the quarter-page handout will have the URLs for this blog’s table of contents pages for the circuits course and the freshman design course, in addition to the two class web pages:

In addition to the quarter-page handout, I also plan to have copies of the prototyping board (both bare boards and ones that I used for testing out EKG or instrumentation amp circuits), one of the pressure sensors (on another PC board I designed), and the hysteresis oscillator boards.  If I can get it working again, I may also wear the blinky EKG while I’m talking.

Preparing this talk has been weird for me—I can’t remember ever having scripted out a talk to this level of detail.  For research talks, I usually spend many hours designing slides, and relying on the slides to trigger the appropriate talk. For classes, I usually think obsessively about the material for a day or two ahead of time, sometimes writing down a few key words to trigger my memory, but mainly giving an extemporaneous performance that relies heavily on audience participation. I had one memorable experience where a student asked me for a copy of my lecture notes after a class—I handed her the 1″ PostIt that had my notes, but warned her that I’d only covered the first word that day, and that it would take the rest of the week to cover the rest.

Doing this short a talk without slides and without time to rehearse will probably require me to read the talk—something else I’ve never done. (I know, I should have rehearsed during our one-week spring break, but I had a 2-day RNA research symposium, a faculty meeting about who we would offer our faculty slot to, meetings with grad students, and feverish rewriting of the first few lab handouts for the circuits course.)

2014 April 12

No-salary adjunct professors

Filed under: Uncategorized — gasstationwithoutpumps @ 16:40
Tags: ,

Katrin Becker in her post Six things that will demoralise staff, which is a response to Six things that will demoralise staff @ smh.com.au, commented on the difficulty of getting adjunct professor appointments at one Canadian university:

Curiously, I actually tried to become an adjunct at my Alma mater—twice. The first time was just after I’d graduated, and won a research award. That time my paperwork mysteriously disappeared—twice. The other time it took months of cajoling to even elicit a response from the Dean, and then they said they weren’t really interested. An adjunct appointment in Canada is a completely resource neutral appointment. It costs the University NOTHING, yet allows them to claim my achievements as part of their “output”. I can’t imagine why anyone would turn that down, but they did.

This phenomenon is not limited to Canada, nor to people whose only connection to the university is as an alumnus or alumna.  On our campus, adjunct professor with 0% salary appointments are up to the deans, and they don’t always make rational decisions.

For example, a few years ago our department had an extremely active researcher and teacher who had created courses for the department, taught them and trained others to teach them, taught one of our core grad courses, written grant proposals (he was on soft money), and basically done all the things that a good assistant professor would do. Our department voted unanimously to give him adjunct assistant professor status with a 0% appointment, which would have no fiscal effect, except that when he taught a course he would be paid like a visiting professor, rather than like a lecturer—the difference is not huge, but it meant that he would not have to take a pay cut from his research position in order to teach.

The dean refused to make the appointment, with the lame excuse that the person had not been a postdoc long enough (despite the fact that most of the tenure-track faculty in the School of Engineering are hired directly from PhD programs without postdoctoral training, and the person had 20 years of industrial experience in a related field).

Who can understand the minds of university administrators?

 

2014 April 11

Impedance, finally

Filed under: Circuits course — gasstationwithoutpumps @ 21:32
Tags: , , , ,

I finally got a chance to cover impedance in class today.

I started with complex numbers and Euler’s formula (e^{j\theta} = \cos(\theta) + j \sin(\theta)), showed them how the amplitude and phase terms could easily be factored out to make a phasor, then had them develop the formulas for impedance of resistors and capacitors.  For capacitors, I had to guide them a bit, but they came up with Q=CV, i = dQ/dt, and taking the derivative of the voltage.  That actually went a bit smoother than I expected, and almost everyone in the class was participating.

I then had them come up with the circuit for an RC filter (they only know one circuit, the voltage divider, so this was pretty easy). They used the voltage divider formula to get the gain of the filter, and I showed them how to plot it by hand (a Bode plot).  The filter we started with was a low-pass filter, so I challenged them to design a high-pass filter.  This is an easy task, and they almost instantly suggested swapping the resistor and capacitor.  They derived the formula for the gain of that filter, and I showed them the how to draw the Bode plot for it also.

Note: we only do amplitude Bode plots in this class, not phase.  For what they are doing, the phase response is not very important.  I did let them know that there were more advanced classes where the phase response was carefully determined, and that filter design got a lot more complicated than simple RC filters.

I covered in class almost exactly what I set out to do, and the students came up with most of what I wrote on the board, so I felt that the Socratic questioning I used to guide them worked fairly well.  I’ll probably have to give them some design exercises soon, to see if they actually understand the material or were just nodding along with their classmates.

This weekend I’ll grade their second design reports (for the microphone lab), and try to rewrite the week-4 labs, which will be characterizing polarizing and non-polarizing electrodes.  I have a bunch of other stuff to do this weekend also (mow lawn, do state taxes, read a thesis proposal, read a chapter of a thesis, replace the rear wheel on my bike, rewrite the designing courses to tech design talk, write a letter of recommendation for a former student trying to get a green card, feed the cats and the fish, … ). I may have a bit less time than usual this weekend, because I’m cooking for myself—my wife and my son have gone to an admitted-students weekend at UCSB.

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