PteroDAQ needs new platform

Today I found out that the boards that I had been using for the data-acquisition system in my electronics course and textbook (the Teensy LC and the Teensy 3.2) have been discontinued, so I’ll need to port PteroDAQ to a new board. I’ve been thinking that I should port to the Seeed Studio XIAO SAMD21, which is a $5.40 board with Arduino IDE compatibility and similar speed to the Teensy LC (it is also an ARM Cortex M0+) with 11 pins that can be configured to be analog inputs. I’m also considering the Seeed Studio XIAO RP2040, which is the same price, with a much faster processor and more RAM, but only 4 analog inputs. I’ve not looked at the ADCs on these two chips, but I think that they are lower resolution than the Teensy LC. I’ve also not looked at what timer options are available for doing PteroDAQ frequency channels.

Sigh, the Teensy LC was such a good board for the course—it is sad to see it gone without a replacement that has as good an ADC at a reasonable price point.

I’m pretty busy with my Cabrillo classes (improv and theater design) and with my other acting stuff, so I probably won’t get a chance to work on PteroDAQ until this summer.

One week into new quarter

We’re one week into the new quarter (10% of the way through!) and the course is going ok. Most of the students have finished the first-week lab, which consists of installing a lot of software and soldering headers onto a Teensy LC board.

The software they had to install was

• Teensyduino
• Anaconda Python
• PteroDAQ
• gnuplot
• Waveforms

Of course, each piece of software has its own installation idiosyncracies, different on Windows, macos, and Linux.  Some people even bumped into some problems because of running old versions of macos or Python (which were luckily cleared by upgrading to slightly newer versions).

The soldering was a bigger problem, because many students plugged in their cheap irons and left them on for a long time without tinning the tips.  The result was a sufficient build-up of corrosion that that they could not then tin the tips—even using a copper ChoreBoy scrubber to clean the tips didn’t help in some cases. In the in-person labs, I often spent most of the first week labs cleaning soldering iron tips that students had managed to mess up, but I can’t do that online.  This was not such a problem last quarter, as most of the students knew how to care for soldering irons from the first half of the course, but it may be a bigger problem this quarter, as most of the students have never touched a soldering iron before.  Some of the ones who are living here in town may be contacting the lab staff to see if they can get access to tip tinner or get some help cleaning their irons.  Those further away may be buying tip tinner on their own—I had not included it in kits, because I nad not expected so many to need it and it costs $8 apiece.

Grading is going fairly well.  My grading team and I have had two Zoom meetings so far (for Homeworks 1 and 2) and I graded Quiz 1 by myself, so we are keeping up with the grading.  He have Homework 3 and Prelab 2a (there is no Prelab 1) both due Monday morning, and we’ll try getting them graded Monday night.  We’re having to do most of our grading in the evening, because one of the graders is living in China, 15 time zones away, and none of us in California is an early morning person.

In other news, I’ve finally finished clearing the blackberries and ivy from behind the garage (a project I started about 2 years ago).  I’ll probably find some more when I cut back the kiwi vine (an annual winter project, in addition to frequent minor pruning during the summer).  I think I either need to get some female kiwi vines and an arbor for them or uproot the male kiwi.  There is really not much point to having just a male kiwi intent on taking over a big chunk of the yard.

There are still a lot of blackberry roots out there that will sprout new vines.  I’ll try uprooting them where I have access (not where they are coming through the cracks in the concrete), but I’ll probably have to do a monthly sweep of the yard to remove blackberries for the rest of my life in this house.

EKG without amplifier

I have done a lot of EKG projects on this blog, mostly for the Applied Electronics course, where an EKG amplifier is Lab 12, but some just for fun. Today I decided to see whether one could do an EKG with just the Teensy LC board or just an Analog Discovery 2 USB oscilloscope.

The project was partly inspired by the Digilent post DIY ECG using a Analog Discovery 2 and LabVIEW, which I saw the title for and assumed that they were using just the USB oscilloscope.  It turns out that wasn’t what they meant—it is just a simple 6-op-amp EKG amplifier looked at with the USB oscilloscope.

The setup I tried was about as simple as you can get—I put on three electrodes wired up as Lead I (see earlier post for this configuration), connected the body electrode to ground, and the other two electrodes to the plus and minus leads of channel 1 of the Analog Discovery 2.

The signal I got was quite small (about 1mV) and buried in 60Hz noise:

The raw signal from the oscilloscope, sampling at 1200 Hz, shows some spiking from the pulse, but a lot of noise. The big R spike is only about 3 LSB, so quantization noise is a problem.

I ran the recording through a digital filter to bandpass filter it to 0.1Hz–100Hz and put in a notch from 58Hz to 62Hz. The 100Hz low-pass had the effect of averaging out the noise, producing a signal with much finer resolution than the raw ADC values:

After filtering, the EKG signal is fairly clear. I don’t recommend trying to use only a couple of the lower-order bits of an ADC, but it is surprising how much information can be recovered by the filter.

I also tried using a Teensy LC board running PteroDAQ, using the A10–A11 differential channel. I had to bias my body between 0V and 3.3V, so I used a pair of 120kΩ resistors (one to GND, one to 3.3V) to connect to the body electrode.

Once again the raw signal was not great:

The signal had less noise than the signal to the Analog Discovery 2, but the signal was smaller also, negating the value of the finer steps of the ADC on the Teensy LC.

Once again, digital filtering restored the signal:

The signal-to-noise ratio here looks a little worse than for the Analog Discovery 2, despite the raw signal looking cleaner.

I managed to get a cleaner signal for the Analog Discovery 2 by turning off the surge protector, so that there was no 60Hz current anywhere nearby. The results after filtering were no better (and possibly worse) than from the signal with the 60Hz interference, so I did not bother plotting them for this post.

My conclusion is that it is possible to get EKG signals without adding an amplifier, but you can only see the signal clearly if you do some filtering.  I’ll have to decide whether to recommend to students that they record signals directly from the EKG electrodes to get an idea what their amplifiers have to work with.

Tape sensors

One of the requirements of the robot for Mechatronics is that it have optical sensors on the bottom to detect black tape on a white background, for detecting the edges of the field, alignment marks for the Ren-ship target, and other marks on the field that seem to be there just to be a nuisance and make the state-machine design harder.

We were issued a number of TCRT5000L reflective sensors, which consist of an IR emitter and a phototransistor in a plastic package that is optimized for a surface 2mm away.  Although the sensors are designed to snap into PC boards and could probably be snapped into carefully sized holes in MDF, I’ll be mounting them on perfboards, which will be screwed onto the top of the baseplate of my robot, with rectangular holes in the MDF for the sensor to poke through.  They’ll barely poke through (6mm high — 5mm MDF), which means they’ll be about 7mm from the floor.  At that distance, the collector current is about half what it would be at the optimum distance.  The MDF provides some extra shrouding from stray light from the sides, which should help with detection.

I’m planning to turn the IR emitters on only when I need to read the sensors, to save power and to allow synchronous detection of reflectance (looking at the difference in light levels between the IR emitter being on or off).  By keeping the duty cycle below 50%, I can run the IR emitters up to 50mA, though I may not need them to be that bright.

If I switch on 5 IR emitters at once, at 50mA each, I’ll be  using 250mA, which is the limit for what I could get from the Teensy 3.2 3.3V regulator (rated for 250mA, though that’s a thermal limit, so 500mA may be available for low duty cycle) and far exceeds the 100mA limit for Teensy LC or Teensy 3.1 boards.  I will be using a 5V regulator to power the Teensy boards, so I could use the 5V supply to power the IR emitters also.

If I use only 10mA per IR emitter, I only need 50mA total and so could power off of either 3.3V or 5V.

For 50mA at 5V, I’d need a current-limiting resistor of ≥(5V–1.2V)/50mA=76Ω, so an 82Ω resistor for 46–48mA would work.  For 10mA at 5V, I’d need ≥(5V–1.1V)/10mA=390Ω, and for 10mA at 3.3V, I’d need ≥(3.3V–1.1V)/10mA=220Ω.

I have a couple of choices for wiring up the sensors:

There are 3 wires to each sensor board, but only the current output is unique to the board—the IR emitter connections can be shared among all sensors.

Low-side switching only requires an nFET, but the power connection for the IR emitter is limited to 3.3V. High-side switching keeps the 3.3V power line from being routed near the motors, but requires a 5v-powered inverter to get a sufficiently large voltage to shut off the pFET.

The expected current from a white background @7mm is about 400µA for 10mA input or 2mA for 50mA input, assuming V_CE is in the active region (above about 0.5V).  If I’m looking for a 1V swing between dark and light, I’ll want about 2.5kΩ for the sense resistor for a 10mA input and 500Ω for a 50mA input.

I did a couple of tests with the TCRT5000L using the low-side switching configuration, with 3.3V and a 220Ω current-limiting resistor.  I was doing the tests in a well-lit room (my breakfast room on a sunny day, but without direct sunlight in the room), so the background light is probably brighter than the robot will ever have to deal with.  I used one of the wheels from the MockRobot to be the target, since I could put a screw through the axle hole to get a more-or-less consistent spacing from the sensor.  I adjusted the nut so that the spacing was about 7mm from the sensor, but the angle was only controlled by eyeballing the levelness of the target, so probably varied by ±10°. I did not have any of the black tape that was used on the fields, so I substituted black electrical tape, which I think may be slightly more reflective.

I initially tried 2.7kΩ as the sense resistor, but after seeing the voltage range for that, I upped it to 5.1kΩ.

With 5.1kΩ, the voltages when illuminated are very different between black and white—a simple digital input would suffice.

The currents can be seen to be about the same, independent of the sense resistor, with the white MDF giving about 10 times current of the black electrical tape. The signal from background illumination is insignificant, though that might not be true if the IR beacon is lighting up the field.

It looks like 10mA (measured as 9.65mA) is more than enough light for the sensors to tell black from white, and that the signal is strong enough that I don’t really need analog input or synchronous sensing for the tape sensors.  If I get really worried about the digital signal in worst-case conditions (like too large a distance), I could increase the size of the sense resistor further.  With a 22kΩ sense resistor, I get a very clean separation between white and black even at 1cm.

I have to decide whether to pulse the IR emitters and wait at least 100µs before reading all the sensors, or just to leave them on all the time (at a cost of 10mA per tape sensor). If they are on all the time, I could use the 5V supply (with a 390Ω current-limiting resistor instead of a 220Ω one), and not worry about overloading the Teensy regulator. This could also simplify wiring, as the 5V regulator could be on the power board on the first level and the battery wiring would be limited to the first level.

The Teensy 3.1/3.2 digital pins are 5V-tolerant, so I could even connect the phototransistor collectors to 5V and put the multiplexer on the first level using 5V power. I could then put the sense resistors on the tape-sensor boards and have three wires for each board: 5V, GND, and WHITE.  I could use 3-pin male headers and standard Futaba servo cables to do the wiring.

I ordered a set of servo cables through Amazon, but they are not expected to arrive for another 2–6 weeks, so aren’t going to do me much good.)  I can make my own, but I’ve been having trouble lately with intermittent contacts from the crimp-on connectors and have taken to wicking tiny amounts of solder into the crimp, which adds a lot of time to making the cables.

With the multiplexer on the first layer, the only cabling from the tape sensors to higher levels would be 5V, Gnd, 3 select signals, and the (now digital) signal for sensing the tape. The motors are also on the bottom level, and they would additionally need 2 wires for controlling each motor and 2 wires per motor for feedback from the Hall-effect encoders. This makes the total wire bundle from the bottom level up to higher levels be only 14 wires (5V, Gnd, 4 for tape sensors, 2×4 for motors), with none of the wires having high-current switching.  The only high current wires are the 5V and Gnd wires, which may be feeding servo motors on higher levels—I may want a separate power wire from one of the switching regulators for powering servo motors on the higher levels, though.

The Teensy LC is not 5V-tolerant, so if I used that for the main controller board, I would need to communicate a 3.3V power line as well for the phototransistors and the motor encoders.  I think that I’ll simply declare that the main controller needs to be 5V-tolerant for digital inputs.

[Update 2017 Nov 18: I soldered up one of the TCRT5000L circuits, so that I could do a little more testing and make sure that I had a layout that worked.  It turned out that 22kΩ for the sense resistor was too large—at some distances the electrical tape reflected enough light to get above the 1.15V which is the guaranteed max V_IL and maybe even enough to get to switch to high. I took out the 22kΩ and used a 10kΩ, and the problem with black reflecting too much light went away, but white still produced a very strong signal. I should be able to detect the white board reliably with just digital inputs out to about 1.4cm, much more than the 0.8cm clearance.

Progress report on mechatronics

I’m falling behind in the mechatronics course—I still don’t have a complete design for the robot, and I’ve built nothing so far except the cat-toy ball track, which is not complete yet—I need to know how much of it to cut away for the ball launcher(s), which I’ve not designed.  I also have the digital beacon detector done, though I think I want to retweak the code for it, as I messed it up trying to get more sensitivity at the distance competition.

I spent yesterday struggling with SolidWorks.  I’m getting better at it, but it is still a terribly complicated, unintuitive interface.  Even trivial things like creating a sphere as a model ping-pong ball took a long time to figure out (I had to Google it, and even then it took me a long time to realize that “centerline” was a special tool, not just a line through the center.  My biggest accomplishment in SolidWorks so far is modeling the helical cat-toy ball track—I’ll have to get a picture of that model to put on the blog.

I did get most of the first layer of MDF designed yesterday, with the motor mounts, the wheel wells, the tape-sensor holes, the battery holder (MDF and velcro), and the M10 ball-tip set screws (to use as skids) all modeled.  I even got one support board for positioning the next layer done, though I’ll probably have to adjust it a bit, as I changed my mind last night about what bumper switches to get.  Instead of spending $3–5 a switch for gold contacts (which is the “right” way to handle low-current switching), I followed the lead of one of the students and am getting super cheap switches from Amazon, which are $7 for 10 switches.  These are standard 5A switches and probably get unreliable with currents below 100mA, though I’ll be using them with 1mA or less.  They are also tiny (specs from the Amazon page):

• Overall Size : 20 x 6.5 x 13.5mm / 0.79″ x 0.26″ x 0.53″ (L*W*H)
• Mounting Hole Dia. : 2mm / 0.08″;Mounting Hole Spacing : 9.5mm / 0.37″;Pin Pitch : 7.2 x 9mm / 0.28″ x 0.35″
• Lever Arm Size : 17.5 x 4mm / 0.69″ x 0.16″ (L*W);Roller Size : 5 x 3mm / 0.2″ x 0.12″ (D*T)

The small size means that the second level will have to be slightly higher to put the bumpers at the right height.

Before I cut the first-layer board, I want to add the rest of the spacer boards between the first and second layer—one of which will have my volt/ammeter and power barrel jack (so I need to measure those and design the panel for them).

I also have to decide how I’m going to lock the boards together.  I don’t want to glue things (messy and hard to disassemble), so I’ll probably clamp everything together with threaded rods.  I’ll need to get the threaded rods and figure out where to put the holes for them so as not to interfere on any level—avoiding the cat-toy ball track will be the hardest, as it takes up most of the second level. Although most of my robot will be using metric parts, I’ll probably go with threaded rod from the hardware store, which will most likely be ¼” rods with 20 threads per inch, as that seems to be the cheapest (at 50¢–$1 a foot).  I think I’ll need 4 pieces, a little under 11″ long each (so that the top of the rod is at 11″ but the bottom doesn’t touch the floor).

I need to laser cut a small test piece with different hole sizes, to see how much increase in diameter there is from the laser cutting.  Most of my holes can be a little loose without problems, but the set screws need to screw into the MDF (unless I get some M10 nuts and put them on either side of the MDF, which might be a better solution anyway).

Today I was going to build tape sensors (soldering the TCRT5000 reflective sensors and current-limiting resistor onto protoboards that have screw holes drilled for mounting), but I’ve not done any hardware work so far today.  Last night I did order the bigger (100mH) inductors for the track-wire sensor, plus a few smaller shielded inductors, in case I decide to add another LC resonator to the sensor, to increase noise rejection, but I forgot to order an 8:1 analog mux, which I think I’ll need for extending the number of pins on the Teensy.  Oh well, another $3–4 in shipping costs and another few days delay.

I’ve spent most of today working on porting the Events and Services Framework to the Teensyduino environment.  I’ve got it all compiling and I’ve got event checkers working, but I haven’t checked the timers or other layers of services.  It turns out that the test harnesses built into the code have not been maintained, and some wouldn’t run (or even compile) even on the Uno32, because the code they supposedly test has been changed and no longer has the entry points they rely on (ES_Timers is one example).

The ES_Framework code is also not very modular, despite being broken up into many files—almost everything is intertwined to the point where you need it all before anything can compile.  Luckily, most of it has very little dependence on the specific processor, so I was able to port it without trouble.  One of the biggest problems was that the Teensyduino environment does not support printf, and the Serial.print() routine is C++, not C.  I added a few C wrappers for different flavors of Serial.print() and converted all the printf statements to a series of Serial_print_… calls.

I’ve not still figured out the best way to handle the analog-to-digital conversion.  The Pic32 processor used in the Uno32 boards for the class have a “scan” mode where the hardware handles scanning the selected analog channels, just doing an interrupt after all the channels have been scanned.  The Kinetis processors on the Teensy boards do not have that feature, so I have several options:

• doing conversions in event detectors or services that need the values.  This will make the individual detectors slower, but no time will be wasted doing unused conversions. It would also make it easy for users to extend the number of analog pins with an external MUX, as the code for doing the analog conversions is not buried inside the system, but in the user-level code.  The disadvantage is that the code will need to busy-wait for each conversion, as the result is needed at the point in the code where it is requested.  I could just use the Teensyduino analogRead(), which already provides support for setting the resolution and averaging levels.  Then there is no need to port the AD.c file.
• do a block of conversions just before calling all the user-defined event detectors.  This will put a largish delay before the event detectors, but several detectors or services can look at the value with needing another conversion. Currently, the event detector loop with just a trivial DummyEventChecker (that reports an event every 100,000 calls and toggles the on-board LED) takes about 3.34µs on a 72MHz Teensy 3.1, but doing 6–8 conversions would add 8–500µs, depending on how much hardware averaging I decide to do (which the user should be given access to control).  This technique is probably the closest to letting me duplicate the existing interface, which is designed around the Pic32 scan.
• do a start-conversion and interrupt for each conversion.  If I do little hardware averaging, the overhead of the interrupt processing is large and nothing is gained, but if I opt for 32× averaging then interrupts have a big advantage over blocking code that busy waits for the conversions.

Currently, I’m leaning towards just using the analogRead() code, discarding all the Pic32-specific AD code.  I still need to decide whether to build in a battery-level check that shuts down the system on a drained battery.  Perhaps the cleanest way to do this would be to build a separate event checker that samples the battery voltage about every millisecond and low-pass filters it (rather than at 9345Hz/number of pins, the way the Uno32 code does).  (Sampling about every millisecond could be done just by watching the clock in the event checker and taking a new sample every time that millis() changed.)  Having a separate checker in a separate file, rather than intertwined with BOARD.c, AD.c , and roach.c would be much cleaner. After I’ve gotten all the basic functions working, I’ll think about dedicating a pin (perhaps A0, which has the fewest other functions) to battery-voltage monitoring.

I’m not sure how to ensure that all the external devices (servo motors, gear motors, … ) are properly turned off in BOARD_End(), before sleeping or halting the processor.  The Uno32 version of BOARD_End() doesn’t seem to do this either—in fact, it keeps the ADC and the serial port on, and just turns off most interrupts and sets pins to be digital inputs, not doing anything about the external peripherals that are the power-hungry ones.