Tag Archives: electronics

MPPT Solar Charger Testing II

It’s time to follow up on the MPPT Solar Charger project. Progress has been slow since I’m currently working full time and doing a master’s degree at the same time. Given that this blog has previously been something close to a 50% job at times things will necessarily slow down a bit. But all the projects, including this one and the ultrasonic anemometer are alive and well and I’m working on them whenever I find some time.

20161012_userinterfacereva_014

User Interface

So what’s new with the MPPT solar charger? First of all, it got this nice user interface. Unfortunately there were still some issues with the first version so there is an identically looking but much-improved and bug-free Rev B.  I’ll do a separate post on that documenting all the things I’ve changed.

I re-used the white-on-blue display from the initial version for the Rev B. as opposed to the black-on-white used with the Rev A. They are pin compatible so you can use whichever you prefer. I more and more start liking that black-on-white look. It’s about 10 bucks cheaper as well. What’s your opinion on this?

img_4334

Solar Panel drains Battery

My intention with this design was to just leave the solar panel connected to the input of the buck converter. There’s a problem with that, however. The input of a buck is always at least at the output voltage minus a diode drop. With a 12V lead-acid battery at the output the input (i.e. where the solar panel is connected) the voltage never drops much below 12 volts.

That’s a serious problem since even a small solar panel like the monocrystalline 30W panel I have here sinks around 10mA at that voltage. So whenever the panel is not providing any power it considerably drains the battery.

Try a Diode

Obviously we need a way to avoid that. The first solution that comes to mind is a diode. But there’s a problem with that, too. To keep the power loss in the diode acceptable, the diode needs to have a low forward voltage. However, any diode with a low forward voltage also comes with a high reverse leakage.  Physics seems to dictate that.

Leakage can easily be in the range of milliamps for a diode that can handle, say, 4A like needed here. And that reverse leakage is also highly temperature dependent. It may be ok-ish at room temperature and then detoriate by an order of magnitude at 50 degrees centigrade.

To get an acceptable reverse leakage one needs to tolerate at least half a volt of forward drop which is something like 3% in efficiency. Since our converter operates at around 97% efficiency that would mean doubling the energy dissipated.

P-Channel or N-Channel Mosfet

So the next obvious thing to try is a mosfet acting as a switch.  Yes, a mosfet also has a body diode that also has a reverse leakage. But the body diode is a bad diode in terms of forward drop and has a correspondingly low reverse leakage.

A nice way of solving our problem would be to use a p-channel high-side switch disconnecting the battery from the converter whenever the panel is not producing. That not only disconnects the panel but powers off the entire buck including the input and output caps and everything. Where there is no voltage no power can get wasted. Perfect.

img_4342

Unfortunately, powerful (i.e. low Rds-on) p-fets are relatively large,  rare and expensive. As a rule of thumb a p-fet needs twice as much silicon area for the same performance compared to a n-fet. Besides that, that would be another type of component and I’m trying to not use too many distinct components.

So the other solution is to use an n-fet as low-side switch just disconnecting the panel. We already have 6 powerful n-fets in our design and they are cheap, too. So this is what I’ll do in the next version.

For now I’ve used one of the power outputs to connect the panel. I’ve cut some traces and soldered in some wires so that the fet is on whenever the buck is powered on.

Buck Software

During early testing of the buck converter I managed to kill the bottom fet several times. Since I was able to run the buck at relatively high power levels for prolonged periods of time without any issues (and without getting hot) I suspected that this was caused by carelessly written software. This seems to have been the case indeed. Whenever I killed a fet it happened at startup or shutdown, usually the latter. In the mean time I’ve written well-defined startup and shutdown routines and never had any issue since.

bootstrapcapacitorcharging

Synchronous converters are dangerous in this respect. It is very easy to short the battery to ground via the bottom fet. All it takes is a duty cycle that’s too low or a timer is running too slow or not at all. Or you stop the PWM module at a time when the bottom fet is on and it will stay on forever.  Or at least a few milliseconds until it’s dead.

Asynchronous topologies are much more forgiving in this respect since you can’t short anything. So an easy solution would be to start up and shut down in asynchronous mode. We can easily make this an asynchronous converter in software by just not utilizing the bottom fet. The diode in parallel with it will then automatically take over.

Unfortunately we cannot start up in asynchronous mode. Why? To enable the upper fet we need a bootstrap voltage above the input voltage. And we don’t have that unless the converter is running. It’s a chicken and egg problem, really. So we need to start up in synchronous mode at least for a few switching cycles for the bootstrap diode to charge up.

That’s exactly what the startup routine does. It completes 16 cycles in synchronous mode at a neutral duty cycle. What do I mean by neutral? Simple: Duty Cycle = Output Voltage / Input Voltage. That means no current actually flows on average. So this is a nice, soft way to start up. After those 16 duty cycles the buck enters asynchronous mode. As the screenshot above shows, the bootstrap capacitor is fully charged after just one full duty cycle so that number is more than sufficient.

This routine is critical. If it doesn’t do precisely what it should chances are high that the buck is destroyed. So I’ve checked it carefully by both looking at the code and the scope. I’ve run it many times and observed closely what it does just to be sure it starts up nicely very time.

Why run in asynchronous mode? Because it’s more efficient at low power levels. Once the current rises above a software-defined threshold the converter will change to synchronous mode. If the current later falls below a second (lower) threshold, the buck changes back into async mode.

The optimal values for the two threshold will have to be determined experimentally but will likely be in the range of a few hundred milliamps.

buckturnoff

The shutdown sequence is simple. The buck is shut down if the current falls below a very low threshold (say, 10mA) below which it cannot run profitably. If the current falls even lower we are better off shutting down the buck and entering a low-power mode. Since the current is low when we shut it off, the converter is already in async mode. So shutting it down is now easy and uncritical. We just turn off the top fet and can then also turn off the timer and supply voltage to the buck. The screenshot above shows an example of that.

Conserving power

A main feature of this solar charger is it’s ability to (hopefully…) run at a very low (<100microamps) current when not in use. So we obviously need to turn off everything we don’t need and run the PIC at a much lower frequency as well.

The PIC’s maximum operating frequency is 48MHz. This is the frequency it runs at when the buck and/or USB (not yet implemented) is on. These two features need that clock frequency to perform their task.

If both the buck and USB are off we can clock the PIC down to 8MHz which already saves a great deal of power. At 8MHz we can still do everything else, including running the user interface. Updating the display and particularly calculating the display content consumes quite some computation time and so we need a few MHz to do this.

If the user interface is not used for a certain time (say, 10 seconds), it is turned off. We can then lower the board voltage to 2.3 volts and lower the CPU frequency even further to 32.768kHz.  This is the frequency of the real-time clock that is running anyway. At such a low frequency the PIC’s computational power is low but still sufficient to do some housekeeping tasks.

One can wake up the user interface at any time by pressing the push button. Turning the encoder won’t help because that, too, has been powered off.

While in this low power mode every 6 seconds the board voltage is raised to 3.3 volts and all temperatures as well as the input and output voltages are measured. That all happens at 32.768kHz. After the measurement is complete the PIC decides if the panel voltage is high enough to start harvesting energy or not. If this is not the case the board voltage is lowered again and a new set of measurements is captured 6 seconds later.

img_4336

Despite all those efforts the board consumes something like 530 microamps which is much more than it should. About 100 microamps comes from the voltage divider used to measure the panel voltge. That’s an easy to solve design problem that I’ve described earlier already. But that still means that something is drawing 400 microamps. Not much at all but still way too much.

I’ve spent an evening trying to find the problem but I still don’t know. I’ve suspected the capacitors but when I measured some spare caps of the same type they hardly drew any current at 12 volts. I also un-soldered the buck’s mosfet driver but that made hardly any difference so that’s also not the problem. It may even just be some near-short on my board homemade board. So I leave that for now and check again once I have a revised design with a proper PCB.

Buck testing

I’ve saved the most interesting part for last 😉 I’ve established before that this converter has a similarly high efficiency than the Arduino version published some time ago. But at that time I was only able to test up to 35 or so watts.

This solar charger is capable of handling much more power than that. Testing how much was a bit more difficult than expected. Unless the battery is very empty it won’t draw as much as this charger is able to provide. And you don’t want to discharge a lead acid battery too much, they don’t appreciate it at all. So once again my constant current dummy load came to the rescue.

img_4339

With the dummy load at one of the power outputs I was able to draw some serious current. I set the dummy load to 4 amps and the battery absorbed (or provided) whatever was left. Obviously, I can only do that for a limited amount of time until the dummy load gets too hot. It automatically shuts down once the heat sink reaches 70 degrees centigrade which is soon the case at 4 amps.

I let the charger draw 4.5 amps at a bit more than 17 volts for as long as I could. The coil got quite warm, slightly hot even, maybe something like 60 degrees. The mosfet only got lukewarm and I’m not sure if this was because of their own power dissipation or due to their proximity to the coil.

img_4341

So from this test I’d conclude that this was about the power which the charger can handle for a prolonged period of time. So I think something like 75 watts is a realistic power rating.

I’ve also had another look at the datasheet of the coil and it pretty much confirmed my findings. 75 watts at (say) 13 volts output corresponds to 5.8 amps through the coil. The datasheet states that the coil starts saturating at 8.2 amps which means we’re still safe with 5.8 amps plus the ripple current. The datasheet also states a 40 degree temperature rise at 5.3 amps. So thermally the coil is pretty much at its limit at 75 watts. That, too, seems realistic to me having touched it after some time at that power.

The road ahead

So how to continue? As we have seen, there is a number of design issues but the general concept works. During testing and debugging I’ve cut and re-soldered many traces and also made some modifications like the one with the n-fet described above. I’ve unsoldered and changed many components as well. All of that doesn’t improve the reliability of the board. At some point it makes sense to design a new version, build it and take it from there. A clean slate kind of.

I think this point has been reached here. So I will work on a new revision from now on. The concept won’t change at all but I’ll try to apply what I’ve learned so far. I also hope to reduce the number of different components, reduce the size and total cost while not sacrificing performance. In other words move a step towards a design that may one day be built as a small series. Let’s see.

Ultrasonic Anemometer Part 28: New hardware tested

20160907_standaloneanemometer_024

I last time proudly presented the new RevB board and got a lot of feedback from people who want one, too. As mentioned I have all the components here to ship up to 10 kits but I was reluctant to send anything until I had the chance to do some hardware testing. Not much had changed since the last revision but I don’t like taking chances on things like this.

20160907_standaloneanemometer_029

In the mean time I managed to do some rudimentary testing and now feel confident to take orders. These tests concern the hardware only. What I said last time about the state of the software still applies. But let me tell you what I’ve been able to test so far.

rxtx_overview

Tests performed so far

  • The PIC32 can be programmed from a PICKit3 via the ICSP header without any issues.
  • Power consumption is as expected. Like the previous version it draws 45mA@12V (programmed) and the other two rails come up with +3.310 and -3.279V, respectively. Also as expected. Regulator stays cool.
  • With the PIC controlling the AXIS, DIRECTION and SIGNAL pins, the transducers receive the 12V signal from the mosfet drivers. HOWEVER: the signals AXIS and DIRECTION are incorrectly labeled both in Eagle as well as on the silk screen. Electrically everything is fine but the names have been confused.
  • The signal from the transducers is properly received (Rec pin on the board) and amplified by a factor of 11 by the first stage of the amplifier (S1 pin on the board).
  • The PIC is able to control the digipot over the internal I2C bus and the second amplifier stage also performs as expected.
  • The zero-crossing detector (ZCD) works.
  • The input to the PIC’s ADC (ADC+ and ADC-) look fine, too. HOWEVER: the labeling on the silk screen is wrong. Eagle is correct, it’s just the silk screen. Plus should be minus and vice-versa. Again, electrically everything is fine.
  • The PIC can communicate (as a slave) with an Arduino UNO connected to the external I2C bus.
  • Communication over USB to a YAT-terminal under Win 7 works.

rxtx_north

rx_closeup

The following has not been tested so far

  • For my tests I still used the transducers already used previously. I believe the ones I ordered last week are of the same type but I haven’t tested them.
  • EEPROM. I haven’t tried the newly added memory yet. I have confirmed that it has power and is connected to the same bus as the digipot so I have no reasons to assume there are issues with it. But testing it would require some software first.
  • The external SPI bus has never been tested, neither on the Rev A board nor with the new one. I don’t expect any problems but I haven’t done any testing so far.

20160907_standaloneanemometer_028

Is there anything important that I forgot to mention? In that case just ask.  A lot still needs to be done but at this point I’m confident that the board has no major flaws and performs much like the prototype.  Want to give it a try? I have some kits left for you.

i2c1_arduino

Continue here to the next post of this series.

Ultrasonic Anemometer Part 26: Rev B Board ordered

I recently ordered my first PCB at dirtypcbs.com and the result was promising. So there was nothing stopping me from finalizing the Rev B of my standalone Ultrasonic Anemometer and ordering a protopack. I’ve placed the order a few days ago and expect the boards to arrive here in 2 to 3 weeks. This should be good news for all those of you who have been asking for kits and want to contribute to the further developement of this project. I’ll build up one or two boards as soon as they get here and do some testing. If everything works as planned I can order some more components and ship some kits soon after that.

EagleSnapshot

So today I’ll go through the changes I’ve made compared to the previous version. All in all the changes are quite minor and only require minimal changes in software. But let’s go through them one by one.

Non-volatile memory

This is the biggest change from a functional point of view. The PIC32MX250 doesn’t have any EEPROM memory of its own. So in order to be able to save some settings an the like I’ve added a Microchip 24AA16 I2C EEPROM providing 16kbit (2kB) of non-volatile memory. That should be more than enough to store any settings and calibrations one might want to make. For example, this gives the user the possibility to calibrate the filter kernels to the transducers used. Of course, the software first needs to make use of that memory but I think it’s great to have the possibility to durably  store a reasonably large amount of data.

dirtypcb_top

Support for 5V I2C communication

I’ve hinted at this in a previous post. On the Rev A board the I2C signals were pulled up to the 3.3 volt rail. This was great as long as one didn’t want to interface to a 5V device such as an Arduino. The microcontroller pins are 5V compatible so you want to be able to pull those lines up to 5V whenever you interface to a 5V device. So I’ve added a diode to allow the SDA and SCL lines to be pulled higher than 3.3 volts. The I2C reference voltage of 3.3 volts minus a schottky diode drop or about 3.1 volts is accessible from the I2C header at the bottom. So just connect that to the external device’s 5V operating voltage and you have a fully compliant 5V I2C bus.

dirtypcb_bottom

Physical layout and connectors

There was a rather large 8-pin connector on the last version to connect to the transducers. Now all the connectors along the edges of the board are standard 100-mil headers. This also allowed me to slightly shrink the physical size of the board to 60x70mm.

The pinout has also slightly changed. The board is now powered from a header on the right-upper side and all three voltage rails are now externally accessible from a newly added header on the right side. The pin order on the I2C and SPI headers on the bottom side of the board has changed, mainly to accomodate an exteral I2C reference voltage.

GerberViewerScreenshot1

Power supply

The tiny (SOT23-5) 3.3 volt linear regulator on the last version worked well but got rather hot when providing close to 50mA from a 12V input voltage. I never had any issues with it at room temperature but decided to be cautious and upgrade to a LD1117 regulator in a much larger SOT223 package. This should be more than sufficient any reasonable ambient temperature..

GerberViewerScreenshot3

Miscellaneous changes

I changed the digipot used to set the amplifier gain to a Microchip MCP4531. This model only has 128 steps but this is still more than sufficient for its task and it’s quite a bit cheaper than the 256-step version.

I also had to change the crystal because the model previously used became unavailable.

GerberViewerScreenshot2

That’s it for now. I’ll let you know as soon as the boards get here.

Bi-Directional Voltage Level Translator – Board House Test

20160706_VoltageLevelTranslator_002

While most of my microcontroller designs run on 3.3 volts there is still the ocasional 5 volt design. Or I do something with an Arduino. So the need may arise to interface between logic working at different voltage levels. There are several ways of doing this, depending on your needs. Things are relatively simple as long as you know in advance which side is transmitting and which side is receiving. It gets more difficult if the communication is bi-directional or with busses such as I2C that are bi-directional by nature. I did a search on farnell.com and identified two chips that can translate between almost any two voltage levels bi-directionally. The Texas Instruments TXB0106 works with up to 6 CMOS (i.e. actively driven high or low) signals for protocols such as SPI. The PCA9306 (also from TI) is intended for protocols such as I2C that rely on pull-up resistors and where a line must never be actively driven high.

20160706_VoltageLevelTranslator_010

I ordered a few of these chips and laid out a simple board that contains the two chips together with the pull-up resistors and some decoupling caps. I also added two larger 10uF ceramic capacitors for a generous amount of bulk capacitance. The result is a tiny little board (45x30mm) that can universally be used to translate between almost any two logic voltages for almost any protocol.

Giving dirtypcbs.com a try

I can think of a lot of situations where it could be useful. But the main reason for this project was to gain some experience with getting a PCB professionally manufactured by a board house. For several years I have done my own designs but I always milled and drilled them myself. This had pros and cons. I never had to worry about silk screens or solder masks because my boards never had any. On the other hand I suffered from the lack of plated-through holes. Vias were always a pain in the arse because I had to manually solder in pieces of wire to connect the two sides. And it was very difficult if not impossible to put a via below a component which made the layout challenging when working with ICs with many and/or tightly spaced pins.

20160706_VoltageLevelTranslator_013

So this simple project seemed perfect to gain some experience in getting a design made by a board house. I chose dirtypcbs.com for their unbeatable prices and read their requirements and restrictions.

20160706_VoltageLevelTranslator_006

Here are some things to watch out for:

  • If you’re working with Eagle you can download a design rule file to automatically check if your design fulfills their requirements. This is great for checking things like minimum via size or trace width/spacing.
  • Make your own gerber files. Yes, you can just send dirtypcbs.com your Eagle files and they will take care of it. But I highly recommend you do it yourself so you know what you’re getting. It’s not difficult. Just download their .cam file and run it. I modified it somewhat so the file names reflect what the file contains. The file types themselves are rather cryptic.
  • View the gerbers online: There are several online gerber viewers available for free. I particularly like the online viewer at mayhewlabs.com. Just drag and drop the gerber files produced in Eagle and get a beautiful 3D view of your design. Check it carefully to be sure the gerbers represent what you are after. My workflow was rather iterative: I produced the gerbers, looked at them online, changed something in Eagle, produced some new gerbers… Again and again until I was happy.
  • Control what’s on your silk screen. A lot of the silk screen comes directly from the component libraries. Typically, the libraries print the outline, orientation, part name and part value on the silk screen. Unless your layout has a loooot of space between components the silk screen will soon be cluttered with all that information. I recommend you make your own libraries. You can then change the components  to only contain the information you want and how you want it. I ended up only putting the outline and orientation of the components on the silk screen.
  • Be aware that the design rule check will not look at your silk screen. You need to make sure your silk screen meets the minimum width of 0.15mm. If you’re using vector fonts (as you should) the math is simple. Just multiply the font size by the font ratio to calculate the line width. Example: A 70mils font size with a 10% ratio gives a line width of 7mils or 0.178mm.
  • Make sure your silk screen doesn’t overlap any of the pads. Again, he design rule check will not check for this. The board house might avoid printing the silk screen over an exposed pad but I wouldn’t bet on it.
  • Eagle seems to do quite a good job with the solder mask without any manual intervention but check the result carefully using the gerber viewer. Pay particular attention to the vias. You generally want your vias to be covered with solder stop – there’s no reason to have them exposed. But you might have some vias that you use as pads to access some signal for testing and debugging. Make sure they are not covered by the solder mask. An easy way to distinguish by the two types of vias is by drill size. You can set this parameter under DRC -> Mask -> Limit.

20160706_VoltageLevelTranslator_020

It took me quite some time until I had my files ready for manufacturing. Especially given the simple nature of this project. I then ordered a proto-pack from dirtypcbs.com. I went for a industry-standard 1.6mm (they default to 1.2mm) FR4 with a cool-looking (and non-standard) blue solder mask at no extra cost. Some 3 weeks later I found 10 of those nice little boards in my mailbox. They are quite precisely how I expected from the gerber viewer. When you look at them close enough you’ll notice that the solder mask and silk screen are not perfectly aligned. The silk screen could also be somewhat cleaner. But don’t get me wrong – I’m not complaining. The quality is absolutely ok, these are nice little boards. And at this price – I paid 14$ including everything – they are simply amazing. Thank you, dirtypcb.

20160706_VoltageLevelTranslator_019

So the board house test was a success but how about the rest? So far I’ve only populated a single board. And while I haven’t actually used it yet I have performed some tests with a signal generator and a scope.

PCA9306 for I2C

So let’s look at the PCA9306′ performance in some more detail.

I2C_50_18_1MHz
I2C: 5V to 1.8V at 1MHz

The chip performs excellently when translating from a high voltage to a lower voltage. In the  example above it is translating from 5 volts to 1.8 volts at a very high 1MHz frequency and the wave forms still look nice.

I2C_18_50_100kHz
I2C: 1.8V to 5.0V at 100kHz

Translating in the opposite direction (1.8 to 5 volts) everything is fine at 100kHz.

I2C_18_50_400kHz
I2C: 1.8V to 5.0V at 400kHz

At 400kHz the output waveform starts to look very jagged, however.

I2C_33_50_400kHz
I2C: 3.3V to 5V at 400kHz

But of course, translating from 1.8 to 5 volts is a rather extreme example. In the much more likely scenario of translating from 3.3 to 5 volts the chip performs reasonably well, even at 400kHz. And I2C doesn’t usually happen at frequencies higher than that. An Arduino, for example, will usually work at 100kHz. So I’m very happy with the performance so far.

TXB0106 for SPI and the like

Now let’s look at the TXB0106.

50_18_100kHz
5V to 1.8V at 100kHz

Translating from 5V to 1.8V at a modest 100kHz is rather undemanding and the waveforms look nothing but perfect.

50_18_1MHz
5V to 1.8V at 1MHz

Even up at 1MHz this translation works totally fine.

18_50_100kHz
1.8V to 5V at 100kHz

Translating the opposite way, from 1.8V to 5V is more demanding but works very well at 100kHz.

18_50_1MHz
1.8V to 5V at 1MHz

However, at 1Mhz the wave forms start to look pretty uggly. And with a protocol like SPI it is much more likely to actually operate at 1MHz. Note that the glitches might be an artefact of my primitive test setup.

33_50_1MHz
3.3V to 5V at 1MHz

As with the other chip, the output signal starts to better when only translating from 3.3 volts to 5 volts. As mentioned above this is a much more likely application and the chip does a decent job even at 1Mhz.

Support this blog – Buy one

By the way: If you need one of these universal voltage level translators you can support this blog by ordering one (fully assembled) for USD 20, including worldwide shipping. Just use the contact form on the about me page.

20160706_VoltageLevelTranslator_004

With the following link you can download the Eagle files as well as the gerbers. Just click here.

Ultrasonic Anemometer Part 25: I2C Interfacing and more

20160706_StandaloneAnemometer_023

It’s been a long six weeks since my last post but that doesn’t mean that I haven’t done anything since. Among other things, I wrote some code to get the I2C interface working and hooked the anemometer up to an Arduino Uno with an LCD display attached. Apart from demonstrating the I2C interface this also nice for testing. For the first time I can see what this thing is measuring in real time without hooking it up to a PC over USB.

20160710_StandaloneAnemometer_025

I2C Interfacing

But let’s look at the setup in some more detail. The PIC has a total of two I2C interfaces and I’ve made both of them accessible via the 100mil headers along the edges of the board. One of them is primarily intended for internal purposes like controlling the gain via the I2C digipot. The other one can then be used to interface to some external logic without having to worry about any internal communication. This external I2C interface also comes with 5V compatible pins which means we can interface to 5V devices like an Arduino without any further logic level translation. All we need is a pair of pull-up resistors pulling the SDA and SCL lines up to 5 volts. The Arduino’s Atmega328 already has built-in (weak) pull-up resistors so that’s not a problem. I didn’t think of interfacing to a 5V device when I designed the board and pulled the I2C signals to the anemometer’s 3.3V supply. So for proper 5V operation I’d have to unsolder the two 10k resistors. Luckily, 3.3 volts is enough to almost certainly be recognized as a logic high by the Arduino so the setup works anyway. But I will think about how to improve this in the next version. I might add a diode to allow the lines to be pulled higher than 3.3 volts.

The I2C interface can be configured to act as a master or as a slave device. For now I’ve only implemented slave mode so the wind meter behaves just like a I2C temperature sensor or external ADC. The Arduino acts as a master and asks the slave for its latest measurement every 250ms. The anemometer then returns 8 bytes of data consisting of 2 status bytes, north-south wind speed, east-west wind speed (both in mm/s as signed 16-bit integers) and temperature (0.01 degrees centigrade as signed 16-bit integer). It’s also possible to send data to the anemometer. That data is then saved to a buffer is otherwise ignored.  In a future version of the software one might use this functionality to set the data format to wind speed and wind direction or to change the temperature unit to kelvin or fahrenheit.

20160710_StandaloneAnemometer_027

It would not be difficult to implement master mode as well but so far I haven’t done it yet. A lot of code could be copied from the module communicating with the digipot where the PIC acts as a master. The anemometer could then push data to an external device whenever a new measurement becomes available. Definitely something I’d like to implement at some point but no priority right now.

Bug fixes

I’ve found (and fixed) a number of bugs while testing. Among other things, the axis and direction signals were not always properly set and so the measurements did not always correspond to the direction they should. So take with a grain of salt some of the test results reported earlier. Some of them were almost a bit too good to be true and this bug might have been the cause. I suspect I might have compared two successive measurements in the same direction when I actually wanted to compare measurements taken in opposite directions. But I’ve fixed the code and I’m confident that now all the directions are set and reported correctly.

Individual filter kernels

EqualAmplitudes

As can easily be seen from the scope screenshots above, the shape of the received signal varies quite a bit from transducer to transducer. Note that the amplifier gain is dynamically adjusted to make sure the peak amplitude is the same for all of them but the shape still differs quite substantially. So the kernel for the matched filter has to be some compromise to suit them all. I have now modified the software to use four individual kernels, one for each direction. This gives us the flexibility to calibrate the kernel to each transducer and so get more reliable results for the absolute phase.

Revised board

20160710_StandaloneAnemometer_026

My main priority at the moment is to complete the revised version of the board and to order a small series of boards. Until recently I never ordered a board from a professional board house so there are quite a few things for me to learn. For the first time I have to worry about silk screen, for example. Or solder mask. On the other hand, a board house can do a lot of things I can’t. Plated-through holes for example. Or smaller vias. Or place vias under an IC. This means a lot more flexibility in my layout that I first have to get used to. But I’m making progress. I got a simple design of mine produced by dirtypcbs.com and got a batch of very usable boards. More on that in a future post.

Wind tunnel

If you’ve been following this project for a long time already you might remember my simplistic wind tunnel made of a 120mm brushless fan and a cardboard tube. I got a number of suggestions from you guys on how to build something better than that and I also found some useful material online. So I’ve started to build a much improved wind tunnel that will hopefully allow me to perform more meaningful tests. That’s also for one or more future posts.

20160706_StandaloneAnemometer_021

Software ideas

I’ve also played around with some software ideas that I believe to have potential. One is dynamically adjusting the frequency. I’m now only working at the transducer’s nominal frequency of 40kHz. But the individual transducer’s resonance frequency might be somewhat different and might change with temperature or age. So I might try to adjust the frequency dynamically using some perturbation and observation algorithm.

I’m also thinking about measuring at two slightly different frequencies (say, 39.5kHz and 40.5kHz) and using the two phase shifts to figure out the absolute time-of-flight. I’ve given it a try and was not very successful but I haven’t given up on that idea yet.

So that’s it for now. The code for the Ultrasonic Anemometer as well as the Arduino are available for download. See the overview page for the respective links.

Ultrasonic Anemometer Part 24: New Microcontroller and Software Controlled Gain

It’s been almost three weeks since my last post and some further progress has been made. I’ve upgraded the microcontroller and can now control the gain of the second amplifier stage in software. But let’s look at the changes in some more detail.

20160625_AnemometerDriver_008

New microcontroller: PIC32MX250

I mentioned last time that I was running out of code space, i.e. flash memory on the PIC32MC220. That particular model has only 32k of flash and I’m using the free version of the XC32 compiler. That free version only performs limited amounts of optimization and therefore produces rather large and slow code compared to the standard and professional version. But the other two are rather costly (around USD 500 and 1000, respectively) and are not really an option here.  And at least so far I’ve always had all the speed I needed so the only issue was flash memory.

So the straight forward solution was to upgrade the microcontroller to an otherwise identical model with more memory. The 250 I’m using now has four times more of both flash (now 128k) and ram (now 32k). I unsoldered the old chip using a hot air gun and soldered in a PIC32MX250 in its place. Now we have all the flash we need to continue our journey.

20160625_AnemometerDriver_005

Getting I2C to work

My design uses a dual op amp for the necessary amplification of the received signal. The first stage is ground-referenced and uses a simple resistive divider to set the gain (currently set at 11).

The second stage is biased to 1.65V (i.e. half way between ground and 3.3 volts) and has its gain controlled by a I2C digipot. The idea is to have the PIC control the digipot so it can adjust the gain as needed to compensate for any effect that might change the amplitude of the received signal.

So in order to control our digipot we first need to get the I2C interface working on the PIC. There are two (identical) I2C peripherals on the PIC32MX2xx and this design uses one of them (#2) exclusively to communicate to the digipot. The other one (#1) is then available to interface to the outside world.

I’ve written a set of simple functions to use the I2C interface. So far I’ve only implemented  master transmit mode since that’s the only thing we need here.

  • void app_i2c_internal_init(void);
  • bool app_i2c_internal_write(uint8_t address, uint8_t *data, uint8_t length);
  • void app_i2c_internal_writeDigipot(uint8_t value);

The functions are entirely non-blocking so they can be called from within the interrupt service routines that do the measurement.

Fixing a design bug

Unfortunately, I made a mistake in the schematic when I referenced the feedback loop of the second op amp stage to ground instead of the mentioned 1.65 volts. Now let’s look at what that did to the signal. First, below is the signal with the digipot at 0 (of 256). The gain is set to one and all is well.

AmpError_Pot0

But at a setting of 64, some clipping starts to occur as shown below.

AmpError_Pot64

With the gain turned up to 4 with a digipot setting of 192 things get really uggly with a completely unrecognizable output signal.

AmpError_Pot192

I was extremely lucky that the correct signal was available right next to the pin where I needed it so all I had to do was to cut the ground connection with a carving knife and to connect the correct signal using a tiny bit of wire.  The change is hardly visible on the photo below. Can you spot it? It’s almost at the center of the photo at pin 5 (bottom right) of the small 8-pin IC. It now connects to the 10k resistor at the bottom instead of the ground plane on its right.

20160626_AnemometerDriver_009

Controlling the gain in software

With that error fixed we can now set the gain as we please. Here some examples. Yellow is the (unamplified) input signal whereas green is the output of the (fixed) first amplifier stage.  The purple signal is calculated as the difference between the DAC+ and DAC- signals. Those two signals is what we feed into the PIC to be measured.

With the gain at 1 (digipot at 0) we get a about 2.6 volts peak-peak. A bit little but sufficient.

Amplifier_Pot0

This is probably about what we want. Digipot at 50 resulting in a peak-peak amplitude of a bit above 3 volts.

Amplifier_Pot50

Needless to say that we can jerk the gain up far more than that if we want.  Below is an example where a gain of 4 results in a seriously clipped signal.

Amplifier_Pot192

Now let’s look a a situation where such a high gain might actually be useful. In the example below I’ve lowered the input voltage to only 5 volts. As a result the received signal is only 90mV in amplitude instead of the 220mV seen above. That’s easily compensated with a bit of help from our digipot. With a setting of 170 we get a nice, clean 3 volt output signal.

Amplifier_Vin5_Pot170

Calculating the necessary gain

For the examples above I’ve explicitly set the gain in the code. But what we really want is to have the software calculate and set the necessary gain automatically.

So I’ve implemented a simple algorithm to do so. I set some target value for the peak-to-peak amplitude of the signal measured by the DAC. Knowing the amplitude and digipot setting of the last measurement I calculate the optimal gain and digipot setting for the next measurement.

AmplitudesGain1

Above is the signal without this automatic gain setting in place (gain=1). As you can see, the peak-to-peak amplitudes vary quite a bit between transducer pairs. So the gain calculations, too, work on a transducer pair basis. Each direction gets its own setting to compensate for different transducer sensitivities.

Below is a screenshot of the result. Notice how now all transducer pairs have identical (and somewhat larger) amplitudes.

EqualAmplitudes

That’s it for today. Click here for my next post.

Ultrasonic Anemometer Part 22: USB up and running

20160514_StandaloneAnemometer_047

Last time I showed you the nice new hardware of the new standalone ultrasonic anemometer. But at that time I had hardly any software written for it so I couldn’t do much with its 32 bit microcontroller. So the last two or three weeks I spend lots of time writing code that I’d like to share with you today.

20160428_StandaloneAnemometer_039

As I mentioned last time I worried most about getting the USB working properly. All the bit-fiddling with timers, PWM modules, input capture modules and the like can be time consuming and at time even frustrating. But I have all the confidence that I finally get what I aimed for. It might take a few nights digging through data sheets but in the end it will almost definitely work.

Now with USB I don’t quite have that level of confidence. The USB specification is quite overwhelming and there are almost countless registers to properly set and USB descriptors to specify. Without some sample code I’m pretty much lost I must admit.

Outdated Application Notes

So I was happy to see that Microchip has published a number of application notes and accompanying software. A handful of .h and .c files that you can include in your project, change some settings to suit your particular application and you’re ready to go. At least that’s what I thought.

20160514_StandaloneAnemometer_048

All those application notes were published around 2008 and the code they are referring to is nowhere to be found nowadays. Aparently, Microchip has been quite pushy trying to get their PIC32 customers migrate to their new library or rather framework called Harmony. MPLAB no longer even includes the the PLIB peripheral library everyone has been using for decades. Microchip has depreciated it and while you can still download it they make very clear that Harmony is the library of the future.

On the road with MPLAB Harmony

So faced with few other alternatives I turned to the aforementioned Harmony library. It’s many hundred megabytes to download and takes up almost two gigabytes of disk space. It integrates nicely into MPLAB X and so I created a first project. You can graphically configure the clock and pin settings so I did that. Clicked on ‘create code’ and was nothing less than shocked by the code I got.  Around a dozend source files scattered around in a dozend deeply nested folders and subfolders. And that was not yet it. Those files referenced dozends more files that came as part of harmony. It took me quite a while to just more or less figure out what the project was doing. And it didn’t do anything yet except setting some clock and port settings. Just a nightmare.

20160514_StandaloneAnemometer_045

But I felt I had few other options so I continued with harmony anyway. And after a few hours I had an almost working USB device. It enumerated just fine but I couldn’t send any data from or to it. It took me more than a week to finally get it working. In the end it was just a single character: a 1 instead of a 2 in one of the configuration files. But until then I had spent a lot of time with that harmony code and was forced to read through a lot of its documentation which is worth thousands uppon thousands of pages of PDFs.

So USB was finally working and I had acquired some rudimentary understanding of MPLAB Harmony. I still hated the whole framework but I thought that I now understand it well enough to change the code to suit my taste. So I spent another few nights trying to do that until it dawned on me that this is leading nowhere. Fortunately, by that time I had read and learned some more about Harmony and was willing to give it another try working with it as opposed to against it. So for the last week or so I was doing that.

And while I still don’t love everyting about it I now feel comfortable working with that library. I’ve tried to follow its conventions and recommendations even where they didn’t make too much sense to me. I think this will make it much easier for others to work with my code and chances are that the conventions and recommendations will start to make more sense as I get more familiar with the framework. And its nice to know that I can integrate another module or migrate to another microcontroller without having to re-invent everything.

20160428_StandaloneAnemometer_037

That was a long introduction but after all that pain the last few weeks I just had to write that down. Now to business.

USB

The anemometer inplements the USB HID (human interface device) class. While mainly aimed at devices such as mice and keyboards, this USB device sees a lot of use in applications that have nothing to do with human interaction. You can transfer up to 64 kbytes of data (64 bytes every millisecond) in each direction with guaranteed latency and that’s plenty for many applications. That data can be anything and in any format you like. And the good thing is that you don’t need to write your own drivers. You can free-ride on the device drivers that came with the operating system just like you don’t need to install a driver when you attach a new keyboard to your computer.

I’ve written a bit of code on top of the Harmony library to provide the main application with a simple to use interface. It nicely encapsulates the the USB logic and data and adds a bit of buffering functionality on top of that. It’s just two files: app_usb.h and app_usb.c in the app_usb subfolder. You can basically include them in your project and they handle all the USB stuff for you.

Once initialized, the necessary code runns inside an interrupt service routine (ISR) so the main application has nothing to worry about. All the USB data including the data buffers are private to the app_usb.c file. The app_usb.h defines just 8 simple, non-blocking functions:

  • void app_usb_init(void);
  • bool app_usb_isReady(void);
  • uint8_t* app_usb_getReceiveBuffer(void);
  • uint8_t app_usb_getReceiveBufferCount(void);
  • uint8_t* app_usb_getTransmitBuffer(void);
  • uint8_t app_usb_getTransmitBufferCount(void);
  • void app_usb_freeReceiveBuffer(void);
  • void app_usb_sendTransmitBuffer(void);

At system initialization you call app_usb_init(). But you can’t expect to have an USB connection. You need to check app_usb_isReady() every time. You never know if there is a USB cable plugged in and even if you had a connection that cable might be unplugged at any time. The module handles all that hot plugging/unplugging gracefully.

20160428_StandaloneAnemometer_038

The module implements a circular buffer for both received packages (aka frames) and packages to be sent. Each package is 64 bytes in size and the ring buffers can be set at compile time but must be a power of 2. Currently the receive buffer consists of 4 frames and the transmit buffer of 8 frames.

The app_usb_getReceiveBufferCount() informs the caller how many new USB frames have been received if any. app_usb_getReceiveBuffer() then returns a handle to the first received frame. FIFO – First In First Out. Once the frame is no longer needed the application calls app_usb_freeReceiveBuffer() and the buffer can be re-used.

Sending data works similarly simple. app_usb_getTransmitBufferCount() returns the number of transmit buffers currently in use. If this  number is smaller than then number of buffers then more data transmissions can be scheduled. Get a handle to then next buffer by calling app_usb_getTransmitBuffer(), write the data to be sent to the buffer and then call app_usb_sendTransmitBuffer().

Taking measurements

This way the main application can focus on more interesting tasks such as measuring wind speed and direction. The first task for doing so is sending some pulses.

SendReceiveOverview

If you’ve alredy followed this project for a while you are probably familiar with the kind of screenshot above. Bursts of a PWM signal at 40kHz (SIGNAL, red) are sent trough ultrasonic transducers. Note that the signal is now active-low (i.e. inverted) since the mosfet drivers used to drive the transducers are themselfs inverting.

As before, the DIRECTION  and AXIS signal determine which transducer is transmitting and receving. The SCLK and MOSI signals are just used for debugging purposes for now. They indicate when an ISR is running which helps a lot since in this example ALL the code is running inside an ISR.

TimeZero

As you can see above, the AXIS and DIRECTION signals are set a few microseconds before we start sending pulses. The first edge of the burst occurs when the 32 bit-timer (consisting of the 16-bit timers TMR2 and TMR3) is cleared. This is the point in time all our measurements are referenced to. The timer runs at the full CPU clock of 48Mhz so the resolution is about 20.8 nanoseconds.

SendReceivePWM

Output Compare module 2 (OC2) handles the PWM pulses. Currently each burst consists of 12 pulses. At 48MHz a full wave equals 1200 clock cycles as opposed to 400 on the Arduino previously used. Note that the frequency of the pulses is precisely 40kHz as expected and that very little time is spend in ISRs despite the unoptimized C code.

SendReceiveCloseUpWithComments

The timer overflows precisely 512 times per second so each measurement takes a little less than 2 milliseconds. Output compare module 1 (OC1) can be used to generate an interrupt at any time during that interval. Currently it does three things: It takes care of the AXIS and DIRECTION signals which are set just shortly before the timer overflows. It also triggers the measurements of the zero-crossings and cancels them if they don’t complete withing reasonable time.

ZCD_Capture

The input capture module 4 (IC4) captures the zero-crossings of the received and amplified signal. The board contains a high speed comparator to detect these events so the microcontroller only needs to measure the precise time whn they occur. The IC4 module has an internal buffer so we don’t need to generate an interrupt after every capture. I’ve currently set things up so that an interrupt occurs after 4 values have been captured up to a total of 32 measurements.

Commicating the results over USB

In order to find the wind speed and direction we need to somehow identify the peak in the received signal which is where the ADC comes in. But I think this is enough for now and I instead show you how the measurements taken so far can be sent to a PC via USB.

The board doesn’t have any display or anything to communicate with a human. There’s not even a status LED we could blink. That would have been nice but there are just not enough I/O pins unfortunately. So connecting the board to a PC via USB is a straight-forward way of communicating with this device.  Actually, USB will be the main testing and debugging tool during development. The in-circuit debugger isn’t very useful in this application since the measurements have to happen in real time.  Setting breakpoints will just mess things up.That’s why I insisted to get USB working first.

I’ve downloaded a simple terminal called YAT (yet another termial) that allows me to easily talk to the anemometer board. The anemometer looks at the first character in any message sent from the host and then decides what to do. So far I’ve implemented:

  • ‘T’: Return the current value of the 32-bit timer
  • ‘Z’: Return axis (0/1), direction(0/1), measurement count (0-31) values as well as the results of the first 9 zero-crossing measurements
  • Anything else: Echo back the received string

Here’s what my chat with the Standalone Ultrasonic Anemomete looked like.

YAT_Capture

Of course, the measurement is far from complete at this stage but I think this is a nice foundation to build uppon. The code is quite clean for this early stage of development and being able to communicate with the device in real-time over USB is really a great advantage.

The software can be downloaded on the project over view page and as always I appreciate any feedback.

The next post on this project can be found here.

Ultrasonic Anemometer Part 21: Standalone Anemometer Hardware

Last time I went through the design of my new standalone anemometer. Now it’s time to build this thing and see if it works as planned.

20160428_StandaloneAnemometer_034

After I fried a couple of chips on my driver circuit testing board due to a wrong chip in the power supply I was a bit more careful this time and built up the board step by step.

20160426_StandaloneAnemometer_016

Only after I confirmed that the power supply was ok I dared to solder some more.

20160426_StandaloneAnemometer_019

The next step was to add the PIC32 with the crystal, the programming header and all the caps they need. This is a chip family that I’ve never used before so I wanted to first see if I can program it. All went well and I managed to get it to run on the crystal’s 8MHz boosted up to 40MHz by an internal PLL. So I was ready for the rest.

20160428_StandaloneAnemometer_031

I wrote some very basic software and confirmed that at least the basics were working ok. I was able to send and receive pulses, the pulses got amplified, the zero-crossing detector worked and so forth.

20160428_StandaloneAnemometer_024

As mentioned, I’m entirely new to the PIC32 microcontroller series. There are a lot of similarities to the PIC16 and PIC18 series that I’m quite familiar with but still it’s always a challenge to work with a new family of chips and the tools that come with it. I took me the better part of an afternoon to master the vectored interrupts with the different priority levels and so on.

20160428_StandaloneAnemometer_040
Driver circuit (front) and standalone anemometer (back) side by side.

By the way, with this project I’m using the free MPLAB X IDE with the also free XC32 C compiler from Microchip. So anyone is able to write, modify or compile code for this thing with free software. At least at the moment you need a programmer to actually burn the chip. But the PICkit3 only costs around 50 dollars and my idea is to write a USB bootloader so that any user can modify the software of a pre-programmed board.

20160514_StandaloneAnemometer_050

So now comes what I think might be the hardest part: Getting the USB to work. I’ve spent quite a few hours so far but haven’t managed to get it working properly yet. If anyone has experience with this kind of software development – Let me know, any help is highly appreciated.

It now works: Click here to view it.

Ultrasonic Anemometer Part 20: Standalone Anemometer Design

Last time I outlined my reasons to ‘go digital’ by adding a powerful on-board microcontroller and designing a standalone wind meter.

20160426_StandaloneAnemometer_001

In the weeks that followed that decision I tried to find a suitable microcontroller and to design a prototype. Today I’ll show you the result of that work.

20160426_StandaloneAnemometer_003

I looked at various series of microcontrollers from different manufacturers and finally decided to go for the Microchip PIC32 series. It offers everything I could possibly ask for: 32 bit architecture, inexpensive, vectored interrupts, integer division, any interface you want (depending on the model of course) , available in large, low pin count, hobbyist friendly packages and so on.

20160426_StandaloneAnemometer_011

As you can see above, the model chosen for my prototype is a PIC32MX220. This is low to mid-end representative of this series but even so the specs are quite impressive. CPU clock up to 50MHz, one instruction per clock cycle, full-speed USB 2.0, 32kB flash, 8kB RAM and all of that in a SOIC28 package at a price of CHF 3.58 in single quantities.

20160426_StandaloneAnemometer_005

After having chosen a chip the next task was to come up with an actual design. I took my anemometer driver design and tried to integrated the new PIC32. That driver circuit had performed extraordinary well so I changed very little. I left away the RELEASE signal since my tests had shown it to be unnecessary. I also replaced the LM5111-1M mosfet drivers by LM5111-2M. The difference is that the 2M is inverting while the 1M is not. The reason for changing this is because the 2M is available at a significantly lower price of CHF 1.35 vs CHF 2.25. Not a big deal if you just build a single prototype but I thought it might be smart to change this anyway. This also required some resistors to be changed from pull-downs to pull-ups. Except these details everything stayed the same with the drive circuit.

20160426_StandaloneAnemometer_012

I also had to re-design the power supply since the PIC needs a 3.3 volt rail as opposed to 5 volts in the driver test circuit. Since I had to re-design it anyway I also downsized the power supply somewhat. I’ve otherwise resisted all temptations to use smaller components but the power supply was just a bit too big with the two  SOIC8 chips and four size C tantalum caps.

The prototype now uses an MCP1755 linear 3.3V regulator in a SOT23-5 package with a 33uF size B tantalum cap at its input and a 10uF 0805 size ceramic cap at the output. A TCM829  (also in a SOT23-5 package) and two more 10uF caps produce the -3.3 volts output. So there is a total of three supply rails: +12V, +3.3V and -3.3V.

20160426_StandaloneAnemometer_006

A major challenge was to make do with the very limited number of I/O pins on the PIC. 28 pins seem like a lot for the task at hand but plenty of them are already used for things like supply voltages and the like.

In the end I managed to still get three independent interfaces to the outside world: USB 2.0, I2C and SPI. All these interfaces as well as just about any other signal of interest is easily accessible from one of the numerous 100mil headers along the edges of the board.

20160426_StandaloneAnemometer_007

The amplifier still uses a LMC6482 dual op amp , now running on a+/- 3.3 volt supply. The first stage amplifies the ground-referenced input signal by a fixed factor (currently 11 but this might change). The output signal is then biased to 1.65 volts and amplified once again. This time the gain can be adjusted digitally via a MCP4561 I2C digipot.

20160426_StandaloneAnemometer_008

Before I forget: The PIC gets its system clock from a 8MHz crystal. That might seem low but there are two (variable)  PLLs inside the PIC that (together with a number of pre- and post-scalers) let us produce the 48Mhz signal needed for USB as well as a reasonable clock speed to run the CPU and peripheral bus on (probably also 48MHz but we’ll see).

20160426_StandaloneAnemometer_013

The board layout proved to be a bit tricky because there isn’t that much space on the board and I had quite clear ideas where I wanted certain headers to be. So I was more than happy when all the traces were finally laid out.

As you can see, the PCB is already milled and the vias soldered. I can’t wait to build this thing up and start to do some programming. That will be the topic of my next post.

20160426_StandaloneAnemometer_014

As always, a zip file with the eagle files and PDFs of the schematic and board layout can be found on the overivew page.

And here‘s the finished board.

Ultrasonic Anemometer Part 19: Testing the Analog Circuit

20160323_AnemometerAnalog_016

In my last post I went through the design of the analog part of the ultrasonic anemometer. Today we will see how the circuit designed last time performs in practice.

20160323_AnemometerAnalog_015

Active Full Wave Rectifier

Let’s first look at the active full-wave rectifier. As a first test I fed the input with a 40kHz sine wave from the scope’s signal generator. Here’s what I got to see on the scope.

Rectifier_40kHz

The input signal is shown in yellow. The pink signal is calculated as the absolute value of the input and is what we are trying to archieve. Finally, the blue signal shows what we actually get on the rectifier’s output. What do you think? The basic wave form is obviously right but the precision leaves quite something to be desired.  So I tried the same setup with an input signal 10 times slower, i.e. 4kHz.

Rectifier_4kHz

As you can easily see, things look much better at 4kHz. That doesn’t make it much better for us since we need it to operate at 40kHz but there doesn’t seem to be anything fundamentally wrong with the design per-se. I think a faster op amp might be that is needed to make this thing work properly at 40kHz, too.

Zero-Crossing Detector

input_rectifier_zerocross

The zero-crossing detector is by far the simplest element of this circuit and has hardly changed since the Arduino anemometer shield. It’s output is shown in blue above. It works just as expected, no surprise here. Well, in the end it’s just a comparator so the sources for error are limited.

Peak Detector Sample And Hold

The input to the peak detector is the rectifier’s output so we should expect to see at least some inaccuracy carried over from the rectifier.

Below is what I got. Again, yellow is the 40kHz sine wave input. Blue and red are the peak detector of the positive and negative half-wave, respectively. Green is the output, i.e. either the blue or red singnal as chosen by the 2:1 multiplexer.

PosPeak_PosGate

The good news is that the circuit basically works as designed. Each peak detector gets zeroed at just the right time. From the screen shot above one might even conclude that the peak detector is fairly accurate. Both half-waves show very similar amplitudes and the output is nicely held stable during the hold period. No droop is visible at least at this level of magnification.

Once you zoom in, things start to look less pretty. There is in deed no droop but the opposite. The output signal rises during the hold period. If you look closely in the screen shot above you can even see it – the segments of the green signal are slightly rising. We’re talking  maybe 10mV here, far from dramatic but still.

Conclusion

20160323_AnemometerAnalog_012

My conclusion is as follows:

  • Zero crossing detector: perfect
  • Active rectifier: poor performance as is but can probably be fixed with a faster op-amp.
  • Peak detector: ok but not good enough. might need lots of trial-and-error to improve it.

I have mentioned in a previous post that I’m still unsure if I should stick to the predominantly analog signal processing or if I should make the switch to a more contemporary, digital, DSP like approach. After having spent an evening or two testing and tweeking this circuit the answer became clear – go digital.

20160323_AnemometerAnalog_014

If you’ve followed my blog for even just a short while you have likely noticed that I enjoy designing and building hardware.  Probably more than I enjoy writing software. But there are several good reasons to to more in software and less in hardware when it comes to this project.

  • The circuit as described above works kind of. But it might take a loooong time to improve it until it really performs well. Chances are that it will never perform as well as I’d like it to.
  • Board space. All this circuitry takes up considerable board space. A later version might use smaller components but this thing will always take up quite a bit of space – maybe a third of the total board area.
  • Even with most of the heavy lifting done in hardware the Arduino is likely not fast enough. We’d need to sample the amplitude at 80kHz which is out of reach for the Arduino’s Atmega328 even when sacrificing some resolution.
  • Further development. Chosing a digital approach enables users to actively contribute to the further development and improvement of this project.
  • Versatility: Eqiping the anemometer with a dedicated microcontroller makes this project much more versatile. It no longer has to be Arduino specific. If you’re an Arduino lover I promise you it will always be Arduino compatible. No need to worry. But equiping it with a easy-to-use SPI or I2C interface makes it useful beyond the Arduino community.
  • Cost.  My analog design uses two op amps among other things. Precision op amps are relatively costly components. It’s not an expensive circuit but it will always cost several dollars for the components alone. And improving the circuit is unlikely to make it cheaper.
  • Processing power is cheap. You might rightly say that a few bucks is not a lot of money to spend on some nice analog circuitry. I fully agree. But one can buy a lot of processing power for far less money nowadays.

20160323_AnemometerAnalog_011

Let me elaborate on some of these points:

Versatility: Arduino Only vs. Anyting, including Arduino

The arduino is a great platform with a great user community. But the Arduino Uno might not be the best choice for this project. A on board microcontroller gives user the option to use this anemometer from whatever platform they chose. Besides: An arduino is a very expensive device even when compared to a quite high-end microcontroller.  32-bit microcontrollers running at frequencies of 40MHz and higher are available for less than 2 or 3 dollars even in small (say, 10 pieces) quantities.  More advanced models might cost, say, 5 dollars.  In comparison, an Atmel Atmega328 as used on the Arduino currently costs CHF 3.10 (USD 3.18 at the time of writing) at Farnell. Not really value for money if you ask me.

Community Driven Developement

There are lots of creat coders out there. I can write decent code but there are plenty of people way better at that than I ever will be.  I see this as an opportuinity to greatly improve this project. Once I get this thing up-and-running I’d like to build a small series and let users share their experience and contribute to the code. Have an idea on how it could be improved? Try it yourself if you have the skills. Share your code if it works. Or share your thoughts and ideas if writing emedded softwar is not for you. So the anemometer could get better and better without having to get new hardware. Just update the firmware.

Next steps

In the weeks to come I will work hard to find a suitable microcontroller and to design and build a board with all that is needed for a stand-alone ultrasonic anemometer.

It’s now ready, click here.