Archaeology Resistivity Meter

Interesting ideas so far.  I meant to chime in earlier, but things have been pretty busy for me lately (building a deck and entertaining my Grandchildren again).

Several years ago, I was asked to sit in with some friends of my sister that work at a geotech company.  They were looking to build impedance measurement devices for soil surveys.  I came across this amazing looking chip from Analog Devices that looked like a great way to measure impedance (including a complex component).

Here is a snippet from the specification:

The AD5934 is a high precision impedance converter system solution that combines an on-board frequency generator with a 12-bit, 250 kSPS, analog-to-digital converter (ADC). The frequency generator allows an external complex impedance to be excited with a known frequency. The response signal from the impedance is sampled by the on-board ADC and a discrete Fourier transform (DFT) is processed by an on-board DSP engine. The DFT algorithm returns a real (R) and imaginary (I) data-word at each output frequency.Once calibrated, the magnitude of the impedance and relative phase of the impedance at each frequency point along the sweep is easily calculated using the following two equations:Magnitude = 22IR+Phase = tan−1(I/R) A similar device, available from Analog Devices, Inc., is the AD5933, which is a 2.7 V to 5.5 V, 1 MSPS, 12-bit impedance converter, with an internal temperature sensor, available in a  16-lead SSOP.

This might be of some help in your planning.

Good luck and let me know if you need any help on this project.

Hi Gene,

It's a super-interesting chip, I was keen to use it a few years ago for plant soil purposes, and for hydroponics - to try to see if the soil or liquid has nutrients. The idea being to have a signature of known good soil or water by sweeping through the spectrum. I never got to try it though sadly, the project moved on to something else.

It was felt that it could have had a lot of merit because then you could publish the signature, so others could try to replicate a yield (it wasn't going to be for farmers, more for home use), and to not waste nutrient. But, I have no idea in practice if the result would have been usable, or too inconsistent/variable.

I wish I'd done some work on it at the time, since it could have been useful for other purposes too.

The proposed design so far is one half of the impedance measuring system, but with digital processing. In theory it could be converted to an impedance measuring system with no additional hardware change, just a software upgrade, since the frequency will be know, and there will be some sync pulse from the FPGA, we just need to internally multiply with a 90 degree out of phase signal from that sync pulse too.

Michael,

If it is desirable to support USB stick or Micro SD (with FAT support) there are a few options that will make the job a bit easier.

1. There is this CH376 chip that handles the FAT structure and lets you write to USB Flash drives or SD cards (not too easy to find, as it does not seem to be handled by any common distributors - seems to only be available direct from the manufacturer).   There is a module available that is intended to interface with an Arduino that could be used ($12.95) from Marlin P. Jones Associates (https://www.mpja.com/USB-to-FAT-File-System-Control-Module-CH376-for-Arduino/productinfo/31813%20MP/)
2. I have used something similar to this (Vinculum) to read/write USB drives (with FAT), the VINC2 parts can support SD cards).  It sure made it a lot easier to handle the task of file system support (and we were in a huge ARM part with tons of RAM).

Good luck,

Gene

Never seen those before !

Not sure about reliability of supply and the data sheet is a bit lacking. Worth thinking about.

The FTDI chips (Vinculum) might be be OK - I'll need to look again - I use their parts for USB serial ports on things quite a lot.

MK

fmilburn  wrote:

 

...

I think this string of comments will potentially become difficult to follow and it would be nice to see it well documented with up to date project description, diagrams, schematics, drawings, design files, etc. in one place. Is that best done on GitHub?  Anyway, I volunteer but request guidance. Do you have an opinion on using GitHub for project documentation other than just code Jan Cumps ?  I am seeing this done quite a bit now.
...

I'd let the ones that do the work choose the tools, in a community effort.

These photos are not really related, except to show an adapter plate. My overall construction of this unit is poor, it was just a quickly-hacked-together low-noise low-current (100mA maximum I think) portable fixed-voltage DC supply project that I only needed for one use-case, but I'd like to re-do it properly sometime. The dual-Lithium battery here is a lot smaller than NP-F, about the size of a 9V PP3 battery:

The photo below shows the adapter plate (these are sold individually, normally used to fit the battery to a universal charger):

Final unit, just banana sockets, LED, power switch and I can charge it from 9-12V using the DC jack on the side:

documented with up to date project description, diagrams

Did someone say PowerPoint? : )

No? Oh well, now it's mentioned : ) I'm happy to put the block diagram so far into that, so that anyone can modify it, since I've got reasonable idea from the thread so far, but it will need refining since it's just ideas currently. It's easy to copy the PowerPoint slide into the same deck, and modify it to express other possible implementations by anyone.

Otherwise I can use Visio although I don't like it as much, and I'm much quicker with powerpoint. Anyway it's easy to copy-paste from Powerpoint or Visio into any other application like Word etc, or generate .png graphics for open documents, and so on. Any good documentation or organising or any other contribution would be great if you don't mind helping fmilburn or Jan or anyone (and collectively we might eventually know about as much archaeology as that kid Short Round in Indiana Jones), and hope your home moving-in is going well Frank!!

I'll post my ideas so far as a hand drawn and scanned block diagram - but not until tomorrow since I have no scanner or squared paper here.

MK

Hi Shabaz,

(1) and (2) in the Snuffler capabilities would seem easy to implement on a relatively low resolution screen In the field as you suggest. For a 20x20 or even 40x40 grid a simple graphic like heatmap in MATLAB could be displayed easily enough with a uC. That would allow an operator to see fliers or anomalies as they are appear.

Thanks Shabaz and Michael,

Readers looking at this for the first time and mechanical engineers without a clue everywhere will thank you for the block diagram. PowerPoint is the universal tool of corporate drones and I know it well :-)

Good news, I searched around for a way to get USB UART with Android, and had some luck! I found some source code for MCP2221, and got it to eventually compile:

That's just the simulator, I need to test on my phone (I couldn't find my USB OTG cable, but I'll search for it later today, and then try it with a couple of baud rates to see what's supported!

Hi a full graphics display wouldn’t be entirely necessary. It was done a few years back on a unit by T R SYSTEMS. They seem to have disappeared though. It was a box of electronics mounted on the side of the frame and an android tablet mounted facing the operator. The connection was remote and I guess Bluetooth. It’s complicated when surveying to work out exactly where you are on a grid, what with 20 rows to survey and 400 readings to take. Seeing numbers relating to your position is good, but it can still be confusing. We also have the added complication of obstructions, such as trees where we can take a reading. A dummy reading is taken in these instances. I’ve posted up my build of the original 2003 resistivity logger and there is also the GEOSCAN RM85 offering, both I feel overcomplicated in the number of buttons and very small displays. Equally the RM FROBISHER unit has a minimalist design, but a nightmare menu system behind it.

Hi a full graphics display wouldn’t be entirely necessary. It was done a few years back on a unit by T R SYSTEMS. They seem to have disappeared though. It was a box of electronics mounted on the side of the frame and an android tablet mounted facing the operator. The connection was remote and I guess Bluetooth. It’s complicated when surveying to work out exactly where you are on a grid, what with 20 rows to survey and 400 readings to take. Seeing numbers relating to your position is good, but it can still be confusing. We also have the added complication of obstructions, such as trees where we can take a reading. A dummy reading is taken in these instances. I’ve posted up my build of the original 2003 resistivity logger and there is also the GEOSCAN RM85 offering, both I feel overcomplicated in the number of buttons and very small displays. Equally the RM FROBISHER unit has a minimalist design, but a nightmare menu system behind it.

Here's my promised block diagram of how I see the story so far:

The bit you can't read on the right says that it could be a different display with an SPI interface.

The Bridgetek thing I have in mind is an ME813A-WH50C (Farnell 2674200) but it is expensive - slightly cheaper from RS and similar things much cheaper from Mouser.

I've used the Microchip BT module and I know it works, an ESP part might be preferred by some because it could do WiFi as well.

I've put an SD card and USB OTG as options - they will need loads of software and probably a processor with more pins (than the 100 I've suggested).

If we want to do much processing and have a big display on the board it might be better to go for a faster processor but it will use more power.

I've suggested a low power version here.

From comments so far it may be that a 4 line LCD character display would do for starters - it has the virtue of cheapness and simplicity.

With care we could keep the interface dual purpose.

I'm thinking of at least 8 Mbytes of directly connected and soldered in flash.

Nothing is set in stone, additional scribbles and crossings out are welcomed.

The basic idea of this design is that it uses sine wave excitation (lots of existing stuff uses square wave which is horrible).

The phase sensitive detector is entirely in software.

The FPGA is tiny (ICE40UP5) and costs about £5 - it is for glue purposes rather than computation.

The proposed operator controls will be buttons and one or two rotary encoder type knobs.

These go through the FPGA to avoid the use of high speed asynchronous interrupts that rotaries require (the FPGA is there so why not use it ?)

Everything is software controlled - there are no hardware switches affecting filters, ranges, signal routing or stuff like that.

The expansion interface could work a multiplexer (sometime far in the future).

MK

This is great.  If shabaz hasn’t already started I can create a PowerPoint version this evening.

That would be good but I was rather hoping that there would be some critical feedback, so it might be worth waiting a day or two so you will only need to do it once.

(Although a Power point version should be easier to mod than mine.)

MK

Hi Michael,

Thanks for this! The diagram is easy to follow. It all makes sense, it's great that there are options to go from character display to an SPI one too. Incidentally there's low-cost and quite large RA8875 displays that could be around for a while (they've been around for several years), I've used one and it has an early 2000 laptop feel to them, i.e. average quality, not bright when viewed from the side etc). They are SPI based too, so with no additional hardware work it could be possible to connect the higher-end BridgeTek panel, or the cheaper one, if one day someone extends the firmware for that. Here's an example of that: https://www.buydisplay.com/5-inch-tft-lcd-module-800x480-display-controller-i2c-serial-spi

Also it's very neat that the Bluetooth module capability and USB UART is there. I'm going to try connecting a phone or tablet to USB UART today, to see if it works, but I don't have a FT230 device to test unfortunately (and will likely need some different Android source). Anyway I believe the phone/tablet capability is an accoutrement and not totally essential, since there's a wealth of display and connectivity options in this design.

I just had one comment, regarding the DAC output, I was wondering if it could be nice handing that off to the FPGA too, to do the DDS, and have external DAC? I know it's not essential, but it could make the microcontroller code easier for anyone to extend it in future, since then there's no SPI writes on timer interrupts once it is set up, and the microcontroller can just read from SPI each time there's an interrupt from the FPGA, allowing developers to just concentrate on processing the results via DSP and then storing or displaying the data and UI controls. It also means that entire microcontroller portion could be replaced in future with (say) Pi for future non-archaeology uses too, e.g. as a lab instrument, provided it is fast enough to respond to an interrupt to read the data from the ADC via FPGA.

fmilburn it's great if you can get this into a ppt, but will think about the design more today and might have a few more comments later this evening.

I was going to draw the block diagram up this evening on cad.

Hi kltm,

I will leave the block diagram to you then...  Can stick a placeholder in there for the power as well.

Frank

I'd wondered about using an off chip DAC - it doesn't need to be fast -  1kHz update rate would do, 8kHz would be really nice. The processor has 12 bit DACs built in and can refresh via DMA so th processing burden is very light.

On the other hand if the FPGA does it then synchronising is easier (the uP can run at whatever speed it likes) .

I'll check out what DACs are cheap and easy to get.

Those displays look nice and cheap but the processor has to work hard if you use graphics. The cute thing about the Bridgetek controller is how much easier they make doing graphics and how little data you need to transfer over SPI.

MK

TI do a nice DAC available in 16 or 12 bit versions fro £4.61 or £2.03 (DAC80501 or DAC60501).

SPI in and voltage out. It has a built in voltage reference which we can use for everything else as well.

So we need to connect the DAC to the FPGA in the block diagram rather than to the micro.

MK