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.

Hi Michael,

I tried out the Snuffler software, and I feel it's definitely too much effort to replicate it, since it's not open source (but is free-to-use) and seems to be Windows-only. It does three main things:

(1) accept data (either in real-time via COM port or from a flat file such as CSV), and the format is straightforward for at least one of the devices it works with,

(2) plots the data,

(3) graphically processes the data or manipulates it (for stuff like edge detection, interpolation and so on).

For an on-board display, just speculating, perhaps only (1) and (2) could be needed, along with fewer basic graphic manipulation options, and as you say just use laptop if the full capabilities of (3) are needed. For on-board display to work I guess it does need to be a pretty nice display, since seeing the data is so core to it. I can try to investigate UART connected via USB to Android as an option (it is supported in Android Studio based apps apparently).. but depends if that's a usable practical scenario all day or if the built-in display is preferable.

Also, incredible the price Sony is charging for what is still a consumer battery...

The Ansmann 6.6 Ah battery is £35, it's a high-quality consumer brand too, so trustworthy I think. I have one of those batteries for about half a year, using this dual fast charger (£24). I don't have any Sony-manufactured gear that uses NP-F, but have ended up with a couple of items that can use the battery (a camera monitor, and a camera light both from different large brands (Atomos and Manfrotto) which optionally can have an adapter plate attached for NP-F, but the plate for the light is a separate purchased item which I don't own).

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!

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!

115200 baud confirmed functioning.

So, one day the snuffler software can be replaced (or at least partially) with Android code if desired. Or could be used to e-mail or upload the data in real-time so someone can remotely check it while the survey is still being performed.

The test setup was as shown here:

Hi Guys,

I've been reading your plans on Element14 for an Archaeology Resistivity Meter after seeing it on Facebook.

Its very exciting. Have you considered using a Raspberry Pi unit? I don't know much about the electronics but you can get a GPS unit to fit it. The Pi will also run QGIS software which is 'A Free and Open Source Geographic Information System' which runs on Pi's and Windows Laptops. I have some experience of adding features to QGIS using Python. You could use QGIS to show your results, your position and the grid in the same way as 'High End' Gradiometer Carts do. (Carts can cost £40K or more.) You could use QGIS to display results on a grid, or map, in real time.

I hope this helps.

Paul D

The Pi has been suggested on a couple of occasions.

By the time you have built a Pi and a display and the required power supply and other goodies need to run a serious graphics type application you end up with something rather big and quite power hungry.

For some applications that's no problem at all but for others it is.

The current plan is that the basic box will have interfaces that will allow it talk to anything - USB, logic level UART, BlueTooth and possibly WiFi.

It will have its own controls and display options (simplest display being 4 lines x 20 characters, maximum 800 x 480 pixels with capacitive touch.)

With a simple display I would expect it to use less than 1W (averaged over a day's work).

There would be no problem making it talk to a Pi or anything else.

We haven't got to talking about control software architecture yet but the plan in my head would have the core resistance measuring engine controlled by a simple protocol that can be exposed either to an internal GUI or to the external interfaces.

A Pi would be able to use an external interface to fully control the instrument.

The idea is to rule nothing out but still make the basic system simple enough to get going reasonably cheaply and quickly.

I'm assuming that it will all be open source so access to protocols won't be an issue.

I've come at this from the electronics side of things so QGIS is  new to me - I'll take a look.

It would be great to have Pi based data processing software route and any help in that respect would be massively welcome.

MK

Hi Paul,

I like using Python too. It will be awesome to see what code could be used or developed for this project.

This is a rough sketch of some (not all) of the connectivity methods and how they might be used:

GPS expansion will be feasible by plugging the GPS module directly into the Pi, but it might become unnecessary, since there's the possibility of either sending GPS position from a phone (via Bluetooth or BLE) or sending the measurements into the phone. The only limitation is what's coded, since the underlying hardware covers lots of popular interfaces (USB, BLE, etc). There's memory on-board too, so in theory the data is reliably stored and the SD Card is for transferring stuff (edit: there's no SD card option, it's unnecessary since there are better wired and wireless options) . Also, if it's more comfortable for someone to analyze remotely from a larger computer, then any captured data can be sent in real-time from the phone - assuming it is coded!

I wasn't thinking of supporting an SD card interface, unless others feel this to be essential:

1) The low power Cortex M4 processor doesn't support it

2) If we change processor to one that does we'll need probably to go to 144 pin chip

3) The code to support SD card is complicated

4) You have to support a full on disc filing system

5) Could use a support chip as Gene suggested but it will use a lot of pins.

6) All the above if we support USB OTG except the low power processor is OK with it.

Probably not clear on my original block diagram that the options in the dotted processor outline might need to use a different processor.

On the other hand if we go for a more powerful processor we do get the benefit of faster processing all the time. The H7s support

dual voltage SD card at up to 104MHz - there is some free software from ST which seems to support FAT.

An H7 will burn more power speed for speed and about 300mW more at full (400MHz) speed.

See ST app note AN5200

I'm agnostic on this one - I've got quite used to STM32H7s now

MK

Oh, I see.. sorry I'd blindly copied that and not noticed the dashed line. I'll remove it, I cannot see it ever being needed, since there's more flexible wireless and wired options to transfer data.

A lot of the hand drawn lines look a bit dashed.

I was just warming to the idea of an STM32H7 ...... but I think lower power is better really.

MK

Shabaz,

I have changed your outline. In the UK two of the probes are 'fixed' and can be in the same position for the day (or longer)

The QGIS team have vast experience to call on in displaying this type of data  Many groups in the UK use QGIS and some run training days in Python.

You can have a table and power at P2C2 and use existing equipment (including a tea and coffee facilities - YES most groups have a tent that could be pitched close to the 'fixed' probes if needed) My editing of your original is very bad, sorry.

When I said "What about a Pi" I meant "QGIS would be good. it runs on Laptops and a Pi"

Paul D

Hi Paul,

I've updated it to incorporate your comments regarding the probes and software. One underlying difference (won't make a difference to the user apart from a major improvement hopefully!) is that the older existing designs use analog circuitry for most of the signal generation and processing. In this design however, more of that is done digitally, and so needs precise timing and regular fast calculations, which would be an overhead for the Pi, so all that underlying processing needs a microcontroller (and an FPGA), hence the separation between that and the Pi. The Pi has full control via USB Serial, which is a key difference to Snuffler. The Pi can instruct the desired frequency, start/stop measurements and acquire the data in real-time, i.e. the Pi is in full control using Python, C or any desired language. Also, the interface can still work with existing software in real-time (I've run Snuffler to examine what data is expects), so I've listed those in the diagram now.

For those who want to use a laptop, they will find it equally seamless with Windows and existing software, it's just more convenient to use USB rather than have the older RS232 DB9 connector. Just like with the Pi, they can have full control using any programming language if desired.

For those who want to use Android, initially there could be just a simple skeleton app which doesn't do much but provides some basic visuals, or just collects up the data to e-mail it.

Shabaz,

What software are you using to create your drawings? They are great.

Paul D