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 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

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

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