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 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

Hi Paul,

Thanks! I'm using Powerpoint. I've used it a lot and so already had things like the Pi and display ready to use.

For items like the phone graphic, I usually google search for (say) 'phone transparent png' and then it finds stuff with the transparent background.

michaelkellett I do agree with you, but

1. The Pi4 can support Debian (Linux)
2. I disagree with Shabaz about using USB between PI and control head, As said before CAN bus is robust and you don't have to write the protocol.
3. I disagree with Shabaz about  using WiFi ( sucks power)
4. If you go with the Pi, then with a USB3 to SATA adapter (this is what my VOIP server is running on) and run a notebook drive, for the OS and Data The Pi can be set not to boot from the SD card if you wish.
5. if you use a tablet, that's overkill + you would have to write an app.
6. if you use a small control box (operator panel) you could put Arduino in it. The would drive the CAN bus communications,  4xx20 I²C display, and keyboard, plus and switches etc.
7. So if you go with my suggestion you need two cables in the waterproof cover.
   1. (5 Volts and Ground)
   2. two-wire shielded twisted-pair (50-ohm stuff works fine.

~~ Cris

.

michaelkellett  and of course the PI would handle the GPS.

michaelkellett  and of course the PI would handle the GPS.