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 Dave,

The proposed design meets these requirements, the principle of the design is identical to the EPE article (i.e. the block diagram at the top of this post in Ken's discussion), however the proposed design does the majority of it digitally. The waveform can be a square wave, or any waveform, and the end result will come out the same - just more accurate with a sine-wave since there's less to filter. But, if desired, the waveform can be set to a square wave, with no hardware changes, because the FPGA bitfile is encoded with the waveform (Edit: could even be software configurable in theory). By having this arbitrary capability, there's re-use for other projects/purposes too - for instance, scenarios where a deliberate bias may be wanted (i.e. AC riding on DC) - no problem for this new proposed design just by changing the waveform. (I'm not saying anyone would ever do that, but the possibility exists to do it in code.) So, there's nothing to stop it being put into a mode where you cannot tell the difference in the output waveform from this design, or the EPE design, nor anything stopping the algorithm being identical to the EPE one. The archaeology requirements have to drive the design otherwise it would be useless, so your user requirements are critical.

Similarly, although some of the comments discuss impedance, this is purely a software configuration issue. The hardware can measure and output a value that should match any other meter, but optionally it can be set to measure impedance too. Likewise, at zero cost, the frequency can be changed to one of many values, so 40 Hz square wave is no issue. The impedance could be handy for soil or liquid measurements for home agriculture, but can be totally turned off so that the output from the device via serial will be compatible with Snuffler, and provide the same values that the EPE meter would, since the block diagram and circuit is available for that. As you say, the system will repeatedly take measurements and store when stable.

The proposed design does everything the EPE design does, but hopefully better and with more relevant features, with these requirements that are coming in from you, Ken, Paul and hopefully others.

Dear All,

I have been following this thread and have some concerns about some of the comments.

To give an illustrative example.:-

If you want to design a chair most designs would look similar four legs, padded seat etc. The height would be one of the recommended "preferred" heights and it would have to meet legal requirements for strength and flammability. Given this information, most of us would sketch out similar designs. If one of our designs looked like the "Barcelona Chair" (designed by Mies van der Rohe and Lilly Reich) we should expect legal letters for design theft within a few months of putting them on sale.

I worked for a company that received such a letter and we were outraged that someone would lay claim to our work. After hours of work with our legal advisor we were shocked to discover how difficult it was to prove ownership of our work. Even having an RS catalogue (when they sent print editions out) was enough to suggest that we had the competitors information prior to our design and could have knowingly, or unknowingly, copied it. For the past three years I have been working on what BREXIT and trading on what World Trade Organisation (WTO) terms would mean for us. Its not so much 'WTO' as 'WTF' some of the examples are beyond belief.

For example:-

An American company designed and manufactured Multi meters and imported them into the USA. The meters were impounded by US Customs because a second US Company had convinced them that the colour of the faceplate was synonymous with their product. They had to pay to have them destroyed. The cost of a few weeks storage would have been more than the cost of the shipment and legal costs would have exceeded that within hours.

Many companies are now becoming aware of the value (or at least the cost) of their Intellectual Property (IP) and are willing to protect it. Phrases like "Product X does it this way", "I have downloaded their product information", "They have wired this way" or "We should not do it the way they did" are enough to cast doubt on the originality of your design. I am not even sure that the original magazine articles are not protected by copyright etc.

The Rolling Stones' legal team have written to D. Trump to "Demand" that he stops using their music - he must have bought a CD (or vinyl) but how he can use it is still controlled by the copyright owner.

In short, and probably too late, make sure you can prove the work that you "Open Source" is ALL your own work.

Paul D

In this case the project was initiated by a request from Mr MacDonald to address specific deficiencies – cost and the user interface.

There’s absolutely nothing wrong with a theoretical project, or the entry in a ‘show us a novel way to use XYZ chip’. Similarly there’s nothing wrong with folk throwing ideas because they may spark someone else’s thoughts.

There’s no doubt one could build a mousetrap controlled by an iPad or a Cray – but would it work any better? And would it be suitable to leave in a dark corner of an infrequently-visited room in the house, where it sometimes gets damp and occasionally someone with two legs accidentally stands on the trap? And why not replace the spring with a stepper-motor – that way it could be self-arming (ignoring the fact that a stepper-motor driven arm would have difficulty closing quickly enough before the mouse had walked away again) and yes, it would be multi-purpose, it could be used as a nut-cracker as well.

There’s nothing wrong with a multi-purpose solution – but just remember that a pair of pliers or mole-grips or a Stillson-wrench never does as good a job as a spanner of the correct size.

Dave

To add a little to the remarks from Shabaz and Dave,

The EPE design uses a synchronous rectifier and averages cycles using a switchable time constant of 1, 0.1 or 0.01 seconds.

The Geoscan research RM85 uses a phase sensitive detector (according to its manual) but doesn't say how it's implemented.

The Frobisher doesn't say.

The design approach we have been discussing can operate in all these modes (and others too).

The rationale behind the design is:

Since any instrument will need box, battery, connectors, pcb, controls, display, input and output amps and a processor

- probably costing between £100 and £180, depending on the display

then we may as well spend £30 on the FPGA, ADC and processor and DAC and have all possible options open, rather than £5 and box ourselves in.

This (I hope) makes the project attractive to the largest possible audience, since it offers the opportunity to do some reasonably state of the art electronics

while still offering a useful end result for several applications.

From my point of view there is no interest at all in working on a clone or component update of an old and limited design.

I hope to bring something new and improved to the party -  this is an open source hardware and software effort - so it's open to anyone to tweak, copy, build whatever.

MK

Interesting comments Paul,

Since this is open source with no commercial plans I'm not (very) bothered.

I have instinctively avoided linking or posting extracts of manuals etc and based on your comments I shall definitely avoid doing so.

If any one wants to sue me on the basis that it might enhance my professional reputation at their expense - bring it on

(or maybe they'll sue me for mentioning them in the same post as my own ideas )

More seriously, I feel that it's a positive good to society to seed the world with as much open source IP as possible so that

it becomes harder for companies and IP predators to make the kind of crazy claims they do.

So here's my stake in the ground:

What I claim is a thing for measuring stuff that uses an ADC, a DAC and a processor which might measure the stuff using a software phase sensitive detector or might not.

Thanks.

MK

You are confusing Apple and Microsoft et.al. with small companies and individuals who have invested their time and money (and their house or children's school fund) to produce stuff.

Even Apple and Microsoft et.al. deserve some protection - or who would decide who gets protection and who doesn't?

Even if you "Give" it away you could still get a legal letter - Farnell/Element14 would disappear it to the distance and let you worry about it.

You can "Open Source" your work but not the work of others.

Paul D

Re Mike and Paul’s comments:

Agree that there is a danger in attempting to clone or even reverse-engineer another design, especially if the actual task and domain are imperfectly understood.

I would absolutely advocate not trying to ‘improve’ the EPE designs as, irrespective of copyright issues, they were constrained by components and techniques available at that time.

What is needed is to understand the needs, possibly then some form of gap and opportunity analysis, and then design an exciting instrument that is usable, which potentially could be constructed by amateurs.

Dave

paul_d_arch  wrote:

 

 

For example:-

An American company designed and manufactured Multi meters and imported them into the USA. The meters were impounded by US Customs because a second US Company had convinced them that the colour of the faceplate was synonymous with their product. They had to pay to have them destroyed. The cost of a few weeks storage would have been more than the cost of the shipment and legal costs would have ex...

 

 

Fluke has trade marked the grey-and-yellow look and feel for handheld instruments in the USA.

Either adafruit or sparkfun (I forgot which of the 2) imported meters from China that were designed to closely mimick Fluke's look&feel.

Thats should not be a Ooops. Importing goods that violate a standing trade mark is not allowed since long time.

I the symposium I was at didn't say that the look and feel was a registered trademark (or more likely, I didn't remember).

It's a minefield. I always say respect the rights of others as you hope they would respect yours. The pain always occurs later and the delay always magnifies the expense.

I've consider it to be "bad manners" to copy the work of others. How do you know you are copying the best, or even good stuff?

The companies referred to don't seem big enough to aggressively protect their work (unlike Apple et. al.) but that doesn't seem to be a reason to rip off their ideas/products when you can do it yourself  - if you are willing to do the work

Paul D   

Yes, the old joke is - "if you want to go there, you shouldn't have started from here"

The physics hasn't changed but the technology we have has.

Paul D

Yes, the old joke is - "if you want to go there, you shouldn't have started from here"

The physics hasn't changed but the technology we have has.

Paul D