Archaeology Resistivity Meter

Hi any comments welcome on using Chris Stanton’s proposal to use Maker, Hacker etc. group temporarily, or should we remain as we are?

Ken

We could give it a go.

We could just start a new thread every month the keep the size reasonable.

We could put the project on GitHub Free.

Using a general group for project documents would be a mess.

I rather felt that e14 wanted to encourage this kind of project but it if we use GitHub the project will gain better document management but lose coherence since GitHub isn't a forum in the usual sense.

The reason I've put what otherwise would have been separate documents all in this one thread is to keep them together. In the past projects  have become very hard to follow as different blogs and documents get posted and comments get spread over all of them.

On a purely personal theme I don't really want my work associated with Maker, Hacker, Inventor & Hobbyist labels, all of which have seriously non professional connotations.

Sorry, more questions than answers - I thought (and still think) as dedicated group would be the way to go.

MK

michaelkellett A word about GitHub, yes there are some "Hobbyist" stuff,  there but there are also folks like NASA and other large contributors. Remember the Git is for OpenSource Projects.  Or you can check out Sourcefoge.net It is friendlier

Plus is give you a way to create pages ie j661.sourceforge.net which is the home for ARINC 661 which is an Aircraft Standard for exchange of info and is a server/client model, which both Boeing and Airbus  have imbraced.

~~Cris

Hello Cris

I don't have any problem re. GitHub, my reference to "Maker, Hacker, Inventor & Hobbyist labels"  was entirely in relation to using that group on E14.

I have my own GitHub account which I use for professional stuff - and it's not just for open source, although the Earth Resistance project is.

MK

There is this one: Open Source Hardware

Hello Jan,

Yes - if we can't have our own group I'd rather this one than the MHIH !

MK

Hi Shabaz,

Thanks for feedback

shabaz

wrote:

 

Hi Dave,

 

I can help here, since these are mainly software-related questions:

how the control processor would know when current was actually being delivered

On the 'source' side, there is hardware circuitry to alert if the demanded current cannot be delivered, because the voltage will raise to a limit if that condition ever occurs (barring damaged circuitry - everything has a failure rate, and there can be a test procedure or perhaps even a self-test to identify that - it's quite easy to do that in software, perhaps prompted on the display, prior to using the instrument). The software will receive the alert.

 

and had reached some kind of stable state, so that it could initiate a measurement cycle.

Again, super-easy for the software. All measurement instruments based on source/sense will wait for the reading to be stable by taking multiple measurements rapidly. The precise algorithm is implemented in software.

 

The control process is in a simplified state machine

1)Try to inject commanded current

Understood.

 

2) When current is being satisfactorily delivered, initiate a measurement cycle and beep when completed.

Understood. That means sound capability needs to be added in hardware.

 

If contact with the earth is believed to have taken place but current can't be injected satisfactorily, raise distinctive* audible alarm to user as one of the probes on the frame might have landed on a stone or piece of pottery etc., so the operator will re-plunge to re-attempt the reading.

From your description there doesn't seem to be any relevant (to this requirement) sensing inputs missing compared to existing systems, so there's no risk of a downgrade, only an upgrade (since the behaviour can be better implemented in software). The logic could sense instability or no current flowing in the measurement for a defined period of time, and if either of these occur to raise the distinctive audible alarm. If this doesn't meet your requirement, more input is definitely needed here.

 

3) Monitor current and after period of no current, start again at (1)

Understood.

 

It needs to be a distinctive alarm

Understood. The hardware needs to implement sound capability with some flexibility.

 

not to produce an instrument which uses marvellous and with high, indeed exciting, potential, to emulate an instrument that has limited resolution and would be a downgrade for almost all, if not all, users.

Please can you specifically point to the downgrades so they can be addressed?

 

If this can be brought to fruition it could not only address the issues Ken original posed (usability and cost) but once basic facilities in place to match exiting commercial kit, then there should be scope to open up so many more avenues.

That makes sense, agreed.

Re functionality and usefulness for actual archaeology:

There is great - and eagerly awaited - undoubted opportunity to improve on the performance of equipment currently used for archaeological resistivity survey. In particular, the two aspects identified in the very first post by Ken (price and UI), plus potential for innovative measuring techniques (not just square wave), and ease of expansion (multiplexing, cart-mounting etc.).

The benchmark for actual archaeological detection and discrimination is the commercial kit used by professionals and volunteer groups alike. Whilst the spirit of the two EPE projects cited earlier in this to bring affordable res survey to enthusiasts (and they will, in certain cases, allow you to see 'something') the performance of those EPE instruments is very severely constrained; and any instrument which takes its lead from them may well result in an instrument that may be affordable, and may have a great UI, and may have great expansion potential - but if it doesn't have the fundamentals necessary to inject and measure the current in the range and way that commercial instruments do, then - in terms of archaeological prospection - it will likely be a downgrade as it won't have the same detection abilities in at least the main use cases.

Whilst the UI has improved on some models (not always for the better) and ADCs can be more sensitive, the fundamental measurement techniques haven't. I'm pretty sure that isn't for want of trying by manufacturers either.

Low injection power was a serious shortcoming of those EPE projects. Low-power can deliver results over limited distances, and may work reasonably for self-contained rigs (Wenner, Wenner-α, Wenner-β, double-dipole) - but the main use case for archaeological prospection is the twin-probe scheme with one pair of fixed (for a grid at least) probes and one pair of mobile probes. Those probes can be 50m apart; and commercial kit may need to use 40, 50, 60 or more volts of either polarity to get a usable current to flow. Any rig which can't deliver, say, 10mA at +/- 50v, will severely limit the use of the new instrument. It needs to be able to deliver 10mA with 50 or 60v DC of alternating polarity.

The instrument needs to be able to measure the currently actually being injected C1C2. In the simplest DC square-wave (periodically reversed DC), the current will hopefully stabilise towards the end of the injection period. The voltage measured P1P2 may never appear 'stable' due to noise or stray currents (mains, telluric, electric fencing etc.). The time taken to stabilise is why some kit offers ability to reduce the polarity switch rate to allow longer for the DC current to settle. The control processor needs to know when the current has settled, in order to take P1P2 voltage readings, otherwise its just guesswork and hope (and some early kit did operate that way). If non-square-wave excitation is being tried, it will be even more important to know the instantaneous current actually flowing C1C2. If the instrument has the needed current measurement, then measuring cycles can be optimised and that will also facilitate taking, for example, muxed sets of readings at each point.

Dave

Thanks Michael, some feedback below:

michaelkellett  wrote:

 

Looks good.

Do you think we need a spec next ?

Might be worth hanging on a couple of days to get comments on the block diagram and thoughts so far.

 

Things to think about on spec:

 

Operating temperature range

Endurance on one fully charged battery

Max output current and voltage (if 20mA RMS and nearly 60V pk-pk won't do it's going to get a lot more expensive )

Input common mode range (max +/- 14.5V including signal) ref instrument internal ground.

Max signal combined signal and common mode +/- 29V differential

Input impedance - could be 100M if wanted but I think this is a bad idea - should be lower , maybe 1M - problem is that you want the option

of input caps for AC coupling. If you have 100M input Z then you get -3dB at 10Hz with a 160pF input cap, but its impedance at

150Hz is 6.6M which will make things noisy. I'd like the capacitor impedance to be no more than 50k at 150Hz = 22nF, with 100M input that gives

a time constant of 2.2s so it may take ages to settle, I'm tempted to go for 10M or even lower input resistance to reduce that effect. (NB, the settling time

of the AC coupling RC is nothing to do with the normal settling time of measuring - its to do with how long it takes the system to settle after a large DC

bias is applied to the input.) Ideas / comments etc welcome. (AC coupling will be switchable.)

 

Input protection - static discharge (8kV ?, 16kV ? - full lightning ? (please not !) - any ideas from the field.

 

Operator controls - buttons - membrane key pads are expensive - nice push buttons are also expensive -  any ideas.

Would a touch screen be any good in the field ?

 

Well there's a good collection of random thoughts and comments.

 

MK

Power output: needs to be able to inject chosen constant current (up to at least 10mA and possibly 20mA) at least 50v if not 60 of either polarity into the ground, so in terms of pk-pk 100 to 120v.

Note P1P2 measuring will allow DC option, great, thanks.

Settling times: not sure if I'm fully grasping but the excitation most usually used for archaeological prospection is 137 Hz reversing DC (square wave) so each injection is around 3.5 mS.

Input range: P1P2 measuring needs to allow for the +/- say 60v used for injection, plus margin for that resulting from stray / telluric currents.

Input protection: not certain what would be justified. Have to trust that the user doesn't spear an inadequately-buried mains cable!  One scenario increasingly common in rural areas is presence of electric fencing (typical discharge 7kV upto 25J in 3mS), normally fence network has +ve pulse, any return is via the earth; in theory only return if an animal, standing on the ground, touches the fence; in practice leakage all the time, not so much from poor insulators but from grass etc. touching lower wires. You don't need to be alongside fence-line, return current will take shortest (or at least lowest resistance) path back to energiser's earthing point, so current can flow across fields. Assuming user doesn't actually touch instrument to electric fence, then effect is the extra voltage from return current flow., Have seen about 10v over 50m from this when unwittingly near (other side of the hedge from) an earthing array.

UI: not certain about touch screen - keypad array or buttons may be longer-lived and easier to operate when gloved. I did post earlier about the option, if a 'graphics' or bit-mapped screen was used, that could ease reversal. My suggestion would be a suitable small set of buttons either side of the screen, don't 'hard' label them, but label them as soft-keys as, for example, seen on say a scope screen. Then, if the instrument is reversed to go back down the next line, rotate the screen and hence key labels/functions.

Temp range: suggest aim for same as current commercial instruments.

Dave

Hi Shabaz,

Re:

shabaz  wrote:

...

There was mention of more sense probes, but maybe the expansion interface is sufficient for that, since one could drive (say) a latching relay via that. Or maybe space for on-board relay?

...

Not sure about relays, especially latching ones...

There are two probe-switching scenarios. The much-less frequent use case 2, a static line of probes used to 'sound' and derive a cross-section, uses 'N' where N maybe a dozen or twenty probes, and that is the scenario where I was suggesting the interface allow driving an external mux (as various probes will be used as the C and P probes in turn). That could use relays, but they would be in the expansion mux, not the main instrument.

The on-frame scenario in the most frequent use case (use case 1) would be muxing between probes on the frame; this would be within a small number, perhaps two or three. All of the measurements would be done in time that the frame is in the ground and in the normal rhythm of using a res frame (commercial auto-triggered, not EPE style) the frame may only be in the ground for 500mS, so for each of the, say, 800 points per grid, those relays would cycle quite a lot!

Dave

WRT the maximum output voltage, right now I'm proposing differential output from amplifier using +/- 15V supplies, giving output voltages of 30Vpk or 60V pk-pk.

If you are correct that in excess of 60V pk (120V pk-pk) may be required this raises serious safety issues. While there is no clear single accepted standard for maximum safe

voltage on exposed terminals there is no doubt whatsoever that 60V can be lethal. The implication of this is that if the possibility exists that 60V or more appears between the output

terminals the current must be limited (even under fault conditions) to less than 10 mA.

This is not impossible but it is difficult and expensive - and arguably not a suitable project for home building.

On the other hand - it almost certainly is not necessary - the measuring technique proposed is fully capable of accurate measurements operating at -60db signal to noise ratio,

(ie operating correctly when the noise is 1000x or more bigger than the wanted signal.)

I'm thinking that the best route to get this thing to fly is to get quickly to a prototype and I certainly think it should stick with 60V pk-pk since this is simple and totally safe.

This gives twice the voltage of the Beck design (and is comparable with some commercial offerings (in so much as their specs make any sense)).

If necessary a higher voltage output power amplifier could be designed but I don't think we should kick off with that.

It would also complicate the design of the input stage which would then need high impedance switchable attenuators (more relays).

The instrument needs to be able to measure the currently actually being injected C1C2. In the simplest DC square-wave (periodically reversed DC), the current will hopefully stabilise towards the end of the injection period. The voltage measured P1P2 may never appear 'stable' due to noise or stray currents (mains, telluric, electric fencing etc.). The time taken to stabilise is why some kit offers ability to reduce the polarity switch rate to allow longer for the DC current to settle. The control processor needs to know when the current has settled, in order to take P1P2 voltage readings, otherwise its just guesswork and hope (and some early kit did operate that way). If non-square-wave excitation is being tried, it will be even more important to know the instantaneous current actually flowing C1C2. If the instrument has the needed current measurement, then measuring cycles can be optimised and that will also facilitate taking, for example, muxed sets of readings at each point.

A current source will force the current to be what you set (unless it runs out of voltage compliance).

Try running the TINA model for the current source that I've already posted (TINA is a free download from TI) .

If the voltage between P1P2 is not stable, and the current is stable then one thing that is definite is that you are not measuring current though a resistor.

Ohm's Law: I = V/R.

The measuring technique you suggest offers no discrimination against interference at all  - synchronous and  phase sensitive detectors just don't work like that. All (and it really is all) rational detectors require that measurements be made over multiple cycles and the discrimination against interference signals at other frequencies increases the more cycles are used per measurement.

(I think I did post some fairly clear models and explanation of this before).

MK