Archaeology Resistivity Meter

Great thank you.

Hello Ken,

mk2 block diagram.

Several changes,

The DAC is driven by the FPGA

The ADC has changed type to a TI single channel audio type (much cheaper (about £3.30) and OK for our purpose I think.)

The ESP chip provides Bluetooth and WiFi, chosen because its widely available, dead cheap and there is a lot of app material on the web.

The battery type is defined as Sony NP-F970 equivalent, 5AH gives about 35Whrs, and I estimate a WiFi and display on power

consumption of 3W so the available batteries should give at least 10 hours per charge. I bought 2 and a charger for about £40.

The display is ideally a Riverdi 4.3" which uses the Bridgetek controller and has SPI interface. You can get a bare bones one for about £36

but the nice one with the touch screeen and flat glass front (like the PI display) is about £50. Or you can use a sub £5 quarter VGA type from

China - but I won't be writing code for it

Any illegible and unguessable features, please ask.

MK

Great, thanks.

Might be worth adding:

the ADC type - PCM4201

note "overload monitor" on the analogue signals going into the level shifters from the input and output amps (I forgot them on the second diagram)

MK

Hi Ken and Michael,

Nice diagram!

Michael,

Quick observation on the block diagram, ADC monitors voltage probes, but one thing that I cannot spot is provision for measuring/monitoring the injected current? If running in mode of conventional resistivity kit (square wave with constant current) then need a way to set and then monitor; if square wave with constant voltage, or a non-square-wave other waveform, need to continuously measure current to compute res?

Dave

Well, sort of yes and no, the output amplifier is a current source, (see posts 42 - 44 in this thread), so the amplifier input is a voltage from the DAC and low pass filter

but it forces a current, proportional to the input voltage, into the load. If you try an force too much current the amplifiers might not be able to generate a high enough output

voltage, which is why there is voltage feedback to the processor analogue ports (which will use the processors rough and ready ADC to keep a check that the voltage

isn't too high).

So the output current is continuously referred to the demand voltage, by the nature of the feedback arrangement of the amplifier. (If you are interested in looking more deeply into this

try Googling "Howland Current Pump", my circuit is a differential version of that venerable design.)

The block diagram shows a digital gain control from the processor to the output amplifier, by switching the current sense resistors and  varying the codes to the DAC we will be able

to set the current anywhere between 20 mA and 50uA with reasonable accuracy and good repeatability.

MK

Michael,

Re ability to know the current to calculate resistance/impedance. Thanks for the explanation, so the current isn't explicitly being measured but constant, or at least predicted current, is being supplied by the output stage in accordance with DAC voltage, and there is monitoring by the processor's ADC to see that the voltage doesn't get too high. What would happen when the processor detects such a too-high voltage?

Thanks

Dave

Hi Dave,

Once the high-voltage is detected, it could flag an alert on the display (or sound an audible alert, I guess that needs to be on the block diagram if there is a requirement for this), to indicate that there is likely a probe fault, or a lower current setting may be required, because that condition should only occur when the probes are not in the soil, or all other reasons for the resistance being too high, making it not possible to pass enough current, reaching the supply limits.

The current is measured as explicitly as in any other system, in the circuit in thread 44 it is sensed by R3 and R8, currrent in the load has to flow through these resistors.

The feedback is applied immediately and continuously to adjust the drive voltage to maintain the current.

If the amplifier runs out of voltage "headroom" this is detected by the voltage monitoring and the operator can be warned, the measurement tagged as an error (if it was taken)

or the current demand adjusted.

The Beck meter uses a similar system with much simpler (and less accurate) current sources. It compares the max output voltage detected with a reference and illuminates

an LED if the output is too high. All the operator can do is change range - and reduce the current by a factor of 10.

There is a potential problem with this or any other system that if the current is set too high at the start the amplifier may run out of headroom in a different position. The

operator will have to balance the lower noise of a large current against the risk of having to reduce it paert way through a series of measurements.

I don't know how comparable resistance reaadings at different currents are.

MK

Hi We had an old commercial meter, but I can't for the life of me remember the brand. It was very similar to the Beck design, except it had a built in logger. It had the red fault light, just as you describe. It would always illuminate with no contact on the probes, but would go out when soil contact was made. If it didn't we had to adjust the current. This was never a good idea half way through a grid as it would change the calc. Our current meter has no controls for setting the current, so I can only assume there is some form of auto ranging. We do occasionally get out of range reading warnings and nothing is recorded. In this instance you just try again or mark the grid location as a dummy.

shabaz  wrote:

 

Hi Dave,

 

Once the high-voltage is detected, it could flag an alert on the display (or sound an audible alert, I guess that needs to be on the block diagram if there is a requirement for this), to indicate that there is likely a probe fault, or a lower current setting may be required, because that condition should only occur when the probes are not in the soil, or all other reasons for the resistance being too high, making it not possible to pass enough current, reaching the supply limits.

Thanks Shabaz,

This is one aspect where I fear either I don't understand of what's being proposed from an abstract basis, or that there is a gap.

The survey mechanism in the main use case (manually advanced frame) (see post 161 for more details).

1) The mobiles probes are inserted into the ground by dropping the frame

2) The instrument takes a reading

3) The instrument beeps to indicate to the operator the reading has been taken

4) The operator lifts the frame clear of the ground, advances to the next location and - if not at end of a grid - drops in the next location as at step 1.

So, the measurement cycle is probes in ground for perhaps 1/2 to at most 1 second; then 'handfuls' of seconds as the frame is lifted, re-positioned and re-plunged into the earth. There is a meaningful measurement window of say 0.7 seconds, then say 3-4 seconds before the frame is in contact with the earth again. During descent through the vegetation, there will be intermittent contact, and similarly during the extraction after a measurement. So, there will a period of totally open circuit (possibly the majority of the time), a period of noisy connection on the way down, a period in the earth to take a measurement, and then another noisy connection as the frame is lifted. (if the use case is a wheeled or other cart, the same phases will apply but maybe not the exact same timings).

If an alarm sounds every time the voltage maxes out then the alarm will only be silent when the probes are in the earth and are making a valid measurement.

Dave

michaelkellett  wrote:

 

The current is measured as explicitly as in any other system, in the circuit in thread 44 it is sensed by R3 and R8, currrent in the load has to flow through these resistors.

The feedback is applied immediately and continuously to adjust the drive voltage to maintain the current.

If the amplifier runs out of voltage "headroom" this is detected by the voltage monitoring and the operator can be warned, the measurement tagged as an error (if it was taken)

or the current demand adjusted.

The Beck meter uses a similar system with much simpler (and less accurate) current sources. It compares the max output voltage detected with a reference and illuminates

an LED if the output is too high. All the operator can do is change range - and reduce the current by a factor of 10.

There is a potential problem with this or any other system that if the current is set too high at the start the amplifier may run out of headroom in a different position. The

operator will have to balance the lower noise of a large current against the risk of having to reduce it paert way through a series of measurements.

I don't know how comparable resistance reaadings at different currents are.

 

MK

Thanks Michael.

OK, I understand the V=>I output stage has feedback, fully understand. What I was trying to understand was how the control processor would know when current was actually being delivered and had reached some kind of stable state, so that it could initiate a measurement cycle.

The control process is in a simplified state machine

1)Try to inject commanded current

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

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

You really really don't want to just tag reading as an error in the data as if you go back you really need to repeat the whole grid as ground conditions will almost certainly have changed.

* It needs to be a distinctive alarm as the operator is used to dropping the frame, hearing a beep, lifting it and moving on. In using current commercial kit, the alarm may not be that different so the first you know is the operator gets to the end of a line and instead of getting a double-beep to indicate end-of-line, you only get one beep. You don't know which one was duff, and subsequent ones may well have been offset, so you have to delete the line, walk back to the start and repeat the line. The problem is that muscle memory takes over, it may well be the two thousand three hundred and twenty seventh time that afternoon...

More on headroom in a moment, but I would urge 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. 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.

Dave

michaelkellett  wrote:

 

The current is measured as explicitly as in any other system, in the circuit in thread 44 it is sensed by R3 and R8, currrent in the load has to flow through these resistors.

The feedback is applied immediately and continuously to adjust the drive voltage to maintain the current.

If the amplifier runs out of voltage "headroom" this is detected by the voltage monitoring and the operator can be warned, the measurement tagged as an error (if it was taken)

or the current demand adjusted.

The Beck meter uses a similar system with much simpler (and less accurate) current sources. It compares the max output voltage detected with a reference and illuminates

an LED if the output is too high. All the operator can do is change range - and reduce the current by a factor of 10.

There is a potential problem with this or any other system that if the current is set too high at the start the amplifier may run out of headroom in a different position. The

operator will have to balance the lower noise of a large current against the risk of having to reduce it paert way through a series of measurements.

I don't know how comparable resistance reaadings at different currents are.

 

MK

Thanks Michael.

OK, I understand the V=>I output stage has feedback, fully understand. What I was trying to understand was how the control processor would know when current was actually being delivered and had reached some kind of stable state, so that it could initiate a measurement cycle.

The control process is in a simplified state machine

1)Try to inject commanded current

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

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

You really really don't want to just tag reading as an error in the data as if you go back you really need to repeat the whole grid as ground conditions will almost certainly have changed.

* It needs to be a distinctive alarm as the operator is used to dropping the frame, hearing a beep, lifting it and moving on. In using current commercial kit, the alarm may not be that different so the first you know is the operator gets to the end of a line and instead of getting a double-beep to indicate end-of-line, you only get one beep. You don't know which one was duff, and subsequent ones may well have been offset, so you have to delete the line, walk back to the start and repeat the line. The problem is that muscle memory takes over, it may well be the two thousand three hundred and twenty seventh time that afternoon...

More on headroom in a moment, but I would urge 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. 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.

Dave

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.

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