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

Re. Sound,

I did think about this a little while ago but forgot about it.

The processor has an on chip DAC with good DMA control  - so we need to add a simple filter and power amplifier

( and a louspeaker) and we can make any sounds you can imagine.

Perhaps Ken can add these to the block diagram.

Re. Quality of connections to the ground and estimating such,

The other point to bear in mind is that the processor is using it's 12 bit ADC to monitor the output voltage from the current source,

so it can make a cycle by cycle (or even faster) assessment of how the current source is coping. To facilitate this there will need to be

a digital signal from the FPGA to trigger the processors ADC. I always try to have a few uncommited signals available between

FPGA and processor for when this sort of thing crops up.

MK

Hi kltm

It will end up looking something like this on the block diagram:

Many thanks!

Great.

I think I'm ready to draw up schematics.

(You might think a formal spec should come first  - but we have a lot of stuff in this thread already and I find

that trying to make a a complete design often helps focus on issues that abstract planning misses.

Think of it as an Agile approach to hardware design And schematics are easily changed.)

I'm wondering if we should start new threads or make a new group for this project.

MK

Hi Michael,

Hehe by random coincidence I was reading last night that it's a valid approach, to have elements of architecture and design co-occurring like you intuitively suggest! it's ISO 42020.

Regarding group/thread I've never created a group on the platform so I'm unsure how it works but sounds like the right thing. Maybe Dudley or cstanton can advise the best method for such a project.

Also, either way if a tag is also used (e.g. some unique codename/project name) and it is added to the new content and to this old thread, then it can be conveniently grouped up by Jive perhaps (like with a custom banner and intro etc, or at least just searchable based on that tag).

Another option is to create a Document. It can be maintained by several people and has versioning.

You can set who has edit right.

Editing it is identical as with a blog post.

If there's a good reason for a group to be made then I can help that to happen.

If the problem is trying to trawl through this entire thread, then there's an URL you can use which helps to display all of the comments, and that is Archaeology Resistivity Meter where ?displayFullThread=true is at the end of the URL.

Hi Christopher

I would welcome your assistance in helping to set up a group for this project please.

Ken

I need to know:

- Who will administrate the group

- Whether it should be publicly visible, or private

- Depending on that, whether or not there is a list of people (links to their profiles or of their usernames are useful) who you want to be able to create content in the group, for example you might want a limited set of people to create discussions and blogs and documents, but only allow the public to comment on them.

- If you have a preference for 'where' the group is listed or created on the site, which will be taken into consideration

This can be sent in a private message.

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

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