Archaeology Resistivity Meter

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 in using our current kit we have on the display as soon as the probes are lifted 'PLACE PROBES AT ...' The unit is sensing that there is no contact with the soil on the current probe.

Hi Ken,

That makes sense, that's a fairly straightforward condition to detect.

However, I'm wondering if a sensor should be added to assist, It's a cheap (in terms of hardware) thing to attach, to get better sensing of the physical state of the equipment (e.g. if it is physically moving or raised). Hi michaelkellett it sounds like a MPU-9050 (or similar, since I think that's obsolete) might be useful : ( or at least easy-to-access pins on the I2C bus. A bit of feature creep though.

EDIT: just to throw another idea out there, maybe it is better to just have a generic socket, e.g. PMOD or (perhaps even better since there's a huge range) Mikro-E Click, rather than a MPU.. sensor. Then people can experiment with add-ons such as additional sensors to adapt the design further.

Re headroom in the current injection:

Instruments typically offer range of constant-currents to inject - such as 0.1mA, 1mA, 10mA.

When you are about to start actual measuring, best practice is to take a couple of test readings at the most distant points of the (typically three) grids that you intend to measure from one C1P1 fixed probe location, and effectively check that you're maybe not using more than, say half your range to achieve that. Hopefully then you won't hit a range problem mid-grid.

Just to answer Michael's question about changing the injection current during a survey.

It is absolutely a fact of life that if you survey a non-trivial area, conditions will change. What you strive to do is to ensure that any changes are between grids, rather than mid-grid. The normal/expected discontinuity occurs when you relocate C1P1 after three grids or whatever. (Good) processing software will look for step-discontinuities between grids and will adjust them out. The not-uncommon error is that the operators forget to advance C1P1 in time so they run out of cable part-way through a grid. If you just relocate them, the readings are inevitably different due to distances and also possibly different soil conditions. You can then see operators doing the hokey-cokey with the probes - one watching the res on the screen whilst their oppo moves the probes in and out; or sometimes the operator will try (if the machine allows) try fiddling the calibration. The problem is that a mid-grid change in measured resistivity is usually associated with an underground change, so you can't risk auto-adjusting it; whereas a similar change between all twenty or forty points on each side of a grid boundary are usually associated with a change in the reference conditions.  That is usually C1P1 relocation, but could also be a change in injected current.

I have seen a change mid-grid due to the operator running out of range, and it was repeatable; the effect wasn't enormous, and if the data points also all log the conditions such as the constant-current used then the discontinuity could be a documented/visible event - but it would be better if it didn't occur; and if it did occur, it shouldn't be a silent auto-ranging, the UI should prompt the operator along the lines of 'Max range exceeded - change current now mid-grid? or change current and re-start the grid?'

Dave

shabaz  wrote:

 

Hi Ken,

 

That makes sense, that's a fairly straightforward condition to detect.

However, I'm wondering if a sensor should be added to assist, It's a cheap (in terms of hardware) thing to attach, to get better sensing of the physical state of the equipment (e.g. if it is physically moving or raised). Hi michaelkellett  it sounds like a MPU-9050 (or similar, since I think that's obsolete) might be useful : ( or at least easy-to-access pins on the I2C bus. A bit of feature creep though.

Shabaz/Ken,

Its not an instrument dynamics device or sensor, its whether or not current is passing C1 to C2 ...

Dave

Hi Dave,

That's understood. This isn't to replace detection using the probes. It's about the potential to make the software more intelligent about precisely when it sounds an alert, for instance if the user decides to take a few seconds longer before setting the instrument down.

Yes agreed, but as Shabaz says a fairly easy condition to detect.

Ken

I have other instruments which make sensitive measurements, and (since I'm working on a desk and not in a field) I end up having to use rigid wire so that even the cabling doesn't move more than required to get the tests done. A movement or change in the bend of the wire causes a detectable difference for those types of measurements, partially because reactance changes.

In the field, perhaps some of this can also be reduced, by (say) having an alert at the beginning to advise the user to verify the location of C1P1 is adequate and to use the specified wire technique (for instance not to loop the spare cable in your hand for part of the grid and not the rest of the grid), and so on, i.e. assist in training the user based on whatever best practices you have. Anyway, some of this can come later when the software or user procedures are documented by someone.

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!