A new method for measuring lots of resistors using a minimal amount of cables

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daqq
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Posted by daqq: Wed. Aug 23, 2017 - 06:20 PM
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Hi guys, long time no see :-D

 

So, after a long while I have finally finished one of my projects - a new method for measuring lots of resistors ( N*(N-1)/2 ) using only N wires! This means that with 8 wires you can measure up to 28 resistors/resistive sensors. With 16 wires this could be 120 resistors... minus a few reference resistors :-) The application range is a very niche area, where you are extremely constrained by the physical size or cost of extra cabling, such as a microscopic probe with thermistors along its length, or similar. 

 

 

In summary, the method uses a mesh of resistors, connected between N nodes where every node is connected to every other node with a maximum of one resistor - topologically, a complete graph, as shown in the attachment. The measurement system connects the nodes to either one of the at least two excitation voltages (the simplest being GND and VCC) or floats that node. This limits the scope of any one measurement, allowing the creation of a single linear equation describing that particular situation. With enough of the measured combinations (I call them situations), and an addition of special equations describing the reference resistors, an overdetermined system of linear equations can be formed, and solved iteratively. And that's that :-)

 

The method works, has been tried both in simulation and on a real device. And that device is of course an AVR ATMega32 :-)

 

For more details, see the write up on my page:

Theory start: http://www.daqq.eu/?p=1333

Theory continuation: http://www.daqq.eu/?p=1412

Practical implementation: http://www.daqq.eu/?p=1676

 

Coming soon: Software, firmware descriptions and detailed results. 

 

Enjoy!

 

David

Attachment(s): 

Image icon g5970.png
Image icon MeasurementSystem_top.jpg

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Tags:

Learning and Information, General Electronics, #Math, #Atmega32
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clawson
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Posted by clawson: Wed. Aug 23, 2017 - 06:24 PM
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Hey welcome back! Good to hear from you again.

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awneil
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Posted by awneil: Wed. Aug 23, 2017 - 06:40 PM
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daqq wrote:
 a microscopic probe with thermistors along its length, or similar.

having difficulty thinking of things that would be "similar" to such a device!!

 

cheeky

 

 

 a mesh of resistors, connected between N nodes where every node is connected to every other node with a maximum of one resistor - topologically, a complete graph, as shown in the attachment. The measurement system connects the nodes to either one of the at least two excitation voltages

Sounds a bit like Charlieplexing in "reverse" ?

 

 

Jolly impressive.

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awneil
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Posted by awneil: Wed. Aug 23, 2017 - 06:45 PM
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awneil wrote:
Sounds a bit like Charlieplexing in "reverse" ?

I just looked at http://www.daqq.eu/?p=1333 - I see that is where the idea came from!

Top Tips:

  1. How to properly post source code - see: https://www.avrfreaks.net/comment... - also how to properly include images/pictures
  2. "Garbage" characters on a serial terminal are (almost?) invariably due to wrong baud rate - see: https://learn.sparkfun.com/tutorials/serial-communication
  3. Wrong baud rate is usually due to not running at the speed you thought; check by blinking a LED to see if you get the speed you expected
  4. Difference between a crystal, and a crystal oscillator: https://www.avrfreaks.net/comment...
  5. When your question is resolved, mark the solution: https://www.avrfreaks.net/comment...
  6. Beginner's "Getting Started" tips: https://www.avrfreaks.net/comment...
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daqq
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Posted by daqq: Wed. Aug 23, 2017 - 07:36 PM
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awneil wrote:

awneil wrote:
Sounds a bit like Charlieplexing in "reverse" ?

I just looked at http://www.daqq.eu/?p=1333 - I see that is where the idea came from!

Yup! I wondered if I could recycle charlieplexing for this kind of measurement, turns out, you can use some of the concepts, but you have to add some nasty math. 

 

clawson wrote:
Hey welcome back! Good to hear from you again.
Thanks! I'm mostly on the EEVBlog now, mostly since I use AVRs only rarely these days. But I lurk here occasionally :-)

There are pointy haired bald people.
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sparrow2
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Posted by sparrow2: Thu. Aug 24, 2017 - 07:41 AM
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My question is :

IF ADC is real 9 bit (10 bit AVR ADC), how many real bit do you have left, with 28 sensors on a normal micro wirh 8 ADC channels ?

 

 

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daqq
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Posted by daqq: Thu. Aug 24, 2017 - 04:14 PM
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sparrow2 wrote:
IF ADC is real 9 bit (10 bit AVR ADC), how many real bit do you have left, with 28 sensors on a normal micro wirh 8 ADC channels ?
 

Good question. Well, the resolution in this measurement is kinda tricky. There is an insane amount of oversampling, but the resolution is much greater than that of the ADC alone. That said, the error can be significant and there are many factors contributing to it. 

 

Here is a measurement of a static, accurate 10kOhm resistor. 

 

Note that the measured point can be pretty much anywhere (since the outputs are true floating point values), and the distribution is not very gaussian. 

Attachment(s): 

Image icon HighImpedance10k_Refereces.png

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Paulvdh
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Posted by Paulvdh: Mon. Aug 28, 2017 - 02:31 AM
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When I first saw this on hackaday my initial reaction was I can measure thousands of resistors with a DMM and it's 2 leads.

 

then I read some more and qickly concluded that the whole thing goes haywire if a bunch of the resistors are 0 Ohm and quickly lost interest.

This also suggests that the resoluton reatly depends on the values of the resistors in the network.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

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daqq
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Posted by daqq: Mon. Aug 28, 2017 - 06:18 AM
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Paulvdh wrote:
then I read some more and qickly concluded that the whole thing goes haywire if a bunch of the resistors are 0 Ohm and quickly lost interest.
Yup, that is a problem. That said, most measurements go 'haywire' if you short circuit them. Also, if you have a bus with a bunch of resistance measurement devices hooked up to it and you short the bus, you aren't going to get much useful data out of it.

Paulvdh wrote:
This also suggests that the resoluton reatly depends on the values of the resistors in the network.
This is true-ish. As I said, resolution in this kind of measurement is a tricky concept. There is no real concept of how many ohms is an LSB.

 

 

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ossi
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Posted by ossi: Mon. Aug 28, 2017 - 08:59 AM
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Hi daqq,

it somehow reminds me on "impedance tomography"

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daqq
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Posted by daqq: Mon. Aug 28, 2017 - 09:07 AM (Reply to #10)
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ossi wrote:

Hi daqq,

it somehow reminds me on "impedance tomography"

Yup, as was mentioned in the article:

Quote:
The technique bears some similarity to electrical impedance tomography, but is at its core different for being used on discrete parts (resistors) rather then volumes, uses DC excitation and uses reference resistors to enable the measurement of the exact resistance values. Also, it can be used with a single, cheap microcontroller.

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Paddy
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Posted by Paddy: Wed. Dec 13, 2017 - 03:05 PM
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Thanks for documenting and sharing. When using it for measuring thermistors, you could also add an extra option : bipolar measurement. You can use this to eliminate seebeck failures effects. 

 

 

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