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Programable commutator

Discussion in 'General Electronics Discussion' started by Gerard, Jul 10, 2015.

  1. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Gerard, I don't think it would be reliable, on my part, to answer that accurately. That question would vary dramatically just between States in the U.S. and possibly even geographic areas of the same State. It's also going to be vastly different when comparing old seasoned engineers vs. young but talented graduate engineers. Commenting on typical R & D cost in another country would be quite presumptuous of me.

    Do you have a Tech College in your area?

    Chris
     
  2. Gerard

    Gerard

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    Apr 22, 2015
    Yes I do have a Tech College in my area.
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Then that's where you will most probably find the hungriest prospects willing to work with a minimal budget. If your work involves non profit I would approach the instructor. He may see this as a viable team project.

    Chris
     
  4. Gerard

    Gerard

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    Apr 22, 2015
    Hello again,

    It’s been 3 years, and I want to retake the project.

    When I stopped posting I found an engineer in my zone and we talked about the project. What seemed to be kind of simple ended up costing 10k euros. The problem came with the wires, they were very long and needed to be well insulated.

    BUT now I want to go back to the small original project, with 8 electrodes (maybe 10, or maybe 12, the most I can achieve without getting things too complicated) and a few meters of wire, using relays.

    Now I still don’t know how to control that number of relays (say 32, 40 or 48) with a microcontroller.

    I came back here looking for advice on what equipment should I get undertake the project.
    I prefer something common (arduino, raspberry pi, etc.) because I think they will have larger communities to help me on this or other projects I might undertake in the future.

    I’m a beginner but I’m interested on this project because I love geophysics and I think it will be better for me to learn doing something that I find interesting and useful.

    If anyone is joining the theard now, it is about making a geophysics equipment.

    I think that on electronic level it all comes down to control 32 relays, opening 4 at a time in a preprogramed order. Each time a set of relays is activated take an amperage and voltage measurement, save the data, then switch to the next (see video in the first post). To take the measurements the signal has to be conditioned so that it can be taken in the same mictrocontroller.

    I think that’s all about it,
    I’m looking forward for your suggestions and advice!

    Cheers,
    Gerard
     
  5. duke37

    duke37

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    Jan 9, 2011
    A relay needs a positive connection and a negative connection.
    Put your relays in a square so with 16 relays you will have four rows and four columns. Thus eight connctions will be needed.
    25 relays will have 5 rows and 5 columns so 10 connections will be needed.
    The bigger the array, the more efficient this way of connecting is.
    32 relays in 8 blocks will need an array of 4 by two so six connections.

    You may be able to map more than one relay to a single position or use multiple pole relays, I assume that a pair relays are equidistanced from the centre,

    It should be possible to use high current, high voltage fets,instead of relays to eliminate contact sparking. Alternatively use a single fet to switch off the power during the relay switch time.
     
  6. Gerard

    Gerard

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    Apr 22, 2015

    Okay, I think you overestimated my electronics knowledge.

    What I understood from your post is that you can place the relays in matrix, and they need both inputs (negative AND positive) to be activated (and close the interruptor inside the relay). And then you activate them like coordinates in Battleship game.

    That is what I understood:

    Battleship_example.png
    Red are postive connection and blue negative, greed relays are the ones that are closed right now.

    I couldn't descipher that. Don't know what do you mean by the center. Tomorrow I'll study what multiple pole relays are,

    From there I understood that there's a thing called "fet" which is better suited for what I want to do than relays.

    I'm sorry, I need to process things slow, I don't know anything about electronics, starting right now.
    Although I'll take my time to study that stuff later.

    Thanks a lot!

    Gerard
     
  7. BobK

    BobK

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    Jan 5, 2010
    You cannot use a matrix like that in the general case to activate 4 relays.

    Say you want to activate the 4 on the diagonal. You would have to activate all rows and all columns, but this would activate all of the relays in the matrix:

    upload_2019-3-4_15-2-1.png

    Bob
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    It seems to me that what you have here is a multiplicity of earth electrodes, with the requirement to make only two of four possible connections to a particular electrode at any given time: (1) a voltage excitation connection to either the positive or negative power supply terminal and (2) a potential measuring connection to either the positive or negative measuring terminal.

    Each earth electrode could be fitted with a small addressable circuit board containing just four single-pole, normally-open (Form A) contacts on four separate relays. One side of each of the four contacts would connect to the earth electrode. The other side of each of the four contacts would connect to a dedicated wire that connects remotely to (1) the positive power supply terminal, or (2) the negative power supply terminal, or (3) the positive measuring terminal, or (4) the negative measuring terminal. During operation in the field, only two of the relays on any given earth electrode would be energized to determine how voltage is applied to the earth electrode (with respect to a second earth electrode) and how potential is measured at the earth electrode (with respect to a second earth electrode).

    To determine which relays are energized to make measurements, a serial data stream consisting of electrode address and relay configuration would be sent to all the electrodes simultaneously. Only one pair of electrodes would be programmed to respond to this data stream, consisting of power and data on a single pair of wires, along with the four wires used for actual measurements. Note that all wiring for the electrodes is in common. Depending on data acquisition requirements, it may be necessary to add a pair of sense wires to the voltage excitation to obtain constant potentials at the remote earth electrodes, bringing the total number of wires for each electrode to eight.

    This is a minimal, expandable, design with all analog data acquisition functions performed remotely. Each electrode will have a small addressable circuit board containing a microprocessor that monitors the serial data link and turns on the appropriate relays, according to a program the end-user generates and transmits to all earth electrodes simultaneously. This could be implemented as simply as an Arduino linked to a PC for program development, and then operated stand-alone in the field for data acquisition and control. More likely, however, would be to have the PC directly mediate the data acquisition and control through a USB port connected to an Arduino or Raspberry Pi or PICAXE microprocessor. A laptop or tablet PC would probably be necessary to crunch and display data collected in the field anyway, even if the raw data is later downloaded to a desktop PC for analysis and publication.

    By implementing just four relays at each earth electrode, along with the necessary microprocessor serial data link control, the system becomes easily expandable to as many earth electrodes as desired. Since the relays are de-energized and their contacts are open on unused earth electrodes, there should be minimal noise injected into the excitation and measuring system. The circuit boards should be attached to the earth electrodes with copper cable clamps. Inexpensive DB-style connectors could be used to make cable connections back to the motherboard that mediates the data acquisition and control functions.
     
  9. duke37

    duke37

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    Jan 9, 2011
    A fet is a type of transistor with three connections, source, drain and gate. They are polarity sensitive and normally n-type is used. When the gate is connected to the source, the fet will be turned off. When the gate raised by a few volts, the fet will be turned on and will have a very low resistance between source and drain so can be used to replace a relay contact.
    Relays can have several contacts but the fet only one. Relays take significant power to switch on which may cause a problem if it is a mobile system. A relay may be better for voltage detection since the insulation can be very good.
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    I like the idea of using electro-mechanical relays, instead of solid-state switches, for this application. There will only be four relay coils energized at any particular time for a measurement of soil conductivity (or its reciprocal, resistance), with two relay contacts closed on each of two earth electrodes. Relay coil power consumption will be minimal, especially if magnetically-latching bi-stable reed relays are used. Using inexpensive Microchip PIC processors to implement the serial data links and relay selection logic will add almost negligible additional power consumption. I estimate (this is a pure rectal extraction) that 95% of the design effort will be devoted to PIC programming and serial data-link design. However, once the programming is solid (complete and error-free) for just one earth electrode, it will be solid and can be duplicated for as many additional electrodes as required.

    The only thing that changes among the earth electrodes is the address of each individual earth electrode. This address could be specified at start-up and downloaded (via the data link) to non-volatile memory on each PIC, or each individual earth electrode address could be "hard coded" into the application software of each PIC, instantiated when each PIC is programmed. There are many variations on this theme, but the LAST thing the OP needs is a circuit board with the added expense of a bunch of slide-switches (or jumper wires) to specify the earth electrode address. Hopefully, this sort of thing disappeared in the previous century, where AFAIK it was most recently used to "program" residential garage door openers.

    The earth electrodes are presumably separated by some distance, perhaps ranging from less than a meter to several meters. A shielded multi-conductor (six to eight conductors) cable will be mandatory for connecting each earth electrode to a motherboard containing the analog signal wiring. To be determined is what gauge wire will be adequate to excite a pair of earth electrodes. That will depend on how much current is expected to be drawn by the electrode pair, the amount of voltage drop tolerable in the connecting cables, and the spacing between pairs of earth electrodes. Commercial cable exists that contains a pair of heavier gauge wire for power and lighter gauge wire for low-level, low-current signals. And of course it is always possible to use two cables, with appropriate gauge wire in each cable.

    While FETs could certainly be used in place of relay contacts, I think that less design effort is required to implement relay switching, with a PIC microprocessor performing the logic and setup function at each earth electrode. Given how infrequently the relays need to be switched (to configure for a new set of measurements), small magnetically latching reed relays rated for 1000 VDC and a few hundred milliamperes of contact current would be more than adequate, although perhaps a bit pricey. Ordinary reed relays can also be used because the "latching" function is only used to reduce relay coil power consumption in the field. Reed relays are already low-power devices, so eeking out that last bit of wasted energy to minimize their power consumption could be unnecessary.

    It all depends on how the equipment is powered in the field, and how long it remains powered without connection to a mains power source. With re-chargeable LiPo (lithium polymer) cells, a suitably designed, fully-charged, portable system should be able to acquire at least eight hours of unattended data acquisition and ideally a much longer period, perhaps days or even weeks, to allow for measurement changes effected by weather conditions. It's really a matter of how much money you want to throw at rechargeable batteries to determine how much operating time in the field is possible before batteries must be charged again.

    You could add "bells and whistles" fairly easily, some inexpensively, others not so much. For example, active earth electrodes could have an illuminated LED on their circuit board to indicate that their relay contacts have been configured. Perhaps a pair of red and green LEDs could be used to designate positive or negative excitation connection to an earth electrode, and another pair of red and green LEDs could be used to designate whether an earth electrode is being measured with the positive or the negative input to the measuring device. Or perhaps this pair of LEDs could indicate the polarity of the measured earth electrode with respect to the other measured earth electrode. These are all simple, inexpensive, possibilities. Most could be configured during circuit board design for later inclusion in the project if desired. A more expensive whistle would be remote RF monitoring of the motherboard data acquisition, with data sent back to a camp site or motel room. Of course it is always better to "nail down" specifications up-front to prevent "design creep" and over-budget additions.

    As far as cost is concerned, that would depend on factors that we have not considered yet. Cost of parts for, say, eight or ten pairs of earth electrodes, is perhaps ten percent of the overall cost of parts for the data acquisition and control motherboard, whose cost is influenced by the measurement accuracy and excitation accuracy required. There is a "sweet spot" for analog-to-digital conversion that currently is somewhere between twelve and fourteen binary bits for several thousand conversions (samples) per second. Higher resolution and faster conversions jack the cost up rapidly, so it pays to be sure what is necessary for publishable results, not just what is affordable.

    A major cost of this project will be paying for the design effort. I am pretty sure it should require less than US$12,000 and less than a year to complete, but that all depends on the qualifications of the designer. A retired, experienced and interested designer could cost less, overall, than someone who is only interested in the renumeration that is obtained. That all depends on what kind of working relationship is established initially. Good luck finding someone local (ideal) who can do this work, but don't be afraid to look abroad either. There is a lot of good engineering coming out of the former Soviet republics, which is the first place I would look if based in Europe. Don't neglect Asian talent either, although there could be huge communication problems caused by language and cultural differences. Best of all would be if you learned to do the project yourself. There are members here who can help with that.
     
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