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Programable commutator

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

  1. Gerard

    Gerard

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    Apr 22, 2015
    Hello,
    First I want to introduce myself and my project. I'm a geologist from Spain and i trying to make a geophysics equipment, but I need to do some wierd switching in my equipment and I don't know how to do it. It's hard to explain but I think the video explains it better. I'm sorry about the text in Spanish but it takes a lot of time to redo the whole gif thing.

    The circuit works at 220 DC; the geophysics I'm doing requires high potential, so I take 12V DC battery and I use an regular inverter to boost the voltage to 220V AC, and I need direct current, so I use a rectifier bridge to put it back to DC.

    There's 4 electrodes plugged on the ground (Because I want to measure resisitivity of the ground). one of them injects the current and other closes the circuit. Between those 2 there's another 2 mesuring the voltage. Then I need to switch those electrodes to take data at different depths and diferent sites, and there is where the problem comes. I have no idea of what could I use to achieve that. The sequence of switching is the one on the video. If anyone is interested this is called earth resisitivity imaging (ERI).



    If you have any doubts just ask them freely.
    Thanks a lot,
    Gerard
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Bienvenido a electronicspoint, Gerard.

    What you want to achieve is probably not too difficult to achieve. A set of relays controlled by a microcontroller should be able to do as you require. Would you be comfortable programming a micorcontroller (e.g. Arduino) or do you know someone who could do this for you? The microcontroller would be programmed to control the relays in the required sequence. It also can be used to log measurement results during the sequence. A set of relays can be connected to the microcontroller using either a rather simple self-made circuit or using an off the shelf relay board (called "shield" in the case of an Arduino).
    The microcontroller can be started from a PC and relay the results to a PC usinf a serial communication port.

    Alternatively you can use a relay card that is directly controlled from the PC (so you don't need the microcontroller), e.g. as found here. YOu will have to write your own piece of software to control the relays from the PC.
     
  3. Gerard

    Gerard

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    Apr 22, 2015
    I don't know how to program a microcontroller, but I could find someone to do it (or maybe learn how to do it). If I understood what you said, if I need all of the 12 electrodes to be plugged to the 4 outputs, that is 48 relays, and then the idea is to open/close the relays on a configured arrangement so they do just like in the video, so I need a microcontroller with 48 outputs.

    What I have to buy to do it is 3 relay boards of 16 relays each and a 48 (analog?) outputs microcontroller? Was that your idea or this is then connected a diferent way to use less relays?

    I don't understand how to log the measurement results with the microcontroller, I was going to use an AD converter that would take measurements each 0.5 seconds, so just measure everything as a bulk, but controlling it seems a promising idea (so that I can know which measurement corresponds to each array of electrodes).

    Also, are the relays ready to handle 220V? Most of what I have seen looking for them on the network were like 12V or 24V, I'm afraid of burning them.

    Sorry for so much questions, I don't know much about electronics.

    Thanks for such a fast answer!
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,069
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    Nov 17, 2011
    Not necessarily. The boards I linked are controlled via a serial port. This means you have to send commands via a serial port (or USB, depending on the exact type of board) to activate and deactivate the relays in the required sequence.

    Yes, 48 relays will be required.
    No, a controller with 48 outputs is not required, see above.

    How would you control the AD converter and read its measurements? The AD converter alone will not be helpful, you most likely will have to use some analog electronics to prepare the signal from the electrodes to be suitable for measurement by the AD converter (that's called signal conditioning).

    220V relays are nop issue at all. They are commonly used in household appliances. Just make sure the relay board you use has the right relays on it. Do not attempt to relays with a lower rating (12V, 24V, 48V etc.) with 220V. This can be lethal as the relays are not designed for such a voltage and not only can break down but injure persons.
     
  5. Gerard

    Gerard

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    Apr 22, 2015
    Thanks Harald I really appreciate all of this altruistic help, and I'm really sorry but I have to ask more questions because I can't find the information about this by myself.

    I can buy 48 relays but I don't have enough serial ports (and even less on my field laptop computer) to plug them all at the same time, so I can't imagine the circuit I have to design to actually control them all. That's why I prefer the microcontroller.

    All examples that I could find around the network work with like eight relays, For me it would be easier to find a controller with 48 outputs (does that exist? Arduino mega maybe?) I would be really grateful you if you told me what system I do need (or what specs or what pieces or so).

    Most of relays are 12V DC and 220V AC but I use 220 DC. The guy on the shop told me it would be also okay.

    I was thinking of plugging the AD converter to the voltmeter and to the ammeter, but I'm not sure if that is possible. If I can't find a way to this I can just write up the results on the screens (although it's really vulgar)

    Thanks a lot again, you're really helping me to progress on this matter.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    9,069
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    Nov 17, 2011
    You don't plug the relays into the serial port. You will need a controller board for the relays, follow the link in my post #2. If the board has 48 relays (see below), you need only one serial connection to the PC.

    48 channel relay boards do exist, e.g. this one. Unfortunately thís one is rated at max. 120V only. I'm sure you can find ones for 220V, to. The arduino, even Mega, does not provide 48 outputs, even less relays.

    You have to distinguish between the coil voltage (12V is rather common here) and the contact voltage (e.g. 220V AC).
    220V AC is equivalent to a peak voltage of 220*sqrt(2)V=311V. THis does not, however, mean that you can use this relay with 220V DC. The DC rating of a relay is often way less than the AC rating. The reason is the electrical arc across the contacts which will extinguish by itself with AC due to the zero crossings of the sine wave of the current, but this arc will stay on with DC because there are no zero crossings. It mainly depends on the current you're switching. Small currents can be switched at higher voltages than high currents. The datasheet of the relay will tell you the details.

    That is not the way an AD converter is used. A digital meter (for voltage or current) already has an AD converter plus the logic to display the measurement. If you're lucky, your meter(s) also have a serial connection so you can control them and read results via a PC. If not, you will need a so called "data acquisition" card which ideally also has the required signal conditioning circuitry.
    What are the voltages and currents you want to measure?
    Which instruments are available to you?
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Periodically we get an interesting project that falls outside the typical (ho - hum) run of the mill. This one promises to be one of them.

    Gerard, you strike me as a quick study. If you want to be in total control of your project and do it in the shortest possible time I would suggest you look into PICAXE.
    Picaxe PICs are the absolute easiest PICs to learn. The software and integrated pdf manuals are FREE. You don't even need an RS232 Serial Port on your PC, as they sell a nice compact USB/RS232 converter cable at a very reasonable price.

    The benefits of programming yourself will be many because you will understand and be able to modify your code at will. Even in the field!

    Chris
     
  8. Gerard

    Gerard

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    Apr 22, 2015
    Thanks both for your answers, and sorry for taking so long to answer, I have been busy.

    I think I can summarize my actual problems in 2:

    1. The number of relays I need to control. I have been thinking, and if I have to do the equipment I will need more electrodes, and that means a lot more relays (I calculated like 128). I can get the relays, the problem will be what to control them with. I need enough outputs to control them all. I have been browsing the network, and as there's no microcontroller that haves so many exits (Hey, if there's one please correct me, I'm wishing there's one), the ideas I have found is to use several microcontrollers, one controlling over the other in a pyramidal way. I don't even know if that is possible, if it would be, that would be the only solution that I have found to problem 1 till now.

    2. The way to take measures. This one seems a lot easier. It seems like microcontrollers have to capability to measure voltage. So I can measure voltage fitting it in the 0 - 5 volt range with a simple OpAmp circuit or a voltage divider maybe. To measure current I will need a very low ohmic value resistor, and just measure the voltage it uses and simply calculate the current. This might also need conditioning. The problem 2 is almost solved IF my speculations and the things I found in internet are good, but I need your aprovation (basically because I have no idea about electricity).

    Also I didn't know my project was out of the mill, I'm happy to hear it's interesting to you.

    Thanks a lot,
    Gerard
     
  9. Gerard

    Gerard

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    Apr 22, 2015
    Bump.

    Are relays the only way to do this? Commercial systems control over 60 electrodes, which is like 240 relays. Are there commercially availible controller boards for so many relays? (this is a yes or no question)
    - Do I need a computer to control relay controller boards or I could do with a microcontroller?
    - What in the techincal specification do I have to search to know if a relay will be able to work on 220V DC and 2A?

    Thanks!
    Gerard
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Power MOSFETs are commonly used to do the work that relays once did.

    Considering that you're now talking about 240 outputs I'm wondering if a Stepper Motor connected to a 240 pole radial commutator would be more practical?

    Cheers,
    Chris
     
  11. GPG

    GPG

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    Sep 18, 2015
    If you reduce the supply there are some analog switch ICs that will handle ~ 200 v
     
  12. Gerard

    Gerard

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    Apr 22, 2015
    Chris, I'm talking about 240 outputs but 4 inputs. I have tried to find how a radial commutator works and I didn't find it, but the way I imagine it works I would actually need 4 Stepper Motors connected to four 60 pole radial commutators. Also are those motors programable to jump to a given position or how does it work?

    I have been searching what power MOSFETs do and I found out it's a weird device that in some circumstances can act as a relay, so I don't understand the purpose of using power MOSFETs instead of relays.

    GPG about your suggestion of using analog switches (I don't know what IC is, can't find it out), that was my original idea, but I don't understand how do multiplexers and so work. Can you control/program them and they do the same job as relays but smaller?


    How many relays can I realitiscally control at the same time without spending over 1000€?
     
  13. Gerard

    Gerard

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    Apr 22, 2015
    I'm sorry, I think I understand how analog switches work, but I don't know much about electronics (yeah, I'm in the wrong project). I need to physically imagine how to plug it all and make it work. Could you send me some kind of example/tutorial on how to control high channel count with analog switches?
    I mean how do you plug them into your system? How do you control them? Via microcontroller?

    It's just I see those black boxes with pins that claim to act as switchers but if I would have one in my hands I wouldn't know how to work with it.

    In the other hand it seems like a more compact option versus the bulky relay system. I guess lowering the voltage a bit wouldn't be a problem, althought the higher to voltage versus intensity, the more precise the measuraments are.

    Thanks a lot for the altruistic help!
    Gerard
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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  15. Gerard

    Gerard

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    Apr 22, 2015
    Ok, I think I kinda have an idea:

    I'm gonna use multiplexers on regular 12V (or whatever arduino uses, I have no clue) to be able to control 240 (or whatever the final number I choose, probably less) power MOSFETs. That will allow to have a control system working at 12V and an external system working at whatever the voltage and amperage MOSFETs (or relays) can handle

    I am very interested on controlling it via microcontroller instead of USB because I want the system to be portable (rather than carrying a computer around).

    Now can you tell me if the above idea it sounds like a viable plan to you? I don't know much about arduino, muxes, and power MOSFETs, so it might be impossible for some reason I don't know. I need to have a plan before I start studying electronics deeply and buying things.

    I'll be expecting your answer!
    This advice would cost me some money outside here, so thanks a lot again to you both.

    Cheers,
    Gerard
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Your project is anything but a novice project. So before we go any further may I ask who is going to prototype it and who is going to build it? This would not be trivial project for even the seasoned veterans here.

    We have some members that will do contract work, including making PC Boards. IMHO you should seek them out. See if Bob K. is interested.

    Cheers,
    Chris
     
  17. Gerard

    Gerard

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    Apr 22, 2015
    Okay Chris, sorry for the delay. After your last reply I had to wonder about it for a while. I asked Bob K and he's not interested, can you suggest me more people to contact with?
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I suggested Bob because he's skilled and has been known to do contract work. That said, Bob has aged along with many of us here and there comes a time when you don't want the pressures of a major development project. So, I do understand Bob's reluctance. This is, as I've stated, not a trivial undertaking. I must admit that I myself have little desire, even with $$$$ as incentive, to leave my comfortable retirement and reenter the development field. I'm enjoying the bliss of screwing off! :)

    Perhaps this post will prompt other young, hungry and skilled members to jump in. We do have a deep well. .... Anyone???

    Chris
     
  19. Gerard

    Gerard

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    Apr 22, 2015
    Do you have any idea of aproximately how much should I expect this to cost? I'm trying to find a professional to develop it around my area, and I would like to know what would be a fair price.

    thanks again,

    Gerard
     
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