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RPM sensor for rotating shaft

Discussion in 'Sensors and Actuators' started by Jeroen Humasol, Nov 8, 2015.

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  1. Jeroen Humasol

    Jeroen Humasol

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    Nov 8, 2015
    Hello everyone,

    I'm a student working on an Electric Load Controller (ELC), for a water turbine in Peru. If you know ELC's already you can skip this part of the post i'll just explain it quickly :)

    (ELC: An ELC is a devise that balances the production and consumation of electrical energy in an isolated power grid. A group of engineering students who went to the peru a few years ago made a water powered turbine (5kW), to give the people of an isolated mountain village acces to electricity (mainly used for lighting). The problem with this turbine is that during daytime little energy is consumed and the generator experiences almost no 'counter torque'. As a result the turbine starts to speed up, hereby generating a huge centrifugal force that tears the turbine apart and greatly reduces the lifespan. An other problem with the turbine and generator when the RPM is to high, is that the voltage on the net goes beyond the required 220V. Therefor we made an ELC. The working of this device is quite simple: it measures the frequency of the AC voltage on the net (should always be 60Hz). When the frequency goes beyond the 60Hz it activates a simple resistance R. This resistance dissipates part of the generated energy as heat. The resistance is cooled by water. When the net frequency goes back to 60Hz the resistance is switched off.)

    A very important part of the ELC is the frequency measurement of the net voltage. Since the net frequency has a linear relationship corresponding to the rotation of the turbine shaft, we would like to somehow measure the RPM of the turbine shaft. The measurements would than be read by an Micro controller which on his turn decides how many resistances (dumploads) to turn on. So here comes my question finally: What methods do you guys advice to measure the shaft frequency? (we would make the sensor ourselves with basic components)

    Some restrictions in order of importancy:
    1) It should have a long lifespan (at least 5 years) and require little if possible, no maintenance.
    2) The shaft vibrates quite a lot, this shouldn't cause any errors
    3) It should be as cheap as possible (altough lifespan is more important)
    4) It should be able to read out it's data to the MC

    Some idea's we had so far:
    1) Use the same system as a cycle computer (they use some sort of little magnet passing a sensor i guess)
    2) Using a black strip on the shaft, in combination with a light sensor somehow, a bit like this youtube video:

    If i forgot any data or you have any questions don't hesitate to ask :)

    Thanks!
     
  2. Minder

    Minder

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    Apr 24, 2015
    There are photo slot sensors such as the H22A1 or the Honeywell SS40 series of hall effect sensors which require a small magnet on the rotating part.
    If there is a protruding ferrous metal part such as a fan blade or gear toothed wheel, a ferrous Hall effect detect sensor could be used. There are many sources for the latter type.
    A CCP capture module on a Picmicro can then be used to capture the input and calculate the count or interval.
    M.
     
  3. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yes I agree with M. I would go for magnetic or optic sensing. I have used a Honeywell opto reflector with a very shiny piece of tape attached to a shaft before to sense the rotational speed.
    Adam
     
  4. Jeroen Humasol

    Jeroen Humasol

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    Nov 8, 2015
    Thanks for the quick responses!
    I have a couple of questions for both options (i'm not an expert in those electric components :) ):
    For the optic sensing:
    1) what did you use as a light source?
    2) Would some vibration of the shaft cause errors in the measurement?

    For the magnetic sensing:
    1) If you use a cheap small permanent magnet what would be about the distance between the Hall sensor and the magnet? (can't be too close because of the vibrations once again)
     
  5. Arouse1973

    Arouse1973 Adam

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    The device has an IR LED and photodiode built in. Let me see if I can find a datasheet.
    Adam
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Why can you not measure the AC output frequency directly? Just use a zero-crossing pulse generator and feed the 120 Hz pulses to a microcontroller that measures their frequency (or, more easily, their period) and controls the "load" resistance accordingly.
     
    CDRIVE likes this.
  7. Arouse1973

    Arouse1973 Adam

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    Jeroen Humasol likes this.
  8. Minder

    Minder

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    The light source is built in but if vibrations are too severe a slot opto may not have enough tolerance on the slot, the other option is the retro-reflective sensor as shown in the hand held unit, these have greater range, the down side is degrading or loss of the retroreflective tape or paint.
    The SS400 style hall sensors are good for around 5-8mm.
    M.
     
    Jeroen Humasol likes this.
  9. Jeroen Humasol

    Jeroen Humasol

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    Nov 8, 2015
    The current ELC's use this method, but the problem is that we're having some serious harmonic distortion. This causes multiple zero-crossings where there should have been only one. As a result the MC thought the frequency was way higher then it actually was and all the dumploads were activated. Now we implemented a digital low pass filter (mainly to filter a third and fifth harmonic), this system appeared to be working fine but after 2 days there was a mechanical breakdown, so it isn't thoroughly tested.

    We would keep on using the digital low pass filter but we would like to have a checkup of the frequency with a real RPM sensor, we hope that this will be more accurate. Another positive aspect of this approach is that if we have one of the systems failing, then we can just rely on the other one.
     
  10. Minder

    Minder

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  11. Jeroen Humasol

    Jeroen Humasol

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    Nov 8, 2015
    Thanks for all the info guys! Really amazing responses and so quickly. i'm gonna take some time to discuss it with my team members, but if you have any other ideas don't hesitate to post them ;)
     
  12. Minder

    Minder

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    CDRIVE likes this.
  13. duke37

    duke37

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    Combine harvestors monitor the speed of several shafts. Those I have seen use the magnetic method, I would prefer this to optical because of the possibility of dirt contamination.

    You could try Massey Harris, New Holland or Claas. Also cars have speed monitors in various places.
    I used a magnet and reed switch to measure a shaft speed up to 1000rpm, this worked for several years without changing the switch.

    The original speed measurement on this machine was a belt driven DC generator giving a voltage proportional to speed.
    Magnets fixed to the shaft would negate the need for a drive system.
     
    Arouse1973 likes this.
  14. Bluejets

    Bluejets

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    I've found if the magnet is too close one can get inaccurate readings.
    Usually 2 to 3 mm works for me but again, the data sheets should show the limits maybe depending on the brand/type.
     
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  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I certainly hope that the heated water is put to good use. For it not to be would be sadly wasteful.

    Chris
     
  16. GPG

    GPG

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    Sep 18, 2015
    What type of turbine?
     
  17. Jeroen Humasol

    Jeroen Humasol

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    Nov 8, 2015
    Sadly enough it's not put to good use at the moment. We're still looking into some good uses for it. We were thinking about smart sockets and stuff like that, but last year there was no time left after the construction of the hydro. If you have any great ideas to use this energy in a small peruvian mountain village don't hesitate to post ;)

    We have 2 pelton turbine's and a crossflow turbine
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I don't see how SmartSockets(R) relates to utilizing the heated water. That's a totally different issue.

    It's difficult to make suggestions without any idea of the logistics involved here. IE: Where is the DumpLoad in relation to the village? If it's not within the village can it be moved there?

    I can tell you this.. I spent over 2 years in the bush where hot showers (I use the term loosely) was an iffy proposition. It was provided by sunlight that heated a battery of 50gal drums mounted atop a wood frame. This system supported a Battalion. During Monsoon season we rarely had showers that I would even call warm. I would think that a similar system backed up and supplemented by the DumpLoad would be a good use.

    Chris
     
    Last edited: Nov 9, 2015
    hevans1944 likes this.
  19. Jeroen Humasol

    Jeroen Humasol

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    Nov 8, 2015
    I just started 3 weeks ago with this project, so i still don't know all the details yet (i'm studying every weekend for it). But the main purpose of this post wasn't the dumploads either. We know that it's silly to just waste the energy but the priority is to have a proper working powered grid. And that we'll be enough of a challenge for now ;) if we have time later on then we will focus on the dumploads.

    If i have some more time then i'll send you some details about the location of the powerhouse (i don't know it by hard yet) and stuff like that.

    Thanks for the showertip anyway i will propose it on our next meeting with the students of last year.
     
    hevans1944 and Arouse1973 like this.
  20. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
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