Measuring the Shaft speed of a motor

Discussion in 'Homework Help' started by __alex__9, Apr 30, 2011.

  1. __alex__9

    Thread Starter New Member

    Apr 30, 2011
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    For this piece of coursework i've been asked to purely design a 'monitoring and measuring system' for the speed of a drive motor. This system would need an item to display the speed and then a method of warning when the speed exceeds a set level. I will not be making this, all i need to do is to understand what is needed etc to a reasonable level.

    I'm not exactly an electronics whizz, but i did have a few ideas as to what needed to be included. However there are gaps in areas i am not 100% sure about, so if i've missed anything please tell me!

    The main idea I had reveloved around a 'tachogenerator', where you attach a generator to the shaft, this rotates and gives a voltage out, you do this at a number of speeds and produce a set of calibration data which can be used to compare voltage to shaft speed.

    This generator would then be in line with a A/D converter that converts this analogue signal (voltage) into a digital value. - I'm not 100% sure how this exactly fits in to the circuit?

    There could then be a pre-programmed pic chip that can take this digital value and output it as a number on an 8 bit display or something similar, and when the value is higher than a set level this could then set off an LED warning display and a simple 'warning' on a display screen or something along those lines. - I uncerstand that the pic chip can be programmed using 'circuit wizard,' which uses a flow chart like format, but i'm not sure exactly what i would need to put in to make this work

    I think that is all i should need, but like i've said above i'm not great by any extent with electronics etc so i would very much appreciate if when i've missed something could someone please point it out, or even if its giving me some more details on a particular part that i'm unsure of or have got wrong in my own understanding. Also if someone could tidy this up into a more neat example of what i mean i would really much appreciate it.

    Or alternatively if you have your own idea for a system that would work i would be very interested to hear it!

    Thanks guys,

    Alex~
     
    Last edited: Apr 30, 2011
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    To what extent do you plan to design this yourself? do you have to construct the tachometer? do you actually have to or want to design the circuit? If you are looking for an easy out, there are ready made solutions for this I can tell you about if you are interested.
     
  3. __alex__9

    Thread Starter New Member

    Apr 30, 2011
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    No, I have to design this thing myself, its a pure design piece and i have no plans on constructing it, i've just got to understand to a reasonable level how this thing works, what it needs to contain and that type of thing. Sadly i don't think i can take the easy out, but these ready solutions may be interesting to see
     
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Then I would reccommend rather than using an analog tachometer which is expensive to buy and time consuming/difficult to make, that you use a rotary encoder. Interface this encoder to a microcontroller. The encoder will give you a certain number of pulses for 1 revolution, say 100 pulses for example. You can write a simple program in the microcontroller to scale the number of pulses recieved in a given time period to the desired speed units (RPM assumptively, or MPH if it's in a vehicle, or wind speed if it's a fan, whatever) and transmit this scaled value to an LCD. Should be fairly easy especially if you have a little programming experience. The encoder can also indicate (via programming) what direction the motor is turning and what the current shaft orientation is. This is how things are done mostly now days (with encoders) as analog tachs are being phased out. The encoder will probably be the costliest part. The microcontroller, LCD & assosciated components probably wouldn't break a 50$ budget, even if you went with a preassmbled microcontroller board like Arduino.

    EDIT: you could also design your own rotary encoder if that is a requirement; they are pretty simple, containing an LED, slotted disk, and a couple of phototransistors.
     
    Last edited: Apr 30, 2011
  5. __alex__9

    Thread Starter New Member

    Apr 30, 2011
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    I am not planning on making or buying either a tachometer or an encoder based design, i am just after finding a solution to the design brief that is the most relatively easy to understand, but more importantly it needs to be the way in which i can use more precise about what is actually going on in the design if you get what i mean. Also i'm not great at all with programming so i would probably struggle with this proposal sadly :/

    The exact brief is:

    Design a monitoring and measuring system for the speed of a drive motor. The system must have a digital display and give warnings when the speed exceeds a pre-set value which would indicate possible mechanical problems.

    Produce a feasible design solution for the system that should include:

    • a block diagram showing all system elements
    • singal pathways, inputs and outputs
    • a detailed explanation of how your system functions
    • the requirments of the design brief, taking into account health and safety considerations
    • details of the sensors, transducers, display equipment and associated circuits
     
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    So it's only theoretical? If you only need to design the system and not the individual components of it, and you don't need to buy anything and you want it to be simple & safe both in operation and explanation then I would recommend designing around the ready made products I mentioned previously. have a look at this:
    http://www.redlion.net/Products/DigitalandAnalog/DigitalPanelMeters/DCVolt/PAXD.html
    This universal process meter can directly accept the output of any DC analog tachogenerator up to 300V. You enter the scaling points (calibration data as you referred to) which is pretty simple; usually all you need is 2 data points, for example (0RPM, 0V) - and - (1750RPM, 50V) and the meter will calculate and display any value between the 2 points. In the event that your tachogenerator output is nonlinear, there are 16 scaling points available to plot segments along your curve. It can be shipped with an output relay card to close a contact when the voltage (RPM) reaches a certain level or falls below a certain level, triggering alarms. It is NEMA rated & UL Listed, purpose made device being used for it's inteded purpose, so you shouldn't need to dwell on safety in your brief, as it's already covered.

    EDIT: note that the theory of operation of this meter is almost exactly as you described in post #1. The tach voltage is scaled down, goes through an A/D converter and then the raw data is multiplied by the scaling factor
     
    Last edited: Apr 30, 2011
    __alex__9 likes this.
  7. amilton542

    Active Member

    Nov 13, 2010
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    For simplicity you can accurately measure the speed of a motor with a strobe light and if the rotor speed falls out of sync with the strobe it will act as your warning
     
  8. bluemarvin

    New Member

    Apr 19, 2011
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    How about putting a magnet on the motor shaft and a hall effect sensor as the tach? The hall effect sensor output goes to a frequency to voltage converter or a counter, which respectively go to a voltmeter or LED lightbar. Use a zener or a digital comparator as the max speed alarm...that's pretty simple and no programming required. I'm an Occam's Razor fan myself.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Yes that is another way of measuring RPM, but there is always the potential to measure incorrectly. Imagine if your shaft is rotating 100rpm, you set your strobe to 100ppm, everything looks good. Everything would also look good if your shaft was going 200RPM, or 400RPM, or 800rpm
     
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