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Thread: Open Source Scanfail Unit (phase 1)

  1. #41
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    LOL ....thanks dave .... ive just sprayed Corona Extra all down the front of my shirt ......

    all the best ... Karl

  2. #42
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    Hehe

    Couldn't help myself...
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  3. #43
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    haha

    poor audrey

    do we really want to be complicating the circuit that much ? I mean, isn't the whole thing about failing safe etc etc to keep it as simple as possible ?
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  4. #44
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    Hi All,

    I would say Steve is on the right track. If you think about it more, there is a clever way to combine the absolute value circuite with a time delay, thus giving you the 5ms (or so) delay. The comparator can then be placed afterward, giving you both the delay function and also the minimum scan rate function (both of which are necessary). Remember, comparators have open-collector output, so they can be used in a similar way that Steve used the FET to extinguish the beam. Simple and reliable.

    Audrey's circuit is interesting, and one which I certainly have not thought of before. I get a bit nervous with a circuit like that though, because, remember, all circuits have noise. So this means that Audrey's delay would generate noise itself (notorious for those bucket briggades by the way) and also even the incoming signal from the scanner amp will have a bit of noise. Thus, Audrey's circuit will never detect the two signals being the same because of noise makes the two signals never the same. In fact, from what I have seen, even HB's circuit is a bit sensitive to noise. The beam is never completely extinguished because noise coming from the scanner amp makes it think that the beam is always moving just a little bit.

    Bill

  5. #45
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    Great topic!

    But isn't it better to use a mechanical safetyshutter instead of blanking the drivers?





    Lets take this circuit twice,
    One for the X and one for the Y then put a comparator between both outputs to detect if they are both missing the pulses and if so we can trigger something.

  6. #46
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    A mechanical shutter is way too slow.

  7. #47
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    Lightbulb

    Hi Mc,

    Nice circuit you have here!
    But this will only detect a missing puls, so a signal of 0V.
    The SafeScan should also shut down the laser if the signal is 'hanging' at another level.
    Please take the picture: if the X and Y-signals are 2,5V constantly, your beam is not moving too, but will shine right into the audience.

    If you can design another circuit that will detect a change in the signal, this can then be added to the input of your NE555 circuit.

    So, the block diagram would then be:
    X-TRUE minus X-ACTUAL ->
    Signal change detector ->
    NE555 timer ->
    Beam shutter.


    Are we almost there?

    Xxx Audrey

  8. #48
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    Cool

    OK,
    so far, to also add "hanged galvo detect"

    absolute rectify both scanner control and scanner feedback,
    run into a window comparitor so we have some room for Bill's noise,
    and then use the differntiator for the motion.

    that way we have both a hangup detect and a minimal motion detect. Audrey,, Bill said if we have no motion less then 10 hz, we have no motion, as far as audience scanning is concerned, so thats why I didnt put DC hangup in the origional circuit.

    you need the differntiator to make short fast pulses to trigger the missing pulse detector.

    Steve Roberts

  9. #49
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    Lightbulb Audrey's second ScanSafe solution

    Hi Steve,

    If I can still be your Chief Engineer, here is my next solution:

    How it should work:

    1.
    X is the input signal. The AD8003 converts the analog signal into an 8 bit digital signal. Because X will have a value between 0 and 5 volt maximum, the resolution will be 5/2^8=20mV. Bill's noise lie within this 20mV.

    2.
    The 8 bit databus is they splitted in two ways. They both latched by a couple of 74LS77's. The 74LS76 flip-flop will provice the clock signal for the latches:the upper latch group 1 is ON, the lower group OFF. Next clock upper group is OFF, lower group is ON. Its alternating.
    The clock for the this flip-flop is generated by the NE555 and be set to 1ms. (1000Hz)

    3.
    The 74LS684 is een chip that can compare 2x 8 bits parallel signals.
    Group P and group Q, see the circuit below.
    Because the latches are alternating, the P input signals are always 1 ms before or after the Q signals.
    Example:
    at 1ms: P is filled with data.
    at 2ms: Q is filled with data of 1ms after 1ms.
    at 3ms: P is filled with data of 1ms after 2ms.
    at 4ms: Q is filled with data of 1ms after 3ms.
    and so on....
    This 74LS684 has an output that can have two states: High or Low.
    High will say: P and Q are the same.
    Low will say: P and Q are not the same.

    4.
    The last chip is the well known CD4017 decade counter.
    At every clock pulse (by the NE555) the CD4017 output signal is at the next output pin starting at O0, then O1, O2,... until O6 is reached.
    This will give the signal to the opto coupler and the laser is shut off.
    If the P and Q are the same, the 4017 will be reset by the P>Q signal of the 74LS684 and the counter will start over.


    Please don't hang me if some things are wrong about this circuit, it's just a concept for others. According to the title of this thread this is just phase 1 .


    Xxx Audrey
    Last edited by Miss A.; 01-07-2009 at 10:09.

  10. #50
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    Intresting aproach.

    I found the folowing interesting text:

    The simplest scanfail interlock derives the vector velocity as the sum of the squares of the and Y velocity signals, comparing this against a threshold voltage. When the vector velocity is below the thresehold the beam is shuttered.

    I have now idea how to do this with electronic components.

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