Experiance Scannersafety V1 - V3?

[dave]

Hey guys.

I reckon I should have a chance after the weekend to check the board out

[greenalien]

If the price is right, I'm up for 3-off V2 boards, which should be enough for present and future plans for a couple of years! I'm particularly interested to know how easy it is to set up the 'zone' on the V2 board - all I'd ever want to do is to set it at half way, so that the scanfail works on the lower half of the display area, where the punter's eyeballs are...

[mccarrot]

I was reading trough the 10MPE rules and i found this:

A scan-fail circuit should be required which can detect and terminate a static beam in the audience area within 1 millisecond

this scanfail is 20ms.

Guido is it possible to get it react faster?

[Guido]

Hi,

first: Setup is very easy. All what i need is a Feedback - Signal
with not more than +/-10V DC. That´s what most of the Scanners
give you.

Then scan a small Rectangle or Circuit and adjust the Density Resistor until
the Safety blanks this Figure.

About the Zone: The one Zone is full Width of the Projection and only the
height ist adjusted by the 2nd adj. Resistor. Sometimes if the Feedback is inverted
there is a Bridge to be set on the 6pin Connector to invert the Zone. Thats not
noticed in the English Translation this Time but possible.

The Reaction Time is <=20ms. It it possible to get down to 1ms but than i can
and will happen that some Parts of the Lasershow can be lost.

Here in Germany we need <100ms. Most Safety work at <= 20ms and this
is a practicable Value.

Did you need 1ms in the USA ?

[-bart-]

within 1 millisecond

You have to have to determine detection and beam shut-off time separately.

How fast would you imagine a mechanical shutter would be ?

Fast detection will only make sense if you use the optocouplers on the modulation inputs.

[mccarrot]

A 1 msec response time is ideal but even 25 msec is 24 msec better than nothing

Let me try to explain.
When you need a MPE safe show its as follows:
response time to shut your eye is about 0.25 seconds, if we calculate a safe MPE for this we arrive at the common 2.5 mW/cm^2

Now Greq Makhov have developed a simplified a method to aproximate MPE.
With his simplified method you can measure a static beam at 10mW/cm^2 for a show to be safe withing MPE when scanning
when using the 10mW/cm^2 approach you must make sure a static beam is never longer than 1ms at one location (scanning the beam)

But in case of a scanner fail you must also ensure a static beam cannot be static for longer than 1ms.


If you have a failsafe doing 25 ms (and de beam is scanning of course), you can calculate a safe MPE of 4.5mW/cm^2.

So simply put you need to half the power to be save for MPE if youe failsafe react in "only" 25ms.


Here are some calculations for people who are interested:
If you consult the ANSI Z136.1 (and the IEC 60825.1), one finds that the MPE is directly related to the time of exposure, using the formula of 1.8 x 10^-3 x t^3/4 Joules/centimeter squared. Solving this for a "t" of 1 second gives a result of 1.8 millijoule/cm^2, which is equivalent to 1.8 milliwatt/cm^2 for the 1 second of exposure.

If "t" is 1 millisecond, then the MPE becomes about 10 microjoules/cm^2, which when divided by the 1 millisecond of exposure is about 10 milliwatts/cm^2. Similarly, at 0.25 seconds, we arrive at the common 2.5 mW/cm^2, which relates to the aversion response (1/4 second).
In other words, going from an exposure time of 0.25 seconds down to 1 millisecond, our MPE goes from 2.5 mW/cm^2 to 10 mW/cm^2.

[mccarrot]

How fast would you imagine a mechanical shutter would be ?

Fast detection will only make sense if you use the optocouplers on the modulation inputs.

Thats why you should use optcouplers and shutter as backup.

Or wire your shutter only to the emergency button.

[mixedgas]

The copy of the CDRH packet I have says 200 nanoseconds for a failsafe for things such as mirror balls. So clearly a 1 mS range is desirable in the US, unless you can do the engineering report and due diligence to otherwise justify a longer time scale.

Steve

[Guido]

Hi,

as i programm the Safety i just did a "fast Calculation" for the Switch-Off Time.
Its really no problem to get the Reaction-Time <2ms. But i need to test this
in practice ( Looking some Shows and see what happen ..)

This Time i did this: I measure the position and store X and Y Values.
Then I wait some Time and repeat the measuring. The differece
(= the movement ) must be bigger that the adjusted Value.
If not a Variable "Errorcounter " will be increased. If the movement is bigger than
the adjusted Value this "Errorcounter" will be decreased. If the Value ist
higher than a.e. the value 20 the Lasers will be switched off, if the Value is lower
than a.e. the value 5 the Lasers will be turned on. This is done to get something
like "Hysterese" an noch flickering Image in the near of the Switchingzone.
The wait-Time between 2 Measurements if always changing in little Steps
because otherwise it can happen that you scan a rectangle with a fixed Speed
that the first Measurement is on the upper left corner, than the Scanners
scan the figure and the next measurement is on the same upper left Corner
--- ERROR :-) . To clear this Problem there is a variable Time.

In Simulation the Switch Off Time is less than 2ms but i need to check this
with some measurements with my Scope. Measuring 20 Values in less than 1ms is
really no Problem today. Because this works so fast i dont need to use the Intensity
Signal to check if the Laser is on or not.

About Shutters: It is possible to change the Circuit in this Way that
an aditional TTL Pin can go high if the Error is longer than "Time X" .
But i dont like to make new PCBs only for this. This "Time X" cant be to short,
otherwise the Relay / Shutter always goes CLICK-CLACK-CLICK-CLACK....

A Shutter should be the "last Part in the Chain.." ..driven by the
Emergency Switch. Thats my Opinion based on the German Laws for Lasershows.

[mccarrot]

Dave, any progress with testing?

Guido, did you managed to get a faster response time?
If ILDA gets the 10xMPE rules approved we need to aim for the 1ms shut-off time as you can read in my previous posts.