Ok, Here's my take on a very simple scan fail detector
I've tried to keep it down to just two chips - the Bill Benner " two chip challenge"
1 x LM423 quad opamp and 1 x LM339 quad comparator
It seems to work in simulation, at least with the waveforms I've tried feeding into it.
By placing this circuit on the forum I'm making no comment about it's suitability for any purpose at all.
So- !!AT YOUR OWN RISK !!
I imagine that it's similar to what Dave's working on, except his actually works with real signals.
Some notes:
1/ If you're going to try making this circuit, be carefull as all the component values are guessed.
'Cos it works in simulation means that it may be super unstable in real life.
2/ It would be bad if any signal were to be fed back into the feedback output of the scanner.
Better choice of R11,C4 ,R15, C5 would help , or another opamp as a voltage follower
would stop that, but would add another chip.
3/ The negative supply rail for the opamp reads "+V-15". Don't ask. It's a Circuitmaker2000 thing.
4/The summing amplifier is a bit stodgy, but it seems to work
5/ The three 5.1v zeners D1,D2,D3 on the output stop badness from getting to the mod. inputs of the lasers.
Waveforms:
1/ Position detector inputs.
Green is point A on the schematic (X axis) - a flat sheet that periodically collapses
Yellow is point B (Y axis) - a few random jumps at varying speeds
2/After differentiation.
Green is point C - super clipped
Yellow is point D - small signals.
3/ After Rectification -
Green is point E
Yellow is point F
4/ Output stuff
Green is point G - the output of the summing
Yellow is point H - the threshold voltage
Cyan is point J - the "shutter" output, floats high through the LED, pulls low during fault.
The first collapse of the X axis is deemed safe , as the Y axis is moving fast, whereas the 2nd,3rd,4th and 5th collapse
are deemed unsafe because the velocity of the Y axis is too slow.
The ringing on the cyan output trace is some sort of interaction between the comparator and the LED to the +15v supply. The modulation outputs shouldn't be effected like this. But if they are the 5.1v zeners are there...
Sometime over the next week I'll try to make a proto of this circuit, but if anyone feels adventurous.....
Cheers,
BF
( first post with images - sorry if I've blown the page width out. I'll learn for next time)