PSN-L Email List Message

Subject: Re: wonderful invention of R. V. Jones (phase detection)
From: Charles Patton charles.r.patton@........
Date: Thu, 28 Feb 2008 07:36:51 -0800

Hi Chris,
You asked, "...can you explain why we are pursuing this method please?"
I'm not pursuing it.  I was just adding a tidbit of information for 
those whose might like to.  Many years ago before they were widely 
available, I designed an optical pick-up for a steering wheel for an 
arcade game.  I used the multiple Ronchi rulings and offset patch 
techniques to resolve direction.  The multiple lines help considerably 
with signal to noise in as much as they provide more light to work with. 
  In the case of this implementation for seismometry, I'd be more 
concerned with the needed optical path for the optical lever effect.  I 
played with a Schlerian (the optical effect Randall was described of 
seeing heat waves from your hand) setup as a kid and found they're hard 
to do both in setup and basically wanting better mirrors than the 
shaving mirrors I was trying to use.  One of the problems as I see it is 
that you've separated the measurement of interest from the point of the 
measurement, i.e., introduced distance with intervening, uncontrolled 
movements (read floor expansion, vibration, etc.)   I personally feel 
that Randall’s SDC method is one of the best sensor methods, other than 
being a bit bigger in area than I’d like, but it has high resolution, it 
can sense direction, and is essentially sensitive only to the axis of 
movement it is designed for.
Charles R. Patton

ChrisAtUpw@....... wrote:
> In a message dated 2008/02/27, charles.r.patton@........ writes:
>> An additional technique that might be useful for this sensor is to
>> construct the detector ruling – make it of two sets of rulings, with a
>> 90 degree offset (1/2 line offset between the sets -- i.e., 1/4 period
>> offset).  Then use two photodetectors, one for each set.  This way you
>> can eliminate much of the gain variation due to temperature, etc.
>> Additionally the sine wave phase difference from the detectors will
>> allow the detection of which way the mirror is moving so you have a
>> sense of positive/negative, north/south, etc.
> Hi Charles,
>        Fine, I have ordered a copy of R V Jones' article on Ronchi gratings.
>        In the meantime, can you explain why we are pursuing this method 
> please? Are Ronchi gratings available commercially? I have no doubt that 
> it can and has been made to work. However, using a wavelength / 
> orientation sensitive method to measure movements of tiny fractions of a 
> wavelength, when you do not need to do it this way, seems just a bit 
> dumb to me.
>        A couple of large area photocells eg VTD34 / BPW34 in a 
> differential setup with a simple moving slit + a small tungsten filament 
> bulb can give perfectly adequate stability for amateur seismometers and 
> a resolution of about 10 nano meters or less. This is easy to make / set 
> up with readily available components. The photocells are about $2 each. 
> Add a filament bulb and a voltage regulator. The slit can be made using 
> razor blades. It is simple, cheap and effective.
>        Regards,
>        Chris Chapman

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