----- Original Message -----=20
From: ChrisAtUpw@..........
To: psn-l@.................
Sent: Friday, August 16, 2002 4:48 AM
Subject: Re: DRIP (diamagnetically restored inverted pendulum)
In a message dated 15/08/02, gjharris@............. writes:=20
A very sensitive and inexpensive two axis sensor can be made as =
follows:=20
Above the top surface of the pendulum, place an LED with the front =
surface=20
flattened so that it become a point source about 1/4 inch above the =
surface.
On the top surface cement a 6mm square mirror (stock H43866 from =
Edmund).=20
On each of the four sides of the LED place small solar cells (stock =
980-0150=20
from allied) in such a position that each will be half illuminated =
by the LED.
Hello George Harris,=20
Nice to have some extra input! I am having a problem in =
visualising the optical layout you suggest and the orientation and =
position of the various components. Is the LED 'water clear' plastic or =
what, please? What size? Are they IR or visible? =20
When you cut the end off a plastic LED and polish the end, =
looking into it, you see a bright central square chip surrounded by a =
ring of light from the plastic case. Putting a mirror on the end just =
reflects most of the light back through the base.=20
Is that any chance of a quick sketch / 'paint' drawing, please?=20
Edmund optics seem to be at http://www.edmundoptics.com/ There =
is no response to H43866, but there is a 10 mm square surface mirror =
NT45-517 @..........
=20
Response By George***************8
The LED I used was the small clear type. I sandpapered the surface =
until it was
very close to the LED, then used finer sandpaper and polishing =
compound (tooth
paste works) until it was bright. This LED was mounted on a small PC
board above the top surface of the pendulum looking down. The spacing =
should be approximately 1/4 inch above the mirror which was attached to =
the upper
surface. The mirror used was 6 mm square (1/4 inch). The resulting =
reflection is=20
approximately a 1/2 inch square at the surface of the PC board. If now =
small silicon photosensors are placed on the same board approximately =
1/2 inch apart, they will each be illuminated on 1/2 of their surface =
when the mirror is centered. =20
The sensors I used are about 3mm (1/8 inch) square and moung flat to =
the board.
The photocells can be connected in parallel, plus to minus, on the =
opposite sides across the inputs to a low noise operational amplifier =
with about a megohm in the feedback. The resulting output is very =
sensitive (nanometers) and linear.
The photo output of LEDs varies exponentially with temperature =
and you get about a factor of five reduction in light output at a =
constant current if you increase the temperature from 0 C to 100 C. =
Since you can expect ambient changes of >10 C deg and the LEDs =
themselves can heat up considerably, the variations are significant. How =
do you stabilise the photo output please?=20
Response**********
Since the photocells are both half illuminated, the null is not =
sensitive to the
output of the LED. The changes in the LED ouput would only slightly =
effect the gain of the system. Since I was using the sensor in a =
feedback mode to recenter
the mass, this effect was minimal. My system was in a heated area, so =
the=20
LED output change was very small.
Regards,=20
Chris Chapman=20
----- Original Message -----
From:=20
ChrisAtUpw@.......
Sent: Friday, August 16, 2002 =
4:48=20
AM
Subject: Re: DRIP =
(diamagnetically=20
restored inverted pendulum)
In a =
message dated=20
15/08/02, gjharris@.............=20
writes:
A very sensitive and inexpensive two axis sensor can be =
made as=20
follows:
Above the top surface of the pendulum, place an LED =
with=20
the front surface
flattened so that it become a point source =
about 1/4=20
inch above the surface.
On the top surface cement a 6mm square mirror (stock =
H43866 from=20
Edmund).
On each of the four sides of the LED place small solar =
cells=20
(stock 980-0150
from allied) in such a position that each will =
be half=20
illuminated by the LED.
Hello George Harris, =
Nice=20
to have some extra input! I am having a problem in visualising the =
optical=20
layout you suggest and the orientation and position of the various =
components.=20
Is the LED 'water clear' plastic or what, please? What size? Are they =
IR or=20
visible?
When you cut =
the end=20
off a plastic LED and polish the end, looking into it, you see a =
bright=20
central square chip surrounded by a ring of light from the plastic =
case.=20
Putting a mirror on the end just reflects most of the light back =
through the=20
base.
Is that any chance of a =
quick=20
sketch / 'paint' drawing, please?=20
Edmund optics seem to be =
at=20
http://www.edmundoptics.com/ There is no response to H43866, but =
there=20
is a 10 mm square surface mirror NT45-517 @.......
Response By George***************8
The LED I used was the small clear type. I sandpapered the =
surface=20
until it was
very close to the LED, then used finer sandpaper and polishing =
compound=20
(tooth
paste works) until it was bright. This LED was mounted on a =
small=20
PC
board above the top surface of the pendulum looking down. =
The=20
spacing should be approximately 1/4 inch above the mirror which =
was=20
attached to the upper
surface. The mirror used was 6 mm square (1/4 inch). =
The=20
resulting reflection is
approximately a 1/2 inch square at the surface of the PC =
board. If=20
now small silicon photosensors are placed on the same board =
approximately 1/2=20
inch apart, they will each be illuminated on 1/2 of their surface when =
the=20
mirror is centered.
The sensors I used are about 3mm (1/8 =
inch)=20
square and moung flat to the board.
The photocells can be connected in parallel, plus to =
minus, on=20
the opposite sides across the inputs to a low noise operational =
amplifier=20
with about a megohm in the feedback. The resulting output is =
very=20
sensitive (nanometers) and linear.
The photo output of LEDs =
varies=20
exponentially with temperature and you get about a factor of five =
reduction in=20
light output at a constant current if you increase the temperature =
from 0 C to=20
100 C. Since you can expect ambient changes of >10 C deg and the =
LEDs=20
themselves can heat up considerably, the variations are significant. =
How do=20
you stabilise the photo output please?
Response**********
Since the photocells are both half illuminated, the null is =
not=20
sensitive to the
output of the LED. The changes in the LED ouput would only =
slightly=20
effect the gain of the system. Since I was using the sensor in a =
feedback mode to recenter
the mass, this effect was minimal. My system was in a =
heated area,=20
so the
LED output change was very small.
Regards,=20
Chris Chapman=20