In a message dated 17/02/2009, tchannel@............ writes:
I asked this question earlier but got no response. Just wondering if the
coil and magnet, widely used, is better than an LED, and why?
Another question about this site..... he uses a LED and Photo Transistor in
place of the coil and magnet. I have never used this. Is one better than the
other? This approach would be smaller and lighter.
_http://tinkeringcaveman.googlepages.com/home_
(mhtml:{6B067749-3EC5-4114-A01F-ABF5F9C52356}mid://00000007/!x-usc:http://tinkeringcaveman.googlepages.com/ho
me)
Hi Ted,
You are talking about two different types of sensor system. Photo
detection systems detect position. Coil + magnet systems detect velocity and are
likely to suffer noise problems at very long periods. They can be made
comparable or better than photo detection at the short periods discussed here.
For very low noise position sensors, variable capacitor detection
systems are used.
Kevin will get a poor performance out of his proposed LED +
Phototransistor seismic system. It will be very noisy and it will show temperature drifts
due to the components he has chosen. He has used a bundle of fibres as the
suspension. This will not give a constant torque with time. The area of the
copper in the damping magnet changes as it rotates, so the damping will not be
constant with changes in position.
You CAN make photo detection systems with resolutions of maybe 10 nano
metres, if you design them correctly. Quad magnet damping systems can give
constant damping with changes in angle. Single wires and foil strips can give a
constant torque / angle relationship.
The manufacturers of LEDs don't often tell you that the light output at
a constant current varies by >x10 for LED substrate temperature changes
between 0 and 100 C. Can't think why! The unstabilised thermal drifts are huge and
the photo noise is considerable. You can stabilise the output fairly well
with a large area Si photocell + a driver amplifier. Phototransistors have very
high noise levels compared to large area photodiodes. They also have metal
mask contacts which may make the output both position and rotation dependant.
You need large area differential photocells and an intense stable light
source to give the high photo currents required (50 to 100 micro A) to get
the photo noise and drift down. The current is proportional to the number of
electrons in a sample. The photo noise is the square root of this number. You
can do it OK with differential Si photocells BPW34 or larger and an under run
tungsten filament bulb. They are 'quiet', but do choose straight / tensioned
filaments and use a voltage regulated supply. Use about 3/4 the rated voltage
to give ~infinite filament life. Maybe put the bulb outside the glass jar to
keep down the heating / air convection? There is a differential photo
amplifier on John Lahr's website.
Remember that this is a position detector, NOT a velocity detector.
If you use a coil + magnet block sensor, you have to get the output
current away from the armature. Maybe you can use the two suspension wires?
You need very small diameter suspension wires to get the longer periods.
The Wood-Anderson seismometers used about 0.8 thou OD Tungsten wire. The
smallest that I can get off the shelf is 2.4 thou OD. It is used in laser
printers. Check used cartridges? _www.wires.co.uk_ (http://www.wires.co.uk) You
might also be able to use small diameter NiChrome wire. I have some 2 thou OD.
What do _www.smallparts.com_ (http://www.smallparts.com/) keep?
The smallest steel music wire is about 7 thou diameter - much too large.
Smaller diameters are produced, but I don't know of a source. Glass
filaments tend to be brittle and don't conduct electric current. S glass is often
used for fibreglass construction instead of O glass. Suspensions are often made
from fused silica, but pyrex is also used.
Regards,
Chris Chapman
In a message dated 17/02/2009, tchannel@............ writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
I asked this question earlier but got no=20
response. Just wondering if the coil and magnet, widely used, is bett=
er=20
than an LED, and why?
Another question about this site..... he=20=
uses a=20
LED and Photo Transistor in place of the coil and magnet. I have never use=
d=20
this. Is one better than the other? This approach would be smaller an=
d=20
lighter.
http://tinkeringcaveman.google=
pages.com/home
Hi Ted,
You are talking about two different types of se=
nsor=20
system. Photo detection systems detect position. Coil + magnet systems detec=
t=20
velocity and are likely to suffer noise problems at very long periods. They=20=
can=20
be made comparable or better than photo detection at the short periods discu=
ssed=20
here.
For very low noise position sensors, variable=20
capacitor detection systems are used.
Kevin will get a poor performance out of his=20
proposed LED + Phototransistor seismic system. It will be very noisy and it=20=
will=20
show temperature drifts due to the components he has chosen. He has used a=20
bundle of fibres as the suspension. This will not give a constant torque wit=
h=20
time. The area of the copper in the damping magnet changes as it rotates, so=
the=20
damping will not be constant with changes in position.
You CAN make photo detection systems with=20
resolutions of maybe 10 nano metres, if you design them correctly. Quad magn=
et=20
damping systems can give constant damping with changes in angle. Single=
=20
wires and foil strips can give a constant torque / angle=20
relationship.
The manufacturers of LEDs don't often tell you=20=
that=20
the light output at a constant current varies by >x10 for LED substrate=20
temperature changes between 0 and 100 C. Can't think why! The unstabili=
sed=20
thermal drifts are huge and the photo noise is considerable. You can stabili=
se=20
the output fairly well with a large area Si photocell + a driver amplif=
ier.=20
Phototransistors have very high noise levels compared to large area photodio=
des.=20
They also have metal mask contacts which may make the output both posit=
ion=20
and rotation dependant.
You need large area differential photocells and=
an=20
intense stable light source to give the high photo currents required (50 to=20
100 micro A) to get the photo noise and drift down. The current is=
=20
proportional to the number of electrons in a sample. The photo noise is the=20
square root of this number. You can do it OK with differential Si photocells=
=20
BPW34 or larger and an under run tungsten filament bulb. They are 'quie=
t',=20
but do choose straight / tensioned filaments and use a voltage regulated sup=
ply.=20
Use about 3/4 the rated voltage to give ~infinite filament life. Maybe put t=
he=20
bulb outside the glass jar to keep down the heating / air convection? There=20=
is a=20
differential photo amplifier on John Lahr's website.
Remember that this is a position detector, NOT=20=
a=20
velocity detector.
If you use a coil + magnet block sensor, you ha=
ve=20
to get the output current away from the armature. Maybe you can use the two=20
suspension wires?
You need very small diameter suspension wires t=
o=20
get the longer periods. The Wood-Anderson seismometers used about 0.8 thou O=
D=20
Tungsten wire. The smallest that I can get off the shelf is 2.4 thou OD=
.. It=20
is used in laser printers. Check used cartridges? www.wires.co.uk You might also be able t=
o use=20
small diameter NiChrome wire. I have some 2 thou OD. What do www.smallparts.com keep?
The smallest steel music wire is about 7 thou=20
diameter - much too large. Smaller diameters are produced, but I don't know=20=
of a=20
source. Glass filaments tend to be brittle and don't conduct electric curren=
t. S=20
glass is often used for fibreglass construction instead of O glass. Suspensi=
ons=20
are often made from fused silica, but pyrex is also used.
Regards,
Chris Chapman