[sci.electronics] pressure transducers

sbcd@earvax.UUCP (Scott Dynes) (11/30/88)

I'm looking for help on the following: I would like to get/make an
airspeed sensor for R/C models. This would require 1) a small
accurate (and light!) pressure sensor that would run off a small set
of batteries, and 2) a similarly small, light, low-power way to get
the information down to earth. I'm thinking of having the output be
a tone whose frequency is proportional to the airspeed. Does anyone
know of such a system, or where to look for the individual pieces?
Comments?? 

Thanks.

Scott Dynes

ornitz@kodak.UUCP (barry ornitz) (12/05/88)

In article <848@earvax.UUCP> sbcd@earvax.UUCP (Scott Dynes) writes:
>I'm looking for help on the following: I would like to get/make an
>airspeed sensor for R/C models. This would require 1) a small
>accurate (and light!) pressure sensor that would run off a small set
>of batteries, and 2) a similarly small, light, low-power way to get
>the information down to earth. I'm thinking of having the output be
>a tone whose frequency is proportional to the airspeed. Does anyone
>know of such a system, or where to look for the individual pieces?
>Comments?? 

Sensors capable of detecting the small pressure differences you need tend to
be expensive and fragile.  You might look into surplus radiosondes which
already have the tone encoding (or alternative telemetry).

A much simpler approach would use a thermistor in its self-heating mode.  
The thermistor heats due to its current draw, but is cooled by the air flow.
To compensate for the change in ambient temperature, a bridge circuit using
two matched thermistors could be used.  A differential input op-amp stage
could then feed a simple voltage controlled oscillator.

Honeywell (Microswitch) makes a minature air flow sensor that should work
well in your application, but I have no idea as to its cost.  Good luck.
                                               Barry  WA4VZQ
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|  ___  ________  |
| |  / /        | |  Barry L. Ornitz  UUCP:..rutgers!rochester!kodak!ornitz
| | / /         | |  Eastman Kodak Company
| |< < K O D A K| |  Eastman Chemicals Division Research Laboratories
| | \ \         | |  P. O. Box 1972
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|                         |
 -----------------

larry@kitty.UUCP (Larry Lippman) (12/05/88)

In article <848@earvax.UUCP>, sbcd@earvax.UUCP (Scott Dynes) writes:
> I'm looking for help on the following: I would like to get/make an
> airspeed sensor for R/C models. This would require 1) a small
> accurate (and light!) pressure sensor that would run off a small set
> of batteries, and 2) a similarly small, light, low-power way to get
> the information down to earth. I'm thinking of having the output be
> a tone whose frequency is proportional to the airspeed. Does anyone
> know of such a system, or where to look for the individual pieces?

	As you are no doubt aware, airspeed requires differential
measurement (i.e., pitot tube), and therefore requires a differential
pressure transducer.  I'm afraid that I don't know about speeds of
model aircraft, but "big aircraft" will provide a pitot differential
pressure of 0.17 in-Hg (0.0835 psi) at 60 knots (the lowest airspeed
for which I know a value).
	A model aircraft application will require a small and lightweight
pressure transducer having some rather respectable sensitivity (as
comparared to most other pressure transducers) - so you will not have
much to choose from in an affordable price range.  However, it just so
happens that I have a catalog from SenSym (Sunnyvale, CA, 408/744-1500)
on my desk.  It appears that their P/N SCX01DN temperature compensated
monolithic transducer will do the trick, and it carries a small quantity
price of $ 49.50.
	You will have to provide a bridge-type signal conditioning
circuit since this transducer has a sensitivity of 18 mv/psi, and
you will have to exercise some care in circuit design to prevent
RFI/EMI from the engine magneto.
	For telemtry, the simplest method is through the use of
FM/FM telemetry; i.e., a V-F converter to modulate a transmitter,
and a F-V converter connected to an appropriate FM receiver.
	More exact circuit details would be difficult to provide over
the Net, but hopefully I have steered you in the right direction.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
<>  UUCP:  {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
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<>  FAX:   716/741-9635 {G1,G2,G3 modes}   "Have you hugged your cat today?" 

steve@eos.UUCP (Steve Philipson) (12/06/88)

In article <848@earvax.UUCP> sbcd@earvax.UUCP (Scott Dynes) writes:
>I'm looking for help on the following: I would like to get/make an
>airspeed sensor for R/C models. This would require 1) a small
>accurate (and light!) pressure sensor that would run off a small set
>of batteries, and 2) a similarly small, light, low-power way to get
>the information down to earth. I'm thinking of having the output be
>a tone whose frequency is proportional to the airspeed. Does anyone
>know of such a system, or where to look for the individual pieces?

   Ace, RC manufactures a device named the "Thermic Sniffler" that
is a rate-of-climb device.  It uses a bridge circuit with a heated
wires as elements of the bridge, and senses the rate of inflow and
outflow out of the sealed enclosure.  The output is a tone that rises
or descends proportionately to change in altitude (rate of flow into
or out of the box).  The transmitter weighs one ounce, as does a typicaly
9 volt battery, so total airborne wieght is two ounces.  The system lists 
for $145 including a reciever for the downlink.

   This device could probably be easily modified for your needs as
an airspeed sensor.  If your clever, you could probably have it
switch back and forth between ROC and airspeed.  At the very least,
you could examine one for ideas about how to build your own.  Best 
of luck to you on this project.
						Steve.
-- 

						   Steve
					(the certified flying fanatic)
					    steve@aurora.arc.nasa.gov

ciciora@tramp.Colorado.EDU (Ciciora; Steven joseph) (12/06/88)

In article <2810@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>In article <848@earvax.UUCP>, sbcd@earvax.UUCP (Scott Dynes) writes:
>> I'm looking for help on the following: I would like to get/make an
>> airspeed sensor for R/C models. This would require 1) a small
>> accurate (and light!) pressure sensor that would run off a small set
>> of batteries, and 2) a similarly small, light, low-power way to get
>> the information down to earth. I'm thinking of having the output be
>> a tone whose frequency is proportional to the airspeed. Does anyone
>> know of such a system, or where to look for the individual pieces?

  -a)  Bust a small light bulb.
  -b)  Run a constant current through it.
  -c)  Measure the voltage accross it vs air speed.

I once wanted to measure air speed and this was the method sugested.
Never really calabrated it, though.  Not sure how.  It was interesting
to send the output to a O'scope and look at the fluctuations of a fan,
person walking by, blowing, etc.  Made a nice lunch-break hack.

Hope this helps,

Steven Ciciora

michael@taniwha.UUCP (Michael Hamel) (12/06/88)

Does anyone know if it is feasible/cheap to get a solid-state sensor
to read barometric pressure accurately enough for a weather station?
I see Arrow distribute some Motorola sensors that seem to cover 0-30
psi but I have absolutely no other information on them, like how to
interface to them and how accurate/repeatable/durable such things
might be...

nobody@tekecs.TEK.COM (-for inetd server command) (12/07/88)

In article <5084@boulder.Colorado.EDU> ciciora@tramp.Colorado.EDU (Ciciora) writes:
>In article <2810@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>>In article <848@earvax.UUCP>, sbcd@earvax.UUCP (Scott Dynes) writes:
>>> I'm looking for help on the following: I would like to get/make an
>>> airspeed sensor for R/C models. This would require 1) a small
>>> accurate (and light!) pressure sensor that would run off a small set
>>> of batteries, and 2) a similarly small, light, low-power way to get
>>> the information down to earth. I'm thinking of having the output be
>>> a tone whose frequency is proportional to the airspeed. Does anyone
>>> know of such a system, or where to look for the individual pieces?
>
>  -a)  Bust a small light bulb.
>  -b)  Run a constant current through it.
>  -c)  Measure the voltage accross it vs air speed.
>
>I once wanted to measure air speed and this was the method sugested.
>Never really calabrated it, though.  Not sure how.  It was interesting
>to send the output to a O'scope and look at the fluctuations of a fan,
>person walking by, blowing, etc.  Made a nice lunch-break hack.
>
>Hope this helps,
>
>Steven Ciciora


Didn't the Voyager Mars Lander (talk about r/c!) use something like this? 
My understanding was that four were used spaced at 90 degree intervals 
which allowed direction as well as speed to be measured. I don't quite 
see how one would determine the direction but this might be neat to play 
around with. If anyone has ever done anything with this I'd like to hear 
about it. It would make a neat addition to a home weather station.

regards,

Phil Biehl

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markz@ssc.UUCP (Mark Zenier) (12/07/88)

In article <848@earvax.UUCP>, sbcd@earvax.UUCP (Scott Dynes) writes:
> I'm looking for help on the following: I would like to get/make an
> airspeed sensor for R/C models. 

Why not put a loud but stable audio oscillator/loudspeaker in your model 
and use the doppler shift.  'Course you may need one of those fancy 
HP fourier analyzers to get accurate results.

Mark Zenier    uunet!nwnexus!pilchuck!ssc!markz    markz@ssc.uucp
                      uw-beaver!tikal!