The choice of sensors for serious altimeter+airspeed project

[kbosak]

Here is what I have found after a month of digging on forums and different datasheets:
Altimeter:
MPX2100AP has 0-100KPa absolute range., and only 0-40mV. needs some amplifier magic or 24bit ADC (http://www.kapelec.com/altivie1.htm). has -40-125C operational temp, even if calibrated for positive temp. only.
I would use it If I would be sure I can get rid of noise, supply precise voltage reference, and well stabilised power supply over temperature (its measurements are proportional to supply voltage). All in all it could be used for very high alt flying, but would require very precise power supply and ADC (how to do it? - help!). if has potentially excellent linearity .25% but who know about its true error.

MPX4115A is nice, but the AP version that has air inlet (nice for silicon tube) is harder to order in Poland. It has 15-115KPa, so not good for over 18.000m if I caluclated well.1.5% max error over 0-85C. The output is 0-5V so can be connected to 16bit ADC to give good results, easy to integrate for somebody without analog electronics skills.

At hands I have MPX5100AP which is almost the same as 4115A, its precision over 0-85C is limited to 2.5%.

Airspeed:
MPX5010DP used by some has 0-10KPa range what is said by KansasFlyer to be able to measure up to 300mph. operating temp -40...125C

in order to have more precision I jumped to MPX4006DP and MPX5004DP with 0...6 or 0...3.92KPa range just in time to discover they have silly 0-85C operating temperature. (edit: oh silly me just discovered I got 5004DP on the table, well, at least I will get exact airspeed at summertime)

my actual preferences:
A.
MPX5010DP and maybe some fan-based anemometer for airspeed
like this one http://www.kapelec.com/altivie1.htm
(anemometer should be in theory more stable at 20-35mph airspeeds)
B.
a mixture of MPX2100AP and MPX5100AP to cover full range alt and have some noise immunity in case something goes wrong with ADC and 40mV levels with MPX2100AP
C. discrete slip indicator based on horizontal tube in the fuselage, DS1820 on both sides and heating element (voltage reg?) in the middle.
I am thinking about adding a few outside DS18s20+ (for median-based failsafe operation, they are cheap), and adding SHL75 humidity+temperature sensor.
All this just to make sure I wont get that altimeter working too fast.

Any ideas? :)

[LukeZ]

MPX4115A is nice, but II've never seen AP version that has air inlet (nice for silicon tube). it has 15-115KPa, so not good for over 18.000m if I caluclated well.By my estimation you'd only get up to about 13,000 meters with the MPX4115, or roughly 45k feet. Unless you're doing one of those high altitude drops from a weather balloon that should be plenty for any UAV. However, if you are doing the weather balloon drop, then you might want a sensor that can go even twice that height, or more.

Interesting link on the anemometer - thanks for sharing.


Luke

[zenoid]

Do you realy need to go over 18 km ? I doubt you could do that in the european sky. In france you can't fly modele over 150 meters. Anyway.
I've build a telemetry to PDA (glider fligth computer) system and I'm happy intersema stuff.
http://www.intersema.com/site/technical/ms5536.php
for airspeed

http://www.intersema.com/site/technical/ms5540.php
for alt

regards

[kbosak]

Do you realy need to go over 18 km ?
regards
Affirmatif. Weather balloon will drop a glider. ;)

Intersema are cool stuff, but not available in Poland in online shops, contrary to MPX.

BTW
http://www.intersema.com/site/applic_altimeter.php
states:
'The easiest way to develop an altimeter is by measuring directly the atmospheric pressure (see an501.pdf "Using MS5534 for altimeters and barometers")'
'The MS5534 or the MS5540 have a resolution of 0.1 mbar representing 1 meter of altitude (when near the sea level). The wide operating range (10 mbar to 1100 mbar) makes them suitable for any application from the sea level up to the top of the Mount Everest.'

So why did you proposed MS5536 for spd? it is not differential so rather fits altitude metering schema?

[kbosak]

Interesting link on the anemometer - thanks for sharing.
Luke
What stinks about anemometer is that you must find a very small propeller.
in order to have linear characteristics I think such propeller should have the blades almost parallel to airflow.
Plus you need long tube compared to diameter, and small spacing between propeller and tube, plus null central cone if possible. This is not what ducted fans are.
In order to measure RPM I think it would be best to use optical system since brutal method of pluggin DC motor and measuring voltage could add strange nonlinear bias.
At the end we must remember that anemometer has quite large drag, on the other hand must be placed far from fuselage. Well it doesnt mix well with stable flying platform.

[zenoid]

Sorry, think you got it wrong, 5536 is differential and have port ; look down :
http://www.intersema.com/site/technical/products_guide.php#modules

[kbosak]

Aha fantastic. What is very nice with those Calibrated Digital Modules is that they have integrated ADC as I understand.

[LukeZ]

The Intersema do include ADC and as you say are very nice. I was able to order a sample directly from the company. These were not cheap, about $25 US for a MS5534. The altimeter sensors do not come with port (and the airspeed sensors do not come with static port, only pitot). However on the airspeed sensor it would be possible to fit a tube around the metal ring attached to the small board, and thus create a port.

You may try contact the company directly, perhaps they will ship you a sample to Poland.

Luke

[kbosak]

They 'samples' are as costly as any regular online store:

http://www.intersema.ch/site/samples.pdf
"The parts will be shipped the following day of receipt of your remittance. To speed up the matter you may send us a copy of
your bank’s transfer advice."

[SkynetAir]

Hi kbosak,

I'm looking at various sensors for precision altimeters at the moment. You can get the Motorola MPX4115AS from Farnell in the UK. I think they will send to Poland for you. They are about £15 GBP. I will be ordering one of these. I like the fact that they have temperature compensation and linear op-amp built-in. Also, the AS version has a port that you can put a silicon tube on.

Look here:

http://uk.farnell.com/1457152/semiconductors/product.us0?sku=FREESCALE-SEMICONDUCTOR-MPX4115AS

Andy.

[SkynetAir]

One more thing...

It is better to get a pressure sensor that does NOT have a built in DAC as this will limit the sensitivity of the device. I bought a Sensym ASDX015A sensor, but realised afterwards that it has an ADC - Microcontroller - DAC. The sensitivity over 4096mV was 3mV..... - only giving 1300 or so quantizations. It worked out that the sensitivity would have been about 6 metres! (21ft), hence why I am going to order the MPX4115AS which is completely analogue.

Andy

[HALE_UAV]

Hey guys,

In my (currently very raw) sensor package project for a high altitude UAV I've used the MPX5010DP for airspeed and All Sensors 15PSIA-4V for altimeter.

Both are working ok - I've been driving around with them in my car.
The airspeed registers the right speed but then drops down to a very low speed. Any suggestions why? I have one port connected to a 3mm tube outside of the car with the other port unconnected inside.

Here is a link to some results I took:
http://smithmatthewguy.googlepages.com/click165.html

Click the polyline to display data in the sidebar.

Ignore the temp and "other" outputs and I'm waiting to use the wind tunnel before I convert the altimeter output to meters as opposed to raw ADC output.

Any help is greatly appreciated.

Cheers guys

[kbosak]

Hi kbosak,

I'm looking at various sensors for precision altimeters at the moment. You can get the Motorola MPX4115AS from Farnell in the UK. I think they will send to Poland for you. They are about £15 GBP. I will be ordering one of these. I like the fact that they have temperature compensation and linear op-amp built-in. Also, the AS version has a port that you can put a silicon tube on.

Look here:

http://uk.farnell.com/1457152/semiconductors/product.us0?sku=FREESCALE-SEMICONDUCTOR-MPX4115AS

Andy.
I can get most MPX from many online shops in Poland (say www.tme.pl), for much less shipment.
The problem is with availability of Intersema.
The problem with MPX is that I have to add and external ADC in order to get more than 10 (12bit) accuracy that is the limit for uC built-in ADC.
It is easier to solder one element than two. Look at SCP1000 from sparkfun, has built-in reasonnable ADC.
My problem with Polish 'supply chain' is that while I can get many analog sensors, getting good ADC from analog devs is not that easy: they are either cheap 8 bit (I can get the same from uC in sleep mode), or they are 16 or 24 bit, very fast, 8 or 4-channel, and cost some 90USD. No slow sensors (few kHz) at 14-16 bit - this is painful. I mean I can order anything from anywhere - the problem is I cannot get rid of 2-4 week delivery time for the most basic 16bit ADC.

[zik]

Yeah I agree with kbosak, the SCP1000 is a winner. I'm using it in my SkyBot autopilot.

If you want something cheap the HP03D is pretty good too. This is the one I'm using on Flying Fox. Like the SCP1000 it has an on-board A->D, something I consider pretty important in highly sensitive sensors.

The HP03D doesn't have the same accuracy as the SCP1000 but it's hard to beat on price/performance.

http://www.hoperf.com/pro/hp03d.html
http://www.futurlec.com/Pressure_Sensors.shtml