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matt116000

MPX5010DP for Airspeed

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Hey guys,

As part of a project I'm doing I'm building an airspeed sensor based on the MPX5010DP. I read in an old post that some of you had done similar things.

I've been testing it in my car with a 3mm tube connected to one port and held out the window and the other unconnected inside the car. The sensor registers the right speed (110km/hr, say) and then drops back down to 14km/hr and then back up to 110km/hr despite being on a highway.

Mr RC-Cam you said that you calibrated your sensor in the car is there something I'm missing?

I know I should have it connected to a pitot-static tube but since its a differential pressure sensor and the other sensor is open to atmos pressure in the car should that not be the same sort of thing?

The tube is fairly temperamental - when I hold it in front of a fan its fairly difficult to maintain a steady speed but I thought that might have been just the fan.

Someone suggested to me that it might be resonance in the tube or the aerodynamics of the car...

Any help would be appreciated.

Cheers.

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Do you have any more info ?

How long dose it drop to 14kmh for ?

Is it steady at 110 and 14 or very bouncy ?

How windy is it on the test day, cross winds ?

Are you holding it well out from the car ?

Dose tilting it make a big difference to the readings ?

Is the front of the tube sharp or rounded ?

Are you using anything to damp the fluctuations ?

Terry

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For accuracy: Both ports should measure the same air space. The pressure inside the car might not be representative due to its methods of ventilation and so on. Also, many pressure sensors are extremely light sensitive, so try blocking ALL light from the air entry hole.

But having said that, I think that there is something else going on. The pressure fluctuations seem a bit extreme (hopefully your firmware is utilizing some data averaging techniques). So, suspect and check everything, including Terry's list of things.

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For accuracy: Both ports should measure the same air space. The pressure inside the car might not be representative due to its methods of ventilation and so on. Also, many pressure sensors are extremely light sensitive, so try blocking ALL light from the air entry hole.

But having said that, I think that there is something else going on. The pressure fluctuations seem a bit extreme (hopefully your firmware is utilizing some data averaging techniques). So, suspect and check everything, including Terry's list of things.

Having been an air balancer for some time, the pitot tube should be outside in undisturbed (inas much as possible) pointing into the airstream, but y'all know that

and the open port inside the car with the windows rolled up as much as possible to keep drafts to a minamum and all ventilation ducts within the car should be shut off so there will be no perceptible drafts

Edited by W3FJW-Ron

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Do you have any more info ?

Yep.

http://smithmatthewguy.googlepages.com/AC13FEB.html

Ignore temp and "other" and altimeter needs windtunnel testing for a proper transfer func

Data stored every 10secs

http://smithmatthewguy.googlepages.com/AC08FEB_1.html

Data stored every sec. Seemingly VERY accurate at these speeds.

How long does it drop to 14kmh for ?

I haven't done a one every sec test at 110km/hr...it is pretty much just one entry...it could be 1 sec or 9 sec or even 19sec...

Is it steady at 110 and 14 or very bouncy ?

similar sort of reasoning as above. Look at the second link - it looks jumpy but thats because I was in a bit of traffic, lights etc so yeah I'd say pretty stable

How windy is it on the test day, cross winds ?

Not very, Fairly light. Would have thought 110km/hr would have dominated

Are you holding it well out from the car ?

No, it was cold :-) held it just outside the window - further testing will most likely involve it extended on a pole

Does tilting it make a big difference to the readings ?

Yeah a bit...tested that in front of the fan but the fan gave off enough emf when I changes settings that it halted the ICD2 in debug - figured the fan brought in a raft of new probs

Is the front of the tube sharp or rounded ?[/u

straight cut off - trying to line up a pitot-static

Are you using anything to damp the fluctuations ?

Only what was recommended by freescale/motorola in the data sheet.

I can use a code averager but wanted to see what raw was before I started that.

An aero guy said virtually the same thing about both ports in the same airflow so I'm gonna try that at well....

Cheers,

Matt

Edited by matt116000

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OK Matt,

I would try to bevel the end or the tube inner and outer.

Solid mount the tube at least 12" from the car body.

See how you get on.

You could also try some filtering/averaging of the readings over maybe 0.5 sec to 1 sec.

Terry

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Hey guys,

Looks like the problem was a combination of the tubing angle change and the aerodynamics of the car. I took your advice Terry and trimmed the edges to try and minimise the effect of angle change and that seemed to work a lot better as well.

I did a few late night constant speed runs while I was basically hanging out of the window (I wasn't driving :-) ) and the sensor responded quite well.

here's a link to one of the result sets.

http://smithmatthewguy.googlepages.com/24Mar08_90_1.html

and one using the camera:

http://smithmatthewguy.googlepages.com/cam10sec.html

Now its onto the next issues: Altitude sensor calibration and program conversion times...there's always something isn't there?! :-)

Thanks for your help guys,

Matt

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Thats good to hear Matt, I'm interested to see how you get on as I'm sure this is a project that I will revisit myself at some point. well done.

Terry

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Hi!

1) I am also working on this type of project and using MPX12DP pressure sensor for measuring air speed. i have a question that when i suck at pitot tube the voltage increases so this means that total pressure increases according to graph provided in the data sheet , is this behavior true or not please reply me about this

2) my second question is this that when no pressure applied or suck at pitot tube the sensor gives me the speed of 97.2 km/hr is this behavior is true or not.

please reply me about this question

Any help would be appreciated.

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