Does anyone know any module on the market that can measure the vertical speed? It is not nessecary for me to have absolute altitude measuring but only to know if the heli is going up or down and how fast. The sensor i m looking for must give measurement at low speeds like 10centimeters/second. If i just know this data i can hover the heli and when i want to land i would go slowly down until i ll get signal from the ultrasonic sensor. If flying high i ll have the altitude by the GPS and just avoiding climb or descent so fast (by the GPS errors) , with such a sensor.

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I can't think of any. The usual speed sensors (pitot tubes) won't work in the vertical dimension on a heli because of propwash. I think this is something you'll have to derive by integrating your static altitude measurements over time.
This is exactly how the vertical speed indicators of certified aircraft work -- mechanical VSI's are essentially an altimeter with a calibrated leak in the case, causing the case pressure to lag behind the real-time pressure by defined time (a second or so, IIRC.)

Since we use electronic sensors, we can use a circular buffer to store altitude readings over time, and use that list to integrate for rate.
What you're looking for is called a "variometer", which is a sensitive vertical speed indicator. There are miniature ones intended for use in R/C sailplanes, but these typically transmit an audio tone to a ground based receiver, so they would require some adaptation if you want to use the signal on-board the helicopter.

I've able to measure vertical speed down to a resolution of less than 0.1 knot (5 cm/sec) using an absolute pressure sensor, 24 bit A/D converter (needs about 18 to 19 bits effective resolution), and digital differentiation, but it takes a lot of specialized code to separate the signal from the noise...
Thank you for replies:)

Can you suggest me a website or model to see the specifications of the miniature variometers please? Are these modules also uses barometer sensor?

Can i use MPX4100A which i have in hand, to achive similar results like your project? I also worry about the noise. What do you think If i narrow the A/D voltage range on the PIC at Vref+ and Vref- and direclty apply to the PIC A/D input just with a simple low pass (RC) filter?
It looks like the EagleTree one sends digital data. I've got their standard Seagull and it's a great bit of gear; I'd never noticed their sailplane products until just now.
the scp1000 has 17 bit of resolution

A resolution of 0.1 knot is impressive Marc.  Can you describe the specialized code you use to pull the signal from the noise?

Jeff, did you notice this thread is nearly 6 years old?  

By "specialized" I meant specific to the implementation of a variometer.  In our case, using the microcontroller onboard ADCs and digital filters, combined with simple linear boxcar filtering in the code was good enough to achieve that level of resolution on a desktop.  In a glider we use the variometer in conjunction with a total energy compensator, which mixes altitude with airspeed to calculate total energy altitude, either before the variometer pressure sensor sees it (using a pneumatic compensating probe), or in the variometer code (using airspeed sensor data).  When in turbulent air, a glider encounters horizontal gusts, causing transient fluctuations in airspeed which, in turn, show up as (sometimes large) fluctuations in variometer reading.  This was addressed by implementing a "gust filter" as the final step in the variometer code.  We tried a variety of filter implementations, but simply rate limiting the change in variometer indication was sufficient to damp down the transients, while not interfering with the all important task of finding rising air.

Hi friends some news abaut vario on 3dr?Ì use from long time this autopilot for make fpv long range and i ask if is possibile use audio input for trasmit to the ground one variometer signal

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