APM / Ardupilot 專用空速計

APM / Ardupilot
專用空速計 

Breakout Board MPXV7002DP

(Differential Pressure Sensor
board)

這個模組是副廠製造的,
品質上與原裝產品一樣, 因都是使用 Freescale 原廠的
MPXV7002DP 壓力傳感器!
  

The MPXV7002 series piezoresistive
transducer in the small outline package (SOP) is a state-of-the-art monolithic
silicon pressure sensor designed for a wide range of applications, but
particularly those employing a microcontroller or microprocessor with A/D
inputs. This patented, single element transducer combines advanced
micromachining techniques, thin-film metallization, and bipolar processing to
provide an accurate, high level analog output signal that is proportional to the
applied pressure.

The MPXV7002 is
designed to measure positive and negative pressure. In addition, with an offset
specifically at 2.5V instead of the conventional 0V, this new series allows to
measure pressure up to 7kPa through each port for pressure sensing but also for
vacuum sensing (refer to the transfer function in the data sheet for more
detailed information).

Features:

– -2 to 2 kPa
(-0.3 to 0.3 psi).

– 0.5 to 4.5 V
Output

Weight: 0.1 oz;
4 g

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使用步驟：

http://code.google.com/p/ardupilot-mega/wiki/Airspeed

Using an Airspeed Sensor

ArduPlane supports the use of an airspeed sensor (shown above), which can help in windy
condition, slow flight and autonomous landings. It is not recommended for most
new users, however, as it does require additional tuning and adds one more layer
of control to set up.

The way it works is that the top tube is “active”
(measures air pressure from the pitot tube that is open at the front and has air
driven into it by airspeed) and the bottom one is “static” (measures ambient air
pressure from tube with intakes on the side).

Here’s how to hook it up:

On APM 2

Plug it into the pins on the “A0” port, as
shown:

Software configuration

You need to enable the airspeed sensor as part of
the Mission Planner setup as shown below. You can test that it is working in the
CLI by using the “airspeed” command in the “test” menu.

Note: Oscillation between zero and small values
(2-3) is normal. The airspeed varies with the square root of the pressure, so
for differential pressures near zero it varies quite a bit with very small
pressure changes, while at flying speeds it takes much greater pressure changes
to produce a similar change in speed. If you see mostly 0, 1, 2, with an
occasional bounce to 3 or 4, consider it normal. You will not see that sort of
variability at flying speeds. As a check, you can take the fleshy part of your
fingertip and press it against the pitot tube to raise the airspeed reading up
to say 15 m/s. It is easy to see that holding a significant constant
differential pressure like this the reading does not bounce around (if you keep
constant pressure).

Installing the Pitot Tubes

When you place the airspeed sensor in your
aircraft, use this pitot tube set. The kit comes with a single
tube to measure both static and total pressure. In the case of the EasyStar,
you’ll need to push it through the foam in the cockpit so it points straight
into the airstream. Make sure the holes in the side of the tube are not covered.
They should be at least 1 centimeter out past the nose. First connect the two
tubes coming out the back to the airspeed sensor. The tube coming straight out
the back should go into the top port and the tube exiting at an angle should
connect to the bottom port on the airspeed sensor. Drill or cut a small hole in
the foam and push it through to the front.

If you are using APM in an aircraft with the
propeller in the nose, the pitot tube must be mounted out on one wing, at least
a foot from the fuselage to be outside the prop flow.

Checking operation

You can check the airspeed reading with Mission
Planner or another ground station. Just blow on the pitot tube and observe the
response. In still air oscillation between zero and small values (2-3) is
normal. The airspeed varies with the square root of the pressure, so for
differential pressures near zero it varies quite a bit with very small pressure
changes, while at flying speeds it takes much greater pressure changes to
produce a similar change in speed. If you see mostly 0, 1, 2, with an occasional
bounce to 3 or 4, consider it normal. You will not see that sort of variability
at flying speeds.

You can also check it with the ArduPlane CLI in
the Mission Planner’s terminal screen. Just go into CLI/test menu and type
“airspeed”. Detailshere.

Finally, you can run a small Arduino sketch that
will also test the airspeed sensor. It’s here.