14.02.2013 Views

Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Chapter 3 / Sensors / Air Data System<br />

_ _<br />

3.13.4 Measured Air Data<br />

The total and static pressure, and the total temperature are subject to the<br />

same functional error model, characterised appropriately. The injection of<br />

independently errors is controlled by the bit pattern of MS_AD_ER. The<br />

reference angles are corrupted at the reference rate by a time delay of 0.05 s<br />

followed by constant bias and random noise.<br />

M ~<br />

: =<br />

ϕ<br />

SF<br />

( ϕ ( M ) + ϕ + ϕ )<br />

TD<br />

3.13-3<br />

CB<br />

RN<br />

Equation 3.13-4<br />

The constant biases are initialised from a Gaussian distribution with<br />

pressure and temperature deviations of 50 N/m 2 and 20 deg K respectively.<br />

The respective Gaussian noises have a PSD of 25 N 2 /m 4 /Hz and 20 (deg<br />

K) 2 /Hz at the reference frequency. The constant, quadratic and asymmetric<br />

scale factor deviations are set to 0.005, 0.02 and 0.02 respectively and<br />

applied to all measurements.<br />

D<br />

M ~<br />

: =<br />

AD<br />

ϕ<br />

LIM<br />

AD AD<br />

AD<br />

( ( ( ( M ) ) , HSf<br />

O ) QN )<br />

~<br />

ϕ ϕ ϕ<br />

+ ϕ<br />

Q<br />

ZOH<br />

AA<br />

Equation 3.13-5<br />

The analogue measurements are converted into digital form using a 40 Hz,<br />

4 th order Butterworth filter, and a 12 bit ADC subject to 3 bits of noise with<br />

a sampling rate and range matched to the output interface.<br />

3.13.5 Derived Air Data<br />

Air data measurements are pre-processing to determine the pressure altitude,<br />

Mach number and true airspeed.<br />

M : =<br />

P<br />

( ) T<br />

GZ<br />

P , M , V<br />

d,<br />

m<br />

m<br />

TAS<br />

Equation 3.13-6<br />

The process used is simple compared with civil air data systems based on<br />

Bernouilli’s Equation and air data provided by NASA. The variation in<br />

geodetic height with static pressure shown in Figure 3-64 is determined by,<br />

P<br />

GZ<br />

d,<br />

m<br />

<<br />

11_<br />

000<br />

11_<br />

000<br />

≥<br />

P<br />

GZ<br />

d,<br />

m<br />

<<br />

⇒<br />

P ~<br />

GZ<br />

d,<br />

m<br />

23 _ 000<br />

: =<br />

⇒<br />

44331⋅<br />

~<br />

P<br />

GZ<br />

d,<br />

m<br />

⎜⎛<br />

1 −<br />

⎝<br />

: =<br />

~<br />

−5. 255879<br />

( P ) ⎟⎞ S 101325<br />

⎠<br />

Equation 3.13-7<br />

⎛ P ⎞<br />

− ⋅ ⎜ ⎟<br />

⎜ ⎟<br />

⎝ 22632 ⎠<br />

~<br />

S<br />

11000 6342 ln<br />

Equation 3.13-8

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!