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

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Chapter 3 / Sensors / Gyroscopes<br />

_ _<br />

E_W_CB<br />

1<br />

+<br />

-<br />

H +<br />

-<br />

TORQUER<br />

DYNAMICS<br />

4000<br />

s+4000<br />

1/J<br />

C<br />

1<br />

s<br />

1<br />

s<br />

3.4-7<br />

E<br />

PWM<br />

GAIN<br />

FLOAT ANGLE<br />

1<br />

+<br />

+<br />

1/z<br />

Figure 3-28 : Torquer Model<br />

512 D/S 0.25 D/S<br />

+<br />

-<br />

3<br />

D_W_CB<br />

The PWM and torquer can be reduced to a 2 nd order lag with a damping of<br />

0.67 and natural frequency of 160 Hz range followed by limiting and<br />

quantisation. The linear model response to a 1° step is compared to this 2 nd<br />

order closed loop response in Figure 3-29.<br />

LINEAR SYSTEM STEP RESPONSE<br />

1<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

0<br />

0 2 4 6 8 10<br />

TIME (MSEC)<br />

Figure 3-29 : Closed Loop Step Response<br />

The maximum float angle depends on the PWM gain and the maximum<br />

expected angular rate,<br />

G ⋅ γ : = ω<br />

PWM<br />

MAX<br />

MAX<br />

C,<br />

B<br />

Equation 3.4-5<br />

The PWM gain is 500 (°/s)/° for a saturation limit of 250°/s and a maximum<br />

float angle of 0.5°. The quantisation for this saturation limit, given 2 11<br />

pulses per output cycle, is 0.244°/s. The cross-coupling misalignment angle<br />

(γ) is determined from the 2 nd order PWM dynamics when bit 3 of<br />

MS_GS_ER is set. The input to the mechanical gyroscope triad is therefore,<br />

ω<br />

U<br />

C , B :<br />

=<br />

⎛<br />

⎜<br />

⎜<br />

⎜<br />

⎝<br />

ω<br />

ω<br />

ω<br />

IR<br />

C, Β<br />

IP<br />

C, Β<br />

IY<br />

C, Β<br />

⎞<br />

⎟<br />

⎟<br />

⎟<br />

⎠<br />

−<br />

⎛<br />

⎜<br />

⎜<br />

⎜<br />

⎝<br />

γ<br />

γ<br />

γ<br />

R<br />

P<br />

Y<br />

⎞<br />

⎟<br />

⎟<br />

⎟<br />

⎠<br />

⊗<br />

⎛<br />

⎜<br />

⎜<br />

⎜<br />

⎝<br />

ω<br />

ω<br />

ω<br />

SR<br />

C, Β<br />

SP<br />

C, Β<br />

SY<br />

C, Β<br />

⎞<br />

⎟<br />

⎟<br />

⎟<br />

⎠<br />

Equation 3.4-6

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