c) Explain why a proportional controller would not be appropriate for this system.

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6. The attitude of a satellite is maintained at its reference value by thrusters mounted on its outer rim, a
distance r from the central axis, Figure Q6. Gravitational forces tend to rotate the satellite out of its
correct attitude, in effect by the application of a disturbing moment D. The actual attitude 0, is measured
by a sensor which has gain K, and is fed back to be compared with the reference value 0,. The error
in the attitude E is passed to a controller with transfer function G.(s) which then activates the thruster
to produce an appropriate force F. The total moment applied to the satellite is then the torque from the
thrusters, plus the disturbance torque. The satellite has a moment of inertia about its axis of rotation of
J kg m².
a) Draw the block diagram of this system.
b) Show that the transfer function relating the attitude of the satellite, to a disturbance, D, for the case
where the reference attitude is zero is given by
0(s)
1
D(s)
Js² + KrG.(s)
c) Explain why a proportional controller would not be appropriate for this system.
d) If a proportional plus derivative controller is used, determine the values of the proportional and
derivative gains, K, and Ka respectively, if a system damping of 0.8 is required and the transient
response is to settle to its steady state within 5 seconds of any disturbance. The system parameters
are J = 2000 kg m², r = 2 m, K = 5.
e) Derive an expression for the transient response of the satellite to a step change of 20 Nm in the
disturbing moment D(s).
F
Figure Q6.
Transcribed Image Text:6. The attitude of a satellite is maintained at its reference value by thrusters mounted on its outer rim, a distance r from the central axis, Figure Q6. Gravitational forces tend to rotate the satellite out of its correct attitude, in effect by the application of a disturbing moment D. The actual attitude 0, is measured by a sensor which has gain K, and is fed back to be compared with the reference value 0,. The error in the attitude E is passed to a controller with transfer function G.(s) which then activates the thruster to produce an appropriate force F. The total moment applied to the satellite is then the torque from the thrusters, plus the disturbance torque. The satellite has a moment of inertia about its axis of rotation of J kg m². a) Draw the block diagram of this system. b) Show that the transfer function relating the attitude of the satellite, to a disturbance, D, for the case where the reference attitude is zero is given by 0(s) 1 D(s) Js² + KrG.(s) c) Explain why a proportional controller would not be appropriate for this system. d) If a proportional plus derivative controller is used, determine the values of the proportional and derivative gains, K, and Ka respectively, if a system damping of 0.8 is required and the transient response is to settle to its steady state within 5 seconds of any disturbance. The system parameters are J = 2000 kg m², r = 2 m, K = 5. e) Derive an expression for the transient response of the satellite to a step change of 20 Nm in the disturbing moment D(s). F Figure Q6.
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