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Compact vibration isolation and suspension for Australian International Gravitational Observatory: Local control system
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Image of FIG. 1.
FIG. 1.

Isolation stages of the AIGO suspension chain. The preisolation stages include a, b, and c. The isolation stack is defined as the three identical stages of self-damped pendulums with Euler stages. (a) Inverse pendulum preisolator (Ref. 18), (b) LaCoste Linkage (Ref. 18), (c) Roberts Linkage (Ref. 11), (d) Euler springs (Ref. 16), and (e) self-damped pendulums (Ref. 12).

Image of FIG. 2.
FIG. 2.

The shadow sensor is a simple device, where a LED shines a beam onto two photodiodes, and an intermediate shadow mask is attached to the part to be measured.

Image of FIG. 3.
FIG. 3.

The magnet-coil actuator. A magnet mounted on an isolation stage is placed in the center of two coils that are mounted on the support frame.

Image of FIG. 4.
FIG. 4.

The inverse pendulum is controlled through shadow sensors and magnetic actuators.

Image of FIG. 5.
FIG. 5.

The LaCoste stage is controlled through a shadow sensor and magnetic actuator as well as the heating of the suspension coil spring.

Image of FIG. 6.
FIG. 6.

The Roberts linkage is controlled through shadow sensors and the heating of the four suspension wires.

Image of FIG. 7.
FIG. 7.

The control mass has three actuators and shadow sensors collocated on the horizontal plane, in a 120° arrangement. These three signals are converted to an orthogonal reference frame , , , and by a sensing matrix.

Image of FIG. 8.
FIG. 8.

The Pitch of the control mass is actuated by two vertical magnetic actuators.

Image of FIG. 9.
FIG. 9.

The optical lever setup, using a quadrant photodiode placed outside the vacuum envelope.

Image of FIG. 10.
FIG. 10.

Block diagram of the isolation local control system. The signals from shadow sensors and a quadrant photodiode are used to feedback to several stages using magnetic actuators or high current heating.

Image of FIG. 11.
FIG. 11.

Plant gain , loop gain and closed loop transfer function of inverse pendulum horizontal DoF. The diagonalized signal from the inverse pendulum shadow sensors is fedback at the inverse pendulum actuators ( and in Fig. 14, respectively). The digital compensator with a low pass filter at 0.7 Hz.

Image of FIG. 12.
FIG. 12.

Measurement of the frequency response of the test mass in its two angular DoF (the resonant modes are highlighted). (a) Pitch with test mass suspension mode at 3.3 Hz. Note that the broad peak at 280 mHz is due to the rocking mode of the control mass. (b) Yaw with suspension mode at 1.75 Hz. The dotted line shows the measurements with the optical lever feedback off, using only the shadow sensor signal for feedback. The solid line shows the measurements with the optical lever control loop on. The optical lever was used for the measurement of both curves.

Image of FIG. 13.
FIG. 13.

Test mass Yaw angular motion using different feedback loops. The optical lever feedback greatly improves the limit imposed by the low signal to noise ratio of the shadow sensor when sensing the suspension normal modes.

Image of FIG. 14.
FIG. 14.

A simple two pendulum system illustrating the preisolation feedback using shadow sensors. The inverse pendulum position is referenced to the ground, for low frequency position control . The Roberts linkage is referenced to the inverse pendulum, and can be feedback to lower the first resonant mode, and damp the second .

Image of FIG. 15.
FIG. 15.

(a) The measured integrated residual motion of the cavity [, where is the displacement spectrum ]. It is at the nanometer level above 1 Hz. Note that the measurement is limited by laser noise above 2 Hz (Ref. 1), due to the free running laser. (b) A model with the same feedback scheme as used for the measurement. (c) The modeled performance with an optimized preisolation feedback scheme, using the superspring concept. (d) A modeled performance if no feedback was implemented. (e) Assumed input spectrum.


Generic image for table
Table I.

I/O channel allocation usage of DSP.

Generic image for table
Table II.

Stages and relevant DoFs in the control scheme.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Compact vibration isolation and suspension for Australian International Gravitational Observatory: Local control system