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).
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.
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.
The inverse pendulum is controlled through shadow sensors and magnetic actuators.
The LaCoste stage is controlled through a shadow sensor and magnetic actuator as well as the heating of the suspension coil spring.
The Roberts linkage is controlled through shadow sensors and the heating of the four suspension wires.
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.
The Pitch of the control mass is actuated by two vertical magnetic actuators.
The optical lever setup, using a quadrant photodiode placed outside the vacuum envelope.
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.
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.
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.
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.
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 .
(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.
I/O channel allocation usage of DSP.
Stages and relevant DoFs in the control scheme.
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