Mirror Suspension

(A simplified model of an Advanced LIGO mirror suspension)

Ludovico Carbone
Dave Hoyland
Steve Brookes

2012

Gravitational wave detectors use cascades of pendulum to reduce the motion of the suspended mirrors that are caused by ground vibration, due to human and seismic activity. The benefits of this vibration isolation technique are well shown in our Processing sketches Pendulum and Augmented Reality Pendulum: for vibrations which are faster than the natural period of oscillation of the pendulum (called "resonance"), suspending the mirror as a pendulum makes the mirror much quieter than the ground.

For our exhibition Looking for black holes with lasers we have developed a hands-on exhibit, the "Mirror Suspension'" to show how the "mirror masses" that make up interferometric gravitational wave detectors are isolated from ground vibrations. Our exhibit is a double suspension system, i.e. a cascade of two "masses". For this replica we use perspex cylinders as "mirror masses", whereas in a real detector these are made of very high purity - and very high cost - super-polished glass cylinders. The wires hanging the masses are made of the same steel wire which was used during the first three observational runs of the gravitational wave detector VIRGO, from 2007 to 2010.

Suspension model Detail of a suspended `mirror mass'
Left: Our "Mirror Suspension" exhibit: the support frame, the "mirror masses" and two electronic units are visible. Right: Side view of the top stage "mirror mass". The four steel wires are clamped on the sides of the perspex cylinder. The two-mirror system hangs from the support frame.

In our exhibit we shine a laser beam on the "mirror mass" hanging at the bottom in order to emphasize the effect of seismic motion, in the same way this would manifest on the mirrors of a full scale GW detector. A 1 inch glass mirror was mounted in a recess of the hanging mass and it reflects the laser beam that can be observed jittering when the pendulum is shaking.

A photograph of the `mirror mass’ hanging at the bottom of the suspension exhibit. The red laser spot is visible on the 1” glass mirror mounted on the perspex mass.

Active control of the pendulum

In a GW detector, it is necessary not only to passively suppress the disturbing vibrations caused by nature and by human activity and that we generically refer to as "seismic noise", but also to control actively the positions and the motion of the suspended masses. For example, one may need to correct the static position of the mirrors to optimize the angular and longitudinal alignment of the interferometer, or compensate for long term drifts to keep the relative distance of mirror masses constant across the km-long interferometer arms. Occasionally, one may be interested in inducing a motion of the mirror masses in a particular frequency range of interest with the purpose of measuring a specific effect or for simple calibration purposes. Furthermore, when using cascades of pendulum suspensions, one needs to damp the motion of the mirror masses at the pendulum resonant frequencies: while seismic motion is very efficiently suppressed for frequencies above the resonance, it is vice-versa amplified at the pendulum resonance.

This can be achieved by means of active feedback control systems where displacement sensors are employed to read the position of the suspended mirrors and "sense" their motion. The sensors information is an electric voltage signal proportional to the mirror mass position that is then amplified, filtered and optimized in intermediate electronics stages. This is eventually sent back to a set of "actuators", devices that can apply forces on the mirrors to purposely correct the position of mirror itself to the desired values.

BOSEMs

At the University of Birmingham, we have designed, developed and produced an integrated system of sensors, actuators and analogue electronics that will be used for the dynamic control of the suspended masses of the 4km Advanced LIGO detectors in the USA.

Our demonstration model shows a functioning example of such `sensing and control scheme’, featuring some of the actual instruments that we have produced. On the stand we have four BOSEM units, which are the sensors that sense the motion of the top cylinder. The BOSEM signal is acquired by the "Satellite Box", which works as amplifier and for filtering. The "actuator" too is integrated in the BOSEM, so that it can apply forces directly on the "mirror mass", in the same location where the mass motion is measured. Therefore the Satellite Box signal is sent to the "Coil Driver", an electronic unit designed to supply the BOSEM actuator, and eventually applied on the hanging mass.

Two of the four BOSEMs (#1 and #3) used to sense the motion of the top mass, which is visible on the left.
The `Satellite Box' The `coil driver' electronic unit
Left: Photographs of the Satellite Box, the electronic unit used to power the BOSEM sensors and to amplify their signal. Right: Photograph of the Coil Driver, the electronic amplifier used to supply signals to the BOSEM coil actuator.

The working principle of the BOSEMs sensing and acting is shown in the images below:

Cartoon of the BOSEM readout working principle Cartoon of the BOSEM actuator working principle
Left: Working scheme of the BOSEM position sensor. An opaque object called "flag" I mounted on the suspended "mirror mass". The flag is used to obscure a fraction of the light cast by a LED onto a single-quadrant photodiode (PD), generating a current in the PD that is related to the position of the flag. As the mirror moves, the current on the photodiode changes accordingly. A set of lenses and a mask are also included to improve the collimation of the beam emitted from the LED. Right: Working scheme of the BOSEM actuator. A magnet is located between the flag and the mirror mass whose motion is monitored by the BOSEM. A purposely chosen current is sent to the coil that surrounds the magnet, generating a magnetic field that produces a force on the magnet itself. The mirror mass will move accordingly.

A simplified scheme of the integrated sensor and actuator system is shown in the scheme below:

A schematic of the integrated BOSEMS sensing and actuator system.

Controls

In the `Mirror suspension’ exhibit, the user can activate the feedback control system by enabling the "CONTROL" button on a control pad. When the control is enabled, the electrical signal fed to the actuator is such that a "force opposite to the motion of the pendulum" is exerted on the pendulum itself: the effect is a rapid damping of the pendulum oscillations.

The Advanced LIGO UK Project

The BOSEMS and the related electronics for the Advanced LIGO suspensions were developed within the "Work-Package 4" of the Advanced LIGO UK Project grant funded by the STFC in the period from 2004 to 2011. The Advanced LIGO UK Project is a collaboration between the University of Birmingham, the University of Glasgow, the University of Strathclyde and the Rutherford Appleton Laboratories.

Within the project, several people have contributed to the successful development and production of the BOSEM system (in alphabetic order): Stuart Aston, Ludovico Carbone, Ron Cutler, Andreas Freise, Justin Greenhalgh, Jay Heefner, Dave Hoyland, Nick Lockerbie, Deepali Lodhia, Norna Robertson, Clive Speake, Ken Strain and Alberto Vecchio.

Further reading

Further reading about the pendulum suspension system currently in use in Advanced LIGO and about the sensors and actuators developed at University of Birmingham for Advanced LIGO is available from the following articles:

Acknowledgments

We acknowledge the contributions from Stuart Aston, Ron Cutler and Paul Fulda who helped at various stages in the development of this exhibit.