Download G050486-00 - DCC

Wireless optical controls of LIGO static
suspensions actuators
Maria Paola Clarizia
University of Sannio
Mentor: Riccardo De Salvo
Calum Torrie
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LIGO R&D
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The aim of this project
A wireless control of a stepper motor to
be used to balance the MIRROR
suspension system of Advanced LIGO
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Seismic noise

At very low frequencies, seismic motions may
dominate the detectors' noise, degrading their
sensitivity

The suspension system provides an isolation
of the mirrors from the ground motion

For Advanced LIGO a quadrupole pendulum
mirror suspension is used
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The suspension system
ATTENUATED
OPTICAL BENCH
22 Kg
22 Kg
40 Kg
40 Kg
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The tuning mass
THE
TUNING
MASS
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PROBLEM: no tuning is possible once the
vacuum chamber is close
SOLUTION: remote control of the mass to
adjust the pitch of the mirror
METHOD: a stepping motor,which will move
its own mass by means of a threaded
shaft
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The idea
wireless circuit to control the
motor, in order to eliminate mechanical
A
and electrical perturbations on the
suspended masses, as well as an
excessive load of outgas on vacuum.
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What is a stepping motor?

It’s an electric motor that rotates in
small discrete steps (1.6O)

An internal rotor containing
permanent magnets is controlled by
a set of stationary electromagnets
that are switched electronically

Stepping motors provide position
holding torque while not in motion.
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How to make steps
A burst of current in the two coils is
required to generate a step.
Steps
Coil 1
Coil 2
Coil 1
1
+i
2
3
+i
-i
4
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Coil 2
-i
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How to test the motor
•
A charge is stored on a
capacitor
•
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A burst of current is injected
in the coils by means of FET
switches
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The feeding circuit
PULSE
GENERATOR
SW
LASER
PD
TRANSFORMER
RECTIFIER
and
CAPACITOR
SW
MOTOR
SW
SW
The LASER signal used is a
square wave with frequency
680 Hz
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In real life the laser would
be coupled to the diode via
a
telescope
through
vacuum view port
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Some specs
The laser:
Max control freq.: DC to 300 kHz
Output power: 1 mW
Wavelength: 635 nm
The silicon detector:
Responsitivity (at 635 nm) = 0.42 A/W
The output current of the detector is
about 0.42 mA
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The transformer
The main formula:
V1 N1 I 2


V2 N 2 I1
...but actually it’s not so simple!!!
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Having fun…
WINDING IT ALONG
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The wire used is
very thin (AWG 35,
that is a diameter of
0.203 mm)
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Influences on the voltage
The voltage that can be obtained on the
secondary strongly depends on:

The wire choice

The section of the FERRITE core

The excitation frequency
Increasing the section of the FERRITE core, the
frequency corresponding to the maximum voltage
decreases
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The voltage on the secondary
20
15
10
V
5
0
-5
-10
-15
-20
-5
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-3
-2
-1
0
sec
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3
4
5
-3
x 10
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The rectifier
1
2
this kind of rectifier gives a DC voltage which is the peak
peak voltage of the wave coming from the transformer (a
SMALL voltage drop is due to the diodes).
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Impedance matching
The rectifier impedance changes while the
capacitor charges up.
6
0
V
V
2
6
4
4
2
2
0
0
V
4
6
-2
-2
-2
-4
-4
-4
-6
-5
-4
-3
-2
-1
0
sec
1
2
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4
5
-3
x 10
-6
-5
-4
-3
-2
-1
0
sec
1
2
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4
5
-3
x 10
-6
-5
-4
-3
-2
-1
0
sec
1
2
3
4
5
-3
x 10
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Choice of the capacitor
The generation of a MOTOR step requires a
certain amount of charge.
The bigger the capacitance, the slower the
charging up TIME, but the smaller the
capacitance, the higher the voltage required
for A motor STEP.
The capacitance has to be big enough to
defeat stictions
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The capacitor chosen
Several charge up times have been measured for
different values of the capacitance and different
transformers.
A capacitor of 100µF seems to be a good tradeoff!
Voltage needed: 5 V
Time of wait for one step: approximately one
minute WITH 1 mW LASER POWER
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The charging up curve
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Further improvements

The transformer used gives a good voltage, but it’s not
optimized: a higher voltage could allow a smaller
capacitor or a faster charge up time to be used.

Impedance mismatching should be taken into account

A laser with a larger power can be used to provide a
stronger signal and to lower the waiting time for one
step
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Next steps
Developing the FET SWITCH control circuit
of the motor.
The principle is to use frequency jumps to
control gates of FET switches and four
bandpass filters to route the current pulses.
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acknoledgements
My mentors Riccardo De Salvo and Calum Torrie
My professor Innocenzo Pinto
Dave Grimmett, Paul Russel and Peter King
Marco Tarallo
Ilaria Taurasi
Yuri, Chiara, and all the italian people (including John HONORARY
ITALIAN)
United states for:
•Wonderful
places
•Wonderful
people
G050486-00-R •Wonderful
cookiesLIGO R&D
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THAT’S IT !
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