1. Sub-nanometer displacement measurements in seismic sensors.
2nd PACMAN workshop
Debrecen, Hungary
Peter Novotny
This project is funded by the European commission under FP7
Project number

2. Content What are seismic sensors. Seismic sensors in PACMAN.
Limitations of state of the art sensors.
Understanding the sensor resolution.
Measurement of the sensor resolution.
How to improve sensor resolution.
Content

3. What are seismic sensors and their working principle.
Instruments which measures ground motion.
What are seismic sensors and their working principle.
Not only earthquakes but also motion in nanometres.

4. Why do we need seismic sensors in PACMAN?
To know how ground motion influences characterization of BPM.
Why do we need seismic sensors in PACMAN?
Seismic sensors
Ground

5. Force Balanced optical
Limitations of state of the art sensors for PACMAN.
Seismometers
Bandwidth
Resolution
PZT Accelerometers
Bandwidth
Resolution
Geophones
Bandwidth
Resolution
PACMAN requirements:
Bandwidth = 0.1 ~ 200 Hz
Resolution ≤ 0.1nm
Magnetic fields resistance
Force Balanced optical
accelerometers
Bandwidth
Resolution
!! Point out the resolution!
MEMS accelerometers
Bandwidth
Resolution

6. How do we measure sensor’s resolution?
Main problem is that perfect quiet place doesn’t exist!
ADC
How do we measure sensor’s resolution?
Compare to a microphone acoustic noise which can be measured in soundproof chamber
Resolution is given by sensor’s self noise
Ground

7. How do we understand resolution.
RMS or “average” motion in the bandwidth of interest.
How do we understand resolution.
= 0.14nm
𝑹𝑴𝑺= 𝒇 𝟏 𝒇 𝟐 𝑷𝑺𝑫 𝒇 ×𝒅𝒇
Bandwidth = 0.1 ~ 200 Hz
Resolution ≤ 0.1nm

8. PSD resolution of different sensors.

9. How to improve sensor resolution?
Implement high resolution sensor for displacement measurement
Options:
Resistive, Capacitive, LVDT, Optical encoders, Piezoelectric, Eddy current, Interferometers, …
After applying following
requirements:
Contactless
Sub-nanometer resolution
Magnetic field resistance
How to improve sensor resolution?

10. How to improve sensor resolution?
All technologies implemented before but results are influenced by:
Ambien environment (temperature, humidity, air refraction index, …)
Mechanical design
Data acquisition hardware
Signal processing algorithm
This makes comparison very difficult and unclear.
How to improve sensor resolution?

11. Direct comparison = no data ambiguity
Capacitive
sensor
Implementation of displacement sensors to same mechanical body
Optical encoder
Seismic mass
Interferometer

12. Increasing sensitivity to improve resolution
Implementation of multi-reflection Michelson interferometer
Increasing sensitivity to improve resolution
Displacement [nm]
Corr. Dif vs mass locking M2 M3

13. Seismic sensors will be used to check how ground motion influences BPM characterization.
Resolution is the main limit for state of the art sensors.
What we understand under resolution and how do we measure it.
Direct comparison of sub-nm displacement sensors to overcome data ambiguity.
Increasing the resolution with multi-reflection Michelson interferometer.
Conclusions

14. Thank you for your attention!

15. RMS of sensors noises

16. Self-noise measurement problem
Mass locking vs. corrected difference??
Self-noise measurement problem