vibration platform for the calibration of optical sensors

Roland Kruse, Tiedo Meyer:

Vibration platform for the calibration of optical sensors

Roland Kruse, Tiedo Meyer (Oldenburg University): Vibration platform for the calibration of optical sensors

Overview

The project KOLOS

Vibration sensor

Measurement principle

Vibration platform I

Concept

Characteristics / Performance

Vibration platform II

Characteristics / Performance

Summary


The project KOLOS

(Cost-Effective Laser Sensor for Optical Vibration Measurements)

• Low cost, contactless sensor for measuring the amplitude of in-plane vibrations

• Based on the evaluation of the speckle pattern of the surface

• Alternative to laser vibrometers (large, expensive but contactless) and accelerometers (low cost, triaxial measurement, high precision but has to be bond to surface)

Device required for

• Calibration of the new sensor

• Check it’s performance (amplitude and frequency range)

• Check the effect of surface - normal vibrations(Can the sensor surely distinguish vibrations in different directions ?)

Needed: Device for reproduction of vibrations in 3D


The vibration sensor I

h(x)

raue Oberfläche

Laserstrahl

Phasen-

differenz

Beobachtungs-

Schirm (CCD)

sphärische Wellenfronten

resultierendesSpecklemuster

interferierende

Wellen

h(x)

raue Oberfläche

Laserstrahl

Phasen-

differenz

Beobachtungs-

Schirm (CCD)

sphärische Wellenfronten

resultierendesSpecklemuster

interferierende

Wellen

Rough surface

Laser beam

Screen / CCD

Speckle pattern

Interfering waves

Path length difference


The vibration sensor II

Typ A Typ B Typ C

Laser LaserLaser

CCD

Probenbewegung

CCD CCD

Specklebewegungnach rechts

KeineSpecklebewegung

Specklebewegungnach links

ProbenbewegungProbenbewegung

Typ A Typ B Typ C

Laser LaserLaser

CCD

Probenbewegung

CCD CCD

Specklebewegungnach rechts

KeineSpecklebewegung

Specklebewegungnach links

ProbenbewegungProbenbewegungSample movement Sample movement Sample movement

Specles move right

Specles move right

No specle

movement


The vibration sensor III

No motion With linear motion


The vibration sensor IV

0

20

40

60

80

100

120

140

160

180

200

0 0.05 0.1 0.15 0.2 0.25

f = 50 Hz

f = 100 Hz

f = 130 Hz

f = 200 Hz

Vibration amplitude [µm]

Arb

ita

ry m

ea

su

re o

f c

orr

ela

tio

n


The vibration sensor V

Diode laser

Lens

Camera


Vibration platform: Concept

• Aluminium plate (≥ 5 * 5 cm²) driven by electro-dynamical shakers

• Elastic suspension

• 3D accelerometer for vibration monitoring

• PC (Matlab) providing GUI and vibration control,

including crosstalk compensation

• Amplitude limited by nominal shaker force,

frequency by first eigenmode of plate

• Requirement: 10 µm amplitude up to 500 Hz (~ 10 g)


Vibration platform I: Model

Rubber damper Shaker, 9N Accelerometer

Aluminium plate, 10 * 10 cm²,

edge polished

Teflon insert


Vibration platform I: Photo

For adjusting contact force shaker / plate


Vibration Platform I: Characteristics

Frequency response

Amplitude range

Crosstalk

Motion pattern


Vibration platform I: Frequency response

50 100 2005

10

15

20

25

30

35

40

45

Frequency [Hz]

Se

nsiti

vity

[ µ

m/V

]

0.1

0.5

1

2

4

Non - linearity: Dependence of frequency response on driving voltage

Mount softening with increase deflection


Vibration platform I: Amplitude range

20 50 100 200 5000.1

1

10

100

1000

Frequency [Hz]

Dis

pla

ce

me

nt [ µ

m ]

FShaker

< FMass

+ FSpring

Mass and spring system

X - direction: Amplitude range for THD+N < 1%

FShaker, Max = Fspring deformation + Fmass acceleration

Performance unexpectedly poor


Vibration platform I: Crosstalk compensation

0 50 100 150 200 250 300 350-30

-20

-10

0

10

20

30

Dis

pla

ce

me

nt [ µ

m ]

0 50 100 150 200 250 300 350-30

-20

-10

0

10

20

30

Dis

pla

ce

me

nt [ µ

m ]

Before compensation

After compensation,

inversely phased signal output


Vibration platform I: Crosstalk compensation

Crosstalk for 10 µm vibration in X - direction

20 50 100 200 5000

2

4

6

8

10

12

Frequency [Hz]

Cro

ssta

lk [%

]

Y

Z

Y comp

Z compx → y

x → z

No compensation

With compensation


Vibration platform I: Motion pattern

Velocity vectors

XY

Z


Vibration platform I: Motion pattern (Z)

X

Y

Z

Position vs. time

Shakercontact

Shakercontact

Freeedge


Vibration platform I: Summary

• Design generally adequate

• Crosstalk cancellation working

• Non – linear behavior means control system required (Proportional controller)

• Vibration amplitude above 300 Hz too low

• Z – vibration not uniform on the surface

• Reduce mass of vibrating parts

• Make design more symmetric


Vibration platform II: Photo


Vibration Platform II: Characteristics

Amplitude range

Crosstalk

Uniformity of vibration


Vibration platform II: Amplitude range X

30 50 100 200 300 50010

-2

10-1

100

101

102

103

Frequency [Hz]

De

flectio

n [µ

m]

a ~ 0.14 m/s²

Y removed

X - direction: Amplitude range for THD+N < 1%

Transition stick -> slip ?


Vibration platform: Model

Rubber damper Shaker, 9N Accelerometer

Aluminium plate, 10 * 10 cm²,

edge polished

Teflon insert


Vibration platform II: Amplitude range Z

30 50 100 200 300 50010

-2

10-1

100

101

102

103

Frequency [Hz]

De

flectio

n [µ

m]

a ~ 0.59 m/s²

Z - direction: Amplitude range for THD+N < 1%

Higher stiffness of suspension compared to X, Y


Vibration platform II: Crosstalk

Crosstalk for 10 µm vibration in X - direction

30 50 100 200 300 5000

1

2

3

4

5

6

7

Frequency [Hz]

Cro

ssta

lk [%

]

x -> zx -> y

No compensation

With compensation


Vibration platform II: Z - vibration

20 50 100 200 500-3

-2

-1

0

1

2

3

Frequency [Hz]

Le

ve

l diff

ere

nce

[d

B]

Difference in vibration amplitude between two positions (edges) on the surface

Difference in stiffness of rubber mounts


Summary

• Platform for reproduction of 3D vibrations has been constructed

• Useful for testing and calibration of (optical) vibration sensors if 1D calibration is not sufficient

• Frequency range of 20 – 500 Hz with an amplitude ≥ 10 µm (maximum 200 µm)

• Simple and effective crosstalk compensation

• Reasonable uniform vibration amplitude on the surface


Thank you for your attention !

vibration platform for the calibration of optical sensors

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