OFV-3001 Modular Vibrometer Controller

For high accuracy velocity and

displacement measurements

OFV-3001 Laser Vibrometer Controller

A vibromeler system comprises an optical sensor head and a controller which provides power for the measuring head and processes the vibration signal.

All Polytec vibrometer optical heads produce the same type of output signal. This signal is decoded in the controller to achieve velocity and! or displace­ment information. Linearity, accuracy, resolution, dyna­mic range etc. are deter­mined strictly by the type of decoder used.

Realistically. no single decoder can cover the immense measurement range (140 dB) and bandwidth (> 20 MHz) which the optics head provides. This is why Polytec offers multiple choice decoder options for its outstanding OFV-3001 vibrometer controller.

2

The Polytec OFV-3001 modular vibrometer con­troller has been designed with ease of use in mind. The operator is guided by a menu displayed on the large LCD front panel. Selecting measurement ranges, filters and controlling remote focus (for OFV-303 head only) is performed by touch pad keys or remotely via the built-in PC interfaces. A signal strength indication for optimizing the focusing of the optics head is shown on the LCD and also available as an analog output on the rear panel. There are IEEE-488 and RS-232 ports for easy interfacing to a computer system.

The OFV-3001 controller comes in a 19- rack mount housing which can accom­modate up 10 4 different velocity and displacement decoders. This high degree of modularity guarantees long term benefits. leaving room for customer upgrades as applications demand and new technologies allow.

In conjunction with the OFV·)03 optical head remote foc:uJ control can be achieved viii the buttons on the OFV-]OOl front panel. via the built-In PC interfilce or with the optlonill handheld key-pad as shown above.

Velocity decoding

A laser Doppler vibrometer is intrinsically a velocity sensor. The Doppler shift from the vibrating object modulates a 40 MHz carrier signal generated by the Bragg cell in the optical head. The velocity decoder recovers the Doppler information from the frequency modulated carrier and generates the velocity output.

Polytec offers multi­scheme decoding because different measurement ranges place different demands on the decoder.

Possible combinations of velocity decoders (maximum of 2 velocity decoders at the same time) and displacement decoders are shown below:

--- eve·Ol

OVO·Ol ---eVD·02 •

OVD·06

OVD·lO •

QVD·20 •

eVD·30 •

,-"""

, ""

,00_

"...,.

, ...,.

'00 .....

" ....

, "" M

""

OVD·02

•

----• •

• •

4

Range diagram OVO-Ol

, � . . 1 0 rnfs - "'" "". "''' IIXIO mm's I V IIr9I

,." "" , 25 mm's I V I.-.pII ... -25 ....... ''1 .... ,,-$-,'1 .... " .....

• '''''''1-' / � i •

$ t •

"

, It/: 10 Hl 100 H.z 1 o.ttz 10 kHz 100 I:Hz , MHl hq,

OVD-06 Ove·1O

•

• • --- •

• ---• •

OVD-Ol decoder The Polytec OVD-01 is a ve!ocity output modu!e which p!ugs into the OFV-3001 modular controller. It consists of one printed circuit board.

The OVD-Ol uses a special PLL decoding scheme which converts the Doppler signal from the optical sensor into a calibrated velocity output. The OVD-Ol is characterized by it's very high linearity (0.5 %) and low noise level.

The maximum operating frequency is 50 kHz. There are 5 different velocity ranges to choose from:

10 mm/so 50 mm/s, 250 mm/s, 1.25 m/s and 10 m/s full scale peak.

Decoder combinations:

Ove·20 evo-JO

• •

• • • •

---

Range diagram OVO-02

" .. ""'" +-Hf-'---,-Iu I-,-IL IL.J--'-u�"""",- , ""''''''''--Lirl loro_/ V ....

'" "'" , ."

125 ........ /V ....

'00_ 2$IIIIVt/VIWIQII SI_

• ...... ,V ....

"""'" • 0

'""'" j '00""" 1

""'"

, ."'" ..,

'"

1Hz 10Hz 100Hz 1kHz 10kHz 100kHz lMHz tr.:!.

OVD-02 decoder The QVO-02 is the most popular velocity decoder. It uses a special FM decod­ing scheme. Its high band­width (1.5 MHz) and high amplitude (velocity 10 mls and acceleration 9.600,000 g) ranges make it ideal for all applications ranging from low frequency measure­ments 10 ultrasonics and high-g evenls such as pyroshock.

The OVO-02 can be installed side-by-side with the QVO-01 PLL or QVO-06 velocity decoder. As there is only one velocity output at a time the user can toggle between the two decoders using the LeO menu and front panel push buttons.

'I Thoe Ild. scale .aIues corre-spond 10 PM!< output 8t me BNC �tOt. F .... """'" OUlput voItBge is � ID V.

.. Thoe resoIuIlon .. deforMld ... me sig<IaI

amplitude at _ me signaI.to-noise ratio is 0 dB in 8 10 Hz $l)ClCtraI te$OIodion.

' ... '00_

"""'"

'""'"

,00_

,,-

,­"

'"

Range diagram OVO-06

5_/V""",

1�/VIWIQII

• ;; r I J

- IIW.. 8CCl8I.: � 9_

,,-

, � " •

0.1Hz 1Hz IDHz 100Hz till-\: 10kHz lookHl lMHz �

OVD-06 decoder The OVO-06 is a new decoder with a demodulation circuit very tolerant to signal dropouts. The decoder is bandwidth optimized for low velocities and low frequen­cies. The demodulation threshold has been signifi­cantly reduced resulting in an increased optical sensi­tivity. The QVO-06 is ideally suited for long-range measurements on buildings. bridges etc. and all other low frequency applications

demanding highest pos­sible sensitivity. With this demodulator, vibration measurements over 250 m distance were carried out on Italy's famous Leaning Tower of Pisa. The OVO-06 can be combined with the QVO-02 decoder and is standard in the PSV-200 Polytec scanning system. The new OVO-06 demon­strates Polytec's commit­ment to continuous perfor­mance improvements.

Module Measurement Full scale output Resolution .. Maj(. Signal Ma.. Unearity errOf range (Peak to Peak) 'I frequency Acceleration (maximum)

mm/oN mm/, ,m/, kH, 9 % of reading

OVO-01 1 20 0.3 25 150 0.25

5 100 0.6 50 3.000 0.25

25 500 0.8 50 25,000 0.25

125 2500 1 50 25,000 0.25

1,000 20.000 2 50 200.000 0.25

OVO-02 5 100 0.5 250 8,000 1

25 500 1,5 1,500 240,000 1.5

125 2,500 2 1,500 1,200.000 1

1,000 20,000 5 1,500 9,600.000 1

� 1 20 0,3 10 kHz 70 1

5 100 0.3 511H1: ___ " .. » 200 1 """

5

Module

OVO-20

OVO-tO

"d Ove-20

OVD-30

Displacement decoding

Displacement information can be calculated by integration of the velocity signal. The OFV-3001 does not rely on such an integrator which can easily cause faulty measurement due 10 integration errors at low frequencies. In the OFV-3D01 displacement is derived completely independently of the velocity output by it's own separate decoders. Even for highest frequencies, displacement information can be achieved from the phase of the Doppler signal. This principle is utilized in the OVO-30 ultrasonic decoder. The displacement decoders OVO-, 0 and OVD-20 are based on a digital method called fringe counting, where the number of fractional wavelengths is counted while the surface moves. The same method is established as a highly accurate reference for accelerometer calibration in many certified test labs.

The OVO-' 0 and OVO-20 displacement decoders utilize a digital to analog converter to transform the digital fringe counts to an analog output voltage.

Either the OVO-l 0, OVO-20

or the OVD-30 decoder can be installed in the OFV-3001 at the same time. Subsequent instaJlation or up-grade of the OVD-l 0 10 OVO-20 is easily accomplished.

---

, .. �

""

Range diagram OVO-I 0

...... �..., ..... 5120 pIIIlV ""11' la ..... 1280 pili 1 V ""11'

� .. ..... , � ,

......

.'" .....

0,02511'" 001 pili

.......

OVD-1 0 decoder The QVD-l0 displacement decoder has 5 range settings with resolutions of 80 nm, 320 nm, 1.3 IJm, 5 IJm and 20 IJm. The full scale output of the ranges is determined by the 12 bit counter of the AID converter, I.e. 20 IJm resolution )( 4096 (= 12 bit) equals 82 mm peak-to­peak displacement.

The full scale analog output corresponds to a voltage of +1- 8 volts.

Naturally the displacement measurement is more sensitive to DC movements, thermal effects and envi­ronmental influences. The analog output can be resel 10 zero by pushing a button on the front panel or by applying a trigger pulse 10 the clear BNC connector.

'I The ,.. ICM ...... COI1'eIQOIId 10 '*'" CUIPuI -' 11-. BHe �Of F .. ICaIe 0UIpuI \IOItIIgtI iI * e �

• Tt. ..o/I,Itlon 11 unOIrsIOOd .. , digI1 ot 11-. rmge __ 0UIp.Il.

Measurem6flt Full scale output Resolution ., Max. Signal MaJ(. Velocity Unearity efTOI' range (Peak to Peak) " bandwidth (-3 dB) ""IV mm ,m kH, mI. % of full scale

0.5 0.008 0.002 25 0.06 0.1

2 0.032 0.008 75 0.25 0.1

• 0.13 0.032 75 1.0 0.1

20 0.32 0.08 2'" 2.5 0.1

80 1.3 0.32 '''' 10 0.1

320 5.1 1.3 '''' 10 0.1

1280 20.5 5 '50 10 0.1

5120 B2 20 '50 10 0.1

SO MW 1SO"", O.OOt AI VHz 20MHz Oml. <-1 %1n:25nm

6

Range diagram OVO-l 0 + OVO-20 combined

----

".�

".�

�120�/Vf3t9' 10"...

1280I""/Vf3t9'

320 I"" I V f3t9'

I!OI""IV'-9'

201""1V'-9'

'�IVra"'lO'

21A1V .......

1Hz 10Hz 100Hz IIJ-Iz 10kHz loolJ-lz IMHl nq.

OVD-20 decoder

The OVD-20 displacement module is identical to the 5- range OVD-1O, but includes three additional range settings with 2 nm, 8 nm and 32 nm resolution. Again lull scale outputs are detennined by the 12 bit ADC and are tabulated above.

The high resolution of the OVD-20 combined with it's excellent accuracy makes it suitable lor calibrating displacement sensors and accelerometers. For appli­cations requiring a larger displacement range with the same high resolution 01 the OVD·20. an external fringe counter board with a 32 bit counter is available (see separate OFV-600 brochure).

7

Range diagram OVO-30

The OVD-20 has similar reset features as the OVD-l0. Disturbing environmental vibrations can be eliminated by a trigger signal connected to the clear input.

The OVD-l 0 displacement module can easily be up­graded to a high resolution OVD-20 module without lactory return 01 the controller.

OVD-30 ultrasonic decoder

The OVD-30 displacement decoder module covers the ultrasonic range from 50 kHz to 20 MHz. Different Irom the digital decoders (OVD-l0, OVD-20) the OVD-30 measures in an analog way the phase of the Doppler signal. This phase is directly related to the displacement of the vibrating surface. The OVD-30 detects displacements ranging from less Ihan one Angstrom up to +/- 75 nm. The decoder is not sensitive 10 environmental vibration,

i.e. disturbing frequencies below 25 kHz are effectively suppressed. The OVD-30 module inslalls in the OFV-3001 controller. It has 2 signal bandwidth sett­ings: A 2 MHz bandwidth limited output and an unfil­tered 20 MHz oulput. Typical applications of the OVO-30 are non-destructive testing and calibration of piezo ceramic sensors.

The OVD-30 ultrasonic module cannot be combined with the OVD-l 0 I OVD-20 fringe counter type displacement decoders. Subsequent installation of the OVO-30 requires return of the OFV-3001 controller to Polytec.

Polytec: companies

POLYTEC GmbH Polytec-Platz 1-7 76337 Waldbronn Tel.: (07243) 604-0 Fax: (07243) 69944 Email 101457, 1541 @ eompuserve.eom

UK LAMBDA Photometries Ltd., lambda House Batford Mill, Harpenden, HEATS AL5 5BZ Tel.: 01562-764334 Fax: 01582-712064

France POLYfEC PI, S.A. Division AMP 32 rue Delizy F-93694 PANTIN Cl!dex Tel.: 1-46103930 Fax: 1 -46100603

Japan PI-POLYfEC KK Akebono-eho 2-38-5 Taehikawa-shi TOKYO 190 Tel.: 0425-267300 Fax: 0425-267301

USA (West) POLYfEC PI, INC. 3152 Aedhill Ave. Suite 110 COSTA MESA, CA 92626 Tel.: (714)650 1835 Fax: (714) 8501831 Email: vibrometers@polytecpl.com

USA (East)/Canada POLYTEC PI, INC. 23 Midstate Drive Suite 212 AUBUAN, MA 01501 Tet.: (506)832 3456 Fax: (506) 832 0506 Emall: vibrometers@polytecpl.com

Visit our Web site: http://www.potytec.eom

Combined range dia9ram for all decoders '0 •

." , ""

10' ��. M '�" x.. '"

r>: � W·

)<'>-.. � � '0 .

� � �� X

1 X • ,0

:x: <% •

•

m -,

10-3

�

� � IX, ,

\><: • ,r$ /

III ,

!>;' , , , /

:? ,0

103 Ht 10'

.I#' ,

.p

Vibtallon Fntqueocy

o Velocity decooer D Displacement decoder

o Expansion of the amplitude measuring range using OFV-600

OVD-30 High frequency decoder

CAUTION LASER RADIATION -

DO NOT STARE INTO 8EAM

1 § • •