Reduction of Pseudo Vibration in Rotational Measurements Using Sensor Arrays

7/29/2019 Reduction of Pseudo Vibration in Rotational Measurements Using Sensor Arrays

1/2

2T21. peckle Interferometry 405/ - -- - - --- - - - -- - --REDUCTION OF PSELIDO VIBRAT IONS IN ROTATIONALMEASUREMENTS, USING SENSOR ARRAYS

M.L. Jakobsen and S.G. HansonDepartment of Optics and Plasma Research.

Risoe National Laboratory, DK-4000 Roskilde, [email protected] and [email protected]

Angular velocity measurements obtained with optical technologies, using coherent sources, suchas laser Doppler velocimetry (LD V) (Halliwell et a[. I ] ) and speckle shearing velocimetry (SSS)(Takai ef al. [I]), are often contaminated by a noise phenomenon called pseudo vibrations( ~ ~ t h b e r gt a/.2]). Pseudo vibrations are minor fluctuations appearing in the angular velocitymeasurements, and caused by laser speckles and their influence through the measurementalgorithm. In essence, they appear as the speckle pattern repets itself for each revolution, and theprocessing algorithm will repeat the same "errors" not connected to a real variation in therotational speed but not being distinguishable from a real torsional vibration. Therefore, theyappear as a noise contribution, which strongly correlate with the angular position of a rotatingtarget, through the fine structure of its surface. It is often observed in e.g. the power spectrum ofangular velocity time series, where it will appear as a comb function with harmonics of thefundamental frequency of the rotating target. Therefore, pseudo vibrations will interfereparticularly with any meas urements o f fluctuations in angular velocity, w hich correlate to the cycleof the target rotation . In the light of the option of using co mpact, high-perform ance sensors at low-cost prices, the prospect of imp lementing senso r with multiple laser sources or ev en sensor arraysappears. Therefore, this paper will study these new practical opportunities to reduce speckle-related noise.

A compact, high-performance optical sensor could be based on optical spatial-filteringvelocimetry (Jakobsen and Hanson [I]) of the dynamical speckles arising from scattering off arotating solid object with a no n-specular surface. A schematic drawing of the sensor is illustratedin Fig. 1 . The speckles are produced when illuminating a non-specular target surface with coherentlight. The scattered Iight propagates through a Fourier-transforming optical arrangement to thesensor inlet (Rose ef 01.' [I). The sensor for measuring speckle velocity is here based on alenticular array and im plements narrow spatial band-pass filtering of thc intensity distribution o fthe translating speckle patterns. The spatial filter extracts a given spatial frequency in terms of aquasi-sinusoidal intensity distribution. The quasi-sinusoidal spatial intensity distribution ismonitored at various phase steps by an arrangement of a couple of photodetector pairs. Thephotodetector signals have a similar quasi-sinusoidal response as a function of the target angle andallow for simple processing schemes based on e.g. zero-crossing detection. Zero-crossingdetection of the frequen cy of the signals provides real-time measurem ents of the angu lar velocityof a rotating target.

Fig. 2. demonstrates the pseudo vibrations. The power spectra of two angular velocity timcseries, obtained over several cycles of target rotation, are plotted while probing a surfa ce when it isdry and when it is intentionally coated with a thin film of oil. In the first case, the surface will sh owa clear "finger print" for each cycle in the time signal, and this appears as a detrimental combfunction in the power spectra. W hen coating the surfac e with a thin film of oil, the speck le patternsdo not repeat themselves for each cycle, and the "finger print" of the surface is dramaticallyreduced. The presentation will discuss other ways to reduce th e pseudo vibrations, and q uantitativeresults will be presented.I

7/29/2019 Reduction of Pseudo Vibration in Rotational Measurements Using Sensor Arrays

2/2

406 M.L. Jakobsen and S .G Hanson

FlGURE 1. A schematic of the sensor design

FlGURE 2. The power spectra of tw o angular velocity time series illustrate the pseudo vibrations.To the left, the surface is dry , while to the right, a thin oil film coats the surface.

References1. N. A. Halliwell, C.J.D. Pickering and P.G. Eastwood, "The laser torsional vibrometer: a new

instrument", Journal of Sound and Vibration, vol. 93,588-592 (1984).2. N. Takai, T. Iwai and T. Asakura, "an effect of curvature of rotating diffuse object on the

dynamics of speckle produced in the diffraction field", Appl. Phys., B, vo l. 26, 185-192(1981).

3. S. J. Rothberg, J. R. Baker and N. A Halliwell, "Laser vibrometry: Pseudo vibrations",Journal of Sound and Vibration, vol. 1 35 (3), 516-522 (1989)4. M. L. Jakobsen and S. G. Hanson, "Micro-lenticular arrays for spatial filter velocimetry on

laser speckles fiom solid surf aces", Meas. Sci. Tech., vol. 15, 1 949-1957 (2004 ).5. B. Rose, H. Imam, S. G. Hanson, H. Yura and R. S. Hansen, "Laser-Speckle angular-

displacement sensor: theoretical and experimental study", Applied O ptics, vol. 37,2 119 -2 129(1998).

Reduction of Pseudo Vibration in Rotational Measurements Using Sensor Arrays

Documents

Transcript of Reduction of Pseudo Vibration in Rotational Measurements Using Sensor Arrays