Phung LeCong, Al Joseph, Erie Udd, and Paul Theriault MeDonnell Douglas Astronauties Company 5301 Bolsa Avenue Huntington Beaeh, California 92647

Transcription

1 EMBEDDED FIBER OPTIC STRAIN SENSOR Phung LeCng, Al Jseph, Erie Udd, and Paul Theriault MeDnnell Duglas Astrnauties Cmpany 5301 Blsa Avenue Huntingtn Beaeh, Califrnia INTRODUCTION In a variety f space related applicatins, a stable platfrm is essential in supprting and maintaining accurate cmpnent alignment and verall system stability. Any structural disturbance such as bending, strain r vibratin must be detected and crrected in a timely manner. In the experiment described herein, such disturbances were detected by fiber ptic sensrs ernbedded in a cmpsite structure. The utputs were fedback t ernbedded piez-electric actuatrs t actively dampen the disturbances. EXPERIMENTAL PROCEDURE These tests used a cmpsite beam "smart" structure made f twelve layers f DMS Type 8 prepregnated glass clth (equivalent t 181 Glass Clth). The mdulus f elasticity and density f this material after curing is apprximately 3.2 x 106 Ib/in. 2 and.07 Ib/in. 3 respectively. The beam dimens ins are 24 in. x 4 in. x.120 in. The f ixed end 0 f the beam was clamped between tw aluminum plates with HYSOL EA934NA bnding epxy and tw plies f 181 Glass Clth separating the beam frm the plates. The elamping length was 4 in. The fiber ptie 'sensr ensists f aluminum jacketed ptical fiber bnded t the uter surface f the beam using HYSOL EA 3990 adhesive. The sensr is f Sagnae interfermeter type. The interfermeter cnsists f a light surce, tw beamsplitters, aplarizer, a lead-zircnate-titanate (PZT) cylinder, an acust-ptic frequency shifter, and a detectr. The Sagnae interfermeter with the eantilever beam in place in the sensing lp is shwn in Figure (1). Light frm the sur ce f frequency f is split int tw cunterprpagating beams by the secnd beamsplitter. The clckwise beam f light passes thru~h the sensing fiber at frequency f then is frequency-shifted, while the cunterclckwise beam travels thrugh the sensing fiber at f + M (after being frequency-shifted). Once bth beams have been shifted by the frequeney mdulatr, they recmbine at the beamsplitter at a frequency f f + ßf and are directed t the detectr. 1061

2 Cantilever Beam Sensing Lp Light Surre PZT Figure 1. Optical Phase Shift: ßct> = ß(jJ [(L-2y)-ßL] nc AO-Mdulatr Sagnac Strain Sensr The ptica1 phase difference after the tw beams cmplete the lp is: ßcll = ~ [(L-2y) - where ßW is the amunt f angular frequency-shift, y is the distance frm the AO mdulatr t the secnd beam-splitter, L is the ttal lp length and ßL is the change f fiber length during the prpagatin interval. The strain ßL/L in the fiber is prprtinal t the strain n the uter surface f the beam. T ensure true prprtinality, aluminum-cated fibers were used instead f plastic-jacketed fibers t minimize slippage. The resultant ptical phase difference is cnverted int a vltage prprtinal t the change in length f the fiber when the beam is deflected. The cnstant phase shift prduced by ß<Dc = ~ nc (L-2y) cnstitutes a dc bias. In the absence f temperature variatins, this bias des nt affect measurements f the ac strain but it dwarfs the dc strain. It can be calculated if (L-2y) is knwn. (L-2y) is determined by varying ßW until ßcllc increases by 2n rad. In practice ne fine-tunes ßW t set the cnstant phase equal t zer when n strain is applied t the system. In this experiment, the nminal ßW 80 MHz was changed t 76.4 MHz t null the excess phase shift prduced by (L-2y) = 296m f fiber. Thus any applied strain will manifest as a deviatin frm the zer phase shift. In null detectin, the strain-prduced phase shift is nulled by varying ßWi the change in ßW that brings back t null cnditin is the reading fr the strain signal. Remval f the excess phase shift als increases the sensr dynamic range. T cmpensate fr the slwly-varying temperature effect which als changes the fiber length, ßwwas allwed t drift by a temperature-mnitring clsed-lp circuit. At 80 MHz the tuning range is apprximately 300 KHz befre the AO ßL] 1062

3 mdulatr deflects the ptical beam ut f the fiber acceptance angle. T aid in the detectin prcess, the PZT in the Sagnac lp injects an ac-phase shift t dither the signal at a frequency F. The lp electrnics is s adjusted that, when the system is free f any disturbance, the dithering ccurs at the tp f the phase curve hence the secnd harmnie f F appears n the detectr. When the cmpsite is subjected t sme type f disturbance such as astrain r vibratin, the first harmnie signal f F falls nt the detectr that is synchrnusly demdulated t btain the size f the disturbance (Figure 2). In suppressing the structural disturbance, the sensr utput was sent t the piez-electric actuatrs embedded in the beam. The actuatrs then acted t dampen ut the vibratins in the beam. The piez-electric actuatrs cnsist f tw thin sheet ceramies f lead-zircnate-titanate with dimensins f 1.55 in. x.600 in. x.010 in., embedded in equal distance frm the neutral axis. Bth faces are cated ~ 0; ~ "in J!! E. Phase Dither Induced by Frequency Mdulatr Relative Phase <; g 0; Phase Dither Induced by. Frequency Mdulatr Relative Phase Figure 2. Dithering Technique Prcesses Strain-Induced Phase Shift5 1063

4 with nickel and serve as electrdes. The plling gemetries f the tw actuatrs are arranged such that a cmmn vltage causes ne t expand and the ther t cntract thus inducing a bending mment n the cmpsite crss-sectin. Figure (3) shws a plt f passive damping versus active damping in the cantilever structure. The damping is imprved by apprximately 15 times. CONCLUSIONS It has been demnstrated that an embedded ptical fiber can measure minute strain f a cmpsite structure. Since ptical phase shift f 10-6 rad can be detected, the sensr perating at 76.4 MHz can measure ßL dwn t 10-6m. In measuring dc r slwly varying strain, the sensr sees a detectin nise flr f apprximately 10-4 rad. The sensr sensitivity can be increased by perating at high ßffi and by embedding large amunts f fiber int cmpsite material (e.g., running the fiber back and frth). Even thugh the reprted sensr measures strain distributed alng the embedded fiber, lcalized strain can be determined thrugh a grid netwrk f multiplexed fiber sensrs t i---i---f.----i J I..L_--L--_...L..-.--l._-'-_ TIME A: 3. le-ol V O.OOSEC SPAN:O.OOOOOOHZ HZ Figure 3. AVG= SEC / SN:3.5+00V FS:5.0+00V V/ Active Damping vs. Passive Damping 1064

5 ACKNOWLEDGEMENTS The cntributins f R. Michal, D. Jlin, and Phil Gretzkwski are gratefully acknwledged. 1065