P A P E R

Demonstration of Biofouling MitigationMethods for Long-Term Deploymentsof Optical Cameras

A U T H O R SJames JoslinBrian PolagyeNorthwest National MarineRenewable Energy Center,University of Washington

88 Marine Technology Society Journa

A B S T R A C T

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Biofoulingmitigationmeasures for optical ports can extend the duration of ocean-ographic deployments, but there have been few quantitative studies of field perfor-mance. Results are presented from a 4-month field test of a stereo-optical camerasystem intended for long-term environmental monitoring of tidal turbines. A com-bination of passive (copper rings and ClearSignal antifouling coating) and active(mechanical wipers) biofouling mitigation measures are implemented on the opticalports of the two cameras and four strobe illuminators. Biofouling on the optical portsis monitored qualitatively by periodic diver inspections and quantitatively by metricsdescribing the quality of the images captured by cameras with different antifoulingtreatments. During deployment, barnacles colonized almost every surface of thecamera system, except the optical ports with fouling mitigation measures. The effec-tiveness of the biofouling mitigation measures suggests that 4-month deploymentdurations are possible, even during conditions that would otherwise lead to severefouling and occlusion of optical ports.Keywords: biofouling, optical cameras, environmental and remote monitoring,field testing

attenuationmeters (ACmeters), photo-synthetically active radiation (PAR)

Introduction

Biofouling is often a limiting factorfor long-term deployments of ocean-ographic optical instrumentation.While this study focuses on the foulingof camera optical ports, the methodsand outcomes may be relevant toother instruments that rely on lighttransmission, such as absorption-

sensors, or fluorometers (Manov et al.,2004). The sensitivity of each opticalsensor to biofouling can vary greatly,and results from one instrument shouldnot be extrapolated to another. As bio-logical growth colonizes a camera’s op-tical port, image quality degrades, andthe monitoring mission may be com-promised. With the proliferation ofcabled ocean observatories (Chaveet al., 2009; Howe & McGinnis, 2004;Woodroffe et al., 2008), long-term de-ployments of optical instrumentationare becoming more common, and bio-fouling mitigation methods are receiv-ing more attention. Research in thisfield is generally focused on improvingunderstanding of fundamental bio-fouling mechanisms (such as adhesionand growth) (Phang et al., 2007; Salta

et al., 2013) or development of bio-fouling mitigation measures. For ex-ample, Manov et al. (2004) discuss theuse of copper to prolong deploymentsof open, enclosed, or semi-enclosed andshuttered optical instrumentation, andDebiemme-Chouvy et al. (2011) de-scribe applications of electrochemistyto produce a biocide on the optical portsurface. Whelan and Regan (2006) andDelauney et al. (2010) provide reviewsof existing biofouling mitigation tech-niques and their implementation ondifferent sensors.

Marine renewable energy, includ-ing wave, tidal and ocean current,and offshore wind energy, is a growingsector of the electricity generation in-dustry that requires robust approachesto biofouling. Energy converters and

their support structures are deployedin the marine environment for multi-year periods and cannot expect to re-ceive significant maintenance if theircost of energy is to be competitivewith conventional forms of electricitygeneration. While biofouling is possi-ble on any of the converter surfaces,general-purpose biofouling mitigationmethods may be different from the ap-proach taken for more sensitive com-ponents, such as sensor transducers.Optical camera observations havebeen proposed to inform a numberof critical environmental questions(Polagye et al., 2014), and the shorecables for the energy converters pro-vide sufficient power and data band-width to support high-resolutionoptical measurements over extended

periods. This paper discusses the im-plementation of biofouling mitigationmeasures on the optical ports of a cam-era system developed for long-termmonitoring of marine energy con-verters (Joslin et al., 2014a). This sys-tem will be recovered periodically formaintenance (Joslin et al., 2013), andit is expected that optical port foulingwill be the limiting factor for the inter-val between maintenance. Methods toquantitatively evaluate the effective-ness of these biofouling mitigationmeasures are developed and applied toa multi-month endurance test of thecamera system.

MethodologyField Deployment Configuration

Figure 1 illustrates the stereo-optical camera system developed formonitoring marine renewable energyconverters ( Joslin et al., 2014a). Theintegrated system combines two Allied

Vision Technologies Manta G-201machine vision optical cameras, fourExcelitas Technologies MVS-5000strobes, and the supporting powerand communications infrastructure tocable the system to a shore station. Thesystem is controlled in real time bya computer on shore that can adjustcamera settings (e.g., frame rate, ex-posure time, digital gain, and strobetriggering) and archive acquired stereoimagery. The optical cameras andstrobes are marinized by enclosingthem in aluminum pressure housingswith planar acrylic optical ports. Mate-rial selection has been shown to influ-ence the rate of biofouling on opticalports (Manov et al., 2004), and al-though not optimal for biofouling,abrasion resistant acrylic is used heredue to its transparency for optical im-agery and ease of manufacturing forintegration in the pressure housings.

A multi-month field trial was con-ducted during early 2013 to evaluate

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overall system endurance (hardwareperformance, software stability, corro-sion, and biofouling). After an initialcalibration in a tank, the systemwas de-ployed from January 24 to February 8in freshwater off of a dock on LakeUnion, WA. Subsequently, the sys-tem was deployed in a saltwater envi-ronment from March 3 to July 2 offEdmonds, WA.

For the salt-water endurance trial,the camera system was mountedto the test frame shown in Figure 2.The Applied Physics Laboratory vesselR/V Jack Robertson lowered the testframe to the seabed in approximately20 m of water at a point 100 m fromshore. Mounted to this frame, the cam-era system was suspended 5 m abovethe seabed in a downward-lookingorientation. The power and fiber um-bilical was terminated on shore andconnected to a data logging com-puter. Divers from the Applied Physics

FIGURE 1

Prototype imaging system showing principal components and scale.

FIGURE 2

Five-meter-tall field test frame on the deck ofthe deployment vessel R/V Jack Robertson.

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Laboratory at the University of Wash-ington visually inspected the system forbiofouling and corrosion on March 3,April 11, May 3, and June 26.

Biofouling Mitigation MeasuresA combination of active and pas-

sive biofouling mitigation measuresare implemented on the optical portsof the two cameras and four strobehousings. As shown in Figure 3, each4-inch optical port has a ring of copper(Cu) around its perimeter, which is in-tended to suppress biofouling at theedge of the optical port. Each housingis also equipped with a mechanicalbrush wiper (Wi) manufactured byZebra-Tech Ltd. (http://www.zebra-tech.co.nz/Hydro-Wiper). In addition,each of the camera and strobe ports iscoated with the ClearSignal foulingrelease coating (CS) produced bySevern Marine Technologies (http://www.severnmarinetech.com/). Thiscombination of biofouling mitiga-tion measures is selected based on thedemonstrated effectiveness of coppershutters (Manov et al., 2004) and thepotential additional benefit of theClearSignal coating. Interactions be-

90 Marine Technology Society Journa

tween the copper, wiper, and releasecoating are theorized to increase theantifouling effectiveness over eachmeasure individually. A fully shutteredsystem was considered but deemedundesirable for optical cameras sincefailure in the closed position would ob-viate any data collection and largeshutters would be subject to significantstructural loads while open in an ener-getic environment (wave or current).Ultraviolet lights were similarly con-sidered but not implemented due tothe uncertainty in the appropriatewavelengths for preventing foulingand limited documentation of thisfield in the literature (Manov et al.,2004).

The wiper, when triggered by thecontrol computer in the shore station,sweeps a 90° arc across the copper ringand optical port before reversing di-rection and returning to its “home” po-sition. This action may transfer traceamounts of copper from the ring acrossthe optical port over the course ofmany wipe cycles, thus increasing theeffectiveness of the wiper in isolation.This is, however, a hypothesis thatwas not part of the test matrix duringthis field trial (e.g., removing the cop-per ring from one of the pressurehousings with a wiper). Throughoutthe endurance test, the wipers actuatedonce per hour during normal systemoperation. Electrical interference inthe serial communication bus betweenthe shore computer and camera systemrequired the system to be shut downon six occasions, during which thewipers were not actuated. For the finalmonth of the deployment, the systemdid not run continuously because ofcontinued degradation of the commu-nication bus. To continue collectingbiofouling data during this period, thecameras were brought on-line manuallyeach night to capture images. For this

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month, the mechanical wipers on thecameras actuated once every 24 h, andthe wipers on the strobes were inactive.During this same period, the mechan-ical wiper on Camera 2 (Cu and Wi)malfunctioned and would periodicallystop in front of the optical port aftera wipe cycle, thereby blocking part ofthe image. This did not affect thewiper’s ability to remove fouling butdid complicate quantification of bio-fouling rates (see the following section).This malfunction was caused by agradual increase in friction betweenthe wiper and the optical port causedby fouling on the wiper brush andmay be avoided by decreasing the in-terference spacing between the wiperbrush and port.

Figure 4 illustrates the arrange-ment of the biofouling mitigationmeasures on the six optical ports inthe system. Strobe 3 (Cu) was intendedto serve as a control with minimal anti-fouling protection by disabling thewiper. However, an interruption tothe bottle’s power supply would cause

FIGURE 3

Biofouling mitigation measures on an opticalcamera port (preendurance test).

FIGURE 4

Arrangement of antifouling measures on cam-era system optical ports (S denotes strobe, Cdenotes camera, and MEB denotes the mainelectronics bottle).

the wiper to automatically actuate, andsince the system was power cycled onsix occasions, the results for this opticalport cannot be considered a controlcase.

Qualitative and QuantitativeEvaluation of BiofoulingMitigation Measures

Performance of biofouling mitiga-tion measures was monitored quali-tatively during the endurance trial bydiver inspections and quantitativelythrough the images captured by thecameras. During the inspections, thedivers visually checked for the pres-ence or absence of macrofouling onthe six optical ports and adjacent sur-faces but did not disturb the surfacesor attempt to quantify the degree ortype of fouling. More precise methods(ASTM D6990-05, 2011; Dobretsovet al., 2014) to quantify fouling dur-ing inspections were not attempteddue to diver limitations. A final quali-tative assessment of the biofoulingon the system and all of the opticalports was conducted post-recovery onJuly 2.

During testing, the optical cam-eras collected sequences of 10 imagesat 10 frames per second once every15 min to monitor interactions be-tween marine life and the frame (suchas fish, crabs, and starfish) and providesome indication of test platform integ-rity between inspection dives. To mon-itor the biofouling levels on the cameraoptical ports, a ring of LED lights isinstalled within the camera housing,at the perimeter of the camera lens.On an hourly basis, sequences of10 images were captured with theseLEDs backlighting the optical port,as shown in Figure 5. Biofouling onthe optical port is illuminated by theLEDs and shows increased brightnessrelative to clean conditions. This

allows the extent and severity of bio-fouling to be contrasted for the twocombinations of mitigation treatmentsapplied to the camera optical ports(Cu, Wi, and CS on Camera 1 versusCu and Wi on Camera 2).

The brightness, B, of an image col-lected at time t with the LED illu-mination activated is calculated as a

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summation of pixel grayscale values,p(x,y), as

B tð Þ ¼Xnx¼1

Xmy¼1

p x; yð Þ ð1Þ

For this camera configuration, theimage resolution is n = 1624 andm = 1234 with a pixel grayscale rangeof 0–255.

A time-varying biofouling metric,F(t), for each acquired image is cal-culated as

F tð Þ ¼ B tð Þ � B 0ð Þð ÞBmax � B 0ð Þð Þ : ð2Þ

where B(0) is the baseline value corre-sponding to the brightness levels onthe first night of the deployment andBmax is the maximum possible bright-ness value (255 × n × m). This metrictakes on values between zero (for thebaseline images) and unity (for a fullyobscured optical port).

Figure 6 demonstrates the effec-tiveness of this method for quantifying

FIGURE 5

Cross-sectional schematic of camera bottledemonstrating the photon path from LEDlights to camera lens as a reflection of biofoul-ing or flocculent.

FIGURE 6

Demonstration images for biofouling metric calculations with LED backlighting. (a)–(c) show rep-resentative image quality for (a) a clear optical port with the LEDs inactive, (b) a partially obscured(F = 0.37) optical port, and (c) a fully obscured (F = 1.0) optical port. (d)–(f) show the correspond-ing image brightness with the LEDs active.

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fouling on a camera’s optical port byshowing the quality of images capturedwith external illumination alongsideimages captured with the internalLED backlight for a clear (F = 0), blur-ry (F = 0.37), and fully obscured (F =1.0) optical port. The artificial foulingin these images is simulated in the lab-oratory using a light coating of siliconegrease as adhesive and fine sand to ob-scure the image. The maximum ac-ceptable biofouling metric for opticalimages depends on the monitoringmission intended for the system. Mis-sions requiring high image resolution(e.g., precise measurements of targetsize and speed using stereo processing)will have a lower threshold than sim-pler missions (e.g., fish detection with-in a few meters of the camera).

The hourly biofouling images werecollected in three sets of 10 imageswith camera exposure times of 10,25, and 50 ms. This range of exposuretimes is used to evaluate the method’ssensitivity to camera configuration.For all three exposures, images were ac-quired at a rate of 1 frame per second,no digital gain was used, and thestrobes were not triggered. By averag-ing the sets of 10 images collectedeach hour, the variations in back-scattered light caused by moving floc-culent in the water are reduced. Adaily mean biofouling metric is cal-culated as

F tð Þ ¼XNi¼1

Fi tð Þ�

N ð3Þ

where Fi(t) is the fouling metric foreach image and N is the number ofimages used from each day. Only im-ages collected during nighttime hours(N = ~80) are used for each camera con-figuration to avoid the confounding ef-fect of variable external illumination.

92 Marine Technology Society Journa

ResultsField Deployment

Diving inspections confirmed in-creasing macrofouling on the testframe throughout the deployment(also observed in camera imagery),while the optical ports were observedto remain clear of fouling until thefinal (June 26) inspection. Duringthis final inspection, the strobe opticalports (which were no longer being ac-tively wiped) were observed to havevarying degrees of macrofouling whilethe camera ports remained clear. Fig-ure 7 shows the increasing level of bio-fouling on the test frame from cameraimages acquired over the course of thedeployment.

Biofouling Mitigation MeasuresFigure 8 shows the calculated daily

mean biofouling metric for each cam-era from the images collected with50-ms exposure times throughout theendurance trial. Qualitatively similartrends were observed with 10- and25-ms exposure settings, suggestingan insensitivity to exposure time.Highlighted periods represent inter-ruptions in system operation due tosoftware errors, electrical interferencewith serial communications, and wipermalfunctions. As previously discussed,during the last month of the deploy-ment, the system operation was reducedto a short period every night such that

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the number of images used for thenightly average was reduced to 10 from~80. On three occasions during thissame period, the wiper on Camera 2(Cu and Wi) malfunctioned and par-tially obscured the images that werecollected, preventing the calculationof a metric for that night.

The biofouling metric valuesshown in Figure 8 are consistentlybelow 0.04, indicating that bothcamera optical ports remained clearthroughout the deployment, consis-tent with diver observation and post-recovery inspection. Variation in thecamera metrics is primarily attributedto changes in the water quality duringthe deployment because flocculent orturbidity in the water close to the opti-cal ports is illuminated by the LEDbacklight and increases the value of F,without actually fouling the port.Days with F value variations duringthe first 3 months of the deploymentalso have increased standard devia-tions, indicating that the variationis within the uncertainty of the mea-surement. During the last month ofthe deployment, the F values forCamera 1 (Cu, Wi, and CS) increasewithout an increase in the standarddeviation, indicating a true change inthe signal. Camera 1 (Cu, Wi, and CS)images are consistently brighter thanthose from Camera 2 (Cu and Wi) andhave a larger range of variation. It is hy-pothesized that the light diffraction

FIGURE 7

Camera 1 (Cu, Wi, and CS) images of biofouling on the field testing frame from (a) March 19,(b) May 15, and (c) July 2 prior to recovery.

through the ClearSignal coating causesa “halo effect,” increasing the numberof bright pixels and magnifying anyvariation in brightness. Images ob-tained with external strobe illumina-tion from the coated camera port arenot of markedly lesser quality than theuncoated port, so this does not suggestthat the coating degrades operationaleffectiveness. Counterintuitively, withthe wipers activated only once per dayat the end of the deployment, the foul-ing metric increases for Camera 1 (Cu,Wi, and CS), even though one wouldexpect that the ClearSignal coatingwould mitigate fouling more effectivelythan the bare acrylic.

Post-recovery inspection of the sys-tem revealed severe macrofouling of

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every surface (including the back ofthe wiper blades) except for the cameraoptical ports. Figure 9 shows the centerof the camera frame with a closeupview of the Camera 1 (Cu, Wi, andCS) optical port. Fouling on the sys-tem generally consisted of barnaclesand algae and was independent of thesurface orientation. The fouling releasecoating onCamera 1 (Cu,Wi, andCS)and Strobe 1 (Cu, Wi, and CS) opticalports was found to be slightly abraded(abrasion grooves in the arc of thewipers). The degradation of the coat-ing, while not apparent in review ofimages acquired with strobe illumina-tion, may have contributed to the foul-ing metric increase on Camera 1 (Cu,Wi, and CS) when illuminated byLED backlighting. As the wiper abradedthe coating, the diffraction of lightthrough the coating may have changed,or the surface may have become moresusceptible to microfouling, both ofwhich could contribute to an increasein the fouling metric for Camera 1(Cu, Wi, and CS) over time.

FIGURE 8

Averaged nightly biofouling metric values with shaded standard deviations on (a) Camera 1(Cu, Wi, and CS) and (b) Camera 2 (Cu and Wi) optical ports throughout the endurance test.

FIGURE 9

Post-recovery biofouling on aluminum frame and camera optical ports.

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The strobe optical ports, whichwere notmonitored during the deploy-ment, other than qualitatively for thepresence of fouling by the diver in-spections, are shown after recovery inFigure 10. Due to the wipers being dis-abled over the last month of the de-ployment for the strobes (whereas thecamera wipers were still actuated onceeach night), all four optical ports ex-hibit some barnacle growth. Strobe 1(Cu, Wi, and CS) exhibits the mostgrowth, though upon recovery, thewiper blade for this bottle was notedto have rotated out of the plane ofthe optical port, thusmaking it ineffec-tive for a longer portion of the test thanthe wipers on the other strobe ports.Coincidentally, the Strobe 1 (Cu, Wi,and CS) optical port is also coatedwith ClearSignal, suggesting that the

94 Marine Technology Society Journa

passive fouling mitigation measures(Cu and CS) would not be sufficientfor multi-month deployments in ad-verse fouling conditions. Qualitativecomparison of the camera and strobeoptical ports demonstrates the effec-tiveness of the mechanical wiper tomitigate fouling (Figures 9 and 10).

Discussion andConclusions

There are expected to be strong sea-sonal and spatial variations in biofoul-ing within a region the size of PugetSound (Dickey & Chang, 2001).The dates of this field deploymentwere chosen to span the spring andsummer seasons, during which foulingis most severe. Since the endurance

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trial took place in calm waters withthe camera system entirely within thephotic zone, the biofouling observedduring the trial was likely to be moresevere than would occur at the antici-pated deployment depth of 50 m.

Monitoring biofouling levels oncamera optical ports in a quantitativemanner is complicated by the variablenature of the imagery. This method ofbacklighting the optical ports withLEDs in the absence of external illumi-nation provides a means to quantifytemporal changes in the optical portclarity. While there was no apparentdegradation in image quality duringthe deployment, the biofouling metricwas able to detect more subtle changesthan the human eye. An upward trendin this metric could, therefore, be usedto predict the need for system mainte-nance before the optical port is visiblydegraded, thus affording more time toplan system recovery (essential in ma-rine renewable energy environments)and reducing system downtime.

The combination of mechanicaland passive biofouling mitigationmethods effectively prevented macro-fouling growth that would have other-wise degraded system performance.The clarity of the camera optical portsin comparison to adjacent surfaces, asshown in Figure 9, is intuitively repre-sentative of this effectiveness. Withno discernible difference between theclarity of the two camera optical ports,the benefit of the ClearSignal coatingwas minimal under the test conditions.The abrasion of the coating by thewiper brush suggests that the combina-tion of these two antifouling measuresis potentially detrimental with a wiperactuation frequency shorter than anhour. If a system deployment were tobe power constrained, and the wiperscould not be run as frequently (aswould be the case for an autonomous

FIGURE 10

Biofouling on strobe optical ports with (a) Strobe 1 (Cu, Wi, and CS; though wiper rotated out ofplane during test and was not effective for unknown period), (b) Strobe 2 (Cu andWi), (c) Strobe 4(Cu and Wi), and (d) Strobe 3 (Cu with six wiper actuations). Ordering identical to treatment sche-matic in Figure 4.

deployment), then a transparent coat-ing may be helpful to maintain opticalport clarity.

Separating the antifouling con-tributions of the copper rings fromthe wipers is difficult due to the lackof a true control. However, evaluationof the surfaces on the outside of thecopper rings, which were not in con-tact with the wipers, suggests that thecopper rings directly reduce foulingwithin a proximity of several milli-meters. Similarly, for the optical porton Strobe 3 (Cu), which had fewerwiper activations, the most severe bio-fouling was at the center of the opticalport, furthest from the copper ring.One hypothesis is that the presenceof the ring contributed to this pattern,but without further testing, it is notpossible to distinguish causality fromcorrelation.

While the mechanical wipers playan important role in biofouling miti-gation, they are only effective if theydo not fail themselves. Integrating thewiper mount into the camera pressurevessel to control the interference spac-ing between the wiper and optical portwould reduce the chance of wiperfailures. For the prototype system de-scribed here, the spacing was set byhand prior to deployment, whichmay have resulted in inconsistent in-terference and caused the wiper mal-functions. The modular design of theZebra-Tech Hydrowiper allows foreasy integration, but caution shouldbe taken to ensure proper and securealignment. These modifications havebeen incorporated into the camera sys-tem design for the next iteration in sys-tem development (Joslin et al., 2014b;Rush et al., 2014).

For future deployments, measuringturbidity independently from the cam-eras and overlaying the measurementwith the biofouling metric may allow

correlations to be identified. This shouldbe possible during subsequent systemdeployments, since the overall moni-toring package can support additionalinstrumentation for an optical turbid-ity measurement. Biofouling of thisturbidity measurement must be con-sidered similarly to the cameras toavoid confounding the results.

The clarity of images obtained dur-ing this endurance trial suggests thatoptical camera deployments of atleast 4 months are possible evenunder adverse fouling conditions withthese biofouling mitigation measures.The results for these biofouling mitiga-tion measures are consistent with theresults for copper shuttered systemsdescribed in Manov et al. (2004),which have been shown to be effectivefor multi-month deployments. Whilethe mechanical wiper does not protectthe optical port by covering it betweencycles like the shutter, it does not haveto be activated every time data areacquired. The cleaning effect of thebrush may also be greater than non-contact shuttered systems. While thisresult is most applicable to opticalmonitoring in Puget Sound, projectselsewhere involving long-term deploy-ments of optical cameras may benefitfrom similar biofouling mitigationmeasures. Future deployments of thiscamera system for environmentalmonitoring of tidal energy projectswill provide additional informationabout seasonal effectiveness of themeasures employed.

AcknowledgmentsThe authors would like to thank

Jeffery Thomas for use of his facilitiesat Sunset Bay Marina, Jim Thomsonfor coordination of the field deploy-ment and inspection dives, SandraParker-Stetter and Sharon Kramer for

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recommendations on system testing,Capt. Andy Reay-Ellers for captainingthe R/V Jack Robertson, Alex DeKlerkfor designing and fabricating the im-aging frame, Joe Talbert and TimMcGinnis for building the shore cableumbilical, Keith van Thiel for the de-sign of the pressure housings and op-tical ports, Rick Towler and KresimirWilliams for insight into componentselection and stereographic calibration,and last but certainly not least, RandySindelar for his adaptable custom elec-tronics for power and communication.

Author Information:James Joslin and Brian PolagyeNorthwest National MarineRenewable Energy CenterUniversity of Washington,Box 352600, Seattle, WA 98195-2600Email: jbjoslin@u.washginton.edu;bpolagye@u.washington.edu

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