IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-23, NO. 3, SEPTEMBER 1974

Sonic Vibration Technique for Rot Detection in Wood PolesA. DOUGLAS SHAW

Abstract-A new method is proposed for rot detection in woodpoles such as those used for electricity distribution purposes. Thetest method has been developed to meet the problem of detectionof concealed ground-line rot in full length preservative treated polesard appears to be capable of successfully achieving this end. Aresonant sonic vibration technique is employed which is capable ofdetecting seriously defective poles not at present accessible to in-spection. The technique is essentially nondestructive in nature, andprovides a degree of precision and security not formerly available.A device employing the technique is described, which is suitablefor use by workers of the caliber of those now employed in thedetection of defective power poles.

I. INTRODUCTIONTHE present state of the art of rot detection in wood

poles in the electricity supply industry can scarcelybe regarded as satisfactory. The techniques of rot detec-tion have remained, for all practical purposes, unchangedsince it was first found necessary to institute regular poleinspection and replacement programs.

Present methods commonly used throughout the worldare of two general types: 1) aural interpretation of soundwaves, and 2) direct investigation with auger, pick, andaxe, or other special device designed to penetrate theinner fibers of the pole to detect areas of rot. The use ofsound waves depends on the interpretation placed on thesound of impact made when the pole is struck by a hammeror similar tool. Interpretation of the result is necessarilyentirely subjective. Direct investigation involves cuttinginto the pole with the consequential possibility of furtheraccelerating the rate of decay of permitting free access forbacteria. Simultaneously, the strength of the pole may beaffected by the removal of good wood.Such methods are essentially vague, and instances are

known where linesmen have had poles collapse underthem after having satisfied themselves that the poleswere safe to climb. Consequently, pole inspectors tendto be conservative, and poles are frequently condemnedwhen they have years of life remaining. This is indicativeof the need for a more precise testing procedure to replaceexisting methods.An investigation commenced in 1967 in an effort to

approach the problem in a quantative way. The systemdeveloped is a comparative one, and the principle em-ployed is that of transmission of a resonant vibratoryforce through a diameter of sound wood several feet aboveground. The resonant signal emerging on the opposite sideof the pole is monitored at several points between the top

Manuscript received January 8, 1974; revised May 28, 1974.The author is with the Retail Supply Branch, Hydro-Electric

Commission, Tasmania, Australia.

VARIABLEFREQUENCYOSCILLATOR

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Fig. 1. Schematic arrangement.

position, opposite the exciter, and at the region of theground line and is compared with that emerging at thetop position. The values of velocity or acceleration whichemerge from a pole having serious rot near the groundhave been found to be significantly larger than those asso-ciated with good poles.By assessment of tests conducted on a large sample of

poles having serious rot in this region, it has been possibleto arrive at a characteristic increase in the observed signal,which always indicates a seriously rotten pole. The as-sumption made is that the condition of a pole several feetabove ground is satisfactory.

EXPERIMENTAL MODEL, EXPERIMENTALTEST EQUIPMENT, AND TESTS ON

GOOD POLESThe measurement system is shown in Fig. 1. The pat-

tern of vibratory energy which would emerge at thevarious transducer positions for good poles was deter-mined by a physical investigation of vibratory phenomenaon typical short pole specimens which were 4 ft long. Asimilar range of experiments was also conducted on typicalnew standing electric power distribution poles. Poles andshort pole sections were selected from Eucalyptus Globuus(Southern blue gum) and Eucalyptus Obliqua (Messmatestringy bark). The air dry densities of the short polespecimens ranged from 62.8 to 66.8 lbs/ft3 and diametersranged from 11 to 14 in.The test equipment consisted of a commercially avail-

able shaker with an input of 10 VA converted to operate asa hammer of very low mass, a variable frequency oscillator,range 20-1000 Hz, an amplifier to drive the hammer, aread-out device, and a 12 V automotive battery. Theoscillator circuit was arranged to provide a square wavewhich was found to produce better signals for the sameinput than a sinusoidal wave form.

Initially, a cathode ray oscilloscope was used in parallelwith a vacuum-tube voltmeter as the output, both being

240

SHAW: SONIC VIBRATION TECHNIQUE

PR.M SVELOCITYOUTPUT

600

PERCENT

40

20

6" 41 2-l 0 i- 61CIRCUMFERENTIAL DISTANCE FROM 'RUE CENTRE

Fig. 4. Velocity distribution about true diameter of typical powerpole.

PERCENTAGE RMS VELOCITY AT TRANSDUCER

Fig. 2. Typical velocity variation curve of good pole specimenswithout radial cracks, resonant, and off-resonant frequencies.

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TYPICAL RESONANT CURVE

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GROUND LINE

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0 100PERCENTAGE R.MS VELOCITY AT TRANSDUCER

200

Fig. 3. Typical velocity variation curves of cracked new specimens.

driven by an electromagnetic transducer, but as the vi-bration patterns became well known, it was found simplerto use the voltmeter alone. Later, the electromagnetictransducer and vacuum-tube voltmeter were replaced bya vibration meter and piezoelectric transducer which pro-

duced similar, but somewhat more refined results. Bothinstruments registered rms values of velocity.The electromagnetic hammer was firmly supported on

the specimen and allowed to vibrate against a small flat-headed nail driven in almost flush with its surface. On theopposite side of the specimen on a diameter through thisnail, another nail was driven which was left with a suffi-cient amount of its length protruding to enable the trans-ducer to be solidly attached to it. Other nails were placedat 3 in intervals down the specimen in a vertical linethrough the top nail over a distance of 36 in.

Values of output in terms of velocity were observed byprogressively moving the transducer down the specimenfrom nail to nail, the hammer remaining fixed with un-

varying power and frequency of excitation at an arbi-trarily selected value. It was found that the velocitycurves were similar for all specimens without cracks and

were independent of frequency of excitation as long as thefrequency was within the range of 50-150 Hz, in spite ofmechanical resonances, which had an effect on the strengthof the signal transmitted to the transducer. Fig. 2 illus-trates the observed results.

However, when specimens had radial cracks, randomresults often eventuated, except when the system was

brought to a resonant frequency of excitation. This was

achieved by tuning the oscillator in order to produce a

maximum velocity at the transducer before the com-

mencement of readings. Fig. 3 illustrates the observed re-

sults in this instance.No differences could be detected between the results

obtained from short specimens or full length specimensin either random frequency or resonant frequency tests,for both treated or natural specimens. Results were alsoindependent of diameter within the stated range.

As it would be likely in practice that the electromagnetichammer and the transducer would not always be on an

exact diameter, the effects of departures from the truediameter position required investigation. The resultsshown in Fig. 4 indicate that the level of transmission ofenergy is unaffected to any significant extent by the pointof excitation being away from a precise diameter, so longas the departure from the true diameter is small. However,the lateral level of energy at the transducer is drasticallyreduced by departures greater than 2 in from the truediameter.

III. TESTS ON POLES CONTAINING DECAY

Similar resonant tests were conducted on a few poleswhich were known to be in poor condition near the groundline. Quite large increases in velocity were noticed at sometest points when compared with readings obtained forcorresponding positions of short specimens and good polesunder conditions of resonance. It was tentatively con-

cluded that rot might be the cause of the observed differ-ences; consequently, a further test program was com-

menced. This consisted of an investigation of 100 untreatedpoles of various sizes and pole top construction, which

were selected by an experienced foreman after the poleinspector had seen them. Each of these poles was bored

- GROUND LINE

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241

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, SEPTEMBER 1974

-- -GRDUND LINE---DANGEROUS POLES-ROOD POLES (NEW)

GDOD POLES ("EED)

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Fig. 5. Velocity variation curves of good and dangerous poles underresonant conditions.

in the suspect region and found to be decayed in somedegree, although not necessarily condemned by the poleinspector, or regarded as being excessively decayed andat the end of its useful life. Rot sought was that of a con-cealed nature, although some poles also showed evidenceof external rot. The tests were taken under conditions ofresonance, and observations were taken at 4 equidistantpoints from 2 to 6 ft above ground to 6 ft below ground.Two sets of observations were taken for each pole ondiameters 90 degrees apart.

After testing, the poles were extracted whole and sec-tioned at each point. The percentage reduction of area ofgood wood due to rot was measured, and an individualassessment of each pole was made in terms of whether itwould have been dangerous or not. It was considered thatthe danger point would be reached, allowing a reasonablemargin of time between condemning and renewal, if apole's strength was reduced to a value approaching 50percent of its original strength, or when the annulus ofsound timber was reduced to 1 or 11 in in thickness. Forthe sonic test, a minimum value of 150 percent of theoriginal observed velocity at any of the six positions oftest below the two used as standards, was used as the cri-terion for condemning a pole.The results of this test showed that for the poles exam-

ined, the system was capable of detecting concealed decaysufficient to reveal a dangerous pole, but where bad ex-ternal decay was present, it failed. It was also shown thatfor poles having only internal decay, the sonic test pro-duced much more accurate results than the pole inspector,and yet erred on the side of safety at all times, using the150 percent criterion [1].

Fig. 5 depicts a series of typical graphs of poles havingdangerous internal defects near the ground line, and similargraphs of good poles, illustrating wide differences in char-acteristics which would have enabled the detection ofdefective poles in this experiment. It will be noted thatwhile the curve for aged but good poles shows some dis-tortion, this will not confuse the issue between the char-acteristics of good and seriously decayed poles.

Typical sections of bad pole are seen in Fig. 6, sections1-4 grading from 2 ft 6 in to 6 in below ground, respectively.

Fig. 6. Typical sections of a bad pole. (1) 2 ft 6 in above ground,(2) 1 ft 6 in above ground, (3) 6 in above ground, (4) 6 in be-low ground.

It is of interest to note the wide variations in the geometryof rot between sections and also the extent of some cracks.It was found that decayed pressure-treated poles re-sponded in the same manner as decayed poles in thenatural condition. Although the main trials had beenconducted on hardwood poles, a few good and bad soft-wood poles were also investigated and behaved in a similarway to hardwoods.

IV. DEVELOPMENT OF FIELD PROTOTYPEA device suitable for use by pole inspectors, apart from

being reliable, would need to be much reduced in weight,simpler to read, and with a selfcontained power source.By changing the read-out parameter from velocity to ac-celeration, it would be possible to considerably reducemass and power input for the same range of meter sensi-tivities.

Utilizing the fact that a 50-percent increase in velocityfrom the original observed velocity had denoted a seriouslydecayed pole in the previous tests, the meter scale nowrequired only "good" and "bad" zones and a datum linecorresponding to the 100-percent reading for the toptransducer position. Detailed graduations could be elim-inated. If midscale were selected for the datum line, the"reject" zone would occupy the last 25 percent of themeter scale towards the right-hand end.A general specification for a group of components com-

prising a transmitter, variable frequency oscillator ampli-fier, receiver, and rms acceleration readout was suggestedto the Fanner Manufacturing Company of Northmead,New South Wales, who produced a prototype for evalu-ation. The vibrator in this instrument had an input ofabout 5 VA and consisted of a shaker having a muchsmaller permanent magnet than the original shaker. It

242

SHAW: SONIC VIBRATION TECHNIQUE

Fig. 7. Equipment in use on a pole.

Fig. 8. Meter dial.

incorporated within itself a striking mechanism, and thewhole was encapsulated in a steel case fitted uith an ex-

ternal clamp arrangement. The complete unit was knownhenceforth as the transmitter. It was light enough to beadequately supported by a 3-in nail using the clamparrangement, thereby eliminating the former flat-headednail and massive support required for the original shaker.The transducer was housed in a metal case with a similarexternal clamp arrangement to that used for the trans-mitter, the whole arrangement being termed the receiver.The transducer used was a piezoelectric accelerometerusing a lead zirconate crystal.To cope with varying physical properties of different

Fig. 9. Equipment layout in its case.

poles, three meter ranges of varying sensitivities wereprovided via a selector switch. The same switch also hadon-off and battery-check positions. The oscillator-ampli-fier was fitted with a vernier frequency control-an essen-tial feature because of the closeness of some resonantfrequencies-and a gain control.An automatic switch ensured that the equipment was

switched off in transit. The meter movement was a stand-ard 1-mA FSD moving coil instrument fed by a preampli-fier. Leads to the transducer and receiver were coaxial.The equipment was designed to operate from a dc sourceof 12 V, provided by rechargeable batteries within thecase, which was of wood sheathed in a synthetic laminatefor durability. An alternative power supply from a vehiclebattery could be provided via terminals on the controlpanel, in the event of the regular power supply failing.The total weight of the final arrangement was 13 lbs.Figs. 7 and 8 show a typical field arrangement of equip-ment for a pole under test, and a close-up view of themeter dial. On this particular model, some coarse gradu-ations were retained for additional research, but wouldnot be necessary as a normal requirement. Fig. 9 illus-trates the equipment layout within the case. The trans-mitter is on the left.

V. FIELD TRIALS USING PROTOTYPE UNITS

Three prototype units were purchased and placed inservice late in 1972, and data obtained of the performanceof the system relative to conventional methods of poleinspection. Both untreated poles and treated poles werechecked in several districts of southern and northernTasmania.

Table I indicates the relative effectiveness of the newsystem compared with conventional methods and covers12 months of operation. Because of the magnitude of theprogram, it was no longer feasible to remove each pole

243

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, SEPTEMBER 1974

TABLE ISUMMARY OF COMPARISONS OF RELATIVE EFFECTIVENESS BETWEENCONVENTIONAL TESTING METHODS AND RESONANT SONIC SYSTEM

Pole Type

Total Poles InspectedPoles assessed as being unsound after

boringPoles rejected by pole inspectorsPoles rejected by pole inspectors andsound

Poles rejected by pole inspectors andunsound

Poles cleared by pole inspectors andunsound

Poles rejected by resonant sonicsystem

Poles rejected by resonant sonicsystem and sound

Poles rejected by resonant sonicsystem and unsound

Poles cleared by resonant sonicsystem and unsound

Treated Untreated

147017

842172

30 30515 138

15 167

2 5

89 178

72 6

17 172

0

TABLE IICOMPARISON OF THE RESULTS FOR SOME EXCESSIVELY SPLIT

TREATED POLES BEFORE AND AFTER APPLYING ATOURNIQUET DURING TESTS

Before AfterModification Modification

Total Poles 822 822

Poles assessed as having advanced 16 16decay after boring

Poles rejected by resonant sonic 79 25system

Poles rejected by resonant sonic 63 9system and sound

Poles rejected by resonant sonic 16 16system and unsound

Poles cleared by resonant sonic 0 0system and unsound

0

and section it, therefore, final assessments were madewith poles standing, by careful penetration in a numberof places in the test region corresponding with the resultsfrom vibration tests.The tests showed that while the performance of the

equipment was identical for seriously decayed poles ofboth treated and untreated types, the incidence of pre-mature rejections of good treated poles was much higherthan for untreated poles.

In the latter stages of the test program, this problemwas closely investigated and found to occur only withexcessively cracked poles. The cause was found to be thatbecause some poles were so badly cracked, some partsvibrated virtually as separate entities. If the receiverhappened to be placed on one of these parts of such a pole,signals often became abnormally high. The problem was-practically eliminated by tightly banding such poles witha tourniquet in the form of a lineman's line stringingtensioning tool, which was placed just above the testregion during tests. When applied to poles containingsevere decay, the tourniquet had very little effect onreadings and tended to enhance the "reject" reading.Table II shows the effect of applying a tourniquet to splitpoles during tests.The results of the 1972 trials were particularly interest-

ing so far as the 150-percent criterion was concerned. Noinstances were found where the annulus of sound timberwas reduced to 1l in before a strong indication in the"reject" zone was observed. The general situation wasthat an annulus of between 2 and 3 in of sound timbercorresponded to a reject reading. It would therefore ap-pear that the criterion selected from the 100 pole samplewas very conservative and erred on the safe side, andthat the equipment was more sensitive to the presence ofrot than had been previously thought. The strength lossin most of the poles of the extended trials which werefound to contain serious rot would have been only of theorder of 25 percent.

VI. EFFECT OF FIELD CONDITIONSThe main problems in the field arise from weather.

Continual expansion and contraction of poles resulting

from temperature, rainfall, and humidity variations mayaffect pole structure to the extent that parts of a polemay be physically separate from one another at one timeand closed tightly together at another. This problem ispractically eliminated when a tourniquet is applied to thepole just above the test zone before commencing the test.Another problem is wind. Gusts above 10 mi/h tend

to make testing difficult. It is therefore important thatthe strongest possible input signal be generated to mini-mize wind effects. Aeolian vibration of light conductorsin long tightly strung spans occasionally causes large spuri-ous readings, but it is easily heard and can be readilyidentified by removing the transmitter from the polebeing tested.

Waterlogged poles, especially those standing in water,may be incorrectly rejected, but this is the exceptionrather than the general case.The comparative nature of the system was found to

hold good for poles containing a tapering hollow heart.Many such poles were encountered during field trials, butnone containing serious decay remained undetected.The system can be operated after a short training period

by personnel of the background of present inspectors, anddetermination of pole condition is very simple. Once ac-cepted by the pole inspectors, the units have shown thatthey are very satisfactory; portability is good, and theinstrumentation is completely reliable.

ACKNOWLEDGMENTThe author wishes to thank his colleagues and the staff

of the University of Tasmania's Physics and ElectricalEngineering Departments and the scientific officers andengineers of the State Electricity Commission of Victoriawhose facilities yielded so much valuable data on decayedcreosote treated poles. Particular thanks are due toR. J. W. Oakden for his interest in producing the proto-type instrument and to the Commissioner of the Hydro-Electric Commission of Tasmania, Sir Allan Knight, forhis kind permission to publish this paper.

REFERENCES[11 A. D. Shaw, "Vibration technique for rot detection in wood

pole," in Proc. Electron. Instrum. Conf., I.E.A. Hobart, Tas-mania, May 1972, p. 57.

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