A New Proximity Probe to OvercomeEddy Current Probe Limitations

By Jackson Lin and Marc R. Bissonnette, VibroSystM Inc., Qubec, Canada

ABSTRACT

A new proximity probe has been designed that overcomes the technical limitations ofEddy current probes. This new probe uses a proven type of capacitive technology tomatch or exceed the performance of Eddy current probes in many critical aspects.Two separate tests comparing the performance of the capacitive proximity probe withthe performance of an Eddy current proximity probe were conducted. The resultsshow the capacitive proximity probe to be immune to many of the effects that influencethe vibration signal of an Eddy current probe. For environments where shaft surfaceroughness, magnetic field, inconsistent target metallurgical irregularities, or shaftcurrents may exist, the capacitive proximity probe offers a strong alternative to anEddy current probe for accurate vibration measurement.

Introduction

To provide effective machinery vibration protection, a reliable and accurate meas-urement must be provided by the vibration transducer. For large rotating machineslike generators and turbines that have low levels of rotor-to-bearing vibrationtransmissibility, the key transducer type is the proximity probe. However, Eddy currentproximity probes, the standard non-contact probe for 35 years, have alwayspossessed certain inherent design limitations that have remained unaddressed.

A new proximity probe using proven capacitive technology has been designed thatovercomes the technical limitations of Eddy current probes. The performance of thenew capacitive probe and an Eddy current probe are compared and discussed basedon two tests : the first at a large motor manufacturing site; the second at a hydro-electric generating facility.

Comparison of Eddy Current vs Capacitive Technology

Eddy current probes operate upon a magnetic operating principle. Basically, a high-frequency signal is transmitted by the probe driver (oscillator-demodulator) to the tip ofthe probe. The coil in the probe tip radiates the signal into the observed target as amagnetic field. As the conductive target nears the probe tip, Eddy currents are gener-ated which diminish the strength of the magnetic field which, in turn, weakens the DCoutput of the probe driver. The probe driver linearizes this DC output over a certainmeasuring range. This linearized signal has both an AC and DC component.

The AC component represents the motion of the target relative to the probe tip(i.e. relative vibration) whereas the DC component represents the average gapbetween the target and probe. While both signal components provide valuableinformation, it is the AC component that is of interest for vibration measurement.

Capacitive proximity probes operate uponan electrical field principle. Anelectricalfield is created in the air gap between theprobe tip and a target, i.e. the rotor shaft.As the gap between the target and theprobe tip changes, the modulatedcapacitive current is monitored andlinearized. Thus, the key parameter thatthis vibration measurement dependsupon is the capacitance of the air gap.Capacitive measurement technology isindependent of magnetic field, targetsurface irregularities and themetallurgical properties of the target. Fig. 1: PCS-102 Capacitive Proximity Probei

1. CASE STUDY #1 : Probe Comparison at Large Motor Manufacturing Site

Allowable electrical and mechanical runout levels

Rotating machinery manufacturers must often meet very tight specifications forallowable electrical and mechanical runout for rotor shafts. API-670 requires thatcombined total electrical and mechanical runout does not exceed 25% of themaximum allowed peak-to-peak vibration amplitude or 6 micrometers (0.25 mil),whichever is greater. Since some users set maximum vibration amplitudes as lowas 0.80mils, the allowable runout may be as low as 0.20mils. Even with diamondburnishing of the rotor shaft, runout levels this low may be very difficult for machinerymanufacturers to achieve.

Fig. 2: Probe Test Arrangement - Capacitive proximity probeshown below LVDT probe observing rotor shaft motion.

Description Of Test Set-Up

Figure 2 above shows the probe arrangement. The probes utilized were an LVDTprobe, an Eddy current proximity probe and the PCS-102 capacitive proximity probe.The capacitive and Eddy current probes were alternately paired with the LVDT probe toobserve the motion of the rotor shaft. The probes were set up to measure themechanical and electrical runout of a 4-pole rotor shaft mounted on a lathe. Probereadings were taken along the shaft circumference next to the rotor journal whereproximity probes are normally installed. It should be noted that the test shaft was notmachined on the test lathe so the mechanical runout level is higher than it wouldnormally be. A probe yoke device was used to hold the LVDT probe and proximityprobes firmly in place.

The LVDT is a transducer which at all times is in contact with the rotor shaft surface.As the rotor turns, it traces out the actual mechanical profile of the rotor circumference.The two proximity probes, being non-contact sensors, also attempt to detect themechanical motion of the rotor. However, the proximity probes due to theirnon-contact design incur some degree of error in their measurements of rotor motion.The difference between the readings taken by the LVDT transducer and each proximityprobe is by definition the level of electrical runout for that proximity probe.

A reference point was marked at a point on the rotor circumference. The rotor wasturned at slow speed (7 RPM). Under different conditions, one set of measurementswas taken for the Eddy current probe matched with the LVDT probe and one set ofmeasurements was taken for the capacitive probe matched with the same LVDTprobe. Since the LVDT was next to the proximity probes, the difference in readingswould provide the level of electrical runout for each type of proximity probe.Measurements were taken over one rotation under the following conditions:

1. Eddy current probe observing the unburnished shaft.2. Capacitive probe observing the unburnished shaft.3. Eddy current probe observing the burnished shaft.4. Capacitive probe observing the burnished shaft.

Additionally, it was intended to scratch the observed shaft surface to test the sensitivityof the two proximity probes to this type of irregularity in their observed path. Since anexisting scratch was found on the shaft, this was used for the comparison.

Results

A summary of the test results is provided in Figures 3 through 6. For each condition,three sets of measurements were taken over one revolution of the shaft. Visualinspection showed no significant discrepancies between the readings of each set ofthree so for brevity, only the first set of readings for each test condition is presentedherein.

Condition 1 Proximity probes observing the unburnished rotor shaftIt was decided to test the probe measurements on an unburnished shaft. The capa-citive and Eddy current probe results are given in Figures 3 and 4 respectively.

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Electrical RunoutEddy Current ProbeLVDT Probe

Vibr

atio

n (m

ils)

Angle (degrees)

Eddy Current Probe Unburnished Shaft

Figure 3: Eddy current probe observingthe unburnished shaft.

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Electrical RunoutCapacitive ProbeLVDT Probe

Vibr

atio

n (m

ils)

Angle (degrees)

Capacitive Probe Unburnished Shaft

Figure 4: Capacitive probe observing theunburnished shaft.

The Eddy current probe (Figure 3) exhibits a peak-to-peak level of electrical runout of0.54mils which is fairly typical for an unburnished shaft. The waveform of the Eddycurrent probe signal does have the same general shape as the LVDT signal butappears to lag the movement of the shaft. There also seems to be significant elec-trical noise from the Eddy current probe signal. The source of this electrical noisemay be due to either mechanical irregularities on the shaft surface or to metallurgicalimpurities in the shaft material. Whatever the exact cause, we can see that theelectrical runout level is fairly high for the unburnished rotor shaft surface.

On the other hand, the capacitive probe (Figure4) tracks the motion of the shaft quiteclosely and without the phase lag exhibited by the Eddy current probe. This is due tothe fact that the larger probe tip on the capacitive probe (typically 5 times the surfacearea of the 8-mm diameter Eddy current probe tip), sees a larger portion of the shaftand is able to average out any mechanical surface irregularities. Additionally, thecapacitive probe does not have to be calibrated for the specific shaft material itsmeasurement technology works equally with all conductive and semi-conductivetargets. Thus, its signal is immune to any metallurgical differences that may exist onthe shaft surface.

The most remarkable aspect of the capacitive proximity probe measurement is thatthe probe exhibits a peak-to-peak level of electrical runout of only 0.13 mils (roughlyone-quarter of the Eddy current reading). This would pass runout specifications(in particular API-670) without the need for diamond-burnishing. Thus, false readingsand their expensive consequences can be avoided.

The Scratch TestRotor surface scratches can induce significant error in the Eddy current probereadings and are highly undesirable for non-contact vibration measurement. Duringthe course of the test, it was noticed a scratch already existed at 90 from the refer-ence marker on the rotor shaft. The scratch was very shallow and its groove could notbe felt with a fingernail.

It was decided that this scratch would be sufficient to compare the scratch sensitivityof the two probes. The Eddy current probe picks up this scratch as can be seen fromthe roughly 0.20 mil valley in the Eddy current signal at 90 (Figure3). On the otherhand, no significant discrepancy is noticeable for the capacitive probe. This may bedue to the larger tip of the capacitive probe which averages out the anomaly over awider area thus reducing the impact of the scratch.

Condition 2 Proximity probes observing the burnished rotor shaftDiamond-burnishing, a method of smoothing the shaft and work-hardening itssurface, is a required standard procedure whenever Eddy current proximity probes arebeing used. Burnishing results in reduced electrical runout levels picked up by theEddy current probes. After the shaft was diamond-burnished, the measurements witheach type of proximity probe were repeated.

From Figure 5, it can be seen the electrical runout of the Eddy current probe has beenreduced to 0.27 mils. However, the capacitive probe electrical runout (Figure 6)remains approximately 0.13 mils. Comparing Figures 5 and 6 with Figures 3 and 4,diamond burnishing halved the electrical runout level for the Eddy current probe buthad essentially no effect on the capacitive probe. From this, it can be inferred thatdiamond-burnishing is not necessary when using the capacitive proximity probe.This has been corroborated by other field tests conducted by the capacitive probemanufacturer showing a much cleaner vibration signal for the capacitive probe thanan Eddy current probe despite that the capacitive probe observed an unburnishedsurface while the Eddy current probe observed a burnished surface.

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Electrical RunoutEddy Current ProbeLVDT Probe

Vibr

atio

n (m

ils)

Angle (degrees)

Eddy Current Probe Burnished Shaft

Figure 5: Eddy current proximity probeobserving the burnished shaft.

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Electrical RunoutCapacitive ProbeLVDT Probe

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Angle (degrees)

Capacitive Probe Burnished Shaft

Figure 6: Capacitive proximity probeobserving the burnished shaft.

Interpretation Of Results

It would seem reasonable to infer from the test results that the capacitive proximityprobe is immune to electrical runout. The discrepancy between its readings andthose of the LVDT probe is difficult to explain with the limited equipment available forthe test. However, since the level of electrical runout was the same (0.13mil) bothbefore and after diamond burnishing, it is reasonable to conclude that when using acapacitive proximity probe, shaft burnishing is not required.

The scratch test shows the insensitivity of the capacitive proximity probe to scratcheson the shaft surface. For both the machinery manufacturer and the machinerymaintenance staff, this characteristic could lead to significant cost saving by eliminat-ing the need to disassemble a machine to reburnish the rotor shaft whenever a toolaccidentally scratches the rotor surface in the observed path of the proximity probe.

Since the 0.13 mil of electrical runout for the capacitive proximity probe is below themaximum limits specified in industry standards, for cases where suspected falsevibration readings due to high electrical runout or a magnetized rotor shaft is aproblem, a capacitive probe might be installed to determine the true motion of therotor shaft. These cases are currently being investigated by the probe manufacturerand the results shall be presented in a future paper.

2. CASE STUDY #2 - Installation At Carillon Hydro Station (Qubec), Canada

Description of Test Set-Up

A summary of the test set-up is provided in Figure 7. The Eddy current and capacitiveproximity probes were located at 180 apart at the turbine guide bearing of generator#13. Thus, both probes should have theoretically given the same AC vibration level(i.e. the same time-based vibration waveform) but out-of-phase by 180 .

VibroSystM and existing Eddy Current (EC) probes located @ 180 apartPCS-102 probes located just above the guide bearing cover

EC probes located beneath the guide bearing cover

Test Equipment:Existing MMS rack

SKF Microlog Data Collector/Analyzer Tektronic Oscilloscope TDS-320Toshiba 2200 Laptop Computer

y

PCS-102

PCS-102

Guide bearing cover

EC Probe

EC Probe

Figure 7: Test set-up of field comparison performed on Unit #13 atHydro-Qubecs Carillon Hydro Station on November 14, 1996

One problem encountered was that the eddy-current probe which had been installedduring unit commissioning was located underneath the turbine bearing cover.The capacitive probe could not be installed in this exact location without removing themachine rotor. Thus, the capacitive probe was installed just above the turbine guidebearing cover. However, it is common knowledge that the rotor shaft of hydromachines is by conventional design smoothly burnished at the rotor journal(i.e. where the Eddy current probe was located); thus, if anything, one should expectthe Eddy current probe to offer a cleaner, smoother vibration signal.

The existing Eddy current probe was connected to an existing vibration instrumen-tation rack. The capacitive probe was interfaced (Figure 8) into the same rack via asignal conditioning device to provide the standard 200mV/mil sensitivity required bythe rack instrumentation.

PCS-102Proximity

Probe

ExtensionCable

TappingBox

IntegralCable

Existing MMSInstrumentation

Rack and Modules

LIN-102Linearization Module

and Enclosure

Figure 8: Retrofitting the capacitive proximity probe to the existing instrumentation

An oscilloscope was also used to examine the raw waveforms. Additionally, aportable vibration meter was used to examine the FFT plot of the vibration signals.

Hypothesis

One should expect to see the same waveform from both probes but out of phase by180 . Keeping in mind the greater shaft-smoothness at the location of the Eddycurrent probe, the Eddy current probe waveform should exhibit less noise and offer apurer sine wave.

Summary Of Results

The vibration rack results (indicated under the MMS Rack title in Figure 9) indicate alarge discrepancy (5.9 mils for the eddy-current probe vs. 3.9 mils for the capacitiveprobe). This very simple presentation of the vibration data has to be reconciled withthe other data before a conclusion can be drawn.

The portable vibration data collector (indicated under the SKF Microlog title) helps usto understand the high discrepancy in the results from the vibration rack. We can seehere that the fundamental vibration frequency is very similar for the two probes. Thediscrepancy is accounted for from the elevated amplitudes for the Eddy current probe

from the higher order frequencies. This is the main reason explaining the higher peakvibration level for the Eddy current proximity probe.

All measurements given in units of "mils peak"

MMS Rack

ModuleDisplay

(Peak Vibr.)

SKF Microlog

ProximityProbe

Amplitude atFundamental

Frequency

Sum ofAmplitudesfrom 2nd to20th Order

Sum ofAmplitudesfrom 1st to20th Order

5.90 0.2Eddy Current 3.704 1.945 5.649

3.94 0.2Capacitive(PCS-102) 3.639 0.497 4.136

Figure 9: Summary of test results, Y-position probes

The discrepancy between the two signals is most clearly seen in the comparison ofthe two vibration waveforms taken by the oscilloscope (Figures 10 and 11). It is veryimportant to note that, unfortunately, these two waveforms are not to the same scale sodirect comparison of the amplitudes is impossible. However, the shape of the twowaveforms provides us information which is quite interesting.

Eddy Current Probe Results

Figure 10 is the waveform for the Eddy current probe. The first thing we notice is thatthe overall shape of the waveform is very irregular and very far from being a pure sinewave. While one cannot expect to see in a real world application a pure sine wave, atleast for a large, low-RPM hydro machine, one would find it hard to believe that thiswaveform truly reflects the actual motion of the rotor.

Second, it is remarkable the level of noise that exists in the signal from the Eddycurrent probe. In the indicated Area #1, we see a lot of noise and signal roughness.These irregularities may be due to shaft surface roughness to which eddy probes arehighly sensitive. However, the parameter of interest in vibration measurement isthe motion of the rotor shaft and not the surface characteristics of the shaft itself.This noise can be misleading and direct one to wrongly interpret the waveform.

Third, in the indicated Area #2, there is significant variation in the trough of thewaveform. Eddy current probes, based upon their magnetic operating principle,are sensitive to irregularities in the metallurgical properties of the target surface.This seems to be a plausible explanation for the noise from this area. This glitchcould have been caused by a surface scratch as well but since it was not possible toshut down the machine, the exact nature of this glitch could not be identified.

Overall, the waveform of the Eddy current probe is surprisingly noisy given the well-burnished shaft where the probe was located. Given the large size of the machineand the low RPM, it is highly unlikely that this waveform reflects the true vibrationbehavior of the rotor shaft.

Area #2

Area #1

Figure 10: Plot of the Eddy currentsignal on the oscilloscope

Figure 11: Plot of the capacitiveproximity probe signal

Capacitive Probe Results

On the other hand, in Figure 11, we see the waveform of the capacitive proximityprobe. Intuitively, this is the type of motion we would expect from a large, low-RPMrotating machine. The signal is clear and free of noise. Capacitive probeperformance is not affected by stray magnetic field or residual surface magnetism.The probe performance is independent of the metallurgical properties of the targetsurface. The probes wider-tip surface averages out any surface irregularities to givea cleaner, almost noise-free signal. The capacitive probe gives a clear picture of thetrue motion of the shaft which is the variable of interest, rather than confounding theresult with errors due to sensitivity to the properties of the target surface.

3. Conclusion

Due to the importance of the vibration transducer in acquiring accurate vibration data,careful study of the environment in which the proximity probe is to be installed woulddictate the selection of the best probe for the application. For environments whereshaft surface roughness, magnetic field, shaft metallurgical irregularities, or shaftcurrents may exist, the capacitive proximity probe should be preferred over Eddycurrent probes. Considering the capacitive probe's competitive pricing, it should nowbe considered a superior replacement for Eddy current probes for almost all non-contact vibration measurement applications.

References

1. Vibration, Axial Position, and Bearing Temperature Monitoring Systems,API Standard 670, Third Edition, November1993;American Petroleum Institute, 1993.

2. Stephen J. Chapman,

Electric Machinery Fundamentals, McGraw-Hill, 1985. 3. John F. Lyles and G. Bruce Pollock, "Vertical Hydraulic Generators Experience with

Air Gap Monitoring On Large Hydro Generators",Proceedings of IEEE Winter Meeting, New York, NY, January 1992.

4. Condition Monitoring Catalog, SKF USA Inc., 1991.

Biographical Details Of The Authors

Jackson Lin graduated in Mechanical Engineering from Queens University (Kingston,Canada) in1991. He completed his Masters in Business Administration at McGillUniversity (Montral, Canada) in 1996. He has also studied for one term at the InstitutSuprieur des Affaires (Jouy-en-Josas, France) in 1995. Currently, Mr. Lin is theProduct Manager for Industrial Applications with VibroSystM.

Marc R. Bissonnette is an Electrical Engineering graduate of the University ofSherbrooke. Since1987, he has been involved with the on-going development andmarketing of monitoring systems for large rotating machines. Mr. Bissonnette ispresently Sales Manager for VibroSystMs Machine Condition Monitoring Division.

Acknowledgments

The authors would like to thank Mark DeBlock, Michael Mladjenovic and Hugh Fife ofGeneral Electric Motors and Industrial Systems (Peterborough, Ontario, Canada) fortheir generous assistance in the preparation of this paper.

i The capacitive proximity probe is manufactured by VibroSystM under the VibraWatch brand name.