A Review of Current Electrostatic Measurement Techniques

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A REVIEW OF CURRENT ELECTROSTATIC MEASUREMENT TECHNIQUES

AND THEIR LIMITATIONS

BY: WILLIAM E. VOSTEEN

MONROE ELECTRONICS, INC.LYNDONVILLE, NEW YORK 14098

PAPER PRESENTED AT THE ELECTRICAL OVERSTRESS EXPOSITION

APRIL 24-26, 1984

SAN JOSE, CALIFORNIA

LT-19 586WEVrca


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APNE-0011-(LT-19)-11/17/2000-WEVrca Page 2 of 7

A REVIEW OF CURRENT ELECTROSTATIC MEASUREMENT TECHNIQUES

AND THEIR LIMITATIONS

by

WILLIAM E. VOSTEEN

Monroe Electronics, Inc.Lyndonville, NY 14098

In electrostatic measurements, as in any otherscientific measurements, there are two areas oflimitations.

1. The limitation of the methods orinstrumentation used to gather the data.

2. The researchers understanding of thesubject under investigation.

In this paper I will attempt to review somebasic electrostatic principles in order to givethe reader a necessary understanding to makeaccurate electrostatic measurements. Then Illreview the current state of the art in ofelectrostatic voltmeters and field-meters andtheir limitations.

It is well known that all matter is made up ofpositively and negatively charged electricalparticles. Whenever there is an abundance ofone polarity in a certain area, an electricfield E is produced. An electric field is saidto exist at a point if a force of electricalorigin is exerted on a test charge at thatpoint.

Please note that electric field is a vectorquantity with force and direction for which theunits are Newtons per coulomb or volts permeter. This is the definition of electricfield.

When we move one coulomb of charge from onepoint to another in an electric field, we aredoing work on that charge. The term we use forthis is electric potential or voltage.

Again, a volt is the amount of work it takes tomove one coulomb of charge a certain distancethrough an electrical field E.

This now leads us to capacitance. A capacitoris normally viewed as any two conductors

separated by an insulator but can be moregenerally defined as follows:

The capacitance C of a capacitor is the ratio ofthe magnitude of the charge Q on two bodies tothe potential difference between the bodies.

C in Farads, Qin coulombs, V in volts.

The energy stored in a capacitor is:

2

W (work)= 1/2 CV

I feel these are the most important concepts tograsp in order to make scientific electrostatic

measurements. I will give two examples to showhow these laws are important in electrostaticmeasurement.

The first example I will use is where acontinuous sheet (web) of plastic or othernon-conductor runs over rollers as in a usualprocessing situation. It is very common to havesignificant charge build-up in this type ofsituation. First let us assume that the web isuniformly charged. The capacitance (C) of anysmall area of the web is proportional to itsarea (A) divided by the distance the web isfrom any grounded object.

If the web is in close contact with the rollers, is very small and C is maximum. As the webmoves off the roller into free space,increases dramatically from almost 0, thethickness of the material to tens ofcentimeters. The capacitance C changes inverselyproportional as noted above.

From this we can see that changes of capacitancein order of 100 to 1000 is not unusual.


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The web being uniformly charged changes inpotential or voltage proportionally to thecapacitance of the web.

Variations can be seen in voltage measurements

in factors of hundreds in this type ofsituation. Over the rollers voltage measurementswould be very low at maybe 100 volts. Away fromthe roller voltages can exceed 10kV.

If a ground referenced fieldmeter is used inthis situation, the capacitance of the field-meter probe can be significant and alter thevoltage measurements. In general this is not aproblem because we are most concerned withrelative measurements and as long as the probeconfiguration is consistent in relation to theweb, reproducible results can be obtained.

The second example is a variation of the first.Today we commonly hear of the need to keep thevoltage levels on printed circuit boards belowsome set number because of the sensitivity of

the components. Lets say we have a circuitboard sitting 1.00 mm above a bench top and wemeasure 50 volts on that board. If we now liftthat board to 10 cm without discharging theoriginal charge the voltage will then rise to5000 volts because the capacitance drop isinversely proportional to the distance of theprinted circuit to the bench.

The point here is that the voltage isnt thewhole story. If there is one equation that mustalways be kept in mind during electrostaticmeasurements, it is Q=CV.

THE ELECTROSTATIC FIELDMETER:

The ideal electrostatic fieldmeter would exhibitthe following characteristics:

1. It would be very very small

2. It would be capable of orientation todetermine the direction as well asthe magnitude of the field.

3. It would be capable of assuming thepotential in space of the point wherefield intensity measurements aredesired.

4. It must telemeter data to ground asany interconnecting wire wouldgrossly distort the field in thevicinity of the probe.

In general the ideal fieldmeter would have thecapability of measuring the field withoutdistorting that field in any way. Clearly suchan instrument is quite impractical for everydayuse.

The practical fieldmeter used today is a groundreferenced measuring device in which readingsare proportionally related to the distance fromthe probe to the surface or object under test.

This trait is one of the limiting factors of allfieldmeters and if accurate readings are to beobtained, the distance from the field-meterprobe to the surface under test must beprecisely known.

Another common characteristic of fieldmeters is

the field of view of the probe. Figure 1 showsus a graph in which a square target or surfaceunder test with side S is positioned one unit ofdistance away from the probe. We can see fromthis curve that for accuracy, the target sizeshould be three to four times this distance fromthe probe to the surface under test. Thisdictates that the field-meter probe should be asclose as possible to the test surface unless theneed is to measure over a large area. Rememberas spacing gets smaller, the variations inspacing becomes much more critical to theaccuracy of the readings. For example: at 5.0 cmspacing a variation of 0.1 cm yields a change of2%, at 1.0 cm spacing the same variation of 0.1cm yields a change of 10%.

Range:

The range of sensitivity of electrostaticfieldmeters currently on the market is from0.1 voltsper centimeter to 20K Volts percentimeter. The upper limit of fieldmetermeasurement is usually dictated by the breakdownof air, which is around 20kV per centimeter.

Calibration:

Commercially available electrostatic field-meters, although responsive to electric fieldintensity, are often calibrated to read voltagewhen held at a fixed spacing with respect to a

plane surface at constant voltage. The moresophisticated instruments are calibrated to readfield intensity directly, usually in volts percentimeter or volts per meter.

A common calibration technique is to establish auniform electric field by using two largeparallel conductors separated by a smaller fixedknown spacing across which is applied a knownvoltage. The electric field established in thecenter of the plates is then simply V/.


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One of these plates is grounded and a hole isprovided in this plate just adequate to permitthe fieldmeter probe to be held flush with thegrounded surface. The fieldmeter probe is thensubjected to the same electrostatic field as itssurroundings and can be easily calibrated.

ProbeToSurface geometry is very important andshould always be taken into account whenattempting to determine charge or voltage from afield intensity measurement.

A technique that is commonly promoted to obtainmore accurate fieldmeter readings is to mountthe probe looking through a grounded metalplate. This normalizes the meters readings byreducing the electric field convergence on theprobe thus creating a more uniform fieldcondition at the probe. (For a more detailedstudy of fieldmeter theory and application,obtain a copy of Mark Blitshteyns paperMeasuring The Electric Field Of Flat SurfacesWith Electrostatic Fieldmeters.)

Types of Fieldmeters:

Fieldmeters are available in three basicvariations. They are the electrometer type, theradioactive sensor type and the A.C. carriertype of fieldmeter.

The electrometer or the pocket size electro-static locator type of fieldmeter will bereviewed first. This is basically a capacitivelycoupled D.C. amplifier with a shunt capacitorfor calibration.

All amplifiers draw a finite amount of current,therefore this instruments readings will driftover time at the rate of dv/dt=I/C where I isthe electrometer input current and C is theshunt capacitance. A more severe drift can alsobe caused by a minute ion imbalance if this type

of field-meter is used in an ionized airenvironment. To counteract this problem theshunt capacitor must be periodically dischargedin a zero field condition or a known fieldcondition that may be used as a reference.

The main advantages of this type of fieldmeterare low cost, simplicity, small size and theability to make extremely high speedmeasurements.

The disadvantages are the inability to monitoran area over an extended period of time, theneed to periodically zero and, also because ofits D.C. coupled circuitry, the inability to usethese in an ionized air environment.

The second fieldmeter type utilizes a radio-

active sensor. This sensor will ionize the airin its immediate vicinity. When this is exposedto an electric field, a current will flow thatis proportional to the electric field. Thecurrent is then measured to obtain an electricfield reading.

This is a very simple system and it is D.C.stable. Its drawbacks are the fears associatedwith radioactive materials, the possibility of

errors in readings due to dirt accumulation onthe radioactive material affecting itsionization ability and the half-life of theradioactive material.

Fieldmeters utilizing an A.C. carrier typesystem are most common in high quality electric

field monitoring systems. This A.C. signal isproduced by modulating a capacitance pickup inan electric field. This is done normally by twomeans:

1. By a fieldmeter (see Fig. 2) whichutilizes a rotor that chops theelectric field and a stator which isthe sensitive electrode in thissystem. This system is simple indesign and principle. Its drawbacksare that they are usually ratherlarge in size, and may have limitedmotor life. The speed of response forthis instrument is limited by thechopping speed of the rotor.

2. By vibrating capacitor fieldmeterswhich are probably the most commonlyused for long term monitoring. Theseutilize an electrode which isvibrated perpendicularly to anelectric field. For good readingstability the modulation amplitudemust remain stable throughout thecalibration period of the instrument.This is because the amplitude of theA.C. signal created is proportionalto the modulation amplitude for anygiven electric field. This problem iscommonly overcome by utilizing a nullseeking feedback technique that mini-mizes any calibration variations dueto modulation amplitude changes. (SeeFig. 3.)


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These instruments can be made rather small withthe probe size being one to two cubic inches.They are extremely reliable and require littlepower. The speed of response is typically around0.5 seconds.

Minimizing Fieldmeter Drift -Purging:

Most fieldmeter probes must be kept clean tominimize drift (zero instability or offset). Thedrift can readily be understood by visualizingwhat happens when undesired charged dielectricmaterial accumulates within the sensitive volumeof the field-meter. The lines of force

associated with this unwanted chargeaccumulation produces an additional uncontrolledelectric field component which results in a zerooffset in the instrument output.

An additional source of zero offset, althoughnormally much less severe, results when thecomponents of the fieldmeter probe are permittedto develop contact potential differences becauseof the adhesion of conductive materials on theelectrodes. These effects can be minimized ifthe atmosphere within the fieldmeter is keptclear of particulate matter and is operated in astable gaseous atmosphere. Usually thisatmosphere is simply clean, filtered air whichis fed to the sensitive volume to keep itcontinuously purged of foreign matter (solids,liquids, and gasses). If the explosion hazard is

minimized by operating the entire system underinert gas, the probe can be similarly purged bythe same filtered inert gas. Purging is a veryimportant aspect which is commonly overlooked.

THE ELECTROSTATIC VOLTAGE FOLLOWER

OR VOLTMETER

Static Accuracy:

The electrostatic voltage follower is anelectronic servo which functions to create anull in the DC electric field at its sensitiveelectrode. Zero electric field intensitynecessitates that the sensitive electrode

(probe) must be at the same potential as thesurface it is sensing.

Modern circuitry permits even a modest system tofunction with a follow-up error of a smallfraction of one percent at practical probe-to-surface spacings. Followup errors of less than0.01% are routine for sophisticated circuitry.

Range:

The useable range of electrostatic voltmeters isfrom millivolts to tens of thousands of voltsdepending on what the system was designed for.

Spacing Sensitivity:

Theoretically, with sufficient gain, no change

in indicated voltage should exist as anelectrostatic voltage follower probe is movedaway from a large uniformly charged surfaceundertest.

In Figure 4, we see an accuracy vs. spacingmeasurement. This shows that voltmeters canindeed be used over a wide range of spacingswith excellent results when viewing a largeuniformly charged surface.


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In the practical situation, any of the fol-lowing can produce an error in the followupvoltage:

1.

Finite dimensions of surface-under-test.

2. Finite dimensions of probe in com-parison with dimensions of thesensitive electrode or sensitiveaperture.

3. Proximity of surfaces at voltagesother than the unknown (typically atground).

The above influences can be readily grasped ifone realizes that as the probe is moved awayfrom the surface-undertest, fringing fieldsdue to influences other than the surface undertest can influence the probe and thus produce achange in indication.

Adding a conductive shroud around the probewith the shroud electrically connected to theprobe can very significantly reduce the in-fluence of these fringing fields and, there-fore, the probe-to-surface spacing errors.

Drift:

Although some drift may be attributable toinstability of electronic circuitry, in a welldesigned system the major sources of drift areundesired charge accumulation and contactpotential changes on the electrode of theprobe.

1. Undesired Charge Accumulations:

This accumulation is generally someforeign undesired dielectricmaterial such as dust or toner whichaccumulates on or in the vicinity ofthe probe. Care must be taken tominimize this accumulation. Forstable, high sensitivity measure-ments, the probe should be purgedwith a clean, stable supply of airor other gas.

2. Contact Potential Changes:

The sensitive electrode of an elec-trostatic modulator is normallyplated with some noble metal (typi-cally gold) to create a stable,corrosion resistant surface.

Unfortunately, a change in theconstituents of adsorbed gases onthese sensitive electrodes can pro-duce a contact potential differenceof hundreds of millivolts. This canproduce a very undesirable drift. Ifvery low drift measurements are to bemade, an extremely stable atmospheresurrounding the probe must beprovided.

Using a selected probe and a purgekit to purge the probe with filteredroom air has resulted in a drift ratein the laboratory in the order of onemillivolt per hour.

Resolution:

In the field of electrophotography it iscommonly desired to make high resolutionmeasurements of charge patterns on photoreceptors. High resolution necessitates a smallsensitive aperture in the probe. It is alsoessential that the probetosurface spacing besmall compared to the aperture size or thebenefits of the small aperture will be lost.Unfortunately, probe-to-surface spacing must atall times be adequate to prevent sparking*between the surface-under-test and the probe.This dictates, for a given surface voltage, aminimum spacing which in turn dictates a limitto resolution.

Figure 5 shows what probe-to-surface spacing is

needed for 99% accuracy measurements of varyingstrip widths.

*Most instruments can normally withstand thissparking but it is destructive to the surfaceundertest.


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Speed of Response:

High resolution measurements can be very timeconsuming unless the system exhibits good speedof-response.

Instruments currently available can exhibit 10%

to 90% speeds-of-response in hundreds ofmicroseconds.

Noise:

The laws of physics dictate that as moresensitive measurements are attempted thesensitivity is limited by noise. Therefore, highsensitivity, high resolution and high speedofresponse are not mutually exclusive.

Fortunately, in typical electrostatic meas-urements, noise is not too serious a problem asvoltages are normally in the range of hundredsof volts.

It is possible to make non-contacting voltagemeasurements with a resolution of 100 micro-

volts. This can only be done by slowing thesystem down to the degree that its speed-of-response must be reckoned in seconds.

REFERENCES

1.Blitshteyn, M. Measuring the ElectricField of Flat Surfaces with ElectrostaticFieldmeters, Electrostatics Society ofAmerica Conference on Electrostatics, June2022, 1984.

2.Haase, Heinz. Electrostatic Hazards,Their Evaluation and Control, VerlagChemie, N.Y. 1977

Types of Voltmeters:

There are basically two types of electrostaticvoltage follower voltmeters available today.There is the modulated capacitor sensor typewhich is by far the most common (See figure 6).

This utilizes either a vibrating capacitor or atuning fork chopper type system for its sensor.This is a well proven system that was originallydesigned thirty years ago.

In the past few years a sensor using a weakradioactive source has been available. These areextremely simple and are capable of extremelyhigh speed readings. The drawbacks are the usualfears associated with radioactive material, theneed to keep radioactive elements clean becausethe element is so weak that any contaminationcan impair the systems function, and theability of the radioactive material to damage oralter the material under test.

If accuracy better than 2% is needed and

resolution of less than one inch is necessary,the electrostatic voltage follower or voltmetershould be used. If less accuracy and resolutionis acceptable the electrostatic fieldmeter maybe used.

If the user keeps in mind the basic physics ofelectrostatics and properly utilizes todaysinstrumentation, satisfactory results should beeasily obtainable.

3.Sears, F.W. and Zemansky, M.W., Univer-sity Physics Fourth Edition, Addison-Wesley Publishing Co., Reading, Mass. 1970

4.Vosteen, R.E., D.C. Electrostatic Volt-meters and Fieldmeters, IEEE-lAS, 1974.

5.Vosteen, R..E., Capabilities and Limi-tations of Electrostatic Voltage Followersand Electrostatic Fieldmeters,Electrostatic Society of America, 1975Conference on Electrostatics.

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A Review of Current Electrostatic Measurement Techniques