AU445285B2 - Method and apparatus for detecting the charge pattern onan electrically chargeable surface - Google Patents

Description

11.65570o This invention relates to apparatus which may detect

through frictional force variations the potential or charge

pattern a semiconductor, dielectric or charge carrying

surface and convert the force variations into electrical

signals.

A system for scanning the charge patterns or potential

on an electrophotographic plate is exemplified in U.S. Patent

No. 2,583,546. In that patent a probe having a potential

thereon is placed in close proximity to but not contacting

a charged surface of a drum. Suitable drive mechanism and

carriage means are furnished so that probe may scan the

drum surface. When a charged area of the drum lies

adjacent the probe tip a voltage is induced in the probe.

The probe is directly connected to the grid of an electron

tube and the induced voltage on the probe changes the

bias on the tube as the probe scans the electrically charged

drum surface. The signal variations are amplified and

sent to a detecting device or other utilization means,

a facsimile recorder.

In the field of graphic reproduction the limit of

resolution and sharpness at which an edge can be reproduced

is of prime importance. Both resolution and sharpness are

degraded by several factors. First, the optical image to

which the electrostatic photoconductor is exposed; secondly,

the degree to which the photoconductor can form and sustain

the imposed optical charges; thirdly, the nature of the

electrostatic field characteristic of the sharpness of charge

definition found on the photoconductor.

The scanning of the electrophotographic plate in the

above patented device, as already noted, is based upon

11.6 5 5 n induction coupling. One of the disadvantages of a system relying on induction coupling is the problem of.maintaining

sharpness of charge definition found on the charge carrying

surface. In a system employing induction coupling the reso- lution is directly related to the spacing between the probe

tip and the charged surface. So that if a 1 mil edge resolution is desired it will require a 1 mil spacing between

the probe and the drum. Closer spacing, although it would improve resolution, is hard to achieve because of mechanical

)reasons such as machine tolerances. In addition, where the charged surface is on a drum, spacing variations would occur

as the fixed probe scans the drum since it is practically

impossible to construct a perfectly rounded drum. Moreover, with induction coupling there is the additional undesirable

effect of current drift in probe bias which may arise from

insulation leakage. The patent in question does not eliminate drift but seeks to avoid its effect by periodically grounding the grid of the electron tube.

It has been discovered that where two electrodes separated by a section of electrically chargeable material

have the voltage potential applied between them increased,

the frictional force between the electrodes and the electrically

chargeable material is increased. This phenomenon is called the Johnsen-Rahbek effect.

The present invention applies the Johnsen-Rahbek effect in practical devices. It donverts the frictional forces resulting from the Johnsen-Rahbek effect into electrical

signals giving high resolution and avoids current drift.

One embodiment of the invention contemplates a micro-electro-

meter probe for scanning an electrically chargeable surface

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11,6 and detecting the charge pattern thereon. Other embodiments

employ a microprobe in a simplified transmission system

and in a coincidence detector.

Examples of the invention will now be described with

reference to the accompanying drawings wherein:

Fig.1 shows a method of measuring frictional force;

Fig. 2 shows the relationship between frictional

force and voltage of the elements of Fig. 1;

Fig.3 shows a force transducer with oscillator means;

Pig. 4 shows a force transducer with a simplified

transmission system; and

Fig. 5 shows a force transducer used in a coincidence detector. The same elements have the same reference numerals

in the various figures-;

The application the Johnsen-Rahbek effect is proposed

as a means for measurement or readout of potential or charge

patterns on photoconductor, semi-conductor, dielectric

surfaces or the like. This experiments which Johnsen and

Rahbek conducted were made using semiconductors such as lithographic stone, agate, marble, flint and ivory. However, later investigators of the effect have used electronic

semiconductor material which do not rely on the presence of

moisture as the materials employed by Johnsen and Rahbek did.

Some of the electronic semiconductor material which have

been successfully tested are disks pressed from a powdered

mixture of magnesium and titanium oxides fired and reduced

in hydrogen and selenium, coating or overlying a base plate.

These electrically chargeable materials yield results which

essentially conform to the results achieved by Johnsen and

Rahbek. Other usable semiconductors might be flexible

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11.6 555m

materials of high resistivity such as Mylar or Teflon

discussed in U.S. Patent No. 3,023,731.

Referring now to Figs. 1 and 2, there is shown an

apparatuswhich in general demonstrates the attractive forces

exerted between the electrodes and a charged semiconductor.

Reference symbol 11 is a grounded wire connected to a block 2 12 having a 1 cm contact area. In the experiment, tests

were conducted using both aluminum and brass blocks. The

charged selenium semiconductor 13 which in this instance

may be approximately 50 microns thick is supported on or

permanently affixed to conductive base plate 14. The

contacting surfaces of the brass or aluminum block 12

and the semiconductor 13 are polished to a smooth condition

such that there are several points of intimate contact

between their surfaces. Plate 14 upon which the selenium

is coated or affixed is at ground potential and may be any

suitable metal which does not deleteriously react with the

photoconductor or chargeable material, e.g. zinc, aluminium

or brass. A thread 16 is connected at one end to block 12

and the other end is connected to cup 18 containing weight

material 19. The thread 16 is stretched over idler roller 17

so that a.force, F, is exerted downward on the thread by the

cup 18 containing the weight material.

In the experiment, the selenium semiconductor o was charged and its.potential measured with a suitable

vacuum type voltmeter. The amount of weight necessary to

maintain constant motion of te block 12 across the surface

of the charged plate was found to be a function of the

potential applied between the block and the plate serving as

electrodes. The friction of the thread over the idler roller

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11.6 55 r

was not determined but it is expected that it would

approximate a constant error.

It is understood that the description of the above

experiment is merely to explain the Johnsen-Rahbek phenomenon

and not meant to be rigidly definitive of the effect. There

are in the literature reports by investigators who have

performed exhaustive experiments on the Johnsen-Rahbek

effect, e.g. see "Johnsen-Rahbek Effect With An Electronic

Semiconductor", C. Balak rishnan, British Journal of

Applied Physics, pp. 211-213 (1950). This article also

provides examples of how to make the semiconductor materials

discussed above.

Fig. 2 shows graphs of test runs using brass and

aluminum blocks. The ordinate axis of Pig. 2 gives the

force in grams exerted upon the block and the abscissa

gives the potential on the semiconductor in volts.

The frictional force is clearly shown from the two graph

curves to be dependent upon the potential. More specifically,

the graph for the brass block test run shows that a

volt increase in potential caused an approximately two

gram increase in the frictional force. The aluminum

block test run graph shows a slope which was not as steep

as that of the brass block. However, both graph curves

confirm the teaching of the Johnsen-Rahbek phenomena.

Referring now to Fig 3, there is showh a force 0

transducer applying the Johnsen-Rahbek effect having a

grounded microprobe 22. Numeral 21 is a semi-rigid non-

conductive mounting which may be made of rubber or the like

for yielding supporting microprobe 22. A selenium

photoconductor 24 is coated or deposited on a drum base 26

11,655/7o which is shown at ground potential. Vhile in this

instance probe 22 and drum 26 are shown at ground potential

successful results can be achieved where these elements

are at a potential other than ground. Furthermore,

although a selenium semiconductor is described it will occur

to those skilled in the art that any electrostatically

chargeable material could be substituted in lieu thereof.

Drum base 26 may be made of zinc, aluminum, brpss or any

suitable metal which will not drain off the charge placed

on the selenium photoconductor. In capacitive relationship

with the ricroprobe is a plate 23. Plate 23 in conjunction

with microprobe 22 form a variable capacitor c' with air

as the dielectric medium. Plate 23 is connected to an LC

tank circuit comprising inductor 28 aid capacitor 29. The

LC tank circuit is connected in parallel with oscillator

control circuits. The oscillator means 31 may be any conven-

tional FM modulator or the like having suitable amplification

and control means.

In the operation of Fig 3, the drum base 26 carrying

the selenium photoconductive layer 24 is rotated at a suitable

speed in the direction of the arrow. The selenium coating

shown in contact with microprobe 22 has a charge along its

surface as indicated which may be placed thereon by any

suitable charging device. A force F, is exerted on the

microprobe as a function of the charge on the selenium

photoconductive layer 24. The variation in force on the

microprobe alters the capacitance between it and plate 23.

The changes in capacitance cause oscillator mepns 31

connected to plate 23 to be frequency modulated. 'he

oscillator provides at its output a signal which varies in

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accordance with the Johnsen-Rahbek effect. That is,

as the potential between the electrodes changes the frictional

force, F between the microprobe and the selenium photoconductor

also changes.

The fact that microprobe 22 is grounded and in contact

with the photoconductive material 24 might make it appear

that the charge on the photoconductor would leak off through

the grounded probe. Although there is some small leakage

present the amount is negligible. 'he area of the microprobe

in contact with the photoconductor is quite small and since

the microprobe and phftoconductor are solids there are few

points of intimate contact between them which would permit

a large leakage. Whatever leakage there is occurs only in the

contact areas of the microprobe and photoconductor.

Consequently, those charged areas not in contact with the-

microprobe are unaffected. Moreover, the photoconductor

is an extremely good insulator and is strongly resistant to

current flow.

Referring to Fig. 4, there is shown a method of

utilizing the Johnsen--Rahbek effect in a simplified

transmission system. A plurality of Remote Units, 32, 33,

34, 35 and 36 contain microprobe devices as discussed in

Fig. 3 above for sensing the charge pattern on photoconductive

belt or web 40 of a transmission system. The .outputs from

the remote units are then fed to a Central Output Unit for

monitoring, processing, recording or utilization. Alternptively,

the device disclosed in Fig. 4 could form the basis for a

simplified facsimile system.

In operation of the remote unit, the photoconductive

belt or web 40 as it rotates in the direction of the arrow is

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11.655 no

uniformly charged by a corotron 30 or any other suitable

electrostatic charging device. At a predetermined time

strobe lamps 38 and 39 are turned on to expose a document

to be reproduced. Other exposure devices such as a shutter

arrangement may be satisfactorily used in lieu of strobe

lamps 38 and 39 and would be considered within the scope

of the invention. The image from the document is focused

on to the photoconductive belt by lens 41 whereupon a latent

electrostatic image is formed on the belt according to

conventional electrophotographic principles. Subsequently,

the charged belt is detected by the scanning microprobe in

accordance with Johnsen-Rahbek effect already described and

is amplified. An output signal representing the charge

pattern on belt 40 is transmitted from the remote unit

to the cntrol output unit 42 for recording, processing

or utilization.

Referring now to Fig. 5, there is shown yet another

application of the Johnsen-Rahbek effect. More specifically,

there is shown a means to accomplish coincidence detection

over long periods of time. A first light source 43 is

disposed at one end of an optical shied 46. Disposed at

the other end of shield 46 is an optical grid 51 which emits

light focused into parallel beams by lens 48. Lens 48

is spaced from light source 43 by a predetermined amount

necessary to concentrate the light into.parallel beams and

lies close to optical grid 51. A second light source 44

disposed within shield 47 is separated a predetermined

distance from the shield 46. Lens 49 as in the case

of lens 48 focuses the light emitted from optical grid 52

onto the photoconductor4. A corotron 53 or other suitable

445.285

11.6 54 charging device uniformly charges the photoconductor/- as

it is rotated past the light sources and detecting means.

The microprobe 22 in conjunction with plate 23 and oscillator

31 detect in accordance with the Johnsen-Rahbek effect the

exposed photocunductor surface for coincidence. More

specifically, the two light sources 43 and 44 sequentially

expose a given area of the photoconductor surface through the

optical grids. The coincident exposure of the same areas

of the photoconductor by 43 and 44 is determined by the

Sphotoconductor surface speed and/or the adjustable physical

spacing between 46 and 47. A null condition whereby light

sources 43 and 44 in sequence, totally discharge a given area

of the photoconductor could also be detected by the detecting

means. Moreover, light'sources 43 and 44 could be actuated

by receive and transmit signals respectively of a monitoring

device. In such a case the detection system could monitor

the time lapse between the transmitted and the received signal.

An alternate coincidence detector may comprise a discharging

means such as an infra red generator for applying heat in order

to discharge the electrostatic material in lieu of the light

sources and optics described. In that system a suitable

electrostatic material would be substituted for the

photoconductor described herein.

From the foregoing, it is understood that the

transducer apparatus described applied the Johnsen-Rahbek

effect to measure or read out the potential or charge patterns

on semiconductor dielectric or photoconductive surfaces.

Claims (4)

11.6 55 /7 The claims defining the invention are as follows:

1. Apparatus responsive to the charge on a free

surface on a dielectric comprising: a conductor having a

layer of said dielectric thereon; a probe adapted to move

in contacting relation across said dielectric whereby a

frictional force varying as a function of the charge on

said dielectric is exerted upon said probe, and means asso-

ciated with said probe for converting said force variations

into electrical signals.

2. Apparatus as claimed in claim i, wherein said

means comprises a plate in close proximity to said probe,

whereby said plate and said probe constitute a capacitor

for detecting force variations, the said probe being movable in response to said frictional force so as to vary the capacitance of the capacitor formed by the probe

and the plate.

3. Apparatus as claimed in claim i, wherein the

probe is conductive and is yieldingly mounted on a support

means.

4. The apparatus in accordance with claim 3,

wherein said converting means comprises a plate connected

to an oscillator said plate being in capacitive relation

with said probe and said oscillator being modulated by

variation in capacitance between said plate and said probe

as it moves in its mounting in response to force changes.

A remote transmission unit comprising: a

photoconductive belt adapted to hold an electrostatic

charge thereon; means for placing a uniform electrostatic

charge on said belt; exposure means situated to expose a

document to be electrostatically recorded on to said

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1.6.5 5 no

uniformly charged belt to form an electrostatic image

on said belt corresponding with said document;

apparatus

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as claimed in any one of claims 1 to 4 the photo-

conductive belt and its backing constituting the dielectric

and the conductor respectively, and means for moving said

belt in sliding contact with respect to said probe.

6. A plurality of transmission units as claimed in cla.im

and a central output unit for receiving and recording the

signals transmitted by said remote transmission units.

7. A coincidence detector comprising; photoconductive

means adapted to hold an electrostatic charge thereon;

means for placing an electrostatic charge on said photoconduc-

tive means; a first means for exposing a given area of said

photoconductive means; a second means for exposing seid

given area of said photoconductive means at a predetermined

time following the exposure by said first means; apparatus

as claimed in any one of claims I to 4 wherein the photocon-

ductive means and its conductive backing respectively

constitute the dielectric and the conductor and means for

moving said photoconductive means relative to said probe

means, the electrical signals from the means associated

with the probe being indicative of the coinciding exposed

given area of said photoconductive means.

8. The apparatus of claim 7 wherein said first and

second exposure means are adjustably separated from each

other by a predetermined amount and wherein each comprises

an optical shield containing a light source at one end and

a lens and optical grid at the other end.

9. The apparatus of claim 8 wherein said first exposure

means is activated by a first signal and wherein said

A8 445,285 second exposure means is activated by a second signal.

The apparatus of claim 9 wherein said first exposure means discharges by a certain amount said given area of said

photoconductive means and said second exposure means dis-

charges the remainder of the same area of said photoconductive

means whereby said detecting means detects a null condition.

11. A method of converting a frictional force into an elec-

trical signal comprising; contacting a first conductive means

with an electrostatic material having a charge thereon,

connecting said electrostatic material to a second conductive

means, applying a voltage potential between said first and

second donductive means and moving a probe in frictional engage-

ment across said electrostatic material to sense frictional force

variations and connecting said sensed force variations to an

electrical signal generating means responsive to said sensed

force variations.

12. Apparatus responsive to the charge on a dielectric sub-

stantially as herein described with reference to figure 3 of

the accompanying drawings.

13. A remote transmission unit substantially as herein described

with reference to figure 4 of the accompanying drawings.

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14. A coincidence detector substantially as herein described with reference to figure 5 of the accompanying drawings.

A method of converting a frictional force into an

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electrical signal substantially as herein described with reference'ofthe accompanying drawings.

16. The force transducer substantially as herein

described with reference to figure 3,e-d figure 4 or figure of the accompanying drawings.

Dated this 20th February, 1970.

XEROX CORPORATION By its Patent Attorneys DAVIES &~COLLISON

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