Electrostatic pin hole detector

A surface capable of holding electrical charge wherein at least one sensing electrode having an edge is located in close physical proximity to the surface and is disposed so that upon relative movement between the surface and sensing electrode, charge on the surface crosses the edge of the electrode. The sensing electrode has a surface area facing the surface which is sufficiently small so as to minimize electrical noise when the electrode is in close proximity to the surface. Electrical charge is applied to the surface and relative movement is provided between the surface and sensing electrode while maintaining a constant distance therebetween. A current signal is induced in the sensing electrode in response to a variation in the surface charge crossing the edge of the electrode, and the signal is detected and electrical parameters thereof are measured to provide information on the charge density of the surface to determine the physical uniformity of the surface. Preferably a plurality of sensing electrodes are provided in a path extending along the surface in a direction generally cross-wise of the direction of relative movement between the surface and the sensing electrodes, and the detected current signals from the electrodes are scanned and then measured to obtain the aforesaid information. As a result, the size, number, and location of surface defects are readily determined.

BACKGROUND TO THE INVENTION 
This invention relates to the electrostatic measurement art, and more 
particularly to a new and improved method and apparatus for determining 
the physical uniformity of a surface capable of being electrically 
charged. 
One area of use of the present invention is in determining the surface 
quality and uniformity of photoconductive drums used in photocopiers, 
although the principles of the present invention can be variously applied 
to inspecting any surface capable of holding electrical charge. 
Heretofore, photoconducting drums have been inspected visually or by light 
scattering techniques to determine the presence of surface defects such as 
holes. These methods, however, are indirect in that they provide no 
measure of charge properties of the drum surface and they provide 
information only about the reflective properties of the surface. 
It would, therefore, be highly desirable to provide a method and apparatus 
for quickly and reliably determining the physical quality and uniformity 
of a charged surface in a manner providing a measure of the charge 
properties of the surface. In providing such method and apparatus, 
important considerations involve minimizing electrical noise and 
maximizing resolution of detected signals. It would also be highly 
desirable to provide in such method and apparatus the capability of 
determining the size, number, and location of surface defects. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of this invention to provide a new and 
improved method and apparatus for determining the physical uniformity of a 
surface capable of being electrically charged. 
It is a further object of this invention to provide such a method and 
apparatus which rapidly and directly measures the charge density on the 
surface. 
It is a further object of this invention to provide such a method and 
apparatus wherein detected electrical signals are of high resolution. 
It is a further object of this invention to provide such a method and 
apparatus wherein electrical noise is minimized. 
It is a further object of this invention to provide such a method and 
apparatus having the capability of determining the size, number and 
location of surface defects. 
The present invention provides a method and apparatus for determining the 
physical uniformity of a surface capable of holding electrical charge 
wherein at least one sensing electrode having an edge is located in close 
physical proximity to the surface and is disposed so that upon relative 
movement between the surface and sensing electrode, charge on the surface 
crosses the edge of the electrode. The sensing electrode has a surface 
area facing the surface which is sufficiently small so as to minimize 
electrical noise when the electrode is in close proximity to the surface. 
Electrical charge is applied to the surface and relative movement is 
provided between the surface and sensing electrode while maintaining a 
constant distance therebetween. A current signal is induced in the sensing 
electrode in response to a variation in the surface charge crossing the 
edge of the electrode, and the signal is detected and electrical 
parameters thereof are measured to provide information on the charge 
density of the surface to determine the physical uniformity of the 
surface. Preferably a plurality of sensing electrodes are provided in a 
path extending along the surface in a direction generally cross-wise of 
the direction of relative movement between the surface and the sensing 
electrodes, and the detected current signals from the electrodes are 
scanned and then measured to obtain the aforesaid information. As a 
result, the size, number, and location of surface defects are readily 
determined. 
The foregoing and additional advantages and characterizing features of the 
present invention will become clearly apparent a reading of the ensuing 
detailed description together with the included drawings wherein:

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
FIG. 1 illustrates the method and apparatus according to the present 
invention for determining the physical uniformity, integrity, or 
homogeneity of a surface capable of holding electrical charge. A 
photoconductive drum 10 of the type employed in photocopiers is shown in 
cross-section and has an outer surface 12 which is to be inspected for 
defects such as holes. The apparatus according to the present invention 
comprises at least one sensing electrode generally designated 14 means 
generally designated 16 for locating electrode 14 in close proximity to 
surface 12. As shown more clearly in FIG. 2, electrode 14 according to the 
present invention has an edge 18 and is disposed so that upon relative 
movement between surface 12 electrode 14, electrical charge on surface 12 
crosses edge 18 of electrode 14 in a manner which will be described in 
further detail presently. 
The electrode supporting and locating means 16 is in the form of a printed 
circuit board rectangular in overall shape having a length commensurate 
with the axial length of drum 10 or corresponding dimension of any other 
surface being inspected, and having a width sufficient to accommodate 
circuit components on the side opposite electrode 14 as will be explained 
in further detail presently. Electrode 14 can be formed on the surface of 
board 16 by deposition, etching, or other suitable techniques well-known 
to those skilled in the art. In accordance with the present invention, 
electrode 14 has a surface area facing surface 12 which is sufficiently 
small so as to minimize electrical noise when electrode 14 is in close 
proximity to surface 12. In particular, electrode 14 is rectangular in 
shape wherein edge 18 extends lengthwise thereof and a second edge 20 
extends parallel to edge 18 along the opposite side of electrode 14. Edges 
18, 20 are joined by a pair of opposite edges, one of which is designated 
22 in FIG. 2. By way of example, in an illustrative apparatus, electrode 
14 is of tin-plated or gold-plated copper having a length of about 6 mm 
and a width of about 0.381 mm. Electrode 14 is connected by an electrical 
lead or conductor 28 to circuit components on the opposite side of load 
16, one such component being designated 30 in FIG. 2. 
For inspecting most surfaces, the apparatus of the present invention 
includes a plurality of sensing electrodes each like electrode 14 arranged 
in a path extending in a direction generally cross-wise of the direction 
of relative movement between the electrodes and the surface. For example, 
in the arrangement illustrated in FIG. 1, the direction of rotation of 
drum 10 indicated by arrow 32 is in the plane of the paper, and therefore 
the plurality of sensing electrodes like electrode 14 extend along a path 
generally normal to the plane of the paper. FIG. 3 illustrates a typical 
arrangement of sensing electrodes on board 16 wherein, for example, 12 
sensing electrodes are arranged along a path generally parallel to the 
longitudinal axis of board 16. This path extends cross-wise of the 
direction of relative movement between sensing electrodes and surface, 
i.e., in a direction parallel to the axis of rotation of drum 10 in the 
arrangement in FIG. 1. Three successively adjacent sensing electrodes are 
designated 14, 14', and 14", respectively. Alternate electrodes, i.e. 14 
and 14" are in end-to-end alignment and the intermediate one, i.e. 
electrode 14' is slightly offset laterally and in slightly longitudinal 
overlapping relation with the corresponding ends of electrodes 14 and 14". 
This arrangement ensures that the entire portion of the surface will pass 
along the arrangement of electrodes and be sensed thereby. Each of the 
electrodes is connected to a corresponding lead or conductor, i.e. 28, 
28', 28", to circuit components or the opposite side of board 16 which 
will be described in detail presently. While 12 sensing electrodes have 
been shown in the illustrative arrangement of FIG. 3, the total number is 
variable, depending upon the sizes of the various surfaces being measured, 
for example on the range of axial lengths of photoconductive drums being 
inspected. 
The apparatus of the present invention further comprises means 40 for 
applying electrical charge to the surface being inspected. In the 
illustrative arrangement of FIG. 1 for inspecting surface 12 of the 
photoconductive drum 10, charging means 40 typically comprises a charging 
electrode in the form of a wire 42 extending longitudinally along and in 
closely spaced parallel relation to drum surface 12, a source of high DC 
voltage 44 connected electrically to wire 42 and a shield 46 located 
outwardly of wire 42 and surface 12 are typically connected electrically 
to ground. 
In accordance with the present invention, a constant distance is maintained 
between sensing electrode 14 and surface 12 during relative movement 
therebetween. This can be accomplished by various suitable means, and in 
the arrangement of FIG. 1, board 16 is pivotally movable about point 50, a 
roller 52 rotatably mounted on board 16 contacts drum 10 and establishes a 
predetermined constant direction or spacing between the surface of the 
board 16 containing electrodes 14, and a biasing means 56 acts on board 16 
urging roller 52 against drum 10. Various other mechanical arrangements 
can of course be employed. 
There is also provided means for causing relative movement between surface 
12 and sensing electrodes 14. In the arrangement shown, surface 12 is 
moved and to this end drum 10 is rotated by means including a drive motor 
60 drivingly coupled through a pulley 62 to a shaft 64 upon which drum 10 
is mounted for rotation. Various other mechanical drive arrangements can 
of course be employed. While in the present illustration surface 12 is 
moved relative to sensors 14, there may be other applications of the 
present invention wherein it may be feasible to move the sensors relative 
to the surface. 
Referring now to FIG. 4, the apparatus of the present invention further 
comprises circuit means generally designated 70 connected to the sensing 
electrodes 14 for detecting current signals induced in the electrodes in 
response to variations in the surface charge crossing the edges of the 
electrodes. In particular, circuit means 70 includes a current to voltage 
conversion or averaging stage 72 and an amplification stage 74, and there 
is provided a circuit means 70 for each sensing electrode 14. As shown in 
FIG. 4, conversion stage 72 includes a differential amplifier 76, the 
inverting input of which is connected through an input resistor 78 to 
sensing electrode 14. The non-inverting input of amplifier 76 is connected 
to an electrical ground or reference, and a pair of parallel, 
oppositely-poled diodes 80, 82 are connected across the amplifier inputs. 
The output of amplifier 76 is connected through the parallel combination 
of resistor 84 and capacitor 86 to the amplifier inverting input terminal. 
Amplification stage 74 includes a differential amplifier 90, the inverting 
input of which is connected through a resistor 92 to the output of 
amplifier 76 of the conversion stage. The non-inverting input of amplifier 
90 is connected to an electrical ground or reference. The output of 
amplifier 90 is connected to one end of a voltage divider comprising the 
series combination of resistors 94 and 96. The other end of the voltage 
divider is connected to the wiper arm of a potentiometer 100 serving as an 
adjustable voltage source and connected between a source of positive DC 
voltage and ground. An intermediate point on the voltage divider, in 
particular the junction of resistors 94 and 96, is connected by line 104 
to the inverting input terminal of amplifier 90. 
As shown in FIG. 4, there is provided an identical circuit means for each 
sensing electrode, for example circuit means 70' for sensing electrode 14' 
and including identical components each designated by the same reference 
numerals provided with a prime superscript. Accordingly, for the 
illustrative arrangement previously described including 12 sensing 
electrodes, there would be provided 12 circuit means each identical to 
circuit means 70. 
The apparatus of the present invention further includes scanning means 
generally designated 110 operatively connected to the circuit means for 
scanning the detected current signals. The scanning means 110 preferably 
comprises a multiplexer having an input connected to the output of circuit 
means 70, control or selection inputs connected to a source of control 
signals 112 and an output 114. In a typical arrangement including a large 
number of sensing electrodes, several multiplexers are provided, each one 
being associated with a group of sensing electrodes and corresponding 
circuit means in a manner which will be described. 
The apparatus of the present invention also includes means generally 
designated 120 operatively connected to scanning means 110 for measuring 
at least one electrical parameter of the scanned signals to provide 
information on the charge density of surface 12 to determine the physical 
uniformity of surface 12. As shown in FIG. 4, the output of multiplexer 
110 is connected through a buffer 122 to an input of measuring means 120. 
Buffer 122 comprises a differential amplifier 124, the non-inverting input 
of which is connected to the output of multiplexer 110. The output of 
amplifier 124 is connected through a resistor 126 to the inverting input 
which, in turn, is connected through a resistor 128 to an electrical 
ground. The output of buffer 122 is converted to the input of an 
analog-to-digital converter 129, the output of which is converted to 
measuring means which can comprise, for example, a video display or a 
computer. The former can provide a visual display of pulses having 
amplitudes indicative of variations in surface layer directly associated 
with surface defects in a manner which will be described. The latter can 
provide a histogram containing information as to size, location, and 
number of surface defects in a manner which will be described. 
By way of example, in an illustrative apparatus including 48 sensing 
electrodes arranged in a manner like that of FIG. 3, there would be four 
scanning means 110 each associated with 12 sensing electrodes. Assuming 
consecutively numbered electrodes 14 extending along board 16, electrodes 
Nos. 1 through 4 would be associated with corresponding ones of the four 
scanning means 110, electrodes 5 through 8 would be associated 
corresponding ones of the scanning means 110, etc. By way of example, each 
scanning means is an AD8526A microprocessor. The circuit means 70 of 
electrodes Nos. 1 through 4 are connected to the S1 ports of the four 
microprocessors, the circuit means 70 of electrodes Nos. 5 through 8 are 
connected to the S2 ports thereof, etc. up to the S12 ports. The A0-A3 
ports of the four microprocessors are connected together as are the WR, EN 
and RS ports. The D port of each microprocessor is connected to the 
corresponding buffer. 
In the foregoing illustrative arrangement, buffer amplifiers 124 are type 
AD712, differential amplifiers 90 and 76 are type O84KU, resistors 94 
and 96 both have magnitudes of 10K, potentiometer 100 has a maximum 
voltage of about 15 volts, resistor 78 has a magnitude of 1k and resistor 
84 has a magnitude of 10 Megohms. 
The apparatus of FIGS. 1 through 4 is operated to perform the method of the 
present invention in the following manner. Drum 10 is rotated by drive 
motor 60 and charging means 40 is operated to apply electrical charge to 
drum surface 12 in a known manner. Typically, about 390 volts DC is 
applied to drum 10. Charge is applied to the entire portion of surface 12 
which is to be inspected or measured. Board 16 is moved to place sensing 
electrode 14 in close proximity to drum surface 12 as shown in FIGS. 1 and 
2. Drum 10 is rotated clockwise as viewed in FIG. 1 so that the edge 18 of 
each sensing electrode is first exposed to charge as surface 12 passes 
relative to electrode 14. 
The basic principle of the measurement relies on the relationship q=CV 
where "q" is the charge coupled into a sensitive electrode 14 (sensor) 
from the surface 12 to be measured. "C" is the capacitive coupling to the 
surface and "V" is the voltage difference between the surface 12 and the 
sensor 14. Moving the surface 12 past the sensor 14 at a fixed distance 
gives 
EQU i=CdV/dt 
where "i" is the current in the sensor 14, "C" is the capacitive coupling 
between the surface 12 and the sensor 14 and "dV/dt" is the rate of change 
of the voltage on the surface 12. 
The spacing between the detector 14 and the drum surface 12 should be 
constant and on the order of 100 microns or less. A defect moving past the 
detector 14 will induce a current in the detector because the charge 
crossing the edge 18 of the detector per unit time will vary, due to i=C 
dV/dt where V is the voltage on the drum surface 12 and C is the 
capacitive coupling of the sensor 14. The noise picked up by each detector 
14 having dimensions 0.381 mm.times.6 mm is relatively small because the 
area is only 2.286 mm.sup.2. The signal is proportional to the speed at 
which the pin hole or defect crosses the edge 18 of the detector. In other 
words, since i=cdV/dt=C(dV/dx)(dx/dt), the signal i is proportional to 
dx/dt, the speed at which the surface defect crosses edge 18 of sensing 
electrode 14. Maintaining the distance between the drum surface 12 and the 
detector 14 at a constant distance is important so that the signal i is 
proportional only to dV/dx and dx/dt. 
As previously described, multiple sensors 14 are employed and digitally 
scanned in a drum sensor system. In a typical system there may be 12 to 
128 sensors, more or less. The sensors 14 may be fabricated by known 
printed circuit techniques on one side of a circuit board 16 as previously 
described. The detection circuitry necessary for each sensor 14 can be 
placed on the other side of the printed circuit board 16. Each sensor 14 
is the sensing element and input to the inverting mode of the circuit 
shown in FIG. 4. The geometry of each sensor electrode 14 is typically 0.4 
mm by 6 mm as previously described. The long dimension is located 
perpendicular to the direction of motion of the drum 10 to provide 
sufficient room for the circuits on the opposite side of board 16. 
As previously described, the basic signal is given by: 
1) i=C(dv/dt); 
2) i=C(dv/dx)(dx/dt); and 
3) q=CV. 
Putting the relationship dV/dx=l/C(dq/dx) into equation 2) gives: 
EQU i=(dq/dx)(dx/dt) 
This equation indicates that the signal is proportional to the charge 
variation, dq/dx, around the drum and the speed dx/dt that the charge 
variation dq/dt passes the detector. 
The noise is generated by: 
EQU i.sub.noise =C.sub.N (dV/dt) 
Where C.sub.N is the capacitance of the detector with respect to the noise 
source. C.sub.N is proportional to the area of the sensor. Capacitance is 
a geometrical quantity and given a fixed spacing becomes large when the 
area of the detector is increase. However, the signal remains the same, 
therefore minimizing the area of the detector will minimize the noise and 
maximize the signal to noise ratio. 
The signal generated at the output of the circuit 70 shown in FIG. 4 is: 
EQU i=C.sub.h dV/dt 
where C.sub.h is the capacitance of the hole to the sensor. For 100 
microns.times.100 microns hole (defect) C.sub.h can be approximated as a 
parallel plate capacitor: 
EQU C.sub.h =C.sub.o A/d=8.85.times.10.sup.-12 (10.sup.-4).sup.2 
/7.62.times.10.sup.-5 
EQU C.sub.h =10.sup.-15 farads 
If dV=100 volts and an 8 cm diameter drum is rotating at 2 revolutions per 
second, dt is 5.times.10.sup.-5 seconds and 
EQU i=(10.sup.-15 farads) 100 Volts/5.times.10.sup.-5 sec=2.times.10.sup.-9 
amperes 
This magnitude of current is easily measured. If the area of the hole is 
reduced by a factor of 16 corresponding to a 25 u by 25 u defect, the 
current generated would be approximately 125 pA. These current levels can 
be measured easily at a rate of more than 100 k sample/sec. The readings 
from a typical drum form a matrix of numbers typically 48 by 15,000. This 
array of numbers contains the information necessary to locate all the 
effective pin holes or defects on a surface 250 mm.times.280 mm or an 
equivalent area corresponding to the drum 10 having an axial length of 28 
cm and a diameter of 8 cm. The size of the effective pin hole (defect) in 
this case is 50 u by 50 u and a .DELTA.V=50 volts. 
The method and apparatus of the present invention is illustrated by FIGS. 5 
through 7 which show the types and behavior of signals induced in sensing 
electrodes 14 and ultimately displayed in apparatus such as that 
designated 120 in FIG. 4. FIG. 5 shows three sensing electrodes or 
detectors 14, 14', and 14" which typically are offset from each other as 
previously described with a group or pattern 140 of surface holes or 
defects about to infringe on the edge 18' of sensing electrode 14'. FIGS. 
6a through 6d illustrate the type of signal that would be displayed in 
apparatus 120 when a hole or defect crosses the boundary or edge 18 of a 
sensing electrode 14. Each graph presents length (L) vs. width (W) of the 
defect and amplitude vs. time of the corresponding current signal. Thus, 
FIG. 6a illustrates the signal 144 resulting from a long rectangular hole 
146, FIG. 6b shows the signal 148 resulting from a square hole 150, FIG. 
6c depicts the signal 152 resulting from a small square hole 154, and FIG. 
6d illustrates the signal 156 resulting from a short (in length) 
rectangular hole 158. The length of the hole, i.e. the vertical dimension 
as viewed in FIG. 6, determines the width of the resulting pulse. That is 
because the length of the hole is in the direction of relative movement 
between surface 12 and sensing electrode 14, i.e. the length of the hole 
is orthogonal to the electrode edge 18. The width of the hole, i.e. the 
horizontal dimension viewed in FIG. 6, determines the height or amplitude 
of the resulting pulse. That is because the current signal amplitude is 
determined by the amount of charge passing by the edge 18 of sensing 
electrode 14, i.e. dq/dt, and the wider the hole the more dq/dt passing 
edge 18. 
FIGS. 7a and 7b illustrate the time sequence of current signals generated 
when two different sized holes or defects pass by the sensing electrode. 
Referring to FIG. 7a, a square-shaped hole or defect 164 in surface 12 
moves in the direction of arrow 166 toward edge 18 of sensing electrode 
14. The first broken line representation of hole 164 is after it has 
entered entirely past edge 18 and is entirely in registry with electrode 
14. The second broken line representation of hole 164 is after it has left 
or past by electrode 14. Waveform 170 is the current pulse generated as 
hole 164 enters the sensitive region of electrode 14 and passes edge 18, 
and waveform 172 is the current pulse generated as hole 164 leaves the 
region of electrode 14 passing the opposite edge. Waveform 170 is 
positive-going because as hole 164 passes edge 18 and enters the region of 
electrode 14, dq/dt relative to electrode 14 is negative. Waveform 172 is 
negative-going because as hole 164 passes the opposite edge and leaves the 
region of electrode 14, dq/dt relative to electrode 14 is negative. The 
time between the fall of pulse 170 and the rise of pulse 172 is a measure 
of the width of electrode 14 minus the length of defect 164, provided the 
width of detector 14 is greater than the length of defect 164 measured in 
the direction of relative movement between surface 12 and electrode 14. 
Referring now to FIG. 7b, a square-shaped hole or defect 180, four times 
larger in area than defect 164, moves in the direction of arrow 182 toward 
edge 18 of sensing electrode 14. Waveform 184 is the current pulse 
generated as hole 180 enters the sensitive region of electrode 14 and 
passes edge 18, and waveform 186 is the current pulse generated as hole 
180 leaves the region of electrode 14 passing the opposite edge. The 
amplitudes of pulses 184, 186 are larger than the amplitudes of pulses 
170, 172 due to the fact that hole 180 is larger in area than hole 164. 
The widths or durations of pulses 184, 186 are larger than the widths or 
durations of pulses 170, 172 because the length of defect 180 is greater 
than the length of defect 164, the length being measured in the direction 
of relative movement between electrode 14 and surface 12. The time between 
the fall of pulse 184 and the rise of pulse 186 is a measure of the width 
of electrode 14 minus the length of defect 180, provided the width of 
detector 14 is greater than the length of defect 180 measured in the 
direction of relative movement between surface 12 and electrode 14. This 
parameter is shorter as compared to that for the sequence of FIG. 7a due 
to the fact that defect 180 is larger than defect 164. 
In view of the foregoing, in order to obtain useful information about the 
surface holes or defects, the width of each sensing electrode, i.e. the 
dimension parallel to the direction of relative movement between surface 
12 and electrode 14, should be greater than the maximum length of the hole 
or defects being measured. 
The edge effect resulting from holes or defects crossing the edge 18 of 
each sensing electrode 14 provides current pulses with fast rising leading 
edges as shown in FIG. 7 thereby resulting in a detector having a high 
resolution. In fact, it has been determined that the detector arrangement 
in the foregoing configuration has a resolution about 240 times better 
around the circumference of drum 10 than along the axial length of drum 
10. 
Providing a plurality of sensing electrodes 114 along a path as previous 
described, each electrode having a relatively small surface area facing 
surface 12, results in a significant reduction in electrical noise as 
previously explained. 
The current signals induced in sensing electrode 14 in response to surface 
holes or defects have several electrical parameters, as described in 
connection with FIG. 7, which can be inspected or measured to provide 
information as to the holes or defects. Pulse amplitude provides a measure 
of the hole size, in particular the width of the hole a mentioned in 
connection with FIG. 7. Pulse width also provides a measure of the hole 
size, in particular the length of the hole. 
Current pulses induced in the sensing electrodes 14 in response to movement 
of surface 12 relative thereto can be arranged in a matrix as previously 
described by means of computer to form a histogram. The horizontal axis 
contains points corresponding to the location of each of the sensing 
electrodes along the one dimension of the surface being measured, i.e. the 
axial length of drum 10. The vertical axis contains points corresponding 
to the number of holes or defects sensed during travel of the electrode 14 
along the other dimension of the surface, i.e. along the circumference of 
drum 10. 
The method and apparatus of the present invention provide a rapid and 
reliable determination of the quality, i.e. physical uniformity, of a 
surface such as the surface of a photosensitive drum. The approach is 
direct, providing information of the charge properties of the surface. 
High resolution information signals are obtained and electrical noise is 
minimized. 
It is therefore apparent that the present invention accomplishes its 
intended objects. While an embodiment of the present invention has been 
described in detail, that is for the purpose of illustration, not 
limitation.