Download Implementation of the ESVM

Implementation of the ESVM
The output quality of the xerographic image is determined by the functionality of each
subsystem. Monitoring the voltage on the photoconductor right after the charging station will help to
characterize the photoconductor’s dark decay property as well as charge uniformity as the exposure
input noise source. It can also be helpful in terms of creating a voltage vs. exposure level plot when the
measurement is taken after exposure.
Trek 344, the Electro-Static Voltage Meter (ESVM) as shown in Figure 1 is the measuring device.
Its probe works like a capacitor that charges itself up to the same level as the object being measured.
Therefore it allows potential on the photoconductor plate to be determined without physical contact.
The probe (Figure 2) has to be placed 2mm from the surface of the photoconductor. It cannot be
held by hand since it will charge itself up with respect to the object being measured; therefore, the
probe needs to be positioned by a fixture. The solution is to use the old camera holder along with a
probe mount (Figure 3) as the fixture since it can be easily adjusted and repositioned while staying at
the same distance from the photoconductor. Readings of the charge will be taken immediately after
charging and exposure station by two separate ESVMs. Figure 2 also shows the two locations for the
probe placement.
The material used for the probe mount is aluminum, ASTM 6061.
Figure 1: the ESVM
Figure 2: placement of the ESVM probes
Figure 3: ESVM probe mount
ESVM ADC
The ESVM has a DC output that is 1/100 of what it is reading from the probe. This signal cannot
be read directly by the DAQ since it only recognizes digital inputs. Therefore, an analog to digital
converter (ADC) has to be designed in order for the DAQ to read the output of the ESVM. The simplest
solution found was to use a voltage to frequency converting chip (LM331) as shown in Figure 4. This
solution has not yet been implemented, however, preliminary tests have been conducted and output
frequency does increase linearly with the input voltage.
Figure 4: LM331 chip
The operation of the LM331 chips is best understood by going through the operating cycle of
the basic V-to-F converter, Figure 5, which consists of the simplified block diagram of the LM331 and the
various resistors and capacitors connected to it. The voltage comparator compares a positive input
voltage, V1, at pin 7 to the voltage, Vx, at pin 6. If V1 is greater, the comparator will trigger the 1-shot
timer. The output of the timer will turn ON both the frequency output transistor and the switched
current source for a period t=1.1 RtCt. During this period, the current i will flow out of the switched
current source and provide a fixed amount of charge, Q = i x t, into the capacitor, CL. This will normally
charge Vx up to a higher level than V1. At the end of the timing period, the current i will turn OFF, and
the timer will reset itself. Now there is no current flowing from pin 1, and the capacitor CL will be
gradually discharged by RL until Vx falls to the level of V1. Then the comparator will trigger the timer and
start another cycle. The current flowing into CL is exactly IAVE = i x (1.1xRtCt) x f, and the current flowing out
of CL is exactly Vx/RL . VIN/RL. If VIN is doubled, the frequency will double to maintain this balance. Even a
simple V-to-F converter can provide a frequency precisely proportional to its input voltage over a wide
range of frequencies.
Figure 5: LM331 basic operation cycle
The simple stand-alone V-to-F converter shown in Figure 6 includes all the basic circuitry of
Figure 5 plus a few components for improved performance. A resistor, RIN=100 kΩ±10%, has been
added in the path to pin 7, so that the bias current at pin 7 (−80 nA typical) will cancel the effect of the
bias current at pin 6 and help provide minimum frequency offset. The resistance RS at pin 2 is made up of
a 12 kΩfixed resistor plus a 5 kΩ(cermet, preferably) gain adjust rheostat. The function of this
adjustment is to trim out the gain tolerance of the LM331, and the tolerance of Rt, RL and Ct. For best
results, all the components should be stable low temperature-coefficient components, such as metalfilm resistors. The capacitor should have low dielectric absorption; depending on the temperature
characteristics desired, NPO ceramic, polystyrene, Teflon or polypropylene are best suited. A capacitor
CIN is added from pin 7 to ground to act as a filter for VIN. A value of 0.01 μF to 0.1 μF will be adequate in
most cases; however, in cases where better filtering is required, a 1 μF capacitor can be used. When the
RC time constants are matched at pin 6 and pin 7, a voltage step at VIN will cause a step change in fOUT. If
CIN is much less than CL, a step at VIN may cause fOUT to stop momentarily. A 50Ωresistor, in series with the
1 μF CL, provides hysteresis, which helps the input comparator provide the excellent linearity.
The output frequency followed this formula:
𝑓𝑜𝑢𝑡 =
𝑉𝑖𝑛 𝑅𝑆 1
2.09𝑉 𝑅𝐿 𝑅𝑡 𝐶𝑡
U1
R5
Rin
7
100k
5
2
Vin
3V
Cin
0.1u
6
8
C2
1u
Rs
12k
RL
100k
Rt
Vs
IN
FRQOUT
R/C
REF
IOUT
3
1
10k
THRS
VS
Vlogic
5V
LM331
15V
5.6k
R4
50
Ct
.01u
0
Figure 6: voltage to frequency converter schematic using LM331
Additional information on LM331 can be found on the following website
http://www.national.com/ds/LM/LM231.pdf
Charging Station Characterization
The voltage experienced by the PC differs as the supplied voltages for the Gird and Corona
change. An experiment was done to characterize the voltage viewed by the PC as a function of the
supplied Grid and Corona Voltage. This was accomplished by placing the ESVM probe as shown in Figure
7. While changing the supplied voltages, the ESVM reading was hand copied. A 3D plot of the data is
available in Figure 8.
Figure 7: probe setting for charging station voltage measurement
Voltage Experienced by PC vs. Corona and Grid Voltage
Voltage Experienced by PC (V)
1200
1000
800
600
400
200
0
1
4
0.8
0.6
2
0.4
Grid Voltage (kV)
0.2
0
0
Corona Voltage (kV)
Figure 8: voltage experienced by PC vs. Corona and Grid voltage
Dark-Decay Analysis
Dark decay is one of the properties of the PC. It is the amount of charge lost on a charged
photoconductor without light exposure. This property is very important as it directly affects the image
quality. Therefore, an experiment was done to characterize the dark-decay property of the PC.
The PC was charged up using Grid voltage = 1.5kV and Corona voltage = 6kV. It is then moved
above an ESVM probe to be measured. A graph of the voltage measured for 3 minutes after the
photoconductor is charged shown in Figure 9 displays the dark decay characteristics of the
photoconductor.
PC Voltage Drop Due to Dark-Decay
1100
1050
1000
PC Voltage (V)
950
900
850
800
750
700
650
0
20
40
60
80
100
120
Time (sec)
140
Figure 9: PC voltage vs. time in a dark room
160
180
200
X-Y PC Voltage Measurement
Measurement throughout the entire photoconductor plane after exposure system is one of the
possibilities that can be done with the ESVM. The whole area can be measured with the photoconductor
moving incrementally along the main drive direction (x-axis) while another servo (Figure 10) drives the
probe back and forth in the y-axis. This method is going to be time consuming, thus prone to dark-decay
effects. However, the dark-decay property of the photoconductor is known by the experiment from the
previous section (Dark Decay Analysis). Therefore, the distorted image can be restored and this is
explained in next section (Image Restoration after Exposure).
Figure 10
The servo shown in Figure 10, MA2512W1-S2.5, needs to be mounted after the exposure
system. It will be driven back and forth with the exact same setup as the main screw motor used to drive
the PC. The ESVM probe will be mounted on the post of the servo. Every sample measurement the
ESVM take will need to be accompanied by a time stamp for the reason of image restoration. The state
machine needed to perform the task is shown in Figure 11
Figure 11: state machine for offline x-y measurement of PC voltage
Data Restoration after Exposure
In the possible offline process where the voltage across the entire PC plane is obtain by the
ESVM, the signal will be a 1D series of voltages as described in the previous section (X-Y PC Voltage
Measurement). Due to the nature of the process being time consuming, dark-decay effect will be
significant. Therefore, the data restoration is required in order to obtain the correct voltage reading.
Each voltage reading sample must has an time stamp indicating when it was measured for the
restoration process to work properly.
The following MatLab code is capable of simulating how dark-decay can affect the data as well
as restoring the original value. The code first loads an image (Figure 12) and create the sampled image
(Figure 13). It then creates the1D series of the voltage readings. Next the code creates a lookup table for
the dark-decayed value with the corresponding original voltage and the time elapsed. It will use this
table to simulate what the dark-decay data will look like (Figure 14). Figure 16 shows the series of the
unaffected voltage in blue and the dark decayed reading in red. The code will then use the same table to
restore the image as shown in Figure 15.
Figure 15: restored image
1000
Figure 12: original image
950
900
850
800
750
Figure 13: sampled image
700
0
10
20
30
40
50
60
70
80
90
100
Figure 16: original (blue) and dark-decayed (red)
samples
Figure 14: dark-decayed image
%the point of this program is to
%demonstrate how to use the
%dark decay data to restore the
%image from the data value
%measured by the ESVM in both
%x and y direction
%lower case letters
%refer to continuous image
%upper case letters
%refer to sampled image
clear all
clc
%creating an sampled ideal image, I
i =
double(imread('cameraman.tif'));
XsampleN=30;
YsampleN=50;
[x,y]=size(i);
Xinc=round(x./XsampleN-0.5);
Yinc=round(y./YsampleN-0.5);
n=1;
for Y=1:YsampleN
for X=1:XsampleN
I(X,Y)=i(Xinc*X,Yinc*Y);
n=n+1;
end
end
figure;
imshow(i,[min(min(i)),max(max(i))])
figure;
imshow(I,[min(min(I)),max(max(I))])
%generate the 1D series of voltage
%values read from ESVM, I1d(1,:)
n=1;
for Y=1:2:YsampleN
for X=1:XsampleN
I1d(1,n)=I(X,Y);
n=n+1;
end
n=n+XsampleN;
end
n=1+XsampleN;
for Y=2:2:YsampleN
for X=XsampleN:-1:1
I1d(1,n)=I(X,Y);
n=n+1;
end
n=n+XsampleN;
end
%assigning time field, t, to
I1d(2,:)
Xscan_time=2;
%time it takes to scan across x-dir
Yscan_time=2;
%time it takes to scan across y-dir
Xsps=Xscan_time/XsampleN;
%second per sample in x-dir
Ysps=Yscan_time/YsampleN;
%second per sample in y-dir
n=1;
t=0;
for Y=1:YsampleN
for X=1:XsampleN
I1d(2,n)=t;
n=n+1;
t=t+Xsps;
end
t=t-Xsps+Ysps;
end
%adding dark decay effect to I1d
load LookUpTable.mat
I1d(1,:)=...
I1d(1,:)*.2+(1000max(I1d(1,:)*.2));
%normalize I1d(1,:)
%setting values to the closed
%there is from teh look up
table
for n=1:length(I1d)
Diff=abs(SV-I1d(1,n));
Min=min(Diff);
k=find(Diff<Min+1e-6);
I1dNew(1,n)=SV(k);
I1dInd(1,n)=k;
end
for n=1:length(I1d)
Diff=abs(TF-I1d(2,n));
Min=min(Diff);
k=find(Diff<Min+1e-6);
I1dNew(2,n)=TF(k);
I1dInd(2,n)=k;
end
%adding dark decay effect
for n=1:length(I1d)
I1dDD(1,n)=...
LUT(I1dInd(2,n),I1dInd(1,n));
end
I1dDD(2,:)=I1dNew(2,:);
%removing dark decay effect from
I1dDD
for n=1:length(I1d)
Diff=abs(TF-I1dDD(2,n));
Min=min(Diff);
k=find(Diff<Min+1e-6);
I1dR(2,n)=TF(k);
I1dInd(2,n)=k;
end
for n=1:length(I1d)
Diff=...
abs(LUT(I1dInd(2,n),...
:)-I1dDD(1,n));
Min=min(Diff);
k=find(Diff<Min+1e-6);
I1dR(1,n)=SV(k);
%I1dR=1d recovered signal from
dark decay
end
%wrap up the 1D signal to 2D to
create image
%image without dark decay fix
n=1;
for Y=1:2:YsampleN
for X=1:XsampleN
I2dDD1(X,Y)=I1dDD(1,n);
n=n+1;
end
n=n+XsampleN;
end
n=1+XsampleN;
for Y=2:2:YsampleN
for X=XsampleN:-1:1
I2dDD1(X,Y)=I1dDD(1,n);
n=n+1;
end
n=n+XsampleN;
end
figure;
imshow(...
I2dDD1,[min(min(I2dDD1))...
,max(max(I2dDD1))])
%image with dark decay fix
n=1;
for Y=1:2:YsampleN
for X=1:XsampleN
I2dR(X,Y)=I1dR(1,n);
n=n+1;
end
n=n+XsampleN;
end
n=1+XsampleN;
for Y=2:2:YsampleN
for X=XsampleN:-1:1
I2dR(X,Y)=I1dR(1,n);
n=n+1;
end
n=n+XsampleN;
end
figure;
imshow(...
I2dR,[min(min(I2dR)),...
max(max(I2dR))])
figure;
plot(I1d(2,:),I1d(1,:));
hold on;
plot(I1d(2,:),I1dDD(1,:),'r')
Voltage vs. Exposure Measurement
A very important property of the PC is how much the voltage drop with respect to the change in
exposure level. Therefore a test was conducted to obtain this information. The method involved in this
experiment was to first create a gradient image (Figure 17) onto a transparency so that it can be used as
the mask for the exposure system. The luminance was then measured across the mask. The ESVM probe
placed after the exposure system was used to capture the voltage on the PC after it has been exposed to
the gradient mask. The result is shown in Figure 18.
Figure 17
600
500
Series1
Poly. (Series1)
Voltage (V)
400
Power
(Series1)
300
y = 2E-08x6 - 5E-06x5 + 0.0005x4 - 0.0286x3 +
0.8901x2 - 21.804x + 716.98
200
y = 839.76x-0.211
100
0
0
20
40
Luminance
60
80
100
(cd/m2)
Figure 18
High Power System Rewiring
The work done for the High Power System Rewiring was mainly for getting rid of the old control
box. Removing the old control box required disconnecting and reconnecting all the wires that powers
the entire Iron Butterfly’s subsystems. Documentation of the new high power system flow diagram and
circuitry are shown in Figure 19 and Figure 20 respectively.
Figure 19
Figure 20