Electrostatic imaging device capable of producing high-quality image despite variations in ambient conditions

An electrostatic imaging device such as a printer or facsimile machine has a power source for a transcription roller for transcribing a toner image from a photoreceptor drum to a recording sheet. The power source applies to the transcription roller during an initialization period a power source having a first characteristic between a transcription voltage (V) and a transcription current (I) defined by I/a+V/b=1. The transcription voltage and the transcription current are measured, based on which the power source selects one of a plurality of characteristics of a power source to be applied to the transcription roller during an operation period.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention relates to an electrostatic imaging device such as a 
printer and a facsimile machine using an electrophotographic technique, 
and more particularly, to an electrostatic imaging device capable of 
obtaining a high-quality image regardless of the variation in ambient 
conditions. The present invention also relates to a method for forming an 
image in the electrostatic imaging device. 
(b) Description of the Related Art 
Conventional electrostatic imaging devices are described in 
JP-A-6(1994)-161294 (first publication) and JP-A-5-313515 (second 
publication), for example. In the electrostatic imaging device described 
in the first publication, a driving voltage calculator calculates a 
driving voltage based on the temperature data from a thermal sensor and 
the humidity data from a humidity sensor with reference to data stored in 
a ROM. An I/O controller transmits the calculated driving voltage data to 
a driver of an electrification unit, to control the transcribing potential 
to be supplied from the electrification unit to the transcribing roller 
based on the detected temperature and the detected humidity. 
In the electrostatic imaging device described in the second publication, 
when the temperature and humidity within the device are detected by the 
temperature and humidity sensors and supplied to a CPU, the CPU judges 
which range in the first table data stored in the memory the detected 
temperature and the humidity reside. Then, the CPU reads the transcription 
current data and the voltage control data for removing an electric charge 
corresponding to the toner species of the transcribed toner image with 
reference to the second table data stored in the memory, thereby selecting 
the transcription current and the voltage control data for removing the 
electric charge. 
As described above, the electrostatic imaging devices described in the 
above first and second publications determine a suitable transcription 
current for the ambient conditions based on the detected temperature and 
humidity. Accordingly, it is necessary in the prior art to examine the 
locations for the thermal and humidity sensors before fabrication of the 
device and determine the most suitable ambient conditions for 
transcription. This increases the number of the steps for designing the 
electrostatic imaging device and decreases the available design choices 
for the device. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an 
electrostatic imaging device capable of suppressing any transcription 
error caused by variation of the ambient conditions without using 
temperature and humidity sensors and of increasing the number of design 
choices as well as decreasing the number of design steps. 
It is another object of the present invention to provide a method of 
forming an image in the electrostatic imaging device. 
The present invention provides, in one aspect thereof, an electrostatic 
imaging device comprising a photoreceptor drum for carrying thereon a 
toner layer having an electrostatic latent image, a transcription roller 
for transcribing the toner layer onto a recording sheet, a power source 
for providing a transcription voltage and a transcription current to the 
transcription roller, a voltmeter for measuring the transcription voltage, 
an ammeter for measuring the transcription current, a ROM for storing 
first data for a first characteristic between the transcription voltage 
and transcription current and second data for a plurality of second 
characteristics between the transcription voltage and the transcription 
current, the power source providing power to the transcription roller 
based on the first characteristic during an initialization of the 
transcription roller, the power source selecting one of the second 
characteristics based on the transcription voltage and the transcription 
current measured during the initialization period, the power source 
providing power to the transcription roller during a normal operation of 
the transcription roller based on the selected one of the second 
characteristics. 
The present invention also provides a method for forming an electrostatic 
latent image on a recording sheet comprising the steps of applying a first 
power source having a first characteristic to a transcription roller and 
measuring a transcription voltage and a transcription current during an 
initialization period, selecting one of a plurality of second 
characteristics based on the measured transcription voltage and measured 
transcription current, and applying a second power source having the 
selected one of the second characteristics to the transcription roller 
during an operational period. 
In accordance with an electrostatic imaging device and a method of the 
present invention, the measured transcription voltage and transcription 
current in the initialization can provide a resistivity of the toner 
layer, based on which suitable ambient conditions for the power source can 
be obtained. Thus, the selected one of the second characteristics provides 
a suitable characteristic of the power source adapted for the ambient 
conditions without using a thermal sensor and a humidity sensor.

PREFERRED EMBODIMENTS OF THE INVENTION 
Now, the present invention is more specifically described with reference to 
the accompanying drawings. Referring to FIG. 1, an electrostatic imaging 
device according to an embodiment of the present invention includes a 
photoreceptor drum 21 for carrying an electrostatic latent image while 
rotating in a clockwise direction, an electrification unit 22 for 
electrifying the photoreceptor drum 21 by applying an electric charge, an 
exposure unit (not shown in the figure) for exposing the photoreceptor 
drum 21 to form the electrostatic latent image thereon, a toner collector 
16 for collecting the toner remaining on the photoreceptor drum 21 by 
using a scraping member 15, a development unit 17, and a transcription 
section 25. 
The development unit 17 has a developing roller 23, a housing 18 for 
receiving therein toner 13, a feed roller 11 for supplying the toner 13 
from the housing 18 to the developing roller 23, and a filming member 12. 
A toner stirring member 14 is provided in the housing 18 for rotation in 
the clockwise direction. The developing roller 23 rotates in the 
counter-clockwise direction while being in contact with the photoreceptor 
drum 21, to supply a thin layer of toner to the electrostatic latent image 
formed on the photoreceptor drum 21. The filming member 12 forms the toner 
layer on the developing roller 23 and restricts the amount of toner 
adhered to the developing roller 23 and the electric charge on the 
developing roller 23. 
The transcription section 25 includes a transcription roller 24 rotating in 
contact with the photoreceptor drum 21, a voltmeter 30 for measuring the 
transcription voltage applied to the transcription roller 24, an ammeter 
29 for measuring the transcription current applied to the transcription 
roller 24, ROM 26, CPU 27, and a transcription power source 28 operating 
with a current characteristic stored in ROM 26 and supplied therefrom. 
The transcription roller 24 transcribes the toner image from the surface of 
the photoreceptor drum 21 onto the recording sheet passing through the 
contact area of the transcription roller 24 with the photoreceptor drum 
21. ROM 26 stores data for a first characteristic pattern for the 
transcription power source in which the relationship between the 
transcription voltage V (kV) and the transcription current I (.mu.A) is as 
follows: 
EQU I/a+V/b=1, 
wherein both "a" and "b" are constants larger than zero. In this 
configuration, the actual voltage and the actual current are further 
defined by the resistivity of the toner layer. ROM 26 further stores data 
for a plurality (three) of second characteristics provided for three 
different ambient conditions, one of which is to be selected for operation 
of the transcribing roller 24 based on the measured ambient conditions. 
CPU 27 controls the overall operation of the device. CPU 27 first starts 
the device including the transcription roller 24 for initialization of 
operation based on the first characteristic pattern read from ROM 26, then 
determines in which range the transcription voltage and the transcription 
current measured by the voltmeter 30 and the ammeter 29 reside in the 
first characteristic pattern, and selects one of the second characteristic 
patterns to be used for printing based on the measured ambient conditions 
as detailed below. 
Referring to FIG. 2, the first characteristic I/a+V/b=1 is represented by a 
line L1 passing the coordinates (0 .mu.A, 6 kV) and (20 .mu.A, 0 kV). The 
first characteristic includes three printing ranges including first range 
between coordinates (4 .mu.A, 4.8 kV) and (10 .mu.A, 3 kV), second range 
between coordinates (10 .mu.A, 3 kV) and (14.8 .mu.A, 1.6 kV), and third 
range between coordinates (14.8 .mu.A, 1.6 kV) and (19 .mu.A, 0.2 kV). The 
actual transcription voltage and the transcription current are defined 
also by a line L2 having a slope corresponding to the resistivity of the 
toner under the present ambient condition and passing through the origin 
(O) of the coordinates. The actual transcription voltage and the actual 
transcription current are presented by the coordinates of the point P at 
which the line L2 crosses with the line L1 defined by the first 
characteristic. 
If the temperature and the relative humidity are 10.degree. C. and 20%, for 
example, the transcription roller operates in the first range during 
initialization due to a high resistivity of the toner, or a large slope of 
L2. If the ambient temperature and the relative humidity are 20.degree. C. 
and 50%, for example, the transcription roller operates in the second 
range due to the moderate resistivity of the toner layer. If the ambient 
temperature and the relative humidity are 32.5.degree. C. and 80%, 
respectively, the transcription roller operates in the third range due to 
a low resistivity of the toner layer. 
Referring to FIG. 3, it is known that the hatched range is suitable for 
operating the transcription roller at a temperature of 10.degree. C. and a 
relative humidity of 20% which correspond to the first range in FIG. 2. If 
the transcription voltage and the transcription current measured by the 
voltmeter and the ammeter resides in the first range, the CPU reads the 
data corresponding to FIG. 3 and controls the transcription roller based 
on FIG. 3. That is, the transcription roller is operated while adjusting 
the transcription voltage and the transcription current specified between 
line A1 and line A2, and basically based on the constant current 
characteristic or constant voltage characteristic. For example, if the 
transcription current is 10 .mu.A in the initializing operation, which 
means that the transcription roller is subjected to a low temperature and 
low humidity condition, the transcription current is maintained at a 
constant of 10 .mu.A along line A2 up to a transcription voltage of 5.5 kV 
based on the constant current characteristic, and then decreases toward 
zero with the transcription voltage maintained at 5.5 kV based on the 
constant voltage characteristic. 
If the transcription voltage and the transcription current measured by the 
voltmeter and the ammeter reside in the second range during the 
initialization, the CPU reads the data corresponding to FIG. 4 and 
controls the transcription roller based on FIG. 4. That is, the 
transcription roller is operated while the transcription voltage and the 
transcription current are adjusted between line A3 and line A4, and 
basically based on the constant current characteristic or constant voltage 
characteristic. For example, if the transcription current measured in the 
initialization operation is 14 .mu.A, which means that the transcription 
roller is under a moderate ambient condition, the transcription roller is 
controlled based on FIG. 4, with the transcription current maintained at a 
constant of 14 .mu.A along line A4 up to a transcription voltage of 2.5 
kV, and then the transcription voltage is maintained at a constant of 2.5 
kV down to a transcription current of 6 .mu.A based on the constant 
voltage characteristic. 
If the transcription voltage and the transcription current measured by the 
voltmeter and the ammeter are in the third range, the CPU reads the data 
corresponding to FIG. 5 and controls the transcription roller based on 
FIG. 5. That is, the transcription roller is operated while the 
transcription voltage and the transcription current are adjusted between 
line A5 and line A6, and basically based on the constant current 
characteristic or constant voltage characteristic. For example, if the 
transcription current measured in the initialization operation is 18 
.mu.A, which means that the transcription roller is subjected to a high 
temperature and high humidity condition, the transcription roller is 
operated for printing while being controlled based on FIG. 5, with the 
transcription current maintained at a constant of 18 .mu.A along line A6 
up to a transcription voltage of 1.5 kV, and then the transcription 
voltage is maintained at a constant of 1.5 kV along line A6 down to a 
transcription current of 12 .mu.A. 
As described above, in the electrostatic imaging device according to the 
present embodiment, before the recording sheet enters the contact area of 
the photoreceptor drum 21 with the transcription roller 24 during an 
initialization operation of the transcription roller 24, a suitable 
combination of the transcription voltage and the transcription current can 
be obtained, without using a thermal sensor or a humidity sensor. Thus, 
location of the sensors need not be determined during the design of the 
device, as a result of which the number of design choices can be increased 
and the number of steps in the design can be reduced. 
Since the above embodiments are described only as examples, the present 
invention is not limited to the above embodiments and various 
modifications or alterations can be easily made therefrom by those skilled 
in the art without departing from the scope of the present invention.