Method of and apparatus for controlling transfer voltage based on specific resistance of paper in laser beam printer

A method of and apparatus for controlling a transfer voltage based on a paper specific resistance in a laser beam printer is provided. In the transfer voltage controlling method, the resistance of the feed rollers is detected as a first resistance before a paper sheet passes between the feed rollers. The resistance of the paper sheet and the feed rollers is detected as a second resistance when the paper sheet passes between the feed rollers. The specific resistance of the paper sheet is obtained as a third resistance by determining the difference between the first resistance and the second resistance. The resistance between a transfer roller and a photosensitive drum is detected as a fourth resistance before the paper sheet passes through the transfer roller. A composite resistance is detected as a fifth resistance by adding the third resistance to the fourth resistance, and the paper sheet is printed by applying a transfer voltage to the transfer roller based on the fifth resistance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a laser beam printer (LBP), and in 
particular, to a method of controlling provision of a transfer voltage 
based on the specific resistance of paper in n laser beam printer (LBP). 
2. Description of the Related Art 
In general, a laser beam printer (LBP) exemplary of contemporary practice 
in the art includes a paper cassette and an image forming device such as a 
developer and a fixer for fixing a toner image onto a paper sheet, to 
print on the paper. 
In a general laser beam printer (LBP) exemplary of contemporary practice in 
the art, plural sheets of paper loaded on a paper cassette are fed sheet 
by sheet by a pickup roller. Then, the paper reaches a pair of feed 
rollers with its leading end aligned, and a toner image is fixed onto the 
paper with high temperature and high pressure while the paper passes 
through a developer and a fixer. The image-formed paper comes out to a 
discharge plate through a discharge roller. 
The above described laser beam printer (LBP) exemplary of contemporary 
practice in the art typically uses a conductive transfer roller, measures 
the specific resistance of the transfer roller varied with an environment, 
and applies a transfer voltage based on the transfer roller resistance. In 
fact, image quality is greatly affected by a variation in the specific 
resistance of paper, which is in turn influenced by the thickness, 
humidity, and quality of the paper. However, despite the great influence 
on image quality, the paper specific resistance is unduly neglected in the 
process of setting a transfer voltage in the laser beam printer (LBP) 
exemplary of contemporary practice in the art which gives consideration 
only to the resistance between the conductive transfer roller and a 
photosensitive drum. 
U.S. Pat. No. 4,511,240 to Suzuki et al., entitled ELECTROSTATIC RECORDING 
APATUS, discloses an electrostatic recording apparatus having a sensor 
for detecting a surface potential of a photosensitive member on which an 
electrostatic latent image is formed and a controller for controlling a 
developing bias voltage in accordance with the detected surface potential. 
The developing bias voltage includes an AC component and a DC component 
which are selectively used in accordance with a latent image potential. It 
is disclosed that the frequency of the AC developing bias and the 
magnitude of the DC developing bias are variable so that an optimum 
quality of image is reproduced. 
U.S. Pat. No. 5,099,287 to Sato, entitled TRANSFERRING VOLTAGE CONTROL 
SECTION, discloses an electrophotographic printing apparatus which 
includes a paper supplying mechanism for supplying recording paper, and a 
transferring section for charging the recording paper supplied from the 
paper supplying mechanism by means of a transferring voltage and for 
transferring development material adhered to the surface of a charging 
body of the apparatus to the charged recording paper. It is disclosed that 
the electrophotographic printing device further includes a transferring 
voltage control section for controlling the transferring voltage level 
according to the type of the recording paper. 
U.S. Pat. No. 5,155,501 to Fujita et al., entitled ELECTROPHOTOGRAPHIC 
APATUS WITH FREQUENCY AND DUTY RATIO CONTROL, discloses an 
electrophotographic apparatus wherein a photosensitive body charged by a 
charger is exposed to light emitted from an exposer, for the formation of 
an electrostatic latent image, and wherein the electrostatic latent image 
is developed by a developer and the image developed by the developer is 
transferred on a paper sheet by a transfer charger. It is also disclosed 
that the transfer charger of the apparatus is made up of a converter 
transformer, a switching circuit for controlling the excitation of the 
converter transformer, and an error detector, arranged in association with 
the converter transformer, for detecting an error voltage corresponding to 
a transfer voltage. The apparatus is disclosed to include a separately (or 
externally) excited converter which outputs the transfer voltage from the 
secondary winding of the converter transformer, an input section from 
which one of the print density levels that are predetermined stepwise is 
designated, and a control section for controlling the frequency and duty 
ratio of a transfer signal used for causing the switching circuit to 
perform a switching action, in accordance with the print density level 
designated from the input section and the error voltage information 
supplied from the error detector. 
U.S. Pat. No. 5,241,343 to Nishio, entitled CONDUCTIVE FOAM RUBBER ROLLER 
USED IN IMAGE FORMATION APATUS SUCH AS ELECTROPHOTOGRAPHIC APATUS, 
discloses an image formation apparatus such as an electrophotographic 
recording apparatus which uses a conductive foam rubber roller as a 
charging roller, developing roller, toner-removing roller, or transfer 
roller, and comprises a tubular roller element made of a conductive foam 
rubber material and having a central bore defined by a solid skin layer 
having an electric resistivity considerably higher than that of a foam 
structure of the rubber element, and a conductive shaft on which the 
roller element is mounted and fixed. It is disclosed that end sections of 
the skin layer are removed from the roller element such that the foam 
structure thereof is in direct contact with the shaft at end sections of 
the bore thereof. Alternatively, a conductive disc-like member having a 
central opening formed therein is inserted onto the shaft to be abutted 
against an end face of the roller element, whereby sufficient electric 
contact can be established between the roller element and the shaft. 
U.S. Pat. No. 5,486,903 to Kanno et al., entitled IMAGE FORMING APATUS 
WITH PAPER THICKNESS DETECTOR, discloses a detection device for detecting 
the thickness of a recording material by using an air capacitor, and an 
image forming device for forming an image on the recording material on the 
basis of the output from the detection device. 
U.S. Pat. No. 5,809,367 to Yoo et al., entitled METHOD OF AUTOMATICALLY 
CONTROLLING TRANSFER VOLTAGE AND FUSING TEMPERATURE IN AN 
ELECTROPHOTOGRAPHIC PRINTING APATUS, discloses a method of 
automatically controlling an electrophotographic printing apparatus' 
transfer voltage and fusing temperature according to the type of paper by 
an automatic mode change during manual paper feeding which includes the 
steps of following a manual feed option being selected, automatically 
converting the apparatus's normal paper mode into a paper selection mode 
in response to the manual paper feeding; and when a keyboard input is made 
indicating a change in the type of paper, setting the transferring voltage 
and fusing temperature according to the keyboard input and printing images 
corresponding to image data from a host computer system on the paper. 
U.S. Pat. No. 5,848,321 to Roh et al., entitled METHOD FOR AUTOMATICALLY 
CONTROLLING TRANSFER VOLTAGE IN PRINTER USING ELECTROPHOTOGRAPHY SYSTEM, 
discloses an electrophotography machine that consistently produces images 
of optimum image density regardless of whether an ordinary sheet of paper 
or a transparency is used as the recording medium. A photosensor activated 
in response to the recording media passing a first sensor is disclosed to 
be positioned on the paper conveyance path and detects whether or not the 
recording media being processed is an ordinary sheet of paper or a 
transparency. It is further disclosed that a controller automatically 
applies the appropriate transfer voltage depending on whether or not the 
sheet of recording media is a sheet of paper or a transparency. 
U.S. Pat. No. 5,887,220 to Nagaoka, entitled ELECTROPHOTOGRAPHIC PRINTER 
SENSING AMBIENT CONDITIONS WITHOUT SENSORS, discloses when an 
electrophotographic printer is manufactured, the initial electrical 
resistance of its transfer roller is measured and a corresponding value is 
stored in a memory device in the printer during operation, the printer's 
control program estimates the resistance of the transfer roller from the 
stored value, taking aging into account, then measures the actual 
resistance of the transfer roller, and infers ambient conditions from the 
difference between the estimated and actual resistance values. It is also 
disclosed that the electrophotographic printer is controlled according to 
the inferred ambient conditions. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and apparatus for 
efficiently controlling a transfer voltage based on a paper specific 
resistance in a high-speed laser beam printer (LBP), to prevent 
degradation of image quality caused by a variation in the paper specific 
resistance. 
Another object of the present invention is to provide a method and 
apparatus for controlling a transfer voltage based on a paper specific 
resistance in an laser beam printer (LBP), to obtain high-quality images. 
To achieve the above and other objects or the present invention, there is 
provided a method of controlling a transfer voltage based on a paper 
specific resistance in a laser beam printer. In the transfer voltage 
controlling method, the resistance of feed rollers is detected as a first 
resistance before a paper sheet passes between the feed rollers, and the 
resistance of the printing medium, such s as a paper sheet, and the feed 
rollers is detected as a second resistance when the paper sheet passes 
between the feed rollers. The specific resistance of the printing medium, 
such as a paper sheet, is obtained as a third resistance by calculating 
the difference between the first and second resistances. The resistance 
between a transfer roller and a photosensitive drum is detected as a 
fourth resistance before the printing medium, such as a paper sheet, 
passes through the transfer roller. A composite resistance is detected as 
a fifth resistance by adding the third resistance to the fourth 
resistance, and the printing medium, such as a paper sheet, is printed by 
applying a transfer voltage to the transfer roller based on the fifth 
resistance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic diagram of a laser beam printer (LBP) 100 
illustrating an embodiment of the present invention including feed rollers 
3, a transfer roller 11, a photosensitive drum 12, and a scanner unit 14, 
such as a laser scanner unit. Referring to FIG. 1, plural sheets of paper 
P loaded on a paper cassette 1 are fed sheet by sheet by a pickup roller 
2. Then, the paper P reaches a pair of feed rollers 3 with its leading end 
aligned, and a toner image is fixed onto the paper P with high temperature 
and high pressure while the paper P passes first through a developer 10 
and then a fixer 20 having fixing rollers 21 and 22. The image-formed 
paper P comes out to a discharge plate through a discharge roller 4. The 
laser beam printer (LBP) 100 of FIG. 1 typically uses a conductive 
transfer roller 11, and applies a transfer voltage Vo from a high voltage 
supply, such as high voltage supply 210 (FIG. 2), based on a printing 
medium specific resistance, such as a paper specific resistance. 
Referring to FIG. 2, FIG. 2 is a block diagram illustration of a laser beam 
printer (LBP) 2000 according to an embodiment of the present invention. A 
controller 200, such as a central processing unit (CPU) or a 
microprocessor, for example, of laser beam printer (LBP) 2000 according to 
an embodiment of the present invention provides an overall control in 
recording an image on a printing medium, such as a paper sheet P. Also, 
the controller 200 generates a pulse width modulation (PWM) control signal 
for controlling a high voltage Vp which is to be applied to feed rollers 
120 for use in measurement of the specific resistance of the printing 
medium, such as paper sheet P, and another pulse width modulation (PWM) 
control signal for controlling provision of a transfer voltage Vo used in 
detecting the resistance between a photosensitive drum 133 and a transfer 
roller 136 (FIG. 5). A memory 202 has a read-only memory (ROM) for storing 
a program used to control provision of the transfer voltage Vo and a 
random access memory (RAM) for temporarily storing information to be 
printed or voltage values used for determining a composite resistance of 
the printing medium specific resistance, such as the paper specific 
resistance, and the transfer roller-photosensitive drum resistance. A 
pulse width modulation (PWM) controller 204 outputs pulse amplitude 
modulation (PAM) signals SG1 and SG3 in response to the pulse width 
modulation (PWM) control signals received from the controller 200. The 
controller 200 and the pulse width modulation (PWM) controller 204 can 
also be combined into a single controller, as desirable. A power supply 
208 outputs different voltages for input of AC (Alternating Current) 
voltage. Upon receipt of the voltage from the power supply 208, the high 
voltage supply 210 generates a feed roller voltage Vp corresponding to the 
pulse amplitude modulation (PAM) signal SG1 received from the pulse width 
modulation (PWM) controller 204, the transfer voltage Vo corresponding to 
the pulse amplitude modulation (PAM) signal SG3, a voltage SG2 for use in 
detecting the printing medium specific resistance, such as the paper 
specific resistance, and a voltage SG4 indicative of the transfer 
roller-photosensitive drum resistance resulting from applying the transfer 
voltage Vo to the transfer roller 136. An A/D (analog-to-digital) 
converter 206 converts the voltage SG2 or the voltage SG4 received from 
the high voltage supply 210 to a digital signal and feeds the digital 
signal to the controller 200. A printing medium sensor, such as paper 
sensor 212, senses the introduction of the leading edge of the printing 
medium, such as the paper P, between the feed rollers 120 and applies a 
signal representing the sensed result to the controller 200. 
FIG. 3 is a detailed circuit diagram of a portion of the high voltage 
supply 210 of FIG. 2, from which the voltage Vp is applied to the feed 
rollers 120, and FIG. 4 is a detailed circuit diagram of a portion of the 
high voltage supply 210 of FIG. 2, from which the transfer voltage Vo is 
applied to the transfer roller 136. 
Referring to FIG. 3, a transformer T1 includes a primary winding L1 for 
receiving a voltage E of 24 volts (V), for example, from the power supply 
208 and a secondary winding L2 with a larger number of windings than those 
of the primary winding L1, for generating a higher voltage than a voltage 
on the primary winding L1. A diode D1 is coupled to the primary winding L1 
and to ground GND, for clipping a voltage induced from the primary winding 
L1 to the secondary winding L2. A transistor Q1 has a base for receiving 
the pulse amplitude modulation (PAM) signal SG1 from the pulse width 
modulation (PWM) controller 204, a collector coupled to the primary 
winding L1, and an emitter that is grounded, for selectively controlling 
the voltage on the primary winding L1 to be induced to the secondary 
winding L2 based on the pulse amplitude modulation (PAM) signal SG1. A 
rectifying diode D2 has an anode coupled to the secondary winding L2, for 
rectifying the voltage induced to the secondary winding L2. A smoothing 
capacitor C1 smooths the rectified voltage and supplies the high voltage 
Vp at a predetermined level to the feed rollers (F/R) 120. A resistor Rs1, 
such as a 500k.OMEGA. resistor, for example, is coupled between the 
secondary winding L2 and the ground GND, for detecting a current flowing 
through the feed rollers 120 to provide a measure of the current flowing 
through the feed rollers 120. A resistor R1 is coupled between the 
resistor Rs1 and the ground GND, and a voltage E.sub.1 of 18 volts (V), 
for example, is at the junction of the resistor R1 and the resistor Rs1. 
The voltage SG2 output for use in detecting the printing medium specific 
resistance, such as the paper specific resistance, is applied to the 
analog-to-digital (A/D) converter 206 based on the current flowing through 
the resistor Rs1. 
Now referring to FIG. 4, a transformer T2 includes a primary winding L3 for 
receiving a voltage E of 24 volts (V), for example, from the power supply 
208 and a secondary winding L4 with a larger number of windings than those 
of the primary winding L3, for generating a higher voltage than a voltage 
on the primary winding L3. A diode D3 is coupled to the primary winding L3 
and to ground GND, for clipping a voltage induced from the primary winding 
L3 to the secondary winding L4. A transistor Q2 has a base for receiving 
the pulse amplitude modulation (PAM) signal SG3 from the pulse width 
modulation (PWM) controller 204, a collector coupled to the primary 
winding L3, and an emitter that is grounded, for selectively controlling 
the voltage of the primary winding L3 to be induced to the secondary 
winding L4 based on the pulse amplitude modulation (PAM) signal SG3. A 
rectifying diode D4 has an anode coupled to the secondary winding L4, for 
rectifying the voltage induced to the secondary winding L4. A smoothing 
capacitor C2 smooths the rectified voltage and supplies the transfer 
voltage Vo at a predetermined level to the transfer roller 136 (T/R). A 
resistor Rs2, such as 500k.OMEGA. resistor, for example, is coupled 
between the secondary winding L4 and the ground GND, for detecting a 
current flowing through the transfer roller 136 to provide a measure of 
the current flowing through the transfer roller 136, and thereby providing 
a measure of current between the transfer roller 136 and the 
photosensitive drum 133 (FIG. 5). A resistor R2 is coupled between the 
resistor Rs2 and the ground GND, and a voltage E.sub.1 of 18 volts (V), 
for example, is at the junction of the resistor R2 and the resistor Rs2. 
The voltage SG4 output indicating the transfer roller-photosensitive drum 
resistance is applied to the analog-to-digital (A/D) converter 206 based 
on the current flowing through the resistor Rs2. Meanwhile, it is to be 
appreciated that while the high voltage supply circuitry or the high 
voltage supply 200 includes two circuits as shown in FIGS. 3 and 4 in a 
preferred embodiment of the present invention, it can be operable using a 
single circuit by controlling the input and output ports of the high 
voltage supply with a relay switch. 
FIG. 5 is a schematic diagram of a laser beam printer (LBP) 2000 for 
describing a transfer operation based on a printing medium specific 
resistance, such as a paper specific resistance, according to an 
embodiment of the present invention. Referring to FIG. 5, a light source 
scanner unit 132, such as a laser scanner, produces an image on the 
photosensitive drum 133. A charge roller 131 charges the photosensitive 
drum 133, with toner being supplied to the photosensitive drum 133 by the 
developing roller 135. The feed rollers 120 including a upper feed roller 
120a and lower feed roller 120b, are formed of a conductive rubber 
material, such as conductive rubber, and function to transfer the printing 
medium, such as the paper P, fed by a pickup roller 110 to the transfer 
roller 136. The upper feed roller 120a and lower feed roller 120b are in 
contact and the high voltage supply 210 supplies a current to the upper 
feed roller 120a and the lower feed roller 120b. When the pickup roller 
110 is driven, the high voltage supply 210 flows a current through the 
upper feed roller 120a and lower feed roller 120b, so that the resistance 
of the feed rollers 120 themselves can be detected. The printing medium 
specific resistance, such as the paper specific resistance, can easily be 
determined by obtaining the difference between the currents on the feed 
rollers 120 upon presence of the printing medium, such as the paper P, 
between the feed rollers 120 and upon absence between the feed rollers 120 
of the printing medium, such as the paper P. For obtaining the resistance 
of the feed rollers 120, the voltage Vp is a high potential voltage, for 
example, in a range of between 500 and 1000 volts (V), for example, and 
the material of the feed rollers 120 has a resistance at or below 
1.times.10.sup.5 .OMEGA., for example, to resist against environment 
changes. Since the resistance between the photosensitive drum 133 and the 
transfer roller 136 is measured in a similar manner, the constitution of 
the laser beam printer (LBP) 2000 according to the present invention 
permits a low cost. In case the printing medium, such as the paper P, is 
rapidly fed, the printing medium resistance, such as the paper resistance, 
and the transfer roller resistance are separately measured. Alternatively, 
while there is a high voltage transfer system which can provide good 
quality images without influences from the paper resistance, a high cost 
is typically incurred. 
By way of example, if the voltage Vp applied to the feed rollers 120 is 
1000 volts (V) and a current of 0.1 milliampere (mA) is detected from the 
feed rollers 120 upon feeding the printing medium, such as the paper P, 
the resistance of the feed rollers 120 and the paper P as the printing 
medium is, for example, 10.sup.7 ohms (.OMEGA.) (=R=V/I=100 volts 
(V)/1.times.10.sup.-4 amperes (A)) under the Ohm's law (V=I.times.R). If 
the resistance of the feed rollers 120 upon absence of the paper P is, for 
example, 10.sup.5 ohms (.OMEGA.), the specific resistance of the paper P 
is 99.times.10.sup.5 ohms (.OMEGA.) (=10.sup.7 ohms (.OMEGA.)-10.sup.5 
ohms (.OMEGA.)). This measured paper specific resistance is fed to the 
transfer roller 136. In the exemplary contemporary practice in the art, 
since the transfer roller 136 operates according to a predetermined paper 
resistance, neglecting the resistance of a particular paper sheet, the 
image quality can therefore vary with the particular paper sheet or paper 
status. On the contrary, the present invention operates the transfer 
roller 136 according to the printing medium specific resistance, such as 
the paper specific resistance, obtained from the feed rollers 120. 
Referring now to FIGS. 6 and 7, FIG. 6 is a flowchart of controlling a 
transfer voltage based on measurement or determination of the paper 
specific resistance according to a preferred embodiment of the present 
invention, and FIGS. 7A-7G are timing diagrams of signals according to an 
embodiment of the present invention. In FIGS. 7A-7G, FIG. 7A illustrates a 
timing diagram for a motor of laser beam printer (LBP) 2000; FIG. 7B 
illustrates a timing diagram for a printing medium sensor or paper sensor 
212 (feed sensor); FIG. 7C illustrates a timing diagram for the pulse 
amplitude modulation (PAM) signal SG1 from pulse width modulation (PWM 
controller 204; FIG. 7D illustrates a timing diagram for the voltage SG2 
resulting from applying the voltage Vp to the feed rollers for use in 
detecting the printing medium specific resistance, such as the paper 
specific resistance; FIG. 7E illustrates a timing diagram for the transfer 
voltage applied to the transfer roller 136; FIG. 7F illustrates a timing 
diagram for the pulse amplitude modulation (PAM) signal SG3 from pulse 
width modulation (PWM) controller 204; and FIG. 7G illustrates a timing 
diagram for the voltage SG4 indicative of the transfer 
roller-photosensitive drum resistance resulting from applying the transfer 
voltage Vo to the transfer roller 136. Also, in FIG. 7C, reference 
character A indicates a sensing period of the resistance of the feed 
rollers 120 themselves, reference character B in FIG. 7C indicates a 
sensing period of the resistance of the printing medium, such as the paper 
P, and the feed rollers 120, and reference character C in FIG. 7F 
indicates a sensing period of the resistance between the transfer roller 
136 and the photosensitive drum 133. 
Now, referring to FIGS. 2 to 7G, particularly to FIG. 6, there will 
hereinbelow be given a detailed description of provision of a transfer 
voltage based on a paper specific resistance as a printing medium specific 
resistance, for example, in a laser beam printer (LBP) according to a 
preferred embodiment of the present invention. 
In step 601 of FIG. 6, the controller 200 drives a main motor 201. In step 
603, the controller 200 feeds a pulse width modulation (PWM) control 
signal to the pulse width modulation (PWM) controller 204 to apply the 
pulse amplitude modulation (PAM) signal SG1 to the high voltage supply 
210. The controller 200 and the pulse width modulation (PWM) controller 
204 can be a central processing unit (CPU) or a microprocessor, for 
example and can be separate controllers or combined into a single 
controller, as desirable. The high voltage supply 210 then applies to the 
feed rollers 120 the voltage Vp corresponding to the pulse amplitude 
modulation (PAM) signal SG1 received from the pulse width modulation (PWM) 
controller 204 in response to a power supply voltage received from the 
power supply 208, for detecting the resistance of the feed rollers 120 
alone. Specifically, the voltage SG2 resulting from applying the voltage 
Vp to the feed rollers 120 is converted to a digital signal in the 
analog-to-digital (A/D) converter 206, and the controller 200 detects the 
resistance R.sub.F of the feed rollers 120 from the digital signal 
received from the analog-to-digital (A/D) converter 206. The feed roller 
resistance R.sub.F is stored by the controller 200 in a predetermined area 
of the memory 202. 
Subsequently, the controller 200 determines whether the paper P is passing 
between the feed rollers 120 by the paper sensor 212, in step 605 of FIG. 
6. In step 607, the controller 200 detects the resistance R.sub.FP of the 
paper and the feed rollers when the paper P is passing between the feed 
rollers 120 and stores the resistance R.sub.FP in a predetermined area of 
the memory 202. In step 609, the controller 200 determines a paper 
specific resistance R.sub.P from the resistance the paper and feed rollers 
R.sub.FP and from the feed roller resistance R.sub.F as follows: 
EQU R.sub.P =R.sub.FP -R.sub.F. 
The paper specific resistance R.sub.P is stored by the controller 200 in a 
predetermined area of the memory 202. 
In step 611 of FIG. 6, the controller 200 detects the resistance R.sub.X 
between the transfer roller 136 and the photosensitive drum 133. 
Specifically, the controller 200 generates a pulse width modulation (PWM) 
control signal to the pulse width modulation (PWM) controller 204 to apply 
the pulse amplitude modulation (PAM) signal SG3 to the high voltage supply 
210. Then, the high voltage supply 210 applies to the transfer roller 136 
the transfer voltage Vo corresponding to the pulse amplitude modulation 
(PAM) signal SG3 received from the pulse width modulation (PWM) controller 
204 in response to a voltage received from the power supply 208, for 
detecting the resistance between the transfer roller 136 and the 
photosensitive drum 133. That is, the voltage SG4 resulting from applying 
the transfer voltage Vo to the transfer roller 136 is converted to a 
digital signal in the analog-to-digital (A/D) converter 206, and the 
controller 200 detects the resistance R.sub.X from the digital signal 
received from the analog-to-digital (A/D) converter 206. The resistance 
R.sub.X is stored by the controller 200 in a predetermined area of the 
memory 202. In step 613 of FIG. 6, the controller 200 determines the 
composite resistance R.sub.S of the paper specific resistance R.sub.P and 
the resistance R.sub.X between the transfer roller 136 and the 
photosensitive drum 133 as follows: 
EQU R.sub.S =R.sub.P +R.sub.X. 
In step 615 of FIG. 6, the controller 200 applies a transfer voltage to the 
transfer roller 136 when the paper P as the printing medium passes between 
the transfer roller 136 and the photosensitive drum 133 on the basis of 
the composite resistance R.sub.S, the composite resistance R.sub.S being 
stored in a predetermined area of the memory 202 by the controller 200. In 
consideration of variation of image quality depending on paper type or 
paper status, the paper type or status is detected in advance before the 
paper passes through the transfer roller 136 and the photosensitive drum 
133, and the transfer voltage is provided to the transfer roller 136 based 
on the detected paper type or status. In step 617, the controller 200 has 
the paper P printed and completes the printing process. 
In a laser beam printer (LBP) of the present invention as described above, 
the specific resistance of the printing medium, such as paper, fed to feed 
rollers is obtained and fed to a transfer roller before the printing 
medium, such as paper, reaches the transfer roller, so that the transfer 
roller operates based on a printing medium type or status, such as a paper 
type or status. As a result, high quality images can be obtained. 
While there have been illustrated and described what are considered to be 
preferred embodiments of the present invention, it will be understood by 
those skilled in the art that various changes and modifications may be 
made, and equivalents may be substituted for elements thereof without 
departing from the true scope of the present invention. In addition, many 
modifications may be made to adapt a particular situation to the teaching 
of the present invention without departing from scope thereof. Therefore, 
it is intended that the present invention not to be limited to the 
particular embodiments disclosed as the best mode contemplated for 
carrying out the present invention, but that the present invention 
includes all embodiments falling within the scope of the appended claims.