Paper storage device

A paper storage device comprises a sensor for generating a detection signal indicative of the arrival of a top level of a stack of papers on a paper support tray at a predetermined height so that the paper support tray can be lowered in response to the detection signal, and a counter for counting the number of papers successively stacked on the paper support tray so that an indication that the paper support tray is held at an improper position can be provided when the counter counts a predetermined number of the papers during the absence of the detection signal from the sensor. Once the top level of the paper stack on the paper support tray has attained the uppermost limit position, the top level of the paper stack on the paper support tray is lowered to a lowermost limit position to secure the orderly disposition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a paper storage device and more 
particularly to the paper storage device used in an image forming 
apparatus such as, for example, an electrophotographic copying machine or 
a laser printer for accommodating, in a stacked fashion papers which are 
successively discharged from the image forming apparatus. 
2. Description of the Prior Art 
A laser printer is usually used for making a large number of copies from a 
single document at one time and, therefore various paper storage devices 
have been proposed such as having a tray of relatively large capacity or 
stackers. In general, the paper storage device of the type used in 
association with the laser printer comprises at least one paper support 
tray supported for stepwise movement between lowered and elevated 
positions. The paper support tray is generally held at the elevated 
position and is stepwise moved down to the lowered position according to 
the payload on the paper support tray, that is, the number or weight of 
the papers discharged from the laser printer onto the paper support tray. 
It is necessary to descend the paper support tray according to the payload 
imposed thereon for maintaining the distance, that is `drop height` 
between a nip of a pair of discharge rollers and a surface of papers 
stacked on the paper support tray constantly in order to achieve good 
disposition of paper discharged from the laser printer onto the paper 
support tray. 
If an operator of the laser printer removes a portion of the stack of the 
papers from the paper support tray while the laser printer is in process 
of producing copies, the level of the uppermost one of the papers 
remaining on the paper support tray lowers and, hence, the drop height 
referred to above becomes larger. Once this occurs, a problem associated 
with a loss of orderly disposition of the papers on the paper support tray 
tends to occur. 
In view of the foregoing, the prior art is such that the use of either 
upper and lower limit switches for detecting the arrival of the paper 
support tray at elevated and lowered positions, respectively, or a 
detector switch for detecting the surface of the uppermost one of the 
stack of the papers on the paper support tray, has been made to control 
the movement of the paper support tray in dependence on the state of one 
of those upper and lower limit switches or the state of the detector 
switch. See the Japanese Laid-open Patent Publication No. 00-137764, 
published in 1985, and U.S. Pat. No. 4,959,685. 
However, according to any one of those prior art systems, when a drive 
means is driven always according to a change in drop height above the 
uppermost one of the papers, stacked on the paper support tray, to move 
the paper support tray between the elevated and lowered positions, they 
have problems associated with durability noises and others and, therefore, 
require the use of a structure for monitoring the position of the 
uppermost one of the stacked papers at all times. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is intended to provide an improved paper 
storage device employing simplified paper detecting means wherein when a 
portion of the stack of the papers on the paper support tray is removed, 
the orderly disposition of the papers subsequently discharged onto the 
paper support tray can be harmonized with the load imposed on the drive 
means for the paper support tray. 
In order to accomplish the above described object, the paper storage device 
according to the present invention makes use of a detecting means for 
generating a detection signal indicative of the arrival of a top level of 
the stack of the papers on the paper support tray at a predetermined 
height so that the paper support tray can be lowered in response to the 
detection signal. The paper storage device also makes use of a count means 
for counting the number of papers successively stacked on the paper 
support tray so that an indication that the paper support tray is held at 
an improper position can be provided when the count means counts a 
predetermined number of the papers during the absence of the detection 
signal from the detecting means. 
When a pair of discharge rollers are employed for discharging the papers 
successively onto the paper support tray, the drop height as measured from 
the nipping region of the discharge roller pair and the uppermost one of 
the papers stacked on the paper support tray that is, the top level of the 
stack of the papers on the paper support tray, is an important factor to 
be considered for accomplishing the orderly disposition of the papers 
successively discharged onto the paper support tray. Therefore, the 
predetermined height referred to above and detected by the detecting means 
is intended to means an uppermost limit position for the top level of the 
stack of the papers on the paper support tray to occupy for accomplishing 
the orderly disposition of the papers on the paper support tray. In 
general, once the top level of the paper stack on the paper support tray 
has attained the uppermost limit position, the top level of the paper 
stack on the paper support tray is lowered to a lowermost limit position 
to secure the orderly disposition. Accordingly, the paper support tray is 
generally retained at a proper position intermediate between the uppermost 
and lowermost limit positions for the top level of the stack of the papers 
resting on the paper support tray. 
Should the top level of the paper stack on the paper support tray is 
lowered from the lowermost limit position as a result of the removal of a 
portion of the papers from the paper support tray, the position of the 
paper support tray will become improper. In such case, according to the 
present invention, the count means starts its counting operation to count 
the number of papers successively accommodated onto the paper support tray 
subsequent to the removal of that portion of the papers and, when the 
counted number of the papers has attained a predetermined value during the 
absence of the detection signal which ought to be generated by the 
detecting means, that is, during a period in which the top level of the 
paper stack on the paper support tray has not yet been reached the 
uppermost limit position, the paper support tray is deemed as held at the 
improper position. The predetermined count used as a basis for the 
determination of the improper positioning of the paper support tray may be 
determined in consideration of the thickness of each of papers frequently 
used, preferably that of each of papers actually used. In the event that 
the detecting means generates the detection signal before the count means 
counts the predetermined number of the papers, an usual lowering control 
is carried out to lower the paper support tray. 
Thus, according to the present invention, even if the top level of the 
paper stack on the paper support tray is lowered below the lowermost limit 
position, a process of accommodating papers onto the paper support tray is 
continued before the count means counts the predetermined number thereof 
and, in the event that the top level of the paper stack has not yet been 
detected by the detecting means even after the count means counted the 
predetermined number thereof, it means that the top level of the paper 
stack on the paper support tray is abnormally lowered and, therefore, the 
position of the paper support tray is deemed improper. After the 
determination, it is preferred either to elevate the paper support tray so 
that the top level of the paper stack can be brought into alignment with 
the uppermost limit position for the proper positioning or to generate a 
warning signal so that the image forming operation of the image forming 
apparatus can be interrupted.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Before the description of preferred embodiments of the present invention 
proceeds it is to be noted that like parts are designated by like 
reference numerals throughout the accompanying drawings. 
First Embodiment: FIGS. 1 to 7 
FIG. 1 illustrates a paper storage device 60 embodying the present 
invention, which is fitted to a laser printer 1 through a paper inverting 
unit 50. 
The laser printer 1 is mounted on a desk 40 and includes a photoreceptor 
drum 10 supported at a generally central portion thereof for rotation in 
one direction shown by the arrow a. During each complete rotation of the 
photoreceptor drum 10, the latter can move past a plurality of processing 
stations defined in the vicinity of an outer peripheral surface thereof. 
These processing stations includes a charging station at which an 
electrostatic charger 11 is disposed for providing an electrostatic 
potential to the photoreceptor drum 10; a developing station at which 
developing units 12 and 13 of magnetic brush type are disposed for 
developing an electrostatic latent image, formed on the photoreceptor drum 
10, into a visible powder image: a transfer station at which a transfer 
charger 14 is disposed for transferring the visible powder image on the 
photoreceptor drum 10 onto a recording paper supplied from one of 
automatic paper supply units 21, 22, 23 and 24; a separating station at 
which a separating charger 15 is disposed for separating the recording 
paper from the photoreceptor drum 10 after the transfer of the visible 
powder image from the photoreceptor drum 10 onto the recording paper; and 
a cleaning station at which a cleaning unit 10 and an eraser lamp 17 are 
disposed for removing residual toner and residue electrostatic charge 
remaining on the photoreceptor drum 10 in readiness for the next cycle of 
copying operation, respectively. Those component parts of the printer 1 
are well known to those skilled in the art and, therefore, the details 
thereof are not reiterated herein for the sake of brevity. 
The paper supply units 21, 22 and 23 are positioned one above the other at 
a left-hand portion of the machine and accommodate respective paper 
cassettes of different size, while the paper supply unit 24 is of a type 
comprising an elevatable platform on which a stack of recording papers is 
placed. The elevatable paper supply unit 14 may not be always necessary 
and may be employed as an optional element. The size or weight of each of 
the recording papers accommodated by the respective paper supply units 21 
to 24 can be detected by a respective sensor SE11, SE12, SE13 or SE14. The 
recording papers in each of the paper supply units 21 to 24 can be 
successively drawn out therefrom by an associated friction feed roller 25, 
26, 27 or 28 one by one and are then fed towards a timing roller pair 30 
operable to synchronize the arrival of the respective recording paper at 
the transfer station with the arrival of the visible powder image carried 
by the photoreceptor drum 10 at the same transfer station in a manner well 
known to those skilled in the art. The recording paper onto which the 
visible powder image has been transferred is subsequently conveyed by a 
delivery belt 31 towards a fixing unit 32 and is, after the visible powder 
image has been heated and permanently fixed on the recording paper to 
provide a complete copy, discharged by a discharge roller pair 33 onto the 
paper inverting unit 50 positioned exteriorly of the machine housing. 
The paper inverting unit 50 has a plurality of capabilities including a 
capability of feeding one sided copy, i.e., the recording paper having an 
image copied on one side thereof, into a circulating passage 35 including 
a plurality of guide roller pairs 36 and 37 so that the opposite side of 
the recording paper can be formed with another image to eventually render 
the one sided copy to be a double sided copy or so that such one side of 
the copying paper can be formed with another image to eventually provide a 
synthesized copy; and a capability of selecting one of a face-up feed mode 
(a non-inverting mode), in which the resultant copy can be delivered 
straight onto the paper storage device 60 in face-up fashion, and a 
face-down feed mode (an inverting mode) in which the resultant copy can be 
turned face down. 
In order for the paper inverting unit 50 to accomplish any one of the 
foregoing capabilities, the paper inverting unit 50 is provided with an 
infeed roller pair 51, an outfeed roller pair 52, switching roller pairs 
53 and 54, a re-feed roller pair 55, switching pawls 50 and 57 and a 
switchback passage 58. Each of the switching pawls 56 and 57 is supported 
for pivotal movement between two positions and, for this purpose, is 
adapted to be driven by a respective solenoid unit (not shown). 
During a non-inverting mode, the recording paper supplied through the 
infeed roller pair 51 into the paper inverting unit 50 is guided along an 
upper face of the switching pawl 56 and is then feed in face-up fashion 
towards the paper storage device 60 through the outfeed roller pair 52. On 
the other hand during an inverting mode, the recording paper supplied 
through the infeed roller pair 51 into the paper inverting unit 50 is 
guided along a left-hand face of the switching pawl 50 towards the 
switching roller pair 55 then driven in a positive direction, and reaches 
the switchback passage 58 through the switching roller pair 54, then 
driven in a positive direction, after having been guided along a 
right-hand face of the switching pawl 57. When a rear end of the recording 
paper reaches an inverting point Q, the switching roller pairs 53 and 54 
are reversed to rotate in a reverse direction so that the recording paper 
can be further fed in face-down fashion into the paper storage device 60 
through the outfeed roller pair 52 while guided along a right-hand face of 
the switching pawl 56 with its rear end serving as a front end. 
The paper storage device 60 comprises upper and lower trays 70 and 70a 
positioned one above the other and a paper transport unit 61. Each of the 
upper and lower trays 70 and 70a is supported for movement in a direction 
substantially perpendicular to the direction of transport of the recording 
paper and is movable up and down in dependence on the amount of the 
recording papers discharged onto the respective tray 70 or 70a. 
The paper transport unit 61 comprises a plurality of delivery roller pairs 
61, 63, 64, 56 and 66 and a switching pawl 67 disposed between the 
delivery roller pairs 62 and 63. The switching pawl 67 is supported for 
pivotal movement between two positions and is adapted to be driven by a 
solenoid unit (not shown). This switching pawl 67 has an upper face, along 
which the recording paper can be guided towards the delivery roller pair 
63 and then onto the upper tray 70, and a right-hand face along which the 
recording paper can be guided towards the delivery roller pair 64 and then 
onto the lower tray 70a through the delivery rollers 65 and 66. A sensor 
SE1 is disposed following the delivery roller pair 63 for detecting the 
passage of the recording paper therethrough, and a sensor SE1a is disposed 
following the delivery roller pair for detecting the passage of the 
recording paper therethrough. 
As hereinabove described, each of the upper and lower trays 70 and 70a is 
mounted on a respective box 71 or 71a for movement up and down between 
elevated and lowered positions and for horizontally shifting in a 
direction substantially perpendicular to the direction in which the 
recording papers are successively discharged onto the respective tray 70 
or 70a. For this purpose shift motors 73 and 73a and elevating motors 74 
and 74a are both accommodated within the associated boxes 71 and 71a 
immediately beneath the upper and lower trays 70 and 70a and are drivingly 
coupled with the upper and lower trays 70 and 70a through shifting 
mechanisms and elevating mechanisms (both not shown). 
Since the upper and lower trays 70 and 70a and their associated peripheral 
components are of identical structure and reference will be made only to 
the upper tray 70 in describing the details of each of them. Accordingly, 
those component parts of and associated with the lower tray 70a, which 
function in substantially the same manner as those of and associated with 
the upper tray 70 are identified by like reference numerals to which a 
suffix "a" is attached. 
A pair of upper and lower discharge rollers 75 and 75' positioned one above 
the other within the box 71 is operable to feed the recording paper, 
delivered from the paper transport unit 61, onto the upper tray 70. As 
best shown in FIG. 2, a paddle wheel 78 is coaxially mounted on a common 
shaft for rotation together with the lower discharge roller 75' for 
applying a biasing force on a rear end of each of the recording papers, 
successively accommodated in the upper tray 70, in a direction counter to 
the direction in which the recording papers are successively discharged 
onto the upper tray 70. This paddle wheel 78 is in the form as comprising 
a plurality of radially outwardly extending elastic fingers which are, 
during the rotation of the paddle wheel 78 in a direction shown by the 
arrow b, successively brought into contact with the rear end of each of 
the recording papers, which have emerged through the nipping region of the 
discharge roller pair 75 and then fall by gravity onto the upper tray 70, 
thereby to apply the biasing force in a direction counter to the direction 
in which the recording papers are successively accommodated onto the upper 
tray 70. The recording papers so biased by the paddle wheel 78 in the 
manner described above are restricted in position by a back-up plate 70 
positioned on a lower end of the upper tray 70 in face-to-face relation 
with the rear ends of the recording papers then stacked on the upper tray 
70. 
The upper tray 70 has a sensor SE2 supported thereby for detecting the 
absence or presence of the recording paper on the tray 70. This sensor SE2 
has an actuator 80 protruding above a top support surface of the upper 
tray 70 and adapted to be pivoted clockwise about a pivot pin 82, as 
viewed in FIG. 2, in response to a loading of the recording paper or 
papers S on the upper tray 70 to a position shown by the phantom line in 
FIG. 2 with an end 81 thereof consequently retracted out of an optical 
path of the sensor SE2. On the other hand, in the event that the recording 
paper or papers S are removed from the upper tray 70, the actuator 80 
returns to the original position, shown by the solid line in FIG. 2, by 
the effect of its own weight with the end 81 thereof consequently brought 
in position to intercept the optical path of the sensor SE2. 
In order that the top support surface of the tray 70, or the top face of 
the recording paper or papers resting on the top support surface of the 
tray 70 if they are on the upper tray 70, can be maintained at a proper 
position, a sensor SE3 and an actuator 70 are provided for detecting the 
top support surface of the upper tray 70 and the top face of the recording 
papers stacked o the top support surface of the tray 70. The proper 
position referred to above is the position lower than the nipping region 
of the discharge roller pair 75 at a predetermined distance and has a 
tolerance. When the support surface of the tray 70 or the top face of the 
recording papers on the tray 70 is positioned at the proper position, the 
recording papers successively discharged onto the tray 70 can be orderly 
disposed by the paddle wheel 78. The sensor SE3 is used to detect if the 
top support surface of the tray 70 or the top face of the recording papers 
on the tray 70 is in alignment with an upper limit of the proper position. 
The actuator 70 is pivotable about a support pin 77 and has a tip portion 
so shaped as to follow an outer peripheral surface of the lower discharge 
roller 75' of the discharge roller pair 75 while positioned above the 
lower end of the tray 70. This actuator 70 also has a rear end adapted to 
selectively intercept and retract from an optical path of the sensor SE3. 
This actuator 70 is normally biased clockwise about the support pin 77 by 
the effect of its own weight to assume a position shown by the solid line 
in FIG. 1, in which condition the sensor SE3 is in an OFF state. However, 
as the recording papers are successively delivered onto the tray 70 with 
the top face of the stacked recording papers reaching the upper limit of 
the proper position, the actuator 70 is biased by the top face of the 
stacked recording papers to pivot somewhat counterclockwise about the 
support pin 77 with the sensor SE3 consequently brought into an ON state, 
i.e., switched on, to provide an ON signal. In response to this ON signal, 
the elevating motor 74 is powered on in a direction required to lower the 
tray 70 and, therefore, the tray 70 is moved towards the lowered position. 
The actuator 70 pivots clockwise in response to the lowering of the tray 
70 and, when the sensor SE3 is subsequently switched off, the elevating 
motor 74 is powered off. In this way, when the amount of the recording 
papers accommodated by the tray 70 increases and the top face of the 
recording papers stacked on the tray 70 subsequently reaches the upper 
limit, the support 70 can be lowered stepwise by a predetermined distance. 
Whether or not the tray 70 becomes full with the recording papers can be 
determined by detecting the arrival of the tray 70 at the lowered position 
by means of an upper sensor SE4 or a lower sensor SE5 positioned at and 
within a lower region of the box 71. Specifically, the upper sensor SE4 is 
utilized where the recording papers of relatively large size are 
successively discharged from the printer 1, whereas the lower sensor SE5 
is utilized where the recording papers of relatively small size are 
successively discharged from the printer 1. The switching between the 
upper and lower sensors SE4 and SE5 is carried out in response to a size 
signal supplied from one of sensors SE11, SE12, SE13 and SE14 provided in 
and for the paper supply units. When one of the upper and lower sensors 
SE4 and SE5 detects that the tray 70 has become full with the recording 
paper this condition can be indicated or warned and, at the same time, the 
printing operation is interrupted or the switching of one tray over to the 
other tray is carried out. 
In the event that the operator removes all of the recording papers stacked 
on the tray 70 during the use of the printer, the sensor SE2 is switched 
off to switch the elevating motor 74 on in a direction required to elevate 
the tray 70 and, therefore, the tray 70 can be moved towards the upper 
limit of the proper position. On the other hand, in the event that the 
operator removes some of the recording papers stacked on the tray 70, a 
paper handling process is allowed to continue and, according to the first 
preferred embodiment of the present invention now under discussion, unless 
the upper limit sensor SE3 is switched on even though extra 10 recording 
papers are successively discharged onto the tray 70, it is determined that 
the position of the tray 70 is improper, that is, excessively lowered, and 
therefore, the elevating motor 74 is powered on in a direction required to 
elevate the tray 70, allowing the tray 70 to be moved towards the elevated 
position until the upper limit sensor SE3 is switched on. The movement of 
the support 70 will be described in detail later with reference to the 
flowchart. 
On the other hand, each time a group of information is printed out, the 
shift motor 73 is driven for a predetermined time to allow the tray 70 to 
be moved a predetermined pitch in a direction perpendicular to the 
direction in which the recording papers are successively delivered onto 
the tray 70 so that, by this reciprocal movement, a group of recording 
papers can be distributed. 
A control circuit for the entire system is shown in FIG. 3. As shown 
therein, the control circuit included in the printer side includes a 
control processor 100 for controlling the printer 1, a control processor 
101 for controlling a laser beam optical system 2 a control processor 102 
for controlling the paper inverting unit 50, and a control processor 103 
for controlling the paper storage device 60. Print information is 
transmitted from a host computer 110 to an image formation controller 112 
through a host interface 111. The image formation controller 112 is 
operable to transmit image information to be printed out to the optical 
system control processor 101 through a video line 113 and also to transmit 
print mode and others to an interface control processor 115 through a 
control line 114. This interface control processor 115 communicates 
through serial interfaces 116 with the various processors 100 to 103 as to 
the various modes. The interface control processor 115 is also operable to 
control an ON-OFF switching of a display device 117 disposed at an 
operating panel on the printer machine housing. The display device 117 is 
used to provide a visual indication of respective states of the processors 
100 to 103 in response to instructions given by the interface control 
processor 115. 
The control sequence of the processor 103 used to control the paper storage 
device 60 will now be described with particular reference to FIGS. 4 to 8. 
FIG. 4 illustrates a main routine. When the supply of an electric power is 
effected with the program consequently started, initialization takes place 
at step S1 during which various flags timers and counters are reset. Then, 
at successive steps S2 and S3, communication takes place with the other 
processors 100, 101, 102 and 115 through the serial interfaces 116, 
followed by step S4 at which a decision is made to determine if the 
sequence control of the printing process is initiated. If the result of 
decision at step S4 indicates that the sequence control is initiated, a 
subroutine for the print processing is executed at step S5. 
FIG. 4 illustrates the details of the subroutine executed at step S5. 
During the execution of the subroutine of FIG. 4, a control for lowering 
the tray 70, a control for controlling a counter used to count the number 
of the recording papers accommodated by the tray 70, a control for 
elevating the tray 70 and other controls (for example, paper transport and 
detection of a paper jamming) are executed at respective steps S11, S12, 
S13 and S14. 
The subroutine for the tray lowering control executed at step S11 of the 
flowchart of FIG. 5 is illustrated in detail in FIG. 6. As shown therein, 
subsequent to the start of the subroutine of FIG. 6, a decision is made at 
step S21 to determine if the upper limit sensor SE3 is switched on and, if 
it is switched on, that is, if the top face of the recording papers on the 
tray 70 has reached the upper limit of the proper position, a flag A is 
set to 1 at step S22. The flag A is used to indicate that the tray 70 is 
being lowered, and if it is set to 1, the elevating motor 74 is driven in 
a lowering direction at step S23 to lower the tray 70. On the other hand, 
if the result of decision at step S21 indicates that the upper limit 
sensor SE3 is switched off the flag is reset to zero at step S24, followed 
by step S25 at which the elevating motor 74 is powered off, Accordingly, 
the tray 70 being lowered can be held still at this timing and retained in 
position until the upper limit sensor SE3 is switched on as a result of an 
increase of the amount of the recording papers subsequently accommodated 
by the tray 70. 
The counter control subroutine executed at step S11 of the flowchart of 
FIG. 5 is shown in detail in FIG. 7. Subsequent to the start of this 
subroutine, a decision is made at step S31 to determine if a flag is 1. 
This flag B is used to indicate that the position of the tray 70 is 
improper (See steps S35 and S36.) and, if it is set to 1, that is, if the 
position of the tray 70 is improper, this subroutine is immediately 
terminated. On the other hand, if this flag B is reset to zero, another 
decision is made at step S32 to determine if the discharge sensor SE1 
disposed at a position immediately following the discharge roller pair 75 
is on an OFF edge, that is, to determine if the rear end of the recording 
paper has been transported past the sensor SE1. In the first preferred 
embodiment of the present invention, when the sensor SE1 is on the OFF 
edge, it is deemed that a single recording paper has been discharged onto 
and accommodated by the tray 70. Should the result of decision at step S32 
be "YES" the program flow proceeds to step S33 at which a further decision 
is made to determine if the flag A is set to 1, but if it is "NO", a 
further decision is made at step S37 to determine if the flag A is set to 
1. In the event that the flag A is deemed as set to 1 at each of steps S33 
and S37, it means that the tray 70 is being lowered and, therefore, step 
S38 takes place at which a counter is cleared, terminating the subroutine 
of FIG. 7. Unless a further discharge of recording papers takes place, 
this subroutine is also terminated even though the flag A is reset to zero 
(as indicated by "NO" at steps S52 and S37). 
On the other hand, if the further recording paper is discharged and the 
flag A is reset to zero (as indicated by "YES" at step S32 and by "NO" at 
step S33), the counter is incremented by 1 at step S34, followed by a 
decision at step S35 to determine if the count of the counter is "10". If 
the count reaches "10", the flag B is set to 1 at step S36, followed by 
termination of this subroutine. 
FIG. 8 illustrates the details of the tray elevating control subroutine 
executed at step S13 of the flowchart of FIG. 5. Subsequent to the start 
of this subroutine, a decision is made at step S51 to determine if the 
flag B is set to 1, and if it is set to 1, that is, when the number of the 
recording papers accommodated on the tray 10 subsequent to the lowering of 
the tray 70 reaches 10, it means that the tray 70 has been excessively 
lowered below the proper position and, therefore, the elevating motor 74 
is powered on in an elevating direction at step S53 to elevate the tray 
70. Where the result of decision at step S52 indicates that the sensor SE2 
is switched off even though the flag B is reset to zero, it means that all 
of the recording papers are removed from the tray 70 and, therefore, the 
elevating motor 74 is similarly driven in the elevating direction at step 
S53 to elevate the tray 70. Then, if the fact that the sensor SE3 has been 
switched on is confirmed at step S54, the elevating motor 74 is powered 
off. In this way, the tray 70 can be set at a proper upper limit position 
and, therefore, the flag B is reset to zero at step S56, followed by 
termination of this subroutine. 
Thus, in the illustrated first embodiment of the present invention, if the 
upper limit sensor SE3 is switched on at step S21 (that is, if the top 
face of the recording papers reaches the upper limit of the proper 
position) by the time the count of the counter attains "10" the flag A is 
set to 1 and the lowering of the tray 70 is initiated at step S23. In 
general, through this routine the counter is cleared at step S38 and the 
position of the tray 70 is properly controlled. However if the recording 
papers on the tray 70 is partly removed with the top face of the recording 
papers consequently lowered below a lower limit of the proper tolerance, 
the tray 70 will not be elevated (as indicated by "NO" at each of steps 
S51 and S52, in which case the subroutine of FIG. 7 is terminated) up 
until the count of the counter reaches "10" unless the top face of the 
recording papers is detected by the upper limit sensor SE3 during the 
counting operation of the counter. Only when and after the tenth recording 
paper has been accommodated on the tray 70, the position of the tray 70 is 
deemed as improper and, therefore, the tray 70 is elevated to the upper 
limit of the proper position. 
By the foregoing control, even though the recording papers on the tray 70 
are partly removed therefrom, a load imposed on the motor 74 and others 
can be reduced with no need to elevate the tray 70. However, where the top 
face of the recording papers on the tray 70 is excessively lowered more 
than necessary, the tray 70 is elevated to bring it to the proper 
position, thereby avoiding any possibility that the recording papers may 
be disordered on the tray 70. 
Second Embodiment: FIGS. 9 and 10 
The paper storage device according to a second preferred embodiment of the 
present invention is similar in structure to that shown in and described 
with reference to FIGS. 1 to 3 and, however, operates according to a 
control sequence wherein the count of the counter which provides the basis 
for the movement of the tray 70 between the elevated and lowered positions 
is made variable according to the basis weight of the recording paper. It 
is to be noted that the basis weight of the recording paper corresponds to 
the thickness thereof and the count of the counter is selected to be of a 
relatively great value if the recording paper is thin, that is, the basis 
weight thereof is small. By so doing, the tray 70 can be elevated when the 
drop height referred to hereinbefore becomes substantially equal, 
regardless of the thickness of each of the recording papers on the tray 
70. 
More specifically, the paper storage device according to the second 
preferred embodiment of the present invention is controlled based on such 
a counter control subroutine as shown in FIG. 9 and such a paper check 
subroutine as shown in FIG. 10, in addition to the flowcharts shown in and 
described with reference to FIGS. 4, 5, 6 and 8. 
Referring to FIG. 9, subsequent to the start of the counter control 
subroutine, and at step S39 following step S34 at which the counter is 
incremented, the basis weight of the recording paper used in the printer 
for printing images thereon is checked. Then, at subsequent step S40 a 
decision is made to determine if the counter has counted up based on the 
basis weight checked at step S39. If the counter has counted up, the flag 
B is set to 1 at step S36. The other subroutine steps shown in FIG. 0 are 
similar to those shown in FIG. 7 and, therefore, the details thereof will 
not be reiterated for the sake of brevity. 
In the subroutine shown in FIG. 10, respective decisions are made at steps 
S61, S62 and S63 to determine if the basis weight of the recording paper 
is greater than 100 grams, 80 grams and 64 grams. The basis weight of the 
recording paper can be determined on the basis of a signal supplied from 
any one of the sensors SE11, SE12, SE13 and SE14 associated with the paper 
supply units in the printer, or an input signal inputted by the operator. 
If the result of decision at each step S61, S62 and S63 indicates "YES", 
another decision is made at step S64, S65 and S66 to determine if the 
count of the counter is 10, 15 and 30, respectively, but if the result of 
decision at each step S61, S62 and S63 indicates "NO", the basis weight of 
the recording paper is deemed as smaller than 64 grams and, accordingly, 
at step S67 a decision is made to determine of the count of the counter is 
50. Thus, if the result of decision at each step S64 to S67 indicates 
"NO", the operation of the counter is continued at step S68, but if it 
indicates "YES", it means that the top face of the recording papers on the 
tray 70 is lower than the proper position and, therefore, the counter 
counts up at step S69. Based on the count-up at step S69, the flag B is 
set to 1 at step S36, followed by execution of steps S55 to S56 of the 
flowchart of FIG. 8. 
Third Embodiment: FIGS. 11 and 12 
The paper storage device according to a third preferred embodiment of the 
present invention is similar in structure to that shown in and described 
with reference to FIGS. 1 to 3 and, however, operates according to a 
control sequence wherein when the top face of the recording papers on the 
tray is excessively lowered below the proper position a warning is issued 
and, at the same time, the printing operation in the printer 1 is 
interrupted, instead of the tray 70 being elevated as is the case with any 
one of the foregoing preferred embodiments. 
More specifically, the paper storage device according to the third 
preferred embodiment of the present invention is controlled based on such 
a counter control subroutine a shown in FIG. 11 and such a tray elevating 
control subroutine as shown in FIG. 12, in addition to the flowcharts 
shown in and described with reference to FIGS. 4, 5 and 6. 
Referring to the subroutine of FIG. 11, instead of the use of the flag B 
for elevating the tray 70, steps S32 to S35 are executed and, if the count 
of the counter attains 10 at step S35, a warning signal is issued at step 
S40 and the printing operation is interrupted at step S41. The warning 
signal is communicated to the interface control processor 115 of FIG. 3 so 
that light emitting diodes in the display unit 117 can be lit to provide 
an visual warning indication. A stop signal necessary to interrupt the 
printing operation is also communicated to the control processor 100 so 
that the latter can control the printer 1 to interrupt the printing 
operation. 
Referring now to the subroutine shown in FIG. 12, steps S52 to S55 shown 
therein are similar to those shown in FIG. 8. After step S55, and 
subsequent to the elevation of the tray 70, the warning is released at 
step S57 followed by a restart of the printing operation at step S58. 
Although the present invention has been described in connection with the 
preferred embodiments thereof with reference to the accompanying drawings, 
it is to be noted that various changes and modifications are apparent to 
those skilled in the art. For example, the second and third preferred 
embodiments can be combined in the practice of the present invention. 
Also, the shifting motion of each of the trays 70 and 70a is not always 
essential in the practice of the present invention. 
Accordingly, such changes and modifications are to be understood as 
included within the scope of the present invention as defined by the 
appended claims, unless they depart therefrom.