Abstract:
A sheet detecting apparatus provided with a light emitter and a light receiver for receiving light emitted from the light emitter through a sheet conveying path, wherein the light receiver detects any change in a quantity of light from the light emitter caused by a sheet passing on the conveying path intercepting the light emitted from the light emitter, to thereby detect the presence or absence of the sheet is provided with a V/I converting circuit for driving the light emitter, a comparing circuit for effecting negative feedback on the V/I converting circuit by an output signal, and changing the quantity of light of the light emitter, and a limiter circuit for applying a limitation to the negative feedback effected on the V/I converting circuit by the comparing circuit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a sheet detecting apparatus for detecting the presence or absence of a sheet passing on a conveying path by a light emitter and a light receiver. The sheet detecting apparatus is utilized in an image forming apparatus such as a printer, an original conveying apparatus and a paper post-treating apparatus. 
   2. Related Background Art 
   A conventional sheet detecting apparatus shown in  FIG. 9  of the accompanying drawings, as described, for example, in Japanese Patent Application Laid-open No. 2002-267767, has been comprised of a light emitter  510  and a light receiver  500  disposed in opposed relationship with each other with a sheet conveying path  530  interposed therebetween, a CPU  600  for controlling the state of the sheet detecting apparatus, a D/A converter  602  for converting a control signal from the CPU  600  into an analog signal, a voltage-current converting circuit (V/I converting circuit)  603  for converting an output from the D/A converter  602  into a current and generating a driving current for causing the light emitter  510  to emit light, a current-voltage converting circuit (I/V converting circuit)  604  for converting a photoelectric current generated by the light receiver into a voltage, and an A/D converter  605  for converting an output voltage from the I/V converting circuit into a digital signal and transmitting it to the CPU  600 . 
   Such a conventional sheet detecting apparatus has detected the presence or absence of a sheet by the sheet intercepting light between the light emitter  510  and the light receiver  500  disposed in opposed relationship with each other as described in Japanese Patent Application Laid-open No. 2002-267767. 
   In the above-described conventional sheet detecting apparatus, however, there has been the problem that when dust such as paper powder adheres to the light emitter and the light receiver and an output value from the light receiver decreases, it is wrongly recognized that the sheet has passed, in spite of the sheet having not passed. If in this case, the sensitivity of the sheet detecting apparatus is set high in order to prevent the wrong recognition, there has arisen the problem that when a thin sheet is passed, light is transmitted therethrough and the sheet cannot be detected. 
   Also, there has been proposed a sheet detecting apparatus provided with an automatic correcting method of taking out and storing an output signal from a light receiver as the data of an initial state, comparing it with an output signal from the light receiver periodically measured, and adjusting the quantity of light of a light emitter in conformity with the comparison signal to thereby maintain it in an optimum state. 
   In such a sheet detecting apparatus however, it has been difficult to set the timing for monitoring the output signal from the light receiver, and this has led to the undesirable possibility that for example, automatic correction cannot be appropriately effected for such an unexpected cause as a sudden change in temperature or the temporary adherence of dust. Also, there has been the problem that the power supply to the sheet detecting apparatus is cut off during the renewal of correction data and the correction data so far backed up is destroyed. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve the above-noted problems peculiar to the conventional sheet detecting apparatuses, and to provide a sheet detecting apparatus in which the quantity of emitted light from a light emitter is automatically corrected so that the quantity of light from a light receiver may become optimum for detecting a sheet passing on a conveying path to thereby prevent wrong detection. Further, it is an object of the present invention to provide a sheet detecting apparatus which can make automatic correction follow even such an unexpected cause as a sudden change in temperature or the temporary adherence of dust to thereby effect detection appropriately and prevent wrong detection. 
   In order to achieve the above objects, according to the present invention, a sheet detecting apparatus provided with a light emitter and a light receiver for receiving light emitted from the light emitter through a sheet conveying path, wherein the light receiver detects any change in the quantity of light from the light emitter caused by a sheet passing on the sheet conveying path intercepting the light emitted from the light emitter to thereby detect the presence or absence of the sheet is provided with a driver for driving the light emitter, a comparing circuit for effecting negative feedback on the driver by an output signal from the light receiver to thereby change the quantity of light of the light emitter, and limiter for applying a limitation to the amount of feedback effected on the driver by the comparing circuit. 
   The light emitter and the light receiver may be disposed so as to be opposed to each other with the conveying path interposed therebetween. Also, the light emitter and the light receiver may be disposed on one sheet surface side of the conveying path, and a light guiding member such a prism or an optical rod for guiding the light from the light emitter to the light receiver may be provided on the opposite side of the conveying path. 
   According to the present invention, the negative feedback is directly effected on the driver by the comparing circuit based on the output signal from the light receiver and therefore, even if for example, the output value from the light receiver decreases, such automatic correction as will adjust the quantity of light of the light emitter so as to be increased, to thereby being about an optimum state for detecting the sheet passing on the conveying path is always effected. Also, since the automatic correction is always effected, the automatic correction can follow even a sudden change or the like in a measuring environment. 
   The comparing circuit may preferably effect the negative feedback on the driver so that the quantity of light from the light emitter received by the light receiver when the sheet is not passing on the sheet conveying path may maintain an allowable quantity of received light. 
   The optical path from the light emitter to the light receiver may preferably be astride the sheet conveying path at a plurality of locations. 
   The light emitter and the light receiver may preferably be disposed on one sheet surface side of the sheet conveying path, and the light guiding member for guiding the light from the light emitter to the light receiver may preferably be disposed on the other sheet surface side of the sheet conveying path. 
   Other objects and features of the present invention will become apparent from the following description and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a sheet detecting apparatus according to an embodiment of the present invention. 
       FIG. 2  shows the disposition of the sensor portion S of the sheet detecting apparatus according to the present embodiment. 
       FIG. 3  is a cross-sectional view of the sheet detecting apparatus according to the present embodiment taken along the line III-III of  FIG. 2 . 
       FIG. 4  shows the construction of a comparing circuit (differential amplifying circuit). 
       FIG. 5  is a characteristic graph of the sheet detecting apparatus according to the present embodiment. 
       FIG. 6  is a table showing the deteriorated margin of the sheet detecting apparatus according to the present embodiment. 
       FIG. 7  is a plan view showing a state in which the leading edge of a sheet passes on a slit. 
       FIG. 8  is a characteristic graph showing the characteristic of an output voltage V 0LM  when the leading edge of the sheet passes on the slit. 
       FIG. 9  is a block diagram of a conventional sheet detecting apparatus. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of this invention will hereinafter be described in detail by way of example with reference to the drawings. However, the dimensions, materials, shapes, relative arrangement, etc. of constituent parts described in this embodiment, unless specifically described, are not intended to restrict the scope of this invention thereto. 
     FIG. 1  is a bock diagram of a sheet detecting apparatus according to an embodiment of the present invention. 
   The sheet detecting apparatus according to the present embodiment is provided with a light emitter  60  such as an LED and a light receiver  80  such as a phototransistor disposed on one sheet surface side of a sheet conveying path  130 , and a light guiding member  70  disposed on the other sheet surface side of the sheet conveying path  130  for guiding light from the light emitter  60  to the light receiver  80  by reflections. 
   Also, the sheet detecting apparatus is provided with a CPU  200  for controlling the state of the sheet detecting apparatus, a D/A converter  20  for converting a control signal from the CPU  200  into an analog signal, a voltage-current converting circuit (hereinafter referred to as the V/I converting circuit)  50  which is a driver for converting an output from the D/A converter  20  into a current and generating a driving current for causing the light emitter  60  to emit light, a current-voltage converting circuit (hereinafter referred to as the I/V converting circuit)  90  for converting a photoelectric current generated by the light receiver  80  into a voltage, an amplifying circuit  100  for amplifying an output voltage from the I/V converting circuit  90 , a limiter circuit  105  for clamping an output voltage from the amplifying circuit  100  by a predetermined voltage and limiting the output voltage, a comparing circuit  120  for comparing a predetermined reference voltage outputted from the D/A converter  20  with the output voltage of the limiter circuit  105 , amplifying the differential voltage and applying negative feedback to the V/I converting circuit  50 , and an A/D converter  110  for converting an output voltage from the limiter circuit  105  into a digital signal and transmitting it to the CPU  200 . 
   Also, a non-volatile memory (EEPROM)  300  stores therein the set value of the D/A converter  20  and the initial value of the output of the A/D converter  110 . 
     FIG. 2  shows the disposition of the sensor portion S of the sheet detecting apparatus according to the present embodiment. 
   In  FIG. 2 , the sensor portion S is constituted by the light emitter  60 , the light receiver  80  and the light guiding member  70 . The light emitter  60  and the light receiver  80  are mounted on a printed substrate  85 . A sensor hood  75  projected from the printed substrate positions the light emitter  60  and the light receiver  80  at predetermined positions and also, prevents light from the other portion than the light emitter  60  from being received as noise by the light receiver  80 . Further, the sensor hood  75  prevents cross talk between the light guiding member  70  and the light receiver  80 . 
   Conveying guides  131  and  132  are disposed in opposed relationship with each other with a predetermined interval therebetween, and a conveying path  130  for a sheet P is formed therebetween. Also, the conveying guides  131  and  132  are formed with a first slit  133  and a second slit  134  at equidistant positions in the conveying direction of the sheet P with a predetermined interval therebetween. 
   The light guiding member  70  is a prism having reflecting surfaces  73  and  74 , and the light outputted from the light emitter  60  passes through the first silt  133  and enters the light guiding member  70 , and is reflected by the reflecting surface  73  and passes through the light guiding member  70 , and is again reflected by the reflecting surface  74  and passes through the second sit  134 , and enters the light receiver  80 . 
   In  FIG. 2 , the sheet conveyed on the conveying path  130  passes from the upper portion of the plane of the drawing sheet to the inner part of the plane of the drawing sheet, and intercepts the light between the light emitter  60  and the light guiding member  70  and the light between the light guiding member  70  and the light receiver  80  at a time. 
     FIG. 3  is a cross-sectional view taken along the line III-III of  FIG. 2 . The sheet P travels from the left toward the right on the conveying path  130 . The second slit  134  is defined by edges  135  and  136  in the sheet conveying direction. The leading edge of the sheet P passes through the second silt  134  in the order of the edge  135  and the edge  136 . 
   The comparing circuit  120  is a differential amplifying circuit having two inputs, and the specific construction thereof is shown in  FIG. 4 . The comparing circuit  120  amplifies V 2  outputted by the use of a predetermined reference voltage V 1  outputted from the D/A converter  20  and an output voltage V 0  from the I/V converting circuit  90 , and is provided with resistors R 1  and R 2 . The output voltage from the I/V converting circuit  90  becomes V 0  when the limiter circuit  105  does not operate, but becomes V 0LM  when the limiter circuit  105  has operated. Here, the value of V 0LM  can be arbitrarily set by the allowed voltage of the compring circuit  120  and the constructions of the other circuits. It is also possible to utilize the output saturation of the amplifying circuit  100  as the limiter circuit  105 . In the following, V 0LM  is handled as V 0  unless particularly indicated. 
   In this case, the gain of the comparing circuit becomes G 1 =R 2 /R 1 . Accordingly, the output voltage of the comparing circuit becomes
 
 V   2 =(1 +G   1 ) V   1   −G   1   ×V   0   (expression 1).
 
   According to the thus constructed sheet detecting apparatus according to the present embodiment, when the sheet is not conveyed to the conveying path  130 , the light emitted from the LED which is the light emitter  60  is guided to the phototransistor which is the light receiver  80  by the light guiding member  70 , and the light receiver  80  outputs a current conforming to the quantity of received light. The output current from the light receiver  80  is converted into a voltage by the I/V converting circuit  90 , and is suitably amplified by the amplifying circuit  100 . Then, the output voltage from the amplifying circuit  100  is directly negatively fed back to the comparing circuit  120  via the limiter circuit  105 . Then, the comparing circuit  120  compares and amplifies the output voltage from the amplifying circuit  100  in conformity with a control signal from the CPU  200  with the reference voltage V 1  outputted from the D/A converter  20  as the reference, and outputs it to the V/I converting circuit  50 . 
   Therefore, when the output voltage from the light receiver  80  does not differ from the output voltage in an initial state, the output voltage from the comparing circuit  120  is not amplified, but yet if the output voltage from the light receiver  80  drops, the comparing circuit  120  suitably amplifies the output voltage in the comparison with the reference voltage, and the quantity of emitted light of the light emitter  60  is always automatically corrected so that the sensor portion S may assume an optimum state for detecting the sheet passing on the conveying path. 
   While in the above-described embodiment, there has been shown an example in which the output voltage V 1  obtained by the output signal from the CPU  200  being converted by the D/A converter  20  is used as the reference voltage, design may be made such that without resort to the output signal from the CPU  200 , a predetermined reference voltage is generated by a power supply provided independently of the CPU. 
   Here, when the transmission function of the V/I converting circuit  50  is defined as G 2 , and the conversion efficiency of the light emitter  60  is defined as G 3 , and the light quantity transmissibility from the light emitter  60  to the light receiver  80  is defined as η, the input light quantity i 2  to the light receiver  80  is
 
 i 2 =V   2   ×G   2   ×G   3 ×η  (expression 2).
 
   Also, the conversion efficiency of the phototransistor which is the light receiver  80  is defined as G 4 , the transmission function of the I/V converting circuit  90  is defined as G 5 , and the transmission function of the amplifying circuit  100  is defined as G 6 , and the output voltage V 0LM  from the limiter circuit  105  when the limiter circuit  105  is not operating becomes equal to the output voltage V 0  of the amplifying circuit  100  and therefore, V 0LM  is
 
 V   0LM   =V   0   =G   4   ×G   5   ×G   6   ×i 2  (expression 3).
 
   Accordingly, from the above-mentioned expression (1), expression (2) and expression (3), the output voltage V 0LM  from the limiter circuit  105  when the limiter circuit  105  is not operating has a characteristic shown by
 
 V   0LM   =V   0   =[{K ×η×(1 +G   1 )× G   2 }/(1 +K×η×G   1   ×G   2 )]× V   1 (expression 4),
 
where it is to be understood that K=G 3 ×G 4 ×G 5 ×G 6 .
 
   In the above-mentioned expression 4, K, G 1  and G 2  are constants and thus, the output voltage V 0  from the amplifying circuit  100  is determined by the light quantity transmissibility η and the reference voltage V 1 . Also, when the output voltage V 0  of the amplifying circuit  100  reaches the upper limit value of the limiter circuit  105 , it is limited to V 0LM =V H  which is the upper limit voltage of the limiter circuit  105 . Also, when the output voltage V 0  reaches the lower limit value of the limiter circuit  105 , it is limited to V 0LM =V L  which is the lower limit voltage of the limiter circuit  105 . 
   Also, when a current for driving the light emitter  60  is defined as J 1 , the input light quantity i 2  to the light receiver  80  becomes
 
 i 2 =J   1   ×G   3 ×η  (expression 5)
 
   From the above-mentioned expression 3, expression 4 and expression 5, the current J 1  for driving the light emitter  60  has a characteristic shown by
 
 J   1 =[(1 +G   1 )× G   2 /(1 +K×η×G   1   ×G   2 )]× V   1   (expression 6).
 
   The characteristics of the light quantity transmissibility η from the light emitter  60  to the light receiver  80  and the output voltage V 0  from the limiter circuit  105  shown in the above-mentioned expression 4 are shown in the characteristic graph of  FIG. 5 . In  FIG. 5 , V 3.1 , V 2.7 , V 2.5  and V 2.2  indicate the characteristic graphs when the reference voltage V 1  is 3.1V, 2.7V, 2.5V and 2.2V, respectively. Also, in  FIG. 5 , V′ 3.1 , V′ 2.7 , V′ 2.5  and V′2.2 indicate the characteristic graphs of the output voltage V 0LM  from the limiter circuit  105  when not provided with the comparing circuit  120  with respect to cases where the output voltage V 1  from the D/A converter  20  is 3.1V, 2.7V, 2.5V and 2.2V, respectively. Also, in  FIG. 5 , the upper limit voltage of the limiter circuit  105  is V H =V 0LM =4.0V, and the lower limit voltage of the limiter circuit  105  is V L =V 0LM =0.7V. 
   In  FIG. 5 , the broken line L 1  indicates the light quantity transmissibility when the sensor portion S is not light-intercepted by the sheet conveyed to the conveying path  130 , and the light quantity transmissibility at this time is η≈0.00009. In  FIG. 5 , the light quantity transmissibility indicated by broken line L 1  indicates initial light quantity transmissibility η 0  for which the sensitivity is not lowered by paper powder or the like. 
   In  FIG. 5 , the broken line L 2  indicates the light quantity transmissibility when the sheet conveyed to the conveying path  130  is thin paper and the sensor portion S is light-intercepted by this sheet, and the light quantity transmissibility at this time is η≈0.000005. 
   Also, in  FIG. 5 , the broken line L 3  indicates a threshold value when the sensor portion S judges the presence or absence of the sheet, and in the present embodiment, it is set to 2.0V which is ½ of the upper limit voltage V OLM =4.0V of the limiter circuit  105 . The threshold value is a value which can be arbitrarily set. 
   Accordingly, according to the characteristic graph shown in  FIG. 5 , the sensor portion S is lowered in sensitivity by paper powder or the like and the light quantity transmissibility is lowered, and the light quantity transmissibility when the output voltage V 0  from the limiter circuit  105  has coincided with a threshold value L 3  is defined as ηth, and the value of ηth becomes small as compared with the value of light quantity transmissibility ηth′ when output voltages V′ 3.1 , V′ 2.7 , V′ 2.5  and V′ 2.2  from the limiter circuit  105  when not provided with the comparing circuit  120  and the threshold value L 3  coincide with each other. From this, it is seen that the sheet detecting apparatus according to the present embodiment is great in the margin which can measure the presence or absence of the sheet even if the sensitivity of the sensor portion S is lowered. 
     FIG. 6  shows the deteriorated margin M=η0/ηth at which the sensor portion S can detect the presence or absence of the sheet when the threshold value is 2.0V and the output voltage V 1  from the D/A converter  20  is 3.1V, 2.7V, 2.5V and 2.2V. 
   From  FIG. 6 , it is seen that when for example, the output voltage from the D/A converter  20  is 2.7V, the deteriorated margin M is 9.0, and this shows that the sensor portion S can detect the presence or absence of the sheet even if the light quantity transmissibility q from the light emitter  60  to the light receiver  80  becomes 1/9 by dust or the like adhering, for example, to the light emitter  60 . 
   The output voltage V 1  from the D/A converter  20  can be arbitrarily set, but if the value of V 1  is too great, as is apparent from the above-mentioned expression 6, the current J 1  passing through the light emitter  60  will become too great, thus resulting in the shortening of the life of the LED which is the light emitter  60 . Accordingly, it is necessary to determine the output voltage V 1  from the D/A converter  20  with the allowable current of the light emitter  60  taken into account. 
   Also, when the sensor portion S is light-intercepted by the sheet conveyed to the conveying path  130 , the incident light quantity i 2  onto the light receiver  80  infinitely approximates to zero and therefore, η≈0, and from the above-mentioned expression 6, the current J 1  for driving the light emitter  60  assumes a maximum value J 1max  represented below by expression 7.
 
 J   1max =(1 +G   1 )× G   2   ×V   1   (expression 7)
 
   Therefore, it is necessary to determine the output voltage V 1  from the D/A converter  20  so as to satisfy the upper limit of the driving current for the light emitter  60 . 
   Accordingly, the output voltage V 1  from the D/A converter  20  is optimized and determined with the allowable current or the upper limit value of the driving current for the light emitter  60  taken into account while the CPU  200  monitors the output voltage V 0LM  from the limiter circuit  105 . 
   If as described above, the output voltage V 1  from the D/A converter  20  is set to an optimum value during the initial setting, thereafter the comparing circuit  120  works so as to maintain the light quantity transmissibility η of the sensor portion S and therefore, periodical sensor adjustment becomes unnecessary. 
     FIG. 7  is a plan view showing a state in which the leading edge of the sheet passes on the second slit  134  provided in one conveying guide  132  of the conveying path  130  shown in  FIG. 3 . In the sheet detecting apparatus according to the present embodiment, the second slit  134  is a rectangle of which one side is 2 mm. In  FIG. 7 , the front and rear sides of the second slit  134  in the sheet conveying direction are −1 mm and +1 mm, respectively, and the coordinates are determined with the center of the two sides as the zero point. 
     FIG. 8  is a characteristic graph showing the output voltage V 0LM  of the limiter circuit  105  when the leading edge of the sheet passes on the second slit  134  shown in  FIG. 7 . In  FIG. 8 , the axis of abscissas corresponds to the coordinates of the second slit  134  shown in  FIG. 7  in the sheet conveying direction. Also, the axis of ordinates corresponds to the output voltage V 0LM  from the limiter circuit  105 . That is, it shows the output characteristic of the output voltage V 0LM  from the limiter circuit  105  when the leading edge of the sheet moves on the second slit  134  from the left position of −1 mm to the position of +1 mm in  FIG. 7 . 
   In  FIG. 8 , the broken line L 4  indicates a threshold value when the sensor portion S judges the presence or absence of the sheet, and in the present embodiment, it is set to 2.0V which is ½ of the upper limit voltage V 0LM =4.0V of the limiter circuit  105 . 
   As is apparent from  FIG. 8 , the sheet detecting apparatus according to the present embodiment is provided with the comparing circuit  120  and the limiter circuit  105  and therefore, in the second slit  134 , the position at which the sensor portion S judges the presence or absence of the sheet is judged when the leading edge of the sheet has arrived at the vicinity of the downstream side (+1 mm side) of the slit  134  with respect to the conveying direction. 
   Accordingly, even if the width of the second slit  134  in a direction perpendicular to the conveying direction is made great, the sheet detecting position can be near the downstream side end of the second slit  134  with respect to the conveying direction and therefore, it becomes possible to always enhance the detection accuracy of the sheet without being affected by the disposition of the sensor portion S and the conveyed state of the sheet. Accordingly, as compared with the conventional sheet detecting apparatus in which it has been necessary to narrow the slit width in order to enhance the detection accuracy of the sheet, the slit width can be widened and the problem of the wrong detection of the sheet occurring from the slight deviation of the optical axis linking the light emitter  60  and the light receiver  80  together can also be solved. 
   As described above, in the sheet detecting apparatus according to the present embodiment, the quantity of emitted light of the light emitter  60  can be automatically corrected so that the quantity of light from the light receiver  80  may become optimum for detecting the sheet passing on the conveying path  130 , to thereby prevent wrong detection. Further, the automatic correction can be made to follow even such an unexpected cause as a sudden change in temperature or the temporary adherence of dust to thereby effect detection appropriately and prevent wrong detection. 
   This application claims priority from Japanese Patent Application No. 2003-416623 filed Dec. 15, 2003, which is hereby incorporated by reference herein.