JP2009113926A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent lowering of accuracy of double feeding detection according to the kinds (basis weight and surface quality) of a recording sheet. <P>SOLUTION: Pulse number, amplitude and frequency of an ultrasonic signal generated from a transmission circuit 104 are changed based on characteristic information of the recording sheet. The double feeding detection is performed by comparing an output from a reception circuit 106 with a threshold value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus having a function of detecting double feeding of sheets.

  In the image forming apparatus, when double feeding (two or more recording sheets are sent out in an overlapping manner) occurs in the paper feed mechanism, it is reliably detected, and the double feeding paper is recorded. It is necessary to prevent image formation. As one of the methods, an ultrasonic method may be used.

  An ultrasonic element is a piezoelectric element that converts mechanical vibrations into electrical signals and electrical signals into mechanical vibrations. An electrical signal having the same frequency as the ultrasonic region (several hundreds of kHz) is given to the elements. To generate ultrasonic waves. The generated ultrasonic wave is applied to the recording sheet, passes through the recording sheet, and is received by the receiving circuit.

  When there is one recording sheet, an ultrasonic signal attenuated by the recording sheet is input to the receiving circuit, but when two recording sheets are overlapped, the signal attenuated much more than when one recording sheet is present. Entered. Therefore, it is possible to determine whether or not double feeding is performed by comparing a threshold value existing between the reception signal level for one sheet and the reception signal level for double feeding.

  The reason why the received signal level is greatly attenuated during double feeding is that the amount of ultrasonic waves attenuated by the air layer generated between the recording sheets is large. It has been proven to be below the level.

  Image forming apparatuses are currently required to be able to form images on various recording sheets, and at the same time, it is necessary to reliably detect double feeding when conveying various recording sheets.

  However, depending on the recording sheet, the air layer between the recording sheets may become very small during double feeding. For example, media with a surface coating has a very high media adsorption rate during multi-feeding, so the amount of ultrasonic attenuation is small and the ultrasonic reception level is likely to exceed a preset threshold. The problem of becoming.

Therefore, for each recording sheet, a threshold for determining double feeding is calculated from the reception output for one sheet and the reception output for two sheets (double feeding), and double feeding is performed based on the calculated threshold. A method for performing the determination has been proposed (see Patent Document 1).
JP 2004-352390 A

  However, in the above-described conventional example, when the recording sheet is transported in a double feed state, the state of the air layer between the recording sheets is unstable, so the change in the reception level is large and the sheet is stopped. The calculated threshold value has a disadvantage that it cannot be detected accurately.

  Also, the threshold value cannot be uniquely determined because there is a large change in the difference between the reception output for one sheet and the reception output for two sheets (double feed) due to the type of recording sheet and the sensitivity variation of the ultrasonic elements. Therefore, there is a problem that the possibility of erroneous detection is increased.

  Therefore, in order to solve the above problems, the present invention provides an image forming apparatus having means for detecting double feeding of a recording sheet conveyed by an ultrasonic method, based on the characteristic information of the recording sheet. It is an object of the present invention to provide a technique capable of reliably detecting double feed regardless of the characteristics of a recording sheet by changing the characteristics of an ultrasonic signal of a transmission means.

  In order to solve the above problems, an image forming apparatus according to the present invention includes a transmission unit that transmits an ultrasonic signal, and a reception unit that receives the ultrasonic signal transmitted from the transmission unit through a recording sheet that is conveyed. A control unit for detecting whether or not the recording sheet is double-feeded based on a reception output level of an ultrasonic signal received by the reception unit, and the control unit is transported The characteristic of the ultrasonic signal transmitted from the transmission unit is changed according to the type of the recording sheet.

  According to the present invention, by changing the characteristic of the ultrasonic signal based on the characteristic information of the recording sheet, it is possible to reliably detect the double feed regardless of the characteristic of the recording sheet.

(First embodiment)
Embodiments of an image forming apparatus according to the present invention will be described below with reference to the drawings.

  A basic configuration for detecting double feed according to the present invention will be described with reference to FIG.

  As shown in FIG. 1, ultrasonic double feed detection includes a transmission unit 101 and a reception unit 102 with a recording sheet interposed therebetween. The ultrasonic transmission unit 101 includes a transmission ultrasonic element 103 and a transmission circuit 104. The transmission circuit 104 transmits ultrasonic waves synchronized with the frequency in the ultrasonic region (about several hundred kHz). The ultrasonic receiving unit 102 includes a receiving ultrasonic element 105 and a receiving circuit 106, and outputs an electrical signal corresponding to the ultrasonic signal received by the receiving ultrasonic element 105. The ultrasonic transmission unit 101 and the ultrasonic reception unit 102 are controlled by the control unit 107.

  An example in which the control unit 107, the transmission circuit 104, the ASIC 108 that drives the reception circuit 106, and the CPU 109 that controls the ASIC and has control specifications related to the double feed detection operation incorporated in the program is shown. Specifically, during the double feed detection operation, the CPU 109 transmits an electrical signal having a predetermined frequency to the transmission circuit 104 to the ASIC 108 with the recording sheet 210 sandwiching the conveyance line. Set. Then, the transmitting ultrasonic element 103 is driven, and at this time, the ASIC 108 receives an electrical signal corresponding to the ultrasonic level received by the receiving ultrasonic element 105 from the receiving circuit 106. The ASIC 108 converts the received electrical signal into a value indicating the reception level of the ultrasonic wave, and based on the level value, the CPU 109 determines whether or not the recording sheets are double-fed.

  Specifically, as described above, whether or not double feeding is determined by comparing a threshold value existing between the reception signal level when the recording sheet is conveyed by one sheet and the reception signal level at the time of double feeding. to decide.

  FIG. 2 is a timing chart when the control unit 107 executes the double feed detection operation.

  The vertical axis represents the amplitude level (arbitrary unit) of the electric signal, and the horizontal axis represents the time axis. (1) in the figure represents an electrical signal to be transmitted to the transmission circuit 104, and has a predetermined amplitude Am [a. u. ] And a frequency fq [Hz]. (2) in the figure represents a received signal indicating an ultrasonic level received by the receiving circuit 106.

During the double feed detection operation, a predetermined number of pulses N [pls] is transmitted repeatedly with the number of cycles M [cyc] at intervals Ti [s]. Here, the pulse number N [pls], the interval Ti [s], and the cycle number M [cyc] are set in advance. Then, with respect to the transmission signal, the reception signal in the reception circuit 106 indicates that the peak P of the reception waveform shown in FIG. u. The peak P occurs with a delay of a predetermined time Tp [s] with reference to the transmission signal. It is clear from the author's experiment that the following relational expression holds for the peak generation time Tp [s].
Tp = Tc + Td × N [s] (1)
Here, Tc [s] is a signal output delay time of an ultrasonic wave / electric signal conversion circuit (integration circuit) built in the receiving element, and takes a constant value in an arbitrary control block. Td [s] indicates a signal delay time when one ultrasonic pulse is received, and with respect to the ultrasonic frequency fc,
Td∝1 / fq (2)
It becomes.

  At the time of actual double feed detection, the level of the peak Vp at the time of ultrasonic reception is smaller than the state of one recording sheet. Therefore, as an example of the determination condition of the double feed detection, in the state where the recording sheet is sandwiched, “the number of cycles when the reception level Vp of each cycle in the M cycle is lower than the preset threshold value Vth” This is a condition such that m (the number of samples satisfying Vp <Vth) is determined to be a double feed when the number of cycles k that is determined to be a preset double feed exceeds k (m> k).

  FIG. 3 is a graph showing the ultrasonic wave reception level in the basis weight (mass per unit area) of the recording sheet. The horizontal axis represents the basis weight [g / scm] of the recording sheet, and the vertical axis represents the ultrasonic reception level Vp [a. u. ] Is shown.

  Characteristic A in the figure indicates the ultrasonic wave reception level when the recording sheet is plain paper (in this embodiment, the medium is not subjected to gloss processing / coating processing on the surface) and the number of recording sheets is one. In this case, it has been experimentally clarified that the reception level of the ultrasonic signal exhibits a characteristic that is inversely proportional to the basis weight of the recording sheet. The reason for this is that the thinner the recording sheet sandwiched between the conveyance paths (the smaller the basis weight), the smaller the attenuation of ultrasonic waves.

  A characteristic B in the figure indicates an ultrasonic wave reception level when the recording sheet is double-fed under the same detection condition as the characteristic A. In this case, the ultrasonic wave reception level shows a characteristic that is inversely proportional to the basis weight of the recording sheet as in the case of the characteristic A, but the amount of change is relatively small. Therefore, the threshold value of the reception level for determining double feeding is Vth, which exceeds the reception level at the time of double feeding of the recording sheet having the smallest basis weight that can be conveyed by the image forming apparatus, and has the largest basis weight that can be conveyed. It is possible to easily set the line below the reception level for one sheet.

  Characteristic C in the figure indicates the reception level for one recording sheet when the recording sheet type is coated paper and the double feed detection operation is performed with the same ultrasonic characteristic as that of plain paper. A characteristic D indicates a reception level at the time of double feeding of coated paper.

  Looking at this characteristic, when there is one recording sheet, the plain paper and the coated paper have the same characteristics. However, the characteristics at the time of multi-feeding tend to increase the output especially on a medium having a small basis weight relative to the characteristic B of plain paper. In many cases, the threshold value Vth is exceeded. Further, the output has a very large variation.

  FIG. 4 is a graph showing the ultrasonic reception level in the basis weight (mass per unit area) of the recording sheet when the ultrasonic reception level is changed based on the type of the recording sheet being conveyed.

  In the present embodiment, when the recording paper is coated paper, if the basis weight is less than S [g / scm], the super transmission is performed so that the C characteristic shown in FIG. 3 becomes the characteristic C ′ shown in FIG. Change the characteristics of sound waves. As a result, it is possible to reduce the reception level (sensitivity characteristic) as in the case of D ', and to obtain a characteristic that is lower than the threshold value Vth for determining the double feed reliably.

Here, the characteristic C′D ′ shown in the figure can be specifically realized by the following method.
(1) Changing the number of ultrasonic pulses FIG. 5 shows a timing chart when the number of pulses of an ultrasonic signal is changed. When the recording sheet is plain paper, the number of ultrasonic pulses transmitted to the transmission circuit at the time of double feed detection is N0, and in the detection region where the sensitivity of the receiving element should be reduced, the number of pulses of the electrical signal to be transmitted is N A smaller value N1 (N0> N1) is used.

  In this case, as shown in FIG. 5, the ultrasonic wave reception level Vp for one recording sheet is Vp_N0> Vp_N1 at an arbitrary basis weight. Therefore, it is possible to reduce the reception level of the characteristic at the time of double feeding, such as D ', and it is possible to obtain a characteristic that is lower than the threshold value Vth for reliably determining double feeding.

Further, as shown in Expression (1), since the peak generation time Tp of the received waveform is also reduced (Tp_N0> Tp_N1), the reception level acquisition timing is changed.
(2) Changing the amplitude of the ultrasonic pulse FIG. 6 shows a timing chart when the pulse width of the ultrasonic signal is changed. When the recording sheet is plain paper, the amplitude of the ultrasonic pulse transmitted to the transmission circuit is set to Am0 at the time of double feed detection, and the amplitude of the electric signal to be transmitted is set to Am0 in the detection region where the sensitivity of the receiving element should be reduced. This is done with a smaller value Am1.

  In this case, as shown in FIG. 6, the reception level Vp for one recording sheet is Vp_Am0> Vp_Am1. Therefore, it is possible to reduce the reception level of the characteristic at the time of double feeding, such as D ', and it is possible to obtain a characteristic that is lower than the threshold value Vth for reliably determining double feeding.

Since the peak generation time at this time is the same in Tc, Td, and N in Equation (1), it is not necessary to change the reception level acquisition timing.
(3) Changing the frequency of the ultrasonic pulse FIG. 7 shows a timing chart when the frequency of the ultrasonic signal is changed. The ultrasonic element has resonance characteristics (frequency in the ultrasonic region [kHz] on the horizontal axis and output level [au] on the vertical axis) as shown in FIG. 8, and the ultrasonic element resonates around 220 kHz. It has a peak (this property does not limit the invention). When the recording sheet is plain paper, the detection operation is performed by resonating the ultrasonic element at the peak frequency fc0 of the resonance frequency. On the other hand, at the time of detection in a region where the reception level should be reduced, a frequency signal fc1 is transmitted by shifting the frequency of the electric signal to be transmitted from the peak frequency. In the present embodiment, a case where fc0 <fc1 will be described (this does not limit the present invention).

  In this case, as shown in FIG. 7, the reception level Vp for one piece of media is Vp_fc0> Vp_fc1. Therefore, it is possible to reduce the reception level of the characteristic at the time of double feeding, such as D ', and to obtain a characteristic that is lower than the threshold value Vth for reliably determining double feeding.

  Further, as shown in the equation (2), the signal delay time Td [s] when one ultrasonic pulse is received varies depending on the frequency, and Td_fc0> Td_fc1, so that the peak generation time Tp of the received waveform is also reduced. . Therefore, since the peak generation time Tb of the received signal changes, the reception level detection timing is also changed.

  By combining the above methods (1) to (3), it is possible to obtain desired ultrasonic reception characteristics.

  Next, the method and timing of changing the above-described ultrasonic characteristics in the image forming apparatus will be described with reference to the flowchart shown in FIG.

  The control flowchart illustrated in FIG. 9 is executed by a program stored in the control unit 107, and the control unit also controls the entire image forming apparatus.

  First, the control unit 107 performs initial settings necessary for the methods (1) to (3) described above (step 701). Each set value is determined based on the reception level of the receiving circuit when no recording sheet is present, and is determined based on the reception level of the ultrasonic signal that has passed through the recording sheet, and is stored in the control unit 107.

  Next, the control unit 107 determines the type of recording sheet to be conveyed, and determines the condition of the ultrasonic signal corresponding to the determined type of recording sheet (step 702). The type of the recording sheet is determined based on a setting by the user or a detection result by a media detection unit (not shown) installed on the upstream side in the transport direction from the double feed detection area in the image forming apparatus. The control unit 107 executes a paper transport operation and a double feed detection operation based on the determined set value (step 703).

  At this time, the recording sheet to be conveyed is discriminated mainly based on the basis weight and surface property. As an example, the basis weight detects the thickness and weight of the recording paper, and the surface property detects the optical surface reflection characteristics and the surface friction coefficient.

  As described above, by changing the characteristic of the ultrasonic signal based on the characteristic information of the recording sheet, it is possible to reliably detect the double feed regardless of the characteristic of the recording sheet.

Configuration diagram of the ultrasonic double feed detection unit Timing chart for double feed detection The figure which shows the ultrasonic reception level characteristic in the basic weight of a recording sheet The figure which shows the ultrasonic reception level characteristic in the basic weight of a recording sheet Double feed detection timing chart when the number of pulses is changed Double feed detection timing chart when pulse amplitude is changed Double feed detection timing chart when changing the pulse frequency Diagram showing frequency and output level characteristics of ultrasonic element Double feed detection control flowchart

Claims (5)

A transmission unit for transmitting ultrasonic signals;
A receiving unit for receiving the ultrasonic signal transmitted from the transmitting unit through a recording sheet conveyed; and
A control unit for detecting whether or not the recording sheet is double-feeded based on the reception level of the ultrasonic signal received by the receiving unit;
And the control unit changes the characteristics of the ultrasonic signal transmitted from the transmission unit in accordance with the type of the recording sheet being conveyed.   The image forming apparatus according to claim 1, wherein the control unit changes a characteristic of the ultrasonic signal by changing a number of pulses of the ultrasonic signal according to a type of the recording sheet.   The image forming apparatus according to claim 1, wherein the control unit changes a characteristic of the ultrasonic signal by changing an amplitude of the ultrasonic signal according to a type of a recording sheet.   The image forming apparatus according to claim 1, wherein the control unit changes a characteristic of the ultrasonic signal by changing a frequency of the ultrasonic signal according to a type of the recording sheet.   The control unit changes the characteristics of the ultrasonic signal based on the reception level of the reception unit when no recording sheet exists and the reception level of the reception unit received through the recording sheet. The image forming apparatus according to claim 1.

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