JP2575787B2 - Document density detection circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a document density detecting circuit used in a copying machine, a document reading device, and the like.

(B) Conventional technology A document density detecting circuit used in a copying machine or the like receives reflected light or transmitted light from a document illuminated by a lamp by a document density detecting sensor, and receives the signal, for example, of a lamp driving circuit. The light amount of the lamp is controlled in accordance with the document density by feeding back to the phase control unit. In this case, since a considerable amount of light is required to irradiate the original, a commercial power supply or the like is used as a driving power supply for the lamp. However, if the driving power of the lamp has a frequency of, for example, 60 Hz, the output of the document density detection sensor includes not only the density signal of the document but also the flicker frequency component of the lamp, that is, the frequency component of 120 Hz. This frequency component becomes noise with respect to the original density signal, and if the feedback is directly applied to the lamp drive circuit, accurate light quantity control cannot be performed.
This causes problems such as the lamp light intensity becoming unstable.

Therefore, conventionally, in order to improve the above drawback, an interrupt is issued to the CPU at a point synchronized with the phase of the lamp driving power supply, for example, a zero cross point, and the output signal of the document density detection sensor is read at the timing of the interrupt. I was

(C) Problems to be Solved by the Invention However, the method of reading the sensor output in synchronization with a specific point of the power supply voltage considerably complicates the control system,
In some cases, it is difficult to maintain a stable synchronization state due to circuit drift or the like. In this case, there is a problem that flickering of the light source cannot be completely suppressed. Further, flickering may occur depending on the mounting position of the sensor.

SUMMARY OF THE INVENTION An object of the present invention is to provide a document density detecting circuit which can eliminate the above-mentioned problem by using a filter.

(D) Means for Solving the Problems The present invention provides a detection signal processing circuit of a document density detection sensor for a specific document having a white / black pitch of 5 mm or more corresponding to a scanning speed of about 200 mm / sec of an optical scanner. A low-pass filter having a cut-off frequency higher than the change frequency of the document density and lower than the flicker frequency of the document illumination lamp is provided.

(E) Operation The original density detecting circuit according to the present invention does not prevent the output signal itself of the original density detecting circuit from including noise as in the conventional art, but instead reduces the noise included in the sensor output signal. Remove by filter. As a method of detecting the document density, there is generally a method based on a sensor output in a specific range of the document, or a method based on an average value of sensor outputs of the entire document. In any case, since the sensor output signal contains the flicker frequency component of the lamp as noise, this component is easily removed by passing through the filter.

(F) Embodiment FIG. 1 shows a circuit diagram of an embodiment of the present invention.

Light reflected from the document 2 enters the optical sensor 1 serving as a document density detection sensor. In the method of detecting the density based on a detection signal (sensor output) in a specific range of the document 2, the optical sensor 1 receives light based on a detection signal captured by the CPU within a specific fixed time, that is, within the specific time. The document density is detected based on the amount of light thus obtained. As shown in FIG. 2, the optical sensor 1 is disposed below the reflector 3 of the light source,
Document reflected light is received from a slit provided in the reflector 3. Therefore, the amount of light received by the optical sensor 1 within a specific fixed time is the reflected light from the length l of the document 2.

In the method of preliminarily scanning the entire document 2 and averaging the detection signals (sensor outputs) to detect the document density, the light source and the optical sensor 1 move in the range from the leading edge to the trailing edge of the document. The time CPU captures the sensor output and detects the document density by averaging the sensor output.

In FIG. 1, reference numeral 4 denotes an amplifier for amplifying the output signal of the optical sensor 1. The output signal is digitized by an A / D converter 6 through a low-pass filter 5. The CPU outside the figure converts the digitized density data into the internal data at a predetermined timing.
Load to RAM.

The low-pass filter 5 has a sufficient frequency characteristic to remove a flicker frequency component of the lamp driving the light source 7. In the embodiment, the power supply connected to the lamp drive power supply circuit 8 is a 60 Hz commercial power supply 9. Also, lamp 7
, A halogen lamp or a discharge lamp is used. In such a configuration, the flicker frequency of the lamp 7 is 120 Hz. The low-pass filter 5 effectively removes the noise that is the frequency component of 120 Hz.

Next, the operation of the low-pass filter 5 will be described with reference to FIGS.

When Detecting Document Density Based on Specific Range Sensor Output Signal In this method, the CPU takes in the density data output from A / D converter 6 for a fixed time. Now, assuming that the range of l of the original 2 is a black original on the left half and a white original on the right half as shown in FIG. 3A, the length of l is 10 mm, and the scanning speed of the optical scanner is 200 mm /. In the case of seconds, the sensor output signal is 10/200 = 50 msec seconds, that is, a signal of 20 Hz. In FIG. 3, reference numeral 10 denotes a sensor detection signal on which the flicker frequency component of the lamp is superimposed, and reference numeral 11 denotes a signal component when the flicker frequency component of the lamp is removed. As shown in the figure, even if the density change in the range of l of the document 2 is not an impulse from black to white but rather complicated, the sensor detection signal is about 20 Hz.

As described above, in the method of detecting the document density based on the sensor output signal in a certain range of the document, the frequency component of the sensor output signal is at most about 20 Hz, which is one digit smaller than the flicker frequency component of the lamp, 120 Hz. Therefore, the cut frequency of the low-pass filter 5 is at least 100 Hz.
Set to about. By doing so, the noise of the flicker frequency component of the lamp is removed from the sensor output signal, and the A / D converter 6 supplies the detection signal 1 corresponding to the document density to the A / D converter 6.
Only 1 is output. When the length l and the speed of the optical scanner are set to the above conditions, the noise removal can be further improved by setting the cut frequency of the low-pass filter 5 to 30 to 40 Hz.

Method of Scanning the Entire Document by Prescanning and Averaging Sensor Output Signals in That Range and Detecting Document Density FIG. 4 shows sensor output signals in this case. The CPU reads all sensor output signals in the document area and averages the signals to detect the density of the document. In this method, the flicker frequency component of the lamp is also averaged at the time of averaging, so that it is still impossible to detect an accurate original density as it is. Therefore, even in this case, the noise component can be easily removed by providing the low-pass filter 5 in the detection signal processing circuit of the sensor.

(G) Advantageous Effects of the Invention As described above, according to the present invention, the flicker frequency component of the lamp can be removed with a simple circuit configuration only by providing a low-pass filter. There is no need to provide a complicated circuit, the burden on the CPU is reduced, and there is no need to consider circuit drift such as a temperature change.

[Brief description of the drawings]

FIG. 1 shows a block diagram of an embodiment of the present invention, and FIG. 2 shows an example of an arrangement position of an optical sensor (document density detection sensor). FIGS. 3 and 4 are diagrams showing output signal waveforms of the optical sensor (original density detecting sensor). 1-optical sensor (document density detection sensor), 2-document, 5-low-pass filter, 7-document illumination lamp, 8-lamp drive power supply circuit, 20-detection signal processing circuit.

Continued on the front page (72) Inventor Kiyoshi Inamoto 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Kazuyuki Onishi 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation (72 Inventor Kazunori Aida 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Tokiyuki Okano 22-22-22, Nagaike-cho, Abeno-ku, Osaka city, Osaka Sharp Corporation (56) References JP Showa 61-110002 (JP, A) Actually open Showa 62-143945 (JP, U)

Claims (1)

(57) [Claims] A detection signal processing circuit of a document density detection sensor, wherein a white / black pitch corresponding to a scanning speed of about 200 mm / sec of an optical scanner is higher than a change frequency of a document density of a specific document having a pitch of 5 mm or more; An original density detecting circuit, comprising: a low-pass filter having a cutoff frequency lower than a flicker frequency of an original illumination lamp.