JPS5977451A - Detector of picture density - Google Patents

Abstract

PURPOSE:To detect correct picture density from a detection pattern of a small area which has large development unevenness by increasing the time constant of the smoothing circuit of a picture density detector in the early stage of detection and decreasing it in the late stage. CONSTITUTION:The detection pattern 9 on a photoreceptor 6 is irradiated with light from a light emitting element 9a and its reflected light is incident to a photodetecting element 9b. Its output is passed through a filter circuit 10 and a buffer circuit 11 and amplified by an amplifier 12. A capacitor C is connected to a side A in the early stage of measurement and charged immediately following up a detection output, so the amplified waveform of the detection output appears at the output terminal of the amplifier 12. When the output voltage of the amplifier 12 exceeds a threshold value, a signal from a comparator 13 is delayed through a delay device 14 and inputted to an analog switch 15, and the capacitor C is connected to a side B to form an LPF. Therefore, the correct density is detected from the detection pattern 9 of a small area which has large development unevenness.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image density detection device for detecting the image density of a toner image formed on an image bearing member of an electrostatic recording device.

Conventionally, in order to maintain a constant image quality, especially the density of a copied image, obtained using an electrophotographic copying machine, a part of the photoreceptor, which is an image carrier, is first exposed to a standard amount of light, and then this is developed. The toner density of the toner image is detected by an image density detection device, and the amount of toner supplied is automatically controlled based on the toner density detected by the image density detection device. In order to maintain a constant density of a copied image, it is necessary that the measured value by the image density detection device described above be accurate.

The detection pattern, which is exposed and developed on the photoreceptor with a reference light amount, is obtained by forming a latent image of a reference target placed near the document table on the photoreceptor, passing through a developing section, and developing the latent image. K, which detects the image density of this detection pattern, is detected by detecting reflected light from the toner image with a detector. Since the detection pattern is limited to a relatively small portion, in order to accurately detect this reflected light at a predetermined detection timing, a detector is provided to receive and detect the regular reflection component of the reflected light. The detector provided in this way responds sharply to the condition of the surface of the photoreceptor, provides highly accurate detection results, and can accurately detect the density of one image.

However, in reality, the detection pattern has scratches, edge effects, noise, etc. caused by the developing brush, and there is considerable development unevenness. The detection output by the detector of a detection pattern with a lot of development unevenness has many fluctuations and ripples, and it is difficult to immediately detect the correct image density from such a detection output.

In order to solve this problem, a proposal has been made in Japanese Unexamined Patent Publication No. 164353/1983. In this proposal, a smoothing circuit is installed to remove ripples, but in the above proposal, if the time constant of the smoothing circuit is made small, the effect of trying to remove ripples is small, and if the time constant is made large, the signal rises slowly. However, it is impossible to perform image density detection from a detection pattern of one small area within a short time.

An object of the present invention is to provide an image density detection device that can accurately detect image density from a small-area detection pattern with a lot of development unevenness.This purpose is to detect the density of a developed reference image. In an image density detection device comprising a detector and a processing circuit for processing an output of the detector, the processing circuit having a smoother for smoothing the output, the smoothing circuit has a small time constant at an initial stage of detection. ,
In the latter stage, this is achieved by an image density detection device for an electrostatic recording device, which is characterized by being configured with a smoothing circuit having a large time constant.

The present invention will be described in detail below using the drawings.

FIG. 1 shows the structure of an electrophotographic copying machine equipped with an image density detection device of the present invention, in which an original platen 2 for placing an original is provided on the upper part of a main body 1 of the copying machine. Original table 2
A reference target 3 and an exposure lamp 4 are provided below, and the exposure lamp 4 can expose the document and the reference target 3 to light.

The reflected light from the original or the reference target 3 passes through the optical system 5 and reaches the photoreceptor 6, which rotates in a counterclockwise direction, and is charged by the charger 7.
The surface of the photoreceptor 6 charged by this forms an electrostatic latent image. The electrostatic latent image on the photoreceptor 6 is developed by the next developing device 8 to become a toner image.

A detector 9 is provided for detecting the density of a detection pattern S, which is a toner image of the reference target 3 formed on the photoconductor 6, under the downward pressure of the developing device 8.

As shown in FIG. 2, the detector 9 is a reflective sensor consisting of a light emitting element 9a such as a light emitting diode and a light receiving element 9b such as a phototransistor, and is formed outside the effective copying surface on the photoreceptor 6. It is provided at a position corresponding to the detection pattern S, so that the detector 9 can detect the density of the detection pattern S and the density of the background portion.

The detected output is input to a filter circuit 10 consisting of a capacitor C and a resistor R, as shown in the embodiment of FIG.
Buffer 11. It is amplified through the amplifier 12, but at the beginning of the measurement, the capacitor C is connected to the A side, and the capacitor 5-C is immediately charged T'', which follows the detection output. A waveform obtained by multiplying the detection output by an amplification factor is output to the output terminal of the amplifier 12.A comparator 13 and a delay device 14 are connected to the output terminal of the amplifier 12, and the output voltage of the amplifier 12 is set to a threshold value V8H. (Fig. 8), a signal is sent from the comparator 13 to the delay device 14.The signal input to the delay device 14 is sent to the analog switch 1 after an appropriate delay time jd.
The analog switch 15 receives this signal and connects the capacitor C from the A side to the B side, thereby forming a low-pass filter circuit 10.

In other words, the filter circuit 10 has a fast response speed that faithfully responds to the detection output until td after the start of measurement (after -1 td, it operates as a low-pass filter with a sufficiently low cutoff frequency, so it can completely eliminate ripples in the detection output in a short period of time. Can be removed.

In the embodiment shown in FIG. 3, the filter circuit 10 is composed of a resistor R and a capacitor C, but as shown in FIG. - The same effect can be obtained by changing the connection of the sensor Cf from the input side to the B side at the above-mentioned timing.

FIG. 5 is a circuit diagram of another embodiment of the present invention, in which only the filter circuit 10 in FIG. 3 is changed, and at the beginning of the measurement, the analog switch 1
5 is connected to the input side. Here, since the resistor R1 is set to a small value, the cutoff frequency of the filter circuit is a sufficiently high value, and the output of the filter circuit approximately follows the detection signal. At the timing mentioned above, turn the analog switch 15 to B.
When connected to the opposite side, the cutoff frequency of the filter circuit becomes a low value, and ripples in the detection signal are eliminated.

In this case as well, as shown in the circuit diagram of FIG. 6, a filter circuit is constituted by the operational amplifier A2, and
If each resistance value is set so that Rft, Rfx/Rss = Rft/R, the analog switch 15a.

A similar effect can be obtained by simultaneously connecting and switching 15b to the B side at the above-mentioned timing.

Figure 7 shows the amplification degree (vertical axis) versus frequency (horizontal axis).
The frequency characteristics of the filter when 5 is connected to the input side and the B side are shown.

FIG. 8 conceptually shows the function of the image density detection device according to the present invention based on experimental results. The horizontal axis represents time and the vertical axis represents output voltage.

FIG. 8(a) shows a detection pattern S detected by a detector 9 that is a combination of a light emitting element 9a and a light receiving element 9b.As shown in the figure, the detection output is affected by uneven development, etc. It shows that there are many.

Figure 8(b) shows the result detected by a detector equipped with a low-pass Ayrtor circuit with a low cutoff frequency to completely eliminate ripple. The concentration is not obtained.

8(c) and 8(dl are signals detected by a detector equipped with the filter circuits of FIGS. 3, 4, 5, and 6 of the present invention, respectively, and as shown in FIG. Up to td, the signal rises at a fast response speed, and after td, a signal with completely eliminated ripples is obtained, indicating that extremely accurate toner concentration can now be detected.

The present invention described above makes it possible to correctly detect the toner concentration of the detection pattern even when there is uneven development by providing a filter that can be switched to have both a fast response speed and a sufficiently low cutoff frequency. became.

As a result, image forming conditions are controlled based on the detected values obtained here, and stable image quality of copies can be obtained.

[Brief explanation of drawings]

FIG. 1 shows the structure of an electrophotographic copying machine equipped with the toner concentration detector of the present invention, FIG. 2 is an explanatory diagram of the detector, FIG.
FIG. 4, FIG. 5, and FIG. 6 are all examples of the present invention, and are circuit diagrams of filter circuits, FIG. 7 is a frequency characteristic of the filter circuit, and FIG. 8 is an explanatory diagram of experimental results. 2... Original table 3... Reference target 9.
...Detector 1o...Filter circuit 9- 11...Buffer 12...Amplifier 13.
...Comparator 14...Delay unit 15...Analog switch agent Yoshimi Kuwahara-]〇-]Figure 14 Figure 84 Figure 84 Line 7 Line

Claims (3)

[Claims] (1) An image density detection device comprising a detector for detecting the density of a developed reference image and a processing circuit for processing the output of the detector, wherein the processing circuit has a smoother for smoothing the output. 1. An image density detection device for an electrostatic recording device, characterized in that the circuit is constituted by a smoothing circuit having a small time constant in the early stage of detection and a large time constant in the latter stage of detection. (2) The image density detection device according to claim 1, wherein the smoothing circuit comprises two circuits having different time constants. (3) The image density detection device according to claim 2, wherein a common circuit element is used in the two circuits.