JPH05165334A - Print density control system for magnetic developer - Google Patents

Abstract

PURPOSE:To provide the print density control system which prevents an irregu larity in density in high-printing-rate printing and high-density printing by an OD system and respective defects such as erroneous density decision by a TC system. CONSTITUTION:An electrophotographic printer uses a two-component developer consisting of toner and carriers for printing uses both an OD sensor 5 and a TC sensor 8 together. For an area wherein the output of the OD sensor 5 is less than a predetermined density value U, the OD sensor 5 is selected as a main sensor and whether or not toner needs to be replenished is determined by referring to the output of the TC sensor 8. For an area which is larger than the density value U, the TC sensor 8 is used as the main sensor and whether or not toner needs to be replenished or whether or not an excessive density warning output is generated is determined by referring to the past output of the OD sensor 5 corresponding to the high density and the low density of output of the main sensor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a print density control system for a recording apparatus using a charging device. In recent years, with the progress of computerization, there is a great demand for a recording apparatus such as an electrophotographic apparatus, and due to the improvement of recording accuracy, there is a demand for the development of an advanced control unit for printing density.

[0002]

2. Description of the Related Art A magnetic developer composed of a two-component system of toner and carrier is used to form a toner image using a charging device, and the toner image is transferred to an electrophotographic apparatus such as a laser printer. The print density control method is the development method, the photoconductor characteristics, and the restrictions from the copying system.
Various methods have been proposed in consideration of size, cost, and the like.

A typical print density control method is as follows.
A method of replenishing toner by detecting the amount of light reflected or transmitted by a toner mark for density detection formed outside the image area on the surface of the photoconductor, and a method of replenishing toner by detecting the amount of toner in the magnetic developer. Two methods of magnetic susceptibility detection are known.

FIG. 5 shows a conventional light amount detection system (Optical Dru) for reflecting or transmitting a toner mark on a photosensitive drum.
m; hereinafter abbreviated as OD method). In addition,
In order to facilitate understanding of the description of the configuration and operation, the same reference numerals are given to the same parts throughout the drawings, and the duplicated description thereof will be omitted.

In the figure, reference numeral 1 denotes a developing roller which rotates in the direction of arrow A, and the roller surface is composed of a combination of magnets (not shown). Reference numeral 2 denotes a magnetic developer composed of two components, toner and carrier, and shows a state in which the magnetic developer is uniformly attached to the surface of the developing roller 1 using a blade or the like (not shown).

Reference numeral 3 denotes a photosensitive drum which rotates in the direction of arrow B to form a latent image by a charging device (not shown). The magnetic developer 2 uniformly attached to the surface of the developing roller 1 comes closest to the photosensitive drum 3. At the position, only the toner is attracted by the charge of the latent image and moves and adheres to the photosensitive drum 3 side, and the toner image formed by the adhesion is transferred to a sheet by a transfer unit (not shown) for printing.

Reference numeral 4 denotes a toner mark (toner image for density detection) formed for each rotation of the photosensitive drum 3 at a density detection position provided outside the image area of the photosensitive drum 3 (drum edge or the like). A latent image of a reference pattern is formed on a position for density detection on the photosensitive drum 3 by an optical system (not shown), and is developed with a magnetic developer stirred in a developing device.

Reference numeral 5 denotes a photo sensor, which is a pair of light emitting elements.
5a and a light receiving element 5b, the light emitted from the light emitting element 5a illuminates the toner mark 4, and the light receiving element 5b receives the reflected light from the toner mark 4 in the direction of arrow C. Alternatively, a method of receiving transmitted light that has passed through the toner mark 4 may be used.

In the conventional method of detecting the quantity of light reflected or transmitted by the toner mark, the reflected light or the transmitted light corresponding to the irradiation point of the toner mark 4 is detected by the photo sensor 5, and the toner density is detected by the output voltage thereof. This is a system in which toner replenishment control is performed based on the detection result, and the photo sensor 5 is referred to as toner mark detection means 5 or OD sensor 5.

In the OD method, since the amount of development on the surface of the photosensitive drum 3 is directly detected, toner replenishment is performed so as to keep the output of the OD sensor 5 at a predetermined value, thereby making the image density constant. It is a method to control. The toner mark 4 is set to an intermediate density where the change in the development amount changes linearly with respect to the change in the latent image potential, and is considered so as to improve the detection accuracy.

FIG. 6 is an explanatory diagram of a conventional magnetic developer magnetic permeability detection method (Toner Carrier; hereinafter abbreviated as TC method). In the figure, 6 is a hopper for collecting the used toner and temporarily storing the magnetic developer 2 stirred with the carrier, and 7 is provided outside a non-magnetic discharge pipe connected to the bottom of the hopper 6. The obtained ferrite core is magnetically shielded against an external magnetic field. 8
Shows a coil for forming a resonance circuit which is magnetically shielded from the outside by the ferrite core 7 and wound around the outside of the discharge pipe.

A constant magnetic field is generated in the coil 8 by applying a constant current to the coil 8. In the magnetic field, the magnetic developer 2 consisting of two components, toner and carrier, flows downward in the direction of arrow D. To do. The amount of carriers that are magnetic particles decreases when the amount of toner is large, and increases when the amount of toner is small, and the inductance of the coil 8 changes according to the change in the content of the carrier.

The conventional magnetic permeability detection method for magnetic developers is
An oscillation circuit is configured by using a resonance circuit composed of a capacitor (not shown) connected to the coil 8, and a change in the content of the carrier is taken as a frequency change of the oscillation output, and the frequency change is converted into, for example, a voltage to change the toner concentration. Is detected and the toner replenishment control is performed based on the detection result, and a frequency voltage converter (not shown) including the coil 8 is referred to as the magnetic permeability detection means 8 or the TC sensor 8.

That is, this is a method of controlling the print density by replenishing the toner so that the output of the TC sensor 8 is maintained at a predetermined value, which is an advantage that the density can be indirectly measured even when not printing. There is.

By the way, as described above, the density value to be converted into the voltage is generally a low density, normal density,
It is possible to select a target density value of several steps of high density, and in the case of the OD method for expressing in a flowchart, the target density value is a three-step generic code Xd and a low density value is a code Ld.
(Voltage value: about 10 V), and normal concentration and high concentration are represented by Md (about 8 V) and Hd (about 6 V), respectively. The specific voltage value of each density value shown here is an example, and changes depending on the sensitivity and characteristics of the sensor, and is expressed below on the same premise.

Similarly, a generic code of three levels of the same target density value of the TC method corresponding to the density value of the OD method is represented by Xc, and a low density value is represented by a code Lc (voltage value: about 2.4 V), which is a normal density or a high density. Concentration Each concentration value is Mc (about 2.3 V), Hc (about 1.8 V)
Shall be represented by. In this case, there is no correlation in the voltage value between the same density values in both formulas, and in particular, in the case of the OD system, consideration is given so that high and low density values centering on the normal density value have linear characteristics. However, as a whole, the arrangement is non-linear, and the higher the concentration, the lower the detection sensitivity.

FIG. 7 shows a flow chart of a conventional print density control, (a) shows an OD system and (b) shows a TC system. In both methods, the output of the OD sensor or the TC sensor is compared with the target density value Xd or Xc, respectively, and when the result is light, toner is replenished, and when the result is dark, the toner is not executed and a predetermined time T (for example, 40 It is a simple system that repeats the same flow after waiting for an intermittent time of about a second).

The execution of the waiting time is a flow provided to intermittently and continuously supply a predetermined toner supply amount (for example, 10 grams per time) corresponding to the output of the sensor. Is configured to allow variable settings.

FIG. 8 is a diagram showing an example of a conventional print density control characteristic of the OD system, in which the horizontal axis represents time and the vertical axis represents the output voltage of the OD sensor. The output voltage of the OD sensor is 12 V when the density detection position previously provided outside the image area of the photosensitive drum 3 is in a clean state (minimum density) and 4 is the maximum density.
It is possible to set the target density value Xd for printing to three levels of low density value Ld, normal density value Md, and high density value Hd by setting it to about V (runaway state up to 0 V below this), and each level is set individually. It is shown by a three-dot chain line, a two-dot chain line and a one-dot chain line.

In the figure, the characteristic line E shows the normal density control characteristic centered on the normal density value Md, and the characteristic line F shows the high density control characteristic centered on the high density value Hd. In particular, the characteristic line E shows that toner supply is intermittently performed at the timing shown by the arrow when the output of the OD sensor becomes lighter than the centered target density value Md.

[0021]

The OD sensor is of a type in which the toner density on the drum surface is read by optical means as shown in FIG. However, there is a drawback in that the toner concentration in the developing device rises due to stirring after the toner is replenished, and it is reflected in the toner mark on the drum surface to be detected by the OD sensor in time.

Therefore, in the case of the characteristic line E as shown in FIG. 8, after the toner is replenished near the time t ₂ (the position thereof is indicated by an arrow), the toner is consumed and the density becomes “light” again at the time t ₄ . the senses when to continue the toner replenishment, there is a case where the excessive supply as shown in the time t _8. This becomes noticeable after the high printing rate printing. Especially 10%
When printing with a high printing rate as described above, toner consumption is heavy and the toner mark becomes light. The OD sensor detects this and instructs toner replenishment.

Toner replenishment is continued when "light" is detected at time t ₄ . However, the supplied toner is the example time t ₇ until the printing of up to appear as a concentration higher toner mark is agitated in a developing device is decreased becomes excessive replenishment of toner, it becomes excessive concentration as shown in the time t ₈ There are drawbacks. As described above, when the toner consumption amount is large, there is a drawback that the toner replenishment cannot be followed and the print density varies.

Further, as shown by the characteristic line F, when the high concentration control is performed, since the change amount of the output voltage of the OD sensor is small, it is detected as "light" at the time t ₁ when the high concentration value Hd is exceeded. Although it has to be provided, there is a drawback that detection cannot be performed unless the concentration is reduced to a fairly light level at time t ₂ .

Further, even if the toner is replenished when it is judged as “light” at time t ₂ , the rising time (time t) until the output voltage of the OD sensor reaches the target value (high density value Hd = about 6 V) ₂
~ T ₅ ) takes too much, and as a result, the cycle from the lightest point to the next lightest point becomes longer, the fluctuation range of the density also becomes large, density unevenness easily occurs, and it is difficult to control appropriate timing. There is a drawback.

Further, after performing the toner replenishment in the time t _2, some time period until the output voltage of the OD sensor is inclined to the dark direction (time t ₃ ~t ₄₎ in the toner replenishment in is performed, the excess There was a drawback that led to replenishment.

In the case of the TC method, the toner in the magnetic developer passing through the sensor position is partially aggregated or dispersed even though the component ratio is a normal concentration depending on the stirring state of the magnetic developer. May be erroneously determined to be an abnormal concentration. In particular, when it is mistakenly judged as'light ', the toner is replenished, so there is a drawback that it disturbs the normal balance of the toner and carrier component ratios of the magnetic developer and results in over-density printing. .. The present invention has been made in view of the above-mentioned conventional drawbacks, and is a print density control method that prevents the occurrence of density unevenness at the time of high coverage printing and high density printing in the OD method and the density error determination in the TC method. For the purpose of providing.

[0028]

In order to achieve the above object, as shown in FIG. 1, an electrophotographic printing apparatus for printing using a magnetic developer 2 composed of two components of toner and carrier, An OD sensor 5 for optically detecting the density of the toner mark 4 formed at a predetermined position on the drum, and a TC sensor 8 arranged in the developing device for detecting the density by detecting the ratio of the two components of the magnetic developer, A target density value generation circuit 11 for generating a target density value, a threshold generation circuit 12 for generating a predetermined threshold U, a main detection means selection circuit 13 for comparing the threshold U with the output of the OD sensor 5, and its The OD sensor 5 to be selected when the comparison result is lighter than the threshold U
First density value comparator 14 for comparing the output of
And a second concentration value comparator 15 for comparing the output of the TC sensor 8 selected when the comparison result is darker than the threshold value U with the target concentration value, and the output of the first concentration value comparator 14 Is lighter than the target density value, the first true / false determination means 16 for comparing the output of the TC sensor 8 with the target density value is provided, and the output of the first true / false determination means 16 is “light”.
Only when the output of the first density value comparator 14 is judged to be “true” and the toner replenishment control unit 10 is driven, and the comparison result of the second density value comparator 15 is lighter than the target density value. Is
A second true / false determination unit 17 for recording the “presence / absence” of the output variation of the output of the OD sensor 5 in the past several times is provided. Only in the case of "," the output of the second density value comparator 15 is judged to be "true", and the toner replenishment controller 10 is driven. When the comparison result of the second density value comparator 15 is higher than the target density value, the output of the TC sensor 8 is further compared with the output of the overdensity value generator 18 for generating a predetermined overdensity value Z. Third to compare
A density value comparator 19 is provided, and only when the comparison result is'dark ', the recorded content of the second authenticity determination means 17 is further referred to,
Only when the output fluctuation is “present”, the output of the third concentration value comparator 19 is judged to be “true” and the overconcentration alarm device 20 is driven.

[0029]

In the vicinity of the intermediate level between the normal density value and the high density value, the threshold value U provided for sensor switching selection is set. First, when the output of the OD sensor 5 is lighter than the threshold value U, the density control by the OD sensor 5 is performed. If the output of the OD sensor 5 is lighter than the target density value Xd, the output of the OD sensor 5 is checked for authenticity by referring to the comparison result with respect to the target density value Xc of the TC sensor 8, and the comparison result is light Only determine that it is true and supply toner.

Next, when the output of the OD sensor 5 is higher than the threshold value U, the density control by the TC sensor 8 is performed.
If the result of comparing the output of the C sensor 8 with the target concentration value Xc is light, the authenticity check is further performed by referring to the output fluctuation of the past several times of the OD method, and it is true only when there is the output fluctuation. Judge and supply toner.

Next, the output of the TC sensor 8 is set to the target density value.
When the result of comparison with Xc is dark, the output of the TC sensor 8 is further compared with the over-concentration value Z, and only when the result of comparison is dark, the authenticity is checked by referring to the output fluctuations of the past several times of the OD method. Only when there is an output fluctuation, it is judged to be true, and after outputting the overconcentration alarm, the flow of waiting time T is executed together with each of the incomplete flows shown in FIG. 2 to return to the starting point of the repeated execution. One toner supply flow is completed.

[0032]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows a block diagram of an embodiment of the present invention. In FIG. 1, 2a is a toner that is a component of the magnetic developer 2, 9a is a toner supply roller that intermittently supplies the toner 2a,
It is controlled by the toner replenishment controller 10. 9b represents a mixing roller for stirring the magnetic developer 2.

Reference numeral 11 is a target density value generating circuit, which outputs the set target density values Xd and Xc corresponding to the OD method and the TC method, respectively, as analog or digital values. Reference numeral 12 is a threshold value generation circuit for outputting a threshold value U for switching and selecting the main sensor, and in this example, it is set so as to be at a substantially intermediate level between normal density and high density.

Reference numeral 13 denotes a main detection means selection circuit, which has a threshold value U.
And an output of the OD sensor 5 are compared with each other, and a changeover switch (not shown) driven by the output of the comparator is provided. The changeover switch outputs the output of the OD sensor 5 when the output of the comparator is faint. , On the contrary, when it is dark, TC
The output of the sensor 8 is selected and the first concentration value comparator 1 is selected.
4, supplied to one of the input terminals of the second density value comparator 15. The output of the target density value generating circuit 11 is applied to each of the other input terminals of the first density value comparator 14 and the second density value comparator 15.

First density value comparator 14 and second density value comparator 15
When each output of the above outputs "light", it operates so as to refer to the outputs of the first true / false determination means 16 and the second true / false determination means 17, which will be described below, respectively. Performs a "time T standby" through a delay circuit (not shown) configured to delay by a predetermined time T to intermittently perform print density control.

Reference numeral 16 is a first authenticity judging means, which is composed of a comparator for comparing the output of the TC sensor 8 and the output of the target density value generating circuit 11, and outputs the toner only when the comparison result is light. The supply control unit 10 is driven, and the presence or absence of supply is recorded in a shift register (recording capacity is several times) (not shown) that constitutes the second authenticity determination unit 17. After driving the toner replenishment controller 10, the delay circuit executes the "time T standby".

Reference numeral 18 denotes an over-concentration value generation circuit, which outputs an over-concentration value Z for which an over-concentration alarm is required when the high-concentration value is exceeded.
Reference numeral 19 denotes a third density value comparator, which is composed of a comparator for comparing the output of the TC sensor 8 and the over-density value Z, and the second true / false judgment is made when the comparison result is “dark”. Overconcentration alarm device 20
After driving, the “time T standby” (not shown) is executed.

The comparison result of the third density value comparator 19 is "light",
When the recorded content of the second authenticity judging means 17 is "no" fluctuation, the "time T standby" (not shown) is executed. Figure 2
2 is a flow chart of the print density control of the present invention, and FIG. 3 is an explanatory view of the density control method at the time of high print ratio printing of the present invention, and FIGS. 2 and 3 will be described below with reference to FIG.

The steps of this flow chart are normal end confirmation means. In step, first OD sensor 5
Threshold value U which is set in advance (a normal concentration value Md of about 8 V and a high concentration value on the output voltage side of the OD sensor as shown in FIG. 3).
A density value of about 7V in the middle of Hd of about 6V) is compared by the main detection means selection circuit 13, and if the comparison result is lighter than the threshold value U, the process proceeds to step and the density control by the OD sensor 5 is performed.

The area lighter than the threshold value U is an area suitable for printing the low density value Ld of the target density value Xd or the normal density value Md of the OD sensor 5, and the OD sensor 5 in this area.
The above operation has a problem as described in the conventional example when the toner consumption is heavy.

The characteristic line E in FIG. 3 shows the characteristic line E in FIG. 8 and is a conventional control characteristic centered on the normal density value Md of the OD method. A characteristic line G indicated by a broken line is a TC system characteristic line corresponding to the characteristic line E at the same time. When the output of the OD sensor 5 in the above step is strong, it corresponds to the time period after the time Tc to Td and the time Th, but the time Tc to Td where the toner consumption is small is short and the time Ta to Tc before this time. The number of toners to be supplied intermittently is small and there is no problem.

However, when high-printing rate printing is executed, the amount of toner consumption increases drastically, and as a result, toner replenishment cannot catch up and the time zone from time Td to Th at which the toner becomes lighter than the target normal density value Md. become longer. Therefore, when toner is replenished by the OD method, the time period from the lightest state to the target density value Tf to Th becomes longer, and intermittent replenishment is continued during this time. Therefore, when printing with a high printing rate returns to the normal state during the time period from Tf to Th, excess toner is supplied and the overdensity value Z is exceeded as shown at time Ti.

In order to solve this drawback, in a step, the output of the OD sensor 5 and the target density value Xd are compared by the first density value comparator 14, and when the comparison result is lighter than the target density value Xd, the toner Using the function of the TC sensor 8 that can detect the change in the consumption amount earlier than the OD sensor, the first true / false determination unit 16 shifts to the step of checking the authenticity of the output of the OD sensor 5.

That is, the TC sensor 8 is detected by the first authenticity judging means 16.
Is compared with the target density value Tc (in this case, the set normal density value Mc), and if the comparison result is dark, the light detection output of the OD sensor 5 in the step is determined as false information due to the detection delay. Ignoring it, toner replenishment is not performed, and the process proceeds to the time T standby flow of step. If the comparison result is light, it is determined that the toner consumption is obviously increasing, and the toner replenishment control unit 10 is driven to replenish the toner, and then the process proceeds to the time T standby flow of the step. To do.

In FIG. 3, the output of the TC sensor 8 is compared with the target density value Tc (in this case, the set normal density value Mc), and when it is light, it is the time zone from time Te to Tf and the above-mentioned OD. Compared with the time Td to Th in the case of the method, the time period is short and the toner replenishment result is detected promptly, so that there is no chance of excessive replenishment, and it is possible to prevent uneven density.

When the comparison result of the above steps is strong,
The process proceeds to step, and the concentration control by the TC sensor 8 is performed. When the output of the TC sensor 8 that is quick in toner consumption detection is compared with the target density value Xc (in this case, the high density value Hc) by the second density value comparator 15, if the result is light, the process proceeds to step and the determination is made. The result is checked by the second authenticity judging means 17 with reference to the output of the OD sensor 5.

That is, by referring to the recorded contents of a shift register (not shown) which records the presence or absence of toner replenishment of the OD sensor 5 in the past several times, it is judged that there is variation if the recording contents are not continuous, and the toner replenishment controller 10 is determined. After driving, the process proceeds to the step of waiting for time T. This check is a flow for correcting the erroneous determination of the TC sensor. On the contrary, if there is no fluctuation and it is constant, it is determined that the toner consumption is small and it is not necessary to replenish the toner, and the process proceeds to the step of waiting for the time T.

If the output of the TC sensor 8 is dark in the step, the output of the TC sensor 8 is further compared with the overdensity value Z and the third density value comparator 19, and if the result is light, the process proceeds to step. If the result is dark, the process moves to step, and the second true / false determination means 17 refers to the output of the OD sensor 5 and checks the same as in the flow of step.

If there is a change in the record of the toner supply determination of the past several times of the OD sensor, it is determined to be true, the overconcentration alarm device 20 outputs the overconcentration alarm, and then the process shifts to the step of waiting time T. Alternatively, it is possible to execute the print suspension with this alarm.

FIG. 4 is an explanatory view of the density control method at the time of high density printing of the present invention, and the characteristic line H in the drawing shows an example of the control characteristics centering on the high density value Hc of the TC method. .. Times that are lighter than the target concentration value HC are times Tj to Tk
Although it is in the range of Tm to Tn, toner is replenished intermittently during this period as indicated by the arrow.

As described with reference to FIG. 3, the density control by the TC sensor 8 has the advantage that there is no excessive replenishment because the toner replenishment result is detected promptly, while it has the drawback of erroneous determination, as shown at time Tn. May exceed the overdose.
In order to correct this defect, when it is detected that the output of the TC sensor 8 is lighter than the target density value, the second true / false determination means.
With reference to 17, the output variation of the OD sensor 5 can be referred to, and the determination result of the output can be checked for authenticity and corrected, so that the occurrence of density unevenness can be prevented.

[0052]

As is apparent from the above description, according to the present invention, the disadvantages inherent to the OD system and the TC system can be eliminated, and a printing density control system that incorporates only the advantages can be realized. .. As a result, blurring, unevenness in density, fog and T
The malfunction of the C sensor can be prevented, and it greatly contributes to stable long-term use of the printing apparatus.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a flowchart of print density control of the present invention.

FIG. 3 is an explanatory diagram of a density control method at the time of high coverage printing according to the present invention.

FIG. 4 is an explanatory diagram of a density control method during high density printing according to the present invention.

FIG. 5 is an explanatory diagram of a conventional toner mark reflection or transmission light amount detection method (OD method).

FIG. 6 is an explanatory diagram of a magnetic permeability detection method (TC method) of a conventional magnetic developer.

FIG. 7 is a flowchart of conventional print density control.

FIG. 8 is a diagram showing an example of a print density control characteristic of a conventional OD method.

[Explanation of symbols]

 2 magnetic developer 4 toner mark 5 toner mark detecting means (OD sensor) 8 magnetic permeability detecting means (TC sensor) 10 toner replenishment control section 11 target density value generating circuit 12 threshold value generating circuit 13 main detecting means selecting circuit 14 first density Value comparator 15 Second density value comparator 16 First true / false judgment means 17 Second true / false judgment means 18 Over-density value generation circuit 19 Third density value comparator 20 Over-density alarm unit U Threshold value Z Over-density value

Claims (2)

[Claims] 1. An electrophotographic printer for printing using a magnetic developer (2) comprising two components of toner and carrier, wherein the density of a toner mark (4) formed at a predetermined position on a photosensitive drum is Toner mark detecting means (5) for optically detecting and magnetic permeability detecting means (8) arranged in the developing device for detecting density by detecting the ratio of the two components of the magnetic developer.
And a target density value generation circuit (11) for generating a target density value,
A threshold value generation circuit (12) for generating a predetermined threshold value U, a main detection means selection circuit (13) for comparing the threshold value U with the output of the toner mark detection means (5), and the comparison result is the threshold value U. A first density value comparator (14) that compares the output of the toner mark detecting means (5) that is selected when it is lighter with the target density value, and the transparency that is selected when the comparison result is darker than the threshold value U. A second density value comparator (15) for comparing the output of the magnetic susceptibility detection means (8) with the target density value, and when the output of the first density value comparator (14) is lighter than the target density value. Is provided with a first authenticity determination means (16) for comparing the output of the magnetic permeability detection means (8) and the target density value,
Only when the output of the first true / false determination means (16) is'light ', the output of the first density value comparator (14) is determined to be'true' and the toner replenishment control section (10) is driven, When the comparison result of the second density value comparator (15) is lighter than the target density value, the “presence or absence” of the output fluctuation of the output of the toner mark detecting means (5) in the past several times is recorded. An authenticity determination means (17) is provided, and the recorded content of the second authenticity determination means (17) is referred to. Only when the output fluctuation is'present ', the second
A print density control method for a magnetic developer, characterized in that the output of the density value comparator (15) is judged to be'true 'and the toner replenishment control section (10) is driven. 2. When the comparison result of the second density value comparator (15) is darker than the target density value, an overdensity value Z that predetermines the output of the magnetic permeability detecting means (8) is further generated. The third concentration value comparator (1 to compare with the output of the overconcentration value generator (18)
9) is provided, only when the comparison result is'dark ', further refer to the recorded contents of the second authenticity judging means (17), and only when the output fluctuation is'present', the third density value comparator 2. The print density control system for magnetic developer according to claim 1, wherein the output of (19) is judged to be "true" and the overdensity alarm device (20) is driven.