JPH05212906A - Image forming device - Google Patents

Abstract

PURPOSE:To properly correct the influence of a printing ratio on a light emitting power of an LED printing head by a simple circuit. CONSTITUTION:A voltage applied to switching transistors T1-T40 is led to a resistance R through diodes D1-D40 and converted to width-variable pulses in a voltage-pulse width conversion circuit 14. A strobe signal is extended as long as the pulse is generated.

Description

Detailed Description of the Invention

[0001]

This invention relates to LED printheads,
Thermal head, liquid crystal shutter array print head,
The present invention relates to an image forming apparatus such as the above, and particularly relates to correction of output fluctuation due to a printing rate.

[0002]

2. Description of the Related Art In an image forming apparatus, an LED print head in which a large number of LEDs are arranged on a substrate to emit light according to the printing data to expose a photoconductor, and a large number of heating elements are arranged on the substrate to print data. There is a thermal head or a liquid crystal shutter array print head which is designed to generate heat according to the above. It is also known to divide an image forming element such as an LED, a heating element, or a liquid crystal shutter into a plurality of blocks and drive the blocks in time division.

The inventor has found that, when developing such an image forming apparatus, the output of the image forming element varies depending on the printing rate. For example, in the case of LED print head,
When the printing rate (the ratio of the driven elements in the image forming element) increased, the output of the LED decreased and the image density decreased.

As a related prior art, Japanese Patent Laid-Open No. 3-234675 reports the influence of the printing rate on the LED print head. In this publication, the LED is driven at a constant voltage, and the output fluctuation of the constant voltage power source depending on the printing rate is monitored. Then, it is proposed to reduce the clock frequency used for controlling the light emission time in accordance with the decrease in the power supply output to keep the exposure energy from the LED constant.

However, this publication does not disclose time-division driving or constant current driving of the image forming apparatus. Further, if the fluctuation of the power output is directly monitored, the small fluctuation of the power output due to the printing rate is monitored, and the correction accuracy cannot be obtained.

[0006]

An object of the present invention is to (1) accurately measure a change in a driving current from a constant current power source depending on a printing rate, and accurately correct an influence on an output of an image forming element. )
Detect the influence of the print rate on the block during printing so that it is not affected by the print rate of other unrelated blocks, (3) Detect the print rate suitable for constant current drive, (4 ) To realize these things with a simple circuit.

[0007]

According to the image forming apparatus of the present invention, a large number of image forming elements are arranged on a substrate, the image forming elements are divided into a plurality of blocks, and a switching transistor is connected to each block. In an image forming apparatus that drives the image forming element for each, from a constant current power supply for supplying a drive current to the image forming element, and a voltage applied to the switching transistor,
A printing rate detecting means for obtaining a printing rate of each block is provided, and the constant current power source is controlled by a signal of the printing rate detecting means to correct the fluctuation of the output energy of the image forming element due to the printing rate. It is characterized by

As the image forming apparatus, LE shown in the embodiment is used.
In addition to the D print head, a thermal head, a liquid crystal shutter array print head, an EL print head, a plasma print head, or the like may be used. The method of correcting the output energy of the image forming element based on the printing rate is arbitrary, and for example, the time (strobe time) for applying the drive current from the constant current power supply is corrected or the output current of the constant current power supply is controlled. You can do that.

[0009]

According to the present invention, the voltage applied to the switching transistor used for time-division driving, that is, the collector-emitter voltage or collector-base voltage in the case of a bipolar transistor, and the source-drain voltage in the case of an FET transistor. The printing rate is detected from the voltage between the source and the gate. The output voltage from the constant current power source in the image forming apparatus is equal to the sum of the voltage drop on the data line, the voltage applied to the image forming element such as the LED, and the voltage applied to the switching transistor. Further, since the constant current power source is used, the printing rate and the magnitude of the current are in a proportional relationship. On the other hand, the voltage applied to the switching transistor has a characteristic that it strongly depends on the current and rises rapidly with the current. Therefore, the output current of the constant current power source = printing rate can be accurately detected from the voltage applied to the switching transistor. Further, by doing so, a small change in the output current of the constant current power source can be accurately detected. Further, since the characteristics can be made uniform by forming the switching transistor as an IC, the output current can be accurately grasped.

Since the voltage of the switching transistor for block selection is measured, the printing rate can be detected for the driving block and is not affected by the printing rate in other irrelevant blocks. The print rate can be detected by measuring the voltage to the switching transistor with a resistor or the like, and the print rate can be detected with an extremely simple circuit. Further, since the combined current (total current) of the blocks being driven appears in the switching transistor, it is possible to detect the printing rate suitable for constant current driving.

[0011]

EXAMPLES FIGS. 1 to 4 show examples of LED print heads. In FIG. 1, reference numeral 2 denotes a control circuit, which is a clock signal CL from the printer body, for example, the CPU of the printer body.
K, print data DATA, and reset signal RESET are received. Further, the control circuit 2 receives the self-diagnosis command TEST of the print head from the printer main body, and if there is an abnormality, an alarm signal AL
Send the ARM back to the printer. For example, 4 is a 64-bit shift register, 6 is a 64-bit latch circuit, and A1 to A64 are 64 AND circuits. Reference numeral 8 denotes a constant current power source for supplying a light emitting current, which is composed of 64 individual constant current circuits B1 to B64. Reference numeral 10 is a data bus including, for example, 64 data lines, and 40 LED arrays L1 to L40 are connected to the data bus 10. Each LED array L1 ~ L
Each 40 is, for example, 64 LEDs integrated in one chip.

Switching transistors T1 to T40 are connected to the LED arrays L1 to L40, respectively. The switching transistors T1 to T40 include bipolar and MOS
A FET or the like can also be used. The switching transistor is also integrated into a single chip to make it smaller and have uniform characteristics. A block selection circuit 12 receives the clock signal CLK from the control circuit 2 and receives the clock signal CLK.
Is counted, and then the switching transistors T1 to T40 are driven one by one with a width of, for example, 30 μsec.
As a result, the LED arrays L1 to L40 are driven one by one in a time division manner, for example, in units of 32 μsec (of which light emission time is 30 μsec).

Diodes D1 to D40 are connected to the collector sides of the switching transistors T1 to T40, respectively, and these are combined into one and connected to a high resistance voltage detection resistor R. The voltage detection resistor R has a resistance value of, for example, about 1 MΩ to 10 MΩ so as not to affect the light emitting characteristics of the LED arrays L1 to L40. In addition, each LED array L1 to L40
Diodes D1-D40 are used to separate the two from each other.

The voltage detection resistor R is an example of a means for detecting the voltage applied to the switching transistors T1 to T40, and any resistor such as a buffer amplifier or an AD converter can be used in addition to the resistor.

Reference numeral 14 is a voltage pulse width conversion circuit, for example, a voltage applied to the resistor R is sampled in a capacitor or the like, and the capacitor is discharged when the original strobe signal from the control circuit 2 ends, and the potential of the capacitor is lowered to a predetermined value. Until then, an output pulse is generated. This pulse width changes almost linearly with the voltage applied to the resistor R. The control circuit 2 adds this pulse to the original strobe signal and extends the strobe signal only during the output pulse from the voltage pulse width conversion circuit 14. The output pulse from the voltage pulse width conversion circuit 14 is also applied to the block selection circuit 12, and the ON time of the switching transistors T1 to T40 is extended only during this pulse.

Here, the switching transistors T1 to T4
In order to precisely control 0, the output pulse from the voltage pulse width conversion circuit 14 is also applied to the block selection circuit 12. However, this procedure may be omitted. In this case, the ON time of the switching transistors T1 to T40 is set longer than the original strobe time in consideration of the output pulse from the pulse width conversion circuit, and the light emission current becomes 0 when the strobe signal disappears. Therefore, the light emission of the LED array may be stopped.

In the embodiment, each LED array L1 to L40 is time-divisionally driven as a block. However, for example, two LED arrays may be time-divisionally driven as one block, and conversely one LED array may be two. It may be divided into blocks and driven in a time division manner.

Reference numeral 16 is a window comparator which detects whether or not the voltage applied to the resistor R is within a specified range. When the voltage is out of the specified range, an abnormality detection signal is sent to the control circuit 2.
To enter.

The operation of the embodiment will be described. FIG. 2 shows the characteristics of the switching transistors T1 to T40. The collector-emitter voltage Vce of the switching transistors T1 to T40 is
It rises rapidly with the collector current iC. The collector current ic is equal to the light emitting current flowing through each LED array L1 to L40. Therefore, by measuring the collector-emitter voltage Vce, the value of the emission current, that is, the printing rate can be accurately detected.

The inventor has studied to detect the printing rate in other points. However, the current flowing through the LED arrays L1 to L40 flows into the switching transistor,
The switching transistor generates a voltage proportional to the current. However, in the case of a bipolar transistor, it does not have a perfect proportional relationship as shown in the figure. However, since the output of the constant current power source decreases when the printing rate is high, it can be sufficiently used. When it is CMOS, it is linear. Therefore, it is most accurate to detect the printing rate from the voltage applied to the switching transistors T1 to T40. Further, by doing so, it is possible to accurately detect the influence of a slight output fluctuation of the constant current power supply 8.

FIG. 3 shows the characteristics of the constant current power supply 8. The constant current power source 8 is an integrated circuit of 64 individual constant current circuits B1 to B64, and the output of each constant current circuit B1 to B64 is equal, and the output current of the constant current power source 8 as a whole, that is, the printing rate. It changes with. Each constant current circuit B1 ~
The output current of B64 changes depending on the printing rate, and as shown in the lower part of the figure, when the number of printing bits changes from 1 to 64, the output current decreases by, for example, about 7%. If the constant current power supply 8 having a sufficiently large output capacity is used, the influence of the printing rate on the output current is eliminated, but such a power supply is extremely expensive. Therefore, the printing rate is detected and the LED arrays L1 to L40
It becomes necessary to correct the exposure energy from.

FIG. 4 shows operation waveforms of the embodiment. The print signal data is serially transferred, for example, one block at a time from the printer body. The control circuit 2 latches the print data for one block when the print data for one block is input, and then generates a strobe signal. The AND circuits A1 to A64 operate in response to the strobe signal, and the constant current circuits B1 to B64 send light emission currents to the data bus 10.
On the other hand, the block selection circuit 12 calculates the LED array to be operated from the number of clock signals and operates the switching transistors T1 to T40. In this way a given LED
Light the array.

A voltage obtained by subtracting the forward voltage of the diodes D1 to D40 from the collector-emitter voltage of the switching transistors T1 to T40 is applied to the voltage detection resistor R. However, the voltage detection resistor R is 1MΩ to 10MΩ
Therefore, only a slight voltage appears in the diodes D1 to D40 and can be ignored. The voltage pulse width conversion circuit 14 converts this into a pulse width corresponding to the voltage applied to the resistor R, and is triggered when the original strobe signal ends to input this pulse to the control circuit 2. In the control circuit 2, this pulse is added to the original strobe signal, and AND circuits A1 to A6 are added.
Input to 4 and extend the strobe signal according to the voltage applied to the voltage detection resistor R. The extension time of the strobe signal is determined by the print rate, and the strobe time is extended so as to compensate for the decrease in the light emission current in the high print rate area. As a result, the variation of the exposure energy due to the printing rate is prevented.

The circuit of the embodiment can also be used for detecting abnormality of the LED print head. LED array L1 ~
The LEDs of L40 are made to emit light one by one. If there is no abnormality in the print head, the voltage detection resistor R has an ON voltage of the switching transistors T1 to T40, for example, 0.5 to 0.6.
V to the forward voltage voltage of the diodes D1 to D40, for example, 0.
Only the voltage of 01-0.05V appears. On the other hand, for example, LED array destruction or constant current circuits B1 to B6
If there is a breakdown of 4, a disconnection of the data bus 10 or the like, no voltage appears in the voltage detection resistor R. If the switching transistors T1 to T40 are destroyed, an abnormally high voltage is applied to the voltage detection resistor R. Therefore, only when there is no abnormality in the LED print head, the voltage detection resistor
A voltage within the specified range is applied to R.

Next, the window comparator 16 detects whether or not the voltage applied to the voltage detection resistor R is within a predetermined range, and when it is out of this range, inputs an abnormality detection signal to the control circuit 2. .. In this way, if there is an abnormality in even one LED, it can be detected. Further, it is possible to detect an abnormality in a portion other than the LED arrays L1 to L40.

[0026]

[Modification] In the embodiments of FIGS. 1 to 4, the influence of the printing rate on the light emission current is corrected by extending the strobe time. However, such a correction method itself is arbitrary, and the correction may be performed, for example, by feeding back the influence of the printing rate to the emission current itself. Such modified examples are shown in FIGS.

In FIG. 5, 18 is a comparator, A
It may be a D converter or the like. 20 is a multiplexer, R0
~ R4 is a current determining resistor. The light emission currents from the constant current circuits B1 to B64 are collectively determined by the values of the current determination resistors R0 to R4 selected by the multiplexer 20, and the light emission current is controlled by switching the current determination resistors R0 to R4 according to the printing rate. To do. The circuit configuration of the modified example is the same as that of the embodiment of FIG. 1 in other points.

FIG. 6 shows the operation of the modified example. The control circuit 2 applies a strobe signal having a constant width to the AND circuits A1 to A64. The voltage to the voltage detection resistor R accompanying this is decomposed into, for example, five levels by the comparator 18, and the multiplexer 20 is controlled. For example, the current determination resistors are set to five levels from R0 to R4 in the latter half period of the strobe signal. To change the output current from the constant current circuits B1 to B64. Then, in the second half of the strobe signal, the constant current circuits B1 to B6
By controlling the output current from 4 with the multiplexer 20, the influence of the printing rate on the exposure energy of the LED is corrected.

Although the embodiment has been described with respect to the LED print head, the present invention can be similarly applied to other print heads such as a thermal head. Further, the structure itself of the LED print head is arbitrary and can be freely changed within the range of known technology. For example, the time-division driving method itself is arbitrary. What is important here is to accurately detect the printing rate from the voltage applied to the switching transistors T1 to T40, and thereby correct the output energy of the image forming element. Although the collector-emitter voltage of the switching transistors T1 to T40 is used here, the collector-base voltage or the source-drain voltage of the FET transistor may be used instead.

[0030]

According to the present invention, it is possible to accurately detect the slight influence of the printing rate on the constant current power source and prevent the deterioration of the image quality due to the fluctuation of the driving current. Further, since the voltage applied to the switching transistor is measured, the printing rate can be detected with a simple circuit, and the printing rate can be accurately detected only for the driven block.

[Brief description of drawings]

FIG. 1 is a block diagram of an image forming apparatus according to an embodiment.

FIG. 2 is an operating characteristic diagram of the switching transistor used in the example.

FIG. 3 is an operating characteristic diagram of the constant current power supply used in the examples.

FIG. 4 is an operation waveform diagram of the print head according to the embodiment.

FIG. 5 is a block diagram of a main part of an image forming apparatus according to a modified example.

FIG. 6 is an operation waveform diagram of a modified example of FIG.

[Explanation of symbols]

 2 control circuit 4 shift register 6 latch circuit 8 constant current power supply 10 data bus 12 block selection circuit 14 voltage pulse width conversion circuit 16 window comparator 18 comparator 20 multiplexer A1 to A64 AND circuit B1 to B64 constant current circuit D1 to D40 diode L1 to L40 LED array R Print rate detection resistance R0 to R4 Current determination resistance T1 to T40 Switching transistor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location B41J 2/35 29/46 D 8804-2C H04N 1/032 D 9070-5C 1/036 A 9070- 5C 1/23 102 A 9186-5C 103 A 9186-5C

Claims (1)

[Claims] 1. A plurality of image forming elements are arranged on a substrate, the image forming element is divided into a plurality of blocks, and a switching transistor is connected to each block, and the image forming element is driven for each block. In the image forming apparatus configured to do so, a constant current power supply for supplying a drive current to the image forming element, and a printing rate detection unit for obtaining a printing rate of each block from the voltage applied to the switching transistor are provided. An image forming apparatus, wherein the image forming apparatus is provided, and the constant current power source is controlled by a signal from a printing rate detecting unit to correct a change in output energy of the image forming element due to the printing rate.