Light quantity control device.

BACKGROUND OF THE INVENTION

Field of the Invention

0001The present invention relates to a light quantity control device adapted for use in a beam recording apparatus or the like.

Related Background Art

0002In the field of beam recording apparatus equipped with a light quantity control device there is already known a laser beam printer, in which a photosensitive member is scanned with a laser beam modulated according to input information to form an electrostatic latent image, and said latent image is rendered visible with magnetic developing powder, called toner, and is then transferred onto a recording material such as paper.

0003Fig. l2 illustrates an example of such conventional laser beam printer, in which a photosensitive drum l having a surfacial semiconductor layer of selenium or cadmium sulfide is rotatably supported in a housing and rotated at a constant speed in a direction A. There are also provided a semiconductor laser 2 and a control circuit 2A for the intensity and on-off control of the laser beam from said laser 2.

0004The laser beam L emitted by the semiconductor laser 2 is expanded to a predetermined diameter by a beam expander 3, and then enters a polygon mirror 4 having plural mirror faces and rotated at a fixed speed by a scanner motor 5, whereby said laser beam is deflected by said polygon mirror to perform a scanning motion in the substantially horizontal direction. Then the beam is focused, by an imaging lens 6 of f-ϑ characteristic, as a light spot on the photosensitive drum l previously charged to a predetermined polarity by a charger l3.

0005The laser beam reflected by a mirror 8 is detected by a detector 7, of which detection signal determines the timing of modulation of the semiconductor laser 2 for obtaining desired light information on the photosensitive drum l. On said drum l, an electrostatic latent image is formed by the scanning laser beam, according to the above-mentioned input information. Said latent image is rendered visible by toner deposition in a developer unit 9, and the visible image thus obtained is transferred onto a recording material, usually paper, fed from a cassette l0 or ll. The recording material then passes a fixing unit l2 for fixing said image to the material, and is discharged to an unrepresented tray.

0006The semiconductor laser usually employed in such laser beam printer has an I - ℓ characteristic, or the relation of the laser driving current and the emitted light quantity, as shown in Fig. l3. The semiconductor laser does not emit a beam until the current reaches a threshold value (I_th), but emits the laser beam when said threshold value is exceeded. In such beam emission state, the light quantity ℓ shows a certain inclination α as a function of the laser driving current, and said α is called slope efficiency.

0007In such laser beam printer, the control circuit 2A controls the light quantity of the semiconductor laser 2 and determines the laser current I_T so as to obtain a predetermined light quantity ℓ_T, and this light quantity is maintained constant by the supply of constant laser current I_T.

0008However the I - ℓ characteristic of the semiconductor laser, which has an initial form A as shown in Fig. l4, during a constant current drive with a laser current I_T for obtaining a predetermined light quantity ℓ_TA, may vary to a form B or C for example due to an increase in the chip temperature caused by the current in the semiconductor laser.

0009Though a normal operation can be achieved while the photosensitive drum is scanned with the necessary predetermined light quantity ℓ_TA, the above-explained latent image may not be formed if the I - ℓ characteristic changes to B shown in Fig. l4 due to the increase in temperature so that the light quantity of the laser decreases to ℓ_TB despite that the laser current is maintained at I_T. On the other hand the laser chip may be destructed if the light quantity increases to ℓ_TC as indicated by C. The present applicant already proposed light quantity controlling devices in the US Patents 4,20l,994 and 4,443,695 but sufficient effect could not be achieved by the processes disclosed in these patent applications.

SUMMARY OF THE INVENTION

0010An object of the present invention is to eliminate the above-mentioned drawbacks.

0011Another object of the present invention is to provide an improvement on the light quantity control device.

0012Still another object of the present invention is to provide a light quantity control device for enabling image recording of high quality.

0013Still another object of the present invention is to provide a light quantity control device capable of efficient light quantity control.

0014Still another object of the present invention is to provide a light quantity control device capable of precise and accurate light quantity control.

0015Still another object of the present invention is to provide a light quantity control device capable of light quantity control with a simple structure.

0016Still another object of the present invention is to provice a light quantity control device capable of highly reliable light quantity control.

0017Still another object of the present invention is to provide a light quantity control device capable of preventing deterioration of image.

0018Still another object of the present invention is to provide a light quantity control device capable of light quantity control with high safety.

0019Still another object of the present invention is to provide a light quantity control device with improved durability.

0020The foregoing and still other objects of the present invention will become fully apparent from the appended claims and the following description to be taken in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0021<ul id="ul0001" list-style="none"><li>Fig. l is a block diagram of a laser drive current control circuit embodying the present invention;</li><li>Fig. 2 is a chart showing the function of the circuit shown in Fig. l;</li><li>Fig. 3 is a flow chart showing the function of a laser light quantity comparator/controller shown in Fig. l;</li><li>Figs. 4A and 4B are charts showing the relation between the bias count (or light-emission count) and the bias current (or light-emission current) in an embodiment of the present invention;</li><li>Fig. 5 is a chart showing the relation between the laser light quantity and the monitor voltage in an embodiment of the present invention;</li><li>Fig. 6 is a circuit diagram showing an example of the laser light quantity setting circuit shown in Fig. l;</li><li>Figs. 7A and 7B are charts showing the function of the laser light quantity comparator/controller shown in Fig. l;</li><li>Fig. 8 is a block diagram showing other circuit of the embodiment of the present invention;</li><li>Fig. 9 is a timing chart showing the output of the circuit shown in Fig. 8;</li><li>Fig. l0 is a flow chart showing the function of a central processing unit shown in Fig. 8;</li><li>Fig. ll is a timing chart showing the output signal of the circuit shown in Fig. 8;</li><li>Fig. l2 is a perspective view showing a conventional laser recording apparatus;</li><li>Fig. l3 is a chart showing the relation between current and light quantity of a semiconductor laser;</li><li>Fig. l4 is a chart showing the relation between light quantity and current of a semiconductor laser;</li><li>Figs. l5A and l5B are charts showing the characteristic of the embodiment of the present invention in a high-speed high-precision control operation; and</li><li>Fig. l6 is a flow chart showing a high-speed high-precision control operation of the embodiment of the present invention.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0022Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings.

0023Fig. l is a block diagram of the principal part of an embodiment of the present invention.

0024In Fig. l there are shown a central processing unit (CPU) l0l for controlling the entire information recording apparatus; a laser light quantity comparator/­controller l02 composed of a one-chip microcomputer incorporating an analog-to-digital (A/D) converter for effecting light quantity control as shown in Fig. 2 according to a control sequence as shown in Fig. 3; a bias current control circuit l03 having a digital-to-analog (D/A) converter connected to outputs Bl - Bn of the light quantity comparator/controller l02; a bias current regulator l04 receiving the bias current I_B and controlled by the circuit l03; a light emission current control circuit l05 having a D/A converter connected to other outputs Dl - Dn of the light quantity comparator/controller l02; and a light emission current regulator l06 controlled by said circuit l05 and receiving the light emission current I_D through a light emission current switching circuit l07.

0025There are further provided a semiconductor laser l08; a photodiode l09 for receiving the light beam from said laser l08; a light quantity monitor circuit ll0 for receiving a detection signal from the photodiode l09 and supplying the light quantity comparator/controller with a voltage V_M corresponding to the detected light quantity; a light quantity setting circuit lll for setting the light quantity of the laser; an AND gate l2l; and an OR gate l22.

0026Now reference is made to a flow chart shown in Fig. 3 for explaining the function of the light quantity comparator/controller l02 explained above.

0027When an auto light quantity control start signal (APCST) ll3 is sent from the central processing unit (CPU) l0l (step 20l), the light quantity comparator/controller l02 clears, in a step 202, all the input signals Dl - Dn, Bl - Bn for the D/A converter of the bias current control circuit l03 and the light emission current control circuit l05 thereby sufficiently reducing the laser current I_ℓ (light emission current I_D + bias current I_B) in the semiconductor laser l08 (for example setting a laser current I_ℓ in the laser unit) through the light emission current regulator l06 and the bias current regulator l04. A next step 203 shifts the video (image) signal LON ll6 to a true level, to open the gate of the switching circuit l07, thereby supplying the semiconductor laser l08 with a light emission current.

0028As explained above, the light emission current control circuit l05 and the bias current control circuit l03 are provided with digital-to-analog converting circuits, such as the D/A converters, and count up or down the digital output signals Dl - Dn and Bl - Bn from the light quantity comparator/controller l02, thereby varying the laser current I in the laser by an analog value corresponding to said up-count or down-count, through the light emission current regualtor l06 or the bias current regulator l04.

0029In the present embodiment, the bias count X_B counted by the bias current control circuit l03 is represented by an n-bit binary number having the output Bl of the light quantity comparator/controller l02 as the least significant bit (LSB) and the output Bn as the most significant bit (MSB), and is explained as being counted up or down. Fig. 4A shows a state of counting up the bias count X_B wherein "0" indicates a low or false level, while "l" indicates a high or true level. As shown in Fig. 4B, it is assumed that the bias current I_B increases proportionally with the increase of the bias count X_B. A similar relation exists between the light emission count X_D and the light emission current I_D.

0030The light emitted by the semiconductor laser l08 is photoelectrically converted by the photodiode l09 in the laser unit ll2 and is processed by the light quantity monitor circuit ll0 to obtain a monitor voltage V_M, as shown in Fig. 5, for feedback to the light quantity comparator/controller l02.

0031After the bias count X_B and the light emission count X_D are both brought to "0" in the above-explained step 202, a bias current control is conducted in a sequence starting from a step 203.

0032In said control, as represented by a region A in Figs. 2A and 2B, the bias current count X_B is counted up stepwise while the light emission count X_D is maintained at "0" (step 204), whereby a laser current corresponding to thus increasing count X_B is supplied to the semiconductor laser l08 through the bias current regulator l04. The detected value of the light emitted by said laser current is fed back, through the light quantity monitor circuit ll0, to the light quantity comaparator/controller l02.

0033The semiconductor laser l08 emits a beam when the laser current exceeds a threshold current I_th. Then the bias current count X_B is increased to increase the laser current in the semiconductor laser l08. The up-counting of the bias current count X_B is terminated when the monitor voltage V_M reaches a bias current specifying voltage V_B0 (step 205).

0034The bias current count in this state is defined as the bias current specifying count X_B0. In a states X_B = X_B0, the laser l08 emits a laser beam with an intensity which may be enough for forming a latent image on the photosensitive drum l. Consequently the bias current actually employed is defined by a count X_BT obtained by multiplying a constant value (for example 80%) on said specifying count X_B0 (step 206).

0035The current in the laser l08, corresponding to the bias current setting count X_BT, is definded as the set bias current I_BT. The bias voltage V_M is confirmed while the set bias current I_BT is supplied to the semi­conductor laser l08, and the absolute value of the difference of the monitor voltage V_M and the bias current specifying voltage V_B0 exceeds the voltage V_B0 multiplied by a constant α (step 207), the sequence returns to the step 202, in order to reset the bias current count X_B to "0", thereby terminating the laser current, and to repeat the bias current control from the beginning.

0036When a condition |V_B0 - V_M| ≦ _B0 × α is satisfied, the sequence proceeds from the steps 207 to 208. In a sequence starting from the step 208, the light emission current is controlled while said set bias current I_BT is supplied to the semiconductor laser l08. In this state the video signal is shifted to the true level, in order to open the gate in the switch circuit l07, thereby enabling the supply of the light emission current I_D to the semiconductor laser l08.

0037As represented by the region B in Figs. 2A and 2B, the control of the light emission current is conducted by maintaining the bias current count X_B is maintained at the bias current setting count X_BT thereby supplying the semiconductor laser l08 with the set bias current I_BT. The light emission count X₀ is increased stepwise from "0" (step 208). A current I_D corresponding to thus increasing count X_D is added, by the light emission current regulator l08, to the set bias current I_BT. Consequently the laser current Iℓ is represented by the sum of the set bias current I_BT and the light emission current I_D, that is, Iℓ = I_BT + I_D.

0038The intensity of the laser beam caused by the laser current Iℓ is photoelectrically converted by the photodiode l09 incorporated in the semiconductor laser unit ll2 and is fed back, by the light quantity monitor circuit ll0, to the light quantity comparator/controller l02. In response to the up-counting of the light emission count X_D by one, the light emission current I_D increases, thereby increasing the intensity of the laser beam.

0039The increase in the light quantity is detected by the increase in the monitor voltage V_M. The light emission count X_D is counted up to increase the current in the laser l08, and the up-counting of the light emission count X_D is terminated when the monitor voltage V_M reaches the light quantity specifying voltage V_D (step 209).

0040The light emission current count in this state is defined as the light emission current setting count X_DT. The current in the semiconductor laser l08 in this state, corresponding to the light emission current setting count X_DT, is defined as the set light emission current I_DT. The monitor voltage V_M is confirmed while said set light emission current is supplied to the semiconductor laser l08. If the monitor voltage V_M is not within a predetermined range (for example ± 5%) with respect to the light quantity specifying voltage V_D (step 2l0), the light emission current count X_D is cleared (step 2ll) to terminate the light emission current I_D, and the sequence returns to the step 208 to repeat the light emission current control.

0041On the other hand, if the monitor voltage V_M is within said range with respect to the light quantity specifying voltage V_D and the APCST signal from the central processing unit l0l is in the false level, the light emission current setting count X_DT and the bias current setting count X_BT are retained but the gate of the switching circuit l07 is closed to limit the current in the laser l08 only to the bias setting current I_BT. Also in case said APCST signal is in the true level, the gate of the switching circuit l07 is maintained open, and is closed when the APCST signal is shifted to the false level.

0042The light quantity specifying voltage V_D mentioned above is determined in the following manner. At first the light quantity setting voltage V₀ is determined by the light quantity setting circuit lll. Said voltage V₀ may be obtained by a voltage division with resistors as shown in Fig. 6. Said voltage V₀ supplied to the light quantity comparator/controller l02 is corrected by the sensitivity of the photosensitive drum l, supplied by signals ll4 (CSENl, CSEN2) from the central processing unit l. The correcting value for V₀ is represented for example as +l0% or -l0% with respect to the central value V₀, and thus corrected value is defined as the light quantity specifying voltage V_D.

0043As shown by the regions A and B in the flow chart in Fig. 3, the bias count X_B and the light emission count X_D are respectively increased by one step until the monitor voltage V_M reaches the bias current specifying voltage V_B0 or the light quantity specifying voltage V_D (steps 204, 208 in Fig. 3).

0044Though such up-counting enables precise control of the light quantity, a longer time will be required if a higher precision is intended by an increase in the number of bits of the count X_B or X_D. The time available for automatic light quantity control is limited since this operation is usually conducted in the interval between sheet feedings.

0045For this reason the present embodiment employs a high-speed and high-precision light quantity control as shown in Figs. l5 and l6. This control operation will be explained in the following according to the flow chart shown in Fig. l6. In this operation the sequences of the regions A and B in Fig. 3 are replaced by those in Fig. l6. Thus, after the steps 203 and 207 shown in Fig. 3, there is executed a step 30l shown in Fig. l6 to discriminate whether the monitor voltage V_M has reached a value V_B0β8 (or V_Dβ) representing a predetermined proportion of B_B0 (or V_D), and, if V_M is lower than V_B0β (or V_Dβ), a next step 302 increases the bias count X_B (or the light emission count X_D) by 3 counts at a time.

0046Then, when the monitor voltage V_M exceeds V_B0β (or V_Dβ), a step 303 compares the voltage V_M with another proportion V_B0α (or V_Dα) of the voltage V_B0 (or V_D), and, if the monitor voltage V_M is smaller, a step 304 increases the count X_B (or X_D) by 2 counts at a time.

0047Then, when the monitor voltage V_M exceeds V_B0α (or V_Dα), a step 305 compares the monitor voltage V_M with the bias specifying voltage V_B0 (or the light emission specifying voltage V_D), and, if the monitor voltage V_M is smaller, the count X_B (or X_D) is increased by one count at a time until the monitor voltage V_M reaches V_B0 (or V_D). When V_M reaches V_B0 (or V_D), the sequence proceeds to the step 206 or 2l0 shown in Fig. 3. The above-explained sequence enables automatic light quantity control of high precision within a short time as shown in Fig. l5. In the foregoing explanation, the parenthesized portions correspond to the region B in Fig. 3.

0048Then, as shown in Figs. 7A and 7B, the laser current Iℓ is increased by the up-counting of the bias count X_B, and, if the monitor voltage V_M does not reach the bias current specifying voltage V_B0 even at a time T_B where the bias count X_B is at the maximum value, the bias current is determined as the current I_MBAX at said maximum bias count, or a predetermined proportion of said current I_BMAX.

0049The control of the light emission current is conducted under the presence of a predetermined bias current. The laser current is increased by up-counting of the light emission count X_D, and, if the monitor voltage V_M does not reach the light quantity specifying voltage V_D at a time T_D when the light emission count X_D reaches maximum, the control of the light emission current is terminated in the presence of a light emission current I_DMAX at said maximum light emission count.

0050Fig. 8 shows the circuit structure other than the laser controller 50l detailedly shown in Fig. l.

0051In Fig. 8 there are shown a scanner motor control circuit 503; a photosensitive drum rotation control circuit 504; and a photosensitive drum detecting circuit 505. A central processing unit l0l controls the scanner motor control circuit 503 and the rotation control circuit 504, controlled for example through known integrated circuits for PLL control, and the detecting circuit 505 composed of microswitches or photointerruptors. The CPU l0l also receives a print control signal (PRINT) 506 or a video signal 507 from an operation panel or a reader unit, and a test signal (MLON) 508 from an external control unit, and controls the laser control circuit 50l shown in Fig. l in response to these signals.

0052Now reference is made to a timing chart shown in Fig. 9 and a flow chart shown in Fig. l0 for explaining the control sequence of the CPU l0l.

0053In response to the entry of the PRINT signal 506 from the external control apparatus in a step 60l, a step 602 shifts a drum drive signal (DRMD) 5l3 and a scanner motor drive signal (SCNON) 509 to the true level. Then a step 603 confirms a SCNRDY signal 5l0 indicating that the scanner motor is ready, and a step 604 shifts the auto light quantity control start signal (APCST) ll3 to the true level.

0054In response, in a step 605, the light quantity comparator/controller l02 of the laser controller 50l effects the automatic light quantity control of the semiconductor laser l08. Then, when the drum drive signal (DRMD) 5l3 or the scanner motor rotation ready signal (SCNRY) 5l0 is shifted to the false state in a step 606, a next step 607 clears the bias count X_B and the light emission count X_D through the light quantity comparator/controller l02, thereby sufficiently reducing the current in the semiconductor laser l08.

0055The operation of the step 607 may be executed when the drum drive signal (DRMD) 5l3 is shifted to the false level, or within a certain period before or after such shifting to the flase level.

0056Also the APCST signal ll3 assumes the true level, in case of a continuous printing operation, between successive printing operations, i.e. in the non-image area between successive recording sheets as shown in Fig. ll, and the automatic light quantity control is completed within said non-image area. Also if the APCST signal ll3 is not generated over a predetermined period T_APC, the automatic light quantity control is conducted after said period T_APC. A TOPER signal 60l shown in Fig. ll indicates the above mentioned non-image area.

0057The present invention is not limited to the foregoing embodiments but is subject to various modifications whithin the scope and spirit of the appended claims.