Laser scanner control circuit which is used in image forming apparatus

Abstract: A laser scanner control circuit which is used for an apparatus for scanning onto a photo sensitive material by a laser beam and forming an image thereon is constructed as a monolithic driver IC onto a single chip. A reset circuit is provided for a sample-hold circuit so as to minimize the laser power when the image forming apparatus is in a standby mode. A circuit to detect an abnormality of the laser power is further provided. Due to this, the safety of the laser power control of the sample-hold circuit is improved. The operation of the sample-hold circuit can be started at a time point when the laser emitting operation is stable.

Description: BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a laser scanner control circuit which is used in an apparatus for scanning on a photo sensitive material by a laser beam and forming an image onto the photo sensitive material and which comprises a laser drive circuit and a scanner drive circuit. Related Background Art DE 36 18 031 A1 shows a laser scanner control circuit of a laser recording apparatus comprising an applying means for applying a current to a laser device, light power detecting means for detecting a light emitting power of said laser device, and a rotational speed control circuit for controlling a rotational speed of a motor to deflect an output laser beam from said laser device on the basis of a clock signal. In addition to this, a sample-hold circuit and a comparison control circuit are used. Finally, an auxiliary system or switch unit provides the laser energy control unit with a dot diameter control signal and a scanning speed control unit with a FS-PULSE as well as a scanning speed control signal. In a semiconductor integrated circuit for the control portion of a laser beam printer according to GB 2 229 833 A, either a signal at a clock signal input terminal or a signal obtained by dividing the frequency of that signal by a frequency dividing circuit is selected by a selecting circuit responsive to a selection signal supplied to a terminal. Consequently, the frequency-dividing circuit can be bypassed by the selecting circuit. Accordingly, it is possible to cope easily with a case where operations at a frequency exceeding the maximum operating frequency of component devices forming the semiconductor integrated circuit are required, by connecting an external frequency divider capable of performing higher speed operations to the clock signal input terminal. US 4,990,763 describes a light beam recorder comprising a light source for generating a light beam, means for modulating said light source with recording information, means including an oscillating mirror for scanning said modulated light beam in a sequence of scans over a recording medium, means for directly detecting the speed of said mirror during a scan of said light beam and control means responsive to said means for detecting for establishing for each scan the timing of irradiating the recording medium with said light beam and shortening the duration of irradiation with the light beam in accordance with said recording information as the scanning speed is increased, and for intensifying the light beam as the scanning speed increases. In an image forming apparatus such as a laser printer or the like for scanning on a photo sensitive material by a laser beam and forming an image onto the photo sensitive material, a laser drive circuit to drive a laser and a polygon motor drive circuit are formed on different chips. Further, a laser power control circuit which is provided for the laser drive circuit uses an A/D converter, a D/A converter, and the like and controls a laser power by a logic control by a CPU or the like. In a laser printer in which a print density can be switched, there is a case where a rotational speed of a polygon motor (scanner motor) must be changed and a laser beam power must be also changed in accordance with a set print density. Therefore, a laser scanner control circuit which is used in the laser printer or the like uses a construction such that a rotational speed control signal is received and the rotational speed of the scanner motor is changed and that a laser beam power control signal is received and the laser beam power is changed, thereby adjusting the scanning speed of the laser and the laser beam power in accordance with the print density. However, the rotational speed control signal and the laser beam power control signal must be independently given, so that there occurs a problem such that the number of input lines increases and an input circuit becomes complicated. In addition, it is necessary to respectively supply the optimum signals according to the print density to a control circuit to generate the above two signals. It is difficult to optimize such signals unless a memory in which characteristics of a printer engine have been stored or the like is provided in the above control circuit. Since a logic section and an analog section exist in the laser drive circuit, when the polygon motor drive circuit comprising a bipolar device and the laser drive circuit are formed on the same chip, it is necessary to construct the logic section by an IIL or to construct the logic section by a CMOS device and to construct the analog section by a bipolar device by using a BiCMOS process (process to form a circuit in which a bipolar device and a CMOS device mixedly exist). When an IIL is used, therefore, there occurs a problem such that the laser power cannot be controlled at a high speed due to a frequency response of the IIL. When the BiCMOS process is used, there occur problems such that the manufacturing costs are high and the IC becomes expensive. When the laser drive circuit and the polygon motor drive circuit are formed on the same chip, a loss of each driver occurs on the same chip, so that there also occurs a problem such that a temperature of the chip rises. SUMMARY OF THE INVENTION It is an object of the present invention to solve the foregoing technical problems. Another object of the invention is to provide an image forming apparatus which can change a print density and whose construction can be simplified without deteriorating a print quality. By controlling a laser beam power on the basis of rotational speed control information of a scanner motor, the optimum laser beam power according to a print density can be always set, the number of control signals which are sent from a printer engine controller is also reduced, and a recording apparatus can be simplified. Still another object of the invention is to realize the low costs of an image forming apparatus. Since a laser power control circuit of a laser drive circuit is constructed by a sample and hold circuit, the laser drive circuit can be constructed by only bipolar devices. Consequently, the laser drive circuit can be formed on the same chip as that of a polygon motor drive circuit comprising a bipolar device and the low costs can be realized. Further another object of the invention is to solve various inconveniences occurring when the laser drive circuit and the polygon motor circuit are formed on the same chip. According to the invention, a laser scanner control circuit according to claim 1 is proposed. By installing a monolithic driver IC of the invention onto a metal circuit board to construct a polygon motor, an increase in temperature of the chip can be suppressed by reducing a thermal resistance of a junction portion of the IC and its peripheral portion. The above and other objects, features and advantages of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the first embodiment of the invention; Fig. 2 is a schematic diagram showing the first embodiment of the invention; Fig. 3 is a timing chart showing the operation of the first embodiment of the invention; Fig. 4 is a block diagram showing the second embodiment of the invention. Figs. 1 and 2 show the first embodiment of the invention. Fig. 1 is a block diagram of a laser scanner control section of a laser recording apparatus. Fig. 2 is a schematic diagram showing a laser scanner unit of an electrostatic photography recording apparatus for exposing a laser beam onto a photo sensitive material and forming an image thereon. In Fig. 1, reference numeral 1 denotes a semiconductor laser unit having a laser diode LD and a photodiode PD to detect a light amount of the laser diode. Reference numeral 2 denotes a laser scanner control section to control the driving of the laser diode and the polygon scanner motor; 3 a scanner motor for rotating a polygon mirror and scanning a laser beam; 4 a switch circuit to turn on or off a current flowing in the laser diode LD on the basis of a control signal a; 5 a constant current circuit to make constant the current flowing when the laser diode LD is turned on; 6 a current detection circuit to detect a current flowing in the photodiode PD; 7 a current/voltage converter to convert the current detected by the current detection circuit 6 into the voltage by a predetermined magnification based on a control signal b; 8 a sample-hold circuit to sample or hold the output voltage of the current/voltage converter 7 on the basis of an external control signal c; and 9 a comparison control circuit for comparing an output voltage of the sample-hold circuit 8 and an output voltage of a reference voltage generator 10 and for controlling a constant current value which is set by the constant current circuit 5. Reference numeral 11 denotes a rotational speed control circuit to control a rotational speed of the scanner motor 3 on the basis of an external control clock signal d; 12 a drive circuit to supply a current to the scanner motor 3 on the basis of an output signal of the rotational speed control circuit 11; and 13 a rotational speed detection circuit to detect the rotational speed of the scanner motor 3. The detection circuit 13 feeds back the rotational speed information to the control circuit 11, thereby keeping the rotational speed of the scanner motor 3 constant. Reference numeral 14 denotes a frequency/voltage converter to convert a clock frequency of the external control clock signal d into a voltage. In Fig. 2, the same component elements as those shown in Fig. 1 are designated by the same reference numerals and their descriptions are omitted here. In Fig. 2, reference numeral 15 denotes a collimating lens to convert a laser beam from the semiconductor laser unit 1 into a parallel beam; 16 a cylindrical lens to converge the parallel beam in only one direction; and 17 a polygon mirror to scan the laser beam. The polygon mirror is rotated by the scanner motor 3. Reference numeral 18 denotes a toric lens and 19 an f- lens. The laser beam is transmitted through the lenses 18 and 19, so that a corresponding image is formed onto a photo sensitive drum 20. The control signals a, b, c, and d are connected to a printer engine control section (not shown). An adjusting operation of the laser scanner control circuit 2 will now be described. A laser beam power on the drum 20 is first adjusted to a specified value. The control circuit 2 receives the control clock signal d according to the print density and decides a reference output voltage of the reference voltage generator 10 by the output voltage of the frequency/voltage converter 14 and supplies to the comparison control circuit 9. The sample-hold circuit 8 is set into the sampling mode by a control signal c. The switch circuit 4 is turned on by the control signal a. The output voltage of the sample-hold circuit 8 is compared with the output voltage of the reference voltage generator and a control signal b (analog voltage value) of the current/voltage converter 7 is controlled so as to always equalize those output voltages, thereby controlling the current value of the constant current circuit 5. A value of the control signal b is fixed at a time point when the laser beam power on the photo sensitive drum surface is equal to a specified laser power. Ordinarily, the control signal b is formed by using a semifixed volume or the like. In general, there is a proportional relation between a laser output of the laser diode LD and a photo current of the photodiode PD. By detecting the photo current of the photodiode PD, the laser beam output of the laser diode can be monitored. When the switch circuit 4 is turned on by the above adjustment, the light emitting amount of the laser diode is always stabilized to a specified light amount. The operation of the laser scanner control circuit 2 during the printing operation will now be described with reference to Fig. 3. When the printer engine control section receives a print signal, it generates the clock signal d according to the print density. The switch circuit 4 is turned on by the signal a. The sample-hold circuit 8 is set into the sampling mode by the signal c. The current value of the constant current circuit 5 gradually increases and the laser beam power reaches a specified value. After the laser beam power reached the specified value and became sufficiently stable, the input voltage of the sample-hold circuit 8 is held by the control signal c and supplied to the comparison control circuit 9. The current value of the constant current circuit 5 is set by the comparison control circuit 9 so that the laser diode LD emits the laser beam of a specified laser power even when the switch circuit 4 is turned off by the signal a after that. The above operation is called an automatic power control (APC). After that, the switch circuit 4 is turned on or off by the signal a on the basis of the print information. Generally, when an image is formed onto the photo sensitive drum by raster scanning, the laser power is monitored by the photodiode PD so as to keep the laser power to the specified value every scan or every page. The voltage conversion value is sampled by the sample-hold circuit 8, thereby adjusting the laser current. A state of the turn-off of the laser diode can be mentioned as an inhibiting condition of the sampling operation of the sample-hold circuit 8. That is, when the sample-hold circuit starts the sampling operation in the off state of the laser diode, the photodiode PD doesn't detect the laser beam, so that the output of the sample-hold circuit 8 doesn't rise and the comparison control circuit 9 increases the current value of the constant current circuit 5 to the maximum current value. When the switch circuit 4 is turned on after that, an excessive current flows into the laser diode LD, so that there is a possibility of a breakdown. Therefore, there is used a construction such that the sample-hold circuit 8 can execute the sampling operation only when the switch circuit 4 is ON. Namely, the sample-hold circuit is set into the sampling mode only when both of the signals a and c are ON. The case where the print density is changed, for instance, the case where the print density rises will now be considered. It is assumed that the print density in the above example is set to 300 d.p.i. (dots per inch). When the print density rises to 600 d.p.i., the scanning speed is doubled and the clock frequency for print data is increased by four times. Since the scanning speed is doubled, the rotational speed of the scanner motor to scan is doubled and the frequency of the control clock signal d is also doubled. The output voltage of the frequency/voltage converter 14 also changes. The set value of the reference voltage generator changes due to the voltage value. The reference voltage value which is supplied to the comparison control circuit 9 changes. Assuming that the set value of the control signal b is fixed to the above value (300 d.p.i.), when the operation similar to the above APC operation is executed, the set value different from the laser emitting power in the case of the above print density of 300 d.p.i. is held. To efficiently improve the resolution when the print density is raised, it is generally better to set the laser emitting power to a value which is lower than that in the case of the print density of 300 d.p.i.. When the print density rises from 300 d.p.i. to 600 d.p.i., a ratio at which the laser power is reduced from the power at 300 d.p.i. to the power at 600 d.p.i. is unconditionally determined according to a state of the electrostatic photographing process. According to the experimental values, a high efficient resolution was obtained in the case of the laser power at 600 d.p.i. which is lower than the laser power at 300 d.p.i. by about 25%. For convenience of the process of print data, the print density is more generally set to a discrete value such as 240 d.p.i., 300 d.p.i., 400 d.p.i., 600 d.p.i., or the like. In the above control construction, the laser emitting power can be also continuously controlled for the control clock frequency of the scanner rotational speed. However, the values of the laser powers corresponding to the discrete values of the print densities can be also previously set as various reference voltage values in the reference voltage generator 10. It is also possible to perform an inhibiting procedure to inhibit that the laser emits the laser beam when the scanner motor is not rotating or the rotational speed of the scanner motor doesn't reach a specified rotational speed. Fig. 4 shows the second embodiment of the invention. In Fig. 4, the same component elements and signals as those shown in Fig. 1 are designated by the same reference numerals and their descriptions are omitted here. The second embodiment differs from the first embodiment with respect to a point that the output of the frequency/voltage converter 14 to convert the frequency of the control clock signal d into the voltage is supplied to the current/voltage converter 7. The reference voltage generator 10 generates the reference voltage. In a manner similar to the first embodiment, the laser emitting powe'r is adjusted to the value according to the print density of 300 d.p.i.. The rotational speed information according to the rotational speed of the scanner motor is supplied in addition to the current/voltage converter 7 through the frequency/voltage converter 14. A conversion ratio of the converter 7 is set in accordance with the value of such rotational speed information. The value of the control signal b is set by also including the rotational speed information. When the print density is changed to 600 d.p.i., the output voltage value of the frequency/voltage converter also changes and the conversion ratio of the converter 7 changes. The APC is performed in a state in which the photo current value of the photodiode PD changes, thereby setting the laser emitting power to a value different from the laser power at the print density of 300 d.p.i.. Although an effect similar to that in the first embodiment is derived, since the voltage applied to the current/voltage converter 7 is adjusted by also including the rotational speed information of the scanner motor, the rotational speed of the scanner motor changes. That is, when the pr-int density changes, the value of the signal b also changes. By reading the information (voltage value) of the signal b, the laser power can be monitored. When the laser recording apparatus is manufactured, the laser power can be easily checked.

Claims: A laser scanner control circuit of a laser recording apparatus comprising applying means (4, 5) for applying a current to a laser device (LD), light power detecting means (PD) for detecting a light emitting power of said laser device, a rotational speed control circuit (11) for controlling a rotational speed of a motor (3) to deflect an output laser beam from said laser device on the basis of a clock signal input to the control circuit, a sample-hold circuit (8) and a comparison control (9) circuit, characterized in that said sample-hold circuit (8) samples or holds an output of said light power detecting means in response to a control signal (c) from outside, that said comparison control circuit compares an output voltage of said sample-hold circuit and a reference voltage and for controlling a current which is applied to the laser device by said applying means, that means (14) for detecting a frequency of said clock signal and means (10) for changing said reference voltage of said comparison control circuit on the basis of the detected frequency of said clock signal are provided, and that a current flowing in said laser device is changed on the basis of said reference voltage. A laser scanner control circuit according to claim 1, wherein said light power detecting means has photoelectric converting means for converting a light emitting power of the laser device into a current and a current/voltage converter to convert the current which has been converted by said photoelectric converting means into the voltage by an arbitrary gain, and wherein a conversion magnification of said current/voltage converter is controlled in correspondence to said clock signal, thereby changing the current flowing in said laser device. A laser scanner control circuit according to claim 1, wherein said laser scanner control circuit is constructed by a monolithic IC. A laser scanner control circuit according to claim 1, wherein the frequency of said clock signal changes in accordance with a print density of said laser recording apparatus.

Inventors: NAKAHARA TAKASHI, SHIBATA AKIHIRO, SOYA TAKASHI, SUZUKI HIDENOBU, NAKAHARA, TAKASHI, SHIBATA, AKIHIRO, SOYA, TAKASHI, SUZUKI, HIDENOBU, NAKAHARA, TAKASHI, C/O CANON KABUSHIKI KAISHA, Shibata, Akihiro, c/o Canon Kabushiki Kaisha, SOYA, TAKASHI, C/O CANON KABUSHIKI KAISHA, Suzuki, Hidenobu, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: G06K15
IPC codes: G02B26, G06K15