AUTOMATIC EXPOSURE DEVICE FOR COPYING MACHINE

Abstract: A copying machine with an automatic exposure device in which the density of an original is detected by irradiating an original by light from a lamp (4a) and detecting reflecting light with an optical sensor (6a), and in accordance with the detected signal the exposure amount of the lamp is adjusted by means of an automatic voltage adjusting circuit, is equipped with a heater control unit for controlling the energization of a fixing heater (20b) for fixing a toner sensible image which has been transferred onto paper. A processing circuit for controlling the automatic voltage controlling circuit and heater control section, acts to maintain the energizing state of the fixing heater (20b) in a single state, while the operation of detecting the original's density is run with the lamp operating. In this way, fluctuation in the voltage to the lamp due to switching of the fixing heater is prevented from occurring at the time of detecting the density, and the intensity of light from the lamp is kept constant.

Description: The present invention relates to an automatic exposure device for use in an electrophotographic copying machine, and particularly pertains to an automatic exposure device capable of suppressing fluctuations in the luminous intensity of a lamp for irradiation of an original. For example, in an electrophotographic copying machine equipped with a prescanning type automatic exposure device, scanning is made before copying by irradiating the original with light from an original irradiation lamp such as halogen lamp, etc., and the original's density is read by receiving the reflected light with an optical sensor. The automatic exposure device has an automatic voltage adjusting circuit for controlling the applied voltage of the original irradiation lamp so as to be constant, which assures that the intensity of light from the original irradiation lamp be kept constant. However, there is a likelihood in such an electrophotographic copying machine that a fixing heater inadvertently comes ON or OFF, while the original irradiation lamp is operating. Since this fixing heater consumes a relatively large amount of electric power, as compared with other units constituting the electrophotographic copying machine, it is liable to affect the automatic voltage adjusting circuit for assuring a constant voltage to the original irradiation lamp. For example, during the time that the fixing heater is held on, when the original irradiation lamp comes into its working state, the input voltage to the automatic voltage adjusting circuit falls, for example, from 100V to 92V. In response thereto, the circuit acts so as to keep its output voltage constant to apply a constant voltage to the original irradiation lamp. However, when the performance of the automatic voltage adjusting circuit is not highly accurate, it becomes difficult to keep its output voltage constant against abrupt voltage changes. Consequently, the output voltage goes down and then the applied voltage to the original irradiation lamp drops, resulting in a decrease in the luminous intensity of the original irradiation lamp. Accordingly, the automatic exposure device brings about an error in the read-out of the original's density and consequently it becomes difficult to provide information for ensuring a proper copied image. Also, since use of a high performance automatic voltage adjusting circuit raises the cost, it is undesirable to use such a circuit. US-A-4113 375 discloses a copying machine wherein either the exposure lamp is on or the fixing heater is on. This machine does not solve the problem of voltage changes during pre-scanning. This invention, to obviate the aforementioned technical problem, seeks to provide an automatic exposure device for a copying machine which assures a proper copied image by reading the original's density at high accuracy without using an expensive automatic voltage adjusting circuit. An automatic exposure device of the present invention is constructed such that the switching state of a fixing heater is maintained unchanged during the time that an original irradiation lamp is held on to detect the original's density so that a fluctuation in the input voltage of the automatic voltage adjusting circuit is diminished and the output voltage is kept constant. Consequently, the voltage applied to the original irradiation lamp and the intensity of light of the lamp are held constant. Accordingly, the original's density can be detected at a high accuracy with an optical sensor. Thus it becomes possible to obtain a proper copied image in correspondence to the original's density. Furthermore, because fluctuation in the input voltage to the automatic voltage adjusting circuit is reduced, it is not necessary to use an expensive high performance automatic voltage adjusting circuit. Accordingly, a constant output voltage can be obtained with an inexpensive automatic voltage adjusting circuit and thus attain a reduced cost. An embodiment of the present invention will be described below wit h reference to the accompanying drawings. Fig. 1 is a block diagram showing an automatic exposure device for electrophotographic copying machine of the present invention; Fig. 2 is a whole composition diagram of the electrophotographic copying machine equipped with the aforementioned automatic exposure device; and Fig. 3 is a flow-chart showing the operation of the aforementioned automatic exposure device. Fig. 2 is a whole composition diagram of an electrographic copying machine equipped with an automatic exposure device of the present invention. In Fig. 2, the electrographic copying machine of the embodiment has a housing 1, contact glass 2 on which an original is put and an original cover 3, etc. On the underside of one end of the contact glass 2 there is provided a white area 2a. An optical system includes an original irradiation lamp 4a, such as a halogen lamp, a first carriage 4 equipped with a reflection plate 4b and a mirror 4c, a second carriage 5 equipped with mirrors 5a and 5b, a lens assembly 6 equipped with an optical sensor 6a for detecting the original's density and a mirror 7. On the outside of the second carriage 5 is mounted a light-shielding plate 8a for switching the optical sensor 8 to detect whether or not the second carriage 5 is at a home position as shown in Fig. 2. Around a photoreceptor drum 9 which receives reflecting light from the mirror 7 are arranged in the order of a turning direction a pre-exposure lamp 10, a main charger unit 11 including a corona-discharger, a developing unit 12, a transferring unit 13, a separating unit 14, a charge removing lamp 15, a cleaning unit 16, etc. The photoreceptor drum 9 has an exposure region corresponding to the optical system between the main charger unit 11 and the developing unit 12. The developing unit 12 having a toner hopper 12a, a toner supply roller 12b, a toner stirring roller 12c and a developing roller 12d feeds a developer, for example, developing toner onto the photoreceptor drum 9, so that the electrostatic latent image formed by the exposure system is developed as a toner image. In the downstream side of the developing unit 12 in the turning direction of the drum 9 and in the upstream side of the transferring unit 13 and near the photoreceptor drum 9 there is provided a conveying unit 17 including rotationally driven conveying rollers 17a and 17b for conveying copying paper to the transferring unit 13 and a guide plate 17c for guiding the copying paper fed from the conveying rollers 17a and 17b to the transferring unit 13. The cleaning unit 16 has a cleaning blade 16a and a blade solenoid 16b for causing the cleaning blade 16a to come into contact with and move away from the surface of the photoreceptor drum 9. The paper feeding unit 18 includes a copying paper cassette 18a in which copying paper is held, a feed paper roller 18b for feeding out the copying paper and a guide plate 18c for guiding the copying paper fed out by the feed paper roller 18b to the conveying unit 17. In the downstream side of the feeding direction of the separating unit 14 there is provided a conveying belt assembly 19. In the downstream side of the conveying belt assembly 19 there is provided a fixing unit 20. A guide plate 19a is provided between them. The fixing unit 20 includes a fixing heat roller 20a having a fixing heater 20b in the center of the roller 20a and a fixing roller 20c forming a pair by combination with the roller 20a. In the downstream side of the fixing unit 20 are provided a guide plate 21a for guiding copy paper coming from the fixing device 20, discharge rollers 21b and 21c for discharging copying paper coming from the guiding plate 21a and a tray 22 for receiving copying paper discharged from the discharge rollers 21b and 21c. Fig. 1 is a block diagram showing the automatic exposure device of the invention. In Fig. 1, an operation section 41 includes a print key switch which is operated at the time of copying, a switch for setting an automatic exposure mode and a plurality of key switches for inputting information necessary for copying, etc. A processing circuit 42 made up of a microcomputer, etc. receives outputs from the operation section 41, light receiving sensor 6a and a home position switch 43, i.e., the optical sensor 8 shown in Fig. 2, and executes later described processing so as to control the exposure lamp automatic voltage adjusting circuit 44 for operating the original irradiation lamp 4a, an optical system motor control section 45 for controlling a motor 46 of the optical system for driving the first and the second carriages 4 and 5, and a fixing heater control section 47 for operating the fixing heater 20b. The operation of the embodiment will be described below with reference to the flow chart shown in Fig. 3. At step n1, it is executed to determine whether the print key switch of the operation section 41 is turned on. When it is turned on, the operation advances to n2 at which it is determined whether an automatic exposure mode setting switch of the operation section 41 is turned on and whether the automatic exposure mode is set. When the automatic exposure mode is set, the operation advances to n3 at which the processing circuit 42 receives outputs from the home position switch 43 and determines whether or not the first and the second carriages 4 and 5 are set at their home positions as shown in Fig. 2. When the outputs indicate that they are set at their home positions, the optical system motor control section 45 is operated so as to cause the optical motor 46 to rotate in the normal direction. The normal rotation of the optical motor 46 makes the first and the second carriages 4 and 5 move in the direction of arrow A (Fig.2). In other words, the prescanning operation is carried out to detect the original's density before the optical system starts the usual copying operation. The processing circuit 42 controls the exposure lamp automatic voltage adjusting circuit 44 so as to raise the intensity of light from the original irradiation lamp 4a higher than that at the usual copying time to keep the intensity of light constant. Moreover, the circuit 42 controls the fixing heater 20b to forcibly maintain it in an ON or OFF state through the fixing heater control section 47 during the time that the pre-scanning is executed and the original irradiation lamp 4a is operating. More specifically, the fixing heater 20b is usually turned on or off by the fixing heater control section 47 in accordance with a signal from a sensor for detecting the temperature of the fixing heat roller 20a so that the heater is maintained at a predetermined temperature. On the other hand, during the prescanning operation, the fixing heater 20b is held in a switching state (ON or OFF) at the point of time when the original irradiation lamp 4a is put into working operation, irrespective of the temperature of the fixing heat roller 20a. At step n4, a timer is set which is provided in the processing circuit 42 and serves to count a first specified time T1 including the lead time which is the period of time between the time that the original irradiation lamp 4a is turned on and the time that a rated intensity of light is attained and the moving time during which the original irradiation lamp 4a reaches the position of the original from the home position. At step n5, it is determined whether the timer reaches the first specified time T1. When the time elapses, the operation advances to n6. At step n6, after the lapse of the first specified time T1, the light receiving sensor 6a detects the reflecting light from the original and the processing circuit 42 starts the reading of the output from the light receiving sensor 6a. At step n7, the timer in the processing circuit 42 is set to count a second specified time T2 which is the moving time of the optical system for determining the reading region of the original in a copy scanning region where the irradiation light from the original irradiation lamp 4a scans the original. At step n8, it is determined whether the timer reaches the second specified time T2 and when the second specified time T2 elapses, the operation enters n9, in which the first and the second carriages 4 and 5 scan the whole copy scanning region. The processing circuit 42 receives the output from the light receiving sensor 6a and completes the reading process of the original's density. Then the original irradiation lamp 4a is turned off, and the optical system motor 46 is actuated in the reverse direction and the forcibly set on or off state of the fixing heater 20b is cancelled. At step n10, it is determined whether the first and the second carriages have 4 and 5 returned to their home positions by detecting whether the home position switch 43 is turned on. When the carriages return to their home positions, the operation enters step n11 at which time the optical motor 46 is stopped. Thereafter, the operation advances to step n12, at which the usual copying operation is started. The density of the copy image produced at this step corresponds to that of the original. At step n2, when the mode switch is not turned on and an automatic exposure mode is not set, the operation enters step n12 at which the copying operation is started at a density set by the operator irrespective of the original's density. The embodiment in which the switching state of the fixing heater 20b is held unchangeable when the original irradiation lamp 4a is in a working state to detect the original's density eliminates the fluctuation of input voltage of the automatic exposure lamp voltage adjusting circuit 44. Accordingly, the output voltage or the voltage applied to the original irradiation lamp 4a is kept constant. Consequently, the intensity of light from the original irradiation lamp 4a will not fluctuate and the original's density can be detected by the light receiving sensor 6a at a high accuracy. Thus a proper copy image corresponding to the original's copy density is obtainable. Though an embodiment has been described which is such that the on or off switching state of the fixing heater 20b is forcibly maintained when the original irradiation lamp 4a is in a working state, it will be needless to say that a device of the present invention may be constructed such that when the switching state of the fixing heater 20b is on, which is detected when the print key switch of the operation section 41 is turned on, the on-state is maintained. On the other hand, when the state is off, off-state is maintained. Furthermore, irrespective of the switching state of the fixing heater 20b when the print key switch is on, the fixing heater 20b may be forcibly set either its on-state or off-state. The switching state of the fixing heater 20b set in this way is maintained until the read-out process by the light receiving sensor 6a completes at step n9 shown in Fig. 3. In such control, since the switching state of the fixing heater 20b is not changed while the original irradiation lamp 4a is in a working state during the prescanning time, the same effect as that of the aforementioned embodiment is achieved. Though the above embodiment has been described with reference to a copying machine in which an original is scanned by a travelling optical system, it will be noted that the present invention is applicable to a copying machine in which while an optical system is not moved but an original is moved, because it is sufficient to move the optical system and original in relation to each other.

Claims: A copying machine including an automatic exposure device for a pre-scanning operation comprising an optical system (4,5) which executes scanning while irradiating light from a lamp (4a) to an original, an optical sensor (6a) for detecting the density of the original by receiving reflecting light from said original, an automatic voltage adjusting circuit (44) for adjusting the exposure amount of the lamp in accordance with the signal detected by said optical sensor, a photoreceptor (9) on which an electrostatic latent image is formed by the exposure scanning on the original by said optical system, a developing unit (12) for developing the electrostatic latent image on said photoreceptor to a toner image, a transfer unit (13) for transferring said toner image to paper, a fixing unit (20) including a fixing heater (20b) for fixing the toner image transferred on said paper, a heater control means (47) for controlling the energization of said fixing heater between two switching states including an on-state and an off-state and a processing circuit control means (42) for controlling the automatic voltage adjusting circuit and said heater control means; characterised in that: said control means (42) is arranged to maintain said heater control means in a predetermined one of said two switching states during said prescanning operation irrespective of the switching state at the time when said optical system is about to initiate execution of said prescanning, by if necessary forcibly switching said heater control means to said predetermined state. A copying machine including an automatic exposure device according to claim 1 wherein said optical system (45) is arranged to shut off said lamp (4a) upon completion of said prescanning operation. A copying machine including an automatic exposure device according to claim 2, wherein said optical system (4,5) is arranged to irradiate light from said lamp (4a) onto said original during said exposure scanning, and said processing circuit control means (42) is arranged to control said automatic voltage adjusting circuit (44) to raise the intensity of the light from said lamp (4a) during said prescanning relative to the intensity of the light from said lamp during said exposure scanning. A copying machine including an automatic exposure device according to claim 3, wherein said processing circuit control means (42) is arranged to disable maintenance of said fixing heater (20b) in either of said switching states upon completion of said prescanning operation. A copying machine including an automatic exposure device according to claim 1 wherein said processing circuit control means is arranged to maintain said predetermined switching state during said prescanning operation, when said optical system is about to initiate execution of said prescanning and said heater control means is in said predetermined switching state. A copying machine including an automatic exposure device according to claim 1 wherein said fixing heater control means (47) is arranged to switch said fixing heater (20b) between said on and said off - states to maintain said fixing heater at a constant temperature except that during said prescanning operation, said heater control means (47) is maintained in said predetermined switching state and is precluded from switching from said predetermined switching state by said processing circuit control means (42) irrespective of the temperature of said fixing heater (20b). A copying machine including an automatic exposure device according to claim 1 wherein said optical system (4,5) is arranged initially to move from a home position when said prescanning is initiated, and said processing circuit control means (42) comprises a timer for determining a first period of time between the time that said lamp (4a) is first turned on and the time that a rated intensity of light is attained plus the time it takes said optical system to move from said home position to encounter said original, whereby said processing circuit control means (42) is arranged to initiate operation of said optical sensor (6a) subsequent to expiration of said first period of time.

Inventors: KURANDO, SHIGEO
Applicants: MITA INDUSTRIAL CO. LTD.
Application year: 1987
Main classification: G03G15
IPC codes: G03G21, G03G15