INITIALISATION OF A FUSER UNIT IN AN IMAGE-FORMING APPARATUS

Abstract: During warm-up of an image-forming apparatus such as a laser printer, the operation of a fuser unit contained therein is controlled by a method which includes the steps of: energizing a heater (310) in a heated roller (10) at substantially the same time as an initialisation process of the mechanical and electrostatic conditions of the machine is commenced in which process the heated roller (10) and a backup roller (12) are rotated together; stopping the rotation of the rollers (10, 12) after the initialisation process has been completed; monitoring the surface temperature of the heat roller (1) for a first predetermined period (T1 - T9) after the completion of the initialisation, and if the surface temperature has reached a set value within the first predetermined period, determining that the fuser unit (14) is ready for operation, and conversely, if the set value is not reached within the first predetermined period (T1 - T9), carrying out an additional warming-up process of rotating the heat roller (10) and the backup roller (12) again until the set value is reached, unless a second predetermined period (T6 - T1) has expired subsequent to the expiry of the first predetermined period (T1 - T9). By employing such a method, the mechanical and electrostatic stresses imposed on the process elements (216, 220, 226) of the apparatus during warm-up may be reduced.

Description: The present invention relates to initialisation of a fuser unit in an image-forming apparatus, and in particular to the control of such a fuser unit during a warm-up stage of the apparatus. In an electronic image-forming apparatus, such as a copier, printer, or facsimile machine, a toner image formed on a photoconductive drum is transferred to a sheet-form medium and fixed thereon by passage through a fuser unit. In Figs. 7(A) and 7(B) of the accompanying drawings, which illustrate principles of a toner fixing process, a fuser unit 14 includes a pair of rollers, consisting of a heated roller 10 and a backup roller 12. The heated roller 10 is made of a thermally-conductive material, such as an aluminum tube coated with a layer of a heat-durable resin and is fitted with an internal built-in heater. The backup roller 12 is made of an elastomeric material, such as a silicone rubber, and is pressed against the surface of the heated roller 10 to be frictionally driven by the rotation of the heat roller 10, which is in turn driven by a main motor of the apparatus. Accordingly, the rollers 10 and 12 rotate together during the printing process and grip a sheet medium 16 therebetween to heat-fix a toner image 18 carried on the sheet medium 16. To achieve a favorable printing quality, it is important to initialize the mechanical and electrostatic conditions of the machine before starting the printing process. In the initialization procedure, the main motor of the apparatus is rotated to thereby drive rotating elements of the machine including the heated roller 10. At the same time, the built-in heater of the heated roller 10 is energized to elevate the temperature thereof. The initialisation procedure is restricted to just a short period, because it imposes undesirable stress on processing elements such as the photoconductive drum or a developer unit, possibly reducing their operating life-times. To minimise this effect, the main motor is stopped immediately after the initialisation period is completed, and thus all of the rotating elements in the apparatus become stationary. The built-in heater of the heated roller 10, however, remains energized until the surface temperature of the roller 10, which is monitored by a sensor, reaches a predetermined value, whereupon it is determined that the warm-up stage is completed and the apparatus is ready to start the printing process. Nevertheless, a problem arises in the above-mentioned steps in that the heater of the heated roller 10 is energized while the roller 10 is stationary after the initialisation procedure has been completed. As shown in Fig. 7(A), the temperature distribution within the heated roller 10 and the backup roller 12 in this case is such that the entire circumference of the heated roller 10 including the topmost point HT 10 and the bottommost point HB 10 is equally heated by the built-in heater, whereas in the backup roller 12, although a region in the vicinity of the topmost point HT 12 is heated to substantially the same degree as the heated roller 10 by heat conduction from the heated roller 10, the lower region of the backup roller 12 remains at a lower temperature because of the relatively poor thermal conductivity of the silicone rubber forming the same, whereby a temperature gradient is established within the backup roller 12 from the topmost point HT 12 to the bottommost point HB 12. Accordingly, if the printing process is started immediately after the surface temperature of the heated roller 10 has reached its predetermined value, the heat stored in the body of the heated roller 10 is transferred to the lower temperature region of the backup roller 12 in the vicinity of the point HB 12, every time the point HB 12 is in contact with the heated roller 10, as shown in Fig. 7(B), and this causes the surface temperature of the heated roller 10 to drop below a predetermined lower limit value for fixing the toner 18 on the sheet medium 16. This phenomenon can be particularly serious when the apparatus has remained unused for a long time in low ambient temperature conditions. The temperature variation of each of the rollers 10 and 12 during the initialisation and warm-up stage as illustrated in the accompanying Fig. 8, which shows that the surface temperature of the heated roller 10 becomes lower than the lower limit for a period t, even after the predetermined temperature is initially attained. To solve the above problem, Japanese Examined Patent Publication (Kokoku) No. 61-31462 (corresponding to U.S. Patent 4,385,826) proposed image-forming apparatus of the kind including a fuser unit having a heated roller and a backup roller, which rollers are pressed one against the other, there being rotary drive means connected with one of the said rollers to bring about rotation of both rollers so that, when the apparatus is in use, a sheet-form medium bearing a toner image is gripped between the two rollers and carried therebetween to cause the toner image to become fixed onto the sheet-form medium, and also including warm-up control means operable to cause the apparatus to undergo a predetermined initialisation procedure, for setting mechanical and electrostatic conditions of the apparatus into a state of readiness wherein a favourable toner image quality is achievable, substantially throughout which procedure the said rollers are rotated at a time when a heater of the heated roller is energised to increase surface temperature of the roller, the said warm-up control means being operative to cause the rotation of the said rollers to stop upon termination of the initialisation procedure. As shown in the accompanying Figure 9, in this prior apparatus the energisation of the heater in the heated roller is started while the roller is stationary (step 900), this energisation being continued for a predetermined period (step 902), and then the main motor is driven to rotate the heated roller together with the backup roller until the surfaces of both rollers are uniformly and sufficiently heated (step 904). According to this method, however, the mechanical/electrostatic stresses stored in the process elements may be larger than in the usual case because the process elements are additionally driven together with the heated roller for a longer period. Furthermore, even when the ambient temperature is not low or the apparatus is restarted after only a short period of disuse, the energisation of the heater is none the less required to be carried out for a predetermined period as part of a standard routine, which delays the commencement of the printing operation and lowers the machine efficiency. According to a first aspect of the present invention there is provided image-forming apparatus of the kind including a fuser unit having a heated roller and a backup roller, which rollers are pressed one against the other, there being rotary drive means connected with one of the said rollers to bring about rotation of both rollers so that, when the apparatus is in use, a sheet-form medium bearing a toner image is gripped between the two rollers and carried therebetween to cause the toner image to become fixed onto the sheet-form medium, and also including warm-up control means operable to cause the apparatus to undergo a predetermined initialisation procedure, for setting mechanical and electrostatic conditions of the apparatus into a state of readiness wherein a favourable toner image quality is achievable, substantially throughout which procedure the said rollers are rotated at a time when a heater of the heated roller is energised to increase surface temperature of the roller, the said warm-up control means being operative to cause the rotation of the said rollers to stop upon termination of the initialisation procedure; characterised in that the said warm-up control means are operative to maintain energisation of the said heater upon such termination of the initialisation procedure, in dependence upon surface-temperature monitoring means associated with the said heated roller, until the said surface temperature attains a preselected desired value, whereupon the apparatus is switched into a ready-for-use condition; and are also operative to ensure that if the said preselected desired value of the said surface temperature is not attained within a first preset time interval, following termination of the said initialisation procedure, the said rollers are subjected to rotation again, whilst energisation of the said heater is maintained in dependence upon the said surface-temperature monitoring means, until the said preselected desired value is attained or until expiry of a second preset time interval, following expiry of the said first preset time interval, whichever is the first to occur. According to a second aspect of the present invention there is provided a method of controlling a fuser unit of an image-forming apparatus in a warming-up stage, which unit comprises a heated roller and a backup roller pressed against the heated roller, and between which rollers a sheet-form medium is nipped so that a toner image carried on the sheet-form medium is fixed, characterised by the steps of starting the energisation of a heater of the heated roller substantially at the same time as the commencement of an initialisation procedure, for setting mechanical and electrostatic conditions of the apparatus, and rotating the heated roller and the backup roller together during the initialisation procedure; stopping the rotation of the rollers after the said initialisation procedure has been completed; monitoring surface temperature of the heated roller during a first preset time interval commencing after completion of the initialisation procedure; and if the surface temperature has attained a preselected desired value within the first preset time interval, determining that the fuser unit is ready for operation; whereas if the said preselected desired value is not attained within the first preset time interval, carrying out an additional warming-up procedure involving rotating the heated roller and the back-up roller again, whilst energisation of the said heater is maintained, until the said preselected desired value is attained or until expiry of a second preset time interval, following expiry of the said first preset time interval, whichever is the first to occur. In such an apparatus and method, once the initialisation procedure has been completed, the rotation of the heated roller and the backup roller for warming-up is restarted only when the surface temperature of the heated roller is not elevated to the preselected desired value within the first preset time interval, and this additional rotation of the heated roller and the backup roller is immediately stopped after the preselected desired value has been reached. This is based upon the phenomenon that, when an apparatus is warmed up in a usual ambient temperature condition or restarted after a short temporary machine stop, for example to clear a paper jam, the roller temperature is high enough to obtain the preselected desired value within the first preset time interval. Thus an additional rotation of the heated roller is rarely necessary, for example only when the apparatus is maintained under cold ambient conditions for a long time. Accordingly, in most cases the apparatus can be warmed-up without the excess stress that might be caused by an additional initialisation procedure, and without unnecessary wastage of electrical energy. Reference will now be made, by way of example, to the accompanying drawings in which: Fig. 1 is a diagram illustrating features of a control system for a fuser unit in image-forming apparatus operating in accordance with an embodiment of the present invention; Fig. 2 is a diagrammatic side elevational cut-away view of a laser printer; Fig. 3 is a block diagram illustrating a control system for a laser printer; Figs. 4 and 5 show respective parts of a flow chart for explaining the operation of an embodiment of the present invention; Fig. 6 is a time chart illustrating the timing of steps in the operation shown in Figs. 4 and 5; Fig. 7(A) and 7(B) show respective diagrammatic side views of a fuser unit; Fig. 8 is a graph illustrating the variation of the respective temperatures of rollers in a previously-proposed fuser unit; and Fig. 9 is a flow chart for explaining the operation of a previously-proposed fuser unit. The principle of a method of controlling a fuser unit in an embodiment of the present invention will be explained with reference to Fig. 1, wherein a fuser unit 14 of a printer 20 comprises a heated roller 10 and a backup roller 12, between which a toner 18 carried on a sheet medium 16 is subjected to pressure and heat and is fixed on the sheet medium 16. Upon commencement of the operation of the printer 20, a built-in heater of the heated roller 10 is energized and the heated roller 10 is driven to rotate together with the backup roller 12. The rotation of the rollers 10 and 12 is stopped when the initialization of the machine conditions is completed (step 100). Surface temperature of the heated roller is constantly monitored by a sensor (step 102), and if the surface temperature of the heated roller 10 reaches a preselected desired value within a first preset time interval commencing after the rotation of the roller 10 and 12 has been stopped, it is determined that the fuser unit is ready for operation. Conversely, if the surface temperature has not reached the desired value within the first preset time interval, the rotation of the rollers 10 and 12 is repeated and, if the surface temperature of the heated roller 10 has reached the desired value within a second preset time interval, the rotation of the rollers 10 and 12 is stopped. If, however, the surface temperature of the heated roller 10 cannot reach the desired value within the second preset time interval, it is determined that the fuser unit is faulty and the rotation of the rollers is forcibly stopped to avoid imposing excess stress on the process elements in the apparatus (step 104). Figure 2 illustrates a representative internal structure of a laser printer 200 which is operable in accordance with an embodiment of the present invention, wherein individual sheets 202 are stacked in a cassette and conveyed one by one along an S-shaped path 204 to an output tray 206 provided in the upper area of the printer 200. The sheets 202 are lifted out by a pick-up roller 208 and transferred to the path 204 by a supply roller 210. Alternatively, the sheets 202 may be input to the interior of the printer 200 through a sheet insertion slit 212 formed on the left-hand side of the printer 200 as viewed in Fig. 2, and transferred to the path 204 through a supply roller 214. The sheets 202 are moved along the path 204 to pass under a photoconductive drum 216. The surface of the photoconductive drum 216 is first discharged by a discharger 218 and cleaned by a cleaner 220, and then charged by a precharger 222. A laser beam is radiated from an optical-unit 224 and transversely scanned over the surface of the drum 216, to form an electrostatic latent image thereon, and the latent image is developed as a toner image by a developer unit 226. The toner image formed on the surface of the photoconductive drum 216 is transferred to the cut sheets 202 by a transfer-charger 238. Then the cut sheets 202 are fed to a fuser unit 14 comprising a heated roller 10 and a backup roller 12, where the toner image is fixed on the cut sheets 202, and the cut sheets are then discharged to the output tray 206 through two pairs of output rollers 240 and 242. A control unit 244 (warm-up control means) is provided in the lowermost area of the printer 200, and sheet-detecting sensors 246 and 248 are provided and cooperate with the control unit 244. Also, a sensor 250 is provided in the vicinity of the heated roller 10 for monitoring the surface temperature thereof. A control system for the laser printer 200 is illustrated in Fig. 3. All of the rotating elements in the printer 200 are driven by a single main motor 300 through the respective transmission systems shown by solid lines. The rollers 208, 210, and 214 are driven via clutches 302, 304, and 306, respectively, and these clutches can be switched on or off so that the roller 208, 210, and 214 can be operated regardless of the operation of the elements related to the image-forming process, such as the photoconductive drum 216, developer unit 226, or cleaner 220 (hereinafter referred to as "process elements"). The operations of the main motor 300 and the clutches 302, 304, and 306 are controlled by a micro-processor unit 308 (hereinafter referred to as "MPU"). The MPU 308 can forecast whether any of the respective operating lifetimes of the process elements has expired by calculating the total number of rotations of the photoconductive drum 216 from the detected number of rotations of the main motor 300, and comparing the former with respective set values determined for the above respective process elements. In addition, outputs of the sensors 246, 248, and 250 are fed to the MPU 308, and the energization of a halogen lamp 310 used as a built-in heater of the heated roller 10 is controlled thereby. The MPU 308 also controls the photo-unit 224 and a mirror motor 314 for the traverse scanning of the laser beam over the photoconductive drum 216. A memory 312 for this purpose is accommodated in the control unit 244. A main switch (not shown) is provided for supplying electric current to the printer 200. If the main switch is ON, the MPU 308 and other parts operable with a low voltage, such as the memory 312 or a panel for the operator (not shown), are energized. The printer 200 also has an interlock switch (not shown) which is made ON or OFF in accordance with the shutting or opening of a front cover of the printer 200. When the front cover is shut, the interlock switch is closed and the main switch is ON, and thus parts operable with a high voltage, such as the precharger 222 or the halogen lamp 310, are also energized. Conversely, when the front cover is open, the power supply to these high voltage parts is stopped, to avoid the risk of an electric shock. The MPU 308 is programmed to start the initialization of the printer 200 when the main switch is ON and the interlock switch is switched from OFF to ON. Figures 4 and 5 present a flow chart illustrating operational steps in an embodiment of the present invention, and Fig. 6 illustrates a corresponding time chart. When the main switch is closed to supply a current to the printer 200, or when the front cover is shut after a temporary machine stop to close the interlock switch, the energization of the halogen lamp 310 is started (step 400) to heat the heated roller 10. Then as shown in Fig. 6, the initialization steps are carried out sequentially as follows: drive mirror motor 314; drive main motor 300; start operation of precharger 222; apply developing bias; start operation of transfer-charger 238; and, check alarm means (step 402). In the above steps, the main motor 300 is started a period T2 after the halogen lamp 310 and the mirror motor 314 are turned on, to avoid a doubling of an initial peak current, and is driven until the expiry of a period T9 (for example, 17 seconds), whereby the heated roller 10 and the backup roller 12 are rotated for a period T9 - T2. Similarly, a developing roller 230 in the developer 226 and the cleaner 220 are also rotated for a period (T9 - T2). The MPU 308 counts the number of rotations of the rollers. The temperature is detected by the sensor 250 during the initialization process (402), and if the surface temperature of the heated roller 10 has reached the preselected desired value (190°C) within the period T9, the flow jumps to step 500 shown in Fig. 5, immediately after stopping the heated roller 10, and the halogen lamp 310 is turned OFF and the printer 200 is ready to commence the printing operation, provided that the other elements of the printer 200 have been reset to receive a start signal from the control unit 244. After the completion of the initialization process (step 402), the MPU 308 determines whether a predetermined period T1 (for example, 60 seconds) has elapsed since the energization of the halogen lamp 310. If not, the MPU 308 further determines whether the surface temperature of the heated roller 10 has reached the above-mentioned desired value (step 404) and if so the flow jumps to step 500 and the printer 200 is made ready for printing (step 502). Thus, if the surface temperature of the heated roller 10 has reached the desired value within the predetermined period T1 after the energization of the halogen lamp, the MPU 308 determines that the printer 200 is ready for a printing operation without additional warming-up steps, because it is surmised that the printer 200 as a whole is warm enough that an abnormal temperature drop soon after the commencement of printing, as shown in Fig. 8, will not occur. This occurs, for example, when the printer 200 is kept in a normal ambient room temperature before being powered-up or when the printer 200 is restarted after a temporary machine stop. Conversely, if the surface temperature of the heated roller 10 does not reach the desired value within the above predetermined period T1, the initialization process is restarted as an additional warming-up process (steps 408, 414, 420, 426, 432). Namely, after the mirror motor 314 is restarted, the main motor 300 is driven so that the heated roller 10 and the back-up roller 12 are again rotated. The respective warming-up steps are sequentially carried out for respective predetermined periods T2, T3, T4, T5 and T6, defined in the time chart illustrated in Fig. 6. During each step 408, 414, 420, 426 and 432, the MPU 308 monitors the time elapsing and determines whether the associated predetermined period has expired (steps 410, 416, 422, 428, 436). If it has, the next step 414, 420, 426, or 432 is begun. If not the MPU 308 determines whether the surface temperature of the heat roller 10 has reached the set value (steps 412, 418, 424, 430). If the set value is reached in any one of the steps 412, 418, 424, and 430, the flow jumps to step 504 in Fig. 5(B), as illustrated in Fig. 4, and the halogen lamp 310 is turned OFF (step 504), the precharger is turned OFF (step 506), and after expiry of a predetermined period T7 (step 508), the main motor 300 is turned OFF (step 510), and accordingly, the printer 200 is ready to commence a printing operation (step 512). Thus, if the surface temperature of the heated roller 10 has not reached the desired value within the predetermined period T1 after the energization of the halogen lamp 310, the MPU 308 determines that an additional warming-up process is necessary, possibly because the printer 200 has remained in a low ambient temperature condition for a long time so that an abnormal temperature drop might occur soon after the commencement of printing as shown in Fig. 8. If the surface temperature of the heated roller 10 has not reached the desired value even upon expiry of a predetermined period T6 (for example, 90 seconds) after the energization of the halogen lamp 310, i.e. if the outcome of step 436 is positive, the MPU 308 determines that the fuser unit 14 has malfunctioned (step 514 in Fig. 5(C)) and turns off the halogen lamp 310 (step 516). As stated above, in the illustrated embodiment of the present invention, if the heated roller 10 is sufficiently heated during the first initialisation process, the printer can start the printing operation without additional warming-up steps. But even when the additional warming-up process is needed, this process can be interrupted immediately after the set temperature has been reached so that the mechanical and electrostatic stresses imposed on the process elements are minimized. This can also reduce the warming-up time and prolong the life span of the process elements and the halogen lamp. Further the efficiency of the printer 200 may be improved because the time needed to complete the machine warm-up is shortened. In this connection, it has been found that, when the desired temperature is not been reached at the heated roller 10 during the first initialization process, the final temperature distribution at the heated roller 10 and the backup roller 12 can be greatly improved by the simple procedure of displacing the position of both rollers from that shown in Fig. 7(A) to that shown in Fig. 7(B), ie. so as to rotate the rollers 12 through half a revolution, particularly after a period of stationary heating following completion of the first initialisation process.

Claims: Image-forming apparatus of the kind including a fuser unit (14) having a heated roller (10) and a backup roller (12), which rollers are pressed one against the other, there being rotary drive means (300) connected with one of the said rollers to bring about rotation of both rollers so that, when the apparatus is in use, a sheet-form medium (16) bearing a toner image is gripped between the two rollers and carried therebetween to cause the toner image to become fixed onto the sheet-form medium, and also including warm-up control means (244) operable to cause the apparatus to undergo a predetermined initialisation procedure, for setting mechanical and electrostatic conditions of the apparatus into a state of readiness wherein a favourable toner image quality is achievable, substantially throughout which procedure the said rollers are rotated at a time when a heater (310) of the heated roller (10) is energised to increase surface temperature of the roller, the said warm-up control means (244) being operative to cause the rotation of the said rollers to stop upon termination of the initialisation procedure; characterised in that the said warm-up control means are operative to maintain energisation of the said heater (310) upon such termination of the initialisation procedure, in dependence upon surface-temperature monitoring means associated with the said heated roller, until the said surface temperature attains a preselected desired value, whereupon the apparatus is switched into a ready-for-use condition; and are also operative to ensure that if the said preselected desired value of the said surface temperature is not attained within a first preset time interval (T1 - T9), following termination of the said initialisation procedure, the said rollers (10, 12) are subjected to rotation again, whilst energisation of the said heater (310) is maintained in dependence upon the said surface-temperature monitoring means, until the said preselected desired value is attained or until expiry of a second preset time interval (T6 - T1), following expiry of the said first preset time interval (T1 -T9), whichever is the first to occur. Image-forming apparatus as claimed in claim 1, wherein substantially all rotating elements in the apparatus are driven by a single main motor (300). Image-forming apparatus as claimed in claim 1 or 2, wherein the said warm-up control means (244) are operative to cause the said backup roller (12) to be rotated through substantially half a revolution at a time subsequent to the said termination of the initialisation procedure. A method of controlling a fuser unit (14) of an image-forming apparatus in a warming-up stage, which unit (14) comprises a heated roller (10) and a backup roller (12) pressed against the heated roller (10), and between which rollers (10, 12) a sheet-form medium (16, 202) is nipped so that a toner image carried on the sheet-form medium (16, 202) is fixed, comprising the steps of: starting the energisation of a heater (310) of the heated roller (10) substantially at the same time as the commencement of an initialisation procedure, for setting mechanical and electrostatic conditions of the apparatus, and rotating the heated roller (10) and the backup roller (12) together during the initialisation procedure; stopping the rotation of the rollers (10, 12) after the said initialisation procedure has been completed; monitoring surface temperature of the heated roller (10) during a first preset time interval (T1 - T9) commencing after completion of the initialisation procedure; and if the surface temperature has attained a preselected desired value within the first preset time interval, determining that the fuser unit (14) is ready for operation; whereas if the said preselected desired value is not attained within the first preset time interval, carrying out an additional warming-up procedure involving rotating the heated roller (10) and the back-up roller (12) again, whilst energisation of the said heater (310) is maintained, until the said preselected desired value is attained or until expiry of a second preset time interval (T6 - T1), following expiry of the said first preset time interval, whichever is the first to occur. A method as defined in claim 4, wherein the said additional warming-up procedure is a repetition of at least part of the said initialisation procedure. Image-forming apparatus or a method, as the case may be, as claimed in any preceding claim, wherein the said initialisation procedure comprises the steps of: driving a mirror motor (step 408); driving a main motor (step 414); energising a precharger (step 420); imparting a developing bias (step 426); and energising a transfer-charger (step 432). Image-forming apparatus or a method, as the case may be, as claimed in any preceding claim, wherein the said heater of the heated roller (10) is a halogen lamp (310).

Inventors: HOKI, YOJI, INOUE, MAKOTO, WATARAI, SHINICHI, YASUDA, MEGUMI
Applicants: FUJITSU LIMITED
Application year: 1989
Main classification: G03G15
IPC codes: G03G15, G03G21