JP2006039027A - Image forming apparatus - Google Patents

Abstract

An object of the present invention is to control the offset power input at the time of starting the heater to be constant even when the input power supply voltage is different, and to change the offset power according to the target temperature, so that there is no overshoot or power shortage. Providing key control.
A heater lighting duty (limit duty) when a maximum allowable current flows is determined from a current detection circuit flowing in the heater. By supplying the power obtained by the duty obtained by multiplying the limit duty by a predetermined multiplication rate as the offset power (I component) of the PI control, it is possible to always supply a constant power regardless of the power supply voltage. Further, by changing the multiplication rate to be applied to the limit duty in accordance with the target temperature, it is possible to perform favorable temperature control without overshoot or power shortage for any target temperature.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus having a heat fixing device that heat-fixes an unfixed image formed on a recording material by image forming means such as an electrophotographic type or an electrostatic recording type. More specifically, the present invention relates to a temperature control method for controlling the heat fixing device at a stable fixing temperature.

  2. Description of the Related Art Conventionally, in an image forming apparatus employing an electrophotographic system, a recording material carrying an unfixed toner image is heated by passing it through a nip formed by a fixing roller and a pressure roller that are pressed against each other and rotated. A so-called heat roller type heat fixing device for fixing is widely used. In recent years, a film heating type heat fixing apparatus has been put into practical use in which power is not supplied to the heat fixing apparatus during standby and power consumption is kept as low as possible. For example, Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4 and the like have been proposed as a film heating type heating apparatus.

  In these heat fixing devices (hereinafter referred to as “fixing devices”), a heater is generally connected to an AC power source via a switching control element such as a triac, and power is supplied from this AC power source. Further, the fixing device is provided with a temperature detection element such as a thermistor at an appropriate position, and the temperature of the fixing device is controlled to be a target temperature based on the detected temperature information.

  As a general temperature control method for electrical equipment, there is an on / off control in which heating is performed if the detected temperature (actual temperature) is lower than a target temperature, and heating is stopped if the detected temperature is higher than the target temperature. When it is performed on the fixing unit of the device, the actual temperature exceeds the target temperature because the required power differs between the paper passing through and between the papers, and the heat capacity varies depending on the paper type. There were drawbacks such as shooting and increased temperature ripple. Therefore, generally, the duty (time to be lit) for turning on the heater is determined by PI control (proportional + integral control) or PID control (proportional + integral + differential control). The engine controller performs finer temperature control by turning on and off the switching element by wave number control or phase control according to the duty.

A general control equation for performing PID control for temperature control of a heater or the like is as follows.
D (t) = D (t−1) + αe (t) + α / T _i Σe (t) + αT _d (e (t) −e (t−1))
... Equation 1
D (t): Lighting duty in the next cycle e (t): Temperature difference between current target temperature and actual temperature e (t-1): Temperature difference between previous target temperature and actual temperature α: Arbitrary coefficient T _i , T _d : Arbitrary coefficients In order from the second term on the right-hand side of Equation 1, they correspond to proportional control, integral control, and differential control. Here, α is a proportional coefficient for weighting the increase / decrease amount of the duty, and Ti and Td indicate integration time and differentiation time. By setting the above-described α, Ti, and Td according to the characteristics of the fixing device, optimum temperature control is enabled.

  The temperature control of the fixing device includes a start-up section for raising the heater from the room temperature state (or standby state) to the fixing target temperature, a section where the recording material passes through the fixing device, and the recording material does not pass through the fixing device. There is a section (between paper). Depending on the fixing device, in order to perform temperature control with higher accuracy, the proportionality coefficient α, the integration time Ti, and the derivative time Td are changed for each section, or the integral control and the derivative control are improved depending on cases. In many cases, such as simplifying.

  For example, an example of PI control optimal for a fixing device excellent in quick start properties such as a film heating method will be given. The next lighting duty at time t is performed according to the following cases.

D (t) = P (proportional control) component + I (integral control) component Equation 2
e (t): (Target temperature)-(Actual temperature)
P (proportional control) component: Increases or decreases by 2.5 × e (t) percent every 10 msec.
I (integral control) component: + 2.5% if e (t)> 0 ° C. continuously for 500 msec.
-2.5 percent if e (t) <0 ° C continuously.
There is no increase or decrease at other timings.

  Thus, the control is simplified according to the heat conduction characteristics of the heat fixing device.

  In particular, a fixed lighting duty is often given as an I (integral control) component as the initial duty of Equation 2 at the heater start-up timing (at t = 0). The power given by this initial duty is called initial offset power. By increasing / decreasing the P component and the I component with respect to the initial offset power value, it is possible to improve the followability to the target temperature and shorten the heater rise time.

On the other hand, as the speed of the printer / copier increases, the thermal energy required for the recording agent per unit time is increasing. For this purpose, the fixing device is provided with a heater having a larger output (= small resistance value) than the conventional one. When the temperature control of the heater is performed by a temperature detection element such as a thermistor, for example, when a recording agent having a large heat energy absorption amount such as cardboard is passed, a large amount of power is automatically supplied. However, on the other hand, the current flowing through the heater cannot exceed the rated current defined by the safety standard. Therefore, a means for limiting the power that can be supplied to the heater so as not to exceed the rated current is required. For such problems, for example, a current detection circuit that detects a current value flowing through the heater or a voltage detection that detects an input voltage value from a commercial power source, and the duty that energizes the heater does not exceed the rated current. By providing the limiting means, the power exceeding a certain level is prevented from entering.
JP-A-63-313182 Japanese Patent Laid-Open No. 2-157878 JP-A-4-44075 JP-A-4-204980

  To summarize the technical background described above, the fixing device has also improved the heater control method in order to respond to market needs such as speeding up printers and copiers and shortening the rise time of the apparatus. At the same time, in order to handle a larger amount of electric power than in the past, a means for limiting and controlling the method of supplying electric power and current is required from the viewpoint of safety. Therefore, it has been raised as a new subject to make further improvements to the conventional heater control method while effectively using such new means. The problems to be solved by the present invention will be described below.

  As a first problem, when the PID control is performed using the electric power given by the fixed initial lighting duty as the initial offset power when the heater of the fixing device is started up, there are the following drawbacks.

  If the AC commercial power supply for supplying power to the heater is a 100V series, there is a power supply voltage range of 85V to 140V. If the input voltage has a large range, the initial offset power actually supplied to the heater is greatly different even if the initial lighting duty is fixed. For example, when the initial lighting duty is 50% and the heater resistance is 10Ω, the initial offset power is 361 W when the input voltage is 85V. On the other hand, when the input voltage is 140 V, it is 980 W, which is greatly different. Of course, if the input voltage is low, even if the PID control is performed after the initial offset power is turned on, it takes time to reach the target temperature, or the target temperature cannot be reached when the recording material reaches the fixing device. It is also possible to enter the fixing operation. In such a case, image defects such as fixing defects and cold offsets occur. In addition, if the input voltage is high, a large overshoot from the target temperature may occur, resulting in a problem that the fixed image is hot-offset or that a failure such as a high temperature abnormality is determined.

  Such a problem is that if the speed of the image forming apparatus is low as in the prior art, it is easy to return to the target temperature while the recording material is being conveyed to the fixing nip. Since the conveyance of the recording material is finished before the temperature is caught up, the control cannot catch up and image defects often appear remarkably.

  In particular, as the speed of the apparatus increases, the temperature of the heater tends to drop rapidly at the moment when the recording material is conveyed to the fixing nip. This is because the duty is reduced by the PI control when the heater is close to the target temperature, and heat is taken away at the moment when the recording material enters the nip. In particular, the lower the ratio of the I component (offset power component) in the lighting duty of the heater, the greater the temperature drop. In order to improve these problems, for example, in the previous print job, the duty that was input to the heater immediately before the end of the job is reapplied as offset power at the moment when the recording material reaches the nip during the next print. In many cases, such power prediction control is performed. However, in the power prediction control, for example, when the paper type is different between the previous print job and the current print job, problems such as excessive supply of power or power shortage occur. Also, depending on the warming condition of the heat fixing apparatus, it may not always be optimal to reflect the previous job in the current job. Therefore, a first problem according to the present invention is a control method that can always provide an optimal initial offset power even when input voltages are different, and that can perform optimal temperature control without relying on power predictive control or the like. Is to provide.

  The second problem of the present invention is that, in the conventional PID control, the initial lighting duty when the heater is started up is always constant regardless of the input voltage, and the fixing target determined at the start of printing. Even when the temperatures are different, the start-up is performed with the same lighting duty. In recent years, image forming apparatuses are often provided with a plurality of modes in advance so that a good image can be formed on a wide variety of recording materials. With regard to the fixing device, for example, thick paper, rough paper, plain paper, smooth paper, thin paper, etc., a plurality of different target temperatures are set so that fixing can be performed at optimum fixing temperatures for recording materials having different heat capacities and surface properties. A set mode is provided in advance. Further, depending on the image forming apparatus, there is a mode in which image formation is performed at half the normal speed depending on the size and type of the recording material. In such a half speed mode, the fixing target temperature is often set much lower than that at the normal speed. As described above, even if the target temperature differs depending on the mode, if the offset power at the time of starting the heater is set equal, the power is insufficient when the target temperature is high, and the power is excessively supplied when the target temperature is low. Accordingly, a second problem of the present invention is to provide a control that always provides an optimum initial offset power even when the fixing target temperature and the process speed are different.

  The present invention is characterized by the following means configuration.

  (1) An image forming apparatus that heats and fixes a recording material holding an unfixed toner image through a fixing nip portion formed by pressing a heating member having a heater and a pressure member together. There is a power control means for controlling the power supplied to the heater, and a temperature detection means for detecting the temperature of the heater. When the heater is started up according to the fixing target temperature when the recording agent is heated and fixed An image forming apparatus, wherein the power to be input is changed.

  (2) The power control means is a current detection means for detecting a current value flowing through the heater, and the electric power supplied when the heater is started up is based on a duty corresponding to a preset current value according to the fixing target temperature. The image forming apparatus according to (1), wherein the image forming apparatus is determined.

  (3) The power control means is a voltage detection means for detecting an input power supply voltage, and the power to be input when the heater is started is determined by a duty set in advance according to the detected input voltage value and the fixing target temperature. The image forming apparatus as described in (1) above.

  (4) The heater is controlled by PI control or PID control, and the power to be changed when the heater is started up is input as an I control component (integral control component = offset power) of PI control or PID control. The image forming apparatus according to any one of (1) to (3).

  (5) The image forming apparatus according to any one of (1) to (4), wherein the fixing target temperature is determined according to a temperature detected by a heater temperature detecting unit before the heater is started.

  (6) When starting up the heater, first, a fixed power is applied to the heater, the heater is started up to a ready temperature (standby temperature) set at a temperature lower than the fixing target temperature, and the heater is started up to the ready temperature. The image forming apparatus according to any one of (1) to (4), wherein a fixing target temperature is determined according to time, and electric power corresponding to the fixing target temperature is input when restarting from the ready temperature. apparatus.

  (7) The image forming apparatus includes means for integrating electric power supplied from a predetermined timing, and when the heater is started up, the heater is set to a ready temperature (standby temperature) set at a temperature lower than the fixing target temperature. The fixing target temperature is determined according to the integrated power amount input from the start-up and start-up to reaching the ready temperature, and the power corresponding to the target fixing temperature is input when restarting from the ready temperature. The image forming apparatus according to any one of (1) to (4).

  (8) The image forming apparatus according to any one of (1) to (7), wherein the heat fixing device has a configuration with excellent quick start performance without energizing the heater during standby.

  (9) The image forming apparatus according to (8), wherein the fixing member of the heat fixing device is a film heating type heat fixing device having a flexible thin fixing film.

  According to the present invention, by using means such as current detection and input power supply voltage detection, it is possible to control the supply power by controlling the lighting duty of the heater even if the input voltage is different. In particular, in the present invention, it is possible to perform favorable temperature control without causing overshoot or power shortage by supplying an appropriate power to the target temperature of the fixing device when starting up the heater. These controls are particularly effective for speeding up recent image forming apparatuses and shortening the start-up time, and are optimal as a control of a heat fixing device excellent in quick start performance in which power is not turned on during standby. In addition, it is necessary to determine an appropriate fixing target temperature according to the situation in a situation where the environment in which the image forming apparatus is used is different or the recording materials used by the user are diversified. According to the present invention, by using the advantage that the electric power supplied to the heater can be controlled, the total electric energy required to reach a predetermined temperature is compared, or the reached temperature when a constant electric power is supplied is compared. As a result, the animal heat state of the heat fixing device can be accurately grasped, so that the target temperature can be determined with high accuracy.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to examples.

(1) Example of Image Forming Apparatus FIG. 1 is a schematic configuration diagram of an image forming apparatus in this embodiment.

  Reference numeral 1 denotes a photosensitive drum, and a photosensitive material such as OPC, amorphous Se, or amorphous Si is formed on a cylindrical substrate such as aluminum or nickel. The photosensitive drum 1 is rotationally driven in the direction of an arrow, and first, the surface thereof is uniformly charged by a charging roller 2 as a charging device. Next, scanning exposure is performed with a laser beam that is ON / OFF controlled according to image information, and an electrostatic latent image is formed. This electrostatic latent image is developed and visualized by the developing device 4. As a developing method, a jumping developing method, a two-component developing method, an FEED developing method, or the like is used, and image exposure and reversal development are often used in combination.

  The visualized toner image is transferred from the photosensitive drum 1 onto the recording material P conveyed at a predetermined timing by a transfer roller 5 as a transfer device. Here, the leading edge of the recording material is detected by eight sensors so that the image forming position of the toner image on the photosensitive drum 1 matches the writing position of the leading edge of the recording material, and the timing is adjusted. The recording material P conveyed at a predetermined timing is nipped and conveyed between the photosensitive drum 1 and the transfer roller 5 with a constant pressure. The recording material P to which the toner image has been transferred is conveyed to the fixing device 6 and fixed as a permanent image.

  On the other hand, the residual toner remaining on the photosensitive drum 1 is removed from the surface of the photosensitive drum 1 by the cleaning device 7. Reference numeral 9 denotes a paper discharge sensor provided in the heat fixing device 6, and is a sensor for detecting when a paper jam occurs between the top sensor 8 and the paper discharge sensor.

(2) Heat fixing device 6
FIG. 2 is a schematic configuration diagram of the heat fixing device 6. This heat fixing device 6 is basically a film heating type heat fixing device comprising a fixing member (assembly) 10 and a pressure member 20 that are pressed against each other to form a nip portion N. A metal sleeve is used as the body.

1) Fixing member 10
In the cross-sectional view of FIG. 2, the fixing member 10 as a fixing rotator mainly includes a heater 11, a heat insulating stay holder 12 that holds the heater 11, and a fixing film (metal sleeve) 13.

The heater 11 heats the nip portion N by contacting the inner surface of the fixing film (metal sleeve) 13. On the surface of a ceramic substrate 11a such as alumina or aluminum nitride, an energization heating resistance layer 11b made of Ag / Pd (silver palladium), RuO ₂ , Ta ₂ N or the like along the longitudinal direction has a thickness of about 10 μm and a width of about 1 to 5 mm. It is coated by screen printing or the like. On the back surface of the ceramic substrate 11a, a temperature detection element 14 such as a thermistor for detecting the temperature of the ceramic substrate raised in accordance with the heat generation of the energization heating resistor layer 11b is disposed. By appropriately controlling the duty ratio, wave number, etc. of the voltage applied to the energization heating resistor layer from an electrode portion (not shown) at the end in the longitudinal direction in accordance with the signal of the temperature detection element 14, Keep the fixing temperature constant. A protective layer 15 such as a thin glass coat or a lubricating resin layer such as polyimide or polyamideimide is provided on the surface where the heater 11 is in contact with the fixing film (metal sleeve) 13 so that the film can rotate smoothly. Is provided. The heat insulating stay holder 12 holds the heater 11 and prevents heat radiation in the direction opposite to the nip N side, and guides the rotation of the fixing film (metal sleeve) 13. It is made of a resin material such as a liquid crystal polymer, a phenol resin, PPS, or PEEK, which has excellent rigidity, heat resistance, heat insulation, wear resistance, and the like.

  The fixing film (metal sleeve) 13 is a heat-resistant film having a total thickness of 20 μm or more and 200 μm or less in consideration of strength for satisfying quick start properties and film durability. The base layer is formed of a heat-resistant resin such as polyimide or polyamideimide, or a pure metal or alloy such as SUS, Al, Ni, Cu, or Zn having heat resistance and high thermal conductivity. In the case of the resin base layer, in order to improve the thermal conductivity, a high thermal conductive powder such as BN, alumina, Al or the like may be mixed. Further, in order to prevent offset and to ensure the releasability of the recording material, the surface layer is formed by mixing or independently coating a heat-resistant resin having a good releasability such as a fluororesin or a silicone resin to form a releasable layer. In this example, a fixing film using a SUS sleeve as a base layer was used.

2) Pressure member 20
The pressure member 20 is formed by foaming heat-resistant rubber such as silicon rubber or fluorine rubber or silicon rubber on the outside of a metal core 21 such as SUS (steel use stainless), SUM (steel use machinery), or Al. This is an elastic roller comprising the elastic layer 22. A release layer 23 such as PFA, PTFE, or FEP may be formed thereon.

3) Driving Method and Fixing Method The pressing member 20 is sufficiently pressed by the pressing means (not shown) in the direction of the fixing member 10 to form the fixing nip portion N necessary for heat fixing from both ends in the longitudinal direction. Has been. And it is rotationally driven by the direction of an arrow by the rotational drive not shown through the metal core 21 from the longitudinal direction edge part. As a result, the metal sleeve 13 is driven to rotate on the outside of the stay holder 12 in the direction of the arrow in the figure. Alternatively, a drive roller (not shown) is provided inside the metal sleeve 13, and the metal sleeve 13 is rotated by rotationally driving the drive roller.

  The recording material P carrying the unfixed toner image is conveyed into the fixing nip portion N, and receives heat applied from the heater 11 through the metal sleeve 13 and pressure from the pressure member 20, and the unfixed toner is heated and fixed. Is done.

  Further, a lubricant such as heat resistant grease is interposed between the heater 11 and the metal sleeve 13 in order to improve slidability. Fluorine grease and silicone grease with excellent heat resistance are mainly suitable.

(3) Current detection circuit and limit current control using the circuit FIG. 3 shows a heater 11, a current detection circuit 30 for detecting the amount of current flowing through the heating resistor 11b, and temperature based on the detection. It is a schematic circuit diagram showing the relationship of the heater control part 35 of an engine controller.

  There are several methods for detecting the amount of current. In this embodiment, the current detection method when the phase control method is used is adopted. Specifically, the heater current is converted into a voltage by the current transformer 36 and input to the current detection circuit 30. The input voltage is half-wave rectified by the half-end rectifier circuit unit 31 in the current detection circuit, and then the output is integrated by the integrating circuit unit 32 and integrated with the half-wave rectified output by the differential amplifier circuit unit 33. Output differential voltage of output. Furthermore, the maximum value of the differential voltage is held by the peak hold circuit unit 34, and is finally detected as an output value of the heater input current. The detected value is A / D input as an HCRRT signal to the heater controller 35 of the engine controller.

Using the above-described current detection method, the image forming apparatus performs the following current limit control so that the current flowing through the heater 11 of the heat fixing device does not exceed a specified limit value. FIG. 4 illustrates a specific control flow of the current limit control. At the engine controller, a request for starting power supply to the heater is generated (S501), it performs the energization at a predetermined fixed duty _{D 1} to the heating element 11b (S502). Here, in the case of phase control, the phase angle α is determined in advance corresponding to the energization duty D (%) as shown in Table 1 below, and the engine controller controls the heater 11 based on such a control table. Do.

A current is supplied to the heater at a phase angle α ₁ corresponding to the fixed duty D ₁ .

HCRRT signal sent from the current detection circuit 30 when it is energized at a fixed duty _{D 1} by detecting the current value _{I 1} (S503). Fixed duty D ₁ is in consideration of the variation in the input voltage range and the heating element resistance value that is previously assumed, the set does not exceed the allowable current. That is, the input voltage is maximum, the resistance value is set a fixed duty D ₁ on the assumption that the minimum value.

From the detected current value I ₁ , fixed duty D _1, and preset current value I _limit (hereinafter referred to as limit current), the upper limit power duty D _limit ( Hereinafter, the limit duty is calculated) (S504). When the current value sent from the current detection circuit 30 to the engine controller 35 is an execution value, D _limit is calculated by the following Equation 3.
D _limit = (I _limit / I ₁ ) ² × D ₁ (Equation 3)

As the limit current value I _limit , an allowable current value that can be supplied to the heater 11 is set by subtracting the current supplied to a portion other than the heater 11 from the rated current of the connected commercial power source. .

The engine controller 35 determines the lighting duty to be supplied next time by PI control based on the difference between the preset target temperature and the actual temperature detected by the thermistor 14. However, when the duty calculated here exceeds the limit duty D _limit , the next lighting duty supplies power at D _limit (S505). That is, PI control is performed with a duty equal to or less than the limit duty D _limit .

(4) Heater start-up control of the present embodiment In this embodiment, the heater is heated by the following method using the limit duty D _limit determined from the current detection circuit 30 that detects the current flowing through the heater 11. 11 is started.

Although the value of D _limit varies depending on the value of the input voltage, the power supplied when the heater is turned on with the duty of D _limit is equal even if the voltage is different if the resistance value of the heater is the same. In other words, if the light is lit at the duty of D _limit , the limit current I _limit flows to the heater, and thus power of W _limit = I _limit ² × R (heater resistance value) is input.

Therefore, if the power given by the duty multiplied by a certain ratio to D _limit is set to the power at the time of starting the heater when the heater is started, even if the input voltage is different, the same power can be supplied to the heater. It becomes. More details are as follows.
In this embodiment, the heater temperature control is performed by the following PI control.

D (t) = D _P (t) + D _I (t) = D _P (t−1) + ΔD _P + D _I (t−1) + ΔD _I
= D (t-1) + ΔD _P + ΔD _I = D (t-1) + 2.5 × e (t) + ΔD _I
D (t): Duty to turn on next time D _P (t): P (proportional control) component of lighting duty D _I (t): I (integral control) component of lighting duty e (t): ( Target temperature)-(actual temperature)
ΔD _P : Increase / decrease by 2.5 × e (t) percent every 10 msec.
ΔD _I : + 2.5% if e (t)> 0 ° C. continuously for 500 msec.
If continuously e (t) <0 ° C. -2.5 percent.
There is no increase or decrease at other timings.

Here, at the start of heater start-up (t = 0), in order to enable quick start-up to the fixing target temperature, power given in advance by the I component (D _{It = 0} : referred to as initial offset duty) As the initial offset power (W _{It = 0} ). In this embodiment, the duty D _{It = 0} is determined as shown in Equation 4 below. The initial offset power (I component) is given to reduce overpower by the P component (proportional control component) and suppress overshoot when the actual temperature approaches the target temperature.

D _{It = 0} = D _limit × A (%) (Formula 4)
A: Arbitrary multiplication rate (5) Comparison with conventional example The point that the initial offset duty is determined as described above, which is superior to the conventional heater startup method, will be described.

As a specific example of this embodiment, when the allowable limit current I _limit = 11 A (ampere) and the resistance value of the heater 11 is 9Ω, the limit duty D _limit when the input voltage fluctuates from 100 V to 140 V, at that time Table 2 below shows the initial offset duty D _{It = 0} and initial offset power W _{It = 0} . Here, 0.7 is used as the multiplication factor of the limit duty D _limit (constant A in the above equation). In addition, as a comparative example, initial offset power when the heater start-up control is performed by a conventional method is shown for each input voltage. In the conventional method, the initial offset duty D _{It = 0} is set to a constant 50% regardless of the input voltage.

As apparent from Table 2, by setting the multiplication factor of 70% of the limit duty D _limit to the initial offset duty D _{It = 0} , the initial offset power W _{It = 0} can be made constant even when the input voltage is different. FIG. 5 shows the transition of the heater thermistor temperature when the heater is started up. The solid line (a) representing this embodiment has good followability to the target temperature (dotted line). On the other hand, when the initial offset duty D _{It = 0} is uniformly equal even if the input voltage is different as in the prior art, the offset power is lower than in Table 2 when the input voltage is as low as 100 to 110 V. Even if the temperature reaches the fixing nip, there is a problem that the target temperature is not reached, or the heater temperature drops rapidly at the moment when the recording material enters the nip (shown by a one-dot chain line (c) in FIG. 5). . In addition, when the input voltage is increased to 130 to 140 V, the initial offset power is too high, causing a problem that the heater temperature greatly overshoots, and the offset power does not drop suddenly, so it converges to the target temperature. It takes time (shown in broken line (b) in FIG. 5). In this embodiment, since the power at startup can be controlled to be constant regardless of the input voltage, such a problem can be prevented.

In this embodiment, the initial offset duty D _{It = 0} is determined to be 70% of the limit duty D _{limit for} the following reason. That is, in the case of the heat fixing device provided in this embodiment, the power when printing is performed at a target fixing temperature of 220 ° C. is 850 W to 1050 W. For example, when the type of recording material is rough, such as rough paper, the power during sheet feeding is about 850 W, 950 W is the most commonly used plain paper with a smooth surface, and much power such as cardboard Is 1050 W for paper that requires a large amount of paper. In general, when PI control is performed in a film heating type heat fixing apparatus, the initial offset power (I component: integral control) when the recording material enters the fixing nip is 7 to 8 with respect to the total lighting duty. It has been experimentally found that setting up to occupy about 20% and supplementing the remaining 20-30% with P component (proportional control) provides the best follow-up of actual temperature to target temperature. Therefore, in the case of the present embodiment, assuming 80% of the power during paper feeding of 850 W to 1050 W, the power is 680 W to 840 W, so that the center value is adopted so that the initial offset power is about 760 W. Set to.

(6) Best Embodiment of the Present Example As described in (5) above, even if the initial offset power at startup is always set to 760 W, depending on the conditions of the fixing device and the fixing mode, The offset power may be excessive or insufficient. For example, when comparing the optimal power required for the fuser to start up from the room temperature to the target fixing temperature and the power required for the fuser to start up from a very warm state after continuous printing, the latter is more suitable for fixing. Since the amount of heat stored in the vessel is large and the target fixing temperature can be set low, the electric power is naturally low. In addition, depending on the image forming apparatus, in order to obtain a good fixed image for various recording agents, a plurality of image forming apparatuses can improve the image quality by lowering the fixing target temperature or simultaneously reducing the process speed of the image forming apparatus. In many cases, it has a mode. Even in such a case, if the fixing target temperature changes depending on the mode, it is desirable to change the optimum electric power for each.

Therefore, this is dealt with by changing the multiplication factor A of the limit duty D _limit according to the fixing target temperature and supplying the optimum initial offset power W _{It = 0} according to the target temperature. The method for determining the initial offset power W _{It = 0} used in this embodiment will be described below. When the engine controller 35 receives the print signal, the fixing target temperature at the next printing is T ° C., and the limit duty at that time is D _limit , the initial offset duty D _{It = 0} when the heater is started up is given by To be determined.
D _{It = 0} = D _limit × (αT + β) (Formula 5)

  (ΑT + β) is a term corresponding to the multiplication factor A, and α and β are arbitrary constants. α and β may be determined according to the characteristics of the fixing device so as to obtain an optimum multiplication rate according to the target temperature. In this embodiment, α = 0.005 and β = −0.4. Table 3 below shows the relationship between the multiplication rate A, the initial offset duty, and the initial offset power. Here, the calculation is made for the case where the input power supply voltage is 120 V and the heater resistance value is 9Ω.

  FIG. 6 shows a temperature transition of the heater thermistor when the heater is started up at different target temperatures (220 ° C., 200 ° C., 180 ° C.) according to the initial offset power given in Table 3. As can be seen from FIG. 6, since the optimum initial offset power can be supplied for different target temperatures, temperature control with high accuracy without overshoot or lack of power is possible. In FIG. 6, the heater startup angle (inclination) is different from the target temperature, but in practice, the heater startup inclination is set to be constant regardless of the target temperature. However, since the initial offset power at startup is variable according to Table 3, even if the power is corrected with respect to the target temperature by P control (proportional control), the target temperature is actually lower when it is started. The slope becomes gentle.

  As a method of determining the fixing target temperature when the engine controller 35 receives a print signal, a method of detecting the temperature of the fixing device by some means and determining the target temperature according to the detection result is common. is there. In this embodiment, the temperature of the thermistor 14 that detects the temperature of the heater 11 is simply detected. However, as another method, the temperature is detected by a non-contact type temperature sensor that monitors the temperature of the pressure roller 20. You can go.

  Further, when a small-size paper such as an envelope or B5 size paper is passed in a job immediately before the next print job is started, the temperature of the non-sheet passing area of the pressure roller 20 is raised at the end. In such a case, the heater controller 35 of the engine controller takes measures to reduce the initial offset duty at a constant rate only when it is determined that the edge temperature rise is large based on the immediately preceding paper passing information. And good. A similar countermeasure is, for example, to remove a paper jam in the image forming apparatus, and after the user removes and attaches the process cartridge, the heat fixing device performs an action such as pre-multi-rotation, and the pressure roller 20 is raised. The same measures can be taken even in a warm state. In addition, when the image forming apparatus is equipped with an environment detection sensor for detecting the temperature and humidity of the installation location of the apparatus, depending on the environment, the duty is reduced if the environment is high temperature, If so, it is possible to cope with such as increasing a certain duty.

(7) Other embodiments (Method for determining initial offset power)
In the present embodiment, the duty that gives the desired offset power is determined by multiplying the limit duty D _limit by a predetermined multiplication rate. However, in order to achieve the object of the present invention, a certain current value I _{X is set} to If the current detection circuit 30 can detect the duty D _X necessary for flowing, the duty D _Y corresponding to the amount of current (= power) I _Y to be input when the heater is started can be determined by the following equation (6).
D _Y = (I _Y / I _X ) ² × D _X (Formula 6)

Therefore, it is not always necessary to obtain the limit duty D _limit by the current detection circuit 30, and it may be installed for the purpose of controlling the input current (= power).

(When detecting input power supply voltage)
In the examples described so far, the limit duty D _limit is calculated from Equation 1 by detecting the current value I ₁ when the fixed duty D ₁ is input by the current detection circuit 30. For example, if the image forming apparatus has means for detecting the input power supply voltage instead of the current detection circuit, the detection voltage value V is used.
D _limit = (I _limit × R / V) ² (Expression 7)
R: The relationship between the input voltage V and the limit D _limit shown in Table 2 is obtained from the heater resistance value. Here, R is the resistance value of the heater. Since there is a manufacturing tolerance in the resistance value, the D _limit cannot be determined unless the image forming apparatus has individual resistance value information. When the designed resistance center value is substituted as the resistance value R, and the resistance value is actually smaller than the center value in a certain individual, if the current is applied at the D _limit calculated by the center value, the limit current I Excess current will flow more than _limit . Therefore, if the resistance lower limit on the tolerance is substituted as the value of R, there is no fear that the limit current will be exceeded even if the resistance value becomes larger than the tolerance. However, if the heater control unit 35 of the engine controller has resistance value information of the fixing device heater, D _limit may be calculated using the value as the R value.

(Initial offset power input timing)
In this embodiment, the PI control is performed by supplying offset power corresponding to the fixing target temperature from the start of the heating heater 11. In order to further shorten the start-up time, for example, as shown in FIG. 7, it is possible to start up the heater with the allowable maximum power given by the limit duty D _limit . In this case, if it is lit at the limit duty to the limit of the target temperature, it will overshoot, so a ready temperature is provided before approaching the target temperature, the heater is started up to the ready temperature with the limit duty D _limit , and the ready temperature is reached Thus, a startup method that shifts to PI control may be used. In that case, the initial offset duty determined in accordance with the fixing target temperature can be input at the moment of transition to PI control after reaching the ready temperature (timing (A) in FIG. 7), and the initial offset duty is re-measured from the ready temperature toward the target temperature. It may be put in at the moment of start-up (timing (B) in FIG. 7), or can be selected to be put in at the moment (timing (C) in FIG. 7) when the recording material enters the fixing device.

  A second embodiment of the present invention will be described. Since the image forming apparatus and the heat fixing apparatus representing the present embodiment are the same as those in the first embodiment, description thereof is omitted. In the first embodiment, a target temperature is set according to the warming condition of the fixing device, and an initial offset power corresponding to the target temperature is input when the heater 11 starts up. The initial offset power is controlled so as to be constant regardless of the value of the input power supply voltage. Further, as a method for determining the target temperature, the temperature of the thermistor 14 that detects the heater temperature before the heater is driven is used to determine the degree of warming of the fixing device, and the target temperature is determined based on the result. Alternatively, the temperature of the pressure roller surface is detected by a non-contact type temperature sensor or the like, and the target temperature is determined from the result.

  However, if the target temperature is determined only by these temperature conditions, for example, if the number of recording materials passed in the immediately preceding print job is different, the amount of livestock heat of the entire fixing device is different. Also, depending on the installation environment of the image forming apparatus, the method of cooling the fixing device during standby is different, so it is difficult to determine an accurate target temperature for the next print job. A method of performing control according to various situations is also conceivable, but the operation becomes complicated, leading to a cause of malfunction or erroneous detection. As recording materials used by users diversify and image forming apparatuses become faster, it is becoming increasingly important to determine an appropriate target temperature according to conditions.

  Therefore, in the second embodiment, paying attention to the fact that the image forming apparatus used in the present invention can control the power based on the current detection circuit 30, an appropriate fixing target temperature is set by the following method.

FIG. 8 illustrates a heater startup method in the present embodiment. FIG. 8A shows a profile of the thermistor temperature at startup, and FIG. 8B shows a control flowchart. Note that the steps (S501 to S504) until the limit duty D _limit is determined using the current detection circuit 30 are the same as those in the first embodiment, and thus will not be described. Receiving an engine controller heater controller 35 print signal and starts to drive the heater, up to light the heater first duty multiplied by the constant multiplying factor a D _limit to the heater 11 (referred to as a primary duty = D ₀₎ (S601). Here, D _limit was multiplied by 80%. The primary duty D ₀ is not to put an offset power is turned as the actual input power, PI control is not performed. The heater is started up with the primary duty D ₀ until the ready temperature (= T _RD ) set in advance (S601). Here, T _RD = 190 ° C. is set.

If the fixing device is cooled to room temperature, it takes time to reach the ready temperature (= 190 ° C.), and the total amount of electric power input from the heater ON to the ready temperature is increased. On the other hand, if the fixing device is sufficiently stocked, the time until reaching the ready temperature is shortened. Therefore, the time t _RD from the heater ON to the ready temperature reaching is detected, and the fixing target temperature is determined by judging the warming condition of the fixing device according to the length of t _RD (S602 to S603). Specifically, 200 ° C. The target temperature as long time to ready temperature _{T RD} arrival is less than 3.5 seconds from the heater ON, 210 ° C., 4.5 seconds or more is less than to 3.5 seconds 4.5 seconds If so, it was set to 220 ° C. When the fixing target temperature is determined, appropriate offset power shown in Table 3 of the first embodiment is determined (S604). After reaching the ready temperature, the heater shifts to PI control in order to maintain the temperature, and restarts from the ready temperature to the fixing target temperature at the timing when the recording material reaches the nip. At this timing, the offset power corresponding to the target temperature is input again as an I component (S605). In this embodiment, the primary duty D ₀ is determined to be 80% of the limit duty D _limit . However, if the rise time to the ready temperature is to be shortened, the multiplication rate may be increased. Alternatively, the multiplication rate may be decreased in order to suppress overshoot when the ready temperature is reached.

(Comparison with Example 1)
When the target temperature is determined from the heater temperature before the start of printing by the thermistor 14 that detects the temperature of the heater 11 as in the first embodiment, [1] From the state in which the fixing device is cooled, 3 in the first job. When the sheet is continuously printed, the heater is turned off after the job is finished, and the thermistor temperature is 80 ° C. 10 seconds later. [2] Similarly, 50 sheets are printed continuously in the first job. In the case where the thermistor temperature is 80 ° C. after 40 seconds, the images after fixing were compared when the fixing target temperature in the next print job was set to 200 ° C. in the same manner. The images were compared by comparing the degree of fixing of the image when a halftone image was printed and the printed surface was rubbed by hand. As a result, while [2] was sufficiently fixed and there was no peeling or rubbing, the fixed image of [1] had halftone peeled off and the fixing property was weak.

  In the cases [1] and [2], when the heater start-up control of the second embodiment is performed, in the case [1], the time taken from the heater on to the ready temperature in the second job is 4.1 seconds. Met. On the other hand, in the case [2], the time until the ready temperature is 3.2 seconds. These are due to the difference in the heat storage state of the fixing device. From the flowchart shown in FIG. 8B, the target temperature of the second job is determined to be 210 ° C. in the case [1], and is determined to be 200 ° C. in the case [2]. In each case, when the fixed images after printing were compared, both showed good fixability.

  In this way, it is possible to accurately detect the degree of livestock heat in the fixing device by determining the target temperature according to the time it takes to reach the ready temperature by supplying a certain amount of power, so it is possible to set the target temperature accurately. It becomes.

  Further, depending on the fixing mode selected by the user and the type of the recording material to be passed, the fixing target temperature may be lower than 190 ° C. set as described above. In such a case, the ready temperature may be set low and the multiplication rate of the primary duty that is input before the ready temperature may be set low.

  Furthermore, the same effect can be obtained even if control is performed such that the target temperature is determined based on the temperature reached when constant power is applied for a predetermined time.

(Other embodiment of Example 2)
In the above example, when the heater 11 is started, the target temperature is determined by supplying a constant power to the ready temperature and comparing the time until the ready temperature arrival time. Even when offset power is input and the temperature is raised to the ready temperature by PI control, the target temperature is determined by detecting the total amount of power input to the heater 11 until the ready temperature is reached and comparing the total amount of power. However, the same effect can be obtained. That is, if the PI control is started from the heater ON and the duty for actually turning on the heater is integrated, it is possible to grasp how much the total electric energy has been supplied. As a result, if the fixing device is in a cooled state, the total amount of electric power supplied until the ready temperature is increased. In this case, the target temperature is set high, and conversely, the electric power is supplied until the ready temperature is reached. If the total amount of power is small, the target temperature may be set low.

1 is a schematic sectional view showing an image forming apparatus of the present invention. It is a schematic sectional drawing showing the heat fixing apparatus of this invention. It is a schematic circuit diagram showing the structure of the heater and current detection circuit of this invention. It is a flowchart which calculates the heater lighting limit duty _Dlimit by the electric current detection circuit of this invention. It is a heater temperature profile comparing the heater start-up method of Example 1 and the conventional method. 2 is a heater temperature profile showing the best mode of Example 1. FIG. It is a profile of the heater temperature which shows other embodiment of Example 1. FIG. 6 is a heater temperature profile showing a heater start-up method of Example 2. 6 is a flowchart illustrating a heater start-up method according to the second embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging roller 3 Scanner 4 Developing device 5 Transfer roller 6 Heating and fixing device 7 Cleaning device 8 Top sensor 9 Paper discharge sensor 10 Fixing member 11 Heating heater 11a Ceramic substrate 11b Energizing heating resistance layer 12 Stay holder 13 Fixing film (metal) sleeve)
14 Temperature sensing element (Thermistor)
15 Protective layer 20 Pressure roller (Pressure member)
21 Core Bar 22 Elastic Layer 23 Fluorine Resin Release Layer 30 Current Detection Circuit 31 Half-Wave Rectification Circuit Unit 32 Integration Circuit Unit 33 Differential Amplification Circuit Unit 34 Peak Hold Circuit Unit 35 Heater Control Unit 36 of Engine Controller Current Transformer P Recording Material L Laser light

Claims (9)

In an image forming apparatus that heats and fixes a recording material on which an unfixed toner image is passed through a fixing nip formed by a heating member having a heater and a pressure member pressed against each other,
The image forming apparatus includes: a power control unit that controls power supplied to the heater;
Having temperature detecting means for detecting the temperature of the heater,
An image forming apparatus characterized in that the electric power supplied when the heater is started up is changed according to a fixing target temperature at the time of performing heat fixing of the recording agent. The power control means is current detection means for detecting a current value flowing through the heater,
The image forming apparatus according to claim 1, wherein the electric power input when the heater is started is determined by a duty corresponding to a preset current value in accordance with a fixing target temperature. The power control means is voltage detection means for detecting an input power supply voltage,
The image forming apparatus according to claim 1, wherein the electric power to be input when the heater is started is determined by a duty set in advance according to the detected input voltage value and the fixing target temperature.   The heater is controlled by PI control or PID control, and electric power to be changed when the heater is started up is input as an I control component (integral control component = offset power) of PI control or PID control. Item 4. The image forming apparatus according to any one of Items 1 to 3.   5. The image forming apparatus according to claim 1, wherein the fixing target temperature is determined according to a temperature detected by a heater temperature detecting unit before starting up the heater.   When the heater is started up, a fixed power is first applied to the heater, the heater is started up to a ready temperature (standby temperature) set at a temperature lower than the fixing target temperature, and the rise time until the ready temperature is reached. 5. The image forming apparatus according to claim 1, wherein the fixing target temperature is determined, and electric power corresponding to the fixing target temperature is input at the time of restarting from the ready temperature. The image forming apparatus has means for integrating electric power supplied from a predetermined timing,
When the heater is started up, the heater is started up to a ready temperature (standby temperature) set at a temperature lower than the fixing target temperature, and the fixing target temperature is determined according to the integrated electric energy input from the start of startup to the ready temperature reaching. 5. The image forming apparatus according to claim 1, wherein power corresponding to the fixing target temperature is input at the time of restarting from the ready temperature.   The image forming apparatus according to any one of claims 1 to 7, wherein the heat fixing device does not energize the heater during standby and has a quick start property.   The image forming apparatus according to claim 8, wherein the fixing member of the heat fixing device is a film heating type heat fixing device having a flexible thin fixing film.

Images