Patent Description:
As a fixing device used in an electrophotographic image forming apparatus, for example, <CIT> (<CIT>) discloses the fixing device including a fixing belt as a fixing member, a nip formation member to form a fixing nip, and a heater that is disposed inside a loop of the fixing belt and radiates radiant heat to heat the fixing belt and the nip formation member. The fixing device including the heater disposed inside the loop of the fixing belt adopts a reflector structure including a reflector having high reflectance and covering around the heater to reflect infrared rays that are not directly emitted from the heater to an object to be heated and irradiate the object to be heated with all infrared rays radially radiated from the heater.

However, such a reflector structure has problems, a poor control performance for a target temperature and a fixing failure caused by a temperature drop in the fixing nip because the temperature of the nip formation member changes the amount of heat supplied to the fixing belt in the fixing nip.

<CIT> discloses an image forming apparatus including an endless belt, a heater, a first temperature sensor, a second temperature sensor, and a controller.

<CIT> discloses a fixation device comprising a fixation belt; a pressure roller; belt guide members; halogen heaters; and a reflection member disposed between the halogen heaters and the belt guide members that reflects heat from the halogen heaters to heat the fixation belt.

<CIT> discloses a fixing device that predicts maximum power supply to a fixing heater according to rise time of temperature sensed by a main thermistor or a sub-thermistor, and corrects output power according to the maximum power supply at the time of output of power necessary for stable operation of the fixing device.

<CIT> discloses a fixing device in which temperature control on the surface of a heating member is accurately performed.

<CIT> discloses a fixing device to fix an image to a print medium, including: a pressing roller, which is driven to rotate; a heat transferring unit facing the pressing roller, and contacting the pressing roller at a fixing nip position, to heat to a print medium disposed at the fixing nip position; a heating member that presses an portion of the heat transferring unit against the pressing roller at the fixing nip position; and a heat source that heats a preheating position of the heat transferring unit, and heats the heating member.

The object of the present disclosure is improving the temperature control performance of a rotating member such as a fixing belt and preventing a fixing failure caused by a temperature drop in a fixing nip. In order to achieve the object, there is provided a heating device according to claim <NUM>. Advantageous embodiments are defined by the dependent claims.

Advantageously, the heating device includes a rotating member having an endless form, a heater disposed inside a loop of the rotating member, a pressing member disposed opposite the rotating member, a nip formation member, a first temperature sensor, and a second temperature sensor. The nip formation member contacts an inner circumferential surface of the rotating member to form a nip between the pressing member and the rotating member. The first temperature sensor is configured to detect a temperature of the nip formation member, and the second temperature sensor is configured to detect a temperature of the rotating member.

According to the present disclosure, the temperature control performance of a rotating member is improved, and a fixing failure caused by a temperature drop in the nip is prevented.

The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:.

Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable.

With reference to drawings, a description is given of a fixing device according to an embodiment of the present disclosure and an image forming apparatus such as a laser printer using the fixing device. The laser printer is just an example of the image forming apparatus, and thus the image forming apparatus is not limited to the laser printer. That is, the image forming apparatus can be a copier, a facsimile machine, a printer, a plotter, and a multifunction peripheral having at least two of copying, printing, facsimile transmission, plotting, and scanning capabilities; or an inkjet recording apparatus.

It is to be understood that identical or similar reference characters are given to identical or corresponding parts throughout the drawings, and redundant descriptions are omitted or simplified below. The dimensions, material, shape, and relative position in a description for each constituent component are examples. Unless otherwise specifically described, the scope of the present disclosure is not limited to those.

Although a "sheet medium" as a recording medium is described as a "sheet" in the following embodiments, the "sheet medium" is not limited to the sheet made of paper. Examples of the "sheet" include not only the sheet but also an overhead projector (OHP) transparency, a fabric, a metallic sheet, a plastic film, and a prepreg sheet including carbon fibers previously impregnated with resin.

Examples of the "sheet medium" include all media to which developer or ink can adhere, and so-called recording paper and recording sheets. Examples of the "sheet" include thick paper, a postcard, an envelope, thin paper, coated paper (e.g., coat paper and art paper), and tracing paper, in addition to plain paper.

The term "image forming" used in the following description means not only giving an image having a meaning, such as a character or a figure, to a medium but also giving an arbitrary image having no meaning, such as a pattern, to a medium.

A configuration of the image forming apparatus according to an embodiment is described below.

<FIG> is a schematic view of a laser printer as an example of an image forming apparatus <NUM> including a fixing device <NUM> according to an embodiment of the present disclosure. The image forming apparatus <NUM> includes an image bearer <NUM> such as a photoconductor drum and a photoconductor cleaner <NUM>. The image forming apparatus <NUM> further includes a charging device <NUM> as a charger that uniformly charges the surface of the image bearer, a developing device <NUM> that renders visible an electrostatic latent image on the image bearer, a transfer device TM disposed under the image bearer <NUM>, a discharger, and the like.

An exposure device <NUM> as an image forming device is disposed above the image bearer <NUM>. The exposure device <NUM> performs writing and scanning based on image data, that is to say, irradiates the image bearer <NUM> with laser light Lb emitted by a laser diode based on image data and reflected by a mirror 7a to form an electrostatic latent image on the image bearer <NUM>.

A sheet feeder <NUM> including a tray loaded with sheets P is disposed in a lower portion of the image forming apparatus <NUM>. The sheet feeder <NUM> is configured as a recording-medium supply device and can house a sheaf of many sheets P as recording media. The sheet feeder <NUM> is configured as one unit together with a sheet feeding roller <NUM> as a conveyor for the sheets P.

Downstream from the sheet feeding roller <NUM> in a sheet conveyance direction, a registration roller pair <NUM> as a separation and conveyance means is disposed. The registration roller pair <NUM> temporarily stops the sheet P fed from the sheet feeder <NUM>. Temporarily stopping the sheet P causes slack on the leading-edge side of the sheet P and corrects a skew of the sheet P.

The registration roller pair <NUM> sends the sheet P that contacts the registration roller pair <NUM> and has the slack on the leading-edge side of the sheet P toward a transfer nip N of the transfer device TM at a timing to suitably transfer a toner image on the image bearer <NUM> onto the sheet P. A bias applied at the transfer nip N electrostatically transfers the toner image formed on the image bearer <NUM> onto the sent sheet P at a desired transfer position.

The fixing device <NUM> is disposed downstream from the transfer nip N in the sheet conveyance direction. The fixing device <NUM> includes a fixing belt <NUM> as a rotating member, a halogen heater <NUM> as a heater inside a loop of the fixing belt <NUM>, which is described later, and a pressure roller <NUM> as a pressing member that rotates while contacting the fixing belt <NUM> at a predetermined pressure.

Next, operations of the image forming apparatus <NUM> according to the present embodiment are described below.

The sheet feeding roller <NUM> rotates in response to a sheet feeding signal from a controller of the image forming apparatus <NUM>. The sheet feeding roller <NUM> rotates to separate the uppermost sheet from a sheaf of sheets P loaded in the sheet feeder <NUM> and send the uppermost sheet out to a sheet feeding path.

When the leading edge of the sheet P sent by the sheet feeding roller <NUM> reaches a nip of the registration roller pair <NUM>, the sheet P forms slack and temporarily stops. The registration roller pair <NUM> corrects the front-end skew of the sheet P and rotates in synchronization with an optimum timing to transfer a toner image on the image bearer <NUM> onto the sheet P.

The charging device <NUM> uniformly charges the surface of the image bearer <NUM> to high potential. The exposure device <NUM> irradiates the surface of the image bearer <NUM> with the laser light Lb based on the image data and reflected by the mirror 7a.

The surface of the image bearer <NUM> irradiated with the laser light Lb has the electrostatic latent image due to a drop in the potential of the irradiated portion. The developing device <NUM> includes a developer bearer 5a bearing a developer including toner and transfers unused black toner supplied from a toner bottle to the surface portion of the image bearer <NUM> having the electrostatic latent image, through the developer bearer 5a.

The image bearer <NUM> to which the toner has been transferred forms (develops) a toner image on the surface of the image bearer <NUM>. The transfer device TM transfers the toner image formed on the image bearer <NUM> onto the sheet P.

A cleaning blade 3a in the photoconductor cleaner <NUM> removes the residual toner adhering to the surface of the image bearer <NUM> after a transfer process. The removed residual toner is collected to a waste toner container.

The sheet P bearing the toner image is conveyed to the fixing device <NUM>. The sheet P conveyed to the fixing device <NUM> is sandwiched by the fixing belt <NUM> and the pressure roller <NUM>. Then, heating and pressing fixes the unfixed toner image onto the sheet P. The sheet P fixed the toner image is sent out from the fixing device <NUM>.

Next, a description is given of the fixing device <NUM> according to the present embodiment of the present disclosure.

The fixing device <NUM> is an embodiment of a heating device according to the present disclosure. In addition to the fixing device, the heating device according to the present disclosure is also applicable to a dryer to dry ink applied to the sheet in an inkjet type image forming apparatus and a coating device (a laminator) that heats, under pressure, a film serving as a covering member onto the surface of the sheet such as paper conveyed by a belt.

As illustrated in <FIG>, and <FIG>, the fixing device <NUM> includes a thin fixing belt <NUM> having a low thermal capacity and a pressure roller <NUM>. In addition, the fixing device <NUM> includes a first temperature sensor <NUM> as a first temperature detector, a second temperature sensor <NUM> as a second temperature detector, and a third temperature sensor <NUM> as a third temperature detector, as described in detail later. The first temperature sensor <NUM> detects a temperature TN of a nip formation member, the second temperature sensor <NUM> detects a temperature of the fixing belt <NUM> inside a range of a sheet conveyance span in the longitudinal direction of the fixing belt <NUM>, and the third temperature sensor <NUM> detects a temperature of the fixing belt <NUM> outside the range of the sheet conveyance span, that is a non-sheet conveyance span in the longitudinal direction of the fixing belt <NUM>.

The fixing belt <NUM> includes, for example, a tubular base made of polyimide (PI), the tubular base having an outer diameter of <NUM> and a thickness of from <NUM> to <NUM>. The fixing belt <NUM> further includes a release layer serving as an outermost surface layer. The release layer is made of fluororesin, such as tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) and polytetrafluoroethylene (PTFE), and has a thickness of from <NUM> to <NUM> to enhance durability of the fixing belt <NUM> and facilitate separation of the sheet P from the fixing belt <NUM>. An elastic layer made of rubber having a thickness of from <NUM> to <NUM> may be provided between the base and the release layer.

The base of the fixing belt <NUM> may be made of heat-resistant resin such as polyetheretherketone (PEEK) or metal such as nickel (Ni) or stainless steel (Stainless Used Steel, SUS), instead of polyimide. The inner circumferential surface of the fixing belt <NUM> may be coated with polyimide or polytetrafluoroethylene (PTFE) as a slide layer.

The pressure roller <NUM> having, for example, an outer diameter of <NUM>, includes a solid iron cored bar <NUM>, an elastic layer <NUM> on the surface of the cored bar <NUM>, and a release layer <NUM> formed on the outside of the elastic layer <NUM>. The elastic layer <NUM> is made of silicone rubber and has a thickness of <NUM>, for example.

Preferably, the release layer <NUM> is formed by a fluororesin layer having, for example, a thickness of approximately <NUM> on the surface of the elastic layer <NUM> to improve releasability. The pressure roller <NUM> is pressed against the fixing belt <NUM> by a biasing member.

Next, stays <NUM> and the nip formation member <NUM> are described below.

The stays <NUM> as support members and the nip formation member <NUM> extend axially inside the loop of the fixing belt <NUM>. The stays <NUM> are a pair of metallic members that are made of stainless steel or the like, extend in parallel, face each other, and each have an I-shaped cross-section. Both side plates of the fixing device <NUM> support both end portions of the stays <NUM>. The pair of stays <NUM> reliably receive the pressing force of the pressure roller <NUM> acting on the nip formation member <NUM> and stably form a nip SN having a uniform width over an axial direction of the pressure roller <NUM> that is a longitudinal direction of the pressure roller <NUM>. A plurality of types of sheets having different widths in the axial direction of the fixing belt <NUM> can pass through the nip SN. A stay as a support member may be one unit formed by connecting end portions of the pair of metallic members.

The nip formation member <NUM> is a plate bridged between the pair of stays <NUM> and supported by the stays <NUM>. Both end portions of the nip formation member <NUM> in the sheet conveyance direction are each bent in an L shape, and the L-shaped bent portions 380a and 380b are located upstream and downstream in the sheet conveyance direction with respect to the pair of stays <NUM>.

In <FIG>, the bent portion located upstream in the sheet conveyance direction is the bent portion 380b on the right side in <FIG>, and the bent portion located downstream in the sheet conveyance direction is the bent portion 380a on the left side in <FIG>. The first temperature sensor <NUM>, which is described later, is sandwiched between the bent portion 380a located downstream in the sheet conveyance direction and the lower end portion of the stay <NUM>.

The nip formation member <NUM> transfers heat in an axial direction that is a lateral direction in <FIG> in a short time to uniformize temperature of the fixing belt <NUM> in a width direction of the fixing belt <NUM>. The nip formation member <NUM> is made of a thin plate having high thermal conductivity such as copper (<NUM> W / mK), aluminum (<NUM> W / mK) or silver to smoothly transfer the heat in the width direction. In consideration of cost, processing, and the like, the nip formation member <NUM> is preferably made of a thin copper plate.

The inner circumferential surface of the fixing belt <NUM> slides over the bottom surface of the nip formation member <NUM> that is a nip formation surface. In order to improve the abrasion resistance and the slidability of the nip formation surface, the nip formation surface may be treated with alumite or coated with fluororesin material. Additionally, a lubricant such as fluorine grease may be applied to the nip formation surface to ensure slidability over time.

In <FIG>, the nip formation surface of the nip formation member <NUM> is planar. Alternatively, the nip formation surface may define a recess or other shape. For example, a recessed nip formation surface directs a leading edge of the sheet P toward the pressure roller <NUM> as the sheet is ejected from the fixing nip SN, thereby facilitating separation of the sheet P from the fixing belt <NUM> and preventing a sheet jam.

A separator may be disposed downstream from the fixing device <NUM> in the sheet conveyance direction to separate the sheet P from the fixing belt <NUM>. In addition, a pressurization assembly may be disposed to press the pressure roller <NUM> against the fixing belt <NUM> and release pressure exerted by the pressure roller <NUM> to the fixing belt <NUM>.

Next, reflectors <NUM> and the halogen heater361 in the present embodiment are described.

As illustrated in <FIG> and <FIG>, each reflector <NUM> is assembled to each of facing surfaces of the pair of stays <NUM>. The halogen heater <NUM> as the heater is arranged at a center position between the right reflector <NUM> and the left reflector <NUM> and extends in the axial direction of the fixing belt <NUM>.

Each of the reflectors <NUM> is made of a stainless steel plate or the like. Both end portions of the reflector <NUM>, that is, an upper end portion 370a and a lower end portion 370b are bent toward outside and engaged with an upper end portion and a lower end portion of each of the pair of stays <NUM>. The reflector <NUM> is configured to increase heating efficiency for the fixing belt <NUM> and the nip formation member <NUM> and to reduce wasteful energy consumption caused by heating the stay <NUM> with radiant heat from the halogen heater <NUM>.

The surface of the reflector <NUM> facing the halogen heater <NUM> is treated with mirror finish or the like to increase reflectance. In the present embodiment, reflectance is measured using the spectrophotometer that is the ultraviolet visible infrared spectrophotometer UH4150 manufactured by Hitachi High-Tech Science Corporation in which the incident angle is set <NUM>°.

In general, the color temperature of the halogen heater varies depending on the application. The color temperature of the heater for the fixing device is about <NUM>. The reflectance of the reflector <NUM> used in the present embodiment is preferably <NUM>% or more with wavelengths of high emission intensity in the halogen heater <NUM>, that is, specifically the wavelengths of <NUM> to <NUM> and more preferably <NUM>% or more with the wavelengths of <NUM> to <NUM>. Alternatively, instead of the reflector <NUM>, a heater-side face of the stay <NUM> disposed opposite the halogen heater <NUM> may be insulated or given a mirror finish to obtain the same effects. In this case, the reflectance of the stay <NUM> subjected to the mirror finishing is preferably similar to the reflectance of the reflector <NUM>.

Both side plates of the fixing device <NUM> support both ends of the halogen heater <NUM>. The above-described fixing device <NUM> uses one halogen heater <NUM>. Alternatively, the fixing device may use a plurality of halogen heaters <NUM> that are arranged in parallel and have different light emitting sections corresponding to a number of widths of sheets passing through the nip SN. Selecting the plurality of halogen heaters <NUM> in accordance with the width of the sheet and turning on the selected halogen heater <NUM> can effectively prevent overheating the non-sheet conveyance span of the fixing belt <NUM>.

A center portion of the reflector <NUM> in a vertical direction is away from one of the pair of stays <NUM> and bulges inwardly in a curved convex shape toward the halogen heater <NUM>. This curved inward convex surface of the reflector <NUM> reflects infrared rays radiated in the left-right direction from the halogen heater <NUM> upward. A radiant light receiving surface of the fixing belt <NUM> is irradiated with the infrared rays reflected upward by the reflector <NUM>.

The radiant light receiving surface of the fixing belt <NUM> located above the halogen heater <NUM> is directly irradiated with the infrared rays radiated upward from the halogen heater <NUM>. In the present embodiment, infrared rays are mainly described as radiant light, but all light emitted from the halogen heater <NUM> as a heater is equivalent to radiant light.

On the other hand, the inner surface of the nip formation member <NUM> located under the halogen heater <NUM> is directly irradiated with the infrared rays radiated downward from the halogen heater <NUM>. Additionally, the reflector <NUM> reflects infrared rays radiated in the left-right direction and an oblique downward direction from the halogen heater <NUM> toward the oblique downward direction. The reflector <NUM> irradiates the inner surface of the nip formation member <NUM> with the infrared rays reflected toward the oblique downward direction.

As described above, an upper portion of the fixing belt <NUM> and the nip formation member <NUM> under the halogen heater <NUM> are directly irradiated with the infrared rays radiated around the halogen heater <NUM>. Additionally, the fixing belt <NUM> and the nip formation member <NUM> are also efficiently irradiated with the infrared rays reflected by the reflector <NUM>, and the fixing belt <NUM> at the nip SN is efficiently heated to a target temperature Tc.

Next, operations of the fixing device <NUM> are described.

In <FIG>, when the sheet P is conveyed in a direction indicated by a horizontal arrow and passes through the nip SN, the sheet P is heated between the fixing belt <NUM> and the pressure roller <NUM> so that the toner image is fixed to the sheet P. At this time, the radiant heat from the halogen heater <NUM> heats the fixing belt <NUM> through the nip formation member <NUM>.

The reflector <NUM> and the stay <NUM> in the present embodiment open toward the nip formation member <NUM>. The above-described configuration can give a first heat transfer path and a second heat transfer path. In the first transfer heat path, the halogen heater <NUM> directly irradiates the nip formation member <NUM> with the radiant light and indirectly heats the fixing belt <NUM> via the nip formation member <NUM>. In the second heat transfer path, the halogen heater <NUM> directly irradiates and heats the fixing belt <NUM> with the radiant light.

A structure of a conventional reflector around the halogen heater is generally an inverted U-shaped reflector structure that covers upper portion of the halogen heater. In contrast, in the present embodiment, as described above, the upper portion of the halogen heater <NUM> is opened, and each of the fixing belt <NUM> and the nip formation member <NUM> is irradiated with the infrared rays radiated from the halogen heater <NUM> and the infrared rays reflected by the left and right reflectors <NUM>.

A configuration of the present embodiment as illustrated in <FIG> including the nip formation member <NUM> and the fixing belt <NUM> which are separately irradiated with the infrared rays and a controller <NUM> which controls temperatures of the nip formation member <NUM> and the fixing belt <NUM> based on values detected by the first temperature sensor <NUM> and the second temperature sensor <NUM> has a better performance of temperature control about the fixing belt <NUM> and more advantageous in reduction of wasteful power consumption, which is described below, than a conventional configuration in which the halogen heater <NUM> concentratively irradiates the nip formation member with the infrared rays of the halogen heater <NUM>.

Since the radiant light reflected by the reflector <NUM> is less likely to be absorbed again by the sealing body glass of the halogen heater, the present embodiment can give an advantage, that is, improvement of the heat transfer efficiency with respect to the fixing belt <NUM> and the nip formation member <NUM>. Additionally, the controller <NUM> described below improves the performance of the temperature control of the fixing belt <NUM> and reduces the fixing failure due to the temperature drop in the nip and unnecessary power consumption.

Next, the temperature Sensors are described.

Values detected by the three temperature sensors <NUM> to <NUM> described above are input to the controller <NUM> in <FIG>. The controller <NUM> controls an amount of alternating current (AC) power supplied to the halogen heater <NUM> by, for example, phase control. The controller <NUM> is arranged outside the fixing device <NUM> which is one embodiment of the heating device, for example, inside the controller of the image forming apparatus <NUM>. The controller <NUM> may also configured as a dedicated product of the fixing device <NUM> independent of the controller of the image forming apparatus <NUM>.

The first temperature sensor <NUM> detects a temperature TN of the nip formation member <NUM>. A compression coil spring or the like fixed to the stay <NUM> on the outlet side of the nip SN presses the first temperature sensor <NUM> against the inner surface of the bent portion 380a on the outlet side of the nip formation member <NUM> with a constant pressure. A contact position between the first temperature sensor <NUM> and the nip formation member <NUM> is disposed outside the nip SN and downstream from the nip SN in the sheet conveyance direction.

The second temperature sensor <NUM> is arranged upstream from the radiant light receiving surface in the rotation direction of the fixing belt <NUM> to face the outer circumferential surface of the fixing belt <NUM>. That is, the second temperature sensor <NUM> is arranged to face the halogen heater <NUM> via the stay <NUM> and the reflector <NUM> and detect the temperature of the outer circumferential surface or the inner circumferential surface of the fixing belt <NUM>, that is, a fixing belt temperature TB. Both the first temperature sensor <NUM> and the second temperature sensor <NUM> are disposed at the substantially center position in the axial direction (longitudinal direction) of the fixing belt <NUM> in the present embodiment but may be disposed in a range corresponding to a sheet conveyance span of the sheet having the smallest width in the axial direction of the fixing belt <NUM> and downstream from the nip SN in the rotation direction of the fixing belt <NUM>.

The second temperature sensor <NUM> is disposed opposite the outer circumferential surface of the fixing belt <NUM> and downstream from the first temperature sensor <NUM> in the rotation direction of the fixing belt <NUM>. Both the first temperature sensor <NUM> and the second temperature sensor <NUM> are disposed such that a position at which the first temperature sensor <NUM> detects the temperature of the nip formation member <NUM> and a position at which the second temperature sensor <NUM> detects the fixing belt temperature are close to each other in the circumferential direction of the fixing belt <NUM>.

Next, the controller <NUM> is described.

The controller <NUM> in <FIG> controls power supplied to the halogen heater <NUM> by, for example, a proportional integral derivation (PID) control method so that the fixing belt temperature TB changes in the vicinity of a control target temperature Tc determined according to the machine state. The PID control method is a kind of feedback control method and control of an input value based on three factors, that is, a deviation between an output value and a target value, the integral, and the differential.

The second temperature sensor <NUM> detects the fixing belt temperature TB at a position closer to an outlet of the nip SN than the radiant light receiving surface, and the controller <NUM> calculates a difference between the fixing belt temperature TB and the nip formation member temperature TN, that is, (TN - TB) to compare the fixing belt temperature TB with the nip formation member temperature TN. Based on the difference (TN - TB), the controller <NUM> can accurately estimate an amount of heat that transfers from the nip formation member <NUM> to the fixing belt <NUM> in the nip SN and, based on the estimation, determine the amount of power to be supplied to the halogen heater <NUM>.

Specifically, the first temperature sensor <NUM> and the second temperature sensor <NUM> are arranged in an outlet side of the nip SN, that is, upstream in the rotation direction of the fixing belt <NUM> from an upstream end of the radiant light receiving surface. A distance D (mm) from the upstream end of the radiant light receiving surface to each of the first temperature sensor <NUM> and the second temperature sensor <NUM> is equal to or larger than a product of a control cycle S (seconds) of the halogen heater <NUM> and a rotation speed V (mm / second) of the fixing belt, that is, (S × V ≤ D). The above-described configuration enables the controller <NUM> to immediately determine a control parameter of the halogen heater <NUM>, that is, a parameter of an amount of heat supplied to the fixing belt <NUM> and the nip formation member <NUM> at a next timing based on the detected fixing belt temperature TB.

In the present embodiment, both the first temperature sensor <NUM> and the second temperature sensor <NUM> are arranged in the above-described range (S × V ≤ D), but arranging at least one of the first temperature sensor <NUM> and the second temperature sensor <NUM> in the above-described range can provide the effect to some extent. That is, arranging at least one of the first temperature sensor <NUM> and the second temperature sensor <NUM> in the above-described range enables the controller <NUM> to immediately determine the parameter of an amount of heat supplied to the fixing belt <NUM> and the nip formation member <NUM> at a next timing based on the detected fixing belt temperature TB to some extent.

Disposing both the first temperature sensor <NUM> and the second temperature sensor <NUM> at positions close to the outlet of the nip SN enables accurate detection of the temperature difference between the fixing belt <NUM> and the nip formation member <NUM> at the nip SN, that is, (TN - TB) and accurate detection of the amount of heat transfer between the fixing belt <NUM> and the nip formation member <NUM>. As a result, the controller <NUM> can accurately determine the power to be supplied to the halogen heater <NUM>. That is, correction of the power supplied to the halogen heater <NUM> based on the determined power enables accurately keeping the fixing belt temperature TB in the vicinity of the control target temperature Tc, improving fixing property, and reducing the unnecessary power consumption.

As illustrated in <FIG> and <FIG>, the first temperature sensor <NUM> and the second temperature sensor <NUM> are arranged inside the range corresponding to the sheet conveyance span of the sheet having the smallest width, which is used in the image forming apparatus <NUM>, in the axial direction of the fixing belt <NUM>. In the above-described configuration, the controller <NUM> can accurately estimate the necessary power because temperature rise that occurs outside the sheet conveyance span when small size sheets pass through the fixing device does not affect the temperatures detected by the first temperature sensor <NUM> and the second temperature sensor <NUM>.

On the other hand, arranging the first temperature sensor <NUM> and the second temperature sensor <NUM> inside the range corresponding to the sheet conveyance span of the sheet having the smallest width may cause a risk that the temperature outside the sheet conveyance span exceeds the upper limit temperature of the fixing device <NUM>, causing melting and damage of the fixing device <NUM>. Therefore, as illustrated in <FIG> and <FIG>, in addition to the first temperature sensor <NUM> and the second temperature sensor <NUM>, a third temperature sensor <NUM> is arranged outside a sheet conveyance span of the sheet having the largest width, which is used in the image forming apparatus <NUM>, in the axial direction of the fixing belt <NUM> and, in the circumferential direction of the fixing belt <NUM>, in an area radiated by the radiant light from the halogen heater <NUM>.

The third temperature sensor <NUM> is arranged at a position at which the temperature of the fixing belt <NUM> is most likely to rise, that is, the position on the radiant light receiving surface outside the sheet conveyance span of the sheet having the largest width in the axial direction of the fixing belt <NUM>. The third temperature sensor <NUM> can prevent damage to the fixing device <NUM> due to excessive temperature rise of the fixing belt <NUM>. That is, when the temperature detected by the third temperature sensor <NUM> exceeds the upper limit temperature, the controller <NUM> performs a control that reduces the temperature detected by the third temperature sensor <NUM> to be equal to or lower than the upper limit temperature, such as interruption of a printing operation, productivity reduction, and lowering the control target temperature Tc.

The controller <NUM> determines the power supplied to the halogen heater <NUM> using the PID control as described above. However, the temperature TN of the nip formation member <NUM> changes the amount of heat transfer between the fixing belt <NUM> and the nip formation member <NUM> in the nip SN.

Therefore, the controller <NUM> corrects the power based on the difference (TN - TB) between temperatures detected by the first temperature sensor <NUM> and the second temperature sensor <NUM>. The correction of the power may be calculated by using, for example, a relational expression between the temperature difference and the power obtained in advance or corrected by switching coefficients of the PID control or the control target temperatures Tc when the difference (TN - TB) between temperatures detected by the first temperature sensor <NUM> and the second temperature sensor <NUM> exceeds a threshold temperature.

With reference to <FIG>, a description is given of effects of the PID control for the fixing belt temperature TB.

<FIG> are graphs each illustrating a change of the fixing temperature TB and a change of the temperature of the nip formation member TN when the controller <NUM> performs the PID control to control the power supplied to the halogen heater <NUM> based on the fixing belt temperature TB detected by the second temperature sensor <NUM>. <FIG> is a graph illustrating the change of the fixing temperature TB and the change of the temperature of the nip formation member TN when the controller <NUM> performs the PID control to control the power supplied to the halogen heater <NUM> based on the difference (TN - TB) between temperatures detected by the first temperature sensor <NUM> and the second temperature sensor <NUM>.

The sensitivity (responsiveness) to the temperature rise of the fixing belt <NUM> when a predetermined power is supplied to the halogen heater <NUM> depends on the temperature TN of the nip formation member <NUM>. That is, since the nip formation member <NUM> that is sufficiently warmed absorbs only a little amount of radiant heat from the halogen heater <NUM>, increase of the radiant heat from the halogen heater <NUM> immediately causes temperature rise of the fixing belt <NUM>.

In contrast, since the nip formation member <NUM> that is not sufficiently warmed absorbs a lot of radiant heat from the halogen heater <NUM>, sensitivity of the temperature rise of the fixing belt <NUM> to the increase of the radiant heat from the halogen heater <NUM> becomes low.

As described above, the temperature TN of the nip formation member <NUM> changes the sensitivity of the temperature rise of the fixing belt <NUM>. <FIG> is a graph illustrating the fixing belt temperature TB and the temperature TN of the nip formation member <NUM> under the PID control using a temperature control parameter suitably set for the nip formation member <NUM> sufficiently warmed. As illustrated in <FIG>, relatively small power is supplied to the halogen heater <NUM> even when the fixing belt temperature TB is low. This is because the above-described temperature control parameter of the PID control is determined based on the state in which increase of the radiant heat from the halogen heater <NUM> immediately causes temperature rise of the fixing belt <NUM> as described above.

In this case, if the nip formation member <NUM> is sufficiently warmed, the fixing belt temperature TB is properly controlled. However, even if the nip formation member <NUM> is not sufficiently warmed, the controller <NUM> performs the PID control using the temperature control parameter suitably set for the nip formation member <NUM> sufficiently warmed to control the power supplied to the halogen heater <NUM>, and the relatively small power is supplied to the halogen heater <NUM> even when the fixing belt temperature TB drops.

The nip formation member <NUM> that is not sufficiently warmed absorbs radiant heat stored in the fixing belt <NUM>. As a result, the temperature TB of the fixing belt <NUM> does not rise easily. Therefore, as illustrated in a left side of the graph in <FIG>, the fixing belt temperature TB largely drops, and fixing failure may occur.

On the contrary, <FIG> is a graph illustrating the fixing belt temperature TB and the temperature TN of the nip formation member <NUM> under the PID control using a temperature control parameter suitably set for the nip formation member <NUM> is not sufficiently warmed. As illustrated in <FIG>, relatively large power is supplied to the halogen heater <NUM> when the fixing belt temperature TB is low because the sensitivity of the temperature rise of the fixing belt <NUM> to the increase of the radiant heat from the halogen heater <NUM> is low.

In this case, if the nip formation member <NUM> is not sufficiently warmed, the fixing belt temperature TB is properly controlled. However, even when the nip formation member <NUM> is sufficiently warmed, the controller <NUM> performs the PID control using the temperature control parameter suitably set for the nip formation member <NUM> that is not sufficiently warmed to control the power supplied to the halogen heater <NUM>, and too much large power is supplied to the halogen heater <NUM> when the fixing belt temperature TB drops.

Since the nip formation member <NUM> sufficiently warmed does not absorb much radiant heat, most of the radiant heat of the halogen heater <NUM> is transferred to the fixing belt <NUM>, and the fixing belt temperature TB rises at a stroke. As a result, large temperature ripple of the fixing belt temperature TB occurs as illustrated in a right side of the graph in <FIG>. The large temperature ripple results in wasteful power consumption.

As described above, the PID control of the halogen heater <NUM> using the fixing belt temperature TB detected by the second temperature sensor <NUM> is unstable, and the performance of the temperature control is poor because actual temperatures of the fixing belt <NUM> are not estimated. In the embodiment of the present disclosure, using the difference (TN - TB) between temperatures detected by two temperature sensors, that is, the first temperature sensor <NUM> and the second temperature sensor <NUM>, improves the performance of the temperature control of the fixing belt temperature TB. <FIG> is a graph illustrating the fixing belt temperature TB and the temperature TN of the nip formation member <NUM> under the PID control using the difference (TN - TB).

That is, based on the difference (TN - TB) between temperatures detected by two temperature sensors, that is, the first temperature sensor <NUM> and the second temperature sensor <NUM>, the controller <NUM> switches the parameters such as the coefficients of the PID control when the difference between the nip formation member temperature TN and the fixing belt temperature TB becomes small, which is the center position of the graph in <FIG>.

Specifically, in the left side of the center position of the graph in <FIG>, like the left side of the graph in <FIG>, the temperature control parameter of the PID control is suitably set for the nip formation member <NUM> not sufficiently warmed. Additionally, in the right side of the center position of the graph in <FIG>, like the right side of the graph in <FIG>, the temperature control parameter of the PID control is suitably set for the nip formation member <NUM> sufficiently warmed.

The above temperature control can reduce the wasteful power consumption and avoid the occurrence of the fixing failure regardless of the nip formation member temperature TN. In the configuration including a plurality of heat transfer paths from the halogen heater <NUM>, the above-described configuration can accurately keep the temperature of the fixing belt <NUM> in the vicinity of the control target temperature Tc, reduce the wasteful power consumption, and avoid the occurrence of the fixing failure.

Finally, a description is given of a method whether the controller <NUM> controls the power supplied to the halogen heater <NUM> based on the temperatures detected by the first temperature sensor <NUM> and the second temperature sensor <NUM>.

If inputting dummy signals relating to detected temperature values to a signal line extending from the first temperature sensor <NUM> to the controller <NUM> causes any change of a temperature control state of the halogen heater <NUM> such as a change of wave of measured temperatures, the controller <NUM> controls the power supplied to the halogen heater <NUM> based on data detected by the first temperature sensor <NUM>.

Similarly, if inputting dummy signals relating to detected temperature values to a signal line extending from the second temperature sensor <NUM> to the controller <NUM> causes any change of the temperature control state of the halogen heater <NUM> such as the change of wave of measured temperatures, the controller <NUM> controls the power supplied to the halogen heater <NUM> based on data detected by the second temperature sensor <NUM>.

In addition, if the temperature control state of the halogen heater <NUM> such as the change of wave of measured temperatures when dummy signals relating to detected temperature values are input to the signal lines extending from the first temperature sensor <NUM> and the second temperature sensor <NUM> to the controller <NUM> is different from the temperature control state of the halogen heater <NUM> such as the change of wave of measured temperatures when dummy signals relating to detected temperature values are input to either the signal line extending from the first temperature sensor <NUM> to the controller <NUM> or the signal line extending from the second temperature sensor <NUM> to the controller <NUM>, the controller <NUM> controls the power supplied to the halogen heater <NUM> based on data detected by both the first temperature sensor <NUM> and the second temperature sensor <NUM>. The above-described method can confirm whether the controller <NUM> controls the power supplied to the halogen heater <NUM> based on the temperatures detected by the first temperature sensor <NUM> and the second temperature sensor <NUM>.

The present disclosure has been described above on the basis of the embodiments, but the present disclosure is not limited to the embodiments. Needless to say, various alterations can be made in the scope of the technical idea described in the scope of the claims. For example, although one halogen heater <NUM> is used in the above-described embodiments, of course, two or three or more halogen heaters may be used. Alternatively, instead of the halogen heater, for example, induction heating (IH) may be employed as the heater.

Claim 1:
A heating device (<NUM>) comprising:
a rotating member (<NUM>) having an endless form;
a heater (<NUM>) disposed inside a loop of the rotating member (<NUM>);
a pressing member (<NUM>) disposed opposite the rotating member (<NUM>);
a nip formation member (<NUM>) contacting an inner circumferential surface of the rotating member (<NUM>) to form a nip (SN) between the pressing member (<NUM>) and the rotating member (<NUM>);
a first temperature sensor (<NUM>) configured to detect a temperature of the nip formation member (<NUM>); and
a second temperature sensor (<NUM>) configured to detect a temperature of the rotating member (<NUM>), wherein
the heater radiates light;
the rotating member has a light receiving surface which is irradiated with the light from the heater;
the nip formation member has a light receiving surface which is irradiated with the light from the heater,
wherein the first temperature sensor (<NUM>) is disposed downstream from the nip (SN) in a rotation direction of the rotating member (<NUM>),
wherein the second temperature sensor (<NUM>) is disposed downstream from the first temperature sensor (<NUM>) in the rotation direction of the rotating member (<NUM>), and
wherein the heater (<NUM>) is controlled based on a difference in values detected by the first temperature sensor (<NUM>) and the second temperature sensor (<NUM>).