Fixing device including detecting mechanism which detects infrared rays radiated from heating body and image forming apparatus including same

A fixing device includes a heating body, a pressuring body, a heat source and a detecting mechanism. The detecting mechanism is not in contact with the heating body and including an infrared detecting element which detects infrared rays radiated from an outer circumferential face of the heating body. A longitudinal direction of the heating body is a second direction which crosses a first direction as a conveying direction of a recording medium. A detected area is arranged on the outer circumferential face of the heating body so that the infrared rays radiated from the detected area is detected by the infrared detecting element. The detecting mechanism is arranged in a posture inclined to another posture facing the outer circumferential face of the heating body so that a width in the second direction of the detected area is wider than a width in the first direction of the detected area.

INCORPORATION BY REFERENCE

This application is based on and claims the benefit of priority from Japanese Patent application No. 2015-003826 filed on Jan. 13, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to a fixing device configured to fix a toner image onto a recording medium and an image forming apparatus including the fixing device.

Conventionally, an electrographic image forming apparatus, such as a copying machine or a printer, includes a fixing device configured to fix a toner image onto a recording medium, such as a sheet.

For example, there is a fixing device including a heating body, a pressuring body configured to come into pressure contact with the heating body so as to form a fixing nip, a heat source configured to heat the heating body and a detecting mechanism configured to be not in contact with the heating body.

There is a case that the above-mentioned detecting mechanism includes an infrared detecting element, such as a thermopile. In such a case, the infrared detecting element detects infrared rays radiated from an outer circumferential face of the heating body, and a temperature of the heating body is calculated on a basis of a detecting value thereof or the like.

In the fixing device with above-mentioned configuration, temperature distribution of the heat source is normally not uniform, so that temperature distribution of the heating body heated by the heat source is not uniform, either.

Accordingly, there is fear that detecting accuracy of the infrared detecting element is deteriorated, because a detecting value of the infrared detecting element in a case where the infrared detecting element detects infrared rays radiated from the hottest part in the outer circumferential face of the heating body is greatly different from a detecting value of the infrared detecting element in another case where the infrared detecting element detects infrared rays radiated from the coldest part in the outer circumferential face of the heating body.

Further, in the fixing device with above-mentioned configuration, there is fear that it becomes difficult to dispose the detecting mechanism in a case where the detecting mechanism is greatly protruded toward an outer diameter side of the heating body.

SUMMARY

In accordance with an embodiment of the present disclosure, a fixing device includes a heating body, a pressuring body, a heat source and a detecting mechanism. The heating body is configured to be rotatable. The pressuring body is configured to be rotatable and to come into pressure contact with the heating body so as to forma fixing nip. The heat source is configured to heat the heating body. The detecting mechanism is configured to be not in contact with the heating body and including an infrared detecting element which detects infrared rays radiated from an outer circumferential face of the heating body. A longitudinal direction of the heating body is a second direction which crosses a first direction as a conveying direction of a recording medium. A detected area is arranged on the outer circumferential face of the heating body so that the infrared rays radiated from the detected area is detected by the infrared detecting element. The detecting mechanism is arranged in a posture inclined to another posture facing the outer circumferential face of the heating body so that a width in the second direction of the detected area is wider than a width in the first direction of the detected area.

In accordance with an embodiment of the present disclosure, an image forming apparatus includes the above-mentioned fixing device.

DETAILED DESCRIPTION

First, with reference toFIG. 1, the entire structure of a printer1(an image forming apparatus) will be described. Arrows Fr, Rr, L, R, U and Lo appropriately added to each of the drawings indicate the front side, rear side, left side, right side, upper side and lower side of the printer1, respectively.

The printer1includes a box-formed printer main body2. In a lower part of the printer main body2, a sheet feeding cartridge3configured to store sheets (recording medium) is installed and, on the top surface of the printer main body2, a sheet ejecting tray4is mounted. On the top surface of the printer main body2, an upper cover5is openably/closably attached at a right side of the sheet ejecting tray4and, below the upper cover5, a toner container6is installed.

In an upper part of the printer main body2, an exposure device7composed of a laser scanning unit (LSU) is installed below the sheet ejecting tray4. Below the exposure device7, an image forming unit8is installed. In the image forming unit8, a photosensitive drum10as an image carrier is rotatably installed. Around the photosensitive drum10, a charger11, a development device12, a transfer roller13and a cleaning device14are located along a rotating direction (refer to arrow X inFIG. 1) of the photosensitive drum10.

Inside the printer main body2, a sheet conveying path15is arranged. At an upper stream end of the conveying path15, a sheet feeder16is positioned. At an intermediate stream part of the conveying path15, a transferring unit17constructed of the photosensitive drum10and transfer roller13is positioned. At a lower stream part of the conveying path15, a fixing device18is positioned. At a lower stream end of the conveying path15, a sheet ejecting unit19is positioned. Below the conveying path15, an inversion path20for duplex printing is arranged.

Next, the operation of forming an image by the printer1having such a configuration will be described.

When the power is supplied to the printer1, various parameters are initialized and initial determination, such as temperature determination of the fixing device18, is carried out. Subsequently, in the printer1, when image data is inputted and a printing start is directed from a computer or the like connected with the printer1, image forming operation is carried out as follows.

First, the surface of the photosensitive drum10is electrically charged by the charger11. Then, exposure corresponding to the image data on the photosensitive drum10is carried out by a laser (refer to two-dot chain line P inFIG. 1) from the exposure device7, thereby forming an electrostatic latent image on the surface of the photosensitive drum10. Subsequently, the electrostatic latent image is developed to a toner image with a toner (a developer) in the development device12.

On the other hand, a sheet fed from the sheet feeding cartridge3by the sheet feeder16is conveyed to the transferring unit17in a suitable timing for the above-mentioned image forming operation, and then, the toner image on the photosensitive drum10is transferred onto the sheet in the transferring unit17. The sheet with the transferred toner image is conveyed to a lower stream on the conveying path15to go forward to the fixing device18, and then, the toner image is fixed on the sheet in the fixing device18. The sheet with the fixed toner image is ejected from the sheet ejecting unit19to the sheet ejecting tray4. Toner remained on the photosensitive drum10is collected by the cleaning device14.

Next, the fixing device18will be described with reference toFIGS. 2 to 5. Arrow I inFIGS. 2 and 5indicates an inside in a front and rear direction, and arrow O inFIGS. 2 and 5indicates an outside in the front and rear direction. Arrow Y inFIG. 3indicates a sheet conveying direction.

As shown inFIGS. 2 and 3, the fixing device18includes a fixing belt21(heating body), a pressuring roller22(pressuring body) arranged at a lower side (outer diameter side) of the fixing belt21, a pressing member23arranged at an inner diameter side of the fixing belt21, a supporting member24arranged at the inner diameter side of the fixing belt21and at an upper side of the pressing member23, a halogen lamp25(heat source) arranged at the inner diameter side of the fixing belt21and at the upper side of the supporting member24, a detecting mechanism26arranged at an upper front side (outer diameter side) of the fixing belt21, and a heat insulating member27arranged between the fixing belt21and the detecting mechanism26.

A longitudinal direction of the fixing belt21is the front and rear direction (second direction) which is orthogonal to (crosses) a left and right direction (first direction) as the sheet conveying direction. The fixing belt21is formed in a nearly cylindrical shape. The fixing belt21has flexibility, and is endless in a circumferential direction. The fixing belt21is rotatably provided. At both front and rear end parts of the fixing belt21, caps30are attached.

In an outer circumferential face of the fixing belt21, a center area R1and end part areas R2formed at both front and rear sides of the center area R1(closer to an outside in the front and rear direction than the center area R1) are arranged. The center area R1is an area through which first size sheets (e.g. maximum size sheets) and second size sheets (e.g. minimum size sheets) pass. Each end part area R2is an area through which each first size sheet passes and each second size sheet does not pass.

The fixing belt21includes, for example, a base material layer, an elastic layer provided around this base material layer and a release layer covering this elastic layer. The base material layer of the fixing belt21is formed by nickel electroforming, for example. A thickness of the base material layer of the fixing belt21is 35 μm, for example. The elastic layer of the fixing belt21is made of a silicon rubber, for example. A thickness of the elastic layer of the fixing belt21is 200 μm, for example. The release layer of the fixing belt21is made of a PFA, for example. A thickness of the release layer of the fixing belt21is 30 μm, for example. In addition, in each drawing, each layer (the base layer, the elastic layer and the release layer) of the fixing belt21is not distinguished in particular.

A longitudinal direction of the pressuring roller22is the front and rear direction. The pressuring roller22is formed in a nearly columnar shape. The pressuring roller22comes into contact with the fixing belt21so as to forma fixing nip N between the fixing belt21and the pressuring roller22. The pressuring roller22is rotatably provided. To a rear end part of the pressuring roller22, a drive gear31is fixed. A temperature sensor32faces a left side part of the pressuring roller22with an interval. The temperature sensor32is composed of, for example, a thermistor.

For example, the pressuring roller22includes a columnar core material33, an elastic layer34provided around this core material33and a release layer (not shown) covering this elastic layer34. The core material33of the pressuring roller22is made of a metal, such as an aluminum, for example. The elastic layer34of the pressuring roller22is made of a silicon sponge rubber, for example. A thickness of the elastic layer34of the pressuring roller22is 3.5 mm, for example. The release layer (not shown) of the pressuring roller22is made of a PFA tube, for example. A thickness of the release layer of the pressuring roller22is 50 μm, for example.

A longitudinal direction of the pressing member23is the front and rear direction. The pressing member23is made of a heat resistant resin, such as an LCP (Liquid Crystal Polymer). A lower face of the pressing member23presses the fixing belt21toward a lower side (a side of the pressuring roller22).

A longitudinal direction of the supporting member24is the front and rear direction. The supporting member24is made of a metal, such as a SUS, and is formed in a square cylindrical shape. An upper face of the pressing member23comes into contact with a lower face of the supporting member24.

A longitudinal direction of the halogen lamp25is the front and rear direction. The halogen lamp25is arranged at a nearly center part of an internal space of the fixing belt21.

As shown inFIG. 4Aand other figures, the halogen lamp25is provided with a heat generating area H. The heat generating area H includes a filament of a coil shape. A width in the front and rear direction of the heat generating area H is 300 mm, for example. In the heat generating area H, a plurality of bright spot parts36and a plurality of dark spot parts37provided between a plurality of the bright spot parts36are formed, because the filament is not wound uniformly. Each dark spot part37has a lower filament winding density than each bright spot part36, and therefore has a smaller heat generating value than each bright spot part36. Widths in the front and rear direction of a plurality of bright spot parts36is not uniform. The width in the front and rear direction of a bright spot part36which has the smallest widths in the front and rear direction among a plurality of the bright spot parts36will be referred to as a “minimum bright spot width Wmin”. In the present embodiment, the minimum bright spot width Wmin is 10 mm.

As shown inFIG. 5and other figures, the detecting mechanism26is not in contact with the fixing belt21. The detecting mechanism26is arranged closer to a front side (an outside in the front and rear direction) than the heat generating area H of the halogen lamp25.

The detecting mechanism26is housed in a housing41which composes a part of a main body frame (a frame of the printer main body2). At a rear lower part of the housing41, a cutout part42is provided. In addition,FIG. 3shows only a lower part of the housing41, and does not show parts other than the lower part of the housing41.

As shown inFIGS. 4B and 5, the detecting mechanism26includes a substrate44attached to the lower part of the housing41, a main body45of a cylindrical shape fixed to the substrate44, a thermopile46(infrared detecting element) housed in a nearly center part of the main body45, a lens47housed in a rear end part of the main body45and a thermistor48(temperature detecting element) provided at a front end side of the main body45.

The thermopile46of the detecting mechanism26has a function of detecting infrared rays I1(hereinafter, simply referred to as the “infrared rays I1”) diagonally radiated from the center area R1of the fixing belt21, and, in the center area R1of the fixing belt21, a detected area D is arranged so that the infrared rays I1radiated from the detected area D are detected by the thermopile46.

The detecting mechanism26is arranged in a posture inclined to a posture (see a two-dot chain line inFIG. 5) facing an outer circumferential face of the fixing belt21. In the present embodiment, an inclined angle a of the detecting mechanism26with respect to the posture facing the outer circumferential face of the fixing belt21(also corresponding to an inclined angle of the infrared rays I1with respect to infrared rays Iv vertically radiated from the detected area D of the fixing belt21) is 70°.

The detecting mechanism26is arranged in the posture inclined to the posture facing the outer circumferential face of the fixing belt21as described above, and therefore the detected area D of the fixing belt21is formed in an elliptical shape, not a precise circular shape. Hence, a width W2in the front and rear direction of the detected area D is wider than a width W1in the left and right direction of the detected area D. In addition, a point M inFIG. 5indicates a part of the detected area D which corresponds to a center part in the front and rear direction of the heat generating area H of the halogen lamp25with regard to a position in the forward and backward direction.

The lens47of the detecting mechanism26includes a function of focusing the infrared rays I1on the thermopile46. In other words, the lens47includes a function of narrowing a viewing angle β of the thermopile46. Thus, it is possible to prevent the thermopile46from detecting infrared rays radiated from members other than the fixing belt21. In the present embodiment, the viewing angle β of the thermopile46is5° , a distance d from the detecting mechanism26to the outer circumferential face of the fixing belt21is 50 mm, and the width W2in the front and rear direction of the detected area D of the fixing belt21is 44 mm. Thus, the width W2(44 mm) in the front and rear direction of the detected area D is not less than four times the minimum bright spot width Wmin (10 mm). The thermistor48of the detecting mechanism26is a temperature sensor for compensating for a temperature, and has a function of detecting an atmospheric temperature of the detecting mechanism26.

The heat insulating member27(seeFIGS. 2, 3 and 5and other figures) composes a part of the fixing frame (the frame of the fixing device18). In this regard, in addition to the heat insulating member27, the fixing frame includes a part which covers a lower side of the pressuring roller22and parts which cover both front and rear sides of the fixing belt21and the pressuring roller22. However, each drawing shows only the heat insulating member27of the fixing frame and does not show parts other than the heat insulating member27of the fixing frame.

The heat insulating member27includes an upper wall part50which covers an upper side of the fixing belt21, and a left wall part51and a right wall part52which are bent downward from both left and right end parts of the upper wall part50and cover both left and right sides of the fixing belt21. In addition,FIGS. 2 and 5do not show the right wall part52.

The upper wall part50of the heat insulating member27is elongated along the front and rear direction. At a front part of the upper wall part50, an inclined part53is bent up toward an upper side (a side of the detecting mechanism26). The inclined part53is inclined with respect to the front and rear direction. At a front end side of the inclined part53, at a part corresponding to an optical path of the infrared rays I1, an opening54is formed such that the heat insulating member27does not insulate the infrared rays I1.

Between the detecting mechanism26and the housing41, and the upper wall part50of the heat insulating member27, a flow passage55of cooling air is arranged so that the flow passage55is spaced away from the opening54at an interval G. The flow passage55is arranged along the left and right direction (the direction which is orthogonal to (crosses) the front and rear direction). At an upstream end part (a right end part in the present embodiment) of the flow passage55, a fan56is arranged, and cooling air provided from the fan56to the flow passage55flows in the flow passage55along the left and right direction.

As shown inFIGS. 2 and 3and other figures, parts except for the detecting mechanism26, the housing41and the fan56of the fixing device18compose a fixing unit49. The fixing unit49is detachable from the printer main body2.

Next, a control system of the fixing device18will be described with reference toFIG. 6.

The fixing device18includes a control part61. The control part61is connected with a storage part62configured as a storage device, such as a ROM or a RAM, and the control part61is configured to control each part of the fixing device18on the basis of a control program or control data stored in the storage part62.

The control part61is connected to a drive source63configured as a motor or the like, and the drive source63is connected to the pressuring roller22via the drive gear31. Further, on the basis of a signal from the control part61, the drive source63rotates the pressuring roller22.

The control part61is connected to the halogen lamp25. Further, when power is supplied to the halogen lamp25on the basis of a signal from the control part61, the halogen lamp25is lighted up, and the heat generating area H of the halogen lamp25generates heat.

The control part61is connected to the thermopile46of the detecting mechanism26, and, when the thermopile46detects the infrared rays I1, the thermopile46outputs a detecting value to the control part61. The control part61is connected to the thermistor48of the detecting mechanism and, when the thermistor48detects an atmospheric temperature of the detecting mechanism26, the thermistor48outputs a detecting value to the control part61.

The control part61is connected to the temperature sensor32and, when the temperature sensor32detects a temperature of the pressuring roller22, the temperature sensor32outputs a detecting value to the control part61.

When a toner image is fixed onto a sheet in the fixing device18configured as described above, on the basis of a signal from the control part61, the drive source63rotates the pressuring roller22(see arrow A inFIG. 3) . When the pressuring roller22is rotated in this way, the fixing belt21which comes into pressure contact with the pressuring roller22is driven to be rotated in a direction opposite to a direction of the pressuring roller22(see arrow B inFIG. 3).

Further, when a toner image is fixed onto a sheet, on the basis of a signal from the control part61, the halogen lamp25is lighted up. When the halogen lamp25is lighted up in this way, the heat generating area H of the halogen lamp25generates the heat so as to heat the fixing belt21. When a sheet on which an unfixed toner image has been formed passes through the fixing nip N in this state, the toner mage is heated and melts and the toner image is fixed onto the sheet.

When the fixing belt21is heated as described above, the infrared rays I1are radiated from the detected area D of the fixing belt21. This infrared rays I1pass through the opening54of the heat insulating member27, are focused by the lens47of the detecting mechanism26and reach the thermopile46of the detecting mechanism26. When the infrared rays I1reach the thermopile46as described above, the thermopile46detects the infrared rays I1and outputs a detecting value to the control part61. Further, the thermistor48of the detecting mechanism26detects the atmospheric temperature of the detecting mechanism26, and outputs a detecting value to the control part61. The control part61calculates a temperature of the fixing belt21on the basis of the detecting value of the thermopile46and the detecting value of the thermistor38. More specifically, the temperature of the fixing belt21is calculated from the following equation.
Vout=A(Tb4−Ts4)Vout: detecting value of thermopile46A: proportionality constantTb: temperature of fixing belt21(K)Ts: detecting value of thermistor48

Compared to a case where only a temperature detecting member (e.g. thermistor) which comes into contact with the outer circumferential face of the fixing belt21is used as the detecting mechanism26, by applying such a configuration, it is possible to enhance a responsivity of a calculated temperature of the fixing belt21to an actual temperature of the fixing belt21, and support precise control. In the fixing device18whose energy saving performance is considered in particular, it is possible to realize low power upon a standby time of the fixing device18and activate the fixing device18up to a fixing temperature (a temperature at which a toner image can be fixed onto a sheet) at a high speed upon use of the fixing device18.

Further, there is a concern that, when the temperature detecting member which comes into contact with the outer circumferential face of the fixing belt21detects the temperature of the fixing belt21, the temperature detecting member damages the outer circumferential face of the fixing belt21. When the outer circumferential face of the fixing belt21is damaged in this way, an exchange of the fixing belt21or an exchange of the entire fixing device18is required, and causes a rise in running cost of the fixing device18. By contrast with this, in the present embodiment, the detecting mechanism26is not in contact with the fixing belt21. Hence, there is no concern that the detecting mechanism26damages the outer circumferential face of the fixing belt21, and it is possible to reduce a frequency to exchange the fixing belt21or the entire fixing device18and make the running cost of the fixing device18low.

Further, when the above contact-type temperature detecting member is used, the temperature detecting member is generally attached to the fixing unit49. Hence, even when the temperature detecting member can be still used upon an exchange of the fixing unit49, the temperature detecting member cannot not help being discarded together with the fixing unit49, which is not preferable in terms of cost and resource saving. By contrast with this, in the present embodiment, the non-contact detecting mechanism26is attached to the housing41which forms a part of the frame of the printer main body2. Consequently, it is not necessary to discard the detecting mechanism26together with the fixing unit49upon an exchange of the fixing unit49, and it is possible to reduce cost and save resources.

Meanwhile, even when the above non-contact detecting mechanism26is used, there are the two following tasks.

First, the first task in case where the non-contact detecting mechanism26is used will be described. Similar to the present embodiment, to calculate the temperature of the fixing belt21on the basis of two detecting values (the detecting value of the thermopile46and the detecting value of the thermistor48), an arithmetic operation amplifier circuit is necessary. Normally, taking a noise resistance into account, this arithmetic operation amplifier circuit is mounted on the main body45of the detecting mechanism26. The arithmetic operation amplifier circuit has a low heat resistant temperature (normally about 100° C.), and therefore it is necessary to prevent a rise in the temperature of the detecting mechanism26caused by heat from the fixing belt21.

Hence, in the present embodiment, the heat insulating member27is arranged between the fixing belt21and the detecting mechanism26and the flow passage55of cooling air is arranged between the detecting mechanism26and the heat insulating member27, and the cooling air is provided from the fan56to this flow passage55. By applying such a configuration, it is possible to prevent the rise in the temperature of the detecting mechanism26.

However, there is a concern that, when the cooling air provided from the fan56flows to the vicinity of the opening54of the heat insulating member27, viscosity of air attracts a heat near the fixing belt21to a space at the side of the detecting mechanism26via the opening54(the space above the heat insulating member27in the present embodiment), and the fixing belt21is unnecessarily cooled and energy saving performance of the fixing device18lowers.

Hence, in the present embodiment, the flow passage55of cooling air is spaced away from the opening54at the interval G. By applying such a configuration, the cooling air hardly flows near the opening54, and therefore it is possible to prevent a heat near the fixing belt21from being attracted to the space at the side of the detecting mechanism26via the opening54, and avoid that the fixing belt21is unnecessarily cooled. According to this, it is possible to enhance energy saving performance of the fixing device18.

Next, the second task in case where the non-contact detecting mechanism26is used will be described. In the present embodiment, the halogen lamp25is used as a heat source which heats the fixing belt21, and, in the heat generating area H of this halogen lamp25, a plurality of bright spot parts36and a plurality of dark spot parts37are formed because the filament is not wound uniformly. Hence, in the heat generating area H of the halogen lamp25, a temperature distribution is not uniform in the front and rear direction, and, in the fixing belt21heated by the heat generating area H of the halogen lamp25, the temperature distribution is not uniform in the front and rear direction, either. More specifically, as shown inFIG. 4C, at a part which overlaps each bright spot part36of the halogen lamp25with regard to a position in the front and rear direction, a temperature of the outer circumferential face of the fixing belt21is high, and, at a part which overlaps each dark spot part37of the halogen lamp25with regard to a position in the front and rear direction, the temperature of the outer circumferential face of the fixing belt21is low. Hence, there is fear that detecting accuracy of the thermopile46is deteriorated, because a detecting value of the thermopile46in a case where the thermopile46detects infrared rays radiated from the hottest part in the outer circumferential face of the fixing belt21is greatly different from a detecting value of the thermopile46in another case where the thermopile46detects infrared rays radiated from the coldest part in the outer circumferential face of the fixing belt21.

When the viewing angle β of the thermopile46is simply widened to prevent the detecting accuracy of the thermopile46from being deteriorated, there is a concern that the thermopile46detects the infrared rays radiated from members other than the fixing belt21. Further, there is a concern that, when the viewing angle β of the thermopile46is widened, according to this, the opening54of the heat insulating member27needs to be enlarged, heat near the fixing belt21is likely to escape to the space at the side of the detecting mechanism26via the opening54and energy saving performance of the fixing device18lowers.

Hence, in the present embodiment, by providing the detecting mechanism26in a posture inclined to the posture facing the outer circumferential face of the fixing belt21, the width in the left and right direction of the detected area D of the fixing belt21is made wider than the width in the front and rear direction of the detected area D of the fixing belt21. By applying such a configuration, when the temperature distribution in the front and rear direction of the fixing belt21is not uniform because the temperature distribution in the front and rear direction of the heat generating area H of the halogen lamp25is not uniform, it is possible to minimize an influence which this non-uniformity has on a detecting value of the thermopile46. Consequently, it is possible to enhance the detecting accuracy of the thermopile46.

Further, the detecting mechanism26is arranged in the posture inclined to the posture facing the outer circumferential face of the fixing belt21as described above, so that it is possible to sufficiently secure a distance of an optical path of the infrared rays I1from the outer circumferential face of the fixing belt21to the detecting mechanism26, and prevent the detecting mechanism26from greatly protruding toward the outer diameter side of the fixing belt21(e.g. the upper side of the fixing belt21). Consequently, the detecting mechanism26hardly interferes other members, and can simplify a layout of the detecting mechanism26.

Further, in the present embodiment, the width W2(44mm) in the front and rear direction of the detected area D of the fixing belt21is not less than four times the minimum bright spot width Wmin (10 mm). By applying such a configuration, it is possible to further enhance the detecting accuracy of the thermopile46. In addition, to enhance the detecting accuracy of the thermopile46, the width W2in the front and rear direction of the detected area D is preferably the minimum bright spot width Wmin or more, and is more preferably not less than twice the minimum bright spot width Wmin.

Further, the detecting mechanism26is arranged closer to the front side (the outside in the front and rear direction) than the heat generating area H of the halogen lamp25. By applying such a configuration, it is possible to prevent an influence of the heat of the fixing belt21on the detecting mechanism26from causing a rise in the temperature of the detecting mechanism26. According to this, it is possible to set a low heat resistant temperature of the thermistor48, and heat resistant parts are not necessary. Further, it is possible to prevent the atmospheric temperature of the detecting mechanism26from changing and, consequently, enhance the detecting accuracy of the thermistor48.

Furthermore, the detecting mechanism26is housed in the housing41and, consequently, can prevent an influence, such as cooling air flowing in the flow passage55, from changing the atmospheric temperature of the detecting mechanism26. Consequently, it is possible to further enhance the detecting accuracy of the thermistor48.

In the present embodiment, the detecting mechanism26has only the single thermopile46(infrared detecting element). Meanwhile, in other different embodiments, as shown inFIGS. 7 to 9, the detecting mechanism26may include a plurality of thermopiles71and72(infrared detecting elements), and a plurality of these thermopiles71and72may include a first thermopile71(first infrared detecting element) which detects the infrared rays I1radiated from the detected area D1formed in the center area R1of the fixing belt21, and a second thermopile72(second infrared detecting element) which detects the infrared rays12radiated from the detected area D2formed in the end part area R2of the fixing belt21. By applying such a configuration, the single detecting mechanism26can detect both of infrared rays radiated from the center area R1of the fixing belt21, and infrared rays radiated from the end part area R2of the fixing belt21.

In the present embodiment, the fixing device18includes one halogen lamp25. In other different embodiments, as shown inFIG. 9, the fixing device18may include a plurality (for example, 2) of the halogen lamps25(heat source). In this case, for example, a heat generating area H of one halogen lamp25may correspond to the center area R1of the fixing belt21and a heat generating area H of another halogen lamp25may correspond to the end part area R2of the fixing belt21, and a plurality of the halogen lamps25may be selectively lighted up according to detecting values of the first and second thermopiles71and72.

In the present embodiment, each end part area R2of the fixing belt21is an area through which each first size sheet (for example, a maximum size sheet) passes and each second size sheet (for example, a minimum size sheet) does not pass. In other different embodiments, each end part area R2of the fixing belt21may be an area through which no sheet passes.

In the present embodiment, the fixing belt21is used as a heating body. In other different embodiments, a fixing roller may be used as a heating body.

In the present embodiment, the halogen lamp25is used as a heat source. In other different embodiments, a ceramic heater or the like may be used as a heat source.

In the present embodiment, the configuration of the present disclosure is applied to the printer1. Meanwhile, in other different embodiments, the configuration of the disclosure may be applied to another image forming apparatus, such as a copying machine, a facsimile or a multifunction peripheral.

While the present disclosure has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present disclosure.