Image forming device

Abstract

To appropriately select a conveyance mode.SOLUTION: An image forming apparatus comprises: a storage unit; an image forming unit; a fixing unit that has a fixing rotating body and a heater; a temperature sensor that detects the temperature of the fixing unit; a conveying unit that conveys a sheet from the storage unit toward the image forming unit; and a control unit. The control unit executes selection processing of selecting one conveyance mode from a plurality of conveyance modes different in timing to start conveyance of a sheet. In the selection processing, after the heater starts heating, the control unit executes first selection processing of selecting the one conveyance mode based on a temperature H detected by the temperature sensor from the start of rotation of the fixing rotating body until a predetermined period T elapses. In the first selection processing, the control unit compares a first temperature H1 with an initial temperature Hb being the temperature H detected by the temperature sensor at the reception of a print instruction, sets the predetermined period T to a first time T1 when Hb<H1, and sets the predetermined period T to a second time T2 longer than the first time T1 when Hb≥H1.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus capable of selecting a sheet transport mode. [Background technology]

[0002] Conventionally, there is known an image forming apparatus equipped with a control unit that selects a transport mode based on the temperature rise gradient (hereinafter also referred to as "temperature gradient") of the fixing nip (see Patent Document 1). Specifically, in this technology, when a print command is received, the control unit starts energizing the heater while keeping the pressure roller stopped, and starts rotating the pressure roller after a predetermined heating stop period has elapsed. Thereafter, the control unit selects a transport mode based on the temperature gradient of the fixing nip from when the pressure roller starts to rotate until a predetermined mode determination period has elapsed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-223862 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the conventional technology, since the mode determination period is a fixed value, there is a risk that an appropriate transport mode may not be selected depending on the temperature of the fixing nip when a print command is received.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming apparatus that can appropriately select a transport mode. [Means for solving the problem]

[0006] In order to solve the above problem, the image forming apparatus of the present invention comprises a storage section for storing sheets, an image forming section for forming a toner image on the sheet, a fixing section having a fixing rotor and a heater for heating the fixing rotor and fixing the toner image to the sheet, a temperature sensor for detecting the temperature of the fixing section, a conveying section for conveying the sheet from the storage section toward the image forming section, and a control section. The control unit is capable of executing a selection process for selecting one transport mode from a plurality of transport modes that differ in timing at which the transport unit starts transporting the sheet. In the selection process, the control unit can execute a first selection process that selects one of the transport modes based on the temperature detected by the temperature sensor from the start of heating by the heater until a predetermined period of time has elapsed since the start of rotation of the fixing rotor. In the first selection process, the control unit determines whether the initial temperature, which is the temperature detected by the temperature sensor when a print instruction is received, is equal to or higher than a first temperature, and if the initial temperature is lower than the first temperature, sets the specified period to a first hour, and if the initial temperature is equal to or higher than the first temperature, sets the specified period to a second hour that is longer than the first hour.

[0007] According to this configuration, the length of the predetermined period is changed depending on the temperature of the fixing unit when a print command is received, so that the sheet transport mode can be appropriately selected.

[0008] The control unit may execute the first selection process based on a temperature difference between a detected temperature at the start of rotation of the fixing rotator and a maximum detected temperature during the predetermined period.

[0009] According to this configuration, the temperature gradient of the fixing unit over a predetermined period can be calculated with higher accuracy than when determining the temperature gradient at a single point in time, and the sheet transport mode can be appropriately selected.

[0010] The control unit may also execute the first selection process based on a temperature difference between a minimum temperature and a maximum temperature detected during the predetermined period.

[0011] According to this configuration, the temperature gradient of the fixing unit over a predetermined period can be calculated with higher accuracy than when determining the temperature gradient at a single point in time, and the sheet transport mode can be appropriately selected.

[0012] In addition, when the initial temperature is lower than a rotation start temperature, the control unit may perform heating using the heater while stopping the fixing rotor until the detected temperature becomes equal to or higher than the rotation start temperature, and start rotation of the fixing rotor when the detected temperature becomes equal to or higher than the rotation start temperature.

[0013] According to this configuration, when the temperature of the fixing unit is below the rotation start temperature, the fixing rotor is stopped and heated by a heater, allowing the fixing unit to accumulate sufficient heat before starting to rotate the fixing rotor.

[0014] Further, the plurality of transport modes may include a first transport mode in which, after receiving a print instruction and starting rotation of the fixing rotor, the transport unit starts transporting the sheet without waiting for the specified period to elapse, and the control unit may select the first transport mode without performing the first selection process when the initial temperature is equal to or higher than a second temperature that is higher than the first temperature, and may perform the first selection process when the initial temperature is lower than the second temperature.

[0015] According to this configuration, when the temperature of the fixing unit is equal to or higher than the second temperature, the fixing unit has stored sufficient heat, so that the sheet can be started to be transported without waiting for a predetermined period of time to elapse, thereby improving printing efficiency.

[0016] The control unit may also control the power supply to the heater based on an operation amount including a sum of a proportional term proportional to a deviation between a target temperature and a detected temperature and a differential term proportional to a differential value of the deviation, and calculate the operation amount by setting the differential term to 0 during the predetermined period.

[0017] According to this configuration, by calculating the manipulated variable with the differential term set to 0 during a specified period, the variation in the heating capacity of the heater can be correctly evaluated, and therefore the sheet transport mode can be appropriately selected to correspond to the variation in the heating capacity of the heater.

[0018] The control unit may be capable of energizing the heater based on a duty ratio, which is an amount of electricity per unit time, and may set the duty ratio to 100% during the predetermined period.

[0019] According to this configuration, by energizing the heater at 100% duty from the start of rotation of the fixing rotor until a predetermined period has elapsed, variations in the heater's heating capacity can be correctly evaluated, and the sheet transport mode can be appropriately selected to correspond to variations in the heater's heating capacity.

[0020] Furthermore, when the initial temperature is equal to or higher than a third temperature that is higher than the first temperature, the control unit may set a target temperature for heating by the heater higher than when the initial temperature is lower than the third temperature.

[0021] According to this configuration, the target temperature is increased when the initial temperature is equal to or higher than the third temperature, so that it is possible to prevent the output of the heater from decreasing during the predetermined period. [Effects of the Invention]

[0022] According to the present invention, the transport mode can be appropriately selected. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a cross-sectional view showing a laser printer according to an embodiment. [Figure 2] FIG. [Figure 3] FIG. 10 is a diagram showing a table for determining a predetermined period based on an initial temperature. [Figure 4] FIG. 10 is a diagram showing a table showing the relationship between the outside air temperature, the initial temperature, the temperature gradient, and the transport mode. [Figure 5] 10 is a flowchart showing a heating process. [Figure 6] 10 is a flowchart showing a part of a selection process. [Figure 7] 10 is a flowchart showing another part of the selection process. [Figure 8] 10 is a graph showing temperature changes when heating is performed using three types of heaters with different heating capacities when the initial temperature is lower than a first temperature. [Figure 9] 10 is a graph showing temperature changes when heating is performed using three types of heaters with different heating capacities when the initial temperature is equal to or higher than a first temperature. DETAILED DESCRIPTION OF THE INVENTION

[0024] Next, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. As shown in Figure 1, a laser printer 1, which is an example of an image forming device, includes a main body housing 2, a supply unit 3 that supplies a sheet S, an image forming unit 4 that forms a toner image on the sheet S, and a fixing unit 7 that fixes the toner image to the sheet S.

[0025] The supply unit 3 includes a storage section 31 that stores the sheet S, and a conveying section 32 that conveys the sheet S from the storage section 31 toward the image forming unit 4. The conveying section 32 includes a pickup roller 33, a conveying roller 34, and a registration roller 35. The sheet S in the storage section 31 is picked up by the pickup roller 33, and then supplied to the image forming unit 4 via the conveying roller 34 and the registration roller 35.

[0026] The image forming unit 4 includes an exposure unit 5 and a process cartridge 6 . The exposure unit 5 includes a laser emitting unit (not shown), a polygon mirror (depicted by omitted reference numerals), a lens, a reflecting mirror, etc. The exposure unit 5 exposes the surface of the photosensitive drum 61 to laser light emitted from the laser emitting unit.

[0027] The process cartridge 6 can be attached to and detached from the main body housing 2 through an opening formed when a front cover 21 provided on the main body housing 2 is opened. The process cartridge 6 includes a photosensitive drum 61, a charger 62, a transfer roller 63, a developing roller 64, and a toner storage section 65.

[0028] In this process cartridge 6, the surface of the photosensitive drum 61 is charged by the charger 62 and then exposed by the exposure unit 5, thereby forming an electrostatic latent image on the photosensitive drum 61. Toner in the toner storage unit 65 is supplied to the electrostatic latent image on the photosensitive drum 61 by the development roller 64.

[0029] As a result, a toner image is formed on the photosensitive drum 61. Thereafter, the sheet S is transported between the photosensitive drum 61 and the transfer roller 63, whereby the toner image on the photosensitive drum 61 is transferred onto the sheet S.

[0030] The toner image transferred onto the sheet S is fixed onto the sheet S by passing through the fixing unit 7. The sheet S with the fixed toner image is discharged onto the discharge tray 22 by the conveying rollers 23 and 24.

[0031] As shown in FIG. 2, the fixing section 7 includes a fixing belt 71 as an example of a fixing rotor, a heating unit 72 that heats the fixing belt 71, a pressure roller 73 that sandwiches the fixing belt 71 between the heating unit 72, and a temperature sensor SE1.

[0032] The fixing belt 71 is an endless belt. The heating unit 72 is disposed inside the fixing belt 71.

[0033] The heating unit 72 includes a heater 72A, a nip plate 72B, a reflecting member 72C, and a stay 72D.

[0034] Heater 72A generates heat when energized, more specifically, radiant heat, to heat nip plate 72B and fixing belt 71. For example, a halogen lamp can be used as heater 72A. Nip plate 72B is a plate-shaped member that receives radiant heat from heater 72 A. Nip plate 72B sandwiches fixing belt 71 between itself and pressure roller 73.

[0035] The reflecting member 72C is a member that reflects the radiant heat from the heater 72A toward the nip plate 72B. The stay 72D supports the nip plate 72B via a reflecting member 72C.

[0036] The pressure roller 73 sandwiches the fixing belt 71 between itself and the heating unit 72, thereby forming a nip N between itself and the fixing belt 71. The heating unit 72 and the pressure roller 73 are configured to be pressed against each other by one being biased toward the other.

[0037] The pressure roller 73 is configured to be rotated by a driving force transmitted from a motor (not shown) provided inside the main body housing 2. When the pressure roller 73 rotates, the fixing belt 71 is rotated in accordance with the rotation of the pressure roller 73.

[0038] The temperature sensor SE1 is a sensor that detects the temperature of the fixing unit 7. More specifically, the temperature sensor SE1 is attached to the nip plate 72B. The temperature sensor SE1 detects the temperature of the nip portion N by detecting the temperature of the nip plate 72B.

[0039] Returning to FIG. 1, the laser printer 1 further includes an outside air temperature sensor SE2 and a control unit 100. The outside air temperature sensor SE2 is a sensor that detects the temperature outside the main body housing 2.

[0040] The control unit 100 has a CPU, a ROM, a RAM, etc., and is configured to execute various processes in response to receiving a print command, etc., according to a pre-prepared program, etc. The control unit 100 can execute a selection process to select one transport mode from a plurality of transport modes that differ in the timing at which the transport unit 32 starts transporting the sheet S.

[0041] In this embodiment, the plurality of transport modes include a first transport mode, a second transport mode, a third transport mode, and a fourth transport mode. The first transport mode is a mode in which the timing for starting transport of the sheet S is the earliest. When the control unit 100 executes the first transport mode, it starts transporting the sheet S immediately upon receiving a print command. That is, in the first transport mode, the control unit 100 starts rotating the pressure roller 73 after receiving a print command, and then starts transporting the sheet S by the transport unit 32 without waiting for the lapse of a predetermined period T, which will be described later. Here, the fixing belt 71 starts rotating in conjunction with the start of rotation of the pressure roller 73, so "the start of rotation of the pressure roller 73" has the same meaning as "the start of rotation of the fixing belt 71."

[0042] The second transport mode is a mode in which the timing of starting transport of the sheet S is later than that in the first transport mode. When executing the second transport mode, the control unit 100 starts transporting the sheet S before the temperature of the fixing unit 7 reaches a fixing target temperature suitable for fixing.

[0043] The third transport mode is a mode in which the timing of starting transport of the sheet S is later than that in the second transport mode. When executing the third transport mode, the control unit 100 starts transport of the sheet S when the temperature of the fixing unit 7 reaches a fixing target temperature suitable for fixing.

[0044] The fourth transport mode is a mode in which the transport speed of the sheet S is slower than the other transport modes. The timing at which the transport of the sheet S starts is the same as in the third transport mode.

[0045] In the selection process, the control unit 100 can execute a first selection process and a second selection process. The first selection process is a process for selecting one transport mode based on the temperature H detected by the temperature sensor SE1 from when heating by the heater 72A starts until a predetermined period T has elapsed since rotation of the pressure roller 73 starts. More specifically, in the first selection process, the control unit 100 calculates the temperature gradient G that increases during the predetermined period T based on the detected temperature H, and selects one transport mode based on the calculated temperature gradient G.

[0046] The temperature gradient G can be the temperature difference between the detected temperature H when the pressure roller 73 starts to rotate and the maximum detected temperature H during the predetermined period T. Note that the temperature gradient G may also be the temperature difference between the minimum and maximum detected temperatures H during the predetermined period T.

[0047] In the first selection process, the control unit 100 has a function of setting the predetermined period T based on the initial temperature Hb, which is the temperature H detected by the temperature sensor SE1 when a print command is received. Specifically, as shown in FIG. 3, in the first selection process, the control unit 100 determines whether the initial temperature Hb is equal to or higher than a first temperature H1. If the initial temperature Hb is lower than the first temperature H1, the control unit 100 sets the predetermined period T to a first time T1. If the initial temperature Hb is equal to or higher than the first temperature H1, the control unit 100 sets the predetermined period T to a second time T2, which is longer than the first time T1.

[0048] The second selection process is a process of selecting one transport mode based on at least the outside air temperature Ho out of the outside air temperature Ho and the initial temperature Hb, without calculating the temperature gradient G.

[0049] Specifically, the control unit 100 selects one transport mode from among a plurality of transport modes as shown in the table of Fig. 4. First, each threshold value in Fig. 4 will be described. The first threshold value Ho1 and the second threshold value Ho2 are threshold values ​​that are compared with the outside air temperature Ho. The first threshold value Ho1 is smaller than the second threshold value Ho2. The second temperature H2 is a threshold value to be compared with the initial temperature Hb, and is higher than the first temperature H1 shown in FIG. The first gradient G1, the second gradient G2, and the third gradient G3 are threshold values for comparison with the temperature gradient G. The first gradient G1 is smaller than the second gradient G2. The third gradient G3 may be a value different from the first gradient G1 and the second gradient G2, or may be the same value as the first gradient G1 or the second gradient G2.

[0050] When Ho < Ho1, the control unit 100 executes the second selection process to select the fourth conveyance mode. When Ho1 ≤ Ho < Ho2 and Hb < H2, the control unit 100 executes the first selection process and selects one conveyance mode from the third conveyance mode and the fourth conveyance mode based on the temperature gradient G.

[0051] When Ho1 ≤ Ho < Ho2 and Hb ≥ H2, the control unit 100 executes the second selection process to select the third conveyance mode. When Ho ≥ Ho2 and Hb < H2, the control unit 100 executes the first selection process and selects one conveyance mode from the second conveyance mode, the third conveyance mode, and the fourth conveyance mode based on the temperature gradient G.

[0052] When Ho ≥ Ho2 and Hb ≥ H2, the control unit 100 executes the second selection process to select the first conveyance mode. That is, when Ho ≥ Ho2 and Hb ≥ H2, the control unit 100 selects the first conveyance mode without executing the first selection process.

[0053] When the initial temperature Hb is less than the rotation start temperature Hm, the control unit 100 performs heating by the heater 72A with the pressure roller 73 stopped until the detected temperature H becomes equal to or higher than the rotation start temperature Hm, and has a function of starting the rotation of the pressure roller 73 when the detected temperature H becomes equal to or higher than the rotation start temperature Hm.

[0054] The control unit 100 has a function of controlling the energization of the heater 72A based on the operation amount U including the proportional term proportional to the deviation ΔH between the target temperature and the detected temperature H and the differential term proportional to the differential value D of the deviation ΔH. The control unit 100 calculates the operation amount U with the differential term set to 0 during a predetermined period T.

[0055] Specifically, the control unit 100 calculates the operation amount U according to the following formula (1). U = Kp·ΔH + Kd·D ···(1)

[0056] Here, Kp is the proportional gain as a preset fixed value, and it is a positive value. Kd is the differential gain as a preset fixed value, and it is a negative value. D is the differential value of the deviation ΔH.

[0057] The control unit 100 determines the duty ratio, which is the energization amount per unit time, based on the operation amount U, and energizes the heater 72A based on the determined duty ratio. The control unit 100 increases the duty ratio as the operation amount U increases.

[0058] In addition, when the initial temperature Hb is higher than the third temperature H3 which is higher than the first temperature H1, the control unit 100 has a function of setting the target temperature Ht of heating by the heater 72A during the predetermined period T to be higher than when the initial temperature Hb is lower than the third temperature H3. Hereinafter, the target temperature Ht during the predetermined period T is also referred to as the target temperature Ht for determination.

[0059] Specifically, when Hb < H3, the control unit 100 sets the target temperature Ht for determination to the first target temperature Ht1. When Hb ≧ H3, the control unit 100 sets the target temperature Ht for determination to the second target temperature Ht2 which is higher than the first target temperature Ht1.

[0060] In this embodiment, the magnitude relationship of each of the aforementioned temperature thresholds is set as follows. Ho1 < Ho2 < H1 ≒ Hm < H3 < H2 < Ht1 < Ht2

[0061] Next, the operation of the control unit 100 will be described in detail. When receiving a printing command, the control unit 100 executes the heating process shown in FIG. 5.

[0062] In the heating process, the control unit 100 first starts energizing the heater 72A (S1). After step S1, the control unit 100 determines whether the detected temperature H is equal to or higher than the rotation start temperature Hm (S2). The control unit 100 repeats the process of step S2 until H≧Hm holds (No).

[0063] If it is determined in step S2 that H≧Hm (Yes), the control unit 100 starts the rotation of the pressure roller 73 (S3). After step S3, the control unit 100 executes a selection process to select a transport mode (S4). The selection process will be described in detail later.

[0064] After selecting the transport mode in step S4, the control unit 100 controls the energization of the heater 72A based on the target fixing temperature (S5). After step S5, the control unit 100 determines whether printing has finished (S6).

[0065] If it is determined in step S6 that printing has not finished (No), the control unit 100 returns to the process of step S5. If it is determined in step S6 that printing has finished (Yes), the control unit 100 stops the supply of electricity to the heater 72A and stops the rotation of the pressure roller 73 (S7), and ends this process.

[0066] 6, in the selection process, the control unit 100 first determines whether the outside air temperature Ho is equal to or greater than a second threshold value Ho2 (S21). If it is determined in step S21 that Ho≧Ho2 (Yes), the control unit 100 determines whether the initial temperature Hb is equal to or greater than a second temperature H2 (S22).

[0067] If it is determined in step S22 that Hb≧H2 (Yes), the control unit 100 selects the first transport mode as the transport mode (S23) and ends this process. If it is determined in step S22 that Hb≧H2 is not true (No), the control unit 100 executes the first selection process (S24 to S31).

[0068] In the first selection process, the control unit 100 first sets the predetermined period T based on the initial temperature Hb (S24). Specifically, as shown in Fig. 3, if the initial temperature Hb is lower than a first temperature H1, the control unit 100 sets the predetermined period T to a first time T1. If the initial temperature Hb is equal to or higher than the first temperature H1, the control unit 100 sets the predetermined period T to a second time T2 that is longer than the first time T1.

[0069] After step S24, the control unit 100 sets the target temperature for determination Ht based on the initial temperature Hb (S25). Specifically, if the initial temperature Hb is less than the third temperature H3, the control unit 100 sets the target temperature for determination Ht to the first target temperature Ht1. If the initial temperature Hb is equal to or greater than the third temperature H3, the control unit 100 sets the target temperature for determination Ht to the second target temperature Ht2, which is higher than the first target temperature Ht1.

[0070] After step S25, the control unit 100 calculates the temperature gradient G based on the detected temperature H during the predetermined period T (S26). After step S26, the control unit 100 determines whether the temperature gradient G is equal to or greater than the second gradient G2 (S27).

[0071] If it is determined in step S27 that G≧G2 holds (Yes), the control unit 100 selects the second transfer mode as the transfer mode (S28) and ends this process. If it is determined in step S27 that G≧G2 does not hold (No), the control unit 100 determines whether the temperature gradient G is equal to or greater than the first gradient G1 (S29).

[0072] If it is determined in step S29 that G≧G1 holds (Yes), the control unit 100 selects the third transport mode as the transport mode (S30) and ends this process. If it is determined in step S29 that G≧G1 does not hold (No), the control unit 100 selects the fourth transport mode as the transport mode (S31) and ends this process.

[0073] If it is determined in step S21 that Ho≧Ho2 is not satisfied (No), the control unit 100 determines whether the outside air temperature Ho is equal to or greater than the first threshold value Ho1 (S51), as shown in Fig. 7. If it is determined in step S51 that Ho≧Ho1 is satisfied (Yes), the control unit 100 determines whether the initial temperature Hb is equal to or greater than the second temperature H2 (S52).

[0074] If it is determined in step S52 that Hb≧H2 (Yes), the control unit 100 selects the third transport mode as the transport mode (S53) and ends this process. If it is determined in step S52 that Hb≧H2 is not true (No), the control unit 100 executes the first selection process (S54 to S58, S53).

[0075] In the first selection process, the control unit 100 first sets the predetermined period T based on the initial temperature Hb (S54), similar to step S24. After step S54, the control unit 100 sets the target temperature Ht for determination based on the initial temperature Hb (S55), similar to step S25.

[0076] After step S55, the control unit 100 calculates the temperature gradient G based on the detected temperature H during the predetermined period T, similar to step S26 (S56). After step S56, the control unit 100 determines whether the temperature gradient G is equal to or greater than the third gradient G3 (S57).

[0077] If it is determined in step S57 that G≧G3 holds (Yes), the control unit 100 selects the third transport mode as the transport mode (S53) and ends this process. If it is determined in step S57 that G≧G3 does not hold (No), the control unit 100 selects the fourth transport mode as the transport mode (S58) and ends this process.

[0078] If it is determined in step S51 that Ho≧Ho1 is not satisfied (No), the control unit 100 selects the fourth transport mode as the transport mode (S58) and ends this process.

[0079] Next, a specific example of the first selection process of the control unit 100 will be described. 8, when the control unit 100 receives a print command (time t1), if the initial temperature Hb is lower than the rotation start temperature Hm, the control unit 100 starts heating by the heater 72A while keeping the pressure roller 73 stopped. When the detected temperature H becomes equal to or higher than the rotation start temperature Hm (time t2), the control unit 100 starts rotating the pressure roller 73 and sets the predetermined period T to a first time T1 based on the initial temperature Hb. The control unit 100 calculates a temperature gradient G based on the detected temperature H during the predetermined period T, and selects a transport mode based on the temperature gradient G.

[0080] Even if the initial temperature Hb is the same, the way the temperature rises when the fixing unit 7 starts up varies depending on the heating capacity of the heater 72A, the capacity of the power supply, the ambient temperature, etc. Figure 8 and Figure 9, which will be described later, show, as an example, differences in temperature change due to differences in the heating capacity of the heater 72A.

[0081] 8, the solid line indicates the temperature change when heating with a heater 72A having a predetermined heating capacity. The dashed-dotted line indicates the temperature change when heating with a heater 72A having a capacity lower than the predetermined heating capacity. The dashed-two-dot line indicates the temperature change when heating with a heater 72A having a capacity higher than the predetermined heating capacity.

[0082] 8, the graphs shown with the three types of lines also show the time when the detected temperature H reaches the rotation start temperature Hm. Therefore, while the time when the print command is received is time t1 in the graph shown with the solid line, the time when the print command is received is earlier than time t1 in the graph shown with the dashed dotted line, which has low heating capacity, and the time when the print command is received is later than time t1 in the graph shown with the dashed two dotted line, which has high heating capacity.

[0083] When Hb < H1 and the predetermined period T is set to a relatively long time (for example, the second hour T2), since the temperatures of the three types of heaters 72A with different heating capabilities will be near the target temperature Ht after the elapse of the predetermined period T, the temperature gradients G during the predetermined period T will be approximated values for the three types, making it difficult to determine the differences in the heating capabilities of the three types of heaters 72A. In contrast, in this embodiment, since the first hour T1, which is a relatively short time, is set as the predetermined period T, the temperature gradients G for the three types can be made different values respectively, and the differences in the heating capabilities of the three types of heaters 72A can be determined.

[0084] Specifically, in the solid-line graph, the temperature gradient G is the temperature difference "Hmax1 - Hm" between the detected temperature H (≈ Hm) at the start of rotation of the pressure roller 73 and the maximum temperature Hmax1 of the detected temperature H during the predetermined period T. Also, in the one-dot chain-line graph, the temperature gradient G is the temperature difference "Hmax2 - Hm" between the detected temperature H (≈ Hm) at the start of rotation of the pressure roller 73 and the maximum temperature Hmax2 of the detected temperature H during the predetermined period T. Further, in the two-dot chain-line graph, the temperature gradient G is the temperature difference "Hmax3 - Hm" between the detected temperature H (≈ Hm) at the start of rotation of the pressure roller 73 and the maximum temperature Hmax3 of the detected temperature H during the predetermined period T.

[0085] As shown by the thin solid line in FIG. 9, when the control unit 100 receives a printing command (at time t11), if the initial temperature Hb satisfies Hm (H1) ≤ Hb < H3, the control unit 100 sets the target temperature Ht to Ht1, starts heating by the heater 72A, and starts the rotation of the pressure roller 73. Also, the control unit 100 sets the predetermined period T to the second hour T2 based on the initial temperature Hb. The control unit 100 calculates the temperature gradient G based on the detected temperature H during the predetermined period T and selects a conveyance mode based on the temperature gradient G.

[0086] In Figure 9, the graph shown with a thin solid line indicates the temperature change when heating with a heater 72A having a predetermined heating capacity. The graph shown with a thin dashed dotted line indicates the temperature change when heating with a heater 72A with a capacity lower than the predetermined heating capacity. The graph shown with a thin double-dashed dotted line indicates the temperature change when heating with a heater 72A with a capacity higher than the predetermined heating capacity. Note that in Figure 9, the time point at which the print command is received is the same (time t11) in each of the graphs shown with the three types of lines.

[0087] If Hb≧H1 and the predetermined period T is set to a relatively short time (for example, the first time T1), the temperature difference between the temperatures of the three types of heaters 72A with different heating capacities is small after the predetermined period T has elapsed, so the temperature gradient G during the predetermined period T will be similar for the three types, making it difficult to determine the differences in heating capacities among the three types of heaters 72A. In contrast, in this embodiment, the second time T2, which is a relatively long time, is set as the predetermined period T, so the temperature gradient G during the predetermined period T can be different for each of the three types, making it possible to determine the differences in heating capacity among the three types of heaters 72A.

[0088] Specifically, in the solid line graph, the temperature gradient G is the temperature difference "Hmax4 - Hb" between the detected temperature H (= Hb) at the start of rotation of the pressure roller 73 and the maximum temperature Hmax4 of the detected temperature H during the predetermined period T. In the dashed-dot line graph, the temperature gradient G is the temperature difference "Hmax5 - Hb" between the detected temperature H (= Hb) at the start of rotation of the pressure roller 73 and the maximum temperature Hmax5 of the detected temperature H during the predetermined period T. In the two-dot chain line graph, the temperature gradient G is the temperature difference "Hmax6 - Hb" between the detected temperature H (= Hb) at the start of rotation of the pressure roller 73 and the maximum temperature Hmax6 of the detected temperature H during the predetermined period T.

[0089] 9, when the control unit 100 receives a print command (time t11), if the initial temperature Hb is Hb≧H3, it sets the target temperature Ht to Ht2, starts heating by the heater 72A, and starts rotation of the pressure roller 73. Furthermore, the control unit 100 sets the predetermined period T to a second time T2 based on the initial temperature Hb. The control unit 100 calculates a temperature gradient G based on the detected temperature H during the predetermined period T, and selects a transport mode based on the temperature gradient G.

[0090] The thicker lines (solid line, dashed line, and dashed double-dashed line), like the thin lines described above, indicate differences in the heating capacity of the heater 72A. Because the thicker lines change in the same way as the thin lines, by setting the second time T2, which is a relatively long time, as the predetermined period T, it is possible to determine the differences in heating capacity among the three types of heater 72A.

[0091] As described above, the following effects can be obtained in this embodiment. Since the length of the predetermined period T is changed depending on the temperature of the fixing unit 7 when a print instruction is received, the transport mode of the sheet S can be appropriately selected.

[0092] The first selection process is performed based on the temperature difference between the detected temperature H when the pressure roller 73 starts to rotate and the highest detected temperature H during the specified period T. Therefore, compared to, for example, determining the temperature gradient at a single point in time, the temperature gradient of the fixing section 7 during the specified period T can be calculated with higher accuracy, and the transport mode of the sheet S can be appropriately selected.

[0093] When the temperature of the fixing unit 7 is below the rotation start temperature Hm, the pressure roller 73 is stopped and heated by the heater 72A, so that the fixing unit 7 can accumulate sufficient heat before the pressure roller 73 can start rotating.

[0094] When the initial temperature Hb is equal to or higher than the second temperature H2, the fixing unit 7 has stored sufficient heat, so that the conveyance of the sheet S can be started without waiting for the predetermined period T to elapse, thereby improving the printing efficiency.

[0095] By calculating the operation amount U with the differential term set to 0 during the specified period T, the variation in the heating capacity of the heater 72A can be correctly evaluated, and therefore the transport mode of the sheet S can be appropriately selected to correspond to the variation in the heating capacity of the heater 72A.

[0096] Since the target temperature Ht is increased when the initial temperature Hb is equal to or higher than the third temperature H3, a decrease in the output of the heater 72A during the predetermined period T can be suppressed.

[0097] The present invention is not limited to the above-described embodiment, but can be used in various forms as exemplified below.

[0098] In the above embodiment, the heater 72A is controlled based on the manipulated variable U calculated with the derivative term set to 0 during the predetermined period T, but the present invention is not limited to this. For example, the control unit may set the duty ratio to 100% during the predetermined period.

[0099] Even with this configuration, by energizing the heater at 100% duty from the start of rotation of the pressure roller until a predetermined period has elapsed, variations in the heater's heating capacity can be correctly evaluated, and the sheet transport mode can be appropriately selected to correspond to variations in the heater's heating capacity.

[0100] In the above embodiment, the fixing belt 71 is exemplified as the fixing rotor, but the present invention is not limited to this, and the fixing rotor may be, for example, a heating roller.

[0101] In the above embodiment, the present invention is applied to the laser printer 1, but the present invention is not limited to this and may be applied to other image forming devices, such as color printers, copiers, and multifunction devices.

[0102] The heater is not limited to a halogen lamp, and may be, for example, a carbon heater. The heater may be a flat plate heater that includes a substrate and a resistance heating element provided on the substrate and that comes into contact with the inner circumferential surface of fixing belt 71 to heat fixing belt 71. The number of heaters may be multiple.

[0103] The elements described in the above-described embodiment and modified examples may be implemented in any combination. [Explanation of symbols]

[0104] 1 laser printer 4 Image forming unit 7 Fixing section 31 Storage unit 32 Conveyor 71 Fixing belt 72A heater 100 control section H Detected temperature H1 1st temperature Hb initial temperature S seat SE1 Temperature Sensor T predetermined period T1 1st Hour T2 2nd Hour

Claims

1. a storage section for storing a sheet; an image forming unit that forms a toner image on a sheet; a fixing unit having a fixing rotor and a heater for heating the fixing rotor, and fixing the toner image onto the sheet; a temperature sensor for detecting the temperature of the fixing unit; a conveying unit that conveys the sheet from the storage unit toward the image forming unit; a control unit, The control unit a selection process for selecting one conveying mode from a plurality of conveying modes in which the timing at which the conveying unit starts conveying the sheet is different; After the heating by the heater is started, when the temperature detected by the temperature sensor becomes equal to or higher than a rotation start temperature, the fixing rotor is started to rotate; In the selection process, a first selection process can be executed in which a temperature gradient that increases during a predetermined period is calculated based on the temperature detected by the temperature sensor from when rotation of the fixing rotator starts until the predetermined period has elapsed, and the one transport mode is selected based on the calculated temperature gradient, In the first selection process, it is determined whether an initial temperature, which is a temperature detected by the temperature sensor when a print instruction is received, is equal to or higher than a first temperature; If the initial temperature is less than the first temperature, setting the predetermined period to a first time; When the initial temperature is equal to or higher than the first temperature, the predetermined period is set to a second period that is longer than the first period.

2. The control unit 2. The image forming apparatus according to claim 1, wherein the temperature gradient is calculated as a difference between a detected temperature at the start of rotation of the fixing rotor and a maximum detected temperature during the predetermined period.

3. The control unit 2. The image forming apparatus according to claim 1, wherein the temperature gradient is calculated as a difference between a minimum temperature and a maximum temperature detected during the predetermined period.

4. The control unit 4. The image forming apparatus according to claim 1, wherein, when the initial temperature is lower than a rotation start temperature, heating is performed by the heater while the fixing rotor is stopped until the detected temperature becomes equal to or higher than the rotation start temperature.

5. the plurality of transport modes include a first transport mode in which, after receiving a print instruction and starting rotation of the fixing rotator, the transport unit starts transporting the sheet without waiting for the predetermined period of time to elapse; The control unit If the initial temperature is equal to or higher than a second temperature that is higher than the first temperature, the first selection process is not performed and the first transport mode is selected; 5. The image forming apparatus according to claim 1, wherein the first selection process is executed when the initial temperature is lower than the second temperature.

6. The control unit controlling power supply to the heater based on a manipulated variable including a sum of a proportional term proportional to a deviation between a target temperature and a detected temperature and a differential term proportional to a differential value of the deviation; 6. The image forming apparatus according to claim 1, wherein the manipulated variable is calculated with the differential term set to 0 during the predetermined period.

7. The control unit The heater can be energized based on a duty ratio, which is the amount of current per unit time.

6. The image forming apparatus according to claim 1, wherein the duty ratio is set to 100% during the predetermined period.

8. The control unit 7. The image forming apparatus according to claim 1, wherein when the initial temperature is equal to or higher than a third temperature that is higher than the first temperature, the target temperature for heating by the heater is set higher than when the initial temperature is lower than the third temperature.

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