Sheet carrier apparatus

Abstract: A sheet carrier apparatus for conveying a sheet includes a roller (3a) having a cylindrical circumferential surface that is made of an elastic material. The apparatus also includes a temperature sensor (10) for detecting a temperature of the roller, and processing device for setting a rotational speed of the roller based on a detection signal from the temperature sensor.

Description: BACKGROUND OF THE INVENTION The present invention relates to a sheet carrier apparatus for conveying a sheet at a constant speed by using a roller and, more particularly, to a sheet carrier apparatus in which a stencil sheet is conveyed by driving a platen roller to rotate while being pressed against the platen roller by a thermal head. In a conventional perforating machine, such as e.g. known from JP-A-04 189 543 a stencil sheet is conveyed by driving a platen roller to rotate while being pressed against the platen roller by a thermal head. The platen roller is required to contact the stencil sheet closely with heating elements of the thermal head; thus, the platen roller is made of an elastic material such as rubber, thereby ensuring an enough nip area where the stencil sheet is pinched between the thermal head and the platen roller. In this machine, expansion or shrinkage of the stencil sheet in the conveying direction is to be determined by a diameter of the platen roller and a feeding speed by a motor as driving means. Since the platen roller is made of rubber as explained, changes of temperature or working conditions of the platen roller vary a diameter of the platen roller. A change in the diameter of the roller leads to a change in a conveying speed of the roller although a rotating speed of the roller remains unchanged. Thus, expansion/shrinkage ratio of a perforated sheet changes, thereby deteriorating a dimensional accuracy of an image on a printed sheet. The present invention aims to solve the problem described above and therefore, an object of the present invention is to provide a sheet carriage apparatus which provides a constant conveying speed without being badly affected by changes in working condition thereof. SUMMARY OF THE INVENTION A sheet carrier apparatus as defined in the first aspect of the present invention comprises a thermal head; a platen roller having a cylindrical circumferential surface made of an elastic material and pressed against the thermal head, the platen roller being rotated to convey the stencil sheet between the platen roller and the thermal head; a temperature sensor for detecting a temperature of the platen roller; and processing means for setting a rotational speed of the platen roller based on a detection signal from the temperature sensor. In a sheet carrier apparatus as defined in the further aspect of the present invention, the processing means is so constituted that the processing means executes a predetermined computing equation based on the detection signal from the temperature sensor, thereby obtaining a diameter of the roller in the detected temperature, and outputs a control signal for setting the rotational speed of the roller according to the diameter, so that the sheet can be conveyed at a constant speed regardless of a temperature change in the sheet carrier apparatus as defined in the first aspect. In a sheet carrier apparatus as defined in the further aspect of the present invention, the computing equation is as follows: St = Ss÷{1+α(t-T)÷s} where, St is a corrected rotational speed [rpm] of the platen roller; Ss is a rotational speed [rpm] of the platen roller at some temperature T °C; s is a diameter [mm] of the platen roller at the temperature T °C; α is an expansion coefficient [mm/°C] of the platen roller; and t is a temperature[°C] of the platen roller, in the sheet carrier apparatus as defined in the third aspect. In a sheet carrier apparatus as defined in the further aspect of the present invention, the α in a temperature range over a predetermined temperature is smaller than an α in a temperature range less than the predetermined temperature in the sheet carrier apparatus as defined in the fourth aspect. A sheet carrier apparatus as defined in the further aspect of the present invention further comprises a memory which prestores data indicating a rotational speed corresponding to the temperature of the roller, in which apparatus the processing means reads the memory based on the temperature indicated by the detection signal from the temperature sensor so as to obtain the rotational speed corresponding to the temperature, thereby controlling rotation of the roller in the sheet carrier apparatus as defined in the first aspect. In a sheet carrier apparatus as defined in the further aspect of the present invention, the temperature sensor directly detects the temperature of the roller while being in contact with the roller in the sheet carrier apparatus as defined in the first aspect. In a sheet carrier apparatus as defined in the further aspect of the present invention, the temperature sensor is disposed at a predetermined distance with the roller and detects a peripheral temperature of the roller in the sheet carrier apparatus as defined in the first aspect. A temperature sensor detects temperature of a roller conveying a sheet, and the detected temperature is outputted to processing means. The processing means calculates a diameter of the roller based on the detected temperature, thereby determining a rotational speed for a constant circumferential speed of the roller based on the diameter. The roller, while being controlled to rotate at the determined rotational speed, conveys the sheet at a constant speed regardless of the changes in temperature. In the present invention, at least a cylindrical circumferential surface of the roller is made of an elastic material; otherwise, the whole roller may be made of the material. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view illustrating a stencil printing machine to which a sheet carrier apparatus of the present invention is adapted; FIG. 2 is a graph showing a temperature change characteristic of a roller; FIG. 3 is a view illustrating a perforating section; FIG. 4 is a block diagram showing an electrical constitution of one embodiment of the present invention; FIG. 5 is a view illustrating a document reading section; FIG. 6 is a view illustrating a perforating section; FIG. 7 is a flow chart showing an operation of one embodiment of the present invention; FIG. 8 is a flow chart showing an operation of one embodiment of the present invention; FIG. 9 is a graph showing a characteristic line for correcting a rotational speed of a roller; FIG. 10 is a graph showing a characteristic line for correcting a rotational speed of a roller; FIG. 11 is an imaginary view showing storage of a memory; FIG. 12 is a view illustrating a document reader to which a sheet carrier apparatus of the present invention is adapted. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 is a side view showing a stencil printing machine to which a sheet carrier apparatus in the present invention is adapted. A reader 2 is disposed to the upper portion of the printing machine 1. An original is read by an image sensor 2b such as CCD and so on, while being conveyed by an original conveying section 2a comprising plural conveying rollers, and then, an image signal is output from the image sensor to a perforating section 3. In this case the image sensor 2b is fixed to a position shown in a broken line in FIG. 1. Otherwise, the original may be read after being placed directly on a scan table 1a of the upper portion of the printing machine 1, wherein the image sensor 2b scans the original in the arrow direction shown in the drawing. A perforating section 3 perforates a stencil sheet (master) P by a thermal head 3b according to an image signal, while conveying the stencil sheet by a stencil conveying section comprising a platen roller 3a and so on. The platen roller 3a is made of an elastic material such as rubber and so on; therefore, a nip area where the stencil sheet is pinched between the thermal head and the platen roller is enough large. The stencil sheet P, while being clamped by one end thereof by a clamping device of a printing drum, is wrapped around the outer circumferential surface of the drum, and then rotates with the drum. The printing paper (paper) 6 on a paper feed section 5 passes between the drum 4 and a paper drum 4a (press roller). Ink inside the drum is transferred to the paper 6, thereby forming an image on the paper. The printed paper is successively discharged onto a discharge section 7. After printing, the used stencil sheet P is discharged into a stencil discharge section 8. Hereinafter, a constitution of one embodiment of the present invention will be explained. In the embodiment, the sheet carrier apparatus in the present invention is adapted to the perforating section 3 of said stencil printing machine for conveying the stencil sheet P. In the perforating section 3, the platen roller 3a is composed of a metal axis and a rubber material wrapped around the axis; thus, the diameter of the roller changes according to a change in temperature or working conditions. FIG. 2 is a graph showing one example of temperature diameter relationship of the platen roller 3a. This platen roller 3a is of hardness 40° on the spring-type hardness testing machine Type C specified by JIS K 6301(established in 1975). In this way, when perforating operation is conducted with the diameter of the platen roller as changed, expansion or shrinkage arises in a perforated image, thereby deteriorating a dimensional accuracy of an image on a printed sheet obtained. FIG. 3 is an enlarged view ofthe perforating section 3. Adjacent to the platen roller 3a, a temperature sensor 10 is disposed for detecting a peripheral temperature around the platen roller 3a. A detection signal from the sensor is outputted into a processing means 20. A mounting plate 12 is attached to a guide plate 11. The temperature sensor 10 is attached to the mounting plate 12. The temperature sensor 10 may be so constituted that the sensor is in contact with the circumferential surface of the platen roller 3a and directly detects temperature of the platen roller 3a. FIG. 4 is a block diagram showing an electrical constitution of the present invention. An image signal, after being read by the image sensor 2b in the reader 2, is outputted into an image processing circuit 12 and processed in a predetermined method. And then the processed signal is outputted into the thermal head 3b in the perforating section 3. A scanner motor 2c that is controlled by a driver 23 in a processing means 20 drives the image sensor 2b. The sensor reads a document placed on the document table 1a while moving. An original-top sensor 2d detects an initial position where the image sensor 2c starts moving. An original-end sensor 2e detects an end portion where the sensor stops. Signals that outputted from the original-top sensor and the original-end sensor are inputted into a CPU 25 in the processing means 20. The image sensor 2b connected with an endless belt 2f reads a document G, while being driven by the scanner motor 2c to move along the bottom surface of the document G, as illustrated in FIG. 5. Further, the platen roller 3a in the perforating section 3 is driven to rotate by a platen motor 3c. The platen motor 3c controls a rotational speed of the platen roller 3a via the driver 18 according to a control signal from the processing means 20. As illustrated in FIG. 6, the platen motor 3c may be a stepping motor, which is driven to rotate by pulse input, and is connected to the platen roller with an endless belt 3d. Further, the temperature sensor 10 detects a temperature of the platen roller 3a. The detected temperature, after being amplified predetermined-fold (ten-fold, for example) by an amplifier 19, is converted from analog to digital at an A/D converter 24 included in the processing means 20, and then outputted into the CPU 25. The processing means 20 may be a one-chip computer which comprises the CPU, a ROM 21 as a memory, a RAM 22 as a memory, the A/D converter 24, and a timer 26 for the platen motor. The processing means control printing operation of the printing machine according to an execute program stored in the ROM 21, and execute a process of correction for temperature as explained afterward. Pressing a start key 31 on a control panel 30 of the stencil printing machine 1 initiates the perforating operation and the printing operation. The perforating operation starts as soon as the process of correction for temperature is executed. The CPU 25 in the processing means 20 calculates a diameter of the platen roller 3a according to a temperature of the platen roller 3a detected by the temperature sensor 10, thereby outputting a control signal for controlling a rotational speed of the platen roller 3c so that a peripheral speed of the platen roller 3c, i.e. a conveying speed of the stencil P can be kept at a constant value. Next, with reference to a flow chart of FIG. 7, the processing executed by the processing means 20 will be explained. When the start key 31 is pressed (SP1-YES), a process of determining a rotational speed of the platen roller 3c is executed (SP2). Details of the process will be described afterward. As illustrated in FIG. 5, the scanner motor 2c drives the image sensor 2b to start from a left position and move in the X direction. While moving, the image scanner reads an original G placed on the scan table 1a (SP3). At the same time, the platen motor 3c rotates to drive the stencil sheet P at the rotational speed determined at SP2 according to the input control signal (SP4). In this way the image sensor 2b reads the image on the original G, thereby outputting an image signal. The image signal is processed in the image processing circuit 12. The image signal as processed drives the thermal head 3b to conduct a heatsensitive perforation on the stencil sheet P simultaneously with the image processing. A print signal is generated (ON) according to an image forming area of the stencil sheet, and the thermal head 3b perforates the stencil sheet P by heat according to the image signal (SP5). Reading the original G and perforating the stencil sheet P are continued until the image sensor 2b reaches the original-end sensor 2e after moving in the direction X (SP6-YES). When the image sensor reaches the end of the original G to turn the print signal off (SP7), scanner motor 2c halted (SP8) and also the platen motor 3c halted (SP9) simultaneously with the motor 2c. After a predetermined time (100ms, for example) passes (SP10), the scanner motor 2c moves the image sensor toward the initial position (SP11). This time the original is not scanned. When the image sensor 2b moves in the direction Y and reaches the original-end sensor 2d (SP12-YES), the scanner motor 2c halted, thereby being restored to the initial position (being set in an entry mode for SP1). FIG. 8 is a flowchart showing a process how the rotational speed of the platen roller 3c is determined. Firstly, just after the start key 31 is operated, the detecting signal is inputted to the CPU through the amplifier 19 and the A/D converter (SP20). According to a temperature indicated by the detecting signal, a predetermined calculation is executed so that the detecting signal is converted into a timer value (SP21). This timer value is set on the timer 26 for the platen motor (SP22). In the process where the detecting signal is converted into the timer value, the following computing equation (1) is executed so as to obtain a rotational speed St of the platen roller 3a (platen motor 3c). St=Ss÷{1+α(t-25)÷s} where, St = corrected rotational speed [rpm] of the platen roller, Ss = rotational speed [rpm] of the platen roller at 25 °C, s = diameter [mm] of the platen roller at 25 °C, α= expansion coefficient [mm/°C] of the platen roller, t = temperature[°C] of the platen roller. A numerical example for each of the parameters listed above will be shown hereinafter. Suppose that t is 30[°C], Ss is 20 [rpm], s is 22.96 [mm], and α is 0.0034[mm/°C]. These values except "t" are optional parameters that are obtained by an experiment. These values are substituted for the parameters in the computing equation (1), so that St is calculated. The St obtained is 19.9852 [rpm]. FIG. 9 is a graph showing a temperature rotational-speed relation according to the equation (1). As shown in the drawing, the linear characteristic line means that the controlled rotational speed decreases with increasing temperature. In SP22, the timer value is set in the timer 26 for the platen motor. The timer value changes an interruption cycle of the timer 26. According to the changed interruption cycle, the platen motor 3c is driven for a predetermined number of the pulses. Accordingly, the shorter the interruption cycle is, in the higher speed the platen roller 3a rotates; conversely, the longer the cycle is, in the lower speed the roller rotates. In this way, although increase in temperature makes the diameter of the platen roller 3a larger, the circumferential speed of the platen roller 3a can be kept constant by decreasing the rotational speed of the platen roller 3a accordingly. Thus, a sheet expansion/shrinkage ratio during perforation can be kept constant regardless of the changes in working atmosphere and condition. Afterwards, the stencil sheet P is wrapped around the printing drum 4 and then printing starts; however, no deterioration arises in a dimensional accuracy of an image on a printed sheet obtained, since a perforating condition is kept constant. When a rotational speed of the platen roller is actually measured to confirm the equation (1), there is a tendency for a conveying speed by the roller to decrease with increasing temperature due to excessive correction. Thus, the α may be decreased by a half value (mm/°C) in a range over 25°C, so that an effect of the correction in the range can be reduced. In this case, the CPU 25 is so constituted that the CPU switch one α-value to the other after judging whether the temperature is over 25°C according to the detecting signal. A characteristic graph for temperature rotational-speed relation of the platen roller 3a in this constitution is shown in FIG. 10. In the constitution explained above, the equation (1) is executed at every printing operation to determine the rotational speed of the platen roller 3a; otherwise, the ROM 21 may prestore timer values for determining the rotational speed of the platen roller 3a corresponding to the detected temperature. FIG. 11 illustrates an imaginary inner constitution of the ROM having prestorage explained above. In each address of the ROM 21, a timer value corresponding to a detected temperature is stored in a table lookup format. When a resolution of the A/D converter is made by 8 bits, temperature can be detected in 256 gradations, and the timer value corresponding to each gradation of the temperature is stored by 2 bytes in each of the addresses. The CPU 25 reads in an address corresponding to a digital data converted from an analog data at the A/D converter 24, thereby obtaining a timer value. In the embodiment explained above, a speed control of the platen roller 3a of the perforating section 3 has been explained; however, this invention in not restricted to such the platen roller 3a. According to the present invention, also in a mechanism that includes a roller with a variable expansion coefficient due to changeable temperature, a conveying speed by the roller can be kept constant in a similar way by changing the speed of the roller. FIG. 12, for example, is a view illustrating a document reader 12 to which the present invention is also applicable. This reader 2 is an automatic document feeder (ADF). The feeder is so constituted that the feeder reads an original G by a fixed image sensor 2b while conveying the original by plural conveying rollers 2a, thereby outputting an image signal. The conveying rollers 2a are also composed of an elastic material like rubber, so that the enough nip area is ensured. Therefore, the diameter of the conveying roller 2a changes according to the changes in temperature. This results in changes in the conveying speed of the original. Correspondingly, a temperature sensor 10 is disposed adjacent to the conveying roller 2a, and a rotational speed of the conveying roller 2a (an original conveying motor 2g) is variably controlled. The method of controlling the rotational speed may be based on either the computing equation (1) or the table lookup of the ROM. In this way the original G is conveyed at a constant speed regardless of the changes in temperature, thus a stable original reading can be achieved. In the present embodiment, the speed control of the platen motor 3c is conducted in such a manner that the timer value is set in the timer 26 for the platen motor, and the CPU outputs the control signal for controlling the platen motor 3c according to the interruption by the timer. Otherwise, the driver 18 may energize and drive the platen motor 3c directly according to the timer value stored in the timer 26. In the present embodiment, the stencil printing machine is explained as an example to which the sheet carrier apparatus in the present invention is applicable; however, the present invention in not restricted to the stencil printing machine only. Namely, the present invention is also applicable to an apparatus that includes a roller with a variable expansion coefficient due to changeable temperature, such as a sheet conveying device in other types of a printing machine, a copier, and a facsimile. Further, real-time temperature of the roller may be detected so that the interruption by the timer can be executed during conveyance as well as before conveyance, thereby achieving a further higher precision control of the rotational speed of the rollers. According to the present invention, the rotational speed of the roller is variably controlled according to the changes in temperature of the roller, so that a sheet can be conveyed surely at a constant speed. In the case where the sheet is a stencil sheet, which is to be wrapped around the drum for printing, a dimensional accuracy of an image on a printed sheet obtained is stably constant, since perforations can be constantly formed in the stencil sheet regardless of the changes in temperature. Further, in the case of an original as the sheet, an accurate conveying is achieved because of a constant speed in conveying the original. Still further, in the case of an image scanner of moving original exposure type, a high reading-accuracy of an image sensor is ensured to be achieved.

Claims: A sheet carrier apparatus for conveying a stencil sheet (P), comprising: a thermal head (3b), a platen roller (3a) having a cylindrical circumferential surface made of an elastic material and pressed against the thermal head, said platen roller being rotated to convey said stencil sheet (P) between said platen roller (3a) and said thermal head, a temperature sensor (10) for detecting a temperature of said platen roller (3a), and processing means for setting a rotational speed of said platen roller (3a) based on a detection signal from said temperature sensor (10). A sheet carrier apparatus as defined in claim 1, wherein said processing means is so constituted that: said processing means executes a predetermined computing equation based on said detection signal from said temperature sensor (10), thereby obtaining a diameter of said roller (3a) in the detected temperature, and outputs a control signal for setting said rotational speed of said roller (3a) according to said diameter, so that said sheet (P) can be conveyed at a constant speed regardless of a temperature change. A sheet carrier apparatus as defined in claim 2, wherein said computing equation is as follows: St=Ss÷{1+α(t-T)÷s} where, St is a corrected rotational speed [rpm] of the platen roller; Ss is a rotational speed [rpm] of the platen roller at some temperature T°C; s is a diameter [mm] of the platen roller at the temperature T °C; α is an expansion coefficient [mm/°C] of the platen roller; and t is a temperature[°C] of the platen roller. A sheet carrier apparatus as defined in claim 3, wherein said α in a temperature range over a predetermined temperature is smaller than an α in a temperature range less than the predetermined temperature. A sheet carrier apparatus as defined in claim 1, further comprising a memory which prestores data indicating a rotational speed corresponding to said temperature of said roller (3a), wherein said processing means reads said memory based on said temperature indicated by said detection signal from said temperature sensor (10) so as to obtain said rotational speed corresponding to said temperature, thereby controlling rotation of said roller (3a). A sheet carrier apparatus as defined in claim 1, wherein said temperature sensor (10) directly detects said temperature of said roller (3a) while being in contact with said roller. A sheet carrier apparatus as defined in claim 1, wherein said temperature sensor (10) is disposed at a predetermined distance with said roller (3a) and detects a peripheral temperature of said roller (3a).

Inventors: HARA YOSHIKAZU, HASHIMOTO TETSUYA, WATANABE HITOSHI, HARA, YOSHIKAZU, HASHIMOTO, TETSUYA, WATANABE, HITOSHI, HARA, YOSHIKAZU, C/O RISO KAGAKU CORPORATION, Hashimoto, Tetsuya, c/o Riso Kagaku Corporation, Watanabe, Hitoshi, c/o Riso Kagaku Corporation
Applicants: RISO KAGAKU CORP, RISO KAGAKU CORPORATION
Application year: 1998
Main classification: B41L13
IPC codes: B41L13, B65H20, B65H5, B41C1