Sheet thickness sensing apparatus

Abstract: A sheet thickness sensing apparatus of an image recording apparatus is described. The apparatus includes a feed roller 114 for conveying a fed sheet, a movable friction roller guide 136 having a reflective surface 138 and a friction roller 128 carried by the friction roller guide 136, the friction roller guide 136 being biased in such a way as to push the friction roller 128 against the feed roller 114. A thickness sensing circuit has an optical sensor 140 positioned to detect reflections from the reflective surface 138 and to produce a sheet thickness value derived from a property of the detected reflections.

Description: Background of the Invention The present invention relates to an ink jet image recording apparatus. Image recording apparatus, such as printers, copiers, facsimiles, etc., record an image onto a record sheet, for example, paper or an overhead projector (OHP) film. In such image recording apparatus, normal sheets are most widely used, but sheets which are thicker or thinner than normal sheets are also frequently used. The image recording apparatus, on the other hand, have been designed on the basis of using normal sheets. Therefore, if the thickness of the sheet differs from that of the normal sheet, the recording picture quality deteriorates or a malfunction may occurs. For example, in a laser beam printer (LBP) using an electrophotographic developing system, if a fixing temperature is constantly maintained irrespective of the thickness of the sheet, since the fixing function is degraded, the recording picture quality may deteriorate or a paper jam may occur. One of techniques for improving the above described disadvantages is disclosed in Korean patent application no. 94-12167. This patent application describes a technique for automatically sensing the thickness of a fed sheet and adjusting the fixing temperature according to the thickness of the sheet. The thickness of the sheet is sensed by sensing the pressure, generated as a upper roller of a register roller is moved up according to the thickness of the sheet, by a pressure sensor. In this case, the variation in the pressure generated by the movement of the roller corresponding to the thickness of the sheet is small. Therefore, it is difficult to detect accurately the variation in the thickness of the sheet. Furthermore, the cost of the product rises because a pressure sensor is used. Recording systems such as wire dot systems, thermal transfer (thermosensitive recording) systems and ink jet systems, etc., use unique recording heads. The ink jet system jets ink onto the sheet to record an image. The recording head in the recording apparatus using an ink jet system includes a plurality of nozzles each having a minute ejection hole for jetting the ink. The ink within these nozzles is heated and expanded by a heating element installed in each nozzle, thus jetting the ink to the exterior of the nozzle. The ink jet recording apparatus records a desired image on the sheet by selectively driving the nozzles while the recording head moves in the horizontal direction so as to correspond to an image to be recorded. The recording head is fixed by a carriage driven by a carriage motor, and the carriage moves in the right and left horizontal direction along a transversely extended carrier shaft. Therefore, the recording head also moves to the right and left when the carriage moves. Since the ink jet recording apparatus records the image by jetting the ink, it is preferable that the gap between the recording head and the sheet, that is, a head gap, is constantly maintained at an optimal state. This is because the size of the ink drop or the shock force of ink jetting onto the sheet varies according to the head gap, and thus the had gap has a considerable effect on the recording picture quality. If the head gap becomes narrower or wider than an optimal value, the recording image spreads or pixels are deformed, thus lowering the recording picture quality. Therefore, manufacturers have designed and manufactured ink jet recording apparatus having a head gap of optimal size. However, even if the head gap is adjusted to the optimal size, it is not always maintained as such and varies according to the thickness of the sheet. That is, since the thickness of the used sheet is not always the same and varies with the kind of the sheet, the head gap also varies. For example, when the head gap is designed and manufactured on the basis of the normal sheet, if a thicker sheet than the normal sheet, like an envelope or label sheet, is used, the head gap becomes narrower than the optimal value. Therefore, to obtain optimal recording picture quality, the head gap should be adaptively adjusted according to the thickness of the sheet. To this end, conventional ink jet recording apparatus include an adjusting lever which can adjust the head gap. The user of the ink jet recording apparatus can narrow or widen the head gap by manipulating the adjusting lever according to the thickness of the sheet. Therefore, even if the thickness of the sheet varies, the optical recording picture quality can be obtained. However, manual adjustment of the adjusting lever is inconvenient. Therefore, even if the thickness of the sheet varies, the user may not vary the head gap or may manipulate the adjusting lever improperly, thus lowering the recording picture quality. Further, if a thick sheet is used, since the head gap becomes narrow, the carriage may be caught on the sheet and the carriage motor may step out. It is therefore an object of the present invention to provide a sheet thickness sensing apparatus which can accurately sense a variation in the thickness of a sheet. It is another object of the present invention to provide a recording head automatic adjusting apparatus and method which can maintain optimal recording picture quality adaptively according to the thickness of a sheet in an ink jet recording apparatus. Summary of the Invention The present invention provides a sheet thickness sensing apparatus of an image recording apparatus, comprising: a feed roller for conveying a fed sheet; a movable friction roller guide having a reflective surface; a friction roller carried by the friction roller guide, the friction roller guide being biased in such a way as to push the friction roller against the feed roller; and a thickness sensing circuit having an optical sensor positioned to detect reflections from the reflective surface and adapted to produce a sheet thickness value derived from a property of the detected reflections. Preferably, the friction roller is carried on one end of the friction roller guide and the reflective surface is on the other end. The reflective surface preferably moves in the opposite direction to the friction roller when the latter is moved by the fed sheet. Preferably, the gap between the reflective surface and the optical sensor varies substantially linearly with the thickness of the sheet. For example, the friction roller guide may be mounted for pivotal movement about an intermediate position. The present invention further provides a recording head automatic adjusting apparatus of an ink jet recording apparatus, comprising a sheet thickness sensing apparatus according to the present invention and a controller for adjusting a head driving enable time of a recording head according to the thickness of the sheet sensed by the thickness sensing apparatus. The controller may adjust the head driving enable time according to the sheet thickness value generated by a thickness sensing circuit when the fed sheet reaches the nip between the feed roller and the friction roller. Preferably, the controller adjusts the head driving enable time to be in proportion to the sheet thickness value. The present invention further provides a recording head automatic adjusting apparatus of an ink jet recording apparatus, comprising a sheet thickness sensing apparatus according to the present invention, means for adjusting the height of a recording head and a controller for controlling the adjusting means to set the gap between the recording head and the fed sheet in correspondence with the thickness of the sheet sensed by the thickness sensing apparatus. The means for adjusting the height of the recording head may comprise: a carrier shaft along which a recording head carriage moves, the carrier shaft being eccentrically mounted with respect to a frame of the recording apparatus; a gap adjusting motor for turning the carrier shaft so as to alter the height of the carriage; and a motor driver for driving the gap adjusting motor under the control of the controller. The controller may adjust the head gap substantially linearly with the sheet thickness value. Preferably, the controller adjusts the head driving enable time or the head gap in accordance with one range of sheet thickness values among a plurality of different ranges. The present invention finally extends to a method of adjusting a recording head in an ink jet recording apparatus comprising: sensing the thickness of a fed sheet in a sheet conveyance path using a sheet thickness sensing apparatus according to the present invention; and adjusting a head driving enable time of the recording head or the gap between the recording head and the fed sheet according to the sensed thickness of the sheet. In one embodiment, the head driving enable time is shortened when the thickness of the sheet is lower and lengthened when the thickness of the sheet is greater. In another embodiment, the gap between the recording head and the fed sheet is lower when the thickness of the sheet is lower and higher when the thickness of the sheet is greater. Brief Description of the Drawings The present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a front perspective view showing the mechanism of a print engine of a typical ink jet printer; Fig. 2 is a rear perspective view of Fig. 1; Fig. 3 is an exploded perspective view of the line feed mechanism and a friction roller assembly shown in Fig. 1; Fig. 4 is a side view showing the structure for the conveyance of a sheet of a feed roller and a friction roller assembly shown in Fig. 3; Fig. 5 shows the structure of a sheet thickness sensing apparatus according to the present invention; Figs. 6A and 6B show examples of the operation of the sheet thickness sensing apparatus of Fig. 5 according to variations in the thickness of the sheet; Fig. 7 is a block diagram showing a circuit of a first embodiment of the present invention; Fig. 8 is a detailed circuit diagram of a thickness sensing circuit shown in Fig. 7; Fig. 9 is a graph showing the output of the thickness sensing circuit of Fig. 8 as a function of thickness of a sheet; Fig. 10 is a flow chart showing the processing of a CPU shown in Fig. 7; Fig. 11 is a perspective view of a gap adjusting apparatus according to the present invention; Fig. 12 is a block diagram showing a circuit of a second embodiment of the present invention; and Fig. 13 is a flow chart showing the processing of a CPU shown in Fig. 12. Detailed Description of the Preferred Embodiment Front and rear perspective views of the structure of a print engine of a typical ink jet printer are illustrated in Figs. 1 and 2 respectively. The print engine includes a line feed mechanism 102, a carriage mechanism 104, a home assembly 106, a friction roller assembly 108, a carriage motor 116, a line feed motor 118, etc, and these are assembled and installed in a frame 100. A reference symbol S designates a sheet. The line feed mechanism 102 includes, as shown in an exploded perspective view of Fig.3, a frame base assembly 126 and a feed roller 114. The line feed mechanism 102 conveys the sheet fed as indicated in Fig. 2 from a pickup unit (not shown) toward a recording head (not shown) mounted in a carriage 112 of the carriage mechanism 104. Generally, the recording head is provided by an ink cartridge (not shown) unified with an ink container. The ink cartridge is exchangeable, and the recording head is installed opposite to the surface of the sheet at the bottom of the ink cartridge. If the sheet is inserted into the nip between the feed roller 114 and a friction roller 128, the sheet is conveyed toward the recording head by the feed roller 114 rotated by the line feed motor 118 to record an image and ejected to the exterior. Whether the sheet reaches a position between the feed roller 114 and the friction roller 128 is sensed by a sheet sensor 120 consisting of a transmission optical sensor 122 and an actuator feed 124. The actuator feed 124 is installed just in front of the feed roller 114 and the friction roller 128, and cuts off an optical path between a luminous element and a light receiving element of the optical sensor 122. Then, if the sheet is fed, the actuator feed 124 is pushed by a front end of the sheet. Therefore, light from the luminous element of the optical sensor 122 is transmitted to the light receiving element and the sheet sensor 120 senses that the sheet has reached a position between the feed roller 114 and the friction roller 128. The carriage mechanism 104 shifts the ink cartridge mounted in the carriage 112 in the horizontal direction by the carriage motor 116. The carriage 112 moves to the right and left horizontally along a transversely extending carrier shaft 110. The home assembly 106 caps the recording head when the carriage 112 moves to a home position while the printer is not used. The capping ensures that the recording head is covered with a cap to prevent ink within the nozzles of the recording head from drying or being polluted. In this case, the nozzle is typically cleaned up. On the other hand, the friction roller assembly 108 consists of a plurality of sub assemblies each having the friction roller 128, a friction roller guide 130 and a tension spring 130, and sticks the sheet to the feed roller 114. In each sub assembly, the friction roller 128 is installed at an axis parallel to the feed roller 114 and is in contact with the feed roller 114. One end of the friction roller guide 130 is connected to the friction roller 128, and the other end thereof is connected to the spring 132 fixed to the frame 100. Hence, the friction roller guide 130 elastically sticks the friction roller 128 to the feed roller 114. The structure for the conveyance of the sheet of the feed roller 114 and the friction roller assembly 108 is shown in Fig. 4. The opposite side of the friction roller guide 130 to that connected to the friction roller 128, which is in contact with the feed roller 114, is pulled by a spring 132 in the direction of the frame 100, and thus the friction roller 128 presses against the feed roller 114. If the sheet reaches the feed roller 114, the friction roller 128 is lifted by the thickness of the sheet by the insertion of the sheet but continues to elastically stick the sheet to the feed roller 114. Therefore, the sheet is conveyed toward the recording head. The operation of the friction roller guide 130 during the conveyance of the sheet will now be described in detail. If the sheet is inserted between the feed roller 114 and the friction roller 128, the opposite side of the friction roller guide 130 to that connected to the friction roller 128 moves in a direction opposite to the movement of the friction roller 128 shifted by the sheet. That is, a part of the friction roller guide 130 connected to the friction roller 128 is lifted up by the sheet, while a part of the friction roller guide 130 connected to the spring 132 goes down. The amount of movement corresponds to the thickness of the sheet. The present invention automatically senses the thickness of the fed sheet in consideration of the above principles. Furthermore, the recording head is adjusted correspondingly to the sensed thickness of the sheet. Therefore, the optimal recording picture quality can be adaptively maintained according to the thickness of the sheet. Fig. 5 shows the mechanism of a sheet thickness sensing apparatus for sensing the thickness of the sheet according to the present invention. Instead of the friction roller guide 130 shown in Fig. 4, a friction roller guide 136 having a reflective surface 138 is used, and a reflective optical sensor 140 is separately installed opposite to the reflective surface 138. That is, the optical sensor 140 is fixed to the frame 100 so that the luminous element and the light receiving element of the optical sensor 140 are opposite the reflective surface 138. The reflective surface 138 and the optical sensor 140 are installed at a portion to which the spring 132 is not connected from the roller guide 136. In Fig. 5, the spring 132 is not shown to avoid confusion because the spring 132 is overlapped with the optical sensor 140 if it is shown. Furthermore, the reflective surface 138 and the optical sensor 140 are installed at one of the plurality of sub assemblies of the friction roller assembly 108, preferably at the sub assembly beside the home position. This is because the size of the sheet is not uniform. Since the home position is a reference position, even if the size of the sheet differs, the sheet passes through the home position and therefore the thickness of the sheet can be sensed. In the thickness sensing apparatus of Fig. 5, a reflective optical quantity of the optical sensor 140 with respect to the reflective surface 138 is determined by the gap between the reflective surface 138 and the optical sensor 140. The reflective surface 138 moves in a direction opposite to the moving direction of the friction roller 128 shifted by the thickness of the sheet by operation of the friction roller guide 136. In other words, if the sheet is inserted between the feed roller 114 and the friction roller 128, a part of the friction roller guide 136 connected to the friction roller 128 is lifted by the sheet, while the reflective surface 138 goes down. Since the amount of movement corresponds to the thickness of the sheet, the gap between the reflective surface 138 and the optical sensor 140 varies with the thickness of the sheet. In the apparatus described, the reflective surface is arranged at a slight angle. As a result, the locus of the reflected light traverses the optical sensor as the friction roller guide moves and this changes the amount of light detected by the optical sensor. Advantageously, the arrangement is such that the amount of light detected decreases with increasing distance of the reflective surface from the sensor. Alternatively of in addition, the use of a diffuse light source, i.e an uncollimated light source, where the intensity of light varies inversely with the square of the distance travelled, allows the degree of movement of the friction roller guide to be determined, any such movement manifesting itself two-fold as an increase in distance travelled by light from the source before it arrives at the sensor. Consequently, the reflective optical quantity of the optical sensor 140 varies in proportion to the thickness of the sheet. Namely, the reflective optical quantity gradually decreases as the thickness of the sheet becomes thicker, and gradually increases as the thickness of the sheet becomes thinner. If there is no sheet, the reflective optical quantity reaches the maximum. Examples of operational states of the sheet thickness sensing apparatus of Fig. 5 according to variation in the thickness of the sheet are shown in Figs. 6A and 6B. Figs. 6A and 6B illustrate relative operational states when the thickness of the sheet differs, Fig. 6A being the case the thickness of the sheet is thin and Fig. 6B being the case the thickness of the sheet is thick. The optical sensor 140 generates a sensing signal of a voltage level corresponding to the reflective optical quantity supplied to the light receiving element. Typically, since the output characteristic of the optical sensor 140 is linear, the reflective optical quantity and the voltage level of the sensing signal are linear. Therefore, the thickness of the sheet can be sensed by the sensing signal generated from the optical sensor 140. Consequently, unlike the above Korean patent application no. 94-12167, since the thickness of the sheet is sensed by a variation in the reflective optical quantity according to the thickness of the sheet, variations in the thickness of the sheet can be accurately sensed. Moreover, since a relatively cheap optical sensor can be used, as compared with the pressure sensor, the cost of the product is lower. In the above embodiment, although the thickness of the sheet is sensed by the reflective optical sensor 140 using the conventional friction roller assembly 108, it is possible to sense the thickness of the sheet by adding the thickness sensing apparatus according to the present invention to a sheet convey path if the friction roller assembly 108 is not used. Fig. 7 is a block diagram showing a circuit for adjusting the recording head by automatically sensing the thickness of the sheet. In Fig. 7, a thickness sensing circuit 162 constructed as shown in Fig. 8 is added to the typical ink jet printer, and a recording head adjusting function performed by a CPU (central processing unit) 144 of a controller 142 as indicated in Fig. 10 is programmed in a ROM 148. Therefore, the basic functions of other parts are the same as the typical ink jet printer. Referring to Fig. 7, the controller 142 has the CPU 144, an interface 146, the ROM (read only memory) 148 and a RAM (random access memory) 150. The CPU 144 executes a program stored in the ROM 148 and controls each part of the printer through the interface 146. An execution program of the CPU 144 and various reference data are stored in the ROM 148. Data generated during an operation of the CPU 144 is temporarily stored in the RAM 150. The interface 146 interfaces input/output signals between the CPU 144 and input/output devices, that is, signals between the CPU 144, and an operation panel 152, motor drivers 154 and 156, a head driver 158, a thickness sensing circuit 162 and a sheet sensor 120. The operation panel 152 has a plurality of buttons for receiving various commands and a display unit for displaying various operation states by the control of the CPU 144. The motor driver 154 drives the carriage motor 116 by the control of the CPU 144. The carriage motor 116 is driven by the motor driver 154 and shifts the carriage 112 in the horizontal direction. The motor driver 156 drives the line feed motor 118 by the control of the CPU 144. The line feed motor 118 is driven by the motor driver 156 and feeds and conveys the sheet. The head driver 158 drives a recording head 160 mounted in the carriage 112 by the control of the CPU 144 and records an image on the sheet. The recording head 160 has a plurality of nozzles and is driven by the head driver 158. The recording head 160 records the image on the sheet by jetting ink through the nozzles. The thickness sensing circuit 162 includes, as described previously, the optical sensor 140 and supplies a sheet thickness value corresponding to the reflective optical quantity to the CPU 144. The sheet sensor 120 installed as indicated in Fig. 2 senses the sheet and informs the CPU 144 that the sheet is fed. Fig. 8 is a detailed circuit diagram of the thickness sensing circuit 162 shown in Fig. 7. The luminous element of the optical sensor 140, that is, a light emitting diode LED is forwardly connected between a power voltage Vcc through a resistor R1 and a ground terminal. The light receiving element of the optical sensor 140, that is, a photo transistor PT is connected between the power voltage Vcc through a resistor R2 and a non-inverting input terminal (+) of an amplifier 164. Therefore, the sensing signal having the voltage level corresponding to the reflective optical quantity is generated from the photo transistor PT. The sensing signal is amplified by the amplifier 164 and supplied to an analog-to-signal (ADC) 166. The ADC 166 converts the sensing signal into digital data having a value corresponding to its voltage level and supplies the digital data to the CPU 144 through the interface 146 as a thickness sensing value V T . An output state of the thickness sensing circuit 162 as a function of a variation in the thickness of the sheet is indicated in Fig. 9. When classifying the thickness of the sheet into 3 ranges of T1, T2 and T3, the thickness sensing value V T varies. T1 is a thickness range of a relatively thin sheet, T2 is a thickness range of a medium sheet, and T3 is a thickness range of a thick sheet. The CPU 144 confirms the thickness sensing value V T generated from the thickness sensing circuit 162, and judges that the currently fed sheet is a thin sheet if the thickness sensing value V T is within the range of V T1 , that it is a medium sheet if V T is within the range V T2 , and that it is a thick sheet if V T is within the range V T3 . Fig. 10 is a processing flow chart of the CPU 144 shown in Fig.7 according to the present invention. There is shown a process for sensing the thickness of the sheet by the thickness sensing circuit 162 when the sheet reaches the thickness sensing apparatus of Fig. 5 after feeding the sheet and for adjusting a head driving enable time of the recording head correspondingly to the sensed thickness of the sheet. An example of an operation of sensing the thickness of the sheet and adjusting the driving enable time of the recording head 160 using the thickness sensing apparatus of Fig. 5 in the ink jet printer of Fig. 7 will now be described in detail with reference to Fig. 10. The CPU 144 starts to feed the sheet from the pickup unit (not shown) by driving the line feed motor 118 in response to a print start command at step A1. The CPU 144 checks whether the sheet is sensed by the sheet sensor 120 at step A2. Generally, when feeding the sheet, the CPU 144 backwardly rotates the line feed motor 118 to rotate a pickup roller (not shown). When the sheet reaches the sheet sensor 120, the CPU 144 forwardly rotates the line feed motor 118 to rotate the feed roller 114. If the line feed motor 118 is backwardly rotated, power is transmitted only to the pickup roller and not transmitted to the feed roller 114. If the line feed motor 118 is forwardly rotated, the power is transmitted not to the pickup roller but to the feed roller 114. Therefore, if the sheet reaches the feed roller 114, a front end of the sheet temporarily stops until the feed roller 114 starts to rotate when it is inserted between the feed roller 114 and the friction roller 128. Hence, the sheet is aligned. If the sheet is sensed by the sheet sensor 120, the front end of the sheet stops under the state it is inserted between the feed roller 114 and the friction roller 128. Then the CPU 144 reads the sheet thickness value V T generated from the thickness sensing circuit 162 at step A3. As mentioned above, the sheet thickness value V T corresponds to the thickness of the sheet. The CPU 144 retrieves the head driving enable time corresponding to the sheet thickness value V T from a lookup table previously stored in the ROM 148 at step A4. The lookup table is listed below in Table 1. Thickness of sheet Sheet thickness value V T Head driving enable time thin V T1 3.0 µs medium V T2 2.5 µs thick V T3 2.0 µs If the head driving enable time corresponding to the range of the sheet thickness value V T is retrieved from the above Table 1, the CPU 144 alters the head driving enable time of the recording head 160 to the retrieved head driving enable time at step A5. That is, the CPU 144 adjusts the head driving enable time correspondingly to the range of the sheet thickness value V T among a plurality of differently set adjusting ranges. If the thickness of the sheet is thin, the CPU 144 lengthens the head driving enable time, and if the thickness of the sheet is thick, it shortens the head driving enable time. However, if the fed sheet is the same as the previous sheet, there is no need to adjust the head driving enable time. Generally, since the head driving enable time is adjusted by the CPU 144 by controlling the head driver 158, a detailed description of it has been omitted. By the above described operation, if the thickness of the sheet is thin, the size of an ink drop jetted from the recording head 160 becomes larger by lengthening the head driving enable time. If the thickness of the sheet is thick, the size of the ink drop becomes smaller by shortening the head driving enable time. After the recording head is adjusted, a printing operation is started. Therefore, even though the thickness of the sheet varies, since the head driving enable time is automatically adjusted, the size of the ink drop or the shock power dropped onto the sheet is constantly maintained and thus optimal recording picture quality can be obtained. Meanwhile, even if a head gap is adjusted instead of adjusting the driving enable time of the recording head 160 according to the thickness of the sheet, the same result can be obtained. To this end, a gap adjusting apparatus is shown in Fig. 11, and a circuit construction according thereto is illustrated in Fig. 12. In Fig. 12, a motor driver 182 and a gap adjusting motor 172 are added to the circuit construction of Fig. 7. Fig. 11 is a perspective view of the gap adjusting apparatus according to the present invention. A carrier shaft 168 provides a horizontal moving axis for an ink cartridge 170, that is, for the carriage 112. The carrier shaft 168 has a cam structure and both horizontal end portions thereof are rotated at the frame 100, unlike the typical carrier shaft 110. This carrier shaft 168 is rotated by the gap adjusting motor 172. A gear 174 installed at a rotary axis of the gap adjusting motor 172 is connected to a gear 176 installed at one horizontal end portion of the carrier shaft 168. A locked projection 180 projecting from the frame 100 is for setting an initial position of a cam part of the carrier shaft 168 and causes a rib 178 protruding from the gear 176 to be correspondingly locked to the locked projection 180. The carrier shaft 168 shown in Fig. 11 illustrates an its initial state. The CPU 144 shown in Fig. 12 initializes the position of the recording head before adjusting the head gap by the gap adjusting motor 172. In more detail, the CPU 144 rotates the gap adjusting motor 172 clockwise through the motor driver 182 by a constant distance. In this case, the motor driver 182 drives the gap adjusting motor 172 by the control of the driving direction and of the number of pulses of the CPU 144. Then the gear 176 is rotated counterclockwise. If the rib 178 of the gear 176 is locked to the locked projection 180, the gear 176 stops rotating, and the carrier shaft 168 returns to its initial state. In this initial state, the height of the horizontal moving axis of the carrier shaft 168 is lowest. Therefore, the height of the carriage 112 is also the lowest state, and the height of the recording head 160 fixed to the bottom of the ink cartridge 170 becomes the lowest state. An initial distance for rotating the gap adjusting motor 172 is appropriately set in consideration of the previous height of the recording head 160, that is, the previous position of the rib 178. If the gap adjusting motor 172 is rotated counterclockwise, the gear 176 and the carrier shaft 168 are rotated clockwise, and the height of the ink cartridge 170 or the height of the recording head 160 becomes high. When the height of the recording head 160 is higher than the initial state, if the gap adjusting motor 172 is rotated clockwise, the gear 176 and the carrier shaft 168 are rotated counterclockwise, and the height of the recording head 160 becomes low again. That is, the head gap can be automatically adjusted by adjusting the height of the recording head 160 using the gap adjusting apparatus according to the thickness of the sheet. Fig. 13 is a processing flow chart of the CPU 144 shown in Fig. 12. There is shown a process for sensing the thickness of the sheet by the thickness sensing circuit 162 when the sheet reaches the thickness sensing apparatus of Fig. 5 after the sheet is fed and for adjusting the head gap by the gap adjusting apparatus of Fig. 11 correspondingly to the sensed thickness of the sheet. An example of an operation for sensing the thickness of the sheet and for adjusting the head gap by using the thickness sensing apparatus of Fig. 5 and the gap adjusting apparatus of Fig. 11 in the ink jet printer of Fig. 12 will now be described in detail with reference to Fig. 13. The CPU 144 starts to feed the sheet from the pickup unit by driving the line feed motor 118 in response to a print start signal at step B1. The CPU 144 checks whether the sheet is sensed at step B2. If the sheet is sensed, the CPU 144 reads the sheet thickness value V T generated from the thickness sensing circuit 162 at step B3. The CPU 144 retrieves the number of steps of the gap adjusting motor 172 on the basis of the sheet thickness value V T from a lookup table previously stored in the ROM 148 at step B4. The lookup table is listed below in Table 2. Thickness of sheet Sheet thickness value V T Number of pulses thin V T1 20 medium V T2 25 thick V T3 30 If the number of pulses corresponding to the range of the sheet thickness value V T from the above Table 2 is retrieved, the CPU 144 drives the gap adjusting motor 172 by controlling the motor driver 182 at step B5, thereby adjusting the head gap. At this time, the CPU 144 adjusts the height of the recording head to the initial state and rotates the gap adjusting motor 172 counterclockwise by the retrieved number of pulses. Namely, the CPU 144 adjusts the head gap correspondingly to the range of the thickness of the sheet V T among a plurality of differently set adjusting ranges. If the thickness of the sheet is thin, the CPU 144 makes the height the recording head 160 lower by driving the gap adjusting motor 172 by the smaller number of pulses. If the thickness of the sheet is thick, the CPU 144 makes the height of the recording head 160 higher by driving the gap adjusting motor 172 by the larger number of pulses. However, if the fed sheet is the same as the previous sheet, there is no need to adjust the height of the recording head 160. By the above described operation, if the thickness of the sheet is thin, the height of the recording head 160 is lowered. If the thickness of the sheet is thick, the height of the recording head 160 is raised, thereby constantly maintaining the head gap. When the recording head has been adjusted, a printing operation is started. Therefore, since the head gap is adaptively adjusted even though the thickness of the sheet varies, the head gap is maintained at a constant state, and thus the optimal recording picture quality can be obtained. As noted above, the present invention not only senses the thickness of the sheet accurately but also lowers the cost of a product by using the optical sensor instead of the pressure sensor. Moreover, the user need not manipulate the adjusting lever since the recording head is automatically adjusted according to the thickness of the sheet, and the optimal recording picture quality can be maintained. Although the sheet thickness sensing apparatus embodying the present invention is applied to the ink jet printer, it may be applied to all image recording apparatuses requiring a sheet thickness sensing function. Furthermore, a recording head adjusting technique may be applied not only to the ink jet printer but also to all recording apparatuses using ink jet recording, for example, to an ink jet facsimile. When sensing the thickness of the sheet, other sheet thickness sensing apparatuses as disclosed in the above Korea Patent Application No. 94-12167 may be used instead of the sheet thickness sensing apparatus of the present invention. Further, instead of dividing the driving enable time of the recording head or the head gap into 3 ranges according to the thickness of the sheet, its adjusting steps may be minutely classified for more precise adjustment.

Claims: A sheet thickness sensing apparatus of an image recording apparatus, comprising: a feed roller (114) for conveying a fed sheet; a movable friction roller guide (130) having a reflective surface (138); a friction roller (128) carried by the friction roller guide (130), the friction roller guide (130) being biased in such a way as to push the friction roller (128) against the feed roller (114); and a thickness sensing circuit (162) having an optical sensor (140) positioned to detect reflections from the reflective surface (138) and adapted to produce a sheet thickness value derived from a property of the detected reflections. A sheet thickness sensing apparatus according to claim 1 in which the friction roller (128) is carried on one end of the friction roller guide (130) and the reflective surface (138) is on the other end. A sheet thickness sensing apparatus according to claim 2 in which the reflective surface (138) moves in the opposite direction to the friction roller (128) when the latter is moved by the fed sheet. A sheet thickness sensing apparatus according to claim 3 in which the gap between the reflective surface (138) and the optical sensor (140) varies substantially linearly with the thickness of the sheet. A sheet thickness sensing apparatus according to any preceding claim in which the friction roller guide (130) is mounted for pivotal movement about an intermediate position. A recording head automatic adjusting apparatus of an ink jet recording apparatus, comprising: a sheet thickness sensing apparatus according to any preceding claim; and a controller (142) for adjusting a head driving enable time of a recording head (160) according to the thickness of the sheet sensed by the thickness sensing apparatus. A recording head automatic adjusting apparatus according to claim 6 in which the controller (142) adjusts the head driving enable time according to the sheet thickness value generated by the thickness sensing circuit (162) when the fed sheet reaches the nip between the feed roller (114) and the friction roller (128). A recording head automatic adjusting apparatus according to claims 6 or claim 7 in which the controller (142) adjusts the head driving enable time to be in proportion to the sheet thickness value. A recording head automatic adjusting apparatus of an ink jet recording apparatus, comprising: a sheet thickness sensing apparatus according to any one of claims 1-5; means for adjusting the height of a recording head (160); and a controller (142) for controlling the adjusting means to set the gap between the recording head (160) and the fed sheet in correspondence with the thickness of the sheet sensed by the thickness sensing apparatus. A recording head automatic adjusting apparatus according to claim 9 in which the means for adjusting the height of the recording head (160) comprises: a carrier shaft (168) along which a recording head carriage (112) moves, the carrier shaft (168) being eccentrically mounted with respect to a frame of the recording apparatus; a gap adjusting motor for turning the carrier shaft (168) so as to alter the height of the carriage (112); and a motor driver for driving the gap adjusting motor under the control of the controller (142). A recording head automatic adjusting apparatus according to claim 9 or claim 10 in which the controller (142) adjusts the head gap substantially linearly with the sheet thickness value. A recording head automatic adjusting apparatus according to any one of claims 6-11 in which the controller (142) adjusts the head driving enable time or the head gap in accordance with one range of sheet thickness values among a plurality of different ranges. A method of adjusting a recording head (160) in an ink jet recording apparatus comprising: sensing the thickness of a fed sheet using a sheet thickness sensing apparatus according to any one of claims 1-5; and adjusting a head driving enable time of the recording head (160) or the gap between the recording head (160) and the fed sheet according to the sensed thickness of the sheet. A method according to claim 13 in which the head driving enable time is shortened when the thickness of the sheet is lower and lengthened when the thickness of the sheet is greater. A method according to claim 13 in which the gap between the recording head (160) and the fed sheet is lower when the thickness of the sheet is lower and higher when the thickness of the sheet is greater.

Inventors: JU YOUNG-BOK, JU, YOUNG-BOK
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1997
Main classification: G01B21
IPC codes: G01B21, B41J25, B41J2, G01B11, B65H7, B41J11