GB2300290A - Impact dot printer - Google Patents

Abstract

A pulse motor driver transmits a phase drive pulse signal to a paper feeding pulse motor (32) to advance paper material into a print gap (G) by a predetermined extent for every receipt of the phase drive pulse signal. A paper thickness sensor (50) detects paper thickness at a position upstream of the print gap for every input of the phase drive pulse signal. According to the detection result made by the paper thickness sensor (50), a CPU (19), serving as an adjustment circuit, retrieves a print gap position of a print head (10) from a table (76) and loads a buffer (77) to control a print gap adjusting pulse motor (61) correspondingly when the portion of paper whose thickness has been detected is fed into the print gap. The sensor (50) uses optical reflection from a lever (53) contacting the paper.

Description

SPECIFICATION IMPACT DOT PRINTER BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to an impact dot printer comprising a print head moving vertically in response to the thickness of a paper material passing through a print gap formed between a platen and the print head. The impact dot printer causes the print head to move to a vertical position corresponding to the paper thickness so as to carry out a printing operation with a constant print gap to the surface of the paper material.

Description of the Prior Art: An impact dot printer, which generally carries out a printing operation by impacting dot pins against a paper material on a platen, is often used for printing on a bank passbook, a pressure sensitive copying voucher or the like. In such a use, the thickness of paper varies depending on the number of pages of the bank passbook opened up, or the number of vouchers stacked up.

It is preferable to conduct the printing operation while adjusting the print gap between the print head and the platen depending on a variation in paper thickness so as to keep a constant space between the print head and the surface of a paper material on the platen.

An impact dot printer has been known for moving a print head vertically so as to keep a constant print gap between the print head and a paper material. Japanese Patent Laid-Open No. Hei 397579 Publication discloses, for instance, a printer comprising a contact type paper thickness sensor disposed on a carriage, which supports a print head and moves horizontally on a carriage shaft. With this arrangement, since the paper thickness is continuously detected in the vicinity of the print head, it is possible to maintain a substantially constant print gap to a paper material even if the paper thickness varies, as a result of which a favorable printing result can be obtained.

However, this type of printer keeps the paper thickness sensor in contact with the paper material when the print head is moving horizontally during a printing operation. The reference point for a detector is thus displaced due to abrasion of the detector, which may cause an inaccurate measurement of the paper thickness. Further, since the paper thickness sensor is disposed right at the side of the print head, that is, in the path of horizontal movement of the print head, it is necessary to move the print head to a predetermined vertical position corresponding to the paper thickness simultaneously with the measurement of the paper thickness so as to cause the print head to follow the paper thickness.When the fed paper material suddenly becomes much thicker, the print head cannot be instantly raised corresponding to the increased thickness, which may cause the dot pins to bump against the paper material and become damaged.

SUMMARY OF THE INVENTION The present invention therefore aims to provide an impact dot printer capable of continuously measuring paper thickness of a paper material at a position upstream of a print gap as the paper material advances, so as to easily respond to variations in paper thickness.

The present inventors propose a known non-contact type paper thickness sensor 101 as shown in Fig. 14 to be used for gap adjustment in an impact dot printer. The non-contact type paper thickness sensor 101 comprises a light emitting diode 102 for emitting light L1 toward the surface of a paper material PP, and a position sensitive detector, PSD, 103 for detecting reflected light L2 being diffused from the surface of the paper material PP. Fig. 15 illustrates that the PSD 103 determines a centroid position where the reflected light L2 shows the maximum value in the light intensity distribution, and outputs two electrical current values Il and I2 depending on the centroid position.The centroid position is displaced on the PSD 103 in proportion to a distance between the PSD 103 and the surface of the paper material PP, so that the detection of a centroid position leads to measurement of the distance between the PSD 103 and the surface of the paper material PP, namely, the paper thickness from a reference face.

Since a PSD of this type carries out measurement utilizing a light intensity distribution caused by diffusion of reflected light, it shows a good sensitivity to a white paper material. It has a problem of variation of measurement values for a colored or lined paper material. The present inventors have achieved the present invention in light of this problem.

According to a first aspect of the invention, there is provided an impact dot printer comprising: a print head moving vertically in response to the thickness of a paper material passing through a print gap formed between a platen and the print head; a paper feeding pulse motor for feeding the paper material into the print gap by a predetermined extent every time a phase drive pulse signal is received; a pulse motor driver for outputting the phase drive pulse signal to the paper feeding pulse motor; a paper thickness sensor for detecting the thickness of the passing paper material at a position upstream of the print gap for every phase drive pulse signal; a head position adjustment circuit for determining a vertical position of the print head based on a detected result by the paper thickness sensor; a buffer storing the vertical position determined by the head position adjustment circuit; and a control circuit for reading out from the buffer the vertical position corresponding to the detected result of a portion of the paper material when the portion is fed into the print gap; wherein the print head is moved to the vertical position according to the paper thickness previously detected when the portion reaches the print gap, so that the print head carries out a printing operation with a constant print gap to a surface of the paper material.

With the above arrangement, since the paper thickness sensor can detect paper thickness in synchronism with a phase drive pulse signal supplied to the paper feeding pulse motor, the printer can keep a print gap corresponding to the paper thickness irrespective of a considerable variation in the paper thickness.

Further, since the paper thickness is detected at a position upstream of the print gap, it is possible to cope with the variation of the paper thickness before it reaches the print gap.

The print head can therefore be adjusted so as to define a print gap for subsequent printing prior to feeding of the paper material.

According to a second aspect of the invention, there is provided an impact dot printer according to the first aspect, wherein the paper thickness sensor includes: a paper thickness detection lever for tracing the surface of the paper material so as to convert an extent of the paper thickness into a vertical displacement of a reflection face; a light emitting diode for emitting light toward the reflection face; and a PSD for detecting a distance to the reflection face in accordance with a light intensity of the light reflected from the reflection face; the paper thickness detection lever having a supporting section rotatably supported by a stationary base, a curved section formed continuously from the supporting section for contacting the surface of the paper material, and an amplifying section formed continuously from the curved section for constituting the reflection face using an attached white tape.

With the above arrangement, the lever is attached to a stationary base instead of a carriage so that the lever does not move irrespective of the movement of the print head, thereby preventing abrasion of the lever. Moreover, the PSD can detect paper thickness with high accuracy irrespective of the variation of paper color and the existence of lines on the paper material since the reflection face is constituted of a white tape attached to the paper thickness detection lever.

According to a third aspect of the invention, there is provided an impact dot printer according to the second aspect, wherein the paper thickness sensor detects a position of the reflection face with the paper thickness detection lever tracing no paper material prior to feeding the paper material to the print gap, to thereby measure a distance between the PSD and the reflection face, the paper thickness being determined as an extent of a displacement of the reflection face, on the basis of the measured distance.

With the above arrangement, the extent of a sensor gap can be measured with no paper material prior to feeding a paper material into the print gap. Even if the paper thickness detection lever sinks due to abrasion or a supporting mechanism of the paper thickness sensor becoming deformed, both causing the reference level of the sensor gap to be varied, the sensor gap can be detected without error. Furthermore, since the establishment of the reference level can be made every time a new paper material is fed in, an accurate reference level for a sensor gap can be constantly maintained.

BRIEF DESCRIPTION OF THE DRAWINGS The above and the other objects, features and advantages will be apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings wherein: Fig. 1 is a schematic elevation showing an entire impact dot printer of horizontal paper feeding type according to the present invention; Fig. 2 is a schematic elevation showing the frame structure within the printer; Fig. 3 is a schematic sectional view along the line 3-3 in Fig. 1, showing main elements of a driving mechanism of the printer; Fig. 4 is an enlarged schematic view showing a driving mechanism of feeding roller means of the printer; Fig. 5 is a sectional plan view schematically showing main elements of a driving mechanism of the printer; Fig. 6 is a schematic view showing a paper thickness sensor;; Fig. 7 shows main elements of a gap adjusting device; Fig. 8 is a block diagram showing an adjustment circuit of a print gap; Fig. 9 is a flow chart for explaining the overall operation of the printer; Fig. 10 is a flow chart for explaining the operation for correcting the reference; Fig. 11 is a flow chart for explaining the operation for printing a first line; Fig. 12 is a flow chart for explaining the operation for printing a subsequent line; Fig. 13 is a flow chart for explaining a data processing operation for data from the paper thickness sensor; Fig. 14 shows a circuit structure of a known non-contact type paper thickness sensor; and Fig. 15 shows an operating principle of a PSD as a paper thickness sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will next be described with reference to the accompanying drawings.

Fig. 1 schematically shows an impact dot printer of horizontal paper feeding type according to the present invention. The impact dot printer P comprises a print head 10 for impacting dot pins against a paper material for printing. The paper material is slid on a paper feeding table 12 mounted onto the front surface of a housing 11 until the paper material reaches the print head 10 through a paper feeding main slot 13.

As shown in Fig. 2, left and right frames 14 and 15 are fixedly attached to a base plate lla of the housing 11, opposing each other. A carriage shaft 17 extends between the frames 14 and 15 for supporting horizontal movement of a carriage 16, to which the print head 10 is mounted. A platen 18 is provided below the moving path of the print head 10 along the carriage shaft 17, for receiving impacts caused by the dot pins of the print head 10. A print gap G is defined between the platen 18 and the tip end of the print head 10. A paper material which is fed via the paper feeding main slot 13 passes through the print gap G. The print head 10 prints on the paper material passing through the print gap G, while horizontally moving along the carriage shaft 17 according to a control signal output from a CPU 19 mounted on a board on the base plate lla.An ink ribbon, not shown, is mounted on the carriage 16, so that the ink ribbon can be constantly fed to a position between the print head 10 and the paper material.

As is apparent from Fig. 3, a guide plane 21 extends from the paper feeding main slot 13 to a paper feeding sub-slot 20 for horizontally guiding a paper material inserted from the paper feeding main slot 13 to the paper feeding sub-slot 20. The platen 18 is provided on the guide plane 21 opposite to the print head 10 at the substantially central part thereof. First and second feeding roller means 22 and 23 are positioned respectively upstream and downstream of the platen 18 in view of the paper feeding main slot 13 for forcefully advancing the paper material fed along the guide plane 21. The first and second feeding roller means 22 and 23 hold the fed paper material between a respective one of the pairs of upper and lower cylindrical rollers 26 and 27 so that the paper material can be fed by rotating the rollers 26 and 27 in an appropriate direction.

Since the first and second feeding roller means 22 and 23 have a substantially similar structure, only the first feeding roller means 22 will be explained in the following detailed description. Referring to Fig. 4, the first feeding roller means 22 includes a lower cylindrical roller 26 contacting the back surface of a paper material at the same level as the guide plane 21, and an upper cylindrical roller 27 biased toward the lower cylindrical roller 26 so as to hold a paper material between the lower and upper cylindrical rollers 26 and 27. The upper cylindrical roller 27 rotates about a rotation axis 31 which is supported by a swing arm 30 for swinging movement. Bias in this case means biasing the swing arm 30 so as to urge the upper cylindrical roller 27 against the lower cylindrical roller 26.

In this way, the upper cylindrical roller 27 can vary the extent of the gap formed between the lower and upper cylindrical rollers 26 and 27. As a result, if the paper thickness of a paper material passing between the lower and upper cylindrical rollers 26 and 27 varies, it is possible to respond to the variation at any time.

The respective cylindrical rollers 26 and 27 are driven to rotate by a paper feeding pulse motor 32 fixed to the frame 15.

The paper feeding pulse motor 32 imparts a driving force to the respective cylindrical rollers 26 and 27 via a first driving force transmission mechanism 34 using a rubber timing belt 33.

The first driving force transmission mechanism 34 includes a first idler 35, a first rotation pulley 36, a first transmission pulley 37, a second idler 38, a second transmission pulley 39 and a second rotation pulley 40, around all of which an endless rubber timing belt 33 is wound. The first and second rotation pulleys 36 and 40 are fixedly attached to rotation axes 41 and 42, respectively, of the lower cylindrical rollers 26 of the first and second feeding roller means 22 and 23. Transmission gears are coaxially fixed to the first and second transmission pulleys 37 and 39 for causing third and fourth rotation gears 43 and 44, which are fixed to the rotation axes 31 of the upper cylindrical rollers 27, to rotate.When the paper feeding pulse motor 32 rotates clockwise, the first and second rotation pulleys 36 and 40 and the first and second transmission pulleys 37 and 39 are caused to rotate clockwise, while the first and second rotation gears 43 and 44 are caused to rotate counterclockwise.

As a result, a paper material is advanced as a result of cooperation between the upper and lower cylindrical rollers 27 and 26. The rotation speed of the paper feeding pulse motor 32 is set by a control signal output from the CPU 19.

Referring to Fig. 5, three paper thickness sensors 50 are provided adjacent to the first and second feeding roller means 22 and 23 for detecting the thickness of a paper material passing through the print gap G. The three paper thickness sensors 50 are arranged apart from one another in the direction of paper width, with two positioned upstream and one positioned downstream of the platen 18. It is preferable to arrange the sensors 50 depending on the kind of paper material supplied to the printer.

For example, two sensors 50 positioned upstream of the platen 18 would be effective for use in printing on a bank passbook, which is opened and will have different paper thickness on the right and left sides depending on the number of pages.

Referring to Fig. 6, the paper thickness sensor 50 includes a reflecting photo sensor 52 fixedly held by the guide plate 51 above the guide plane 21, and a paper thickness detection lever 53 supported by the guide plate 51 for swinging movement.

Referring to 14, the reflecting photo sensor 52 emits light L1 from an emitting light diode 102 to a reflection face. The sensor 52 detects the distance from the reflection face to the PSD 103 in accordance with the light intensity of the reflected light L2 from the reflection face. The paper thickness detection lever 53 has a supporting section 53a rotatably supported by the stationary guide plate 51 via an axis 54, a curved section 53b connected to the supporting section 53a for tracing the surface of a paper material PP passing through the print gap G, and an amplifying section 53c formed continuously from the curved section 32b for constituting a reflection face by means of a white tape attached thereto.The light emitted from the light emitting diode 102 is reflected at the reflection face on the upper surface of the amplifying section 53c of the paper thickness detection lever 53 so that the PSD can detect the reflected light. During this process, the paper thickness detection lever 53 traces the surface of the paper material PP, so that the paper thickness is transformed into vertical displacement of the reflection face in an amplified manner. The detection of the vertical displacement from the reference level, which is established with no paper material existing, can lead to the detection, the thickness of a paper material passing through the print gap G. The photo sensor 52 outputs a distance signal, corresponding to the paper thickness, to a controller or the CPU 19.

Referring to Figs. 3 and 7, the gap adjusting device GG utilizes a signal derived from the distance signal at the CPU 19.

The gap adjusting device GG thereby moves the print head 10 toward/away from the platen 18 in accordance with the thickness of the paper material supplied, in order to adjust the print gap G such that the print head 10 carries out a printing operation with a constant space between the print head 10 and the surface of the paper material on the platen 18. The gap adjusting device GG comprises a print gap adjustment pulse motor 61 which imparts a driving force to the print head 10 via a second drive force transmission mechanism 60 so as to move the print head 10 toward/away from the platen 18. The CPU 19 controls the amount of rotation of the pulse motor 61 according to a signal, corresponding to the paper thickness, from the paper thickness sensor 50.

The second driving force transmission mechanism 60 includes a displacement gear 62 rotatably supported by the frame 15. The displacement gear 62 eccentrically supports the carriage shaft 17. The displacement gear 62 has a coaxial sector gear 62a. A driving force from the pulse motor 61 is reduced by the two transmission gears 63 and 64, and then acts on the coaxial gear 62b so as to rotate the displacement gear 62 about an axis 62b.

For example, when the displacement gear 62 rotates clockwise in Fig. 7, the carriage shaft 17 is lowered accordingly, thereby causing the print head 10 to descend vertically. On the contrary, when the displacement gear 62 rotates counterclockwise in the same figure, the carriage shaft 17 is raised, thereby causing the printer head 10 to ascend vertically.

A gap lever 65, which serves as an abutment member, is integrally connected to the displacement gear 62 of the second driving force transmission mechanism 60. A guide axis 67 is attached to the tip of the gap lever 65 for moving within an arclike guide slot 66 formed on the frame 15. The extent of swinging movement of the gap lever 65 is restricted by a single stop plate 68. The stop plate 68 has a first stop 68a and a second stop 68b. The first stop 68a is abutted on by the gap lever 65 (as indicated with a dash line) so as to restrict the movement of the print head 10 away from the platen 18 when the pulse motor 61 drives the print head 10 to move away from the platen 18. The second stop 68b is abutted on by the gap lever 65 so as to restrict the movement of the print head 10 toward the platen 18 when the pulse motor 61 drives the print head 10 to approach the platen 18.When the gap lever 65 abuts on the first stop 68a, the pulse motor 61 tries to continue rotating, so that the pulse motor 61 is resultantly brought into the out-of-phase condition, thereby assuming an initial condition. The initial condition is determined to be a reference position of the print head 10. The amount of rotation of the pulse motor 61 is controlled by the number of pulses since the initial condition, whereby the print head 10 can be vertically moved in accordance with the amount of rotation. The minimum gap between the print head 10 and the platen 18 can be defined by abutting.the gap lever 65 on the second stop 68b. The minimum gap is defined so as to accommodate the thinnest paper material to be printed, such as 0.3mm.

Fig. 8 shows a schematic structure of an adjustment circuit for adjusting a print gap G with a gap adjusting device GG. The CPU 19 in the adjustment circuit includes a first pulse motor driver 70 for outputting a phase drive pulse signal to the paper feeding pulse motor 32 and a second pulse motor driver 71 for outputting a phase drive pulse signal to the print gap adjusting pulse motor 61. The first pulse motor driver 70 and the light emitting diode of the paper thickness sensor 50 respectively receive synchronous clock signals from the CPU 19.The paper feeding pulse motor 32 advances a paper material to the print gap G by a predetermined extent for every phase drive pulse signal received, simultaneously with which the light emitting diode emits light, so that the paper thickness sensor 50 detects paper thickness at a position upstream of the print gap G for every input of the phase drive pulse signal.

The CPU 19 is connected with the paper thickness sensor 50 and a photo interrupter 72. The PSD of the paper thickness sensor 50 outputs two flows of electric current according to the light intensity distribution of the reflected light, which are then converted into digital values via an A/D converter 73, so as to be received by the CPU 19. The photo interrupter 72 detects the existence or non-existence of a paper material at the print gap G.

An average value calculation memory 74 repeatedly stores a measurement value of a sensor gap, that is, a distance from the reflection face to the PSD, which has been detected by the paper thickness sensor 50. The average value of the measurement values is calculated from the stored measurement values. A reference data memory 75 stores the position of the reflection face on the condition that no paper material exists, the condition having been established prior to feeding a paper material to the print gap G. The sensor gap in this condition is taken as a reference, and the extent of the displacement of the reflection face corresponding to the paper thickness is detected as paper thickness from the reference. A print gap position table 76 prestores the optimum print gap position, that is, a vertical position of the print head 10, for every expectable paper thickness.Thus, the CPU 19, serving as a head position adjustment circuit, retrieves the optimum print gap position from the print gap position table 76, based on the result detected by the paper thickness sensor 50. A buffer 77 which comprises a FIFO or the like stores print gap positions which have been successively retrieved along with the advancement of a paper material. The CPU 19, then serving as a control circuit, reads out a corresponding print gap position from the buffer 77 when the portion of the paper material whose thickness has been detected by the paper thickness sensor 50 is fed into the print gap G.

In operation, referring to Fig. 9, when the impact dot printer P is switched on in the first step S1, the paper thickness sensor 50 carries out an operation for correcting a reference in a second step S2, thereby determining a reference position for a sensor gap. The print gap adjusting pulse motor 61 raises the print head 10 to the highest position in the third step S3, so that a reference position of the print gap position is determined by bringing the pulse motor 61 into an out-of-phase condition. The print gap position can be defined accurately on the basis of the thus determined reference position. When these processes are completed, the impact dot printer P is in a standby condition, that is, ready for a printing operation (the fourth step S4), awaiting receipt of a print command.

When the impact dot printer P receives a print command from the CPU 19 in the fifth step S5, it drives the paper feeding pulse motor 32 so as to feed a paper material from the paper feeding table 12. In the sixth step S6, a printing operation is carried out to the fed paper so as to print the first line according to the received print command. If another print command follows after the completion of printing the first line, a subsequent line printing operation is carried out to the same paper material (the seventh and eighth steps S7 and S8).When it is detected in a ninth step S9 that a whole page has finished being printed, the paper material is discharged via the paper feeding sub-slot 20 in the tenth step S10, so that the reference is corrected again after the paper material passes through the print gap G (the eleventh step Sol). In the case of a further print command being input, a second sheet of paper material is fed, so that the processes from the sixth step S6 to the eleventh step S11 are repeated. With no print command input, the printer returns to the stand-by condition of the fourth step S4, awaiting input of the next print command.If no print command follows after the completion of printing the first line, the operation proceeds from the seventh step S7 to the tenth step 10 so as to discharge the paper material.

Turning off the impact dot printer P while it is in a standby condition can completely terminate the operation thereof.

Referring to Fig. 10, the operation of correcting the reference of a sensor gap, which is carried out in the second and eleventh steps S2 and S11 of Fig. 9, will next be described in detail. In the first step 91, the photo interrupter 72 detects whether or not a paper material exists at the print gap G. When the paper material exists, the operation proceeds to the second step Q2, where reference data stored in the reference data memory 75, that is, the value of a sensor gap without a paper material, is retained intact. Note that the reference data memory 75 stores reference data measured in advance at a factory.

When no paper material is detected in the first step 91, the CPU 19 retrieves the reference data from the reference data memory 75 at the third step Q3. Subsequently, the paper thickness sensor 50 measures the extent of the sensor gap at n times, e.g., four times (the fourth and fifth steps Q4 and 95).

A measurement value is stored in the average value calculation memory 74 every time of measurement. Upon completion of measurement N times, the CPU 19 reads out all measurement values obtained during the N times of measurement from the average value calculation memory 74, so as to calculate the average value thereof. The calculated average value is compared to the reference data which has been read out in the third step Q3 (the seventh step 97). In this comparison, when the average value stays within a predetermined tolerance range (+a) with respect to the reference data, the operation proceeds to the second step Q2 so as to retain the reference data. This process will eliminate the need for the CPU 19 to frequently rewrite reference data. On the contrary, when the average value deviates from a predetermined tolerance range, the operation proceeds to the eighth step 98, where the CPU 19 rewrites the content of the reference data memory 75 so as to store the average value.

According to a reference correction of this type, the extent of a sensor gap is always measured on the condition that no paper material exists prior to feeding a paper material to the print gap. Thus, even if the paper thickness detection lever 53 sinks due to abrasion or a supporting mechanism of the paper thickness sensor 50 is deformed, causing the reference for the sensor gap to be displaced, the sensor gap can be detected without error.

Further, since the reference correction is carried out every time a new paper material is fed, it is always possible to retain an accurate reference for a sensor gap.

Referring to Fig. 11, the operation of printing the first line in the sixth step S6 of Fig. 9 will next be described in detail. At the first step T1, the number of times of measuring a sensor gap is set as one, or N=1. A paper material starts being fed in the second step T2 so that the impact dot printer P takes the paper material in via the paper feeding main slot 13.

The CPU 19 supplies a signal to the paper thickness sensor 50, the signal being in synchronism with a clock signal supplied to the first pulse motor driver 70 for feeding a paper material.

The paper thickness sensor 50 then measures the extent of the sensor gap for every input of a phase drive pulse signal to the paper feeding pulse motor 32. When a single phase drive pulse signal is applied, a paper material is fed into the print gap by a predetermined extent, e.g., 1/360 inch, simultaneously with which the paper thickness sensor 50 carries out a sensor gap measurement once (the third step T3). As a result, while the paper material advances to the print gap, the paper thickness sensor 50 successively detects the paper thickness.

At the fourth step T4, it is judged whether the latest sensor gap measurement is the fourth one. When the measurement has been carried out only less than four times thus far, the operation proceeds to the fifth step T5 where the measurement value obtained from the latest measurement is stored in the average value calculation memory 74. In the sixth step T6, if it is judged that the paper material has not yet reached a starting position for printing, the operation returns to the third step T3 where a sensor gap is measured every 1/360 inch.

When it is judged in the fourth step T4 that the latest measurement is the fourth one, the CPU 19 reads out measurement values which are stored in the average value calculation memory 74 so as to obtain the average value of the four measurement values (the seventh step T7). The CPU 19 compares the obtained average value and the reference data stored in the reference data memory 75 so as to obtain the difference thereof (the eighth step T8). The CPU 19 then retrieves a print gap position from the print gap position table 76 in the ninth step T9, based on the obtained difference. The retrieved print gap position is stored in the buffer 77 at the tenth step T10.

As is apparent from the branch at the sixth step T6, the processes from the third step T3 to the tenth step T10 are repeated until a paper material reaches the starting position for printing. In other words, the paper thickness sensor 50 detects the extent of a sensor gap for every advancement of the paper material by 1/360 inch, so that a print gap position is sequentially written every four measurements.

When it reaches the starting position for printing, the paper material is stopped. Then, a print gap position measured a predetermined number of times previously is read out from the buffer 77 in the eleventh step T11. That is, since the paper thickness sensor 50 detects paper thickness at a position upstream of the print gap, the print gap position corresponding to the detected paper thickness will be temporarily stored in the buffer 77 until the paper material is advanced to reach the print gap. In this event, the number of measurements stored in the buffer 77, that is, the number of measurements to count back is determined by the number of phase drive pulses required to advance a paper material from the paper thickness sensor 50 to the print gap G.The CPU 19 reads out from the buffer 77 a print gap position corresponding to the paper thickness for a portion of a paper material, previously detected, when the portion is fed into the print gap G. In the twelfth step T12, the print head 10 is adjusted with respect to its vertical position based on the print gap position read out. According to the adjusted position, the print head 10 prints the first line on the paper material.

Next, referring to Fig. 12, the printing operation of a subsequent line relating to the eighth step S8 of Fig. 9 will be described. At the first step U1, the number of the measurement just made is determined, and this number is consecutive to the previous measurement. In the second step U2, prior to advancing the paper material with a finished printing line, a print gap position corresponding to the next line for printing is read out from the buffer 77. Then, based on the read out print gap position, the print head 10 is adjusted to be at a suitable vertical position in the third step U3. After the adjustment is made, the paper material is then advanced by n/360 inch according to the print command (the fourth step U4).Because the vertical position of the print head 10 is thus adjusted prior to paper feeding, a print gap position corresponding to paper thickness is established before the relevant paper portion is actually fed into the print gap G. As a result, dot pins of the print head 10 can be prevented from being bumped against the paper material even if there is a considerable change in paper thickness.

Once the feeding of a paper material is started, the extent of a sensor gap is measured every 1/360 inch (the fifth step U5).

In the sixth step U6, it is judged whether the latest measurement of the sensor gap is the fourth one. When the measurement has not yet been done four times, the operation proceeds to the seventh step U7 where the measurement value obtained from the latest measurement is stored in the average value calculation memory 74. If it is judged that the paper material has not yet been fed by n/360 inch in the eighth step U8, the operation returns to the fifth step U5 so as to resume measurement of the extent of the sensor gap for every advancement by 1/360 inch of the paper material.

When it is judged that the latest sensor gap measurement is the fourth one in the sixth step U6, the CPU 19 reads out measurement values stored in the average value calculation memory 74 so as to obtain the average value of the four (the ninth step U9). The CPU 19 compares the obtained average value with the reference data stored in the reference data memory 75 so as to obtain the difference thereof (the tenth step U10). Subsequently, in the eleventh step Ull, the CPU 19 retrieves a print gap position from the print gap position table 76 based on the obtained difference. The retrieved print gap position is stored in the buffer 77 in the twelfth step U12.

When a judgement is obtained indicating completion of paper feeding in the eighth step U8, a printing operation is carried out to print a line in the thirteenth step U13. The processes from the fifth step U5 to the twelfth step U12 will be repeated till the completion of paper feeding. That is, the paper thickness sensor 50 detects the extent of a sensor gap for every advancement by 1/360 of the paper material, so that a print gap position is written in every four measurements.

According to the above mentioned printing operation, since the paper thickness sensor 50 detects paper thickness in a manner synchronous with a phase drive pulse signal supplied to the paper feeding pulse motor 32, even if the paper thickness varies during the advancement of the paper material, a print gap G can be established corresponding to the variation. Further, since the paper thickness is measured at a position upstream of the print gap G, it is possible to instantly respond to variation of paper thickness, because the print head 10 can be adjusted prior to the paper feeding so as to form a print gap G suitable for subsequent printing.Furthermore, since a print gap position is retrieved with reference to the average value of measurement values of a sensor gap, it is possible to obtain the optimum print gap position irrespective of variation of measurement values due to sensing characteristics or the like.

Fig. 13 is a flow chart showing a process of the paper thickness sensor 50 detecting the extent of a sensor gap.

Referring additionally to Fig. 14, the light emitting diode 102 emits light according to a light emission control signal from the CPU 19 at the first step V1. The emitted light L1 is reflected at a white tape attached to the paper thickness detection lever 53. The reflected light L2 is received by the PSD 103, which in turn outputs two flows of electric current I1 and I2 according to the logic depicted in Fig. 15. The voltages V1 and V2, respectively corresponding to the output electrical currents I1 and 12, are converted into digital values V10 and V20 by the A/D converter 73 (the second and third steps V2 and V3). In the fourth step V4, the light emitting diode 102 is turned off.Two voltage values V10,, and V20,, corresponding to electric currents output from the PSD 103 when the diode 102 is off, are read out (the fifth and sixth steps V 5 and V6) so as to calculate the difference in a voltage value, that is VU1 and W2, from that at the time when the diode 102 is on (the seventh and eighth steps V7 and V8). At the ninth step V9, the extent of the sensor gap is calculated using a predetermined expression, based on the calculated voltage values W1 and W2. Respective constants of the expression are determined so as to secure a linearity characteristic between the sensor gap and the electric current value. The calculated sensor gap is written in during the tenth step V10.

In the foregoing embodiment, the number of measurements made to obtain the average value of measurement values of the extent of a sensor gap can be arbitrarily determined. Although a printing operation is carried out for every single line in the above embodiment, it may be carried out for every group of a plurality of lines at one time.

Claims (4)

What is Claimed is:

1. An impact dot printer comprising: a print head moving vertically in response to thickness of a paper material passing through a print gap formed between a platen and the print head; a paper feeding pulse motor for feeding the paper material into the print gap by a predetermined extent for every receipt of a phase drive pulse signal; a pulse motor driver for outputting the phase drive pulse signal to the paper feeding pulse motor; a paper thickness sensor for detecting thickness of the paper material at a position upstream of the print gap for every phase drive pulse signal; a head position adjustment circuit for determining a vertical position of the print head based on a detection result by the paper thickness sensor; a buffer for storing the vertical position determined by the head position adjustment circuit; and a control circuit for reading out from the buffer the vertical position corresponding to the detected result of a portion of the paper material when the portion is fed into the print gap; wherein the print head is moved to the vertical position according to the paper thickness previously detected when the portion reaches the print gap, so that the print head carries out a printing operation on a surface of the paper material with a constant print gap.

2. An impact dot printer according to Claim 1, wherein the paper thickness sensor includes: a paper thickness detection lever for tracing the surface of the paper material so as to convert an extent of the paper thickness into a vertical displacement of a reflection face; a light emitting diode for emitting light toward the reflection face; and a PSD for detecting a distance to the reflection face in accordance with a light intensity of the light reflected from the reflection face; the paper thickness detection lever having a supporting section rotatably supported by a stationary base, a curved section formed continuously from the supporting section for contacting the surface of the paper material, and an amplifying section formed continuously from the curved section for constituting the reflection face by an attached tape.

3. An impact dot printer according to Claim 2, wherein the paper thickness sensor detects a position of the reflection face with the paper thickness detection lever tracing no paper material prior to feeding the paper material to the print gap, to thereby measure a distance between the PSD and the reflection face, the paper thickness being determined as an extent of a displacement of the reflection face on the basis of a measured distance.

4. An impact dot printer substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.