PRINT HEAD-PLATEN GAP ADJUSTMENT MECHANISM FOR A PRINTER

Abstract: A printer comprises a support frame (5,6,33), a platen (1) mounted on the support frame and a printing head (17) also mounted on the support frame. The printer also comprises means (7,13,20,47; 7,13,50,72) for adjusting the distance between the printing head and the platen. The adjusting means comprise a lever (28) mounted on the support frame for movement towards and away from the platen, sensing means (37,54) for sensing the position of the lever relative to the platen, and means (13) for adjusting the distance of the printing head from the platen in response to the output from the sensing means.

Description: The present invention relates to a printer having means for automatically adjusting the distance between a platen and a printing head of the printer in dependence upon the thickness of the printing paper. Generally, in wire dot impact printers or thermal transfer printers, the printing density is changed in accordance with the gap between a printing head of the printer and the printing paper. Therefore, in recently introduced printers, means have been provided for detecting the thickness of the printing paper in order to adjust the position of the printing head and thereby maintain a constant gap. Japanese Patent Application No. 60-234872 discloses a printer having a pressure sensitive element secured to a carriage, which carries a printing head and which is movable in a direction orthogonal to the axis of a platen of the printer. An electric signal indicative of the thickness of the printing paper is generated by this pressure sensitive element and the distance between the printing head and the platen is automatically adjusted in response to the electric signal. However, when the thickness of the printing paper is detected, the adjustment requires the platen to be moved in the vertical direction, so that the speed of response is low. Further, since the paper thickness is detected using the movable carriage, measuring errors may result and the reliability of the adjustment may be poor. U.S. Patent No. 4,676,675 discloses a further arrangement in which a pressure sensitive element is secured to a carriage for a printing head for generating an electric signal indicative of the thickness of the printing paper. In this instance, adjustment is achieved by rotation of an eccentric shaft on which the carriage is mounted, whereby the printing head is moved rather than the platen. Likewise French Published Application No. 2,267,885 discloses a printing machine having a fixed part for supporting a web of printing paper, and a movable part carrying a sensing element for detecting the paper thickness whereby the position of the movable part may be appropriately adjusted. A further printer having means for adjusting the distance between a platen and a printing head in dependence upon paper thickness is described in Japanese Published Application No. 62-119077. In this printer, a link element is displaceable both to press a detection roller towards a pressure sensitive element for detecting the paper thickness and to adjust the position of the printing head. According to the present invention, there is provided a printer having a support frame, a platen, a printing head, a carriage bearing the printing head and mounted on an eccentric shaft which is rotatable for adjusting the distance between the printing head and the platen, and means for controlling rotation of the eccentric shaft so as to adjust the distance between the printing head and the platen according to paper thickness, characterised in that the controlling means comprise a lever mounted at a fixed location on the support frame for rotation towards and away from the platen and arranged to co-operate with sensing means for generating signals representative of the position of the lever relative to the platen, means for rotating the lever to a first position in which the lever engages the platen and the sensing means generate a first signal and subsequently to a second position in which the lever engages paper on the platen and the sensing means generate a second signal, and detecting means responsive to the first and second signals for determining the paper thickness and for generating an output for controlling rotation of the eccentric shaft. Since the thickness of the printing paper is detected by means mounted on the support for the platen, it is possible to measure the thickness of the printing paper with a high degree of accuracy and to provide a reliable adjustment of the distance between the platen and the printing head. Further, since the means for detecting the thickness of the printing paper are mounted on a substantial support, it is not necessary to make these detecting means small and so a simple overall lever construction is possible. The invention is described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a printer according to the present invention; Figure 2 is a sectional view through the printer; Figure 3 is a side view, which shows one example of means for detecting the thickness of the printing paper; Figure 4 is a block diagram of a control circuit of the printer; Figure 5 is a flow chart illustrating the operation of the printer; Figure 6 is a side view showing another example of means for detecting the thickness of the printing paper; and Figure 7 is a circuit diagram of a control circuit employed with the means of Figure 6. A printer shown in Figure 1 comprises a platen 1 rotatably supported by frame elements 5 and 6 (which serve as substrates of the printer). The platen 1 is connected to a paper feeding motor 2 through a driving pinion gear 3 and a transmission gear 4. A carriage guide shaft 7 is mounted parallel to the platen 1 at a front side of the motor 2. The guide shaft 7 is eccentrically mounted for rotation in holes 8 and 9 in the frame elements 5 and 6 (see Figure 2). An adjusting gear 12 is secured at one end of the guide shaft 7 and is connected to an adjusting motor 13 through a driving gear 10 and a transmission gear 11. A position detecting plate 14 is secured to one side of the gear 12 for movement relative to a photo-sensor 15, which is coupled to a side of the frame element 5 and which comprises a light emitting element and a photo sensitive element. The rotational position of the detecting plate 14 determines the amount of light reaching the photo-sensitive element and hence the output of the photo-sensor 15. A printing head 7 is mounted on a carriage 16. The carriage 16 is supported by the guide shaft 7 and a second guide shaft 18 which is mounted parallel to the guide shaft 7. The carriage 16 is reciprocally movable in the direction of the axis of the platen by a timing belt 23. The timing belt 23 is supported by a pinion gear 22 of a carriage driving motor 21 and an idle roller 24. Further, in response to rotation of the guide shaft 7, the carriage 16 is also movable in the vertical direction relative to the platen 1. A paper thickness detecting mechanism 20 is attached to a substrate 33 forming part of the printer frame as shown in Figures 2 and 3. In Figure 2, the reference numeral 28 designates a paper thickness detecting lever having one end 34 projecting from a window in a paper guide plate 25 and another end coupled to a plunger 32 driven by a solenoid 30. The lever 28 is pivotally supported by a shaft 29. The reference numeral 35 designates a spring, which urges the paper thickness detecting lever 28 away from the platen 1. Reference is now made to Figure 3, which shows the paper thickness detecting mechanism 20 in greater detail. In Figure 3, the reference numeral 37 designates a pressure sensitive element, formed from a pressure sensitive conductive rubber material, whose electric resistance changes in response to applied pressure. One side of the pressure sensitive element 37 is secured to the substrate 33 through a support 40 and a protective plate 39, which also serves as an electrode. The other side of the pressure sensitive element 37 is in contact with a pressure member 36 at the second end of the paper thickness detecting lever 28 through a further protective plate 38. Returning to Figure 2, the reference numeral 27 designates a paper detector, which generates signals in response to the position which a lever 26 adopts upon the insertion of the printing paper. Figure 4 shows a control circuit for controlling adjustment of the carriage 16 in the vertical direction relative to the platen 1 and hence the distance between the printing head 17 and the platen 1. In Figure 4, the reference numerals 45 and 46 designate memory circuits which store first and second signals generated by the pressure sensitive element 37 of the detecting mechanism 20. The adjustment operation is controlled by a CPU 47. Upon the input of the signals generated by the paper detector 27 and signals from the memory circuits 45 and 46, the CPU 47 controls the motors 2 and 13 and the solenoid 30 as shown in the flow chart of Figure 5. Initial adjustment is as follows: On assembly, the eccentric guide shaft 7 is rotated so that the distance between the platen 1 and the printing head 17, or the distance between the platen 1 and the carriage 16, is adjusted to a reference value Gs. At the same time, the position detecting plate 14 is moved relative to the photo-sensor 15 to a position wherein the photo-sensor 15 supplies a reference signal. The printing head 17, the platen 1, the plate 14 and the photo-sensor 15 are now in their reference positions. The paper thickness detecting lever 28 is then lifted by the solenoid 30 to bring the end 34 of the lever into contact with the surface of the platen 1. The pressure sensitive element 37 is thereby pressed down by the pressure member 36, with the applied pressure being in proportion to the initial spacing between the end 34 of the lever 28 and the surface of the platen 1. The output thus generated by the pressure sensitive element 37 is input to the memory circuit 45 to be stored as a first signal E1. Adjustment during operation is as follows: When the printer is turned on in step (a) of the flow chart shown in Figure 5, the motor 13 acts to rotate the eccentric guide shaft 7 and move the printing head 17 in the vertical direction relative to the surface of the platen 1. The printing head 17 is moved until the distance between the head 17 and the surface of the platen 1 reaches the reference value Gs, and the position detecting plate 14 moves to the position wherein the photo-sensor 15 generates the reference signal. Rotation of the motor 13 is then stopped, step (b). The printing paper is then partially loaded along the paper guide 25, the detecting lever 26 is pushed down and the paper detecting signal is output from the paper detector 27 as in step (c). Upon the output of the paper detecting signal, the solenoid 30 is energised by the CPU 47 and the end 34 of the paper thickness detecting lever 28 is urged toward the surface of the platen 1. Thereby, the pressure signal, which is output when no printing paper is present between the lever 28 and the platen 1, that is the first signal E1 representative of the reference value Gs, is output and is stored in the memory circuit 45, step (e). When the first signal E1 is output, the solenoid 30 is de-energised, and the paper thickness detecting lever 28 is released. The paper feeding motor 2 then operates so as to move the printing paper P into the initial printing position. When the printing paper is loaded into the initial printing position in step (f), the solenoid 30 is again energised by the CPU 47 and the end 34 of the paper thickness detecting lever 28 is urged toward the platen 1 in step (g). Since the printing paper is now present between the platen 1 and the lever 28, the end 34 of the lever 28 is held away from the surface of the platen 1 by an amount corresponding to the thickness Δ D of the printing paper as compared with the case where no printing paper is present. A second signal E2 corresponding to the initial distance between the platen 1 and the lever 28 less the thickness Δ D is output and is stored in the second signal memory circuit 46, step (h). The difference between the second and the first signals, that is E2-E1 is indicative of the thickness Δ D of the printing paper. From these signals, therefore, the CPU 47 calculates in step (j) the thickness Δ D of the printing paper P. The CPU 47 then controls the motor 13 in step (k) so as to rotate the eccentric guide shaft 7 and move the carriage 16 until the distance between the printing head 17 and the platen 1 is adjusted to an appropriate value given the thickness Δ D of the printing paper. Reference is now made to Figure 6, which shows a second example of the paper thickness detecting means. In Figure 6, the reference numeral 50 designates a paper thickness detecting mechanism which comprises a light blocking plate 51 and a detector 54. The plate 51 is insertable into the detector 54 and is surrounded by the detector 54 when inserted. The detector 54 includes a light emitting element 52 and a photo-sensitive element 53. As shown in Figure 7, the photo-sensitive element 53 is connected to reference voltage setting means 56 through a resistor 55. The output from the photo-sensitive element 53 is applied through a buffer amplifier 58 to an operational amplifier 59. More particularly, the output from the photo-sensitive element 53 is applied to the non-reversal input terminal of the buffer amplifier 58, and a variable resistor 60 for adjusting the output signal from the amplifier 59 is connected between the reversal input and the output terminals of this amplifier 59. A zero point adjusting voltage is supplied from a reference voltage generator 61 to the non-reversal input terminal of the operational amplifier 59. A circuit 62, in the form of an amplifier, is provided for monitoring the output of the photo-sensitive element 53. The output from the buffer amplifier 58 is supplied to the reversal input terminal of the amplifier 62 and to the output from the reference voltage setting means 56 is supplied to the non-reversal input terminal of the amplifier 62. A time constant setting circuit 64 comprises a high value resistor 65, a capacitor 66, a low value resistor 67, whose value is smaller by an order of two than that of the resistor 65, and a diode 68, which is rendered conductive in response to a reversal in the output of the monitoring circuit 62. The low value resistor 67 and the diode 68 are connected in series. The DC circuit, comprising the low value resistor 67 and the diode 68, is connected in parallel with the high value resistor 65. The time constant setting circuit 64 acts so as to stop the operation of a circuit 69, for adjusting the voltage to the light emitting element 52, during the detection of the thickness of the printing paper and then to restore this voltage to its previous value immediately after the thickness has been detected and the detecting lever 28 is released so that the next operation can then be started. The adjusting circuit 69 controls a transistor 70 in response to the output from the monitoring circuit 62 so as to adjust the current applied to the light emitting diode 52, thereby making it possible to obtain a regular reference value from the photo-sensitive element 53. The operation of the printer including these paper thickness detecting means is explained below. Initial adjustment is as follows: When assembly is complete, the eccentric guide shaft 7 is rotated so that the distance between the platen 1 and the printing head 17, or the distance between the platen 1 and the carriage 16, is adjusted to the reference value Gs as set by a gauge or the like. At the same time, the position detecting plate 14 and the photo-sensor 15 are moved relative to one another to supply the reference signal as described with reference to Figure 1. Upon operation of the solenoid 30, the end 34 of the paper thickness detecting lever 28 is brought into contact with the platen 1. The light blocking plate 51 is thus inserted into the detector 54 so that a minimum amount of light is transmitted from the light emitting element 52 to the photo-sensitive element 53. The voltage adjusting circuit 56 is adapted to provide a sensor output of a predetermined value at this point and the reference voltage generator 61 is set is ensure that a predetermined first signal representative of the reference value Gs is supplied to a CPU 72. Paper having a predetermined thickness is then inserted along the paper guide 25, and the paper thickness detecting lever 28 is lifted, in response to operation of the solenoid 30, so that the end 34 of the lever 28 is brought into contact with the surface of the paper. The light blocking plate 51 thereby enters the detector 54 by an amount corresponding to the initial spacing between the lever end 34 and the surface of the paper, and an amount of light representative of the reference value Gs less the predetermined thickness of the paper is supplied to the photo-sensitive element 53. At this stage, the variable resistor 60 is adjusted to control the output of the operational amplifier 59 so as to enable the CPU 72 to assess correctly the predetermined thickness of the paper and so as to set the CPU 72 for future paper thickness measurement. Adjustment during operation is as follows: Upon turning on the printer, the motor 13 is operated. The eccentric guide shaft 7 is rotated and the carriage 16 is moved to bring the printing head 17 into the position where the distance between the printing head 17 and the surface of the platen 1 corresponds to the reference value Gs. The position detecting plate 14 and the photo-sensor 15 are then in the predetermined reference positions, and the output from the detector 54 results in the first signal being supplied to the CPU 72. The motor is then stopped. When printing paper is now partially inserted along the paper guide 25, the detecting lever 26 is pushed down and the paper detecting signal is output from the paper detector 27. The CPU 72 operates the motor 2 for feeding the printing paper P into the initial printing position. When the printing paper is in the initial printing position, the CPU 72 operates the solenoid 30 so that the end 34 of the paper thickness detecting lever 28 is lifted toward the platen 1 and into contact with the printing paper P. The light blocking plate 51 is moved upward by an amount which, compared with its maximum travel, is reduced by the value Δ D corresponding to the thickness of the printing paper. The output from the photo-sensitive element 53 is supplied to the operational amplifier 59, which in turn generates the second signal for supply the CPU 72. The CPU 72 determines the thickness of the printing paper on the basis of the difference between the second and the first signals and thereafter operates the motor 13 to position the printing head 17 at the most suitable distance from the platen 1 for the thickness of the printing paper. During this time, the light blocking plate 51 allows an increased amount of light to pass to the element 53 by comparison with the situation where no printing paper is present and the diode 68 of the time constant controlling circuit 64 remains unconductive. Therefore, the time constant controlling circuit 64 does not output the signal from the monitoring circuit 62 to the adjusting circuit 69, at least during the detecting period. Hence, the current supplied to the light emitting diode 52 is not changed on insertion of the printing paper so that the detecting operation is not disturbed. When the detecting operation is completed and the light blocking plate 51 is removed from the light passage of the detector 54, the output of the monitoring circuit 62 is reversed so that the diode 68 of the time constant circuit 64 becomes conductive and the time constant is switched to a small value determined by the low value resistor 67 and the capacitor 66. In the non-detecting condition, when the output level of the photo-sensitive element 53 is reduced by changes in the voltage supply or deterioration of the light emitting element 52 and/or the photo-sensitive element 53, the monitoring circuit 62 outputs corresponding signals thereby to operate the adjusting circuit 69 and a transistor 70 for adjusting the current supply to the light emitting element 52. The output of the photo-sensitive element 53 is thus kept uniform irrespective of the deterioration of the elements 52 and 53 and of other variations, so that it is possible to prevent errors which may be caused by the passage of time.

Claims: 1. A printer having a support frame (5, 6, 33), a platen (1), a printing head (17), a carriage (16) bearing the printing head and mounted on an eccentric shaft (7) which is rotatable for adjusting the distance between the printing head and the platen, and means (13, 20, 47; 13, 50, 72) for controlling rotation of the eccentric shaft so as to adjust the distance between the printing head and the platen according to paper thickness, characterised in that the controlling means comprise a lever (28) mounted at a fixed location on the support frame for rotation towards and away from the platen and arranged to co-operate with sensing means (37; 54) for generating signals representative of the position of the lever relative to the platen, means (30) for rotating the lever to a first position in which the lever engages the platen and the sensing means generate a first signal and subsequently to a second position in which the lever engages paper on the platen and the sensing means generate a second signal, and detecting means (47; 72) responsive to the first and second signals for determining the paper thickness and for generating an output for controlling rotation of the eccentric shaft. 2. A printer according to claim 1, characterised in that the sensing means comprise a pressure sensitive element (37). 3. A printer according to claim 1, characterised in that the sensing means comprise a light emitting element (52) and a photo-sensitive element (53). 4. A printer according to claim 3, characterised by means (62) for monitoring the output of the photosensitive element, means (64) for generating a larger time constant during sensing and a smaller time constant on completion of sensing, and means (69, 70, 56) controlled by the monitoring means and the time constant generating means for adjusting the power supplied in use to the light emitting element when sensing is not taking place so as to maintain the output of the photo-sensitive element and thereby avoid errors due to deterioration of the light emitting and photo-sensitive elements.

Inventors: KUMAZAKI, MASAYUKI, MIMURA, TAKAO, NISHIZAWA, ATSUSHI
Applicants: SEIKO EPSON CORPORATION
Application year: 1989
Main classification: B41J25
IPC codes: B41J25