RIBBON PROTECTORS FOR PRINTERS

Abstract: A printer in which a printing head (2) faces a platen (1) and performs a spacing movement between impressions is provided with a ribbon protector (18). Two sheets are provided with through openings for the passage of a pin of the printing head (2). Mutually engageable electrodes are provided on inner sides of the sheets and resilient spacers are located between the two sheets. The protector (18) is secured by a connector (19) to a carriage (4), and the platen gap is adjusted when the protector (18) is made conductive by pressure. This may be done by drive means (23) or manually.

Description: TECHNICAL FIELD The invention relates to ribbon protectors for printers in which printing is performed by a printing head facing a platen and performing a spacing movement between impressions. Such printers are referred to herein as printers of the kind specified. BACKGROUND ART (see e.g. JP-A-61-31275) In such printers, platen gap adjustment is provided for the distance between the platen and the printing head, to take into account the thickness of the paper set in the printer. A ribbon protector is installed between the paper and an ink ribbon for protecting the paper from contamination. In order to protect the printer from a decrease in the quality of printing, breakage of a printing head pin, contamination of the paper, and from inaccurate spacing movements of the carriage, it is necessary to maintain a predetermined distance between the surface of the paper and the tip of the printing head regardless of variations in the thickness of the paper. THE INVENTION The invention provides a ribbon protector comprising two sheets provided with through openings for the passage of a pin of a printing head, mutually engageable electrodes on the inner sides of the sheets, and resilient spacers between the sheets. The spacers may be of rubber or like plastics material, and positioned on one or both of the sheets. Thus the thickness of the paper is detected without involvement of a physical reaction which upsets the adjustment of the platen gap of a printer. The distance between the surface of paper to be printed and the tip of a printing head can be maintained at a specified value regardless of the paper thickness. The adjustment of the platen gap can be accurate and predetermined. The mechanism can be made compact. The invention includes a printer of the kind specified comprising a ribbon protector according to the invention, a connector for securing the protector to a carriage, and drive means for adjusting the platen gap when the protector is made conductive by pressure. In an alternative arrangement, the adjustment means may be manual, and operative on an indication of platen gap adjustment from the ribbon protector. By positioning conductive portions of the sheets to come into contact through compressing of the spacers when the printing head is in contact with the paper at a predetermined pressure, when the printing head approaches the platen, the platen gap becomes zero, the spacer is compressed, and the electrodes conduct. This conduction can be used for detection of the paper thickness, and for detection of a predetermined reference position of the head with respect to the platen, which is the platen gap zero position. Alternatively, the platen gap may be adjusted by moving the entire carriage, on which the printing head is installed, towards or away from the platen. DRAWINGS: Figure 1 is a perspective view of a conventional printer of the kind specified; Figure 2 is a graph which shows characteristics of the printer of Figure 1; Figure 3 is a perspective view of a ribbon protector according to the invention; Figure 4 is a front view of the protector of Figure 3; Figure 5 is a side view of the same protector; Figure 6 is a perspective view of a printer incorporating a ribbon protector according to the invention; Figure 7 is a side view corresponding to Figure 6; Figure 8 is a perspective view of part of a platen gap adjustment device in Figure 6: Figure 9 is a corresponding partially-broken away front view; Figures 10A, 10B and 10C are side views which illustrate operation; Figure 11 is a perspective view of another printer according to the invention equipped with a manual platen gap adjustment device; Figure 12 is an electric circuit diagram of the printer of Figure 11; Figure 13 is a top view of the control portion of the printer of Figure 11; Figures 14A and 14B explain the operation of the printer of Figure 11; and Figures 15A and 15B are front views of alternative control portions for printers according to the invention. In Figure 1, a platen 1 has a printing head 2 in front which prints function by being moved with a carriage 4 which performs spacing movements along a carriage shaft 3. Both ends of the carriage shaft 3 are fixed to cams 5. A pulse motor 6 rotates the cams 5 through gears 7a and 7b against a carriage side wall FW. The printer is also provided with a spring 8, a slit disk 9, and a photoelectric sensor 10 which detects pulses produced by rotation of the slit disk 9. Through the slit disk 9 and photoelectric sensor 10, it is possible to determine the moment when the pulse motor 6 is stepped-out. A sheet of paper 11 is set between the platen 1 and print head 2 adjacent an ink ribbon 12. A ribbon protector, which is intended for protection against contamination by the ribbon, does not have any relation to the platen gap adjustment function, and therefore is not shown in the drawings. There is a constant distance betwen the surface of the paper and the tip of the printing head 2 for all thicknesses of paper inserted. When paper 11 is guided around the platen 1, the pulse motor 6 turns in a forward direction, and through gears 7a and 7b rotates cams 5 rigidly attached to the carriage shaft 3, so that the carriage shaft 3 is turned and moved in the direction of withdrawal from the platen 1. The carriage 4 and print head 2 thereon are also moved away from the platen 1 and stop in a predetermined position. The pulse motor 6 is then turned in a reverse direction, i.e., so that the print head 2 approaches the platen 1, and abuts the platen 1 clamping the paper 11 and ink ribbon 12. The printing head cannot move further towards the platen 1, and the pulse motor 6 is in a step-out state. The slit disk 9 is at a halt, the photoelectric sensor 10 does not generate pulses, and the system detects a step-out state of the pulse motor 6. From this point, for the second time, the pulse motor 6 rotates in the forward direction only a predetermined amount, so that a specified gap is obtained between the surface of paper 11 and the tip of the printing head 2. The predetermined amount of rotation is normally constant, so the gap between the surface of the paper 11 and the tip of printing head 2 remains constant, irrespective of the thickness of the paper 11. However, as shown in Figure 2, the pulse motor 6 continues to rotate even after contact of the printing head 2 with the surface of the paper 11, because even after the contact, the clamped paper 11 and ink ribbon 12 can be further compressed, and the carriage shaft and the carriage frame FR can be bent, until the reaction of the shaft coincides with the pulse-motor torque, when the pulse motor is stepped-out. That is the pulse motor 6 stops only when it overruns the zero-gap condition by a value ΔN. Because the value of overrun ΔN depends on the pulse-motor torque, variation of the initial torque of the pulse motor or reduction of the torque because of an increase in temperature, or fluctuation of voltage causes changes in the amount of overrun. As a result, the return stroke of the printing head is unstable. Because the platen gap is adjusted by indirect means, i.e. as a function of the carriage shaft adjustment position, the gap adjustment does not take into account such factors as distortion of the carriage shaft. This leads to deviations from the prescribed value of the gap in plus and minus directions. In other words, the conventional device does not provide high accuracy of the adjustment. Because the platen gap is adjusted through the rotation of the carriage shaft, the mechanism has large overall dimensions, rotary means such as gears, drive means, such as a motor for driving the rotary means, have to be installed on the end of the carriage shaft. High cost results from the use of the slit disk and photoelectric sensor. BEST MODE In Figure 3, the protector comprises two sheets or films 13,14 formed of substrates made from polyester. The film 13 has a projecting electrode portion 13a, a rhombus-shaped opening or window 13b for passage of a printing-head pin, and a plurality of resilient projections 15 on a surface which faces film 14. The film 13 is coated, e.g. by a vacuum deposition method, with a thin metal film which thus makes its surface a conductive part of the electrode 13a. The film 14 has an electrode 14a which is similar to the electrode 13a. The film 14 also has a rhombus-shaped window 14b for the passage of a printing-head pin. The film 14 also is coated with a conductive metal film, except for portions 16 which are located on the side of film 13 and correspond to the locations of the projections 15. The portions 16 are not coated because the projections 15 of the film 13 are coated during the vacuum deposition with a layer of metal. An insulating tape 17 is adhesive on both its sides for assembling the films 13 and 14 into a unit. Alternatively, a spacer for providing a predetermined distance between the conductive portions of the films 13 and 14, which diminishes the distance upon application of a compressive force applied from outside, can be other than the projections 15, for example a net-like element which is made of a resilient material with insulating properties. When a horizontal compressive force is developed in the protector of Figure 3, the projections 15 are compressed, and the electrodes 13a,14a are brought into contact and make an electrical connection. Because a ribbon protector is installed between the platen 1 and the printing head 2, a compressive force in the horizontal direction is created when the printing head 2 approaches the platen 1, and comes into contact with the ribbon protector. By appropriate positioning of the ribbon protector, the position of the head can be detected through the conduction. In Figure 6, the same reference numerals as in Figure 1 have been used as far as possible, and a ribbon protector as in Figures 3 to 5 is used. The carriage 4 is provided with a base plate 4a and a carriage frame 4b having a cylindrical part mounted rotatably on the carriage shaft 3 and movable in the axial direction. The carriage 4 is thus rotatable around and movable along the carriage shaft 3. The base plate 4a is fixed to carriage frame 4b by screws 4c. A ribbon protector 18 is fixed to the carriage 4 in the space between the paper 11 and ribbon 12 by means of a connector 19 which also serves for connection to the electric circuit. The lower end of the ribbon protector 18 is inserted into the connector 19. Figure 7 shows a spacing-movement motor 20 mounted on the lower surface of the carriage base plate 4a. Through engagement between a pinion 22 and a rack 21, the motor 20 provides spacing motions of the carriage 4 and carriage shaft 3. A pulse motor 23 adjusts the platen gap of the printing head. A slider 24 supports the rear end portion of the carriage base plate 4a and slides along a base plate 25. For adjustment of the platen gap, the carriage 4 is rotated about the carriage shaft 3: when rotated in the direction of an arrow A, the platen gap is increased; when rotated in the direction of an arrow B, the platen gap is decreased. Such rotation of the carriage is accomplished by the pulse motor 23 and slider 24. As shown in Figures 8 and 9, a shaft 26 of the pulse motor 23 has right-hand thread 27. Screwed onto the right-hand thread 27 is a nut 28 which has a pair of parallel flats on its outer surface. The slider 24 comprises a parallel link having a length which is almost equal to the width of the carriage 4. A base plate engaging portion 24a at a first end of the slider is slidably fitted on base plate 25, while a carriage-connecting portion 24b, which is located on a second end of slider 24, is connected to a second end of the rear end portion of the carriage base plate 4a by a screw 30. The sliding portion 24a of slider 24 which slides along the base plate has an oval-shaped opening (elongate hole) 29. A nut 28 of the pulse motor 23 is fitted into the opening 29, so that it cannot rotate, but can slide along the longer dimension of opening 29. When the thread 27 rotates relative to the nut 28, the rotation is converted into movement along the thread 27. Such movement is a rotation about the carriage shaft 3. During such movement, the parallel link formed of resilient material is compressed or released. When the pulse motor 23 rotates the thread 27 in a counterclockwise direction CCW, the nut 28 is pressed downward, and pulses up the thread 27, the pulse motor 23 and the rear end of carriage base plate 4a. The carriage 4 is thereby rotated in the direction of an arrow B. The printing head 2 is disposed above and fixed to the carriage 4, and so is moved forward towards platen 1. The platen gap is thereby reduced. The displacement of the rear end of the carriage base plate 4a pulls up the second end of the parallel link connected to the second end of the rear end portion of the carriage base plate 4a. During the deformation, part of the parallel link having the oval-shaped hole is moved relative to the nut 28 towards the second end of the parallel link. This is permitted because the hole is elongate. As the thread 27 rotates in a clockwise direction CW, the nut 28 is moved towards the pulse motor 23. The parallel link tends to restore its shape and the second end of the parallel link pulls down the second end of the carriage base plate 4a. The carriage 4 is therefore rotated in the direction of the arrow A. The printing head 2 is moved away from platen, and the platen gap is thereby increased. Operation Figure 10A shows the printing head 2 approaching the platen 1; Figure 10B shows the moment of contact with the platen 1; and Figure 10C shows the condition of separation of the head 2 from the platen 1. First, the pulse motor 23 turns for a predetermined amount of rotation in a clockwise direction. This rotation increases the gap between the printing head 2 and platen 1. With an increased gap, the operator inserts paper 11 and guides it around the platen 1. Using as a trigger completion of the insertion of the paper 11, or an input by means of a switch which is not shown in the drawings, rotation of the pulse motor 23 in a counterclockwise direction is initiated. At this moment, which is shown in Figure 10A, the printing head 2 begins to approach the platen 1. When the printing head 2 comes into contact with the platen 1 and assumes the position shown in Figure 10B, the projections 15 of the film 13 are compressed. In a compressed state, the electrodes 13a,14a are brought into contact and make a connection. This makes it possible to detect the moment, when the printing head 2 strikes the paper 11 with a predetermined pressure developed in contact. Thus, detection of the paper thickness is achieved and a reference position for adjustment of the platen gap i.e. a zero gap position is achieved. Then the pulse motor 23 is stopped. Subsequently, the pulse motor 23 is turned for a predetermined quantity of rotation in a clockwise direction, and the gap is set to a value appropriate for printing. Thus the platen gap adjustment mechanism has an entirely automatic control. Figure 11 illustrates manual platen gap adjustment. A carriage shaft 101 has both ends support by a carriage frame 106 movable toward or away from a platen 102. The shaft 101 is constantly urged toward the platen by springs 107. Fixed at an end of the carriage shaft 101 are a cam 108 and a lever 117. Another cam 108 is also provided on the other end of the carriage shaft 101. A projection 118 on the lever 117 is engageable with rectangular holes 119 which are formed on the carriage frame 106 and arranged at an equal pitch. The engagement of the projection with the holes provides adjustment of the position of the carriage in a transverse direction. A carriage 104, on which a printing head 103 and ribbon protector 120 are mounted, is mounted slidably on the carriage shaft 101. An ink ribbon 115 is provided with a ribbon protector as in Figures 3 to 5. In Figure 12, connected in series between the electrodes of the film 121 and film 123 of the ribbon protector 120, are an electric power source 133 and a gap indicating lamp (GAP lamp) 134 which is an LED. In Figure 13, a control panel 135 has a number of control switches 136. Near the switches 136 are various indicator lamps. The indicator lamp which is located on the left side is the GAP lamp 134. Figure 14A shows conduction, and Figure 14B an appropriate gap. First, paper 114 is inserted into the space between the platen 102 and ribbon protector 120, and guided over the platen 102. When the lever 117 is turned in the direction indicated by the arrow, the carriage shaft 112 is moved by the action of the cam 108. As a result, the ribbon protector 120 is shifted forward, and pressed by the printing head 103 onto the paper 114 on the platen 102. Rubber projections 125 of the first film 121 are compressed, and metal of the films 121 and 123 comes into contact to provide electrical conduction. The electric circuit is closed, and GAP indicating lamp 134 is lit. Lighting of the GAP lamp 134 indicates that the reference position for adjustment of the platen gap is achieved. The operator then turns the lever 117 one step back from the position where the indicator lamp 134 is lit. Thus, in a very simple manner, the platen gap is adjusted for one step of the lever shifting range, and this adjustment does not depend on the thickness of the paper 114. Figures 15A and 15B are almost the same as the control panel of Figure 13 except for the control portion of the printer. As shown in Figure 15A, the control panel 135 contains a red GAP lamp 134a and a blue GAP lamp 134b. Lighting of the red GAP lamp indicates the reference position of the platen gap, while lighting of the blue GAP lamp indicates an appropriately adjusted value of the platen gap. The only difference is that the lever is moved one step back after lighting of the red GAP lamp 134a. The gap is confirmed to have been adjusted to an appropriate value if the blue GAP lamp 134b is lit and red GAP lamp 134a is extinguished. In Figure 15B, the control panel 135 has an indicating portion 137, which shows a message, and selective switches 138. First, a selective switch 138 is pushed, and the device is placed into an off-line condition. When the lever is then turned in the gap reducing direction and the first and second films of the ribbon protector become electrically conductive, this condition is indicated by a message on indicating portion 137 shown in Figure 15B. In accordance with this message, the operator turns the lever one step back. As a result, the message disappears, and an appropriate platen gap is obtained. As a modification, a buzzer can be used instead of a GAP lamp.

Claims: A ribbon protector for a printer in which printing is performed by a printing head facing a platen and performing a spacing movement between impressions characterized by two sheets (13,14) provided with through openings (13b,14b) for the passage of a pin of a printing head (2), mutually engageable electrodes (13a,14a) provided on inner sides of the sheets (13,14) and resilient spacers (15) between the sheets (13,14). A printer in which printing is performed by a printing head facing a platen and performing a spacing movement between impressions comprising a printing head (2) mounted on a carriage (4) rotatably and slidably mounted on a shaft (3) parallel to a platen (1) characterised by a ribbon protector (18) according to claim 1, a connector (19) for securing the protector (18) to the carriage (4), and drive means (23) for the adjustment of the platen gap when the ribbon protector (18) is made conductive by pressure. A printer according to claim 2 comprising a link (24) one end (24b) of which is connected to the rear end of the carriage (4), while the other end (24a) is slidably fitted in a guide parallel to the platen (1), the drive means (23) resiliently deforms the link (24). A printer in which printing is performed by a printing head facing a platen and performing a spacing movement between impressions comprising a printing head (2) mounted on a carriage (4) rotatably and slidably mounted on a shaft (3) parallel to a platen (1) characterised by a ribbon protector (120) according to claim 1, a connector (19) for securing the protector (18) to the carriage (4), means (117) for manually displacing the shaft (3), and means (135) for indicating platen gap adjustment when the ribbon protector (120) is made conductive by pressure. A printer according to claim 4 in which the displacement means comprises a lever (117), means movable with the lever (117) for changing the platen gap, and lever fixing means (118,119).

Inventors: HAYASHI, KUNIHARU, KIKUCHI, HIROSHI, MAEKAWA, MASANORI, MIZUTANI, MINORU, WATANABE, SHOICHI
Applicants: OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1987
Main classification: B41J31
IPC codes: B41J31, B41J35