GB1589356A

GB1589356A - Printer with a printing head mounted on a movable carriage

Info

Publication number: GB1589356A
Application number: GB46433/77A
Authority: GB
Original assignee: Facit AB
Current assignee: Facit AB
Priority date: 1976-11-10
Filing date: 1977-11-08
Publication date: 1981-05-13
Also published as: CH624050A5; FR2370592A1; JPS5363126A; SE7612498L; SE403326B; DE2750319A1; IT1090432B; NL7712346A; FR2370592B1

Abstract

The carriage (12) can be moved along an information bearer (14), and the print head (10) can be moved back and forth in relation thereto. It has a sensor (17) which limits the path of the print head (10) to the information carrier. The sensor (17) is arranged on the print head (10) in such a way that a greater spacing of the print head (10, 13) from the information carrier (14) is set with a greater thickness of the information carrier (14). It is thus achieved that, when the thickness of the information carrier changes, the impact force is automatically set correctly as a result of a fluctuating number of impressions. <IMAGE>

Description

(54) PRINTER WITH A PRINTING HEAD MOUNTED ON A MOVABLE CARRIAGE (71) We, FACIT AKTIEBOLAG, a Swedish joint stock company, of S-597 00 tvidaberg, Sweden, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention relates to a printer of the type having a printing head, which head has printing means and is attached to a carriage which is movable relative to a recording medium to move the head along the medium, the head being movable relative to the carriage in a direction towards or away from the medium and having sensing means arranged to co-operate with the medium to limit the movement of the head towards the medium so that in operation the printing means is located at a predetermined distance from that surface of the medium which is to be printed.

Printers of the type referred to are known.

Since the printing head is movable relative to the carriage, different thicknesses of medium can be used. This feature is particularly important in printers having mosaic printing heads, as the printing needles used in such heads have a very short stroke, which requires that the head be situated at a specific distance from the medium.

The known devices for compensating for the medium thickness are designed to maintain a constant distance between the printing head and the surface of the recording -medium, regardless of the medium thickness.

This means that the printing means will always strike the medium with the same speed, and hence with the same striking force. However, as it is often desirable that the striking force varies in accordance with the number of copies being printed, the known devices have the drawback that the adjustment of the striking force is always a compromise between avoiding too hard a striking force when printing on only a single sheet and avoiding the danger of the undermost copies being difficult to read when a large number of copies is being printed.

It is an object of the present invention to eliminate this drawback. According to this invention, in a printer of the type referred to, the sensing means is spaced a considerable distance from the printing means in a direction transverse to the direction of motion of the carriage, whereby the greater the thickness of the medium, the greater is the predetermined distance between the printing means and the surface of the medium. Thus the striking force of the printing means on the recording medium may be automatically adjusted according to the number of copies being printed.

The invention will now be described by way of example with reference to the drawing in which Figures 1 and 2 are diagrammatic side elevations of a first embodiment of the invention operating in Figure 1 with a thin recording medium, and in Figure 2 with a relatively thick recording medium, and Figure 3 is a similar elevation of a second embodiment.

The printer shown in Figures 1 and 2 has a printing head 10 which co-operates with a platen 11. The head is mounted on a carriage 12 which is movable parallel to the platen by conventional driving means (not shown).

The cover of the printing head is provided with an opening for the printing means, in this case a number of printing needles 13, which project through the opening and in operation strike against a recording medium 14 covering the platen 11.

The head 10 is movable relative to the carriage 12 in a direction perpendicular to the axis of the platen. A spring, shown diagrammatically at 15, urges the head 10 towards the platen 11. A sensing means in the form of a roller 17 on an arm 16, is mounted on the upper part of the printing head. The spring 15 will press the roller 17 against the recording medium on the platen and the roller 17 thus restricts the motion of the head 10 towards the platen.

As can be seen, the roller 17 contacts the surface of the medium 14 at a point which is spaced a considerable distance above the needles 13. Therefore, referring to Figure 2, due to the curved surface of the platen 11 a thickness d of the medium 14 will be sensed as a distance a, a being greater than d. If the thickness of the medium is comparatively small in relation to the diameter of the platen, the relation between the distance a and the thickness is given approximately buy a = d/(cosa), where a is the angle between the direction of movement of the head 10 towards the platen 11 and the line between the point of contact of the roller 17 and the axis of the platen (0 < a < 90").

Due to the fact that the sensed distance a is greater than the thickness d, the needles 13 of the printing head will have a stroke from their rest position to the surface of the record medium amounting to a, + (a -d), where al is a predetermined minimum stroke corresponding to a medium thickness of zero. Provided that the above approximation is valid, the stroke may be written as 1 + dk,where k is a constant having the value (1/cos a) - 1.

Thus, the stroke increases as the thickness of the medium increases. This means, provided that the needles are still accelerating when they stroke the medium, that the striking force increases as the medium thickness increases, i.e. when the number of copies to be printed increases.

To prevent the sensing means from causing marks on the medium 14 the roller 17 should be of a sufficient size that it bears against the medium over a comparatively large area or, in a known way, a pressure distributing ribbon (not shown) of for example plastics should be placed between the sensing means and the medium 14.

Figure 3, in which corresponding parts have the same reference numerals as in Figures 1 and 2, shows an embodiment having a printing head 10 pivotably supported on the carriage 12 by means of a leaf spring 18, instead of being slidable on the carriage. A spring 19 is arranged to urge the head 10 forwards, thus also pressing the roller 17 against the medium 14. in other respects the printing head operates in the same way as described with reference to Figures 1 and 2, except that the part of the printing head 10 which houses the needles 13 is movable through a shorter distance than the sensing means.

WHAT WE CLAIM IS: 1. A printer of the type referred to, wherein the sensing means is spaced a considerable distance from the printing means in a direction transverse to the direction of motion of the carriage, whereby the greater the thickness of the medium, the greater is the predetermined distance between the printing means and the said surface of the medium.

2. A printer according to claim 1 wherein the sensing means is so disposed as to sense a distance greater than and substantially proportional to the thickness of the medium.

3. A printer according to claim 2 wherein the medium follows a curved path between the region where it passes the printing means and the region where it passes the sensing means, and wherein the sensing means senses the thickness of the medium along a line parallel to a line which is substantially perpendicular to the surface of the medium in the region of the printing means.

4. A printer according to any preceding claim wherein the printing head is attached to the carriage in such manner that the printing means is movable in a direction substantially perpendicular to the surface of the medium in the region of the printing means, the movement of the printing means being limited by the sensing means contacting the medium.

5. A printer according to any of claims 1 to 3 wherein the printing head is pivotable relative to the carriage about an axis parallel to the direction of motion of the carriage, the printing means being located between the sensing means and the axis.

6. A printer of the type referred to constructed and arranged substantially as herein described and shown in the drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. against the recording medium on the platen and the roller 17 thus restricts the motion of the head 10 towards the platen. As can be seen, the roller 17 contacts the surface of the medium 14 at a point which is spaced a considerable distance above the needles 13. Therefore, referring to Figure 2, due to the curved surface of the platen 11 a thickness d of the medium 14 will be sensed as a distance a, a being greater than d. If the thickness of the medium is comparatively small in relation to the diameter of the platen, the relation between the distance a and the thickness is given approximately buy a = d/(cosa), where a is the angle between the direction of movement of the head 10 towards the platen 11 and the line between the point of contact of the roller 17 and the axis of the platen (0 < a < 90"). Due to the fact that the sensed distance a is greater than the thickness d, the needles 13 of the printing head will have a stroke from their rest position to the surface of the record medium amounting to a, + (a -d), where al is a predetermined minimum stroke corresponding to a medium thickness of zero. Provided that the above approximation is valid, the stroke may be written as 1 + dk,where k is a constant having the value (1/cos a) - 1. Thus, the stroke increases as the thickness of the medium increases. This means, provided that the needles are still accelerating when they stroke the medium, that the striking force increases as the medium thickness increases, i.e. when the number of copies to be printed increases. To prevent the sensing means from causing marks on the medium 14 the roller 17 should be of a sufficient size that it bears against the medium over a comparatively large area or, in a known way, a pressure distributing ribbon (not shown) of for example plastics should be placed between the sensing means and the medium 14. Figure 3, in which corresponding parts have the same reference numerals as in Figures 1 and 2, shows an embodiment having a printing head 10 pivotably supported on the carriage 12 by means of a leaf spring 18, instead of being slidable on the carriage. A spring 19 is arranged to urge the head 10 forwards, thus also pressing the roller 17 against the medium 14. in other respects the printing head operates in the same way as described with reference to Figures 1 and 2, except that the part of the printing head 10 which houses the needles 13 is movable through a shorter distance than the sensing means. WHAT WE CLAIM IS:

1. A printer of the type referred to, wherein the sensing means is spaced a considerable distance from the printing means in a direction transverse to the direction of motion of the carriage, whereby the greater the thickness of the medium, the greater is the predetermined distance between the printing means and the said surface of the medium.