EP0287804A2

EP0287804A2 - Carriage position indicator for a serial printer

Info

Publication number: EP0287804A2
Application number: EP88103998A
Authority: EP
Inventors: Carlo Fare'
Original assignee: Bull HN Information Systems Italia SpA; Honeywell Bull Italia SpA
Current assignee: Bull HN Information Systems Italia SpA
Priority date: 1987-03-25
Filing date: 1988-03-14
Publication date: 1988-10-26
Also published as: IT8719833A0; IT1202688B

Abstract

In a serial printer, where a printing carriage moves along a printing line on guiding bars fixed to two side plates of the printer frame, the carriage position along the printing line is precisely defined by an encoding disk rotatably mounted on the carriage, and by a flexible cable fixed to and stretched between the two side plates, which cable is wrapped around the encoder shaft, so that the carriage movement along the printing line causes a corresponding rotation of the encoding disk which, by cooperation with a detector, steadily fixed to the carriage enables for the generation of position signals which precisely define the carriage position.

Description

The present invention relates to a position indicator for serial printer.
It is known that in serial printers a print carriage moves along a printing line on two parallel guides fixed to side plates of the printer frame.
A printing head, of the impact type, inkjet, thermal or like, mounted on the printing carriage, is periodically energized during the carriage movement along the printing line, so as to perform the subsequent printing of characters along the print line.
In high speed serial printers each character is obtained as a composition of elemental printed dots arranged according to a row/column matrix.
The quality of the printed character and of the set of the printed characters depends on the precision with which the elemental printing dots are located on the printing support.
The printing elements of the printing head, which is steadily fixed to the carriage, have a well defined and permanent position as to the carriage.
Therefore the position of the printed dots on the printing support is essentially defined by the position of the printing carriage in the different and subsequent instants at which the printing elements are actuated. In order to achieve a good printing quality it is therefore of essence to precisely know the carriage position.
To this purpose several arrangements have been used in the past.
The most common one, consists in an encoding disk or encoder mounted on the shaft of the motor which controls the carriage movement, the encoding disk cooperating with a sensor in fixed position.
Since the motor is fixed to the frame and movement to the carriage is transmitted by helicoidal rods and coupling gears, or by pulleys and flexible cables, or gears and toothed belts, the carriage position is not univocally related to the angular position of the motor shaft, but is affected by slacks and elastical deformations of the mechanical transmission members, which in turn depends on the movement direction, the acceleration entity and the friction.
To overcome this unconvenients, an alternative arrangement consists in the use of an encoding rod, or linear encoder, steadily fixed to the printer frame and cooperating with a sensor mounted on the carriage.
This arrangement provides a carriage position indication which is not affected by the mechanical coupling members but which in turns has other disadvantages.
The resolution which may be obtained with such encoders is low, unless particularly expensive (using the Moire' fringe effects) encoders are used.
In addition the encoder is exposed for its entire length to powder and particles present in the atmosphere, which particles may damage it or hamper its response.
In order to overcome these problems it has been recently proposed, in the european patent application published with N. 0107501, to mount a rotating encoder in a fixed position of the frame and to cause the encoder rotation through a coupling system comprising a flexible cable loop held in tension among two pulleys fixed to the side plates of the frame.
The cable is fixed at one point to the carriage and the encoder is mounted on the shaft of one of the pulleys.
Each movement of the carriage along the printing line causes a corresponding rotation of the two pulleys, hence of the encoder.
In other words the movement is transmitted to the encoder by the carriage through a "dedicated" coupling which is not affected by slacks and deformations of the coupling among the motor and the carriage.
Undoubtely some advantages are achieved, but this arrangement too is affected by some elastical deformation induced in the coupling cable by the inertia and the friction torque of the mechanism (pulleys).
It overcomes these unconvenients the position indicator for serial printer which is the object of the present invention and which reduces to a minimum the uncertainty in the position indication with constructive simplifications which render completely neglectable the inertia and the friction of the moving elements, the only moving part being limited to the encoder.
In addition, the encumbrance of the position indicator is reduced to a minimum.
These advantages are achieved by means of an encoder rotatably mounted on the printing carriage.
A flexible cable is wound or wrapped around the encoder shaft and held in tension between the two side plates of the frame, to which its ends are fixed.
The features and the advantages of the invention will appear more clearly from the following description of a preferred form of embodiment and from the enclosed drawings where:

- Figure 1 shows in schematics perspective view a printer incorporating the position indicator of the invention.
- Figure 2 shows in block diagram an electronic control system for a printer incorporating the position indicator of the invention.

With reference to figure 1, a serial printer has a frame comprising a base 1 and two side plates 2,3 parallel each other.
The side plates 2,3 support a printing platen 4, on which the printing is performed.
The shown platen is of fixed type, but it may consists in a rotating platen as well.
Two guiding bars 5,6 are mounted between the side plates 2,3 parallel each other and to the platen.
A printing carriage 7 is slidably mounted on the guiding bars and supports a printing head 8.
The printing head may be of the printing needle type, of the nozzle type for ink jet printing, of the thermal element type for thermal printing, and so on.
The printing head has a nose 9 facing the platen with its end.
The printing elements (needle tips, thermal elements, nozzles) are located at the nose end, generally arranged in one or more vertical columns and the printing support, not shown, is interposed between the platen and the nose end.
A motor 10 is fixed to side plate 3.
A driving pulley 11 is splined to the motor shaft.
A flexible cable 12, having its ends fixed to the carriage 7, extends from carriage 7 to pulley 11, wraps around it, extends to side plate 2 and through a return pulley, not shown, returns to carriage 7.
The rotation of the driving pulley causes, through the sliding of the flexible cable, the movement of the carriage along the printing line.
The carriage movement along the guiding bars causes the movement of the head 8 and nose 9 along the printing line.
The selective energization of the printing elements at predetermined intervals during the carriage movement, causes the printing of the desired characters, with the dot elements composing the characters in their predetermined positions.
According to the invention a rotating encoder 13, enclosed in a protection housing, is steadily fixed to carriage 7 with its shaft 14 parallel to the side plates, in horizontal position, as shown in Figure 1, or in slanted or vertical position depending on convenience.
A flexible cable 15 has one end fixed to side plate 2 and the other end fixed to a screw 16 screwed in side plate 3.
Screw 16, by a greater or lesser screwing in side plate 3 allows to impart a suitable stretching to cable 15.
Cable 15 is wrapped around encoder shaft 14
Owing to the friction between cable 15 and shaft 14, each carriage movement induces a corresponding rotation of the shaft 14 and therefore of the encoder splined thereto.
The encoder may for instance be of the type manufactured and marketed by the hewlett Packard firm with code HEDS-5010. Such encoder has a very small size (28 mm diameter and axial encumbrancy of about 20 mm) and comprises, in addition to the encoding disc, the related sensor.
The resolution of the encoder is of 500 divisions per turn and the shaft diameter is 4 mm.
This means that when the coding disk is rotated by a flexible cable directly wrapped around the shaft, the resolution which can be attained, expressed in mm is 4 π/500 that is about 0,025 mm (25 micron).
It is to be noted that in the described embodiment the only moving part of the position indicator consists in the rotating disk of the encoder, in the related shaft and in that portion of the cable which is wrapped around shaft 14.
These elements have a neglectable inertia. In addition the coupling with the flexible cable does not subjects the encoder shaft to load transverse to the axis and therefore the friction torque developed between shaft 14 and its bearing is neglectable.
As a conseguence, when the carriage is moving, being the movement at constant speed, accelerated or decelerated, the resisting torque due to inertia and friction is minimum and the stretching of the two cable portions comprised between side plate 2 and shaft 14 and shaft 14 and side plate 3 remains practically even and constant.
Therefore no appreciable differential stretching occurs causing differential changes in the cable length, and possible error in the measurement of the carriage position.
The carriage position is univocally determined.
The substantial equality and constance of the cable stretching in winding phase as well unwinding phase from the shaft 14 renders further completely neglectable the possible slipping of cable 15 on the surface of shaft 14 and assures stability and repeatability in the position indicator at long term, that is for hundreds of thousands of carriage reciprocations.
To this purpose it must be noted that, in order to achieve high printing quality, it is not required to precisely know the carriage position relative to a fixed and absolute reference point, but it suffices to know the carriage position relative to an arbitrary reference, for instance a print position at the beginning of a first printed line and that this position control be maintained for the whole operations required to print the full printing support, generally a sheet.
This requirements is fully meb by the position indicator which is the object of the invention.
The "absolute" position of the carriage along the print line, may be identified, even with lesser precision, by a stroke end sensor 17, actuated by carriage 7 when it is in the extreme left position.
Sensor 17 may be used by a control logic in well known manner.
Figure 2 shows a control logic which makes use of the position indicator of the invention.
Encoder 13 provides two pulse signals A,B in quadrature phase each otherm to a counter 18 which, dependings on the order in which signals A, B are received, increments or decrements.
The stroke end sensor 17, when actuated, provides an initialization reset signal RST to counter 18.
Counter 18 provides in output a code SCODE, indicative of a space position identified by the count and related to an arbitrary initial condition.
Together with code SCODE, counter 18 outputs a pulse signal ST for strobing the output code each time it is modified.
Code SCODE and signal ST are input to a control logic 19, which in known and conventional manner, which is beyond the scope of the invention, uses them to define the several operations to be performed.
Substantially the used control procedure is the following:
At printer start-up the carriage position is unknown.
Therefore the control logic 19 controls a carriage return movements until sensor 17 is actuated.
At this point counter 18 is reset or set at a preestablished count value and from this instant and thereafter the moving carriage position is rigorously determined by the count values taken by counter 18, with a precision or resolution of 25 micron.
Then the required printing operations and carriage movement operations are performed as a function of the carriage position and its speed, which may be easily determined by measuring the time interval occurring between two subsequent ST pulses, or by the number of pulses ST occurring in a predetermined time interval.
It is clear that figure 1 shows a preferred form of embodiment only and that several changes can be made.
For instance, if a lower resolution is required in the carriage position indication, cable 15 may be wrapped around a pulley having a suitable diameter and splined to the encoder shaft 14.
Likewise cable 15 may be fixed to the side plates with arrangements other than those described, for instance directly, without using tensioning screws, or by replacing the tensioning screw with a traction spring.

Claims

1) Position indicator for serial printer in which a printing carriage is slidably mounted on guiding bars extending between two side plates of the printer for reciprocation thereon characterized in that it comprises:
- a rotating encoder mounted on said printing carriage with a rotation shaft perpendicular to said guiding bars, and
- a flexible cable having its two ends each fixed to one of said two side plates and extending between said two side plates parallel to said guiding bars, said cable being wrapped with at least a wrapping turn around said rotation shaft,
so that a predetermined rotation of said encoder corresponds to each carriage displacement along said guiding bars.

2) Position indicator for serial printer as claimed in claim 1 comprising further a carriage stroke end sensor for generating an initialization signal for counting means controlled by signals in output from said encoder.