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(11) | EP 0 075 465 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Print element shifter |
| (57) A print element shifting apparatus has a cam (32) movable in an arc for locating
a print element (10) in either of two operating positions. The cam is movable by a
crank arm (42) also movable in an arc between two limit positions. The driving connections
between the crank arm and the cam is effected through a pin-and-slot arrangement such
that there is driving contact between the crank arm and the cam intermediate the two
limit positions, and virtually no driving contact at the two limit positions, so that
vibration of the crank arm is not transferred to the print element. |
[0001] The invention relates to a print element shifter for use in an impact printer having a print element bearing fully formed characters on two circular series. The shifter is designed to move the print element rapidly and accurately from a first operational position, wherein a printer hammer is aligned with the print characters of one circular series, to a second operational position, wherein the printer hammer is aligned with the second series.
[0002] In recent years, serial impact printers utilizing print elements such as "daisy" wheels or "thimbles" have had increasing popularity. The "daisy" wheel comprises a rotary hub from which plural radial spokes or "petals" extend, with each spoke carrying at its end a character pad bearing the impression of a character in relief. The "thimble" is a cup- shaped print element with characters carried around its lip. It may be considered to be a daisy wheel with its spokes bent substantially at right angles. Each print element bears a number of characters including letters. numerals and symbols designed in accordance with a font motif. Printing is accomplished, with both of these types of elements, by mounting the hub of each upon a rotary drive shaft extending from a drive motor indexable by means of suitable control logic to align the desired characterin front of the hammer. As both of these print elements are light and compact, they have relatively low inertia enabling rapid rotation during the printing sequence. This enables high speed printing. Because installation and removal of the print elements of these printers are generally mechanically simple maneuvers, the user is afforded great flexibility in selecting and changing type styles, since in order to change a given font the user merely replaces the element then in the machine with another element.
[0003] It has become desirable, in many instances, to increase the number of characters carried by a given print element- Clearly, one method of accommodating a greater number of characters is by adding more spokes. This will increase the capacity of the element but it will also increase the diameter of the wheel, resulting in an increase in the inertia of the element. Ultimately, the speed of operation of the printer would have to be reduced, as the print element becomes too large. A logical alternative to increasing the size of the print element is to increase the number of characters on an element of a given size. This may be accomplished by placing more than one circle of characters on the element, i.e. inner and outer concentric circles of characters on a "daisy" wheel, or upper and lower circles of characters on a "thimble". With print elements of this type, a greater character capacity is achieved without increasing the inertia of the element In each of U.S. Patents 3 651 915, 3 651 916, 3 921 787 and 4 106 611 there is shown a print element having two circles of characters, in the form of a printwheel. Furthermore, in each of U.S. Patent Nos. 3 651 916 and 4 106 611 there is also disclosed a "thimble"-type print element with two circles of characters. In order to position the selected character to be printed in front of the print hammer, it is necessary to provide some suitable mechanism for repositioning the print element. Thus, in U.S. 3 651 915 the printwheel is secured to, and is moved vertically by means of, a carrying member lifted and lowered on movable rods. In US. 3 651 916 the axial drive shaft of the illustrated printwheel is supported by an arm movable in an arc and the axial drive shaft of the illustrated "thimble" may be oved axially by a positioning electromagnet. In U.S. 3 921 787 the printwheel is shifted by a pivoting motor driven arm acting on the printwheel support shaft. Also, in U.S. 4 106 611 the illustrated printwheel is shifted by a crank arm drawn into both its upward and downward positions by shifting solenoids and the illustrated "thimble" (shown with four circles of characters) is vertically slideable on its axis.
[0004] We have found that a major factor controlling the speed of printing, in these plural circle print element devices, is the settling delay time after shifting during which the print element vibrates and settles at its "home" position. Such a delay is inherent in mechanical linkages systems wherein drivers move connecting links against fixed stops. If printing is carried on immediately after shifting, while the print element is vibrating, the printed characters will not lie upon the typed base line and will render the printed page unappealing. This will be the case, for example, in the system disclosed in U.S. 4,106,611, where the crank arm is slammed against the shifting solenoids and there is a tendency for the crank arm, and the print element intimately connected thereto, to bounce and vibrate before it actually settles into its home position. In the other patents referred to above, it also appears that extended settling will be encountered.
[0005] It is the intent of this invention to provide a shifting mechanism which will move a print element accurately and at a high rate of speed from one character circle to another while eliminating the probhem of vibration at its- home position. Smoother rapid and accurate shifting is accomplished by means of a crank arm, movable in an arc between two end of travel positions defined by fixed limit stops, and connectable to the print element through a shifting cam and slider arrangement. The driving connection is made such that there is driving contact between the crank arm and the shifting cam between the two end of travel positions and the driving contact is substantially zero at the two end of travel positions as the crank arm reaches its limits of travel. Thus, as the crank arm impacts the fixed stops, the impact is isolated from the print element
[0006] This invention may be carried out in one form, in accordance with the following detailed description and with reference to the accompanying drawings in which:
Figure 1 is an isometric view of a print element shifter and driving apparatus of the present invention shown in relation to a platen;
Figure 2 is an elevation showing a printwheel in its normal, lowered position;
Figure 3 is an elevation showing the printwheel of Figure 2 in its shifted, raised position;
Figure 4 is an elevation of the shifter mechanism showing the printwheel in its normal, lowered position;
Figure 5 is an elevation similar to that of Figure 4 showing the printwheel in its shifted, raised position;
Figure 6 is a graphical representation of the time and displacement characteristics of known shifting mechanisms; and
Figure 7 is a graphical representation of the time and displacement characteristics of the shifting mechanism of the present invention.
[0007] With particular reference to the drawings, there is shown in Figure 1 a printwheel 10 provided with radial spokes and two concentric rows of characters such that each spoke has an outer character 12 and an inner character 14. The printwheel is removably mounted on drive shaft 16 which is rotatable either in a clockwise or a counterclockwise direction as shown by arrows A, under the motive force of printwheel motor 18. In this manner, the proper angularly oriented spoke may be aligned with and arrested in front of hammer 20. A hammer, such as that disclosed and fully described in U.S. Patent No. 4,037,532 may be advantageously used herein. Properly timed energization of the hammer 20 will impact the selected character and move it against inked ribbon 22 and record receiving sheet 24 supported by platen 26. Thus ink will be released from the ribbon 22 and transferred to the sheet in the shape of the impacted fully formed character.
[0008] The printwheel 10 is coupled to the output shaft 28 of printwheel motor 18 through a universal coupling 30 which allows the printwheel to be shifted in space, in a direction perpendicular to its axis, while still being positively driven by the motor. Printwheel drive shaft 16, which is connected at one end to the universal coupling 30 and carries the printwheel at its other end, passes through, and is suitably journallcd for rotation in, shifter cam 32. In this manner, the shaft may be moved in the direction of shaded arrow D while still rotating about its axis. Shifter 32 is shown to be of generally "pear" shape with a slot 34 extending from one end inwardly toward its center of rotation, as defined by a pivot shaft 36 fixed to a carriage (not shown). In a manner fully comprehended by those familiar with impact printers of this general design, all the mechanisms related to rotating and shifting the print element, mounting and driving the ribbon and mounting and driving the hammer are supported upon the carriage for reciprocating movement parallel to the axis of the platen.
[0009] Movement of the shifter cam 32 is effected by a slider pin 38 (which may also be in the form of roller 39, as shown in Figures 4 and 5) constrained to move in slot 34, and carried at the end of long leg 40 of crank 42. The generally L-shaped crank 42 is fixed for rotation about pivot pin 44 (secured to the carriage) located at knee 46. A short leg 48 of crank 42 has a slot 50 therein through which a driving connecting pin 52 passes. The pin 52 is fixed at both ends in clevis 54 formed at the end of solenoid drive shaft 56, extending from solenoid 58. In order to retain the solenoid drive shaft in its normally extended position relative to the solenoid 58, when the solenoid is deenergizcd, a return compression spring 60 is mounted to encircle the shaft 56. At one end the spring 60 is biased against the body of the solenoid and at its other end it is biased against the short leg 48 of crank 42. Upper and lower stop pads 62 and 64 (secured to the carriage) are positioned adjacent the path of movement of long leg 40 to limit movement of the crank 42 in a prescribed arc of travel, under the influence of either the solenoid 58 or the compression spring 60.
[0010] The operation of the shiftcr is as follows: In its normal position, as shown, the solenoid is decnereized and the solenoid drive shift 56 is fully extended tinder the influence of compression spring 60. The printwheel is in its lower position as shown in Figure 2. Energization of the solenoid 58 elcctromotively draws in solenoid drive shaft 56 in the direction of arrow B. This movement rotates crank 42 in the direction of arrow C about pivot pin 44, until it hits the lower fixed stop pad 64, causing slider pin 38 to traverse an arc in a clockwise direction. As the slider pin moves in slot 34 the shifter cam 32 is rotated in a counter clockwise direction,as indicated by the arrow D, thus elevating the printwheel 10 toward the position shown in Figure 3. Upon dccncrgiation of the solenoid 58, the return compression spring 60 causes the linkage system to move in a reverse direction, i.e. oppositely to arrows B, C and D, and to arrest crank 42 against upper fixed stop pad 62.
[0011] Movement of the shifting mechanism has been described insofar as it moves the printwheel rapidly in a vertical direction and locates the selected circle of characters in its proper position. An understanding of Figures 4 and 5 will make clear the advantage achieved by the shifter of our invention in completely eliminating the effects of settling vibration on the print element. Each of these Figures illustrates the end of stroke positions of shifter cam 32, At either end of its arc of travel, long leg 40 of crank 42 is driven into the fixed stop pads 62 and 64, by the force of return compression spring 60 and solenoid 58, respectively. As the crank is thus "slammed" home it bounces and vibrates prior to arriving at its quiescent home position. At the time of this jarring contact the slider pin 38 is being moved directly in line with the slot 34 and is, in effect, isolated from the shifter cam. Thus, the shifter cam and printwheel have been moved smoothly and rapidly to their full-up or full-down positions and sit motionlessly while the crank settles,
[0012] A graphic representation of the beneficial operation of our invention, just described, is shown in Figure 7. It should be compared with the graphic representation, shown in Figure 6, of known prior art shifting mechanisms. In Figure 7, the traces representative of the vertical movement of the printwheel drive shaft 16, in both the the lifting and lowering modes, rise and drop smoothly from beginning to end. However, in Figure 6 the traces representative of the vertical movement of the printwheel drive shaft, in both the the lifting and lowering modes, of the prior art devices, clearly show the irregular movement usually imparted to the print element as it settles into its home position. In order to impact a character immediately after shifting from one character circle to another, care should be taken to ensure that the subsequent character is properly nositinned at the tvned baseline. Thus. in the prior art dcviccs one must either operate the printer at a slower speed, or take a chance that the settling print element will not cause the impacted character to be too far off the target Neither of these alternatives is desirable and both are obviated by the novel shifter mechanism of the present invention.
1. Apparatus for shifting a rotary print element (10), provided with two circles (12,
14) of characters, between one operating position in which the characters on one of
the circles are in alignment with an impact element, and an other operating position
in which the characters on the other circle are in alignment with the impact element,
including means (16) for supporting said print element for rotation about its axis,
driving means (18) for rotating support, characterised by
universal means (28,30) for coupling said axial support and said driving means,
means (32) connected to said axial support for shifting said print element between said two operating positions, and
driving means (58, 42, 38) coupled to said shifting means for moving said shifting means between two limit positions of which each is associated with one or other of said two operating positions, such that there is driving contact between said driving means and said shifting means intermediate the limit positions, and virtually no driving contact at the limit positions.
2. The apparatus as claimed in claim 1, characterised in that said shifting means
comprises a cam having a fixed center of rotation (36) carrying said axial support
(16) eccentrically thereon, and having a drive slot (34) therein for receiving said
driving means (38).
3. The apparatus as claimed in claim 2, characterised in that said shifting means
is movable in an arc about said fixed center of rotation, and in that the drive slot
is disposed in a radical direction.
4. The apparatus as claimed in claim 3, characterised in that said driving means comprises
an arm (40) movable in an arc about a fixed pivot (44), said arc of travel being limited
by two fixed stop members (62, 64).
5. The apparatus as claimed in claim 4, characterised in that said arm has a driving
pin (38) thereon slideably engaged in said drive slot.
6. The apparatus as claimed in claim 5, characterised in that as said arm approaches
said two limit positions said driving pin moves in a radial direction relative to
said shifting means.
7. The apparatus as claimed in any preceding claim, characterised in that said axial
support is moved by said shifting means in a direction normal to the rotary axis of
the print element.
8. The apparatus as claimed in any preceding claim, in which the driving means includes
a solenoid (58) movable against a spring bias (60) to pivot the shifting element in
one direction when energised, and which, when deenergised, allows the spring bias
to pivot the shifting element in the opposite direction.