GB2127749A

GB2127749A - Daisy wheel printer

Info

Publication number: GB2127749A
Application number: GB08326105A
Authority: GB
Inventors: Jeffrey P Stein
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-09-30
Filing date: 1983-09-29
Publication date: 1984-04-18
Also published as: DE3335374A1; GB8326105D0; JPS5983663A

Abstract

A daisy wheel printer includes a print wheel (11) with a hub (19). Flexible spokes (21) in the plane of the hub extend from the hub over a predetermined angle less than its whole circumference. At the end of the spokes the print characters (23) have a predetermined displacement (x) substantially defined by: <IMAGE> where theta equals the angle in radians through which the hub rotates in order that a radius forming an angle with the radius through the centre of the hub not having said spokes is perpendicular to the print line, where p equals the pitch of the type, where x is measured on a line tangential to a circle having a radius r at the point where the circle is intersected by an imaginary radius equal to pi radians, where r is the distance from the centre of the hub to the character intersected by an imaginary radius forming an angle of pi radians to the radius through the centre of the sector of said hub not having the spokes. <IMAGE>

Description

SPECIFICATION Daisy wheel printer This invention relates to a printer particularly suitable for use in office typewriters, computers, word processors, and personal computers, and more particularly of the type which uses fullyformed printing characters disposed on the extremity of flexible leaf springs, commonly known as a daisy wheel.

The most prevalent prior art method of implementing daisy wheel printers has been through use of dual motors, whereby the daisy wheel rotates around its axis to bring a particular letter into a print position. The daisy wheel is also mounted upon a carriage which translates along a line of the paper to position the daisy wheel to print at a particular spot on that line. Each of these two movements, namely rotation of the daisy wheel, and translation of the daisy wheel carriage, has been controlled by separate motors.

Examples of such prior art devices are disclosed in U.S.A. Patent Specifications 3,817,367 and 4,030,591.

In some of these devices the motor driving the daisy wheel would do so directly or through some mechanical linkage means, whereas the motor driving the carriage would operate through a belt or other taut band linkages. These devices obviously suffered from at the very least cost disadvantages since they required two motors and the necessary linkages. Furthermore, synchronization and correlation means were required to ensure that each motor performed its function at the proper time. As a result, these devices were more complex.

It is the general object of the present invention to provide a daisy wheel printer which is relatively simple, efficient, durable, and inexpensive to manufacture.

Accordingly, it is a feature of the present invention that a daisy wheel printer utilizes only one motor and can print in either or both directions.

Another feature of the present invention is that the daisy wheel printer does not require complex or unusual mechanical or electrical devices to ensure that the carriage and daisy wheel are in synchronization and correlated.

Another feature of the daisy wheel printer of the present invention is that it provides a view of the entire line being printed, i.e., the daisy wheel does not obscure or cover-up any character positions on the line.

The nature of the invention and many of its attendant features should be readily apparent by reference to the following description with reference to the accompanying drawings.

In these drawings: Fig. 1 is a perspective view of one embodiment of the present invention; Fig. 2 is a plan view of a daisy wheel according to one embodiment of the present invention; Fig. 3 is a partial enlarged view of the daisy wheel of Fig. 2; Fig. 4 is a side view of another embodiment of the invention; Fig. 5 is a side view of another embodiment of the invention; Fig. 6 is a sectional view along 6-6 of Fig. 5; Fig. 7 is a side view of another embodiment of the invention; Fig. 8 is a sectional view along line 8-8 of the embodiment of Fig. 7; Fig. 9 is a plan view of another embodiment of the daisy wheel of the present invention; Fig. 10 is a block diagram of a damping system for the daisy wheel of the present invention; and Fig. 11 is a block diagram of the frequency detector of Fig. 10.

In the drawings it should be noted that parts of the various embodiments which correspond with one another are designated with like reference numerals.

In a first embodiment shown in Fig. 1 a carriage 1 rides upon guide rods 3. Mounted upon this carriage 1 is a motor 5 and a gearbox 7. The motor shaft 9 drives the input of the gearbox 7 and a daisy wheel 11.

Connected to the output shaft 13 of the gearbox 7 is a pinion 1 5 which engages a rack 1 7.

As will become clear from the description below, for every revolution of the shaft 9 the travel of the pinion along the rack 1 7 moves the carriage 1 a distance equal to the inverse of pitch of the font of the daisy wheel 11. This means that pitch is "pica", i.e., 10 characters per inch, for every revolution of the daisy wheel 11 the carriage 1 moves 0.1 inches.

Normally the carriage 1 rests at a point midway between the positions at which two adjacent characters are to be printed. Thus, for example, with respect to pica type, the carriage rests 0.05 inches from the next print position in either direction.

As can be most clearly seen from Figs. 2 and 3 the daisy wheel 11 comprises a hub 19.

Emanating from the hub 19 are a plurality of spokes 21 having at their extremities print characters 23. The spokes 21 are resilient in order to return the characters 23 by spring action to their original position after being deflected onto the paper during the printing process. Spokes 21 and characters 23 can be constructed of materiais all well known to those skilled in the art. For example, synthetic plastics material, sintered metal or cast metals may be used.

A support 12 is mounted upon the motor 5 and carries a solenoid 14 which drives a hammer 16.

As is well known to those skilled in the art, the hammer 16 drives the characters 23 upon the daisy wheel 11 into the surface onto which the characters are to be printed.

In order to permit full viewing of the print line when the carriage 1 is at a rest position there is a sector opening 25 in a daisy wheel 1 When the daisy wheel 11 is at its home or rest position, the radius which bisects the sector opening 25 is perpendicular to the line being printed. Although any sector opening 25 can be provided, for full line viewing the sector opening 25 must be large enough for the first end of the first radius spoke on both sides of the opening to be below the printed line when the carriage 1 is at rest. In the illustrated embodiment, a sector opening of 30 degrees to each side of the home position, or a total of 60 degrees is provided.Thus, if the daisy wheel 11 has 120 spoke positions (every 30), 19 of those spoke positions are consumed by the sector opening 25, leaving 101 spoke positions for characters.

For every revolution of the shaft 9 the daisy wheel 11 rotates one full revolution. As already noted, because of the gear reduction of the gearbox 7, for every rotation of the shaft 9 the shaft 13 rotates sufficiently to drive the pinion 1 5 along the rack 17 to move the carriage 1 exactly the inverse of the pitch of the font of the daisy wheel 11. Thus, if the daisy wheel 11 has the common pica font, the pitch is 10, and for every revolution of the daisy wheel 11, the pinion 1 5 drives the carriage 1 exactly one tenth of an inch.

In conventional daisy wheel printers, a particular character is printed when the character on the radial spoke is carried in front of the print hammer. Normally this occurs when the particular radial spoke is normal to the line being printed.

In the present embodiment, the daisy wheel 11 rotates exactly one revolution at the same time as carriage 1 moves from one rest position to the following rest position. If the characters on the daisy wheel 11 were arranged in the conventional manner, i.e., radially, when a particular spoke assumed such a print position, the carriage 1 may not be in front of the desired position for printing.

For example, when the first character after the sector opening 25 is to be printed, the daisy wheel 11 need rotate only slightly more than one half of the sector opening (in the above example 300 of rotation). However, only with a daisy wheel rotation of 1 80 degrees will the carriage 1 be positioned directly in front of the desired print position which is located one half the distance between rest positions of the carriage 1.

To place characters 23 directly in front of the print position, regardless of the fact that the carriage 1 may not have yet moved into position, each of the characters 23, except for the character at the position directly opposite the home position (a revolution of 1 80 degrees), is offset both circumferentially and radially from the position the prior art would suggest.

This offset can be stated generally. As already discussed the combination of the gearing 7, the pinion 1 5 and the rack 1 7 move carriage 1 exactly the inverse of the pitch of the font of the daisy wheel 11 for each full revolution of the daisy wheel 11. Thus the distance between rest positions of the carriage 1 is equal to 1/p where p is the pitch of the font. Since the carriage 1 rests exactly one half the distance between print positions, the carriage must move a distance equal to 1/2p to move from its rest position to the print position.

The daisy wheel 11 makes one half of a revolution or rotates 180 degrees (n radians) during the period when the carriage 1 moves from its rest position to the position directly opposite the print position. Thus, the distance de which the carriage moves upon a rotation 9 of daisy wheel 11 can be stated to be 0 do= (1 ) 2n:p where Ois expressed in radians.

The offset distance x by which the carriage is displaced from the print position is given by: 9 1 ~~~~~~ - ---- (2) 27rp 2p where xis measured along a line perpendicular to a radius displaced an angle 9 from the radius through the home position of the daisy wheel 11.

Note that when the carriage offset distance is negative, the character must be displaced in one direction in order to line up with the print position, and when it is positive, the character must be displaced in the opposite direction. Also note that when the angle 7r is equal to one half revolution or radians, the offset distance x is zero, as required.

Furthermore, since the first character 31 is located on a tangent to a radius somewhat displaced from the position that it would have in a conventional daisy wheel, its distance from the centre of the daisy wheel 11 is greater than the distance of the characters 23 located on the spoke of the daisy wheel 11 that is placed 180 degrees from the home position. The distance which z character 31 is from the centre of daisy wheel 11 is

where r is equal to the distance the character 23 which is located 180 degrees from daisy wheel 1 's home position, is from the centre of the daisy wheel 11 and p and 9 are as defined above.

Referring now to Figs. 2 and 3, a specific example of this embodiment is illustrated. In this illustration the sector opening 25 is 60 degrees, i.e. an opening is provided in the daisy wheel 11 with no spokes 30 degrees each side of the home position. The daisy wheel 11 assumes this position when the carriage 1 is at rest.

It is assumed that the daisy wheel 11 rotates clockwise as viewed in Figs. 2 and 3 when the carriage 1 translates from left to right as viewed in Fig. 1. The first character 31 should be in the print position after the daisy wheel 11 has rotated 30 degrees. However, as noted above, after a rotation of the daisy wheel 11 of 30 degrees, the character 31 on a spoke collinear with a radius displaced 30 degrees from a radius intersecting the home position of daisy wheel 11 would not be directly in front of the print position.If the daisy wheel 11 has a pica pitch (10 characters per inch), the first character 31 in fact would be displaced an offset distance of 7r/6 7r -1(2) (10) 6=30=- radians 2X10 6 x=.0416667 inches p=1 0 characters/inch (4) Accordingly, as best seen in Fig. 3, the first character 31 is offset a distance equal to .0416667 inches measured along a tangent from the position which that character would have occupied if it was on a radius having an angle of 30 degrees to the radius intersecting the home position of the daisy wheel 11.

Assuming in this specific example that 4 equals one inch, and 0 equals or/6 radians and p equals 10 characters per inch

Additionally, each of the characters 23 would have to be skewed with respect to the radii of the daisy wheel 1 except for the character on a spoke 21 corresponding to the rotation of 1 80 degrees from the home position of the daisy wheel 11. In prior art daisy wheels each of the characters is designed to print on a line perpendicular to the radius intersecting that character. However, in a daisy wheel of the present embodiment a character on the spoke must be rotated slightly to print on the line.It can be shown that each character is rotated to print on a line forming an acute angle A with the radius intersecting that character:

Referring now to Fig. 4 another embodiment of the present invention is shown. In this embodiment the invention automatically compensates for the different pitch of the font on the daisy wheel 1 There is provided a carriage 1 moving upon rods 3. Mounted upon this carriage 1 is a motor 5 having a shaft 9 which carries a daisy wheel 11. In this embodiment the daisy wheel 11 includes a hub 41 comprising, in the illustrated embodiment, 3 gears 43, 45 and 47. Only one of the gears 43, 45 and 47 has teeth. In the illustrated embodiment the gear 43 has teeth, whereas the gears 45 and 47 are smooth; the purpose of this is explained directly below.

The toothed gear of the gears 43, 45 and 47 engages one of the gears 49, 51, or 53. Those gears are carried upon a shaft 55 which enters the gearbox 7. As with the former embodiment, the gearbox 7 reduces the rotation of shaft 55 to cause the output shaft 1 3 to advance the pinion 1 5 along the rack 17 by the inverse of the pitch of the daisy wheel 11 for every full rotation of the daisy wheel 11.

As anyone skilled in the art will now recognise, by providing the proper radius to the gears 43 to 47 inclusive and gears 49 through 53, the hub 41 will permit this embodiment automatically to adjust the travel of the carriage 1 to accommodate up to three different type pitches. Thus teeth are put on the proper one of the gears 43, 45 and 47 to match the pitch of the font on the daisy wheel 11 and only one particular pair of gears engage each other, all others being inactive. When the daisy wheel 11 having one pitch is interchanged with a daisy wheel having another pitch, different pairs of gears will engage and the pinion 1 5 will be driven the proper distance along the rack 17 for each revolution of shaft 9.

Another advantage of this particular embodiment is that whenever the daisy wheel 11 is changed, the driving gears 43 to 47 inclusive are also renewed. Accordingly, with proper selection of materials, i.e., softer, but very inexpensive, material for the gears 43 to 47 and harder material for gears 49 to 53, this embodiment of the invention will ensure a high accuracy of movement through the simple interchange of relatively inexpensive daisy wheels 11 instead of by the complete renewal of all the other parts of the printer mechanism.

Referring now to Figs. 5 and 6 still another embodiment of the present invention can be seen.

In this embodiment the daisy wheel 11 does not need offset spokes but uses standard radial spokes with a sector opening 25. The shaft 9 directly drives the gear 61. Accordingly, the gear 61 makes a full revolution for every revolution of the shaft 9 and the daisy wheel 11.

However, the gear 61 has teeth 63 only over a sector of its circumference having an angle corresponding to the angle of the sector opening 25 of the daisy wheel 11. The gear 61 is keyed to the shaft 9 so that the teeth 63 engage the rack 1 7 only during the period when the sector opening 25 is in the print position. Thus, when the carriage 1 is at its rest position the centre tooth of the gear teeth 63 engages the rack 1 7.

When the shaft 9 begins its rotation to move one of the characters 23 into its print position, the carriage 1 is moved directly to the print position by the rotation of the gear 61 through one half of the teeth 63. The carriage 1 remains at the print position for the remainder of the rotation of the daisy wheel 11 through the characters 23. When the appropriate character is opposite hammer 16, solenoid 14 is activated (by circuitry not shown) and the appropriate character is printed. Daisy wheel 1 1. then completes its rotation when sector opening 25 passes in front of hammer 16 and teeth 63 engage rack 17, moving carriage 1 to its rest position between print positions.

Figs. 7 and 8 show a further variation of the embodiments shown in Fig. 5 and 6. In this embodiment the rack 1 7 has a plurality of appropriately located apertures 71. These apertures 71 are engaged by a detent 73 which is operated by a solenoid 75 mounted upon the carriage 1. When the carriage 1 has moved to the approximate print position, the solenoid 75 is activated (by circuitry not shown) to cause the detent 73 to engage the aperture 71 and precisely position the carriage 1 at the desired print position.

Referring now to Fig. 9 another embodiment of the present invention is shown. As with the embodiment of Figs. 1 to 4 except for the character located 1 80 degrees from the daisy wheel 11 's home position, the characters 23 will either arrive before or after the carriage 1 is directly positioned in the proper print position.

In the first embodiment shown in Figs. 1 to 4 this offset was compensated by displacing the character from the position at which it would conventionally be located in accordance with prior art print wheels. The same effect can be accomplished by using print wheels with radial spokes and either delaying or advancing the firing of the hammer 1 6. In this embodiment the hammer 16 fires only after the daisy wheel 11 has rotated sufficiently to compensate for the offset x.

For example, in the above description for a daisy wheel rotation of 6, compensation must be made of an offset distance x measured along a line perpendicular to a radius at angle 6 to a radius through the daisy wheel's home position.

The angle a between the radius at angle 6 and a radius intersecting a point a distance x along a line perpendicular to the radius at angle 6 is given by (See Fig. 3)

where z is given by formula (3) above. If the display wheel rotates at an angular velocity w, the time T in which the radius at angle 6 will approximately intersect the print position is

Of course, the exact time will be slightly different since during the time that the daisy wheel rotates through an angle a, the carriage 1 moves with rotation of the wheel 11 through the gears 7, pinion 15 and rack 17.

Thus, the distance x is actually composed of two components. The first results from the horizontal movement of the carriage 1 and the second comes from the circumferential movement of the spoke tip. Referring to Fig. 9.

X=Xd+Xa (9) where x-a'/2 p (10) Xd=Ct27 P (lo) and x"'=Z' sin a' (11) One skilled in the art will note that xa', the actual angle of rotation of wheel 11, is not quite identical to a (Equation 7), because of the interaction of linear carriage motion and wheel rotation. The radial displacement (spokes 21) z' of the characters 23 is given by

Thus the exact time for delaying or advancing the hammer firing around the normal time for 6 rotation (13) w Each of the characters 23 is skewed with respect to the radii of the daisy wheel 11 as described above (see equation 6). The solutions to these equations can be derived by any of the techniques well known to those skilled in the art, including reiteration.

The release of the hammer 16 in accordance with this embodiment could be under the control of a microprocessor programmed in accordance with the above teachings. Such programming or the circuitry to control the hammer 1 6 and the motor 5 are a matter of choice as is known to one skilled in the art.

Referring now to Fig. 10, another embodiment of the present invention is shown which may be used to damp the oscillations in the daisy wheel 11. As in the prior descriptions, the daisy wheel 11 is driven by a motor 5 through a shaft 9. The motor 5 is the type of motor known as a stepping motor, i.e., it rotates a discrete angular amount upon application of a pulse. The daisy wheel 11 has a plurality of slits 91.

As will become evident below in the preferred version of this embodiment there are an odd number of slits 91. The centre one of the slits 91 is coincident with a radius passing through the home position of the daisy wheel 11. A light source 93 transmits columinated light, which will pass through a properly aligned slit 91 to a photo detector 95. The output of the photo detector 95 is connected both to a counter 97 and a frequency detector 98. Both the frequency detector 98 and the counter 97 are connected to computer 99. The counter 98 counts the number of slits which pass between the light source 93 and the detector 95. The frequency detector provides signals to a computer 99 representative of the time between pulses from the detector 95.

The computer 99 controls a motor drive logic 101, which pulses the motor 5 either in a clockwise or counter-clockwise direction on lines 103 and 105, r'espectively.

Referring to Fig. 11 a block diagram of a suitable frequency detector 98 is shown. A clock 111 provides pulses to the counter 113 which totals the number of pulses received. Upon receipt of a pulse from the detector 95, the gate 11 5 passes the pulse count from a counter 11 3 to a computer 99. The difference between the count successively passed through the gate 11 5 is proportional to the frequency at which pulses from the slits 91 are received.

The slits 91 can be used to detect when the daisy wheel 11 is at the home position. For example, assume there are 7 slits 91 in the daisy wheel 11. The daisy wheel 11 passes through the home position when the counter 97 registers a count of 4.The computer 99 then switches to looking at the output of the frequency detector 98.

Ideally the daisy wheel 11 should stop at the home position without overshoot. If the computer 99 senses further movement through receipt of the contents of the counter 11 3, the computer 99 recognises that the daisy wheel 11 needs additional dampening. Accordingly, the computer 99 instructs the motor drive logic 101 to pulse the motor in the opposite direction to damp the daisy wheel 11.

As a result of these damping pulses, the daisy wheel 11 will slow to a stop and begin rotation in the opposite direction, gradually picking up speed.

Eventually the daisy wheel 11 will again pass through the home position. The computer 99 then instructs the motor drive logic 101 to pulse the motor 5 to damp the rotation in this new direction. The computer 99 determines the direction of rotation of the daisy wheel 11 by analyzing the output of the frequency detector 99, i.e. the computer 99 detects the daisy wheel li's slowing down to a stop and commencement of rotation. It recognises this as a change in the direction of the daisy wheel 11's rotation. Since the computer 99 knew the original direction in which the daisy wheel 11 was moving, it had enough information to keep track of the direction of the oscillations around the home position and to properly damp out the daisy wheel ii's motion.

The computer 99 stores the pulsing information necessary to damp out the daisy wheel 11's motion. Using this information in a programme prepared by one skilled in the art in accordance with present practices, on the next rotation of daisy wheel 11 where it is not to stop at home, but at one of the characters 23, the computer 99 causes motion to drive the logic 101 to apply dampening pulses to the motor 5 causing the delay wheel 11 to damp out in accordance with the stored pulsing information. When the daisy wheel 11 again is to stop at its home position, the computer 99 again analyzes the dampening motion of the daisy wheel 11 under the effect of the stored pulsing information. Using algorithms known in the art, the computer 99 can derive pulsing information which can more quickly damp out the motion of the daisy wheel 11.This new derived dampening motion pulse pattern is then used to stop the daisy wheel 11 at each character 23.

One skilled in the art will readily recognise various variations on the above embodiments. For example, an up-down counter driven by two photo detectors or one photo detector and direction control detector could be used in lieu of the counter 97 and the frequency detector 95.

The computer 99 would know the daisy wheel 11's relative position to home by the count in the up-down counter and the direction of the daisy wheel's rotation. In accordance with the previous description the computer 99 could complete the corresponding damping pulse pattern to quickly damp the oscillations of the motor 5.

While the invention has been described by specific embodiments in illustrated variations, it is not limited thereto. Obvious modifications will occur to those skilled in the art. For example, various features of one embodiment can be combined with other embodiments. Furthermore, one skilled in the art could adapt existing features in the prior art to be used with the present invention without departure from its scope.

Claims

1. A printer including a print wheel comprising: a hub; flexible spokes in the place of the hub extending from the hub over a predetermined angle less than the entire circumference; print characters at the end of the spokes, the characters having a predetermined displacement x substantially defined by:: 1 X= ~~ 27rp 2p where 0 equals the angle in radians through which the hub rotates in order that a radius forming an angle 0 with the radius through the centre of the hub not having said spokes is perpendicular to the print line, where p equals the pitch of the type, where xis measured on a line tangential to a circle having a radius rat the point where the circle is intersected by an imaginary radius equal to 7r radians, where r is the distance from the centre of the hub to the character intersected by an imaginary radius forming an angle of 7e radians to the radius through the centre of the sector of said hub not having the spokes.

2. A printer as claimed in claim 1 including motive means to turn simultaneously the hub one complete revolution and to move the print wheel from a point 1/2 the distance between two adjacent print positions on the printed surface to the next point 1/2 the distance between two print positions.

3. A printer as claimed in claim 2 where the motive means comprises: a motor to turn the print wheel; first gear means connected to the motor; a carriage carrying the print wheel connected to the gear means, the motor causing the gear means to advance the carriage a distance equal to 1/p during one revolution of the print wheel.

4. A printer as claimed in claim 3 wherein the carriage carries the motor and the gear means includes: a pinion connected to the motor; and a rack engaged by the pinion.

5. A printer as claimed in claim 3 wherein the motor is an electrical motor.

6. A printer as claimed in claim 5 wherein the electrical motor is a stepping motor.

7. A printer as claimed in claim 3 including second gear means connecting the print wheel to the first gear means.

8. A printer as claimed in claim 7 wherein at least one gear of the second gear means is integral with the print wheel.

9. A printer as claimed in claim 1 including motive means comprising: a pinion having teeth on its circumference only over a sector of first predetermined angle; a rack which engages the pinion; a motor connected to the print wheel; gear means connected to the motor and the pinion to cause the pinion to rotate a full revolution for each revolution of the print wheel.

10. A printer as claimed in claim 2 or 8 wherein the print characters print on a line having an angle A to a radius of the print wheel intersecting the character, where A is substantially defined by:

ii. A printer comprising: a print wheel including a hub; flexible spokes in the plane of the hub extending from the hub except for an empty sector of first predetermined angle; print characters at the end of the spokes; a carriage; motor means carried by the carriage and connected to the print wheel and the carriage to turn simultaneously the wheel one complete revolution from and back to a home position and to move the carriage a distance equal to 1/p where p is the pitch of the character font; hammer means to strike the character into a print surface.

12. A printer as claimed in claim ii where the characters have a predetermined displacement x where 2nip 1 2p where 0 equals the angle in radians through which the print wheel rotates in order that a radius having an 0 to the radius through the home position is perpendicular to the print line where x is measured on a line tangential to an imaginary radius having an angle to the radius through the print wheel's home position and intersecting a circle having a radius r where r is radius of the circle centred on the centre of the print wheel and intersecting the character on a radius rr radians from the radius through the home position.

13. A printer as claimed in claim 12 wherein said characters print on a line perpendicular to an imaginary line having an angle A to an imaginary radius of the print wheel intersecting said character, where A is substantially defined by:

14. A printer as claimed in claim 11 comprising:: means to cause the hammer to strike said character at a time T where T is approximately ' a) where a' is an angle selected to satisfy the equations and is approximately defined by

where where 1 2np 2p and and

where 0 equals the angle in radians through which the hub rotates between the centre of the sector of the hub not having the spokes and a radius having an angle 0 to the radius through the centre of the sector of the hub not having the spokes, where p equals the pitch of the type, where x is measured on a line tangential to a circle having a radius rat the point where the circle is intersected by an imaginary radius equal to n radians, and where r is the distance from the centre of the hub to the character intersected by an imaginary radius equal to n radians.

1 5. A printer comprising: a print wheel including a hub and flexible spokes carrying characters in the plane of the hub extending from the hub; motor means to rotate the print wheel; a plurality of odd number slits in the print wheel; detection means to detect the passage of a slit past a predetermined point; and motor control means connected to the detection means and to the motor to damp the oscillations in rotation of said print wheel.

16. A printer as claimed in claim 1 5 wherein: the slits are located in the hub; the detection means include a light source and a light detector; and the control means include a counter, a frequency detector, and a computer connected to the counter and the frequency detector, the control means being such as to detect the speed, location, and direction of rotation of the print wheel and to damp the oscillations in the print wheel.

17. A printer including a print wheel and constructed substantially as hereinbefore described with reference to Figs. 1 to 3, 4, 5 and 6,7 and 8,9 or 10 and 11 of the accompanying drawings.