Disc printers

The type carrying disc has the type characters spaced irregularly along the disc periphery in dependence on their width, but the order in which these characters appear on the disc is independent of their width. Associated on the disc with each type character is an aperture that indicates the angular position of the corresponding type character. A photoelectric detector reads these apertures as the disc spins, and enables, in dependence on a coded signal, the disc to be stopped at the correct position for printing the desired character. In other embodiments, the aperture or other means indicates one or another printing characteristic, such as printing force or size of the printing step.

The invention relates to improvements in disc printers, which printers 
comprise a rotatable support having the type characters in relief, the 
characters being brought to a position opposite the printing surface, 
where the character in the printing position is struck by a hammer to 
print on the printing surface. 
In known printers of this kind, the type characters are carried by the 
flexible radial arms of a rotatable support in the form of a disc or cone. 
These arms occur at a constant angular spacing, so that the type 
characters are uniformly spaced along a circumference. Consequently, the 
type character that is to be printed is selected by a number of constant 
angular steps. Since the rotatable support is usually made of a molded 
synthetic plastic, it is difficult to obtain a highly precise angular 
position of each type character, because the internal mechanical stresses 
distort the support after it is removed from the mold. 
Moreover, the observance of a constant angular step between type characters 
means that the size of this step depends on the widest type character, 
which fact results in a rotatable support that is undesirably large and 
therefore heavy. With a view to obtaining higher printing speeds, it has 
been sought to reduce the diameter and the inertia of the rotatable 
support by alternating the wide and narrow type characters. This leads to 
special orders in which the type characters appear on the support, none of 
which orders corresponds to those of the codes of the American Standard 
Code for Information Interchange (ASCII), which are widely used in 
information interchange. This lack of correspondence complicates the 
electronic circuits that control the rotation of the support. 
An object of the invention is improvements in disc printers, which 
improvements remedy the aforesaid faults. 
In accordance with the invention, this object is attained by irregularly 
spacing the type characters about the rotational axis of the support so 
that the angular arc occupied by each type character is dependent on the 
width thereof, indicating means incorporated by the support for indicating 
the angular position of each type character, and reading means for reading 
the indicating means, and for providing from the reading, a signal for 
controlling the printing of each character so as to obtain a uniform 
distribution of the printed characters on the printed copy.

FIG. 1 shows a rotatable support 1 consisting of a synthetic plastic disc 
composed of flexible radial arms 2, each arm incorporating at its free end 
a type character 3 in relief. Each arm also has an aperture 4, the angular 
position of which about the axis of rotation of the disc 1 is very 
precisely located with respect to the angular position of the associated 
type character 3. 
As shown in FIG. 2, the disc 1 is mounted on the shaft of a servo motor 5, 
so that each type character 3 can be moved to a position opposite a 
cylinder 6, over which passes a sheet of paper 7. This sheet constitutes 
the printing surface. The means for guiding the sheet over the cylinder 6, 
as well as those that control the rotation of this cylinder, are not 
shown, because they are well known in themselves and do not form a part of 
the invention. 
To cause the printing of the desired character, the motor 5 is rotated 
until the character is positioned opposite the sheet 7. After the disc 1 
is stopped, an electromagnetically driven hammer 8 is operated to strike 
the character 3 against the paper 7. The arm 2 is free to move because it 
is flexible. 
In order to ensure that the motor 5 stops at the precise position, the 
printer incorporates a photoelectric arrangement for determining the 
correct position of the character to be printed. This arrangement includes 
a light source 9 placed on one side of the disc 1. When the arm 2 is 
correctly positioned, the light beam from the source 9 passes through the 
aperture 4 and falls on a photoelectric detector 10. Moreover, one of the 
arms 2 of the disc 1 also has an aperture 30 that is closer to the center 
of the disc, and which enables, by means of a second photodetector (not 
shown), to fix a predetermined angular position of the disc, if necessary. 
The control for the servo motor 5 is shown in FIG. 3, which also shows the 
light source 9 and the photodetector 10. The detector 10 receives the 
light beam through the aperture 4 each time that an arm 2 passes before 
it. The beam also falls on the detector each time that a radial slit 
separating two consecutive arms 2 moves past the photo-detector. 
Consequently, the latter furnishes electrical pulses that are shaped by a 
trigger 11. The pulse output of the trigger, after passing through a 
divide by two circuit 31, goes to a counter 12, the outputs of which are 
connected to a comparator 13. This comparator also receives from a 
register 14 a coded signal that indicates the character to be printed. 
When the number of pulses received from the photo-detector 10 since the 
last printing corresponds to the coded signal from the register 14, the 
comparator 13 responds to this coincidence and sends a signal on output 
line 15 to stop the motor 5. 
The comparator 13 also responds to the difference between the position 
given by the signal from the register 14 and the position actually 
occupied by the disc 1. Therefore, the comparator furnishes on one or the 
other of the output lines 16 and 17 signals for causing the motor 5 to 
turn in one or the other direction, so as to bring the desired type 
character into position for printing. The means that enable the comparator 
to determine whether the desired position of the disc 1 is more quickly 
arrived at by a rotation in the one direction or the other are known and 
therefore will not be described in detail. The three output lines 15, 16 
and 17 of the comparator 13 control a circuit 18, which, in turn, controls 
the motor 5. 
The signal on the output line 15 is applied not only to the circuit 18 but 
also to an AND gate 20, the other input of which is connected to the 
output of the trigger 11. Thus, when the motor is stopped while a pulse is 
delivered by the trigger 11, the gate 20 furnishes an electric signal that 
operates the hamer 8 by means of a monostable multivibrator 19 and an 
amplifier 21. 
As shown in FIG. 3, the amplifier 21 has an input 22 that controls the 
amplification factor of the amplifier, this input being directly connected 
to the output of the photo-detector 10. This arrangement enables the force 
driving the hammer 8 to be varied as a function of the amplitude of the 
signal furnished by the photo-detector 10, as will be seen in FIGS. 4A and 
4B. In this way, it is possible to adapt the force with which the hammer 
strikes an arm 2 to the surface of the type character that is to be 
printed. It is quite obvious that the force used to print the character 
"." should be less than that used to a print a "W". Accordingly, the force 
with which an arm is struck (a printing characteristic) is controlled by 
information related to the surface area of the character to be printed. As 
shown in FIGS. 4A and 4B, this can be done by replacing the round 
apertures 4 of FIG. 1 with slits 24, 24' the radial lengths of which 
correspond to the size of the type character surface. FIG. 4B also shows 
that an arm 2 is wider for a character such as "M" than it is for a narrow 
character such as "I" and that the slit 24' is longer than the slit 24, 
resulting in a greater hammer force. This enables the diameter of the disc 
1 to be rounded to a minimum and thus to limit the disc's inertia, thereby 
permitting very fast accelerations and decelerations and consequent high 
printing speeds. 
The apertures 4 and 24, 24' can be simple openings, openings covered with a 
light passing material, or light passing inserts in the arms 2. 
If it is desired to print with the spacing varied (variable printing step: 
another printing characteristic) as a function of the length of each 
character printed, the invention can incorporate some means that indicate, 
for example, the width of each type character. The indication can be 
obtained, for example, from the width of the arm incorporating the 
respective character, with an optical reader detecting the arm width and 
providing an electrical signal that determines the amplitude of the spring 
step (the amount of relative movement between the disc 1 and the surface 
printed on) for the character to be printed. 
In accordance with the invention, this indication can also be obtained by 
use of apertures in the arms 2 in the same way as for controlling the 
printing force. 
There are several possible embodiments for ensuring an exact positioning of 
a type character for printing, as well as for varying the printing force 
in dependence on the size of the type character surface and/or for 
printing with spacing varied in dependence on type character length. One 
such embodiment is shown in FIG. 5. 
With reference to FIG. 5, there is provided at the free end of each arm 2 
at least one projecting stop 25, the height of which varies in dependence 
on the desired printing force for the type character of the arm. The stop 
25 cooperates with a member 26 that is free to move towards and away from 
the center of the disc 1. This member incorporates at least one V-shaped 
notch 27, which, when it engages the stop 25, acts to lock the disc 1 in 
the desired printing position. The downward movement required of the 
member 26 to lock is a function of the height of the stop 25; the 
amplitude of this movement provides a signal that enables control of the 
printing force. 
In the embodiment shown in FIG. 5, the size or the position of the notch 27 
or the size or the position of the stop 25 or any combination of size and 
position of the notch and/or the stop can be used to indicate either one 
or both printing characteristics. 
FIG. 6 shows another embodiment for ensuring the correct printing position 
of the disc 1. Each arm 2 has a ferromagnetic member 28, and the printer 
comprises a magnetic pickup (not shown) positioned alongside the circular 
path through which the members 28 move. 
In accordance with the invention, it may be required to furnish the disc 1 
only with means for indicating the exact position at which the disc must 
be stopped to print a character or there can also be provided means for 
indicating the two aforesaid printing characteristics: namely, the desired 
printing force and/or the printing step as varied in dependence on the 
width of the type character to be printed. 
It will be apparent to those skilled in the field of the invention that the 
means for indicating the width or surface area of each type character need 
not be incorporated on the disc 1; instead, they can very well consist of 
one or more independent members, such as one or more cams that are 
rotationally rigid with the disc 1 and constitute with the latter the 
rotatable support. 
Although the invention has been described and particularly shown with 
reference to the preferred embodiment, those skilled in the art will 
understand that the invention admits of changes in form and detail, 
asidefrom those already described, without exceeding the scope and spirit 
thereof.