Printing apparatus

A printing apparatus having a plurality of printing wires or hammers. The printing apparatus has a main drive driven continuously at a resonance frequency so as to supply energy to the plurality of printing wires or hammers, a plurality of selective drives corresponding to the plurality of printing wires or hammers, selectively energized by printing signals so as to transmit the driving energy to the printing wires or hammers. The printer head assembly can be extremely small sized and operate with low energy.

BACKGROUND OF THE INVENTION 
This invention relates to an impact type printer having a plurality of 
print wires or hammers. Impact printers are well known in the prior art. 
As an example, a wire dot matrix printer, a typical impact printer has a 
plurality of solenoid-driven wires mounted within a movable print head 
which traverses a paper. During movement of the print head across the 
paper, selected solenoids are energized and drive the corresponding print 
wires to impact an inked ribbon and ultimately the paper to form 
dot-column patterns at closely spaced intervals across the print line. The 
print head utilizes seven or nine solenoid driven print wires and 
successively forms five or seven dot column patterns so as to form 
alphanumeric patterns. 
Nowadays wire dot matrix printers are very popular because of their 
superior characteristics such as simplicity of the mechanism, high speed 
of solenoid operation, high reliability, and ability to make at the same 
time. Due to the popularization of readily usable computer systems, dot 
matrix printers functioning as output apparatus for such computer systems 
are required to have at least 16 print wires so as to print not only 
alphanumeric characters but also chinese characters and graphic patterns. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide a novel and small impact printer 
having a plurality of printing members and driven by an amount of electric 
power lower than heretofore achieved in conventional systems. The impact 
printer of the present invention has a main drive means continuously 
driven and generates driving power for operation of printing members, and 
a plurality of trigger driving means selectively energized according to 
print signals so as to transmit the driving power to the print members to 
impact the paper. 
It is therefore an object of the present invention to provide an impact 
printer which is small-sized and consumes a small amount of power. 
It is another object of the present invention to provide an impact printer 
which has a main driving means and trigger driving means for driving 
printing members.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a print head assembly 1 is mounted on a carriage base 
2 capable of sliding parallel with paper 3. The carriage base 2 is moved 
parallel with the paper 3 at a constant speed by means of drive pulley 4a, 
idle pulley 4b and a drive wire rope 5, the drive pulley 4a being driven 
by a carriage drive motor 6. 
FIG. 2 is a cross-sectional view of the print head assembly 1 showing the 
internal structure thereof in detail. Print wires 7 are all arranged to 
converge at the front end of a wire housing 8 and aligned along an 
imaginary straight line. A jewel bearing 9 is mounted on the front end of 
the wire housing 8. The jewel bearing 9 has openings corresponding to the 
print wires 7 for guiding the front ends of the print wires 7. The print 
wires 7 are slidably supported by the jewel bearing 9 and fixed to print 
springs 10 at the rear ends, respectively. Each print spring 10 is made of 
magnetic material and has a magnetic chip 11 on the surface opposite to 
that to which each print wire 7 is connected. The print springs 10 and 
magnets 12 are mounted on the inside of the wire housing 8. A trigger yoke 
13 made of magnetic material and having a release coil 14 wound 
therearound has one end fixed to a corresponding magnet 12, and the other 
end surface faces the corresponding magnetic chip 11 mounted on the 
corresponding print spring 10 with a gap therebetween. 
Each spring 10, magnetic chip 11, magnet 12, trigger yoke 13 and release 
coil 14 constitute a selective (or trigger) driving mechanism. 
FIG. 2 shows the print head assembly 1 impacting a ribbon 15 and indirectly 
the paper 3. An armature 16 carrying a driving member 17 is pulled by a 
main solenoid 18 mounted on the rear part of the wire housing 8 so as to 
move the print wires 7 to the reset position. A main spring 19 is mounted 
on the wire housing 8 for urging the armature 16 to move in the opposite 
direction it is pulled by solenoid 18. The driving member 17 has a 
projection 20 at the front end, which catches and pulls the print spring 
10 until the magnetic chip 11 contacts with the trigger yoke 13 when the 
solenoid is energized as shown in FIG. 3. 
Armature 16, driving member 17, main solenoid 18, main spring 19 and 
projection 20 constitute a main driving mechanism. 
The ratio of the mass of the print wire 7 and the print spring 10 to the 
elastic coefficient of the print spring 10 is equivalent to the ratio of 
mass of the driving member 17 and the main spring 19 to the elastic 
coefficient of the main spring 19. The main solenoid 18 is energized 
continuously at the resonance frequency of the vibrating system 
constituted by the print spring 10 and the print wire 7. 
A position sensor 21 comprising a radiant unit 21a and a light sensor 21b 
facing each other is mounted on the carriage base 2. A positioning plate 
22, having transparent slits 22a corresponding to the position where the 
print wires 7 should be driven to impact, is placed between the radiant 
unit 21a and the light sensor 21b. 
FIG. 4 is a block diagram of a driving circuit. A selection control circuit 
23 delays the time when a selecting circuit 24 energizes the desired 
release coil or coils 14 until after the time when a main circuit 25 
energizes the main solenoid 18. In operation, a main control circuit 26 
controls the input voltage to the main solenoid 18 or the time of 
energization of the main solenoid 18 according to the number of the print 
wires 7 being driven. The selection control circuit 23 and the main 
control circuit 26 are not driven by a print signal from a print control 
circuit 27 until they receive the signal from the position sensor 21. 
In the initial condition, all the print springs 10 are held distorted by 
the force of the magnets 12. The carriage base 2 having the position 
sensor 21 thereon moves across the paper 3 at a constant speed, so that 
the position sensor 21 generates a signal at each position where the print 
wires 7 should be driven. On receiving this signal, the main control 
circuit 26 and the main circuit 25 energize the main solenoid 18 so as to 
drive the driving member 17 until the printing spring 19 moves to the 
reset position where each print spring 10 is held by a magnet 12. The main 
control circuit 26 controls the duration of the input voltage to the main 
solenoid 18, i.e. time of enegization of the main solenoid 18, according 
to the number of the print springs 10 which must be moved so as to be held 
by the magnet 12 i.e. according to the change of the load. 
The vibrating system composed of the driving member 17 and the main spring 
19 is driven after a delay time because of the phase delay due to the 
mass, elasticity and viscosity of the vibrating system. The print springs 
10 therefore are moved after a constant time including the delay time so 
as to move to the reset position. The selecting circuit 24 is energized at 
a constant time later than the time of energization of the main solenoid 
18 by the selection control circuit 23. The selecting circuit 24 sends an 
electric current to selected ones of the release coils 14 which are 
required to produce the desired alphanumeric pattern, which cancels the 
magnetic flux of the magnet 12. Consequently, the attractive force of the 
magnet 12 is caused to disappear, and the selected print springs 10 are 
released so that the desired print wires 7 are driven against the paper 3. 
At this time the main solenoid 18 is no longer energized and the driving 
member 17 has been driven in the direction toward the paper 3 by the main 
spring 19. 
During the next energization of the main solenoid at the next cycle of the 
resonance frequency, the main solenoid 18 is energized by the main control 
circuit 26 and the main circuit 25 so as to move the selected print 
springs 10 which have driven the selected print wires to the impact 
position back to the reset position where the print springs 10 are held by 
the magnets 12. 
By the continuous operation as described hereinbefore, the printing 
apparatus carries out the printing operation. 
In the above operation, the amount of electric energy supplied to the 
release coils 14 is extremely small, being only enough to cancel the 
magnetic flux of the closed magnetic circuit including: the magnet 12 made 
of alnico; the trigger yoke 13 made of high permeability material, for 
example, permalloy; the magnetic chip 11; and the print spring 10, which 
together constitute a small size trigger unit. Consequently, the main 
circuit 25 and the power supply unit (not shown) can be small sized, small 
capacity, and low cost. 
Since the main solenoid 18 is capable of being driven continuously in an 
imaginary constant cycle, it can be driven in the resonance condition 
despite any variation in the vibrating system due to variations in the 
number of print wires 7 being driven to impact. This is because, the 
resonance frequency f.sub.i of the vibrating system comprising the print 
wire 7 and the print spring 10, assuming that the moving mass and the 
elastic coefficient of this vibrating system equal to m.sub.i, k.sub.i, is 
##EQU1## 
In the same way, the resonance frequency f.sub.M of the main vibrating 
system comprising the driving member 17 and the main spring 19 is 
##EQU2## 
where m.sub.M, k.sub.M equal the moving mass and elastic coefficient of 
the vibrating system. The resonance frequency f.sub.T of the total 
vibrating system when n print wires 7 are driven can be calculated by 
using the expression 
##EQU3## 
Thus the relation of each of the vibrating systems 
##EQU4## 
leads to the relation 
EQU f.sub.T =f.sub.M =f.sub.i =f.sub.d 
where f.sub.d is driving frequency. Accordingly, the resonance frequency 
f.sub.T is equal to the driving frequency f.sub.d independently of the 
number of the print wires 7 being driven to impact the paper 3. 
Therefore, the electric power supplied to the main solenoid 18 can be 
greatly reduced because of high efficiency of the main solenoid as an 
electro-mechanical transducer. And the delayed time of the movement of the 
driving member 17 with respect to the time when the electric energy is 
inputted to the main solenoid 18, decreases, so that the reliability of 
the printing operation is improved. 
The same efficiency is given by the main control circuit 26 which controls 
the current intensity to the main solenoid 18 or the time of energization 
of the same according to the number of the print wires 7 being driven to 
impact the paper 3. And the selection control circuit 23 achieves a high 
reliability of the printing operation, because the time when the release 
coils 14 are energized is delayed a constant time including the phase 
difference between the driving force of the main solenoid 18 and the 
movement of the driving member 17 with respect to the time when the main 
solenoid 18 is energized. 
In the print head assembly moving across the paper 3 at a constant speed, 
the main solenoid 18 and the release coils 14 are energized according to 
the selecting signals at constant intervals generated by the position 
sensor, so that the dot-pitch error is reduced, and high quality of 
printed characters is achieved. 
FIG. 5 shows another embodiment of the driving circuit. The same effect is 
achieved by the circuit having a standard oscillator 28 and a motor 
control circuit 29 shown in FIG. 5. The standard oscillator 28 generates 
pulses at a constant frequency in order to energize the main soleniod 18 
and the release coils 14. The motor control circuit 29 controls the 
carriage motor 6 to synchronize the pulses from the position sensor and a 
standard pulse. 
It can be seen from the foregoing description that the present invention 
provides a novel impact printer having a plurality of print members 
comprising a main drive means including the main solenoid, and a trigger 
(or selective) drive means including release coils so as to achieve a 
small size printer using only a small amount of energy. 
Although this invention has been described with respect to its preferred 
embodiments, it should be understood that many variations and 
modifications will now be obvious to those skilled in the art and it is 
preferred, therefore, that the scope of the invention be limited not by 
the specific disclosure herein, but only by the appended claims.