Uniform print density and registration in an impact printer

Commencing with the first print hammer(s) "out", or actuated, on each print line the hammer drive current is boosted 10% for a period, ranging from 25%=650 microseconds for single part forms to 50% for multi-part forms, of the total print hammer flight time. Earlier striking hammers on each print line strike harder, alleviating displaced first-printed characters on single part forms and light characters on multipart forms due to hammer energy loss in forms compression.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally concerns alleviating displaced 
first-printed characters on single-part forms, and light characters on 
multi-part forms, due to the energy loss incurred in forms compression by 
that print hammer(s) first printing upon each line in an impact printer. 
The invention specifically concerns boosting the hammer drive current for 
the first print hammer(s) actuated in the printing of each line by an 
impact printer. 
2. Description of the Prior Art 
There exists a problem with the print quality of an impact printer, most 
particularly a band printer but also any printer such as a drum printer, 
wherein a hammer strikes a form to press such form against a type font 
disposed proximate to such form and oppositely to the hammer, thereby 
printing the character of such type font. This problem with an impact 
printer, particularly visible on multipart forms, resides in the fact that 
the first hammer(s) fired upon each print line must compress the forms 
paper, and thus dissipate energy which would otherwise be used in the 
generation of the printed character. The result of this energy loss is a 
light character on multi-part forms, or a displaced character on 
single-part forms. The displaced character problem is most noticeably 
visible for characters having a strong vertical line component, such as a 
"H" or "F", when such characters repetitively appear in the same print 
position, or column, of many lines upon a printed form. When certain ones 
of the characters, in certain lines, are printed by first, or early, 
hammers fired in the printing of such lines whereas others of the 
characters, in other lines, are printed by later-fired hammers then the 
problem is manifested. The problem is noticeable as a "ragged", 
non-aligned, appearance of the characters in the print columns. The 
problem is so widely prevalent and recognized that it has an ascribed 
name: "the first hammer out problem", meaning that the problem is 
resultant from a condition occurring with the first actuated print hammers 
upon each print line. 
The traditional, prior art, method of attempting to deal with that first 
hammer out problem which results in displaced and/or light characters in 
the print line printed by an impact printer is to attempt to better 
mechanically compress the form. Mechanical forms compressors have been 
used, which can either be passive-type compression fingers or active-type 
devices such as electronically controlled clamps which are enabled only 
during printing. The disadvantages of these mechanical forms compression 
methods are many. For the simpler and more rudimentary methods where the 
form is continually compressed, as by passive-type compression fingers, 
such compression may be in conflict with the reliable and rapid movement 
of the form through the printer in the printing of successive lines. The 
active-type compression devices, which may only attempt to compress the 
form simultaneous with the printing of each line and release compression 
during forms movement, are expensive and unreliable. Further, both methods 
must operate at some distance from the actual strike point of the print 
hammer upon the form, and are thus necessarily imperfect in securing the 
optimal forms registration and compression at that very point(s) which 
counts most, the point(s) opposite the first striking print hammer(s) 
wherein the first character(s) will be printed upon each print line. 
Finally, it should be recognized that any mechanical forms compression 
system separate from the remaining operative parts, and core 
functionality, of the printer will have negative cost and reliability 
implications to the implementation of the basic printer function which is 
simply to print. 
SUMMARY OF THE INVENTION 
The present invention is concerned with obtaining improved uniform print 
density and print registration in an impact printer, particularly a band 
printer but also other types of impact printers such as drum printers 
wherein print hammers do strike a form to force such against a character 
font thereby printing a character upon a print line. Specifically, the 
present invention deals with alleviation of the "first hammer out" problem 
wherein energy loss due to paper compression by the first print hammer(s) 
actuated during the printing of each print line results in, for characters 
printed by such first-fired print hammer(s), a displaced character(s) 
particularly on a single-part form and/or a light character(s) 
particularly on a multi-part form. 
The preferred embodiment of the present invention uses components already 
existing in the printer in order to better compress the form at the very 
point(s) wherein the first character(s) are first printed upon each print 
line. Mainly, the preferred embodiment of the invention uses the 
first-fired, or actuated, print hammer(s) itself (themselves), providing 
it (them) with higher energy in order to compensate for the energy loss 
incurred in paper compression. The drive current is increased for the 
first hammer(s), resulting in increased energy and, thus, impact force. 
Specifically, in an apparatus and method implemented in accordance with the 
present invention a boost of the current drive of all the one or more 
print hammers first fired is enabled. This current boost, nominally a 10% 
higher hammer drive current, is enabled for a period of time commencing 
with the flight of the first print hammer(s) to print upon each print 
line. The current boost is enabled regardless of howsoever far it may be 
into the print cycle time before a first character(s) is to be printed. 
The duration of the time period of current boost will be variably 
predetermined by the magnitude of the current which is being boosted, 
which magnitude is itself predeterminedly fixed by an operator setting of 
the print hammer strike energy in consideration of the number of parts 
within the form being printed. 
For the lowest current setting used with single part forms, the 10% higher 
current boost for the first print hammer(s) is enabled for 25% of the 
print hammer flight time. For the highest current setting used with thick 
multi-part forms, the same 10% current boost is enabled for the first 
print hammer(s) for a duration equal to 50% of the print hammer flight 
time. For a 900-line-per-minute printer, the print hammer flight time is 
typically 2.6 milliseconds, making that a 10% current boost for a minimum, 
25%, portion of such flight time will last for 650 microseconds. Since in 
the same 900-line-per-minute printer a successive one(s) print hammer(s) 
of a nominal one hundred and thirty-six total print hammers may be 
successively fired upon each 173 microsecond subinterval of a 692 
microsecond minor print interval, it is possible for the print hammer or 
hammers which are fired at up to four successive such subintervals 
(commencing with the subinterval of the first overall hammer's (hammers') 
firing) to be affected with a current boost. Such current boost will be 
applied for a progressively smaller duration to print hammer(s) firing in 
each subinterval after the first subinterval upon which any such print 
hammer(s) does (do) fire. It should be understood that all print hammers, 
one or more, fired within each and any of the four subintervals of the 
effected first minor print time interval are boosted. 
The summary effect is that the print hammer or hammers "out" or energized, 
upon the first time subinterval at which any print hammer is energized 
will receive a maximum current boost. If another print hammer or hammers 
fires upon a time subinterval suitably proximate to this first time 
subinterval, such following print hammer or hammers will also receive 
current boost in proportion to the proximity in time at which they do 
follow the first print hammer(s) out. If no print hammer(s) follow 
sufficiently proximate in time, only the very first print hammer(s) will 
receive the current boost. After expiration of the minor print interval of 
four subintervals, no print hammer will be boosted until the printing of 
another line 
Expressed in numbers, from a single 1 print hammer up to a nominal 34 
(one-quarter of the 136 total print hammers) print hammers will fire on 
some first interval. For the first interval, howsoever early or late in 
the overall print cycle, whereupon any print hammer fires, then all print 
hammers fired within that first interval are boosted in current drive. The 
duration of this boost of first interval print hammers is nominally, for 
one operator adjustment, 25% of that print hammer flight time from rest to 
impact, which is nominally 650 microseconds. At a next, second, interval 
some 173 microseconds later, from 0 to 34 print hammers may fire. All 
firings are boosted for 650 microseconds minus 173 microseconds, or 477 
microseconds. If no print hammers fire, none are boosted. Similarly upon 
each of the next, third and fourth, following intervals from 0 to 34 print 
hammers may fire. Third interval print hammer firings (if any) are boosted 
for 304 microseconds, and fourth interval print hammer firings (if any) 
are boosted for 131 microseconds. Subsequent to this fourth interval no 
boost will transpire until first printing on a next subsequent print line. 
Correspondingly it is a first object of the present invention that the 
current drive of print hammers striking a workpiece form in an impact 
printer should be controllable, and that such drive will be controlled to 
apply more current to that (those) print hammer(s) first striking a 
workpiece from in each line printed by such impact printer. 
It is a second object of the present invention that such current boost 
supplied to that (those) print hammer(s) first actuated in each line 
printed by an impact printer will be proportional to the level of drive 
current normally supplied to such print hammers, which normal drive 
current level is variably predetermined by an operator to be higher for 
multipart paper forms and to be lower for single-part paper forms. 
Further, it will be the duration of the current boost which is made 
proportional to the normal drive current level. 
It is a third object of the present invention that the application of a 
boost to the current drive of that (those) print hammer(s) first striking 
on each line printed by an impact printer will be at a predetermined level 
of the normal hammer drive current, nominally 10%, and will last for a 
predetermined fractional portion of the print hammer flight time, 
nominally within the range of 25% to 50% of the normal print hammer flight 
time. 
It is a fourth object of the present invention that the boost applied will 
be sufficient to compensate for the loss of print energy which results 
from the compressing of a paper form by the first print hammer(s) striking 
such paper form upon each printed line, and that, further, a boost of 
diminishing magnitude will be applied to successive print hammer(s) fired 
at successive times which are sufficiently proximate to the time of the 
initial firing. By such a manner of intensity controlled and time 
distributed boost to the current drive of the first print hammer(s) and 
its (their) immediate successor(s), all characters upon a line printed by 
an impact printer will exhibit equal spatial registration and uniform 
print density. This will be true regardless of the time or times, early or 
late within the print cycle, at which such characters are printed upon 
each print line. 
It is a fifth object of the present invention that the apparatus and method 
for alleviating the firsthammer out problem will be implemented at low 
cost and high reliability with those components--the print hammers and the 
current drivers thereof--which are already present within an impact 
printer. The preferred embodiment circuit in accordance with the present 
invention is a modification to that existing current amplifier within an 
impact printer which provides current drive to the print hammers. 
It is a sixth object of the present invention that it should be universally 
adaptable to impact printers of disparate types. The present invention may 
be tailored to provide appropriate boost energy to impact printers of 
disparate band, drum, and other types; to printers with different font 
velocity and/or font size; to printers with print hammers exhibiting 
varying flight time and/or current requirements; to printers employing 
print intervals and print interval timing of various durations; and to 
printers printing forms of various materials and structure of all nature, 
specifically including forms varying in compressibility and resistence to 
compression. When the present invention is installed and simply adjusted, 
which adjustment may be empirically accomplished, within an impact printer 
then it will significantly alleviate problems with the uniform density and 
uniform registration of printed characters. 
These and other objectives are met by the preferred embodiment of the 
present invention, as hereinafter described.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is concerned with providing additional energy to a 
print hammer or hammers first energized in the printing of a first 
character or characters within each print line printed by an impact 
printer. This additional energy is provided in order to compensate for the 
energy loss which this (these) first actuated print hammer(s) particularly 
incurs in compressing the paper struck by such print hammer(s). Since the 
paper form being printed on is significantly compressed by the first print 
hammer(s) striking such, print hammers striking such paper at a later time 
during each print cycle do not suffer this energy loss. Forbearing that 
additional energy should be applied in order to overcome the energy loss 
which is incurred in compressing the paper, the character(s) printed by 
the print hammer(s) first striking upon each printed line (whatsoever this 
(these) character(s) may be) are prone to be positionally displaced and/or 
of substandard print density. The present invention provides additional 
energy to the print hammer(s) first striking upon each printed line by 
boosting the drive current of such hammer(s). Employment of the boost 
current of the present invention to a first firing print hammer(s) in an 
impact printer improves the positional registration and the density 
uniformity of the characters upon a line printed by such printer. 
A conceptual level block diagram of the function of the present invention 
is shown in FIG. 1. A typical band-type impact printer is illustrated in 
FIG. 1a. A side view of the position of a paper form, nominally multi-part 
paper form 10, relative to hammers 11 and band 12 is shown in FIG. 1b. The 
present invention functions to effect the printing of the TEXT TO BE 
PRINTED through DETECT[ing] FIRST TO BE FIRED HAMMER(S) in the impact 
printing of such text. Subsequent to detecting, the invention functions to 
INCREMENT ENERGY OF FIRST TO BE FIRED HAMMERS thereby causing that a 
selected one(s) of print hammers 11 will receive more energy to compress, 
and to print, paper form 10 against print band 12. 
Pictorial representations of the dual problems of character displacement 
and of print density dealt with by the present invention are respectively 
shown in FIG. 1c1 and FIG. 1c2. For the three lines shown in FIG. 1c--LINE 
1 and LINE 2 and LINE 3--it may be imagined that the left most character 
"H" shown in each line is the first character printed within such line. It 
may be noted in line 2 that the character "H" is horizontally displaced 
relative to the occurrence of the identical character "H" in LINE 1 and in 
LINE 3. Although it might well be envisioned that the first character "H" 
in both LINE 1 and in LINE 3 is likewise displaced, it is evident that the 
most visually severe misalignment, or misregistration, problem occurs by 
the comparison of the character "H" in LINE 2 to the like characters in 
LINE 1 and in LINE 3. The displacement of the character is especially 
noticeable because of the vertical line component therein, such as is 
typical of characters like "F", "L", and "T" . The misregistration shown 
in FIG. 1c1 is particularly typical of single-part paper forms, and occurs 
because the print hammer first impacting such form upon each print line 
must press such form against the character font, dissipating energy and 
registering the character at an alternative position than such would have 
been registered by a later struck hammer upon such time as the form was 
already in compression against the print character font. 
Likewise, the problem with non-uniform print density in an impact printer 
is illustrated in FIG. 1c2. Within each of LINE 1, LINE 2 and LINE 3 the 
leftmost "A" may be considered to be the first printed in time, and, the 
print band moving to the right, the letter "A" in the second column may be 
considered to be printed upon a next subsequent print interval, further 
continuing to the "A" in the third column which is printed upon a third 
print time interval. It is intended to illustrate in FIG. 1c2 that the 
density of the printed "A" becomes successively greater, until upon some 
subsequent printing of this character (or any other) at some subsequent 
print time then the print density will achieve substantial uniformity. The 
reason that the light character, the light "A", occurs at the first 
position printed is that the energy in the print hammer impacting such 
position at the first print time interval is dissipated in compression of 
a paper form. The phenomena of non-uniform print density exhibiting 
lightly printed characters illustrated in FIG. 1c2 is particularly 
prominent on multi-part paper forms. Both the problems shown in FIG. 1c1 
and in FIG. 1c2 are dependent upon the font velocity and font size, being 
more evident at higher band or drum speed wherein the print hammer is 
necessarily forcing the form in contact with the character font during the 
printing operation for a minimum period of time. The problems of print 
character registration and print character density respectively shown in 
FIG. 1c1 and FIG. 1c2 can actually become one of the primary limiting 
factors which preclude that impact printers should be operated at ever 
increasing speed, precluding that the print hammers of such printers 
should compress and print a paper form against a moving print font at ever 
faster rates and shorter time intervals. The proper registration and the 
proper density obtained by the present invention is shown in FIGS. 1d1 and 
1d2. 
A block diagram of an apparatus constructed in accordance with the present 
invention is shown in FIG. 2. A PRESET LEVEL OF HAMMER DRIVE which is 
normally set by the operator in consideration of the density, thickness, 
and numbers of copies of the forms being printed, is amplified, nominally 
in a FEEDBACK CURRENT REGULATOR 02, in order to produce a HAMMER DRIVE 
CURRENT which is distributed to all print hammers in parallel. Specific 
ones of such print hammers are actuated by logics in the printer at 
specific print interval times during the print cycle in order that they 
may contact a specific character font upon a moving media, such as a print 
band or print drum, disposed at a specific time oppositely to such print 
hammers. These logics selectively complete a path to current ground 
through selected print hammers at, and during, selected print interval 
times. By such a procedure the well known function of a band, or drum, 
impact printer is realized. 
Continuing in FIG. 2, in accordance with one preferred embodiment 
implementation of the present invention, circuit FIRST HAMMER(S) OUT 
CURRENT BOOST 04 does establish that a signal BOOST HAMMER DRIVE CURRENT 
shall be provided to the FEEDBACK CURRENT REGULATOR 02 upon the time of 
the firing of the first print hammer(s) out on each print line. Such 
signal BOOST HAMMER DRIVE CURRENT nominally connects to the feedback path 
within FEEDBACK CURRENT REGULATOR 02, causing such current regulator to 
produce an augmented drive signal which is the HAMMER DRIVE CURRENT 
(BOOSTED FOR FIRST HAMMER(S) OUT). In the actual operation of the circuit 
FIRST HAMMER(S) OUT CURRENT BOOST 04, signal HAMMER FIRED related to the 
occurrence of a first print hammer firing upon each print cycle, signal 
LAST HAMMER IMT relational to the end of a print cycle, and signal 
PAPER FEED relational to the time at which the paper form is being 
advanced and printing is not transpiring--all of which signals are 
obtained from and normally available within the logics of an impact 
printer--will be logically combined in order to establish an interval of 
time during which the print hammer drive current will be boosted. 
As evidenced by the graphically displayed transform function within the 
circuit FIRST HAMMER(S) OUT CURRENT BOOST 04, the magnitude of the HAMMER 
CURRENT BOOST will be 10%, commencing at the FIRST HAMMER OUT and 
extending for an interval of time which is VARIABLE, NOMINALLY 25% OF 
FLIGHT TIME=650 MICROSECONDS. This transfer function displayed in FIG. 2 
is that nominally occurring for a (1) band printer, with the (2) PRESET 
LEVEL OF HAMMER DRIVE set to a minimum which is particularly suitable for 
a single part paper form, and with (3) a hammer flight time of 2.6 
milliseconds which is typical of a 900-line-per-minute impact printer. 
Other transfer functions are possible in the boosting of print hammer 
drive current, and other forms of interconnection to an amplifier, or 
power supply, are possible alternatively to that current boost transfer 
function, and that interconnection, which are shown in FIG. 2. The 
pictorial showing of FIG. 2 should be understood to be representative of 
one preferred embodiment implementation of the present invention only, and 
not to depict or limit the sole electrical structures or design parameters 
which may be employed in alternative embodiments of such invention. 
A graphical representation of the boost provided to the print hammer drive 
current during the actuation of a first print hammer(s) during the 
printing of each line by an impact printer employing the present invention 
is shown in FIG. 3. The normal level of the PRINT HAMMER DRIVE VOLTAGE 
which is provided to all print hammers in parallel is shown as voltage 
level VCL. Of course, the individual hammer drive current will depend upon 
whether such individual print hammer is actuated, and the collective print 
hammers' drive current will, at any one time, depend upon the total 
numbers of print hammers as are at that time actuated. Howsoever many 
print hammers are actuated, or simultaneously actuated, the power supply 
to such print hammers does maintain voltage level VCL. 
In the preferred embodiment of the invention, to be explained in 
conjunction with the timing diagram of FIG. 5, up to 136 print columns can 
conceivably, the character fonts disposed relative to each such column 
permitting, be printed in as little as one print interval time of 692 
microseconds for a 900-line-per-minute printer. Normally, only a few print 
hammers are actuated during each print interval time, of which the print 
interval times there are normally so many in number as there are 
characters in a character set upon the band or drum and printable by the 
printer. An exception to the uniform maintenance of HAMMER DRIVE VOLTAGE 
at level VCL occurs upon the occasion of the 1st HAMMER(S) OUT. Before 
this time, which can occur at any print time interval within a print cycle 
but which time can occur only once for any line upon which printing 
transpires at all, the level of the PRINT HAMMER DRIVE VOLTAGE to the 
hammer or hammers printing is elevated to VCL+10%. Of course, no print 
hammer is actuated, availing itself of the boosted drive derived from the 
elevated voltage, until the print interval of 1st HAMMER(S) OUT. 
This elevated level of the HAMMER DRIVE VOLTAGE is maintained for a 
proportion of the FLIGHT TIME interval ranging from +25% to +50% of such 
interval The alternative time within the range of +25% to +50% of the 
FLIGHT TIME interval at which the PRINT HAMMER DRIVE VOLTAGE is reduced 
from VCL+10% to VCL is illustrated by the several curves in FIG. 3, and is 
determined, by the circuit of the present invention which will be shown in 
FIG. 7, relative to the nominal level of the PRINT HAMMER DRIVE VOLTAGE 
VCL which is being employed In particular, this level VCL of the HAMMER 
DRIVE VOLTAGE will be predetermined by an operator setting in 
consideration of the type, thickness and numbers of parts of the form 
being printed. In any case, it should be understood from FIG. 3 that the 
boost in the HAMMER DRIVE VOLTAGE, and in the hammer's(hammers') drive 
current will not be for entire of the hammer FLIGHT TIME, but will be for 
a maximum of 50% of such FLIGHT TIME. Such hammer FLIGHT TIME is nominally 
2.6 milliseconds for a 900-line-per-minute printer, or, as represents a 
different physical type of hammer, nominally 3.35 milliseconds for a 300- 
or 600-line-per-minute printer. 
Further to this concept shown in FIG. 3 that the PRINT HAMMER DRIVE 
VOLTAGE, and the hammer current resultant therefrom, may be boosted for a 
varying time ranging, from 25% to 50% of the FLIGHT TIME interval, a 
graphical representation of such PERCENTAGE OF FLIGHT TIME FOR WHICH 
CURRENT IS BOOSTED (BY 10%) is shown in FIG. 4. It may be observed that 
for MINIMUM HAMMER CURRENT (corresponding to) SINGLE T FORM the 
PERCENTAGE OF FLIGHT TIME FOR WHICH CURRENT IS BOOSTED is 25%. 
Alternatively, for MAXIMUM HAMMER CURRENT (representative of a) MULTI-T 
FORM, then the PERCENTAGE OF FLIGHT TIME FOR WHICH CURRENT IS BOOSTED is 
50%. Therefore, in accordance with the present invention when the preset 
nominal print hammer drive voltage, or current, is lower as besuits 
single-part forms, then the duration of the boost applied to such voltage, 
or current, is likewise minimal. Conversely, when the preset nominal print 
hammer drive current is larger, as besuits the printing of multi-part 
forms, then the duration of the boost voltage, or current, applied to 
first actuated print hammers is of longer duration and greater percentage 
of the print hammer flight time interval. 
That this should be the preferred relationship is not obvious. For example, 
if it is even perceived that the level of boost to be applied should be 
variable at all, and that such should be variable in accordance with the 
preset of the nominal print hammer drive voltage (current) occurring by 
operator entry relational to the form being printed (as opposed, for 
example, to a separate operator entry), then it is still not obvious that 
the parameter of boost operation which should be modified is the duration 
of the application of such boost (from 25% to 50%) and not the magnitude 
of such boost (which remains constant at 10%). Experimentation and 
empirical observation are involved in derivation of both the 10% boost 
level and the recognition that such should be variably applied for an 
initial portion (only) of the flight time interval ranging from 25% to 50% 
of the total flight time interval. To the extent that this empirical 
investigation needs be repeated for printers of diverse characteristics 
employing the present invention, it should be recognized that it is 
important in such investigation to study the font velocity relative to the 
hammer velocity at impact. Particularly, it is desirous that the hammer(s) 
first fired during each print interval should have an equal velocity upon 
final displacement of the form into the font as do later fired hammers 
have at the same point of printing. Since the font velocity, upon a band 
or upon a drum, is not appreciably affected by the progression of the 
print cycle, nor by the number of parts within the form being printed, nor 
by the compression of such form, then the controllable factor is most 
appropriately the print hammer flight velocity. The scheme of application 
of drive current boost shown in FIG. 3 and FIG. 4 is tailored to obtain 
best results on a 900-line-per-minute band printer printing commonly used 
paper forms. Measurements, and parameters, of boosted hammer operation 
taken scientifically may be augmented by empirical observation of print 
results obtained. 
A timing diagram for a 900-line-per-minute impact printer in which the 
preferred embodiment of the invention is located is shown in FIG. 5a. At 
900 lines per minute, the total print time for each line is 60/900 
seconds, or approximately 66.7 milliseconds. Of such total 66.7 
milliseconds print time, nominally 20 milliseconds are used for paper 
feeding, leaving an interval of 46.7 milliseconds during which all 
characters upon a print line must be printed. The printer is nominally 
capable of printing a full ASCII character set consisting of 64 
characters, and thus potentially requires up to 64 minor cycle, print 
interval, times within the overall 46.7 millisecond print cycle in order 
to so print up to 64 characters. In actuality, there are an additional 
three minor cycle times, or print intervals, used for synchronization 
within the print cycle. Consequently, a 46.7 millisecond print cycle 
interval is divided into 67 equal parts, each of approximately the 692 
microseconds duration illustrated in FIG. 5a. Such 692 microseconds is one 
print time interval. At each one print time interval the print hammers 
which must be fired to print a then oppositely juxtaposed print font in 
accordance with the print character to be printed at that location will be 
fired. As few as none, or as many as all of the print hammers may be fired 
within one print time interval, or 692 microseconds. The 692 microsecond 
minor print cycle, or print interval, is divided into four 173 microsecond 
subintervals upon which the enabling print signals for every fourth 
character position will be staged. As illustrated, the signal ENABLE PRINT 
0,4, . . . 132 will enable that a first bank of hammers should commence 
movement toward printing up to 34 print positions at zero elapsed time 
into the 692 microsecond print time interval, whereas signal ENABLE PRINT 
1,5, . . . 133 will commence the actuation of up to an additional 34 print 
hammers bank some 173 microseconds later, and so on. 
Such staged actuation of print hammers within a minor print cycle, or print 
interval, is not essential to the present invention. However, it may be 
understood as illustrated by example in FIG. 5a and FIG. 5b that earlier 
firing print hammers will obtain relatively more boost than later firing 
print hammers within the boost interval, and print hammers firing after 
the expiration of such interval will receive no boost at all. For example, 
in FIG. 5b it is suggested that hammers do fire at a first print time 
subinterval from hammers 1 (print position 0, 4, . . . 132), 2 (print 
position 1, 5, . . . 133), and 4 (print position 3, 7, . . . 135), all 
upon a first print time interval. Further, and though it needs not 
necessarily be so, it is shown in the example of FIG. 5b that the fourth, 
fifth, sixth and seventh hammer(s) firings occurring during a second 690 
microsecond print interval do also involve, at successively phased 
subintervals within such interval, hammers 1, 3, 4 and 1. 
Assuming that the hammer flight time within the 900-line-per-minute printer 
is equal to 2.6 milliseconds, and that the hammer current is adjusted for 
the minimum (single-part form) causing that a current boost will be 
applied for 25% of such flight time, or 650 microseconds, then the effect 
of such current boost upon the example firing shown in FIG. 5b is in 
accordance with the analysis shown in FIG. 5c. The first hammer or hammers 
ever fired, in other words those at print positions 0, 4, . . . 132, are 
affected with a boost of exactly 650 microseconds, or 25%, of their 2.6 
millisecond flight time. The time during which the hammers associated with 
the second firing interval are boosted is, however, reduced by the phase 
delay in the enablement of the print hammers within such hammer bank. By 
reference to FIG. 5a, this phase delay subinterval in enablement of the 
print hammers may be observed to be 692/4 microseconds, or 173 
microseconds. Therefore, in accordance with the analysis shown in FIG. 5c, 
the latter-fired print hammers, although still within the first print time 
interval, do receive boost for but 477 microseconds of their total 2.6 
milliseconds flight time. The continuing analysis in FIG. 5c shows that in 
the third print time interval which is empty of hammer firings, and during 
which the 3rd set of hammers would have been fired should the proper 
character have then appeared at any of the print positions (2,6, . . . 
134), would have been affected with a boost for 304 microseconds. The last 
fired hammers (print position 3,7, . . . 135) within the first print time 
interval will have that the one or more hammers fired shall receive 
current boost for the first 131 microseconds of their 2.6 millisecond 
flight time. Finally, note by the continuing analysis of FIG. 5c, that no 
subsequent hammer firing, howsoever early occurring within a next 
subsequent print interval, will receive a current boost. Similarly to FIG. 
5c, FIG. 5d shows an analysis of the current boost provided to up to a 
first eight print hammers when the level of boost is set at a maximum 
suitabale for multi-part forms At such a setting, for example in the 
printing of six-part forms. At such a setting, for example in the printing 
of six-part forms, the first boosted hammer(s) would receive a current 
boost for 50% of its flight time with the print hammers fired at up to 
seven subsequent print intervals also receiving a current boost. 
The summary teaching of FIG. 5 is that although all of the one or more 
hammers as are associated with one or more print cycles which are enabled 
to be fired during a first print time interval (at which any hammers are 
fired) will receive a current boost. The time of the hammer flight over, 
and during, which such first-fired one or more print hammers will receive 
such current boost varies. The time varies with the positions of such 
hammers within the 136 position print line and with the corresponding 
subinterval at which such print hammers are enabled to be fired. Thus, the 
earliest fired print hammers, which contact the paper form first and which 
loose the most energy to the compression of such paper form, receive the 
highest energy boost. This is the desired relationship: those print 
hammers which need the greatest boost receive such greatest boost. Those 
print hammers which are fired closely proximate in time, and which still 
need boost to overcome a paper form which is, as yet, not fully 
compressed, receive correspondingly less boost as they are more distant in 
time from the initiation of the flight of the first hammer actuated during 
the printing of each print line. 
A timing diagram for signals of pertinence to the present invention is 
shown in FIG. 6. A corresponding schematic of a preferred embodiment 
circuit apparatus of the present invention is shown in FIG. 7. In a 
900-line-per-minute printer having an approximately 66.7 millisecond line 
print time, signal PAPER FEED shown in FIG. 6 is high for approximately 20 
milliseconds during the feeding of the paper form. The remaining 46.7 
milliseconds of the line print time is the print cycle, as evidenced by 
the high condition of signal PRINT. The signal LAST HAMMER IMT 
(DELAYED) is a signal which is low-going at the time of the impact of the 
last hammer used in any print line (which impact occurs during the print 
cycle represented by the high condition of signal PRINT) delayed into the 
paper feed interval, represented by the high condition of signal PAPER 
FEED. Logical combination of this signal and signal PAPER FEED will be 
seen, in the logic circuit shown in FIG. 7b, to produce signal ENABLE 
BOOST which goes low in order to enable and initiate the boosting of print 
hammer current in accordance with the present invention. 
Continuing in FIG. 6, at a latter time, which latter time may be and which 
is illustrated to be well into the print cycle represented by the high 
occurrence of signal PRINT, the received signal (FIRST) HAMMER FIRING goes 
low, and remains that way, during the duration of the firing of successive 
hammers during printing. The signal is actually derived from a digitized 
signal of the hammer waveform and is normally and commonly available for 
control purposes in impact printers. Although the HAMMER VOLTAGE SIGNAL 
has been enabled to rise to 110% of its normal value, meaning 110% VCL as 
opposed to VCL, by the low-going occurrence of signal ENABLE BOOST, there 
is, until the time of the first firing of print hammers upon each print 
line as represented by the low-going condition of signal (FIRST) HAMMER 
FIRING, no print hammer actuation which does avail itself of this boosted 
level of HAMMER VOLTAGE. When, however, the first hammers on a print line 
do commence actuation as represented by the low-going occurrence of signal 
(FIRST) HAMMER FIRING, then the logic of the present invention shown in 
FIG. 7b will establish that a timer is set, the duration of which is 
represented by the high occurrence of signal BOOST TIMER. At the 
expiration of this timer, and upon the low-going occurrence of signal 
BOOST TIMER, the boost of the HAMMER VOLTAGE will be disabled, and such 
HAMMER VOLTAGE will return to its normal quiescent level VCL. The logic 
apparatus which does effect the interrelationship between signals shown in 
FIG. 6 will be observed during the discussion of FIG. 7. 
The schematic diagram of the circuit of the present invention is shown in 
FIG. 7. The FEEDBACK CURRENT REGULATOR 02 shown in FIG. 7a, and previously 
seen in FIG. 1, is an existing circuit within an impact printer to which 
the FIRST HAMMER(S) OUT CURRENT BOOST circuit 04 shown in FIG. 7b, and 
previously seen in FIG. 1, does connect. The connection between the two 
circuits is by signal BOOST HAMMER DRIVE CURRENT, previously seen in FIG. 
1. By this signal the boosting of the HAMMER VOLTAGE, and associated 
hammer current, to the print hammers is caused to incur upon the 
appropriate time at the actuation of a first print hammer(s) upon each 
print line. A further signal is communicated, proportional to such HAMMER 
VOLTAGE developed by the FEEDBACK CURRENT REGULATOR 02 back to the FIRST 
HAMMER(S) OUT CURRENT BOOST circuit 04. This signal is for the purpose of 
affecting that such circuit 04 should enable the boosting of hammer drive 
circuit for a variable interval dependent upon the normal, quiescent, 
level of hammer drive (which variable duration of boost was observed in 
FIG. 3 and FIG. 4). 
Considering first the existent FEEDBACK CURRENT REGULATOR 02 shown in FIG. 
7a, such circuit will produce a HAMMER VOLTAGE of a predetermined level 
from approximately 1.4 VDC to 1.8 VDC. The predetermination of the level 
to be supplied is in accordance with signal VCL ADJUST, itself nominally 
of 1.4 to 1.8 VDC. The signal VCL ADJUST arises from a variable resistance 
set by an operator in accordance with a dial indicator upon a control 
panel. Such dial nominally indicates 16 equal increments of print density 
control, and the operator will establish lower settings for lighter, 
single-part, print forms and higher settings with denser, multi-part, 
print forms. The operator predetermined level of signal VCL ADJUST will be 
amplified in the FEEDBACK CURRENT REGULATOR 02, consisting essentially of 
operational amplifiers type LM 1458 and associated circuitry in order to 
provide a high current capacity drive signal to all print hammers in 
parallel. A higher, 1.8 VDC level, of signal HAMMER VOLTAGE will cause the 
print hammers to strike harder, which is appropriate to the printing of 
multi-part forms, whereas a lower, 1.4 VDC, level of signal HAMMER VOLTAGE 
will cause the print hammers to strike less hard as is appropriate to the 
printing of a lighter, single-part paper form. 
Signal PRINT INHIBIT, high going when it is desired to inhibit print 
operations as during printer tests or upon the occurrence of printer 
faults, is inverted in inverter I4 type 7406 and applied to the base of 
transistor Q1 to turn on such transistor Q1, thereby disabling the 
feedback path of the HAMMER VOLTAGE RETURN within the FEEDBACK CURRENT 
REGULATOR 02 and disabling that any voltage, or current, should be 
supplied to the print hammers. Forebearing that such signal PRINT INHIBIT 
is high, the FEEDBACK CURRENT REGULATOR 02 does have a feedback path of 
signal HAMMER VOLTAGE through the print hammers which it does energize 
returning as signal HAMMER VOLTAGE RETURN. Such feedback path, observed to 
proceed through resistor R208, will maintain an essentially equal HAMMER 
VOLTAGE, and consequent individual print hammer current, howsoever many 
print hammers do simultaneously fire upon each print interval time. 
The interaction of the FIRST HAMMER(S) OUT CURRENT BOOST circuit 04 shown 
in FIG. 7b with the existent circuit of the FEEDBACK CURRENT REGULATOR 02 
shown in FIG. 7a is straightforward. The signal BOOST HAMMER DRIVE CURRENT 
is, by such operation of the circuit as will be discussed, of a low 
condition during times when the application of a boost to the current 
drive resultant from signal HAMMER VOLTAGE is not enabled. This level of 
signal BOOST HAMMER DRIVE CURRENT received at the feedback path of the 
FEEDBACK CURRENT REGULATOR 02 will have no effect upon the operation, or 
the HAMMER VOLTAGE produced by operation, of such FEEDBACK CURRENT 
REGULATOR 02. The HAMMER VOLTAGE output therefrom will remain at its 
normal, quiescent, level of VCL (shown in FIG. 7a). 
The low condition of signal BOOST HAMMER DRIVE CURRENT is enabled by the 
satisfaction of OR gate G1, type 7432, which satisfaction will occur at 
all times save when signal LAST HAMMER IMT (DELAYED) is low 
simultaneous that signal PAPER FEED is high. The high condition of signal 
ENABLE BOOST output from satisfied OR gate G1 is received at the clear, 
CLR, input to dual D positive edge-triggered flip-flop FF2, type 74LS74, 
and will accord that such flip-flop FF2 will hold that condition, set or 
clear, which was previously assumed. By operation of the circuit as will 
be discussed, the condition of flip-flop FF2 post the period of enabling 
current boost will be the clear condition, resulting in the maintenance of 
a high clear, or Q, signal output This high signal is inverted in inverter 
Il, type 7406, and applied through resistor R206 as the low condition of 
signal BOOST HAMMER DRIVE CURRENT. The high condition of signal ENABLE 
BOOST will likewise establish the clear condition of dual D 
positive-edge-triggered flip-flop FF1 type 74LS74. 
The commencement of the application of current boost, enabled by the high 
condition of signal BOOST HAMMER DRIVE CURRENT, begins with the 
dissatisfaction of OR gate G1. Dissatisfaction of OR Gate G1, type 7432, 
is required to produce the low condition of signal ENABLE BOOST. Such 
dissatisfaction of AND Gate T1 is enabled by the high occurrence of signal 
PAPER FEED during the paper feeding portion of the print line time 
inverted in inverter I2 type 7414 and applied to the OR gate G1 in 
conjunction with the low occurrence of signal LAST HAMMER IMT 
(DELAYED). This signal LAST HAMMER IMT (DELAYED) represents the delayed 
occurrence of the low signal resultant from the firing of the last hammer 
within the print cycle of the previous line print time. The low signal 
resultant from satisfaction of OR Gate G1 is received as the clear, CLR, 
input into dual D positive-edge-triggered flip-flop FF2 type 74LS74 
simultaneous with the high preset, PR, input to such flip-flop. The low 
clear, and high preset, signals in combination do clear such flip-flop 
FF2, producing a high clear side Q signal output therefrom. This high 
signal is inverted in inverter Il type 7406 and applied to resistence R206 
as the low condition of signal BOOST HAMMER DRIVE CURRENT. The low 
condition of this signal BOOST HAMMER DRIVE CURRENT will cause that 
FEEDBACK CURRENT REGULATOR 02 should boost the output of the signal HAMMER 
VOLTAGE by approximately 10%. The duration of this boost will be for so 
long as the signal BOOST HAMMER DRIVE CURRENT does remain low. 
Continuing in FIG. 7b, it may be noted by momentary reference to the timing 
diagram of FIG. 6 that the signal LAST HAMMER IMT (DELAYED) does return 
to the high condition, satisfying two-input OR Gate G1 and producing the 
high condition of signal ENABLE BOOST. This high condition of signal 
ENABLE BOOST is communicated through open-collector high-voltage output 
noninverting buffer B2, type 7407, to the clear, CLR, input to dual D 
positive-edge-triggered flip-flop FF1, type 74LS74. Meanwhile, the 
continuing high signal output of the clear, Q, side of flip-flop FF2 which 
is buffered in open-collector high-voltage output non-inverting buffer B3, 
type 7407, and applied to the same clear CLR input of flip-flop FF1. Upon 
the low occurrence of signal (FIRST) HAMMER FIRING, representing that a 
first firing of a print hammer(s) within a printed line has occurred, such 
signal will be inverted and stretched by Schmidt trigger ST1, type 7414, 
and applied as a high signal to the clock input of flip-flop FF1. During 
the continuing high presence of the clear, CLR, signal to such flip-flop 
FF1, the high occurrence of the clock signal will enable the flip-flop to 
respond to the high set, D, side signal input, and become set. In such a 
set condition of flip-flop FFl the clear, Q signal output goes low. This 
low clear, Q, side signal output of flip-flop FF1 is inverted in the 
open-collector high-voltage-output inverting buffer I3, type 7406, and 
applied to charge a resistance and capacitance network consisting of 
resistance R203 and capacitance C78. This resistance and capacitance 
network will charge at a rate representative of the drive current provided 
by the industry standard 7406 part. It will gradually assume a 
sufficiently positive level at the plus +, input to comparator type LM393 
relative to the differential minus, -, input to the same comparator so as 
to allow the comparator to develop a positive voltage output signal. This 
signal, when communicated to flip-flop FF2 will be sufficient to clock 
such flip-flop. 
Note at this point that the minus, -, input to comparator A1 type LM393 is 
derived from the signal output of operational amplifier A2, type TL084. 
This signal output is itself derived from a plus, +, signal input to such 
amplifier A2, which signal input is proportional to the signal HAMMER 
VOLTAGE which is developed by the FEEDBACK CURRENT REGULATOR 02. Since 
this signal HAMMER VOLTAGE which is developed by FEEDBACK REGULATOR 02 is 
proportional to the signal VCL ADJUST, and since the signal VCL adjust is 
predetermined by the operator in consideration of the thickness and parts 
of the forms being printed, then the signal produced by operational 
amplifier A2, and received at comparator A1, will be proportional to this 
predetermined setting. Consequently, this proportionality, in combination 
with the fixed signal rise time resultant from the resistor-capacitor 
network consisting of elements R203 and C78 driven by inverting buffer I3, 
will make that the time at which the signal output of operational 
amplifier A1 will become sufficiently positive so as to clock flip-flop 
FF2 will be a function of (1) the preselected components of the resistive 
and capacitive circuit which is charged, and (2) the predetermined 
adjustment as to the number of forms parts being printed by the operator. 
In this manner, the circuit of the present invention is effective to boost 
the drive current for a longer period of time upon operator preselection 
of a higher number of form parts corresponding to a higher preselected 
drive current (voltage), or to boost the drive current for a shorter 
duration of time if such preselection of the number of form parts, and 
corresponding hammer drive current (voltage), is lower. The actual circuit 
components shown will produce a time interval of approximately 50% of 
hammer flight time, or 1.3 milliseconds, when the signal HAMMER VOLTAGE is 
nominally (unboosted) at a level of 1.8 volts. Alternatively, when the 
level of such signal HAMMER VOLTAGE is 1.4 volts, the circuit shown will 
produce a time period of approximately 650 microseconds, which is 25% of 
the hammer flight time of 2.6 milliseconds. 
Continuing in FIG. 7b, the signal output of the comparator A1 will, after a 
duration of time in the range of 650 microseconds to 1.3 milliseconds, 
assume a sufficiently positive level so as to clock flip-flop FF2. This 
clocking of flip-flop FF2 will, in the presence of the high set D, side 
signal input, enable such flip-flop FF2 to become set therein producing a 
low clear side, Q, signal output. This low clear, Q, side signal output is 
inverted in open-collector high-voltage-output inverting buffer I1, type 
7406, and applied through resistance R206 as the high signal BOOST HAMMER 
DRIVE CURRENT to the feedback loop of the FEEDBACK CURRENT REGULATOR 02. 
Such high signal will be overridden by the normal low condition of this 
feedback loop, and will have no effect upon the level of signal HAMMER 
VOLTAGE as is developed by such FEEDBACK CURRENT REGULATOR 02. 
Consequently, by operation of the circuit FIRST HAMMER(S) OUT CURRENT BOOST 
04, the signal HAMMER VOLTAGE developed by circuit FEEDBACK CURRENT 
REGULATOR 02 has been boosted for a fixed period of time and, by selection 
of the component value of resistance R206 versus that resistance R208 
within the normal feedback current path, for a fixed amount Within the 
preferred embodiment of the present invention shown in FIG. 7, such boost 
will occur for a time period between 650 microseconds and 1.3 milliseconds 
depending on the setting of signal VCL ADJUST. The amount of the boost 
will be constant, regardless of the duration of the boost, at a level 
equal to 10% of signal HAMMER VOLTAGE. Finally in FIG. 7b, the low 
occurrence of the clear, Q, signal output of flip-flop FF2 will be 
communicated through open-collector high-voltage output non-inverting 
buffer element B3 type 7407 and received as a low signal into the clear, 
CLR, input to flip-flop FF1, therein clearing such flip-flop FF1. Since 
signal ENABLE BOOST had previously returned to the high condition, 
flip-flop FF2 will remain in the clear condition, and will not reset, so 
as to allow reinstitution of the current boost, until the cycle next 
begins again upon the low going condition of signal ENABLE BOOST. 
In summary, the preferred embodiment implementation of the electrical 
apparatus of the present invention has been seen to be a circuit of 
predominently analog characteristics. A simple logical control of two 
flip-flops controls the time and duration of actuation of analog circuits 
in order to produce, at the time of the firing of a first print hammer 
during each print line, a 10% current boost. It should immediately be 
recognized that much greater sophistication could be applied to the 
digital initialization and/or durational control of the analog elements of 
the present circuit than is evidenced by that rudimentary digital control 
shown in FIG. 7b. For example, the concept of the invention could be 
further extended to selective reenablement of the current boost if the 
hammers within a print line were not fired continuously, or suitably 
proximate in time. In such an event of early hammer(s) firing(s), followed 
by a period of idleness prior to the later firing of an additional 
hammer(s), then the paper form could relax, re-instituting the 
"first-hammer out" syndrome, between the earlier and later hammer(s). 
Further, even those print hammers which are fired continuously, but which 
are fired at some distance apart upon a very large dimension printed form, 
could still, in distant parts of such printed form, exhibit the 
undesirable "first-hammer out" syndrome. The obvious way to overcome both 
problems is by greater sophistication of control, selectively reenabling 
the boost if the time gap between sequentially fired hammers is too long, 
and/or the physical separation of print hammers is so great that print 
hammers fired at one location upon a paper form cannot be considered to 
reliably compress the entire paper form. The manner of obtaining such 
greater sophistication in the control of the print hammer current boost is 
by using microprocessors to monitor signals indicating print hammer firing 
across the entire time of printing, and for indicating the hammer firings 
several physical sections across a hammer bank. It could be envisioned 
that the ultimate adaptive printing scheme would selectively control the 
current to be applied to each single hammer in consideration of the 
priorly, and concurrently, occurring activity of all neighboring hammers 
to such hammer. Such extreme sophistication is, however, not generally 
required to obtain satisfactory results, the apparatus of the present 
invention in its preferred embodiment form alleviating to a substantial 
degree the entire historically occurring "first hammer out syndrome" 
problem. 
In consideration of the preceding teaching, it will be understood that the 
present invention should be interpreted in consideration of the language 
of the following claims, only, and not solely in consideration of the 
preferred embodiment within which the present invention has been taught.