Note: Descriptions are shown in the official language in which they were submitted.
Description
MATRIX PRINTER WITH OPTIMUM PRINTIN(-, VE~I.OCTT'J
ound of Invention
. _ _ _ _
The present invention relates to prlnters and
particularly to serlal printers which have the ca
pability of operating at variable velocities.
Background Art
With the expansion of word process,ing systems
throughout the office systems industry, there has been
an increasing demand for printers which provide letter
quality documents at very high throughput rates, e.g.,
in the order of 100 to A00 net characters per second.
Because oE the high throughput requirement, matrix
printers have been viewed with increasing favor over
the more traditional printwheel or ball types of
printers. In the matrix printer, as the printhead is
indexed from print positiorl to print position, charac-
ters are formed by the selective energization of
elements in a matrix or line in a printhead to provi~e
the ~elected visual pattern forming the character.
The fo~mation of characters on the matrix type of
printhead is customarily faster than the positioning
of the selected characters on the printwhee] or ball
types of printheads because in the matrix printhead,
the selection process is entirely electronic while on
the printwheel or ball type of printer, the selection
is a slower mecnanical one. An additional advantage
of the matrix type of printer is that the selection
time at each print position is uniform while in the
printwheel or ball type of printer, the selection time
will vary based upon the time required to traverse the
distance from the last selected character to the next
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one which of course will be variable based upon the
character positions. As a result, in matrix type
printer~s, the whole print line may be printed at a
constant carrier escapement velocity while in the ball
or printwheel printers, the escapement velocities of
the carrier have to be varied throughout each print
line in order to maximize throughput. Otherwise, the
escapement velocity would be limited to that of the
slowest possible character celection increment.
In view of this background, it is customary to
operate matrix printers at much higher carrier veloci-
ties than ball or printwheel printers. However, since
each line of print commences and ends with a stop
position, the time reauired for the acceleration of
1~ matrix printer escapement from the initial stop
position to selected printing velocity and the corre-
sponding deceleration Crom the printing velocity to
the terminal stop at the end of the print line may
involve a signiEicant portion of the time involved in
printing an entire line.
As will be hereinafter described in greater
detail, we have found that this acceleration and
deceleration and consequently the time for printing an
entire line may be substantially reduced hy selecting
carrier escapement velocities which are dependent upon
the length of the line to be printed. For example, if
the line to be printed is a relatively short one, it
may very well be inefficient to spend the acceleration
and deceleration time required to print at a maximum
velocity. Since the characters to be printed in the
line are few, it may be advantageous to "trade-off"
maximum carrier velocity for relatively short acceler-
ation and deceleration times.
U.S. Patent 3,761,880, Kritz et al appears to
represent the closest prior art. It discloses a
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variable speecl matrix type o~ printer. Control means
in the Kritz et al printer appear to control the
printinq velocity in order to operate the printer as
close as possible to synchronization with the input
being applied to the printer from a data proce~ssina
system. Kritz et al do not wish to operate their
printer at such a high speed that a printer prints the
- data faster than it is received since this woul~ ause
unnecessary stoppage and slow down the printer equip-
ment and consequently the overall operation. In order
to achleve this synchronization between their printer
and the data processor input, ~ritz et al utilize an
input buffer in which the data being input from the
data processing system is loaded. Then, dependent on
the amount of the data backlog in the buEfer, the
printer is operatecl at selected velocities. Jf the
buffer is very heavily loaded, the printer opera-tes at
a fas-ter pace. As the buffer becomes unloadecl, the
printing operation slows down. In this manner, the
printer need never run out of information and be
required to stop. However, in Kritz et al there is no
suggestion of making the printing velocity for each
line dependent upon the length of said line.
~isclosure of the Present Invention
The present invention provides a matrix printer
having a printhead suitably mounted on a carrier
movable across a record medium such as paper document
to be printed upon. The printer is characterized by
having means for determining the length of the next
line of alphanumeric characters to be printed by said
prin-thead on said medium in combination with drive
means which are responsive to the determined length
for moving the printhead across the medium at a
selected velocity based upon this determined length.
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In addition, means are providecl for activating :the
printhead during this movement of the printhead to
print the par-ticular line of characters. The present
invention is most advantageously utilized in matrix
printheads and particularly where the printer is a
wire matrix printer. In the case of matrix printers,
the selected velocity is constant during the actual
printing of the line.
In accordance with a more specific embodiment of
the present invention, means for determining the line
length of the next line to be printed includes storage
means for receiving and storing input data representa-
tive of the alphanumeric characters forming the next
printed line in combination with mean~ for scanning
the stored data and determining the length of the
printed line of the next line to be printed from this
data.
Brief Desc ~ of the Drawin~s
. .
Referring now to the drawings, wherein a pre-
ferred embodiment of the invention is illustrated, andwherein like reference numerals are used thro~lghout to
designate like parts;
Fig. 1 is a diagrammatic representation of the
logic in the printer control system which may be used
to carry out the velocity selection expedient of the
present invention.
Fig. 2 is a flow chart of the process carried out
in the control system in making a velocity selection.
Fig. 3 is a timing graph illustrating the change
in velocity with time during the printing of a typical
line.
Fig. 4 is a graph showing the change in N, net
printing throughput in characters per second for a
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7~L5
print line for a given print line length and acceler-
ati.on/deceleration with different printing velocities.
- Best Mode for Car~yin~ Out The Invention
Now with reference to Figs. 1 and 2, the velocity
selection system of the present invention will be
described in detail. First r with reference to Fig. 1,
velocity selection logic unit 10 is part of the
control system of a printer which is under the control
of a processor 11 which may be any conventional
microprocessor used for this purpose such as the Intel
8085. Logic unit 10 controls the escapement velocity
of a printer 12 which comprises a stepper motor 13
rotating a lead screw escapement 14 to move carrier 1
supporting printhead 16 along a line of the docurnent
: 15 17 being printed. Printhead 16 may be any convention-
al matrix printhead of the type described in U.S.
: Paten-t 3,764,99~ for purposes of the present example.
The data to be printed is loaded one line at a
time into print line buffer 18 from input line 19
coming from any standard data processor or text
processing host CPU to which the printer is attached.
Under the guidance of processor 11, read control uni~
20 serially reads out the characters to be printed in
a given print line from buffer 18. Read control unit
20 also can read the end of print line position in
buffer 18 of the par-ticular line of print currently
being stored in the bufer. Velocity selection unit
21 also under the control processor 11 will make the
appropriate velocity selection from velocity table 22
based upon the line length as will be described in
greater detail hereinafter with respect to the Elow
chart of Fig. 2. Carrier escapement control unit 23
will control the carrier escapement velocity through
stepper motor 13 based upon the velocity selection,
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and printhead control unit 24 will selectively acti-
vate particular elements of the matrix in head 16 to
produce the character provided from read control unit
20 at a position based upon that of velocity selection
unit 21 whereby the printed characters 25 are corre~t
in format and printed at the correct position along
the line being printed in document 17.
Now, using the logic and apparatus described
above with respect to Fiq. 1~ the operations involved
in the present invention will be described with
respect to the flow chart in Fig. 2. First, decision
block 26, a determination is made as to whether there
is a new line in print line buffer 18. This deter-
minat:ion is made in the velocity selection unit 21
based upon data read from line buffer 18 by read
control unit. If there is a new line in the huffer,
then, block 27, the last character position in that
new line is accessed. This is accomplished through
read control unit 20. Then, block 28, the line length
is calculated or determined in velocity selection unit
21. The line length is determined by subtracting the
last character position from the current character
position. This current character position has been
stored in velocity se7ection unit 21 which keeps track
of the carrier position through feedback via line 29
~rom the conventional position sensing device associ-
ated with stepper motor 13.
Next, the print mode is accessed, block 30. The
print mode or pitch of the characters is stored in
]ine buffer 18 and accessed through read control unit
20 to velocity selection unit 21. Then, block 31, the
print velocity is selected through velocity lookup
table 22 by velocity selection unit 21. Selection in
-table 22 is based upon two parameters, i.e., line
length and print mode. As will be hereinafter
describecl in qreater detail, the velocities listed in
table 22 are selected so as to provide the maximum
throughput considering the len~th of the line and the
pitch of the characters. The selected velocity will
be a fi~ed one, i.e., once actual printing begins
after acceleration to the fixed printing velocity and
before deceleration to the stopped position at the end
of the line.
Next, block 32, the print distance is determined,
i.e., distance over which the actual printing is done.
This determination is carried out in the velocity
selection unit and is based upon print mode, i.e.
character pitch, and the difference between the
present and last character position. Next, the
selected velocity and the print distance is trans-
ferred from the velocity selection unit to the carrier
escapement control, as set forth in block 34. Then,
under the combined control of carrier escapement
control unit 23 and printhead control unit 24, both
under the control of processor ll, stepper motor 13 is
appropriately xotated moving escapement lead screw 14
and thus carrier 15 over the distance to be printed
while printhead 16 produces the selected characters in
the conventional manner. Upon the completion of the
printing, the process is returned to decision block
26.
Upon the return to decision block 26, the proce-
dure is repeated. Let us assume that on a given
iteration throu~h decision block 26, a determination
is made that there is no ne~ line in the buffer.
Then~ decision block 35, a determination is made as to
whether or not the carxier is at the last character
position. If the carrier is at the last character
position, then, the carrier is stopped, block 36, and
the current carrier position, i.e., the stopped
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location is stored, as set forth in block 37. This
inEormation is stored in the velocity selection unit
21 to be used or the next determination of line
length. On the other hand, if a determination is made
in decision block 35 that the carrier is not at the
last position, then, the system is returned directly
to decision block 26 for determination as to whether a
new line is now in the buffer.
We have hereinabove discussed in general the
advantages of selecting the velocity at which the
ch~racters are to be printed based upon the length of
the lines to be printed. Now with respect to Figs. 3
and 4, we will explain the theory involved in our
approach as well as how some specific calculations of
optimum velocity may be made. With reference -to Fig.
3,
Vp = Print velocity
Ta = Acceleration time to print velocity = Vp/a
a = Acceleration capability of carrier drive
mechanism
Tv = Time required to print the line of text at
at the selected velocity =
Line Le~th = L
Vp Vp
Td = Deceleration time required to stop carrier =
Vp/a
If the acceleration rate is assumed equal ~o the
deceleration rate, the equation for the total print
time is as follows:
T = ~ JL (1)
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If throughput is defined as the number of charac-
ters printed per unit of time, the equation relatinq
throughpu-t, line length, and velocity is as follows:
Throughput (net characters per second) =
N = LP (2)
Where
L = Length of line
P = Density of characters on line
T = Time to print the line from
Equation (1)
Inserting (1) into (2) yields:
N = ~ (3)
The rate of change of the throughput i.s ohtained by
taking the first derivative of (3) with respect to the
15 print velocity parameter, Vp:
dN _ aLp(aL - 2V 2 )
dVp (2Vp2 + aII) 2
Fig. 4 illustrates a graphical representation of the N
vs Vp curve for a given line length and acceleration
value.
The point 40 on the N vs Vp curve where the
derivative goes to zero dictates the choice of earlier
velocity which yield the maximum throughput. This
value can be obtained by setting the derivative (4) to
zero and solving for VpO
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dV - - aI. - 2V 2
P P
Vp = (aL/2)~
The above calculations are made for a series o~
line va~ues in advance and stored in velocity table 22
of Fig. 1. It should be noted that for a particular
line length the optimum velocity, i.e., poin-t 40 on
the curve of Fig. 4 designated Vpl may be a velocity
which is not one of the velocities available on the
printer. For purposes of the illustration in Fig. 4,
velocities available on the printer are designated as
Vl, V2 V3, and V4. In such a case, the velocity
designated in the table 22 for the line length would
be the velocity closest to the optimum velocity. In
this case, it would be velocity V~. Thus, ~or best
results the selected velocity is such that
Tad = Tv~ wherein
T d is the total time which it -takes the printhead
to accelerate to the selected velocity and
to decelerate from said velocity, and
Tv is the total printing time at said constant
velocity.
However, when the optimum constant velocity is above
the maximum constant velocity at which the printer can
mechanically print, then the final selected velocity
should be such that
,
T c T , where
ad v
Tad ls the total time which it takes the printhead
to accelerate to the selected velocity and
to decelerate from said velocity, and
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Tv is the total printing time at said conStant
velocity~
While the invention has been particularly shown
and described with reference to a preferred embodiment
it will be understood by those skilled in the art that
various other changes in form and detail may be made
without departing from the spirit and scope of the
invention.
