U.S. patent number 3,575,107 [Application Number 04/829,574] was granted by the patent office on 1971-04-13 for underspeed and undervoltage protection for printer.
This patent grant is currently assigned to General Electric Company. Invention is credited to Clifford M. Jones, Earle B. McDowell.
United States Patent |
3,575,107 |
McDowell , et al. |
April 13, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
UNDERSPEED AND UNDERVOLTAGE PROTECTION FOR PRINTER
Abstract
An underspeed and undervoltage protection system for an
electromechanical printer. The printer includes a plurality of
moving characters actuated by stationary print hammers. The speed
of the moving characters is continuously monitored along with the
input voltage to the printer system. A synchronizing signal is
provided which indicates when the characters are positioned to
permit application of buss voltage which is the source of
energization of the print hammers. When the speed of the characters
and the input voltage reach predetermined minimum values, power is
applied to a power buss for energizing the print hammers, the
application of power being delayed until the synchronizing signal
indicates that the print hammers are not in position to be
energized. Similarly, if the speed of the characters or the input
voltage subsequently falls below the predetermined minimum values,
the hammer power buss is synchronously deenergized so as to prevent
damage to the printer mechanism.
Inventors: |
McDowell; Earle B. (Waynesboro,
VA), Jones; Clifford M. (Waynesboro, VA) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
25254899 |
Appl.
No.: |
04/829,574 |
Filed: |
June 2, 1969 |
Current U.S.
Class: |
101/93.14;
361/92; 307/130; 307/149; 101/111 |
Current CPC
Class: |
G06K
15/08 (20130101) |
Current International
Class: |
G06K
15/02 (20060101); G06K 15/08 (20060101); B41j
009/38 (); H02h 003/28 () |
Field of
Search: |
;101/93,93 (C)/
;101/96,95,111,1,426 ;307/120,130,149,9 ;317/31,9,36
;340/267,(Inquired),210 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Penn; William B.
Claims
We claim:
1. A protection system for a printer having a plurality of movable
characters and a plurality of electrically energizable hammers for
engaging the characters comprising:
a. indication means for indicating the speed of said
characters;
b. a speed reference generator for generating a speed reference
signal indicative of the minimum permissible speed of said
characters;
c. speed detecting means operatively connected to said indication
means and said speed reference generator for generating a signal
when the speed of said characters is less than said speed
reference;
d. undervoltage detecting means for indicating when the input
voltage to said printer is below a predetermined value;
e. synchronizing means for generating an output signal indicating
when the hammers are permitted to be energized; and
f. deenergization means operatively connected to said speed
detecting means, said undervoltage detecting means and said
synchronizing means so as to synchronously deenergize the hammers
at a time determined by the absence of an output from said
synchronizing means in response to an output from said speed
detecting means or said undervoltage means.
2. The protection system recited in claim 1 further comprising a
hammer power buss for selectively energizing the hammers and
wherein said deenergization means operates to deenergize said
hammer power buss.
3. The protection system recited in claim 1 wherein said speed
detecting means comprises a bistable device operatively connected
to said indication means to assume a first state upon receipt of an
input from said indication means and wherein said speed reference
generator comprises time delay means operatively connected to said
indication means to begin a timing cycle in response to receipt of
said input from said indication means, the output of said time
delay means being operatively connected to said bistable device so
as to change the state of said bistable device at the end of said
timing cycle.
4. The protection system recited in claim 1 wherein said
undervoltage detecting means comprises a Zener diode operatively
connected to said input voltage.
5. The protection system recited in claim 1 further comprising a
second time delay circuit connected between said input voltage and
said undervoltage detecting means so as to desensitize said
undervoltage detecting means from the effects of temporary
variations in said input voltage.
6. The protection system recited in claim 1 wherein said
synchronizing means comprises means for indicating the position of
the movable characters.
7. A protection system for a printer having a plurality of
characters on a moving belt and a plurality of electrically
energizable hammers for mechanically engaging the characters so as
to print the characters comprising:
a. synchronizing means for generating a plurality of time-spaced
pulses indicative of the position of the moving characters;
b. speed detecting means operatively connected to said
synchronizing means for indicating when the time between said
pulses is greater than a predetermined fixed time so as to generate
an output signal indicating an underspeed condition;
c. undervoltage detecting means for indicating when the input
voltage to said printer is below a predetermined value;
d. a power buss for selectively energizing said hammers; and
e. hammer buss control means operatively connected to said
synchronizing means, said speed detecting means and said
undervoltage detecting means for energizing said power buss
synchronous with said synchronizing means in the absence of outputs
from said speed detecting means and said undervoltage means, said
hammer buss control means being further operative to deenergize
said power buss synchronous with said synchronizing means in
response to an output from said speed detecting means or said
undervoltage detecting means.
8. The belt printer protection system recited in claim 7 wherein
said synchronizing means comprises a photocell detector for sensing
the position of the characters on the belt.
9. The system recited in claim 7 wherein said detecting means
comprises a time delay means, a bistable element, said time delay
responsive to receipt of normal voltage and normal speed conditions
to produce a given time delayed signal, said element responsive to
the simultaneous occurrence of a pulse from said synchronizing
means and said time delay signal to set, means for removing said
time delayed signal in response to an underspeed or undervoltage
condition, said element responsive to removal of said time delayed
signal and the absence of said synchronizing pulse to reset.
10. The belt printer control system recited in claim 7 wherein said
undervoltage detecting means includes a Zener diode having a
breakdown voltage corresponding to a predetermined minimum voltage
and time delay means operatively connected between said input
voltage and said Zener diode.
11. A method of protecting a printer having a plurality of movable
characters and a plurality of electrically energizable hammers for
engaging the characters comprising:
a. indicating the speed of the moving characters;
b. comparing the indicated speed of the characters with a
predetermined lower speed limit;
c. comparing the input voltage to the printer with a predetermined
reference;
d. deenergizing the print hammers at a time when the print hammers
are not permitted to be energized if either the speed or voltage is
below the predetermined limits.
12. The method of protecting a printer recited in claim 11 wherein
the step of comparing the speed of the characters comprises
comparing the time spacing of pulses indicating the speed of the
characters with a fixed time delay.
13. The method of protecting a printer recited in claim 11 wherein
the step of deenergizing the print hammers comprising generating a
signal indicating when the print hammers are permitted to be
energized and then delaying the deenergization of the print hammers
until the first time the generated signal is absent following an
indication that the speed or voltage is below the predetermined
limits.
14. A protection system for a printer having a plurality of movable
characters and a plurality of recurrently, selectively electrically
energizable hammers for engaging the characters comprising:
a. means for providing a first signal only when the speed of said
characters is different from a desired speed;
b. means normally providing a second signal only when selected
hammers are to be energized; and
c. means responsive to said first signal and the absence of a
second signal to prevent a subsequent recurrent energization of
hammers.
15. A protection system for a printer having a plurality of movable
characters and a plurality of recurrently, selectively,
electrically energizable hammers for engaging the characters
comprising:
a. means for providing a first signal only when the speed of said
characters is different from a desired speed;
b. means for providing a second signal only when the input voltage
to said printer is different from a desired value;
c. means normally providing a third signal only when selected
hammers are to be energized; and
d. means responsive to at least one of said first and second
signals and the absence of a third signal to prevent a subsequent
recurrent energization of hammers.
16. A protection system for a printer having a plurality of
characters on a moving belt and a plurality of recurrently,
selectively electrically energizable hammers for mechanically
engaging the characters so as to print the characters
comprising:
a. synchronizing means for generating a plurality of time-spaced
pulses whose time occurrence is indicative of the position of the
moving characters;
b. means for providing a first signal only when the time between
said pulses is different from a desired time to indicate an
undesirable speed condition;
c. means normally providing a second signal only when selected
hammers are to be energized;
d. means responsive to said first signal and the absence of a
second signal to prevent a subsequent recurrent energization of
hammers.
17. The belt printer protection system recited in claim 16 wherein
the source of said first signal comprises a bistable device
operatively connected to said synchronizing means to assume a first
state in response to a pulse from said synchronizing means, time
delay means operatively connected to said synchronizing means to
begin a timing cycle in response to a pulse from said synchronizing
means, the output of said delay means being operatively connected
to said bistable device so as to change the state of said bistable
device at the end of said timing cycle.
18. A protection system for a printer having a plurality of movable
characters and a plurality of recurrently, selectively,
electrically energizable printing elements for cooperating with the
characters to effect printing:
a. means for providing a first signal only when the speed of said
characters is different from a desired speed;
b. means normally providing a second signal only when selected
printing elements are to be energized; and
c. means responsive to said first signal and the absence of a
second signal to prevent a subsequent energization of printing
elements.
Description
BACKGROUND OF THE INVENTION
The present invention relates to protection systems for printers.
More specifically, the invention relates to an underspeed and/or an
undervoltage protection system for a printer having a plurality of
movable characters which are caused to effect printing, as for
example, by electrically energizable print hammers.
A broad variety of protection systems for all types of electrical
and mechanical equipment are known. Among the many examples are
systems which provide for protection by sensing underspeed,
overspeed, undervoltage, overvoltage, etc. The particular needs of
individual types of protective schemes depends, in large part, upon
the particular characteristics of the apparatus to be
protected.
The present invention is particularly slanted toward, and suited
for, protection of electromechanical printers by sensing underspeed
and undervoltage conditions. The need for such protection is
particularly acute in printers which have a number of moving
characters which are selectively engaged by stationary print
hammers. Printers of this type include, for example, chain printers
and belt printers wherein the print characters are carried on
continuously moving chains or belts. The moving print characters
are engaged by stationary print hammers which are electrically
energized by a control system so as to strike the appropriate
character.
The control system controls the energization of the print hammers
by sensing the position of the characters in the chain or belt.
However, since the characters are moving, it is necessary to
energize the print hammers before the characters are directly in
front of the print hammer so as to "lead" the moving character. The
amount of "lead" required depends, of course, upon the speed of the
moving character and the amount of time required for the print
hammer to travel to the printing position. It will be apparent that
too much "lead" or too little "lead" will result in the print
hammer missing the character and may further result in jamming of,
and damage to, the printer mechanism.
For these reasons, the speed of the moving characters and the
amount of voltage applied to energize the print hammers become
extremely important factors in such a control system. If either of
these characteristics fall below predetermined limits, there is a
significant likelihood that the hammers or the print characters
will be damaged.
Not only must a control system for a printer of this type ensure
that the character speeds and hammer voltage are proper but it must
also prevent premature energization or deenergization of the print
hammers. Thus, in certain situations it may be necessary to delay
the energization or deenergization of the power buss which feeds
the print hammers until such energization or deenergization can be
done without damage to the printer mechanism.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved protective
circuit for a printer.
It is an object of the present invention to provide a novel
protection system for preventing damage to an electromechanical
printer.
It is a further object to provide such a protection system which
protects the printer from damage in case of underspeed or
undervoltage conditions.
It is a still further object of the present invention to provide
such underspeed and undervoltage protection which is synchronized
so as to prevent improper energization or deenergization of the
printer.
Briefly stated, these and other objects are carried out by
comparing the speed of the moving print characters and the input
voltage to the printer with predetermined minimum reference values.
A synchronizing signal is provided so as to indicate when the print
characters are positioned so as to be permitted to be engaged by
electrically energizable print hammers. The protection system is
synchronized with this synchronizing signal so as to assure that
the printer is energized or deenergized only when the synchronizing
signal indicates that the print characters are not in position to
be engaged by the print hammers.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly pointing
out and distinctly claiming the subject matter which is regarded as
the invention, an illustration of a particular embodiment can be
seen by referring to the specification in connection with the
accompanying drawings in which:
FIG. 1 is a perspective view of an electromechanical printer and a
block diagram of a control system for the printer,
FIG. 2 is a detailed electrical diagram of a protection system
comprising a preferred embodiment of the present invention;
FIG. 3 is a series of waveforms illustrating the operation of the
underspeed detector of FIGS. 1 and 2; and
FIG. 4 is a series of waveforms illustrating the operation of the
undervoltage detector of FIGS. 1 and 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a perspective view of a printer and a block diagram of a
control system for the printer. The present invention is
particularly suited for use in a printer which has a plurality of
characters which are continuously movable as shown in perspective
view of the printer of FIG. 1. A printer of the type referred to
may comprise, for example, a belt printer as illustrated in FIG.
1.
The print characters 10 are carried on a continuously movable belt
12 which is driven at a predetermined speed by a drive motor shown
generally at 14. The drive motor 14 is coupled by way of a shaft 16
to a power pulley 18 which drives the belt in a counterclockwise
direction as indicated by the arrows 20. The belt 12 bearing the
print characters 10 is continuously driven by the power pulley 18
and a second pulley 22. A recording medium such as a piece of paper
24 is provided and supported against a platen or roller 26. A
plurality of hammers 28 are positioned so as to selectively strike
the print characters 10 and urge them against an ink ribbon 30
which passes between the print characters and the recording medium
24.
The position of the characters on the rotating belt 12 is sensed by
a pair of photocells 34, 36 which are activated by a light source
38. The print characters 10 are supported by flexible arms 37 which
have fingers 39 protruding below the belt 12. The fingers 39 pass
between the light source 38 and the photocells 34, 36 so as to
indicate the position and speed of the characters on the belt
12.
The hammers 28, the photocells 34, 36, and the fingers 39 are
positioned so that the output of the photocells 34, 36 indicate
when the hammers 28 can be actuated. Note that the printer of FIG.
1 has one hammer for each column to be printed, but that the print
characters 10 are separated by the width of a column. The hammers
28 are separated into two groups, even and odd, according to the
particular column with which they are associated. Since the
photocells 34, 36 are similarly spaced, the output of photocell 34
is used to indicate when the "even" hammers can be actuated whereas
the output of photocell 36 is used to indicate when the "odd"
hammers can be actuated.
The printer of FIG. 1 is controlled by a control system shown
generally at 40. The control system 40 includes a first section 42
which is the source of the input data to be printed by the printer.
Input data source 42 may comprise, for example, a keyboard for
directly activating the printer or, alternatively, may comprise
part of a data receiver which receives data from a remote location
to be printed at the printer location. The data from the input data
source 42 is fed to a data storage unit 44 which stores the input
data until it is printed. The contents of data storage 44 are
examined by a data decoding and selection circuit 46 which has the
output of the photocells 34, 36 as its inputs. The function of the
data decoding and selection portion of the control system 40 is to
determine the position of the characters (on the belt) so as to
determine when to actuate the hammers to print the characters
stored. The output of the data decoding and selection circuit 46 is
fed to a hammer actuation circuit 48 which applies power to the
appropriate solenoid 32 so as to activate the hammer and print the
desired character. A system for controlling a printer of this type
may be found, for example, in copending application, Ser. No.
734,501; filed Jun. 4, 1968 (Docket 45-SL-01033), assigned to the
assignee of the present invention. In this application the circuit
46 applies a preconditioning signal to the hammer drive mechanism
to indicate which hammers are to be actuated to print during the
next print cycle. One or more hammers may be actuated at a
time.
Since the print hammers 28 are stationary and the characters 10 are
moving, one of the primary purposes of the control system 40 is to
synchronize the recurrent electrical energization of the print
hammers with the rotation of the print characters on the belt 12.
It is not only necessary to select the appropriate print hammer but
also is important to assure that the print hammers are energized at
the appropriate time. Improper energization of the print hammers 28
can result in damage to both the hammers themselves and also to the
print characters 10. That is, if one of the print hammers 28 is
energized improperly, it may miss the character completely and
instead fall into one of the spaces between the characters 10.
Since the characters are moving, the next character may catch the
side of the print hammer and the result may be a bent character or
print hammer.
In order to ensure that the print hammers are properly energized,
the system must, as pointed out in the above cited copending
application, be appropriately synchronized with the rotation of the
character belt 12. In addition, even if the hammers are
appropriately synchronized, damage may still result if the voltage
applied to the printer and/or the hammer solenoids is too low or if
the speed of the belt 12 is below a predetermined limit.
As to undervoltage conditions, it will be apparent that the effect
of applying a decreased voltage to the hammer solenoids is to
reduce the amount of actuating force applied to the hammers 28 when
the solenoids 32 are activated. Thus, if the hammer is energized by
a voltage below predetermined limits, it will not travel as fast as
is necessary in order to strike the desired character but instead
may miss the print character altogether, resulting in damage to the
printer mechanism. In addition, it is also necessary to assure that
when power is first applied to the system the hammers are not
prematurely energized.
If the belt speed is below normal, it can be appreciated that the
energization of one of the hammers may also result in damage to the
printer mechanism. Since the hammers are stationary and the
characters are moving, the hammers must be energized before the
desired character is directly in front of the hammer. That is, the
hammer must be energized so as to "lead" the desired character.
Thus, if the characters are traveling at a rate below a
predetermined limit, the hammer will arrive at the character
position before the print character itself is there resulting in
damage to either the print characters or the hammers
themselves.
In order to avoid these sources of difficulty, an undervoltage
detector 50, and underspeed detector 52 are provided to monitor the
system voltage and belt speed. The undervoltage detector 50 and the
underspeed detector 52 control the application of power to the
hammer buss by way of a hammer buss control circuit 54. The hammer
buss control circuit 54 operates to ensure that power is not
applied to the hammer buss prematurely and that it is removed from
the hammer buss in a synchronous fashion when either the speed or
voltage falls below predetermined limits. As illustrated in FIG. 1,
fast switching means, such as a solid-state switch 56 is provided
to control the application of power to the hammer buss. Activation
of switch 56 is controlled by the presence of a signal on hammer
buss control 54.
More specifically, the undervoltage detector 50 monitors the input
voltage supplied to an input transformer 62 including a secondary
winding 64 which feeds through a full wave rectifier including
diodes 66, 68 and 70 whose output is supplied to the control system
for the main source of control power. A filter capacitor 72 is also
provided so as to reduce variations in the DC voltage supplied to
the control system 40.
When an undervoltage condition is detected, the output of
undervoltage detector 50 is relayed to the hammer buss control 54
to deenergize solid-state switch 56 in synchronism with the
character belt fingers. At the same time, a second output from
undervoltage detector 50 is also relayed to the data storage
portion 44 of the control system 40 so as to clear data stored in
the control system at that time.
The underspeed detector 52 monitors the output of one photocell 36
(which represents the speed of the character belt) and indicates
when the belt speed is below a predetermined minimum. The output of
the underspeed detector 52 is also fed to the hammer buss control
54 and operates to deenergize the hammer buss when the speed falls
below predetermined limits, again in synchronism with the position
of the character belt fingers.
THE LOGIC ELEMENTS
FIG. 2 is a detailed logic diagram of a preferred embodiment of the
undervoltage detector 50, the underspeed detector 52, and the
hammer buss control 54 of FIG. 1. Before describing in detail the
operation of the logic diagram of FIG. 2, it will be necessary to
briefly describe the operation of certain logic elements contained
therein.
The present embodiment will be explained with respect to a digital
logic system wherein the signals are encoded in two voltage levels.
These voltage levels will be referred to, for the sake of
convenience, as logic 1 and logic 0. Logic 1 may be, for example, a
positive voltage such as +12 volts and the logic 0 level may be
lower voltage such as 0 volts. While the preferred embodiments
shown will be described using this notation, the present invention
is not to be limited to the particular logic levels described since
it will be apparent to those skilled in the art that any type of
logic system, either positive or negative, could carry out the
principles underlining this invention with equal facility.
The logic element 80 labeled FF is a bistable device commonly
referred to and herein defined as a J-K flip-flop. It has three
input terminals SS, RS, and T. The SS input terminal is the set
steering input terminal, the RS input terminal is the reset (or
clear) steering terminal, and the T input terminal is the trigger
terminal. Operation, briefly, is as follows--the presence of a
logic 1 at the set steering terminal SS followed by a trigger pulse
(a signal which goes from logic 1 to logic 0) on the trigger
terminal T sets the flip-flop 80. Conversely, the presence of a
logic 1 on the reset steering terminal RS followed by a trigger at
the trigger terminal T results in resetting (or clearing) the
flip-flop 80.
In addition to the three inputs referred to above, the J-K
flip-flop has direct set and reset input terminals labeled S and R
respectively. The presence of a logic 1 at the direct set terminal
S immediately sets the flip-flop 80. Conversely, the presence of a
logic 1 at the direct reset input terminal R immediately resets the
flip-flop 80.
To indicate its present state, a J-K flip-flop has two output
terminals labeled 0 and 1. These terminals are labeled to indicate
the logic signal present at the terminal when the flip-flop is in
its normal or reset state. That is, the 0 terminal has a logic 0
present at the terminal and the 1 output terminal has a logic 1
present at that terminal when the flip-flop is in the reset state.
When the flip-flop 80 assumes the set state, the logic signals
present at the two output terminals are reversed so that a logic 1
is present at the 0 output terminal and vice versa when the
flip-flop 80 is in the set state.
The logic element 82 in FIG. 2 is a time delay unit. It has a
predetermined time delay which begins when its input (indicated by
the arrow) goes to logic 0. At the end of the predetermined time
delay, the output momentarily switches from logic 0 to logic 1. If
the input, however, goes to logic 1 before the predetermined time
delay passes, the time delay cycle is terminated and restarted when
the input returns to the logic 0 level.
The time delay unit 82 includes an NPN transistor 51 having its
base connected to the input. The collector of transistor 51 is
connected to the junction of a first resistor 53 and a capacitor
55. When the base of transistor 51 is at logic 1 level, transistor
51 is turned full on so as to short out capacitor 55. As long as
transistor 51 is turned on by the presence of logic 1 at the input,
capacitor 55 cannot charge through resistor 53. However, when the
input to time delay 82 goes to logic 0, transistor 51 is turned off
so that capacitor 55 charges through resistor 53 at a rate
determined by the RC time constant of these elements.
The junction of resistor 53 and capacitor 55 is also connected to
the emitter of a unijunction transistor 57. A pair of resistors 59,
61 are connected to the bases of unijunction transistor 57 so as to
provide temperature compensation and means for obtaining an output
voltage.
The RC network consisting of resistor 53 and capacitor 55
cooperates with unijunction transistor 57 and its associated
resistors 59, 61 to form a conventional unijunction relaxation
oscillator. Capacitor 55 charges at a predetermined rate until the
voltage on the emitter of the unijunction transistor 57 exceeds a
predetermined percentage of the base-to-base voltage at which point
unijunction transistor 57 conducts momentarily thereby discharging
capacitor 55. Prior to the time the unijunction transistor 57
conducts, the output of the time delay 82 is at effectively zero
volts or logic 0. However, when unijunction transistor 57 conducts,
the output raises to the positive voltage representing logic 1.
However, since the transistor 51 is connected in parallel with
capacitor 55, capacitor 55 is discharged when transistor 51
conducts. Therefore, the time delay 82 will never change its output
to logic 1 if transistor 51 is energized before the voltage on
capacitor 55 exceeds the predetermined percentage of the
base-to-base voltage of unijunction transistor 57.
The logic elements 104, 114 are time delay units which are somewhat
different than time delay 82. The operation of time delay 114 will
be explained in detail with the understanding that time delay 104
is substantially similar in construction and operation. Briefly,
time delay 114 operates such that a logic 1 at its input is delayed
and, after a predetermined time delay, the output goes to logic 1.
On the other hand, if the input to time delay 114 is a logic 0, the
output assumes a logic 0 with substantially smaller delay.
The input to time delay 114 is fed to the base of an NPN transistor
63. A first resistor 65 is connected from a positive voltage, +V,
to the collector of transistor 63 and the emitter of transistor 63
is connected to 0 volts. An RC network consisting of resistor 67
and capacitor 69 is connected from the positive voltage +V to 0
volts with the junction of resistor 67 and capacitor 69 which is
connected to an inverter 73 serving as the output of time delay
114. Finally, a diode 71 is connected from the emitter of
transistor 63 to the junction of resistor 67 and capacitor 69.
When the input to time delay 114 is a logic 1, the timing cycle
begins. The presence of the logic 1 at the base of transistor 63
turns transistor 63 on. When transistor 63 is turned on, its
collector is at 0 volts so that diode 71 cannot conduct. Under
these circumstances, the RC network consisting of resistor 67 and
capacitor 69 begins to initiate the timing cycle since capacitor 69
begins to charge through resistor 67. After the time delay
established by the RC time constant of these components, the output
of time delay 114 will eventually reach the logic 1 level.
On the other hand, when the input to time delay 114 is a logic 0,
the presence of logic 0 at the base of transistor 63 turns
transistor 63 off. When transistor 63 is turned off, the collector
rises to essentially the positive voltage, +V. This causes diode 71
to become conductive so as to almost immediately discharge
capacitor 69. Thus, the presence of a logic 0 at the input to time
delay 114 causes its output to switch, after a very short time
delay, to the logic 0 level.
The logic element 108 in FIG. 2 is an AND gate. AND gate 108
operates such that its output will be a logic 1 wherever both of
its inputs (indicated by the arrows) are logic 1. Under all other
conditions the output of AND gate 108 will be a logic 0. The AND
gate 108 in FIG. 2 includes a pair of diodes 81, 83 and a resistor
85 which is connected to a positive voltage representative of the
logic 1 level. If either of the inputs are logic 0 (0 volts) the
diode associated with that input conducts thereby pulling the
output of the AND gate down to logic 0. If, however, both inputs
are logic 1 then neither of the diodes 81, 83 will conduct so that
the output of AND gate 108 assumes the positive voltage equivalent
to a logic 1.
The logic element 110 of FIG. 2 is a nonexclusive OR gate. OR gate
110 operates such that its output will be a logic 1 if either (or
both) of its inputs (indicated by the arrows) are logic 1. The OR
gate 110 of FIG. 2 consists of a pair of diodes 87, 89 which are
poled so as to conduct in the positive direction. Therefore, if
either of the inputs is a logic 1 (+12 volts) then the output will
assume that voltage since the diode associated with that particular
input will conduct in the appropriate direction.
Finally, the logic element 106 and element 118 in FIG. 2 are
inverters. Inverters 106 and 118 operate such that the signal on
their outputs (indicated by the circle) will be the inverse of the
signal at their inputs (indicated by the arrow). Thus, if their
input is a logic 1, the output of inverter 106 or 118 will be a
logic 0 and vice versa.
DESCRIPTION OF FIGURE 2
The underspeed detector of FIG. 1 includes a flip-flop 80 and a
time delay 82. The output pulses from the photocell 36 in FIG. 1
are fed through a differentiator 47 to the direct set terminal S of
flip-flop 80 so that flip-flop 58 sets each time these input pulses
go to logic 1. These same pulses are also fed to the input to time
delay 82. As explained above, the presence of a logic transition of
1 to 0 at the input of the time delay 82 begins the timing
cycle.
The output of time delay 82 forms one input to an OR gate 49. The
other input(s) to OR gate 49 come from other protective circuits
which require the synchronous deenergization of the hammer buss.
The output of OR gate 49 is connected to the direct reset input R
of flip-flop 80 so as to reset flip-flop 80 to indicate that the
hammer buss is to be deenergized. Briefly, as will be seen
hereinafter, the minimum belt speed is set by time delay 82 and if
the speed of the belt is above the minimum permissible speed,
flip-flop 80 will stay set. On the other hand, if the speed of the
belt is below the minimum permissible speed, time delay 82 will
reset flip-flop 80 before the next input pulse arrives on the set
input terminal S.
The undervoltage detector of FIG. 1 monitors a portion of the input
voltage to determine whether it is above a predetermined minimum.
The circuit includes a variable resistor 84 which is connected to
the DC output of the power supply shown generally at 86. A diode 88
is connected in series with variable resistor 84 and feeds a time
delay circuit shown generally at 90 including a capacitor 92 and a
resistor 94. Briefly, time delay circuit 90 provides a time delay
so as to prevent an undervoltage indication under temporary
conditions of line droop, etc. As was explained in FIG. 1, the main
DC power supply includes a number of filter capacitors such as
capacitor 72 which can maintain the DC output fed to the printer
system long enough to overcome minor variations in the input
voltage.
The input voltage is monitored by a comparator circuit 96 which
compares the input voltage with a predetermined reference
established by a Zener diode 98. If the input voltage is greater
than the reference voltage established by the breakdown voltage of
Zener diode 98, Zener diode 98 will conduct. Conduction of Zener
diode 98 causes transistor 100 to turn on so that the output of the
comparator circuit 96 will be a logic 1. Therefore, the output of
the comparator circuit 96 will be a logic 1 if the input voltage is
above the predetermined reference. On the other hand, if the input
voltage is below the breakdown voltage of the zener diode 98, it
will not conduct. This causes transistor 100 to turn off, switching
the output of the comparator circuit 96 to logic 0 by virtue of
pulldown resistor 102 connected to 0 volts.
The output of the comparator circuit 96 is fed to a time delay
circuit 104 which is identical to time delay circuit 114 explained
above. The purpose of time delay 104 is to allow the power supply
to stabilize before applying power to the hammer buss and logic.
After the preset time, the output of time delay 104 goes to logic 1
indicating that the input voltage is above the predetermined
minimum and has stabilized so as to allow synchronous energization
of the hammer buss.
As was pointed out in the description of FIG. 1, an undervoltage
condition may result in the storage of erroneous data in the data
storage portion 44 of the control system. For this reason, the
undervoltage detector 50 assures that no data is stored during
undervoltage conditions and further acts to clear all data
previously stored if the input voltage drops below the
predetermined reference. This is accomplished by an inverter 106
whose input is connected to the output of time delay 104. The
output of inverter 106 is connected to the data storage portion 44
of the control system. If the input voltage is below the
predetermined reference, the output of time delay 104 is a logic 0.
This results in a logic 1 at the output of the inverter 106.
The presence of a logic 1 on the CLEAR input to data storage 44
clears all stored data and prevents any additional data from being
entered. It is important to note that clearing the data storage
portion 44 of the control system 40 has the effect of providing
alternative protection for the hammers and print characters since
this prevents any further energization of the hammers. In some
applications, it may not be necessary to remove the power from the
hammer buss in case of an undervoltage since clearing the stored
data may have the same effect as the synchronous energization and
deenergization of the hammer buss.
The hammer buss control 54 of FIG. 1 includes a first AND gate 108
which has as its inputs the outputs of the undervoltage and
underspeed detectors 50, 52. If the belt speed and input voltage
are both correct, the output of AND gate 108 will be a logic 1
since both of its inputs are logic 1. The output of AND gate 108
forms one of the inputs to OR gate 110. The other input to OR gate
110 comes from an OR gate 112 which ORS the outputs derived from
the photocells 34, 36 and acts as a synchronizing signal.
The output of OR gate 110 forms the input to a time delay 114. Time
delay 114 assures that both the voltage and the belt speed have
stabilized before permitting the hammer power buss to be energized.
After the delay of time delay 114 has passed, flip-flop 116 will be
steered to assume the set state since the output of time delay 114
is connected to the set steering input 55. The trigger input T of
flip-flop 116 is connected to the output of OR gate 112. By using
this source as the trigger for flip-flop 116, the system ensures
that the hammer buss is not energized except during those periods
when the hammers are not permitted to be activated.
If either the voltage or speed should fall below predetermined
minimums, flip-flop 116 will synchronously reset since the output
of time delay 114 is fed through an inverter 118 to the direct
reset terminal R of flip-flop 116. Resetting flip-flop 116 removes
power from the hammer power buss by deenergizing the switch in FIG.
1.
THE WAVEFORMS OF FIGURES 3--4
FIG. 3 is a series of waveforms illustrating the operation of the
underspeed detection portion of the preferred embodiment of FIG. 2.
The waveforms shown are lettered A through F to correspond to the
locations in FIG. 2 similarly indicated.
The waveform labeled A is the output of the differentiator 47 in
FIG. 2. As was pointed out, the differentiator 47 differentiates
the positive-going portions of the output of the odd photocell 36
of FIG. 1, and the interval between signals is therefore, an
indication of the speed of the character belt 12. This signal forms
the input to the underspeed detector 52.
The initial portion of the waveforms of FIG. 3 assumes that speed
of the character belt 12 has not yet reached minimum permissible
speed so as to allow the application of power to the hammer buss.
Pulse 130 in waveform A initiates the timing cycle of the speed
detector 52 by setting flip-flop 80 (as indicated in waveform C)
and by starting the time delay 82 by discharging capacitor 55
through transistor 51 (as indicated by waveform B). Capacitor 55 in
the time delay 82 charges at the exponential rate shown until the
voltage on capacitor 55 is sufficient to cause unijunction
transistor 57 to conduct as is shown at 132 in waveform B. At this
point, unijunction transistor 57 conducts, causing flip-flop 80 to
reset as shown at 132 in waveform C.
When pulse 130 arrived and set flip-flop 80, time delay 114 began
the timing cycle of that element, as indicated in waveform D.
However, since flip-flop 80 reset before the time delay 114 was
completed, its timing cycle terminated at 134. Operation is similar
during the next three pulses 136, 138 and 140, since the spacing
between these pulses is more than the minimum permissible speed. As
a result, the time delay of time delay element 114 started anew
upon the arrival of each of these pulses.
Pulse 142, on the other hand, arrived before capacitor 55 reached
the point where unijunction transistor 57 conducts. This indicates
that the speed of the character belt 12 is now above the minimum
permissible speed. Arrival of pulse 142 at the input of time delay
82 caused transistor 51 to conduct and discharge capacitor 55
before unijunction transistor 57 conducted. Therefore, the output
of time delay 82 never reached the logic 1 level so that flip-flop
80 remains set. Since flip-flop 80 remains set, time delay 114 (as
indicated by waveform D) completed its timing cycle at 144. At this
time, the underspeed detector 52 has indicated that the belt speed
has reached the desired level and the system is ready to apply
power to the hammer buss.
Power is not, however, instantaneously applied to the hammer buss
since the application of power is synchronized with the
synchronizing signal of waveform E. Since this waveform is the
combined output of waveforms derived from the photocells 34, 36 in
FIG. 1, the logic state of this waveform indicates when the hammer
busses are permitted to be energized. Whenever waveform E is a
logic 0, the hammer busses are permitted to be energized whereas
the presence of a logic 1 at the E waveform indicates that hammer
busses cannot be energized since the character belt fingers are
not, at that time, in the proper position. In this way, the
energization and deenergization of the hammer buss will be
controlled by timing it with waveform E. That is, the hammer buss
will be energized (or deenergized) when the waveform E switches
from the logic 1 to the logic 0 level.
As was pointed out above, the output of time delay 114 went to
logic 1 at 144. This occurred, however, when the output of waveform
E was a logic 1 so that the hammer buss could not be energized at
that time without risk of damage to the printer mechanism.
Therefore, the flip-flop 116 which controls the energization of the
hammer buss does not set at the precise time when time delay 114
completes its timing cycle. Instead, the setting of flip-flop 116
is delayed until the time shown at 146 when waveform E goes to
logic 0 thereby triggering flip-flop 116 and energizing the hammer
buss. Having set flip-flop 116, the hammer buss is energized and
will remain energized so long as the speed of the character belt
stays higher than the minimum permissible speed established by time
delay 82.
The waveforms of FIG. 3 following the broken lines illustrate the
operation of the underspeed detector 52 when the speed falls below
the minimum permissible speed so as to require deenergization of
the hammer buss. The fact that the speed of the character belt is
below the minimum permissible speed is indicated by the spacing of
pulses 148 and 150 in the A waveform. At 152 the capacitor 55 in
time delay 82 reaches the point where unijunction transistor 57
conducts. At this point, the flip-flop 80 is reset. However, the E
waveform at this time is a logic 1 so that one of the hammers may
be energized at this point. Therefore, power should not be removed
from the hammer buss in order to prevent damaging the printer
mechanism.
Damage to the print mechanism in this situation is prevented
because the E waveform forms the second input to OR gate 110 in
FIG. 2. Therefore, the fact that the output of flip-flop 80 changed
to a logic 0 did not take effect in time delay unit 114 since the
second input to OR gate 110 was a logic 1 at this time. Therefore,
flip-flop 116 does not reset until the point indicated at 154 in
the E waveform, at which time the output of OR gate 110 changes to
a logic 0. Since the output of time delay 114 is fed through
inverter 118 to the direct reset terminal R of flip-flop 116,
flip-flop 116 immediately resets at this point and removes power
from the hammer buss during the period of time in which the E
waveform is a logic 0 so as to safely deenergize the hammer buss at
a time when no hammers are being energized.
FIG. 4 illustrates the operation of the undervoltage portion of the
embodiment of FIG. 2 in synchronously energizing and deenergizing
the hammer buss. The effective DC output of the power supply in
FIG. 2 is represented by waveform H. When the control system is
initially energized, the effective DC voltage at H will rise at the
exponential rate shown.
The voltage on the time delay circuit 90 in FIG. 2 is illustrated
by the waveform I, which is similar to waveform H but delayed
somewhat in time in order to accommodate the charging of capacitor
92 in that time delay circuit. At 160, the input voltage to the
comparator 96 reaches the lower voltage limit at which time the
output of the comparator 96 becomes a logic 1. When the output of
the comparator 96 becomes a logic 1, the time delay of time delay
unit 104 is initiated as indicated in waveform G. Following the
time delay of time delay 104 (indicated at 162), the time delay of
time delay unit 114 is initiated, as indicated by waveform D.
The time delay of time delay unit 114 terminates at the point
indicated at 164. Hence, the undervoltage circuit has indicated at
this time that the input voltage is above the lower voltage limit
and the combined time delays of time delay units 104, 114 have
provided sufficient time to allow the system to stabilize. However,
it will be noted that the output of time delay unit 114 reached the
logic 1 level when the E waveform was a logic 1. Therefore, it is
not permitted to energize the hammer buss at 164 since energization
of the hammer buss must be delayed until the next time the E
waveform goes to logic 0. Thus, while the waveform D arrives at the
logic 1 at 164, the hammer buss is not energized until 166 so as to
ensure that the printer mechanism will not be damaged.
The function of the time delay circuit 90 in FIG. 2 is also
illustrated in FIG. 4. Momentary droops in the input voltage are
illustrated at 168 and 170. The undervoltage circuit of the present
invention need not deenergize the hammer buss during such temporary
droops in the power supply since filter capacitors (such as
capacitor 72 in FIG. 2) are able to maintain the input voltage to
the control system for a period of time sufficient to alleviate the
effects of such temporary droops. Therefore, while the waveform H
is shown as drooping somewhat at 168 and 170, the input to
comparator 96, as illustrated by waveform I, does not feel the
effects of these temporary droops by virtue of the action of the
capacitor 92 in time delay unit 90.
The portion of FIG. 4 following the broken lines illustrates the
operation of the undervoltage detector 50 when the input voltage
drops below predetermined lower voltage limits. As can be seen in
waveform H, the input voltage begins to fall at the point indicated
at 172. The effect of the droop in input voltage is not immediately
felt at the comparator 96 by virtue of the action of time delay 90,
so that the I waveform is shown falling below the lower voltage
limit at a later time indicated at 174. When the I waveform falls
below the lower voltage limit, the output of time delay 104
(illustrated by the G waveform) simultaneously changes to logic 0.
At this point, however, the E waveform is a logic 1 indicating that
it is not permissible to deenergize the hammer buss. The hammer
buss is, therefore not deenergized at this time since the E
waveform forms the second input to OR gate 110. However, at 176 the
E waveform changes to logic 0 at which time the flip-flop 116 is
immediately reset so as to deenergize the hammer buss during the
time when none of the hammers are permitted to be energized.
Although the present invention has been described with respect to a
particular embodiment, the principles underlining this invention
will suggest many additional modifications of this particular
embodiment to those skilled in the art. Therefore, it is intended
that the appended claims shall not be limited to the specific
embodiment shown, but rather shall cover all such modifications as
fall within the true spirit and scope of the present invention.
* * * * *