CA2051571A1 - Hammer energy control for quiet impact printer - Google Patents

Abstract

ABSTRACT

An impact printer including a platen, a carriage mounted for reciprocating movement generally parallel to the platen, a rotatable print element having character imprinting portions disposed thereon, the characters being assigned a class designation according to their imprinting area, a print element selector for moving the print element to position a selected character portion at a printing position, a hammer for driving the character portions to deform the platen, and means for driving the hammer toward and away from the platen. Different impact velocities are assigned to the hammer in accordance with the character portion class designations.
The rate of hammer displacement is varied as it is moved from a home position to an impact position so that the hammer initially moves through a first region wherein it is rapidly displaced at an increasing velocity and subsequently moves through a second region wherein it is slowly displaced at a substantially constant velocity. A
counter is incremented periodically in response to the hammer movement for determining the instantaneous location and velocity of the hammer, and the counter is reset with a predetermined count, notwithstanding the actual count, at a location within the second region.

Description

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HAMMER ENERGY CONTROL FOR QUIET ~MPACT PRINTER

Field of the Invention This invention relates to a controlled low frequency impact system for a quiet impact printer wherein variable impact speeds are utilized to impart different hammer kinetic energy to character elements of different size and to an inexpensive system for precise bidirectional motor control without the need for direction information from the feedback sensor.

Background of the Invention The o~fice has, for many years, been a stressful environment due, in part, to the large number of objectionable noise generators, such as typewriters, high speed impactprinters, paper shredders, and other office machinery. Where several such devices are placed together in a single room, the cumulative noise pollution may even be hazardous to the health and well being of its occupants. The situation is well recognized and has been addressed by governmental bodies who have set standards ~or maximum acceptable noise levels in office environments. Attempts have been made by of fice machinery designers, in the field of impact printers, to reduce the noise pollution. Some of these methods include enclosing impact printers in sound attenuating covers, designing impact printers in which the impact noise is reduced, and designing quieter printers based on non-impact technologies such as ink jet and thermal transfer.

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The low cost personal typewriter is purchased primarily for home usage (including both personal and in-home office) and for school usage. It is particularly desirable in these environments to reduce the acoustic noise level of the printing mechanism, at the source, to levels which are unobtrusive. For example, in the home, other members of the family should not be distracted by the clatter of typing if conducted in common rooms. In a secondary school or college setting, colleagues and others should not be disturbed if the user types in a library, a study hall or a dormitory room. Heretofore such usage has not been possible because typewriters are notoriously noisy devices.
The silent operation of our low cost quiet typewriter will enable such usage because silence transports such useful appliances into new physical settings and enhances portability. A derived benefit will be freer communication among work group members as the user is able to work directly in the group in a nonirritating manner.

We have been introducing design changes in low cost typewriters in order to reduce their output noise levels. Maintaining a competitively marketa~le product becomes a challenge in the consumer, or commodity, market segment where product cost is inthe $100 to $300 range. For example, a $100 typewriter would typically have a unit manufacturing cost of about $65. Clearly, any modification necessitated by the implementation of a sound reduction design will of necessity be extremely low in cost because the incremental increase in product cost to the consumer will not warrant a large percentage rise in this market segment.

Noise measurements are often referenced as dB or dBA ~alues, wherein the "A" scale represents humanly perceived levels of loudness as opposed to absolute values of sound intensity. When considering sound energy represented in dB (or dBA) units, it should be noted that the scale is logarithmic and that a 10 dB difference equals a factor of 10, a 20 dB difference equals a factor of 100, a 30 dB equals a factor of 1000, and so on.

Typical typewriters generate impact noise in the range of 65 dBA to just over 80 dBA, when measured at the operator's position. These sound levels are deemed to be intrusive. For example, the IBM Selectric ball unit generates about 78 dBA, while the Xerox Memorywriter generates about 68 dBA, and the low cost Smith Corona Correcting Portable generates about 70 dBA. When reduced to a dBA in the high 50s, the noise is identified as being objectionable or annoying. It would be highly desirable to reduce the impact noise to a value in the vicinity of 50 dBA. The low cost typewriter of the present invention has been typically measured at about 50 dBA~, representing a dramatic improvement, on the order of about 100 times less sound pressure, over presently available low cost typewriters.

The major source of noise in the modern typewriter is produced as the hammer impacts and drives a character pad to form an impression on a receptor sheet.
Character pads are carried upon and transported past a print station at the ends of the rotating spokes of a printwheel. When a selected character is to be printed, it is stopped to the print station and the hammer drives it against a ribbon, the receptor sheet and a supporting platen, with sufficient force to release ink from the ribbon onto the receptor sheet.

' ' In conventional ballistic hammer impacting typewriters a hammer mass of about 2.5 grams is ballistically propelled by a solenoid actuated clapper toward the character/ribbon/paper/platen combination. After the hammer hits the rear surface of the character pad, its momentum continues to drive it toward and against the ribbon/paper/platen combination and to deform the platen surface. Once the platen has absorbed the hammer impact energy it seeks to restore its normal shape by driving the hammer back to its home position where it must be stopped, usually by another impact. This series of high speed impacts is the main source of the objectionable impact noise in these printers.

Typically the duration of platen deformation by the very small irnpacting hammermass is very short, on the order of 100 microseconds. Intuitively it is known that a rapid impact will be noisy and that a slow impact will be less noisy. Thus, if the impact duration were longer it would be possible to make the device quieter. In low end typewriters with printing speeds in the 10 to 12 character per second range, the mean time available between character impacts is about 85 to 90 milliseconds.
Clearly, more of that available time can be used for the hammer impact than the usual 100 microseconds. If, for example, the platen deformation time were stretched to even 5 to 10 milliseconds this would represent a fifty to one hundredfold increase, or stretch, in the impact pulse width. It is also intuitive that in order for a slow impact to deform the platen by the same amount as does the rapid impact, for adequately releasing the ink from the ribbon, a larger hammer mass ~or effectivemass) must be used. This is because manipulation of the time domain of the deformation changes the frequency domain of the sound waves emanating therefrom.

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As the deformation time is stretched, the sound frequency (actually a spectrum of sound frequencies) emanating from the deformation is proportionately reduced andthe perceived noise output of the lower frequencies is reduced. Since this is a resonant system, the mass will be inversely proportional to the square of the frequency shift.
Therefore, a one hundredfold increase in the time domain (100 microseconds to 10milliseconds) will proportionately reduce the frequency output when a ten thousandfold increase in the mass is effected. Clearly it would not be practical to increase the actual mass of the hammer by such a factor. As an alternative to increasing the harnmer rnass per se, its effective mass may be increased by neans of a mechanical transformer.

The general concept implemented in the present typewriter, i.e. reduction of impulse noise achieved by stretching the deformation pulse and impacting with an increased hammer mass, has been recognized for nany decades. As long ago as 1918, in U.S.
Patent No. 1,261,751 (Anderson) quieter operation of the printing function in a typewriter was proposed by increasing the "time actually used in making the impression". A type bar typewriter operating upon the principles described in this patent was commercially available at that time.

The quiet impact printing mechanism incorporating the theory of operation of thepresent invention is explained in the following two commonly assigned patents either one of whose disclosures is herein fully incorporated by reference. United States Patent No. 4,681,469 (Gabor), entitled ;'Quiet Impact Printer", relates to greatly increasing the ef~ective mass of the hammer, introducing the hammer to the platen at a relatively slow speed and causing the platen deformation to take place over an 2 ~

extended period of time. In United States Patent No. 4,668,112 (Gabor et al) entitled "Quiet Impact Printer" it is taught to control the movement of the hammer throughout its path of movement from its home position to its application of impact force to the platen. As the hammer nears the surface of the platen its velocity is significantly diminished by a braking action of the drive motor so that impact takes place at a very slow speed. Subsequent to initiation of contact, the drive motor is reenergi~ed, increasing the hammer force to deform the platen.

In both the '469 and '112 patents a mass transformer, comprising a heavy rockable bail bar driven by a voice coil motor, urges a push rod toward and away from theplaten in a controlled manner. The push rod in turn moves a print tip (hammer) into deforming contact with the platen. A sensor mounted upon the print tip indicates the moment of contact with the platen so that an additional application of kinetic energy may be provided by the voice coil motor at that juncture. A suitable controller energizes the voice coil motor to move the print tip across a throat distance between its home position and the surface of the platen, where its velocity is very slow. After contact has been sensed, the controller again energizes the voice coil motor forimparting a predetermined pressure force for deforming the platen to release ink from the ribbon with this high effective mass.

A low cost implementation of a quiet impact printer, based upon the principles of operation of the '469 and '112 patents, is described in copending patent application U.S. Serial No. 07/510,654 (Babler et al) assigned to the same assignee as the instant application, whose disclosure is herein fully incorporated by reference. A high effective mass hammer is driven toward and away from the platen, by a D~ motor . .

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acting through a displacement and force modifying mechanism, in a timed manner.
When ~he hammer impacts the platen, contact is sensed as a sudden change in velocity and then a pressure force is applied by the motor~In the low cost quiet impact printer designed substantially in accordance with the teachings of the copendingBabler et al application, wherein a DC motor and cam control the print hammer motion, signif~lcant power is used to accelerate and decelerate the motor armature for prepositioning the hammer immediately prior to contacting the platen. The power requirements are further increased because of the elasticity of the mechanical system which necessitates an extended duration of the post-contact pressure ("squeeze") force for adequate ink release from the ribbon. Furthermore, the precision DC motor control systems, as incorporated therein, typically use expensive servo controls with various types of feedback elements. In particular, such systems requiring bidirectional start/stop motion almost always include some means for deriving directional information about the motor. Without directional signals, along withposition pulses, the controller could encounter instances where counting errors occur due to mistakes about direction. In sum, while the acoustic characteristics are quite attractive, the speed of operation is limited, the power demand on the control electronics is significant, and the control system is expensive.

Therefore it is the primary object of the present inYention to decrease the power requirements of a low cost quiet impact printing apparatus by foregoing the fixed approach velocity and variable "squeeze" force method of operation in favor of varying the contact velocity of the hammer tip for characters of different size. Increases in audible emissions should be barely perceptible because the natural frequency of impact is still very low.

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It is another object of the present invention to further reduce the cost of manufacture of a low cost quiet impact printer by providing a bidirectional startlstop motor control system that has precise control without the need for direction information from the feedback sensor Summary of the Invention The present invention may be carried out, in one form, by providing an impact printer comprising a platen, a carriage mounted for reciprocating movement generally parallel to the platen, a rotatable print elernent having character irnprinting portions disposed thereon, the characters being assigned a class designation according to their imprinting area, a print element selector for moving the print element to position a selected character portion at a printing position, a hammer for driving the character portions to deform the platen, and means for driving the hammer toward and away from said platen. The carriage supports the print element, the selector, the hammer and the means for driving. The improvement comprises means for assigning different impact velocities to the hammer in accordance with the character portion class designations, means for varying the rate of hammer displacement as it is moved from a home position to an impact position so that the hammer initially moves through a first region wherein it is rapidly displaced at an increasing velocity and subsequently moves through a second region wherein it is slowly displaced at a substantially constant velocity, and means for controlling the attainment of the impact velocities, including means for incrementing a counter periodically in response to the hammer movement for determining the instantaneous location and velocity of the hammer, 2~

and means for resetting the counter with a predetermined count, notwithstanding the actual count, at a location within the second region.

Brief Description of the Drawings Qther objects and further features and advantages of this invention will be apparent from the following, more particular, description considered together with the accompanying drawings, wherein:

Figure 1 is a perspective view schematically showing the low cost quiet impact printer;

Figure 2 is a schematic partial plan view of the printer;

Figure 3 is a schematic side elevation sectional view of the printer;

Figure 4 is a enlarged schematic side elevation view showing the relationship between the hammer, its driving cam, the timing disc and a sensor;

Figure 5 is a graphical representation of the hammer cam transfer characteristics;

Figure 6 is a phase diagram showing typical print cycles, for characters of different sizes; and Figure 7 is a state diagram of the print cycle of the present invention.

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Detailed Description of the Illustrated Embodiment The novel, low cost quiet impact printer 10 housed within a cover set (only the base 12 is shown) includes relatively few moving parts. Vertically upstanding left and right side support plates 14 and 16 are each secured to the base 12 and support the ends of platen 18 in seats therein. The platen is driven by a suitable motor (not shown)through a gear train including driving gear 20 and driven gear 22 on the platen sha~t 24. The side plates also support the ends of a highly polished guide rod 26 and the ends of reaction bar 28 having an accurately machined guiding edge 30 facing theplaten. The reaction bar is mounted so as to be capable of adjustment in order to maintain the guiding edge 30 parallel to the platen surface and to accurately establish its distance from the platen.

A printer carriage 32 comprised of carriage frame plates 34 and 36, each having a bearing 38 mounted thereon, is supported upon the guide rod 26 for reciprocatingmovement therealong, across the length of the platen. Carriage reciprocation is controlled by a motor (not shown~ which drives a toothed spacing belt 40 (a cable or rack drive may be used instead) secured to the carriage, over pulleys 42 and 44. As the carriage 32 moves along the guide rod 26 on bearings 38 it will tend to rotate in a clockwise direction thereabout (as viewed in Figure 1), under the influence of gravity, and biases bearing shoe 46 against the guiding edge of reaction bar 28. The shoe is made of a hard, low friction material, such as Delrin~. This carriage mounting arrangement facilitates inexpensive assembly of the printing device because it eliminates criticality in the placement of the guide rod, requiring only one element, ii 7 ~

the reaction bar 28, to be accurately positioned. By adjusting the ends of the reaction bar relative to the side plates 14 and 16, the guiding edge 30 may be accuratelypositioned parallel to the platen, so that as the carriage 32 traverses the printer, all the printing elements carried thereon will remain in their proper position relative to the platen.

The printing elements comprise a printwheel 50, a hammer assembly 52 and a ribbon pack assembly 54 (seen in Figure 3). A printwheel drive motor 56 mounted on the carriage frame plates 34 and 36 has a drive coupling 58 to which a printwheel hub 60 may be connected for rotation of the character pads 62 (located at the ends of printwheel spokes 60 past a print station adjacent to the platen. Selective rotation of the drive motor 56 under processor control, initiated by keystrokes, locates andarrests the desired character pad 62 at the print station. A resilient card guide 66 also mounted on the carriage frame plates holds an image receptor sheet 68 in intimate contact with the platen surface The hammer assembly 52 is best seen in Figures 3 and 4 wherein carriage frame plate 36 has been cut away to better reveal it. A hammer actuating DC motor 70 is mounted upon carriage frame plate 34 with its drive shaft 72 extending through and beyond both frame plates. Hammer drive cam 74 secured to the shaft 72 moves cam follower 76 to rotate bell crank 78 about pivot pin 80. The hammer 82 is pinned at the opposite end of the bell crank and slides through a stationary guide bearing 84. As the motor rotates it also drives timing disc 86 relative to a fixed sensor 88 for generating a location count in motor controller 90, mounted upon circuit board 92.
Although the circuit board is illustrated as being secured to the carriage, it is possible to mount it on the base The motor controller sends signals to the DC motor for ef~ecting cam rotation at a desired velocity and in a desired direction.

As taught in the '469 and '112 patents, in order to achieve low impact noise thehammer must initiate contact at a very slow velocity ~under 16 inches per second), but in order to achieve a satisfactory printing speed it must move rapidly across the throat. These movement characteristics are determined by the profile of hammer drive cam 74 and the DC motor rotational speed as determined by the controller 90. A
representation of the cam displacement characteristics can be seen in Figure 5. A
first cam region 7dsa will result in the illustrated rapid hammer displacement, in which harmonic motion has been selected to move the hammer smoothly for minimizing acoustic noise associated with cam transition points and for reducing cam and cam follower wear. A second, linear, cam region 74b will result in the shallow straight line displacement (e.g. 0.001inch/degree of motor rotation) over the region from xl to x2 (corresponding to angles al and a2 of the cam~ in which impact is expected. i.e. from the surface of a multi-sheet pile (xl) to the surface of a single sheet (x2). The linearity of this second cam region results in a linear relationship between the motor current and the hammer force so that its slope may be selected to yield the maximum force needed for a particular system in view of the torque available from the motor.

The print force is resolved as the hammer 82 is driven against the platen and the shoe 46 is driven against the reaction bar 28. Ideally, if the hammer and the reaction bar were aligned the print force and the reaction force would be equal and opposite and no other system elements would experience any force at impact. However, in view of 2 ~ 7 ~

design constraints it is often not possible to align these forces, in which case there will be a force through the carriage and other elements of the system, including the guide rod 26, all of which should be minimized.

Rather than using a fixed approach velocity which is slowed substantially prior to impact, followed by a variable "squeeze" force, we have found that the efficiency of the quiet impact printing method can be improved by impacting the platen with the hammer approaching at various approach velocities, in accordance with the size of the character to be printed. The former method is less efficient because it requires the motor to accelerate the large hammer mass and then to decelerate it solely to traverse the throat distance, but it is slightly quieter in operation. It is well known in conventional impact printers to impact characters of different size with different hammer forcest but this has always been accomplished with a high impact speed and a relatively small mass, i.e. in very noisy systems. We have determined that by insuring that the contact velocity is relatively slow (i.e. Iess than 16 ips as defined in the '469 patent), our method will result in a controlled low frequency impact. Also, in accordance with the method taught in the '469 patent the effective hammer mass at the moment of impact must be greater than 0.5 pound and the platen deformation period is greater than 1 millisecond.

The following Table indicates four classes of characters, based upon their impact area, and the hammer tip speed required by each for obtaining the printing force needed for good ink release and maintaining quiet operation. It should be understood that afiner grained control is possible by grouping the characters into rnore classes.

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... ,..... . .
Character Force (Ibs) Tlp Speed Curve (Fig. 6) ~.. . _ . _ _.. __ 22 __ A
x 12 4.5 B
. .. ~, ... . . . . .
i 9 3 B/C
. . __ . . .. _ ~ BID

Because our system eliminates the need for hard forward and reverse driving of the DC motor, for moving the hammer tip across the throat distance, it requires significantly lower power. As illustrated in Figure 6 and set forth in the above Table, the motor drives the hammer tip to achieve a desired velocity (and maintain it) just prior to impact with the platen. Curve A represents the velocity response for characters needing the highest energy level and curve B represents the velocity response for the most frequently used impact levels (i.e. the "x" and similar characters). The target velocity for these high energy curves is achieved and maintained prior to the initiation of the impact zone (i.e. the location at which impact may be expected). For the low energy curves C and D, the system is slowed down from curve B to the desired lower target velocity at one or more predetermined points. The alternative of accelerating the system from the starting point directly to the C and D
target velocities is not efficient for these low energy levels since it would signi.~lcantly extend the print cycle time.

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In any system requiring bidirectional startlstop motion it is imperative to maintain precise information regarding velocity and location of the motor/carnlhammer. Such systems almost always include some means for deriving direction information in order to increment and decrement the counter for maintaining accurate position information. Knowledge of direction is a safeguard because there are instances during rotation and counterrotation of these elements when some accidental eventcauses the direction to change by mistake, resulting in counting errors. Usually an optical encoder assembly is incorporated from which position, velocity and direction may be determined. Such an interruptive encoder assembly may comprise a circumferentially slotted timing disc, mounted upon the motor drive shaft, and asophisticated IC sensor mounted in a housing positioned relative to the disc. The sensor typically employs a single light source (e.g. an LED), two accurately positioned photodetectors, and logic circuitry which provides two outputs: a counting pulsegenerated whenever the illumination level on one of the photodiodes passes through a threshold level, and a direction output which is set in response to which of the two channels is illuminated first. Such a device is referred to as a dual channel sensor.

A simple sensor is typically used in systems where the controller is using the output information for counting, determining velocity, and/or detecting a stop position. Such a sensor (referred to as a single channel sensor) employing a single light source and a single detector will not yield direction information. These systems often use aninterrupting timing disc with a unique location feature such as a flag, or wide slot (for use in a transmissive mode~, or a wide reflective elernent (for use in a reflective mode), vhose width dimension is a multiple (such as 3x) of the width o~ the remaining ~ O ' ~ 7 .~

circumferential interrupting features (narrow slots or reflective stripes). The flag may be easily detected and dif~erentiated by a controller. In conventional systems, the location feature is usually phased with a desired stopping position of the mechanism. Thus, its controller simply uses the period (Ts) of the slot count pulses for speed control and the absence of pulses for a predetermined time (TFI~ as the sensor passes the ~lag, for detecting the stopping position. It should be clear that these systems must operate at a uniform speed, particularly when nearing the flag location, so that the comparison of TF with Ts will be accurate, since deceleration in advance of the stop position (increasing the slot time intervals Ts) makes detection of the flag very difficult.

As the result of an aggressive cost reduction design project we have devised a way to achieve accurate timing of motor control events, in our bidirectional system, with only a simple sensor which generates a signal indicative of the presence or absence of light. In Figure 4 there is illustrated the timing disc 86 having circumferential interrupting features (narrow slots) 94 and flag (wide slot, comparable to threenarrow slots) 96 movable past the fixed simple sensor 88. We place the flag relative to the cam at a location intermediate the motion end points where the rotational velocity is expected to be substantially constant and the direction of movement is known.

When the flag is sensed 1TF> >TS) a position counter in the controller 90 is reset to a predetermined count, thereby calibrating the system each time the flag is sensed. By resetting the counter at this point in the cycle, the location is always known at a critical point from which the controller will count to the location at which it will instruct the motor to initiate braking so as to impact the platen at the predetermined velocity for the character to be printed. It will make no dit~erence in the operation of this printer if there have been errors in the count prior to the reset since the slate will be wiped clean and the new information will prevail. This unique sensing system~which is inherently unsophisticated, enables relatively sophisticated control functions to be performed. In addition, by substituting a simple sensor for a sophisticated sensor, the cost saving is substantial relative to the approximately $6a cost of manufacturing a $100 typewriter.

Turning to Figure 7 there is illustrated a state diagram showing the programrnedprint cycle as used in the present system with a simple sensor and employing variable impact velocity for achieving low cost quiet impact printing The program performs the following series of routines, wherein exemplary values for times and location are set forth:

In START UP PREPARATION, a look up table in the controller 90 is used to determine the desired impact energy (velocity) for the character to be printed. As shown in Table 1 and Figure 5, four impact velocity assignments have been selected.
It is certainly possible to assign more, if needed. Frorn the curve of Figure 5, it can be seen that, starting from a home position, an initial target velocity is selected. For high energy characters (curves A and B) the impact velocity will be the initial target velocity. For low energy characters (curves C and D) the lower impact velocity would be too slow to be used for traversing the throat, so the initial target velocity will be the higher velocity of curve B.

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In BEGIN MOVING, a high forward drive along either curve A or B is initiated. Inorder to validate the timing increments used for detecting the flag, an initialization loop is implemented . This is done by setting an initial target location equal to the present location plus three counts and then driving forward until the initial target location is reached. Then the program starts looking for the flag. As the timing disc rotates, a counter is in~remented at each interrupt of the sensor.

In LOOK FOR FIJAG, the program enables a detect flag routine which monitors the time between slots and establishes in memory a flag present time when a slot time value is greater than twice (2x) the time value between the prior two slots (i.e.
TF> >TS) After enabling this routine, the program continues to monitor the velocity of the motor as determined by the time it takes between slots (Ts). A Drive/Coast loop subroutine allows the initial target velocity to be achieved. If the present velocity is slower than the initial target velocity (set at the start), the motor is instructed to drive forward, and if the present velocity is faster than the initial target velocity the motor is allowed to coast.

In FLAG FOUND, when the 2x time is detected, the location counter is RESET to a predetermined value (lepending upon the direction of rotation of the motor and taking into consideration the missed interrupt counts attributable to the wide slot.
Currently we use count 161 when advancing to the platen and count 166 when retreating from the platen. The existence of the flag will disrupt the speed determination because~ when the 2x slot time is detected, it could be interpreted as a slowing of the motor and the controller would attempt to hard drive the motor to bring æ~ A~

it back up to speed. Instead, the program continues to drive the motor at the same state as immediately prior to the flag being detected until several post-flag sensor interrupts occur and the velocity data can again be used.

In APPROACHING IMPACT ZONE, the character to be printed is tested to determine if it is a low energy or a high energy character. ~a) If the character is a high energy character (curves A or B), the velocity is tested to determine if it is ~aster or slower than its target velocity. If it is faster, low current reverse drive and dynamic braking are used until the correct impact velocity is achieved. If the velocity is equal to or slower than the target velocity the Drive/Coast loop is effected until the impact zone. (b) If the character is a low energy character ~curves C or D) the detected speed will be greater than the target impact velocity and a low current reverse drive is initially applied to rapidly decelerate the hammer to a predetermined lower velocity, followed by dynamic braking until the correct target impact velocity is reached. This velocity is then maintained by a Drive/Coast loop until the hammer is at the impact zone.

In AT IMPACT ZONE, the hammer will be in the Drive/Coast loop at the target velocity. The value of the low drive current is chosen such that it is adequate to maintain the velocity, unless the hammer motion is restricted by IMPACT, which is assumed when the hammer velocity drops to 1/2 the target velocity. At this point the hammer is put into a coast state and allowed to remain in that state for 6 milliseconds to allow it to continue its forward progress and begin to rebound. Then the hammer is retracted toward its home position.

In START RETRACT, the program begins retracting the hammer with a low reverse drive for 3 milliseconds in order to get the hammer moving. Typically this routine is only needed with low energy characters because the high energy characters have ahigh enough rebound velocity.

In ~OOK FOR FLAG, the reverse drive is increased to a predetermined value and the return target velocity is maintained by the Drive/Coast loop until the flag is again encountered. When the 2x slot time (i.e. TF> >Ts) is detected (F~AG FOUND), the location counter is again RESET (to count 166 in this direction) and the return target velocity is maintained by the Drive/Coast loop until a predetermined location isreached in the vicinity of the home location (at count 130).

In ENDING CYCLE, at a predetermined location a two step deceleration routine is used to bring the hammer to rest. First, a low current forward drive rapidly slows it until the time between slots is greater than 2 milliseconds, then dynamic braking completely stops the motion, as indicated by the time between slots being greater than 5 milliseconds.

In the high energy cases ~A and B) the target velocity is maintained substantially all the way from the reset point to the platen surface. However, in the low energy cases (C and D) a transition point, at which deceleration is initiated, is selected sorne number of counts after the RESET point. It is important to minimize the time spent at the low target velocity because the low velocity adversely affects print speed.
Optimally, it is desired to achieve the correct target velocity just in time. Therefore, rt ~

the program constantly updates in memory the count at which impact is sensed andhow long it took to decelerate to the predetermined impact velocity. In this way, the transition point may be adjusted based upon performance of the preceding cycle.

It should be understood that the present disclosure has been made only by way ofexample and that numerous changes in details of construction and the combinationand arrangement of parts may be resorted to without departing from the true spirit and scope of the invention as hereinafter claimed.

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Claims (10)

1. An impact printer comprising a platen, a carriage mounted for reciprocating movement generally parallel to said platen, a rotatable print element having character imprinting portions disposed thereon, said characters being assigned aclass designation according to their imprinting area, a print element selector for moving said print element to position a selected character portion at a printingposition, a hammer for driving said character portions to deform said platen, and means for driving said hammer toward and away from said platen, said carriage supporting said print element, said selector, said hammer and said means for driving, the improvement comprising means for assigning different impact velocities to said hammer in accordance with said character portion class designations, means for varying the rate of hammer displacement as it is moved from a home position to an impact position so that said hammer initially moves through a first region wherein it is rapidly displaced at an increasing velocity and subsequently moves through a second region wherein it is slowly displaced at a substantially constantvelocity, and means for controlling the attainment of said impact velocities, including means for incrementing a counter periodically in response to said hammer movement for determining the instantaneous location and velocity of said hammer, and means for resetting said counter with a predetermined count, notwithstanding the actual count, at a location within said second region.

2. The serial impact printer as defined in claim 1 wherein said hammer having aneffective mass of at least 0.5 pounds and an impact velocity no greater than 16 inches per second at the location where said character portion initially deforms said platen, and said character portion deforms said platen for a contact period of at least 1 millisecond.

3. The serial impact printer as defined in claim 1 wherein said means for controlling comprises a signal emitter and collector and a signal interrupter.

4. The serial impact printer as defined in claim 3 wherein said signal emitter and collector comprise a single channel sensor.

5. The serial impact printer as defined in claim 4 wherein said driving means comprises a motor having a drive shaft, said means for varying the rate of hammer displacement comprises a cam mounted upon said drive shaft and having a first portion for moving said hammer through said first region and a second portion for moving said hammer through said second region, and said signal interrupter comprises a timing disc mounted upon said drive shaft.

6. The serial impact printer as defined in claim 5 wherein said timing disc has a row of narrow slots located near its periphery and having a single wide slot in said row which is substantially wider than said narrow slots, said wide slot and said sensor being located relative to said cam such that said wide slot is sensed after said hammer enters said second region in its travel toward said platen.

7. The serial impact printer as defined in claim 1 wherein said means for resetting said counter is actuated as said hammer approaches said platen and as said hammer is retracted from said platen.

8. A method of impact printing comprising the steps of moving a rotatable print element having character imprinting portions disposed thereon past a printing zone adjacent a platen, arresting a selected character imprinting portion at said printing zone, moving a hammer toward and away from said platen for driving said selectedcharacter imprinting portion to deform said platen with a printing force, the improvement comprising assigning a class designation to each character imprinting portion according to its imprinting area, assigning different impact velocities to said hammer in accordance with said class designations, varying the rate of hammer displacement as it is moved from a home position to an impact position by initially rapidly displacing the hammer through a first region at an increasing velocity and subsequently slowly displacing the hammer through a second region at a substantially constant velocity, and controlling the attainment of said impact velocities by incrementing a counter periodically as said hammer is moved and determining the instantaneous location and velocity of said hammer, and resetting said counter with a predetermined count, notwithstanding the actual count, at a location within said second region.

9. The method of impact printing as defined in claim 8 wherein said step of varying the rate of hammer displacement further includes the step of achieving said impact velocity and maintaining it until said hammer impacts said platen.

10. The method of impact printing as defined in claim 8 wherein said step of varying the rate of hammer displacement further includes the steps of reducing the velocity of said hammer to said impact velocity, subsequent to said step of resetting, and maintaining said impact velocity until said hammer impacts said platen.