CONTROL SIGNAL PROFILER FOR MULTIPLE ACTUATOR ASSEMBLY

Abstract: Using a shared profiler (10), a multiple actuator print drive system assembly operates in open loop mode for relatively high performance at very low system cost. A single shared ROM-based control signal profiler (10) provides a tailored current control profile, presented as a clocked series of 1's (long pulse) followed by a pattern of groups of 0's followed by groups of 1's (short pulses). The active components integrate the 1's and 0's into the desired actuator current profile. For each actuator position there is a simple actuator drive (20). The drivers (20) may be a single-transistor driver, or a dual-transistor half-bridge driver, with appropriate passive devices (13, 14). Simple selection logic (17) gates the coil control profile signal to selected actua­ tor drivers (20). Each of the selected actuator drivers (29) provides current to its coil (11) in response to the composite selection and profiling signal. In the case of a half-bridge driver, which has two transistors operating nonsynchronously, a composite selection and profiling signal is provided for each, each with a profile appro­ priate to the transistor function. Change of profile, to adapt to change of condition, may be made by changing digital values in the ROM (5), or by changing the address to select from different sections in the ROM (5), under manual or processor (1) control.

Description: This invention relates to wire matrix print actua­ tors, and more particularly relates to a simplified multiple wire matrix print actuator assembly using a simple shared adaptable pulse width modulator as the drive current pulse profiling control for a group of actuator drivers connected by simple decode logic to the source of data, providing overall economy as a result of open loop mode and yet providing very good performance as a result of adaptable drive current pulse profiling. A great variety of computer print actuator assemb­ lies, described in the prior art, have had high perfor­ mance as their primary design goal. High power consump­ tion, expensive hardware, and sophisticated timing patterns, including complex shaping of the drive pulses, have been provided in the continuing quest for high performance. A significant number of other print actuator assemb­ lies, also described in the prior art, have had low cost as their primary design goal. For example, a single high performance print actuator could print an entire line or page by means of a mechanical scan, but this is prohibitively slow for wire matrix printing. Wire matrix print acuators typically are replicated several times in a matrix print actuator printhead assembly; there may be multiple printheads to share the effort; cost thus becomes an important parameter of design. As the cost has diminished for electronics, parti­ cularly for logic and storage, the cost of print actuator assemblies relative to logic and memory has increased. Designers of printers have accordingly sought overall low cost by increasing the amount of electronics associated with each actuator. The constant search for high quality printing at reasonable cost has not, however, generally minimized both the electronics cost and the actuator cost for overall low cost of a high speed wire matrix printer system. The following publications are representative of the prior art: US-A-3,852,646 shows a high voltage/low voltage sequence drive pulse pattern generator for a solenoid. US-A-3,922,589 shows a capacitor resistor network and pulse width circuit to adjust the drive pulse to a triangular waveform. US-A-4,159,882 shows a wire matrix printing tech­ nique including bit map timing for vertically overlapping dots to achieve horizontal resolution finer than the nominal dot diameter. US-A-4,162,131 shows a circuit for providing a fixed amount of energy for a printhead by making the length of the pulse inversely proportional to the time lapse since last firing. US-A-4,162,858 shows an impact energy control network for a wire matrix printer, using long pulses normally and alternatively using short pulses if the magnet was actuated on the previous cycle. US-A-4,173,030 shows a high voltage/low voltage sequence pulse patterning circuit, with high voltage kept on for a variable time to compensate for variations in the pull-in time of a fuel injector. US-A-4,291,992 shows a wire matrix print pulse patterning network in which each printwire actuator is provided a separate pattern of one long firing pulse followed by a variable number of short damping pulses, as directed by a microprocessor. US-A-4,314,305 shows a fuel injector solenoid drive circuit pulse patterning network using a fixed voltage and two paths of differing resistance. US-A-4,440,079 shows a print hammer flight time control system in which delay values, previously deter­ mined by measurement of individual hammer flight charac­ teristics, are provided by an external memory to a delay register for each hammer. US-A-4,485,424 shows a pulse patterner for applying dither to the drive pulse for a solenoid. US-A-4,514,737 shows the use of a micorprocessor to control timing variations as a function of power supply voltage viariations. R.H. Sweat, Jr, "Impact Force Compensation for Voltage Variations in the Hammer Firing Pulse in Impact Printer", IBM Technical Disclosure Bulletin, Vol. 22, No. 5, Oct. 1979, pp. 1756-1758, shows a pulse patterning technique for controlling impact force despite voltage variations. The voluminous prior art may be summarized as providing for pulse profiling for actuator control, on an individual actuator basis, either through analog control networks or through digital electronics. US-A-4,291,992 and US-A-4,440,079 are representative of techniques for using digital electronics on an indivi­ dual actuator basis. The former shows a digital electro­ nics technieque for profiling the drive pulse program for best operation of each individual printer pin, using shift registers ans a read only memory. A 256 × 1 memory device is electronically configured as seven 16-bit shift registers, one shift register for each pin. The content of each shift register is provided by computer accessing of a 32 word × 24 bit read only memory, which among other things keeps a short term use history for each pin and provides a pattern for tailoring the acutation pulse for each individual pin in response to activation history for the individual pin. The actuation pulse for each print pin is a burst of pulses including a relatively long firing pulse and a series of from zero to seven relativ­ ely short damping pulses, roughly decreasing in number as the interval between pin activations decreases. US-A-4,291,992 uses unminimized electronics to provide a complex actuation pulse pattern profiled to adapt to environmental conditions and to the individual print pin actuation sequence. US-A-4,440,079 shows a timing control system including an external memory device for transfer of pulse profiling information to an individual register for each print hammer. This reference uses unminimized electronics in the form of separate high voltage (unregu­ lated) and low voltage controls gated to the drivers. These digital techniques of the prior art thus teach pulse profiling for each actuator, but do not minimize electronics in applying adaptable pulse profiling to the actuator drivers as a group. US-A-4,162,131 and US-A-4,162,858 are representative of analog control network techniques of achieving print quality. The former reference shows techniques for controlling the amplitude or duration of the actuation pulse so that the energy applied to the print actuator is the same for each stroke; the actuation pulse is inver­ sely proportional to the time lapse since last firing. The unminimized electronics, used provide a separate drive circuit, including energy control, for each print wire. US-A-4,162,858 shows a simpler technique for supplying either a short drive pulse or a long drive pulse to the individual print actuator depending upon whether the actuator was fired during the previous interval. The latter reference uses uniminimized elec­ tronics, providing for each driver separate gating for short and long pulse controls. Chopper drives are con­ ventional. In a representative chopper drive, coil current profiling is done by comparing a feedback signal representing the coil current to a reference signal, thus providing accurate control of the coil current even in the presence of variations in drive voltage or other parameters. Other prior art items show print actuation pulse shaping by a variety of passive element networks, open loop networks and closed loop networks. These analog techniques of the prior art thus teach pulse profiling for performance enhancement, using waveshaping built into network circuits, but do not minimize electronics in applying adaptable pulse profiling to the actuator drivers as a group. The prior art does not teach nor suggest the inven­ tion, which provides overall cost minimization to a high performance multiple wire matrix print actuator assembly by using a single adaptable digital pulse width modulator as the pulse shaping profiling control for the entire group of actuators, an by using simple decoding logic together with the profiling control to operate, in open loop mode, a single-transistor or dual-transistor driver per actuator. Accordingly, the object of the invention is to provide at low cost a multiple actuator assembly with a sohpisticated pattern of actuation current, using a simple transistor driver network per actuator, using a simple switching network per actuator for selection logic and control gating of the profiling signal, and sharing open loop profiling control circuits among all actuators of the assembly. A feature of the invention is its shared use of a precalculated open loop profiling control signal, among a number of simple selection and drive circuits of an assembly of actuators. An advantage of the invention is good performace at very low cost. Another advantage of the invention is durability resulting from elegant simplicity. The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings, in which: Fig. 1 is a schematic diagram of a first embodiment of the invention, using single-transistor drivers. Fig. 2 is a composite waveform of gating profile and coil current profile for the embodiment of Fig. 1. Fig. 3 is a schematic diagram of a second embodiment of the invention, using dual-transistor bridge drivers. Fig. 4 is a composite waveform of gating profile and coil current profile for the embodiment of Fig. 3. Fig. 1 and Fig. 2 show a preferred embodiment. On the left side, a single shared control signal profiler 10 provides an optimally tailored profiling signal to all the actuators 20 in the group. The actuator group normally is the entire printhead; there is thus only one profiler 10 per printhead in the usual case, only one profiler per assembly. The assembly includes a number of print wires, each having its own driver transistor and decode logic. This profiling control signal is adaptable, and may easily be optimally tailored for efficient high speed operation of a wire matrix print actuator assembly. Host computer 1 is a microprocessor which serves as a processor means for the profiler. It preferably pro­ vides overall control, provides input selections for text control, and conveniently also provides clock signals. It is possible to have a dedicated microprocessor as the processor means, within the profiler 10, having only the function of controlling the profiler, but the shared host computer is preferred. The clock signals arrive, along with other control signals from host computer 1, or from an alternate source, on control lines 2. Address counter 3 and the host computer provide profile address signals on address lines 4-1 and 4-2. Address counter 3 accesses the values digitally stored in the selected profile area of read-only memory 5. Read-only memory (ROM) 5 may conveniently be 1K × 1 bits, if a single nominal coil actuation profile will suffice. If a number of different profiles should be desired, ROM 5 may be extended, for example to NK*5 bits or equivalent for N different profile selections, where address lines 4-1 are used to select the active profile. The output of ROM 5 passes on output line 6 to sample-hold buffer 7, which provides the nominal coil current profile signal on profile signal line 8. The nominal coil current profile signal generally consists of a long pulse followed by a number of short pulses, as shown in Fig. 1 and Fig. 2. Actuator selection signals are supplied on selection lines 9, one line per actuator. The term "processor" is used in its broadest sense, to encompass the combination of the character generation function and the timing function. In practice, the processor can work with the address counter, ROM and clock, or can emulate one or more of these functions. All items 1-9 together form profiler 10, the control and selection electronics which are shared by the assem­ bly. Profiler 10 is not normally replicated, although all or portions may be replicated for fanout. The following chart shows a simplified representative digital control signal profiling pattern of 1 and 0 values in ROM 5, which pattern is the profiling signal made available on profile signal line 8, which connects to an actuator gating control node: 01111010101000 It is easy to change the digital values in ROM 5 to change the profile. For example, the drive pulse may be lengthened slightly, by an additional short pulse, by changing the penultimate bit from 0 to 1, as follows: 01111010101010 Such changes, under control of host computer 1, may be preset for a single profile or a limited number of profiles. The coil current profile signal can be shared by the entire group (entire printhead assembly) of serveral actuators because wire-actuators generally fire simultaneously. Some matrix printers, however, do not fire all actuators simultaneously, but rather allow some actuators to be firing slightly earlier, at base time plus 1 T, 2 T, 3 T, etc., while other actuators are firing at base time plus 1.5 T, 2.5 T, 3.5 T, etc., where T is the minimum cycle time of a single actuator. The coil current profiling signal can still be shared by all actuators in this type of printer if the coil current profile signal pattern repeats every 0.5 T, assuming that the required current-on time for each actuator is less than 0,5 T. This contrasts with actuators in an impact line-printer, which do not ordinarily fire simultaneously because their firing time is used to select the desired character from a presentation sequence of characters on a band or wheel. The preferred way to generate and apply the coil current profiling signal is shown schematically in Fig. 1. Address counter 3 runs at a rate which is roughly three orders of magnitude faster than the maximum actua­ tor repetition rate, to allow for high resolution switching to format the profile for the coil actuation current. In the preferred embodiment, adress counter 3, running at 550 KHz, is used to access sequentially a small block (less than 1K) of single-bit ROM 5. The output of ROM 5 is sent to a sample-and-hold buffer 7, with the buffer 7 output serving as the coil current profiling signal to be shared by all actuators. Counter 3 is reset by host computer 1 at the beginning of each printing cycle. In this manner, any coil current pro­ filing signal with a switching-time resolution of 2 micoseconds can be generated simply by storing the appropriate bit pattern in the ROM. Generating the coil current profiling signal in this manner requires some relatively expensive electronics, but the cost is shared by many actuators (up to 24 or more in some printers) and adaptability is convenient. This keeps the overall electronics cost low and keeps the average electronics cost per print wire low. As desired, the bit pattern stored in ROM 5 can be changed easily so that, for example, the coil current decreases as the air-gap in the actuator closes, thus maintaining just enough magnetomotive force to ensure magnetic saturation. Such current-pulse profiling mini­ mizes the heat dissipated in the coil (a serious problem in high-speed actuators) and maximizes the overall actuator efficiency. Changes of the information in ROM 5, either through computer rewriting of a changeable ROM, or through replacement in an innutable ROM, can thus adapt to changes of paper of typefont, or of ambient temperature, or other environmental situation. These changes are provided by host computer 1 during an appro­ priate non-print inverval, in accordance with preset or calculated rules. In summary, the control signal profiler 10 provides a single low-cost, extremely flexible means of con­ trolling the coil current in all actuators of a wire-­ matrix print actuator assembly. The wire matrix print actuator assembly includes, for each print actuator, an actuator coil 11-1...11-n and a driver transistor 12-1...12-n, together with appropri­ ate supporting components 13-15, and a connection 16 to a voltage source. Character selection is by host computer 1 as decoded by decoding switches 17-1, 17-2...17-n, which in the preferred embodiment are AND-gates controlled by signals from profiler 10 at actuator prifiling nodes 18-1, 18-2...18-n. AND-gates 17-1, 17-2...17-n are enabled by signals on actuator selection lines 9-1, 9-2...9-n, respecively, to pass the profiling signal to their respective selection and profiling nodes 19-1, 19-2...19-n. Selection and profiling nodes 19-1, 19-2...19-n connect respectively to control input connec­ tions of driver transistors 12-1...12-n. Each actuator 20-1...20-n thus has its own simple selection and profiling electronics, but shares the profiler 10 with all the other actuators of the print actuator assembly. Profiler 10 provides a pulse-width modulated profiling control signal on signal line 8. The pulse-width modulated profiling signal is generated simply, as a series of 1s and 0s which, when integrated by the reactances normally present, provide for the properly tailored current profile in the actuator coils selected. Each actuator coil 11-1...11-n is driven by combinatorially gating individual actuator selection signals together with the shared coil current profiling signal, and applying the gated selection and profiling signal the the base of the selcted driver transistors 12-1...12-n as shown in Fig. 1. The first pulse in the coil current profiling signal is relatively long to allow the coil current to rise to the desired value. A sequence of shorter pulses is then used to control the current level during the remainder of the actuator drive cycle. This contrasts to a conventio­ nal chopper-drive, in which coil current profiling is done by comparing a feedback signal representing the coil current to a reference signal, thus providing accurate control of the coil current even in the presence of variations in drive voltage, etc. Sacrifice of the advantages of such accurate current control, to keep costs down, is a good tradeoff in some wire-matrix printers. Therefore, according to this invention, the coil current profiling signal is strictly an open-loop control signal, pre-calculated to provide the desired coil-current pulse shape under operating conditions as measured or as calculated prior to selection or setting of ROM 5. Fig. 2 shows how a sequence of relatively simple on-off profile signals results in a coil current profile of reasonably fast rise and fall, and reasonable flat­ ness. Fig. 3 shows a second embodiment of the invention, featuring two-transistor half-bridge drivers as actuator means, and featuring appropriate two-signal ROM and AND-gate select logic. Most elements of the circuit remain unchanged from the first embodiment and are similarly numbered. Half-bridge drivers 21-1...21-n respectively energize actuator coils 11-1...11-n. The half-bridge driver requires first and second profiles, and the selection logic is replicated to provide first and second selections. ROM 5 is an NK*2 array, providing two outputs, on output lines 6, to sample-hold buffer 7. Selection logic for each actuator position includes decoding switches 17-n...17-n-2. In operation, host computer 1 provides a control signal on control line 2 to reset address counter 3. An additional control signal on clock control line 2-1 operates address counter 3 to provide low-order address­ ing signals on low-order addressing lines 4-2 to ROM 5. Host computer 1 provides high-order addressing signals on high order addressing lines 4-1. The terms low-order and high-order are nomenclature only; the nature of address­ ing a read-only memory permits great flexibility in addressing the information stored in ROM. The number of address lines 4-1 is determined by the number of bits desired for each profile. The number of address lines 4-2 is determined by the desired number of profiles. This provides a great number of values, sufficient in preci­ sion to provide all necessary profiling values, via output lines 6 and sample-hold buffer 7 to decoding switches 17-1-1...17n-2. Selection values from host computer 1 are provided on selection lines 9-1...9-n to make appropriate actuator selection for the characters or images desired for printing. A typical half-bridge driver comprises a first transistor 22-n-1 and a second transistor 22-n-2, plus two diodes 23 and 24. In operation, both transistors 22-n-1 and 22-n-2 must conduct to provide full current through coil 11-n. Transistor 22-n-2 is provided with the switching profile desired, while transistor 22-n-1 is provided with an envelope profile as shown. At print time, microprocessor 1 resets counter 3 to start the profiling. Decoding switches 17-n-1 and 17-n-2 select their respective half-bridge driver coils, through preconditioning by their respective select signals, to pass current in response to the profiles. Upon receipt of the first profiling 1 after select at first transistor 22-n-1, and upon simultaneous receipt of the first profiling signal 1 after a select signal at the second transistor 22-n-2, current passes from power source V node 16 through the second transistor 22-n-2, through coil 11-n, through first transistor 22-n-1, to ground. When the profile signal 0 appears, second transistor 22-n-21 turns off. Inductance tends to continue current from ground through grounded diode 23, coil 11-n and first transistor 22-n-1 back to ground. As the first transistor remains conductive, the second transistor tunrs on and off as 1's on 0's appear in its profiling signal, providing appropriately profiled current to coil 11-n- When both profiles become 0, both transistors are off, and the current in coil 11-n decays quickly from ground through grounded diode 23, coil 11-n, discharge diode 24 and node V to the power supply node 16. Fig. 4 shows simplified on-off profiles for transi­ stors 22 and 23, and the resulting coil current waveform. Complex profiles may be provided, and changed as appropriate, simply by changing the 1-0 pattern. A representative set of complex profiles for the embodiment of Fig. 2 is as follows: (8-1) 01111111111111111111...111111111111111111...1000 (8-2) 00111111111111111110...011111110001110001...0000

Claims: 1. Multiple actuator assembly having a number of electromagnetic coils (11), each with individual actuator position driver means characterized by a shared control signal profiler (10) for all of the actuators (20, 21) of the actuator assembly, comprising selection means (1) for providing selection signals, clock means (2), an address counter (3) connected to said clock means (2); and a memory (5) connected for addressing by said address counter (3) to provide a timed sequence of readouts of previously stored values defining an electromagnetic coil actuation control profile; and profiling selection means (6-9 and 17-19) connected to said read only memory (5) to provide open loop profiling selection signals to selected individual actuator position driver means; whereby each selected one of said plurality of individual actuator coils (11) receives controlled profile coil current in response to profiling selection signals in open loop format. 2. Actuator assembly in accordance with Claim 1, characterized in that said memory (5) is a read only memory, said address counter (3) is clock driven by said clock means (2) and is connected to said read only memory (5) on connecting lines (4-2), and that said shared control signal profiler (10) comprises said selection means (1, 4-1) connected to said read only memory (5). 3. Actuator assembly characterized in that the control signal profiler (10) comprises an address counter (3), a read only memory (5) connected for addressing by said address counter (3), and buffering means (7) connec­ ted to said read only memory (5), to provide an open loop profiling signal at an actuator profiling node (18); a plurality of actuators (20-1...20-n), one for each actuator position, each actuator comprising a decoding switch (17) having connections including an output selection and profiling node (19), a selection input on a respective actuator selection line (9), and a profiling input from said actuator profiling node (18); a driver transistor (12) with appropriate support components (13-15), connected to said decoding switch (17), for profile controlled coil actuation in response to a selection and profiling signal at said selection and profiling node (19-1...19-n); and an actuator coil (11) connected to said driver transistor (12). 4. Actuator assembly in accordance with Claim 1, characterized in that said control signal profiler (10) comprises a processor (1) connected to provide control inputs on connecting lines (4-1) connected to said read only memory (5) and further connected on selection lines (9) for driver selection, and that said control signal profiler (10) further comprises said address counter (3) connected to said read only memory (5) on connecting lines (4-2), said control signal profiler (10) providing open loop print selection control signals. 5. Actuator assembly in accordance with Claim 1, characterized in that said selection means (1) comprises a processor to provide clocking signals and address signals; that an address counter (3), connected to said processor (1) is provided for scanning a sequence of addresses; a read only memory (5), connected to said address counter (3) is provided for supplying stored digital profiling control values upon scanning of a sequence of addresses; that there is a voltage supply (16), to supply power at a nominal voltage; that a finite plurality of actuators (21-1...21-n) is provided, each including an electromagnetic coil (11-1...11-n); a similar finite plurality of controllable half-bridge driver actuators (21-1...21-n), each connected to said voltage supply (16) and each connected to supply power to a related one of said plurality of actuators (20, 21) and having a connection to an actuator selection line (9-1... 9-n); and a similar finite plurality of pairs of decoding switches (17-1-1--17-1-2...17-n-1--17-n-2), each connec­ ted respectively to a related one of said actuator selection lines (9-1...9-n), connected for pulse profi­ ling control to said read only memory (5), and connected for selection to a source (1) of control input informa­ tion, to supply to selected ones of said plurality of bridge drivers (21-1...21-n) appropriate selection and profiling signals as a function of predetermined pulse width modulated control values; whereby each selected actuator coil (11-1...11-n) integrates the applied pulse width modulated control values, in effect receiving a profiled current pulse appropriate for operation optimi­ zed as adapted for best performance of the print actuator assembly. 6. Actuator assembly in accordance with Claim 5, characterized in that said control signal profiler (10) comprises a processor (1) connected to provide control input information on connecting lines (4-1) connected to said read only memory (5) and further connected on connecting lines (9) for driver selection, and that said control signal profiler (10) further comprises said address counter (3) connected to said read only memory (5) on connecting lines (4-2), whereby said control signal profiler (10) provides open loop print selection control signals.

Inventors: KARIDIS, JOHN PETER
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: B41J3
IPC codes: B41J9, B41J2, H03K17, H01H47