Print Hammer Energizing and Control Circuit

A number of electromechanical printer systems are now in use in which high speed repetitive operation of a multiplicity of hammer elements is used to control impacting at precisely timed intervals relative to a recording member. In such systems, prevention of erroneous hammer actuation and control of hammer movement are of crucial importance to print quality. In using the energization of a coil to effect printing action, it becomes highly desirable to have a regulated voltage source for supplying the multiple hammer energizing circuits, but it is of course not desirable to utilize expensive or complex circuits. It is desirable to avoid substantial power dissipation, and to protect against the generation of radio frequency interference and other noise effects in regulating the voltage. Furthermore, noise effects and transients within a system may introduce perturbations which cause erroneous hammer firings under various conditions, particularly during startup, shutdown and other intervals. It also can occur that there are various times at which it is desired to prevent possible firing of the hammers, as during paper advance intervals.

Hammer energizing and control circuits in accordance with the invention provide control of the charging of a storage capacitor from a full wave rectifier by sensing the combined voltage variation between the full wave rectified and stored charge signal levels, adjusting these to a low voltage range, and comparing the combined signal to a reference voltage of predetermined amplitude. An SCR controlling the charging of the storage capacitor with the full wave rectified power signal is consequently fired only in an initial ascending portion of the composite wave, thereby increasing the voltage level established by the storage capacitor. This arrangement is substantially free of radio frequency interference, introduces low dissipation, and controls high energy storage with adequately precise regulation.

In accordance with other features of the invention, the voltage reference itself is significantly varied so as to shut off the power during intervals in which printing is to be prohibited. In the event that a signal is present denoting a non-valid printing interval, along with sensing of a current indicating discharge of the storage capacitor, the voltage reference signal is effectively grounded so as to shut off the power. The voltage reference signal may also be used during transient conditions, such as during power failure or startup or shutdown, at which time it is also desired not to permit firing of the hammers. During these intervals, material reduction in power supply voltages for the circuits when compared with the voltage reference signal level causes energization of a separate sensing circuit which controls the provision of energizing signals to circuits which control hammer energization.

A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of circuits in accordance with the invention; and

FIG. 2 is a graphical representation of waveforms occurring at different points in the circuit of FIG. 1.

A print hammer energizing and control circuit in accordance with the invention is described as intended for cooperation with a dot matrix printer of the type disclosed and claimed in a co-pending application for patent entitled "Printer System", Gordon B. Barrus et al, Ser. No. 495,830, filed Aug. 8, 1974, now U.S. Pat. No. 3,941,051 and assigned to the assignee of the present application. As disclosed in that patent, a hammer bank having a plurality of dot printing hammers is reciprocated laterally across a printing form while individual hammers operate at high speed in each direction of movement to imprint successive dot rows for an individual span of characters. Each hammer mechanism includes an energizing coil, erroneous operation of which could cause damage to the hammer, ribbon, paper or associated elements. As described in that application also, a magnetic pickup is used in conjunction with an index wheel which is moved in synchronism with the shuttling hammer bank, to provide positional information, regularly spaced teeth on the index wheel providing reference points with a selected periodicity, except at two regions, 180.degree. apart on the wheel, in which a wider gap demarcates the region in which the reciprocating mechanism is reversing direction, during which time the paper advance movement is effected.

The system of FIG. 1 is specifically intended for use in this particular dot matrix printer context, but those skilled in the art will recognize its applicability to other contexts as well. In FIG. 1, a source of line voltage 10, such as a typically 60 hz supply having a nominal 120 volt level, is stepped down in voltage to an approximately 30 volt peak through a transformer 12 coupled to a full wave rectifier 14, the output terminal of which is coupled to a sensing and control circuit 20. It will be appreciated that the voltage source 10 is subject to its own voltage variations, typically within the range of 10% in excess of or 15% less than the stated voltage. It is desired to provide a regulated 30 volt DC supply of reasonable precision to the coil energizing circuits for the printer, and to this end the rectified output is provided to a very large (78,000 microfarads) capacitor 16 through the sensing and control circuit 20.

The sensing and control circuit 20 includes an SCR 22 having a feedback resistor 24 coupling its gate and cathode, and a control transistor 26 coupled to its gate. A blocking diode 28 provides a return path between the opposite terminals of the SCR 22, the cathode of the blocking diode 28 providing a junction A at which the voltages from the output of the full wave rectifier 14 and the capacitor 16 combine. Conduction of the SCR 22 is determined by the conductance state of the control transistor 26, which has a feedback resistor 30 between its emitter and base, the base receiving a control voltage through a resistor 32 from a comparator 34. The minus input of the comparator 34 receives a voltage reference, here at 4.3 volts. The plus input of the comparator 34 is coupled to the junction B of a pair of voltage divider resistors 36, 38 coupled to the junction A. A resistor 40 couples the capacitor 16 to ground to provide a discharge path for the capacitor 16 when the system is turned off.

The voltage on the capacitor 16 represents the energizing voltage for a plurality of latch and driver circuits 50a, 50b . . . 50z. Each latch and driver circuit 50 includes a hammer coil 52 which is energized by the supply voltage when a series coupled switching transistor 54 is rendered conductive by an associated flip-flop 56. In the system, SET and RESET signals applied to the flip-flop 56 which is energized by a supply voltage E.sub.o, here at +5 volts, control the duration of energization of the hammer coil 52.

The emitter of the switching transistor 54 is coupled to ground, but a line from the emitter is coupled into a status checking circuit 60. The status checking circuit includes a comparator 62 which senses the current drain at the storage capacitor 16 by sensing the current through a small (.01 ohms) resistor 64 in series with the grounded terminal of the storage capacitor 16, the opposite terminals of this small resistor 64 being coupled to the separate inputs of the comparator 62. When the comparator 62 senses sufficient voltage differential across the resistor 64, it is rendered conductive. However, an SCR 66 whose control electrode is coupled to the output of the comparator 62 is rendered conductive only if a switching transistor 68 also coupled to its control electrode at a common junction with the comparator 62 is not conducting. The switching transistor 68 normally conducts because a resettable single shot multivibrator 70 is kept conducting by the recurring timing pulses provided from the index wheel in the system. Only when the timing pulses are absent for greater than a selected interval (here 600 microseconds) is the index position (the motion reversal interval occurring twice per index wheel revolution) detected. Therefore, when the resettable single shot multivibrator 70 becomes inactive, the switching transistor 68 becomes non-conductive, and the SCR 66 is fired, its anode providing a grounding signal V.sub.G for the voltage reference signal V.sub.REF supplied to the comparator 34 in the sensing and control circuit 20.

The system also includes a coil safety circuit 80 for generating the energizing signal E.sub.o for the flip-flops 56 in the latch and driver circuits 50, and this circuit operates in response to the V.sub.REF signal and the voltage of its power supply. If for any reason the power supply voltage is less than V.sub.REF, the E.sub.o signal is terminated and the hammer coils cannot be energized. To this end, a comparator 82 compares the V.sub.REF signal to a signal of normally +5 volt value from a source 84, taken through different input resistors 86, 88 respectively, to provide an output signal HMC when the voltage of source 84 is greater than 4.3 volts. The presence of the HMC signal energizes a series connected pair of transistors 90, 92 energized by the +5 volt supply, and providing the E.sub.o signal for energizing the various flip-flops 56. During power failure, startup, shutdown and other transient conditions when the voltage of source 84 drops below the 4.3 v value of V.sub.REF the HMC signal is not produced. Consequently transistors 90 and 92 are not energized and E.sub.o is not provided. In the absence of E.sub.o the various flip-flops 56 cannot be energized and the various hammer coils 52 cannot therefore be energized.

The system of FIG. 1 not only provides a properly regulated high power DC signal for energizing the hammer coils, but operates in an integrated fashion to assure that the hammer coils cannot be energized under various erroneous conditions of operation. Reference should also be made to FIG. 2, in which typical waveforms occurring at different junctions in the system are depicted for different conditions. The typical full wave rectified signal at the output of the rectifier bridge 14 is applied to the SCR 22 in the sensing and control circuit 20. Assuming the startup condition in which there is no charge on the storage capacitor 16, the combined voltage is substantially zero. However, until V.sub.REF is applied, the minus input to the comparator 34 is also substantially zero and the switching transistor 26 is not turned on to fire the SCR 22. When V.sub.REF is applied to start the system, the comparator 34 causes firing of the SCR 22 each time the combined signal at junction A as reduced in voltage at junction B increases above V.sub.REF. Once fired, the SCR 22 remains conductive until the negative-going portion of the waveform terminates in accordance with normal SCR operation. The charge on the storage capacitor 16 then begins to build toward the desired 30 volt level at a junction C, in successive increments, as shown by the corresponding waveform in FIG. 2. The combination of this signal with the full wave rectified signal at junction A gives the waveform shown in FIG. 2. At junction B, in the start mode, the SCR 22 is turned on each time the wave has a positive-going portion which begins below V.sub.REF and rises through the value of V.sub.REF. Thus, the circuit automatically provides the equivalent of a zero crossing detector, firing the SCR only as it increases above the preestablished reference level set by the charge on the storage capacitor 16, and preventing the generation of substantial RFI noise.

The startup mode continues with increasing charge being built up on the storage capacitor 16 until the charge ultimately provides the desired 30 volt supply for the hammer coils 52. At this point, the junction B is at a voltage slightly greater than 4.3 volts, and until the voltage again drops below V.sub.REF, the SCR 22 is not fired. As the hammer coils 52 are energized to drain current from the storage capacitor 16, however, the steady state component of the voltage at junction B drops down through V.sub.REF, as shown in the curve depicting the regulation mode for this point in FIG. 2, and the comparator 34 is again turned on and the SCR 22 is fired, to again charge the capacitor 16 to a higher voltage level. This operation then continues in like fashion, and the output voltage is maintained with approximately 5% regulation in the present example, on a continuous basis. Such regulation is more than adequate for a multi-hammer system, because the high peak pulses that are needed require substantial amounts of energy but do not require a high precision source.

If there is no significant charge on the storage capacitor 16, firing of the hammer coils 52 is not feasible. On the other hand, if a sufficient voltage level is present, the hammer coils need be protected only during the turnaround interval in which the paper may be undergoing vertical movement relative to the print hammers. The status checking circuit 60 insures against erroneous operation by interrelating these factors with control of the reference voltage V.sub.REF. Current from the charged storage capacitor 16, if the capacitor is being discharged, generates a small current in the low value resistor 64 in the status checking circuit 60, turning on the comparator 62. Except during the index interval, in which the resettable single shot multivibrator 70 is turned off, however, the switching transistor 68 is turned on, so that the output of the comparator 62 is effectively grounded and the SCR 66 is non-conductive. During the index interval the output of the comparator 62 fires the SCR 66, which effectively couples V.sub.REF at the input to the sensing and control circuit 20 to ground, preventing the comparator 34 from conducting and preventing the SCR 22 from firing to charge the capacitor 16.

In the coil safety circuit 80, any one of the conditions that can cause the power supply signal to be less than desired is used to prevent firing of the hammer coils 52 by disabling the flip-flops 56. The power supply signal at terminal 84 will be below 4.3 volts in the event of power failure, startup or shutdown. In each such eventuality, the comparator 82 output goes high, shutting off the normally conducting transistors 90, 92, and terminating the E.sub.o signal required to activate the flip-flops 56 and to turn the switching transistors 54 on.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.