US 3909681 A
This disclosure relates to a driving circuit for driving a coil of an electromagnet for the actuation of a hammer in a high speed impact printer of data handling systems, which driving circuit includes first switching means upstream of the coil, second switching means downstream of the coil, current sensing means for sensing current in the coil, a closed unidirectional current path including the second switching means, the coil in a diode, first control circuit means for actuating the first switching means in response to an input control signal, second control circuit means for switching on the second switching means in response to the first input control signal and for the entire duration of the input control signal, and a bistable device triggered by the current sensing means to assume one of two electrical states when the sensed current reaches a predetermined value and to provide an output signal corresponding to that one state, the output signal being effective to switch off the first switching means even in the presence of the input control signal.
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Description (OCR text may contain errors)
United States Patent Campari et al.
1 1 Sept. 30, 1975 1 DRIVING CIRCUIT FOR PRINTING ELECTROMAGNET  Inventors: Alfredo Campari; Giorgio Vigini,
both of Milan, Italy  Assignee: Honeywell Information Systems Italia, Caluso, Italy  Filed: Nov. 19, 1974 ] Appl. No.: 525,220
 Foreign Application Priority' Data Nov. 28, 1973 Italy 31771/73  US. Cl 317/1485 R; 3l7/DIG. 4; 317/31; 317/33 R  Int. Cl.'- H0111 47/32; HOIF 7/18  Field of Search 317/123, 148.5 R, 148.5 B, 317/149, DIG. 4, 31, 33
 References Cited UNITED STATES PATENTS 3,205,412 9/1965 Winston 317/D1G. 4 3.206.651 '9/1965 Proulxum 317/DIG. 4 3,235,775 2/1966 Winston..... 3l7/DIG. 4 3.549955 12/1970 Paine 3l7/D1G. 4 3,582,734 6/1971 Bryden 3l7/D1G. 4
3.859.572 1/1975 Keidl et al. .1 317/D1G. 4
Prinmry limminer-L. T. Hix Atlorney, Agent, or Firm-Fred Jacob  ABSTRACT This disclosure relates to a driving circuit for driving a coil of an electromagnet for the actuation of a hammer in a high speed impact printer of data handling systems, which driving circuit includes first switching means upstream of the coil, second switching means downstream of the coil, current sensing means for sensing current in the coil, a closed unidirectional current path including the second switching means, the coil in a diode, first control circuit means for actuating the first switching means in response to an input con trol signal, second control circuit means for switching on the second switching means in response to the first input control signal and for the entire duration of the input control signal, and a bistable device triggered by the current sensing means to assume one of two electrical states when the sensed current reaches a predetcrmincd value and to provide an output signal corresponding to that one state, the output signal being effective to switch off the first switching means even in the presence of the input control signal.
6 Claims, 3 Drawing Figures US. Patant Sept. 30,1975 Shet 1 of2 3,909,681
Sept. 30,1975 Sheet 2 of 2 3,909,681
US. Patent DRIVING CIRCUIT FOR PRINTING ELECTROMAGNET The present invention deals with driving circuits for high speed electromagnets, such as those used for hammer actuation in high speed impact printers for data handling systems.
Such driving circuits must fulfill particular requirements. Thus, they must provide fast magnetization of the electromagnet, they must have repeatability with a limited spread of the kinetic energy imparted to the printing hammer in the various and subsequent energi zations, as well as during the entire actuation time of the latter.
Among the various-solutions used to achieve such results, there are voltage controlled energization systems "which use a first higher voltage for energization and a advantages. In particular, the circuit according to the invention:
a. provides magnetomotive force sufficient for magnetic core saturation and which assures the appropriate attracting force;
[1. reduces to a minimum extent the power drained from the power supply and uses to a maximum extent the energy stored in the electromagnet induc tance;
0. enables the electromagnet to be driven by an unstabilized voltage supply;
d. reduces the risk of catastrophic damageand may be kept under voltage supply even in case of failure and has self diagnostic capability.
The driving circuit according to the invention includes in series connection a first and a second current switch respectively upstream and downstream of the electromagnet winding or coil and a unidirectional short circuit path for the coil, in which path the second switch is included. The invention further includes a current detector, a bistable circuit and a control circuit which opens the first switch when the current flowing in the coil reaches a preestablished value.
According to another aspect of the invention the circuit additionally includes a second current detector associated with the bistable circuit to detect current flowing through the first switch and to open it when such current exceeds a safety limit.
According to still another aspect of the invention, the circuit additionally includes a voltage detector to provide an indication of the voltage existing at a coil terminal, the voltage detector and the bistable circuit providing logical information which allows one to check the correct circuit operation and, in case of failure, to identify faults in the circuit components. '9 Y 9 These and other features of theinvention will appear more clearly from the following description of a preferred embodiment and from the attached drawings wherein:
FIG. 1 is a block diagram of a driving circuit according to the invention. I
'- FIG. 2 shows a wiring schematic of a preferred embodiment of the driving circuit according to the invention.
. FIG. 3 is a timing diagram of the operation of the circuit according to the inventiom Referring now to FIG. 1, the circuit according to the invention is shown in block diagram form.
The coil 1 of a printing'electromagnet is connected between a voltage source V and a reference point or ground, through two switches 2 and 3, preferably of the electronic type respectively placed upstream and downstream'of the coil, and. connected respectively to the winding terminals 4 and 5.
A diode 6 is connected between ground and terminal 4 and is conducting, in relation to the voltage supply, which by example has been chosen positive, from ground towards terminal 4.
A current detector 7 is interposed between switch 3 and ground and has an output which controls a bistable device 8. The bistable device 8 has-a second control input in the form of an output 10 of a voltage detecting circuit 9, which provides a signal indicative of the voltage existing at terminal 5.
The bistable device 8 is set to a first electrical state,
designated set" when the current detected by detector 7 reaches a preestablished value, and is reset to a second electrical state when the voltage at terminal 5 reaches or exceeds a preestablished value detected by circuit 9. The two switches 2 and are controlled by control circuits 11 and 12, respectively.
When a suitable electrical signal is applied to an input terminal 15 of the driving circuit, wires 13 and 16 transfer such signal to the inputs of the control circuits 11,12 which operate to close the switches 2 and 3. Switch 3 remains closed for the whole time during which the electrical control signal is present at input 15. However, switch 2 remains closed for a shorter time. This is due to the fact that output 14 of the bistable device 8 is connected to a second input of the control circuit 11.
As soon as the current flowing in coil 1 reaches a preestablishd value, which is chosen as being adequate to saturate the magnetic core of the coil 1, the bistable device 8 issues a signal which is applied to control circuit l1 and commands the opening of switch 2. At this point the currentflowing in the coil 1 continues to flow in the closed path formed by diode 6 and switch 3 and decreases at an exponential rate which depends on the time constant of the circuit. When the control signal applied to input 15 is removed and switch 3 is opened, the overvoltage due to the opening is detected at terminal 5 by-circuit 9 and bistable device 8 is reset. Thereafter, a new operating cycle of the electromagnet may start.
It is clear from the foregoing that the electromagnet is energized by an initial voltage which may be chosen so high as tomagnetize and saturate the circuit in a very short time.
'As soon as magnetizationis achieved, no more energy'is supplied to the circuit.
If a suitably high voltage supply is used, saturation of the magnetic circuit is achieved before the customary electromagnet keeper accomplishes a substantial displacementfrom its rest position, so that the energy stored in the air gap may be converted nearly in its entiretyintoakinetic.energyof the keeper. On theother hand, it is known that in a magnetic circuit'provided with air gap, the energy'storedin the gap is'the most relevant fraction of the magnetization energy supplied to the whole circuit. Therefore, the driving circuit accordingto the invention provides a rational utilization of the energy supplied to the electromagnet. In addition, since the driving-circuit is controlled by a quantity (current) relatedto theimagnetization status of the coil, it is largely insensitive to changed in the voltage vSwitch 2 is now specifically disclosed as a transistor having an emitter connected to terminal 4 of coil 1. A:
collector of the trnasistor switch 2 is connected to a voltage source +V,, having a rather'high value (for instance 45-V),' through a resistor 116 which has a very.
low value (for instance 0.1
Switch 3-is now specifically disclosed as a transistor having its collector connected to terminal 5 of coil 1 and its emitter connected to ground through a resistor 17 which has a very low value (for instance 02 G).
Control circuit 1 1, which controls transistor switch 2, comprisestransistors 23,24, pulse transformer 25 and certain resistors and diodes to be described hereinafter. Transistor 24 has an emitter connected to ground, while itsbaseis connected to the input terminal through a diode 28, whose conducting direction is from the-input to the base. ln'addition, the base" of transistor 24' is connected toa second control terminal 31 through a resistor 30.
When, at least one ofthe'control terminals 15 and 31 is at a positive voltage, transistor 24 is conducting and its collector is'at voltage a very close to ground. Thus, transistor 23, the base of which is connected-to the collector'of'transis'tor 24, is off and no current flows in the primary winding of pulse transformer 25. When both the terminals 15 and 13 are at a'voltage close to'ground (or negative) transistor 24 isoff and transistor 23 is cond ucting. Therefore, transformer 25' is fed by avoltage pulse, which is transferred to the sec-" ondary winding and commands the switchingon of transistor switch 2. The size of transformer 25 is chosen so as to avoid saturation for the maximum fo'reseable duration of the control pulses. v
The collector current flowing through resistor 21 causes a'voltag'e drop'across the resistor, so'that the collector volt'age rises. Thus, the base emitter junction of transistor switch 3, is forward biased to render the switch conductive. Transistor switch'3 remains conductive for the entire duration of the control pulse;
Transistor 23 has itscollector connected to a suitable voltagesource V through the primary winding of the transformer 25 and a current limiting resistor 26. The emitter of transistor 23 is connected to ground. The terminals of the secondary winding of the transformer 25 are connected respectively to the base and Further, the probability of catastrophic failure is reto the emitter of switching transistor 2. The base of Thevoltage drop produced by the current across the resistor 17 (which current, byneglecti'ng the negligible base current of transistor switch 3,'is also the current flowing in the coil) provides a voltage indication proportional to such current.
In order to assure the demagnetization of the transformer 25, a Zener diode 32, connected in opposition to a diode 33,. provides a unidirectional short circuit path.
A. control circuit 12, which controls transistor switch 3, includes a transistor 18, for instance of PNP type,
t and resistors 19, 20, 21, 22. A control terminal .15 is connected to the base of transistor18 through resistor 22. The emitter of transistor l8is connected to a voltage source V and the collector thereof is connected both to the base of transistor 103, through resistor 2 and to ground, throughresistor 21. i
Normally the input control terminal 15 is kept at a positive voltage so that transistor 18 isnon-conductive and-the collector of transistor 18 is at ground potential Therefore, transistor switch 3 too is non-conductive. Upon the application of a control pulse to the input terminal 15, the potential'of the latter approaches ground potential, so that the base-emitter junction of transistor 18 becomes forward biased and transistor 18 becomes conductive.
The the use of a pair of switches, enables one to provide the driving circuit with diagnostic circuit devices in order to detectand toidentify the. faults of the same.
duced due to the fact that the probability of a multiple failure of both switches is lower than the probability of a simple failure of a switch.
Control terminal 31 is driven by the bistable circuit 8. In a preferred embodiment thebistable circuit 8' includes an amplifier 34, a transistor 35 and certain diodes and resistors. The inverting input of the amplifier 34 is connected to the emitter of transistor switch 3 through resistor 36 and receives a voltage signal proportional'to the current flowing in coil 1. The noninverting input of the amplifier 34 is connected on one side to a voltage source +V through resistor 37 and on the other side to ground, through resistor 38.-ln addition the non-inverting input of amplifier 34 is connected to the output of the amplifier through resistor 39 and diode 40. Diode 40 has a conductive direction from the input to the output of the amplifier.
When transistor switch 3 is open, the inverting input of amplifier 34 is at ground potential. Resistors'37 and 38 are chosen of a value so as to apply to the noninverting input a voltage that is slightly positive. The output of the amplifier 34 is therefore at a positive level and diode 40 is reverse biased.
When transistor switch 3 is closed and current flows in the coil 1 and in resistor 17, the voltage drop across resistor 17 tends to raise the voltage applied to the inverting input of the amplifier 34. For a suitable value of the current flowing in the'coil l and corresponding to the saturation current forthe coil,'the voltage applied to the inverting input exceeds the voltage applied to the non-inverting input and the output-voltage becomes negative. Diode 40v becomes conductive and lowers the voltage applied to the non-inverting input, causing a positive feed-back which saturates the amplifier 34 and maintains it in that state even when the current in resistor 17 goes to zero and the voltage'applied tofth'e inverting input goes to ground.
The output of amplifier 34 is connected through resistor 41 to the base of transistor 35, whose collector is connected to the voltage source through resistor 43 and whose emitter is connected to, ground. A diode 44 between the base of transistor 35 and ground, the
direction of conduction of which from ground to.base,'
prevents the application .of too high a reverse bias to the base of transistor 35. The collector of transistor is further connected to the input terminal 31. When the, voltage at the output of amplifier34 is positive, transisrespective of the voltage applied to terminal 15, transistor 24 is kept in its conductive state and transistor 23 and transistor switch 2 are switched off. Therefore, as
soon as the current flowing in the electromagnet, of
which coil 1 is a part, reaches a predetermined value suitable to saturate the magnetic core, the abovedescribed circuits intervene to cause the switching off of transistor switch2 to be switched'off and to remain non-conductive in its stable condition.
However, the current flowing in coil 1 is not switched off and the self-induced electromotive force in the circuit sustains the current which may flow through diode 6. Only at the termination of the control-pulse applied to terminal '15, when transistor switch 3 is opened, is the residual current flowing in the coil 1 switched off. The self, induced voltage caused by this switching action is used to reset the bistable circuit 8. To this purpose, such voltage is appliedthrough a voltage divider formed by series, connected resistors 46,47 (which are connected on one side of terminal 5 and on the other side to ground) and through diode 45 to the noninverting input of amplifier 34. Such voltage is sufficiently high to bring the potential of the non-inverting input to a potential higher than the one applied to the inverting input so that the output of the amplifier 34 becomes positive. In order to limit the self-induced voltage which is generated by transistor switch 3 switching off, terminal 5 is connected to the voltage source V through diode 42 so that the self-induced voltage cannot exceed the value of voltage source V The remainder of the driving circuit consists of protective devices. Maximum current protection is achieved by means of resistor 1 l6, transistor 48 and resistor 49. Transistor 48 (of PNP type) has its emitter connected to the voltage source V and its collector connected to the inverting input of amplifier 34, through resistor 49. Its base is connected to the collector of transistor switch 2.
If, for some reason the current flowing in resistor 116 exceeds a predetermined value, transistor 48, normally open, starts conducting and applies to the inverting input of the amplifier 34 a positive voltage which causes the intervention of bistable circuit 8 and the switching off of transistor switch 2.
Some examples of causes of failure to bring about the foregoing action are as follows:
a. short circuit of diode 6.
b. short circuit of coil 1.
c. transistor switch 2 emitter grounded.
A further device which provides indirect protection consists of a voltage detector connected to terminal 4 of the coil 1. Such a circuit may consist of a voltage dividerincluding resistors 50, 51 and a threshold circuit 52 having an input connectedto such voltage divider so asto provide an output signal of a first level when the input voltage is lower than, a threshold voltage, and an output signal of a second level when the input voltage is higher than the threshold voltage. The threshold circuit 52, as well'as amplifier 34 are conventional circuits which are commerciallyavailable as integrated devices and therefore they need not be described in detail.
An output terminal E of the threshold device 52 provides'asignal which, jointly with the signal present at the collector of transistor 35 (terminal D), provides a heretofore unavailable degree of diagnostic power for detecting failures and for fast tion circuit. 7
FIG. 3 is a timing diagram showing the operation of the driving circuits and enables one to evidence the utility of signals present at terminals D, E. In the diagram, the waveform designated by the reference numeral 15) represents the control voltage applied to input terminal 15. The waveforms designated D) and E)'represent the signals at' terminals D and E respectively, while I) represents the current flowing in coil 1.
At the beginning, theinput control signal is at a positive value, no current is flowing in the coil 1 and terminals and D are at' zero voltage. At instant t the input control signal is lowered to zero level. At instant t,, with a delay which depends on the intervention time of the control circuits, transistor switches 2 and 3 start conducting and current starts flowing in coil 1. There fore, at instant t the signal at terminal E rises to a positive level. When the current I) reaches a predetermined value (instantt the bistable circuit 8 is set and the signal at terminal-D'rises to a positive level. Therefore, transistor switch 2 is cntrolled to switch off, such action occurring with some delay at instant 1 At instant t the voltage at terminal 4 decreases and the signal at terminal E goes to zero level.
With a predetermined delay from T at 1 the input control signal israised again to a positive level so as to command the-opening of transistor switch 3. This occurs with some delay starting from instant t in which the self-induced voltage caused by the circuit opening resets the bistable"v circuit 8, lowering to zero level the signal at terminal D.
In normal operation, at instant t the voltage at terminals ED is zero, while at instant t the voltage at D is positive and the voltage at E is zero. Voltage levels different from the ones indicated above and detected at instants t t, are indicative of failures.
Among the serious failures which may occur are the following:
a. Short circuit of transistor switch 2. It causes the terminal 4 to remain in tension, so that at instant! signal E remains at positive level.
b. Open steady state of transistor switch 2. It prevents the energization of the coil 1, so that the bistable circuit 8 is not triggered at instant t At instant 1 signal D is still at zero level.
0. Short circuit of diode 6, 0r transistor switch 2 emitter grounded. It causes, as already seen, the intervention of the protection circuit and the set of bistable circuit 8. On the other hand there is no current flowing in the coil 1 so that bistable circuit 8 is not reset. Therefore at instant t of a subsequent activation of the protec- 7 input control pulse'the voltage level at terminal D will be. positive instead of zero.
d. Transistor switch 3 steadily in off state. No current can flow in the coil 1: the bistable circuit 8 is not set and at instant t terminal D is still at electrical "levelzero. v
From 'the foregoing description, it will be clear that the circuit described, which forms the subject matter of applicants invention, provides a number of advantages, as discussed above. Specifically, the invention provides greater efficiency, reliability-and safety than heretofore available in comparable circuits, as well as in improved ability to diagnose failures.
The driving circuit described constitutes a preferred form of embodiment of the invention. Numerous changes, modifications and substitutions will now occur to those skilled in the art, all of which fall within the spirit and scope of the invention, as, defined by the ating said first switching means in response to an input,
control signal; second control circuit means connected to said second switching means for switching on said second switching means in response to said input control signal and for the entire duration of said input control signal; a bistable deviceconnected to and triggered by said current sensing means to assume one of two electrical states when the sensed current reaches a preestablished value and to provide an output signal corresponding to said one state, said output signal being connected to said first control circuit means and effective to switch off said first switching means even in the presence of said-input control signal.
2. Driv.ing circuit as claimed in claim 1 further comprisingovervoltage detecting means to detect an overvoltage due to the switching off of said unidirectional current path and to provide an overvoltage signal, said overvoltage signal being applied to said bistable device, said bistable device being reset in that one of the electrical states other than said one by said overvoltage signal.
3. Driving circuit as claimed in claim 1 wherein there is a voltage detector connected to said coil between said coil and said first switching means.
4. Driving circuit as claimed in claim 3 wherein there is voltage signal means connected to and operated in response to the condition of said bistable device for diagnosing element failure.
5. Driving circuit as claimed in claim 1 wherein there is voltage signal means connected to and operated in response to the condition of said bistable device for diagnosing element failure.
6. Driving circuit as claimed in claim 1 wherein said first control circuit means includes a transformer having a primary winding, and demagnetization means for said primary winding including means defining an unidirectional short circuit'path.