US3268810A - Electronic tachometer utilizing tuned signal transducer - Google Patents

Description

ELECTRONIC TACHOMETER UTILIZING TUNED SIGNAL TRANSDUCER Filed Nov. 22. 1960 R. L. REINER 3 Sheets-Sheet 1 Aug. 23, 1966 R. REINER 3,268,810

ELECTRONIC TACHOMETER UTILIZING TUNED SIGNAL TRANSDUCER "mn-21PM United States Patent O 3,268,816 ELECTRNIC TACHMETER U'HLEZING TUNED SHGNAL TRANSDUCER Robert L. Reiner, West Caldwell, Nd. (Glenwood Court, Bethany, Conn.) Filed Nov. 22, 1960, Ser. No. 70,991 6 Claims. (Cl. 324-70) My invention relates to tachometers, particularly to electronic tachometers, and has for its primary object to provide an improved electronic circuit for measuring the number of revolutions per minute of ignition systems ignited by spark plugs.

The need for a tachometer which would not require special calibration or knowledge of the number of cylinders in an engine when used with various types of ignition systems and particularly with respect to two cycle or four cycle ignition systems has resulted in the present invention which samples an electrical condition common to all ignition systems. My invention is unique in that I take advantage of the low impedance produced during the arc discharge to drive the signal impulse and thereby actuate or trigger the subsequent circuitry to result in a reading proportional to r.p.m. I, therefore, use the low impedance of the arc which is a reasonably constant parameter common to all ignition systems. Also, l sense this parameter electromagnetically through only one spark plug lead of the total number of spark plug leads required in an ignition system, so that the number of cylinders in the ignition system is immaterial.

In order to utilize this arc discharge for all types of ignition systems, including magneto, battery and electronic, it is necessary to sense the electrical force which sustains the arc. In this invention it is the sustaining current that maintains the arc, which is utilized as a signal source at a sensing transducer. In addition to the current associated with the arc or spark there is always an associated rapid rise or fall of potential. Consequently, to insure overall system stability it is necessary to isolate potential changes generated from other spark plugs in order to prevent their appearance as signals, and thereby when calibrated for two cycle operation enable my tachometer to be used on all two cycle engines regardless of the number of cylinders and, similarly, when calibrated for four cycle engines. This is important, since among the various ignition systems and during normal operation of a gasoline engine this change in potential varies over very wide limits, such that it would be impractical to try to use it as a means of signalling from one spark plug. Further, I avoid sensing potential changes at the spark plug wire which is necessarily done at a very high impedance level and which is consequently very sensitive to stray electrostatic pick-up particularly from other spark plugs or other ignition systems which may be nearby.

Also, when sensing electrostatic changes as a signalling means in conventional tachometers, capacitive coupling is usually used to produce a driving signal. In practice, any coupling capacitance in the signal circuit has been found to contribute to unsteady meter indication because of charging and discharging time constants. This occurs primarily during transient periods, either during changes in r.p.m. or loading on the gasoline engine. However, when sensing the current, as in my invention capacitive coupling is eliminated. My arrangement is eiective during the arc discharge which is during a short circuited condition of the ignition system and, therefore, during a very low driving point impedance interval. The advantage of such sensing is negligible susceptibility to stray electrostatic pick-up and, by proper design of the sensing transducer, negligible loading at the ignition system. The transducer, which senses these current impulses at a spark plug and produces output signal pulses directly proportional to the r.p.m. of the engine irrespective of the number of cylinders, is the most important component in the entire tachometer system. It was necessary in the design of the transducer to isolate the sensing coil from the signal coil, so that current produced at the signal coil by the current ilowing through the sensing coil during an arc at the spark plug would dominate all other system noise, particularly, electrostatic.

It is therefore a further object of my invention to provide an improved tachometer that is unaffected by potential variations in an ignition system tired by spark plugs.

It is another object 0f my invention to provide an improved tachometer that is stable over a Wide range of temperatures and that requires only one calibration when used with either two cycle or four cycle ignition systems.

It is still another object of my invention to provide an improved electronic tachometer that is simple in construction, reliable in operation and is economically manufactured.

It is a further object of my invention to provide an irnproved tachometer that maintains calibration accuracy over a wide input voltage swing.

Other and further objects will be obvious upon an understanding of the illustrative embodiment Vabout to be described, or will be indicated in the appended claims, and Various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

In accordance with the present invention the foregoing objects are generally accomplished by providing an irnproved universal tachometer comprising a transistorized voltage pulse counter which is stabilized and regulated electrically, and a transducer which senses current pulses in a spark plug lead of a gasoline engine. The transducer, which may be located in the engine compartment and connected directly to the spark plug wire, senses each time the spark plug fires, advantage being taken of the low impedance produced during the arc discharge, and transmits a signal pulse by a shielded cable to the counter, which may be located on a dashboard of a vehicle or bulkhead of a boat and which converts these pulses into a steady meter reading with a linear scale calibrated in r.p.m.

A more thorough understanding of my invention may be obtained from a study of the following description of several specific embodiments. In the drawings:

FIG. l is a schematic diagram showing a signal transducer of my invention connected to a typical ignition system tired by a plurality of spark plugs.

FIG. 2 illustrates typical wave forms of an ignition system supplying a multi-cylinder engine through a distributor having a primary driving force of coil, magneto or electronic.

F'IG. 3 illustrates typical wave forms of output voltage lat the 'output :terminals of fthe signal transducer.

FIG. 4 is a schematic dnawing showing la counter circuit and indicating circuit of my invention, adapted tto be connected to the signal mnansducer of FIG. 1 without capacitive coupling.

FIG. 5 is la :schematic `diagram -of a composite tachometer circuit including complete signal transducer, eonnter and indicating circuits in its simplest form.

Referring now Ito fthe idnawings, panticularly tno IFIG. l, there shown a convencional ignition system having a storage battery 20, primary coil 21, secondary coil 22, breaker points 23 with timer-cam 24, which napidly makes and lbreaks the circuit between the breaker points 23, `and la condenser 26 which shunts the lbreaker points 23. A lead 27 connects ia voltage potential fnom the secondary coil 22 tto la notary larm 28 of la distributor 29 Iwhich includes terminals 3*] 'over which the contact farm 28 rides, as is well known. Each of the terminals 31 is connected 3 to la spark plug I32 for each cylinder (not shown), so that 'the ignition system shown is for a four cylinder engine.

In the present invention connection lof ya signal rtransfducer 3'3 is readily made to the ignition system either at the spark plug or the distributor, -or by cutting the high 'voltage wire 27 along its length, so that the current owing in fthe wire, when the spark plug 62 tires, will iiow through .the signal transducer 3'3. As seen in iFlG. 1, the signal transducer 33 comprises a transformer 34 having a primary Winding or sensing coil 66, bridged by xa condenser 37 (which provides ra low impedance), and ra secondary winding lor signal coil 38. The primary winding or sensing coil 36 is also bridged respectively to distributtor terminal 31 :and `spark plug 32, say for lthe ,first cylinder of the engine, so that the signal transducer 33 is serially interposed between the distributor rand the spark plug.

In order to minimize electrostatic impulses at the output of the transducer I33, 'the sensing coil 36 and the signal coil 38 are so constructed that the sensing coil 36 is physically separated from the signal coil 38 lby sufficient physical distance :to keep the electrostatic coupling to ra very low value. This separation also helps prevent voltage breakdown. In a prototype of my invention the core material was constituted by 'an inexpensive ferrite rod (not shown) approximately three inches long; however, iron core material has been found to Ioperate satisfactorily. In order to further reduce electrostatic sensitivity, the condenser 37 is bridged across the sensing coil `36, whereby electrostatic charging currents liowing between the spark plug 32 and ground are `by-passed and cannot be confused as a true signal impulse. This condenser 37, during the low impedance arc discharge, has little shunt-ing effect of the current produced thereby 'and a strong current signal is Iltherefore magnetically coupled between the sensing winding 36 and the signal winding 38 of the signal transducer 33. In practice the signal transducer 33 is remote from the rest of the tachometer circuitry, since it is convendent to make connections at the distributor lor spark plug lead Within the engine cowl and locate the tachomete-r indicator and circuitry on the dashboard for other par-t of the boat or automobile, etc. The signal from the tra-nsducer 33 is connected by -a shielded cable lor twisted pair comprising leads 39 and 40 to a counter circuit 41, FIG. 4, in order to minimize electrostatic noise. It might be mentioned at this point that my tachometer comprises fou-r distinct parts-a signal transducer 33; an interconnecting cable comprising leads 39 and 40; a counter circuit 41; and a visu-al indicating circuit 42.

Referring now to IF IG. 4, the counter circuit 41 is shown as being based upon the properties of a driven blocking oscillator in which a transistor 43 is used las its active element rather than la vacuum tube. The principle by which n controlled pulse is produced is generally lnnown by those versed 'in the art and can be briefly described as follows: transistor 43 |has la base b, which is .a control electrode corresponding to 'the grid of ya vacuum tube, 'an emitter e which corresponds to the cathode and la collector c which corresponds Ito the plate of a vacuum tube. These elements form, in effect, switching contacts such that for an NPN transistor a positive current flow between fthe base b and the emitter e causes the collector-emitter contact to close allowing .a large signal to ilow. Similarly, the Lack of a positive current flow between base b and emitter e causes the collector-emitter contact to open, whereby no signal current can flow. This, of course, is the ideal case. in practice, however, the transistor A43 is never fully an cpen circuit nor fully a short circuit. However, [by driving the base b with ra large positive current the transistor 43 can be made to 'approach la short circuit; rand `by applying a reverse current between base b `and emitter e, in other words making the base b negative with respect to fthe emitter e, the transistor 43 can be made to approach an open circuit.

In this counter circuit 41, a diode `44, preferably of 4 silicon, is used lto provide the reverse bias. Initially no current iiows from the positive terminal `of battery 20 through .the indicating circuit 42 and winding 46 of transformer 47, since the transistor 43 is assumed open. If a -sma-ll positive current is now made to iiow, say from the transducer 33 through the base b of transistor 43, a voltage drop will be produced across 'the winding 46 of transformer 47, because the collector c is being driven towards the emitter e by the small current. This drop is coupled to winding 48 by transformer notion =and the drop across winding 48 is in such -a direction as to reinforce the original positive current iiow through the transistor base b, thereby making the current cumulative. Transistor 413 is then rapidly driven to saturation, so that the collector emitter contacts can now be considered closed. The full battery voltage .less the drop across diode 44 and condenser 45 now appears across winding 46 of transformer 47, thus causing a magnetizing current to iiow, which current is proportional to the primary inductance of the transformer 47. The vlengt-h of time that the transistor 43 remains saturated or closed is determined by the time that the magnetizing current continues Ito build up in the core yof transforme-r 47. This, then, is the length of time for a unit energy impulse to flow through the indicating circuit 42. It can be shown that 'variations in battery supply volt-age will shorten lor lengthen the time of this impulse resulting in la constant energy situation regardless of battery supply voltage.

After this forward pulse of energy terminates, because of saturation of lthe core transformer 47, an amount of power, proportional to the volume of the core material of transformer 47, is released and produces a potential drop across both windings 46 and 48 of wha-tever magnitude is necessary to cause a discharging current to flow. This is the common inductive kick principle associated with the discharge of any magnetic core and, in all blocking oscillator circuits, can develop high voltages across the transistor 43. If these voltages are no-t properly controlled or limited, the transistor 43 can be ruined. Consequently, protection lof transistor 43 is aorded through diodes 49 and 50, the first of which limits the reverse polarity of winding 48 t-o a few volts and the second of which provides an additional discharge path, as will be described hereinafter. A smal-l resistor 51, not usually required, can be included yas shown to speed discharge of the magnetic core and its subsequent reset to allow the cycle to repeat. Diode 49, in addition to providing transistor protection, has a primary function `of transferring positive signal impulses from the signal winding 38 of transducer 33 to the base b of transistor 43 through a current limiting resistor 52. Once a strong enough positive signal is supplied from the signal Winding 38 of the transducer 33 through diode 49 to the base b circuit of transistor 43, regenerative action takes place for driving transistor 43 full on. The upper end, as viewed in FIG. 4, of winding 48 is driven several volts positive Ito insure su'icient base b current flow in transistor 43. Diode 49 is then also used to decouple the signa-l winding 38 of the transducer 33, so that current produced by ythe positive drop across winding 48 flows through the base b circuit of transistor 43 rather than across the signal winding 3S of the transducer 33.

After a unit energy impulse has terminated, diode 49, as explained hereinbefore, and diode 50 are used to complete discharge paths for Athe energy lcontained in the core of transformer 47. The rst discharge path may be traced from winding 48 of transformer 47 via lead 40, through winding 38 of transducer 33, over lead 39, diode 49, resistor 51 and back through Winding 48. The other discharge path is from winding 48, over lead 40, diode 50 and` thence Via resistance 51 back through winding 48. In general, negative potential can be considered ground or earth; however, for the system to operate properly either battery polarity may be grounded and, if more convenient, the entire tachometer circuit can be left oating.

The indicating circuit 42 comprises a direct current sensing ammeter 53 and an integrating network of sufciently long time constant to smooth a repetition of unit energy impulses into a steady direct current flow through the ammeter 53. Many configurations for producing this are possible, only one of which is shown and which comprises a resistor 54 in series with lthe ammeter 53 and a Variable resistor 56 and condenser 45 each in parallel with said ammeter 53 and resistor 54.

The operation of the circuit will now be considered. Each time upon discharge of the selected spark plug (No. 1) a signal is produced which comprises current ow through the vsignal transducer sensing winding 36 with a corresponding Voltage drop being produced therein and also in the signal winding 38. The signal is then coupled via leads 39 and 40 to the counter circuit 41, as described earlier, resulting in a unit of energy iiow through the indicating circuit 42. The initial polarity of the Voltage drop at the signal winding 38, because of the direction of the current flow ithrough the sensing winding 36, is unimportant, since transformer 34 and condenser 37 form a high Q (quality) ringing circuit which helps the normally oscillatory arc current. Assuming that the initial signal applied to diode 49 is a negative going signal, it will be blocked by diode action of diode 49 and no appreci- `able current will flow until the signal oscillates to a positive going polarity which is proper signal polarity for triggering the counter circuit 41 into conduction. Condenser 37 forms a low impedance path, as described earlier, to by-pass electrostatic changing currents, so that a -drop caused thereby does not appreciably appear across the sensing winding 36 of the transducer 33. Because in a practical signal transducer and associated cable electrostatic voltage transients generated by other spark plugs in the ignition system `cannot be entirely eliminated, an unwanted signal is coupled through diode 49 to tnansistor 43 and this unwanted signal must be kept below a minimu-m threshold, as seen in FIG. 3, so that it cannot trigger the counter circuit into conduction. This is important, since such signals are not of consistently high amplitude and cannot be included in the calibration of even a single ignition system.

In this connection, condenser 57 with resistor' 52 forms an integrating network of such time constant that these unwanted signals are reduced, as seen in FIG. 3, below the discrimination level afforded by diode 44 and therefore cannot trigger the circuit. Resistor 52 serves two additional purposes. It limits the peak positive drive current through the base b of the transistor 43, as explained earlier, and also with lresistor 51 provides a collector circuit load impedance which is ree-cted into winding 46 by transformer action. As long as resistor 52 is made large with respect to the variations in the base b resistance of transistor 43 over a specific operating temperture range, the stability and accuracy of the entire circuit can be predictably controlled and no special compensation is required. In general, a value for resistor 52 and, by proper choice of primary inductance of transformer 47, pulse width can be selected to produce optimal transistor base drive and collector load, so that a practical meter operating current is produced. In general this may be one milli-ampere for full scale deection to reiiect a two cycle or four cycle r.p.m. rate of perhaps 6,000 or 8,000 resulting in a signal repetition rate from 3,000 to 8,000 pulses per second for full scale readings. After resistor 52 is chosen, -condenser 57 is picked to produce the desired intergrating time constant.

For each signal of positive going polarity applied to the transistor 43, closure of the latter sends a similar pulse through the indicating circuit 42. Each such pulse may be traced from positive potential of battery over leads 59, 61, via meter 53, resistor 54, lead 62, winding 46, lead 63, closed collector-emitter Contact, lead 64, diode 44, and thence back over leads 66, 40 and 67 to negative terminal of battery 20. Thus the series `of pulses produced by the spark plug and illustrated in FIG. 2, are translated by the transducer 34 into a series of closures of the transistor 43, which in turn produces a series of repetitive pulses that, as explained hereinbefore, are integrated into a steady reading at the meter 53, which reading is calibrated by resistance 56 into corresponding r.p.m. of the engine. To produce suiiicient current flow through the diode 44 in the above-traced circuit, such that its inherent voltage drop of about 0.7 volt for a silicon device is achieved, a resistor 65 is connected at one end to the positive side of the battery 20 and at its other end to lead 64 between the emitter e contact and diode 44. If this resistor 65 is eliminated, the voltage drop across diode 44 because of leakage current flow in transistor 43 may not be sufficient to reverse bias the transistor at elevated temperatures with the consequence that the circuit will -become a free running oscillator at these elevated temperatures.

In order to provide economical battery supply operation the function of the diode 44 of FIG. 4 can be replaced by a single battery cell 68, as seen in FIG. 5, and the resistor 65 of FIG. 4 eliminated. It is necessary that the battery 68 have a fairly constant voltage drop over its entire operating life, since variations in the bias voltage will directly affect accuracy. The current demand from the battery supply 68 during stand-by will not exceed the very small leakage current ilow in transistor 43.

It is practical to operate my tachometer from an external battery having very low internal impedance, such as a 6 volt or 12 volt lead-acid storage battery 20. If dry cell batteries 69, as seen in FIG. 5, are employed for the power supply, a shunting condenser 71 is provided in order to insure the necessary low supply impedance, so that calibration accuracy can be maintained throughout the discharge life of the battery 69.

Referring again to FIG. 5, which illustrates simplified counter and indicating circuits 41 and 42', respectively, combined with the transducer circuit 33, it will be noted that the resistors 51 and 52 have been replaced by a variable resistor 72 and the diode 50 eliminated. The operation of FIG. 5 is otherwise substantially that described 4in connection with FIG. 4.

An example of component part Values for a representative embodiment of FIGS. 1 and 4, exclusive of ignition circuit, is as follows:

Condensers:

37 microfarad 0.02 45 do 400 57 do 0.01

Diodes:

44 1N483 49 1N805 50 IN805 Resistors:

51 ohrns-- 100 52 do 1,500 54 do 1,000 56 do 500 do 5,600 Transistor:

43 (NPN type) 2N1059 Meter:

53 ma. F.S 0-1 Transformers:

34, ferrite core- Winding 36 turns 250 Winding 38 do 150 47, saturable core- Winding 46 turns 250 Winding 48 do 500 The Values listed hereinbefore are for a particular ernbodiment of the invention and are not to be `considered as limiting the invention. Also, it is to be understood that any other type of switching means provided by the NPN type junction transistor might be substituted therefor Without departing from this invention. As Various changes may be made in the form, construction, and arrangement of the parts herein, without departing from the spirit and scope of the invention and without sacricing any of its advantages, it is to be understood that all matters are to be interpreted as illustrative and not in any limiting sense.

What is claimed is:

1. In an electronic tachometer for an engine having an ignition circuit ired by a spark plug, a sensing coil connected serially to said spark plug in a high voltage circuit of said ignition circuit, a condenser connected in parallel with said sensing coil for providing a low impedance path for current discharges of said spark plug and for providing a time integral of each of said current discharges, whereby the current discharges are transformed into a series of constant peak amplitude signal voltages, an elongated conductive rod carrying said sensing coil at one end thereof, a signalling coil carried by said rod at its other end, whereby said sensing coil and said signaling coil are physically separated from each other so as to avoid arcing, said signaling coil being effective to isolate effects of said noise currents and to transform said constant peak amplitude signal voltages, said sensing and signaling coils being solely electromagnetically coupled, a tuned counter circuit responsive to said transformed signal voltages, said counter circuit including transistor switching means, said transistor switching means including an emitter, collector and base connections, and a meter circuit responsive to said counter circuit for indicating said spark plug discharges and correspondingly the speed of said engine.

2. In an electronic tachometer according to claim 1, wherein said sensing coil serially connected with said spark plug senses each discharge yof the spark plug and said signaling coil signals each said discharge to the counter circuit, and a diode serially interposed between said signaling coil and said base connection of said transistor switching means for providing positive current ow between said base and emitter connections `of said transistor switching means at each said discharge of said spark plug.

3. In an electronic tachometer according to claim 2, wherein a resistor is connected in series with said diode and base connection of said transistor switching means.

4. In an electronic tachometer according to claim 3, wherein a `condenser is connected at one terminal to said resistor and at its other terminal to a terminal of said signaling coil to provide with said resistor an integrating network for reducing unwanted signals.

5. In an electronic tachometer for an engine having an ignition circuit fired by a spark plug, a tuned transducer serially connected to said spark plug in a high voltage circuit of said ignition circuit, said transducer including a capacitor and a sensing winding connected in parallel, said capacitor providing a low impedance path for current discharge of said spark plug and providing a time integral of said current discharge, whereby the current discharge is transformed into a constant peak amplitude signal voltage, said capacitor also providing a shunt by-pass for electrostatically coupled noise currents from adjacent spark plug conductors of the ignition circuit, said transducer also including a signaling coil for isolating etects of said noise currents and for transforming said constant peak amplitude signal voltage, said sensing and signaling coils being solely electromagnetically coupled, a tuned counter circuit responsive to said transformed signal voltage, and an indicating circuit responsive to said tuned counter circuit.

6. In an electronic tachometer according to claim 5, wherein said tuned counter circuit includes filter means for accepting said transformed signal voltage and for rejecting said effects of said noise currents.

References Cited by the Examiner UNITED STATES PATENTS 2,005,992 6/1935 Heaton 324-18 2,226,185 12/ 1940 Sturm 324-70 2,312,840 3/1943 Lansdale 324-16 2,485,666 10/1949 Silver 336-182 2,603,685 7/1952 Bychinski 324-15 2,630,529 3/1953 Mann et al 324-70 2,810,080 10/ 1957 Trousdale 331-112 X 2,817,058 12/1957 Weidner 324-16 2,848,613 8/1958 Green et al 331-112 2,857,518 10/1958 Reed 331-112 2,873,388 2/1959 Trumbo 324-78 X 2,902,647 9/ 1959 Hartung 324-70 2,927,268 3/ 1960 Haggai et al. 324-70 2,934,703 4/1960 Cohen 324-78 X 2,936,383 5/1960 Mees 331-112 X 2,962,654 11/1960 Wilson 324-16 X 3,010,032 11/1961 Carney 331-112 X 3,056,084 9/1962 Parmater 324-70 3,094,656 6/1963 Miles 324-70 X 3,104,329 9/1963 Haas 307-885 FOREIGN PATENTS 602,200 9/ 1934 Germany.

WALTER L. CARLSON, Primary Examiner.

S. BERNSTEIN, Examiner.

A. E. RICHMOND, Assistant Examiner.

Claims (1)

1. 5. IN AN ELECTRONIC TACHOMETER FOR AN ENGINE HAVING AN IGNITION CIRCUIT FIRED BY A SPARK PLUG, A TUNED TRANSDUCER SERIALLY CONNECTED TO SAID SPARK PLUG IN A HIGH VOLTAGE CIRCUIT OF SAID IGNITION CIRCUIT, SAID TRANSDUCER INCLUDING A CAPACITOR AND A SENSING WINDING CONNECTED IN PARALLEL, SAID CAPACITOR PROVIDING A LOW IMPEDANCE PATH FOR CURRENT DISCHARGE OF SAID SPARK PLUG AND PROVIDING A TIME INTEGRAL OF SAID CURRENT DISCHARGE, WHEREBY THE CURRENT DISCHARGE IS TRANSFORMED INTO A CONSTANT PEAK AMPLITUDE SIGNAL VOLTAGE, SAID CAPACITOR ALSO PROVIDING A SHUNT BY-PASS FOR ELECTROSTATICALLY COUPLED NOISE CURRENTS FROM ADJACENT SPARK PLUG CONDUCTORS OF THE IGNITION CIRCUIT, SAID TRANSDUCER ALSO INCLUDING A SIGNALING COIL FOR ISOLATING EFFECTS OF SAID NOISE CURRENTS AND FOR TRANSFORMING SAID CONSTANT PEAK AMPLITUDE SIGNAL VOLTAGE, SAID SENSING AND SIGNALING COILS BEING SOLELY ELECTROMAGNETICALLY COUPLED, A TUNED COUNTER CIRCUIT RESPONSIVE TO SAID TRANSFORMED SIGNAL VOLTAGE, AND AN INDICATING CIRCUIT RESPONSIVE TO SAID TUNED COUNTER CIRCUIT.

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