OPTICAL TRANSMITTER FOR PRODUCING AN OPTICAL SIGNAL INDICATIVE OF TEMPERATURE

Abstract: An optical transmitter (16) for converting a thermocouple voltage into an optical pulse train and a low pass input filter (30) are contained in respective spaces (28, 30) in a soft magnetic housing (22). The input filter (30) is in lead wires (32, 34) from a thermocouple whose hot junction is embedded in the tip of the insulator of a spark plug (10) of an internal combustion engine. The housing (22) itself forms part of or is connected to the H.T. lead to the centre electrode (12) of the spark plug. The housing (22) is surrounded by insulating material (42) which is itself surrounded by an earthed screening casing (44). The low pass filter (30) is symmetrically constructed, both electrically and mechanically. The filter (30) prevents interference from reaching the circuitry of the optical transmitter (16) and the housing (22) and the casing (44) prevent the generation and transmission of radio interference.

Description: DESCRIPTION OPTICAL TRANSMITTER FOR PRODUCING AN OPTICAL SIGNAL INDICATIVE OF TEMPERATURE. State of the Art. The present invention relates to an optical transmitter for producing an optical signal indicative of the temperature of a component, such as a spark plug, operating under high tension, in accordance with the pre-characterizing clause of claim 1. Such an optical transmitter is known (DE-C-2512634) . The thermoelectric voltage from a hot junction on the tip of the insulator of the spark plug is applied to a voltage controlled oscillator (VCO) whose output frequency is thus dependent on and preferably proportional to the temperature to be measured. The oscillator output is fed to a light-emitting diode (LED) and the resulting pulsed optical signal is transmitted along an optical conductor to a light sensitive semi-conductor component where it is re-converted to an electrical signal. The electrical signal can be processed for further use, such as for the digital or analog display of the temperature or for appropriate control of the operation of the internal combustion engine to which the spark plug is fitted. The optical conductor electrically isolates the optical transmitter and thereby the thermoelectric element and its lead wires from the processing circuitry, whereby the risk of arcing between the centre electrode of the spark plug and the thermo-electric element and its lead wires is minimized. By arranging the optical transmitter as close to the spark plug as possible, the radiation of interference and the capacitive loading on the ignition system are minimized. However, the sparks produced at the spark plug themselves have a very high frequency component (> 10^ Hz) and this can interfere with the evaluation of the measured voltage. Advantages of the invention. This disadvantage can be avoided by an optical transmitter in accordance with the characterizing clause of claim 1. The low pass input filter prevents the high frequency interference from reaching the VCO or other circuity of the optical transmitter and the ferromagnetic housing screens any interference radiation from the filter itself. Interference suppression is thus very effective. Filtering of the high frequency interference is further improved by constructing the low pass filter electrically and mechanically symmetrical in accordance with claims 3 to 5. Filtering is also improved by the use of lead-through capacitors between the filter and the VCO in accordance with claim 6. To avoid radiation of interference from the ferromagnetic housing, an outer casing can be provided in accordance with claims 7 and 8. Faulty readings can be further avoided by arranging the components of the optical transmitter, including a battery, within a separate ferromagnetic housing in accordance with the features of claim 10. A particularly convenient arrangement is achieved by adopting the features of claim 11. Drawings The invention is further described, by way of example, with reference to the accompanying drawings, in which:- Fig.l is a somewhat diagrammatic representation of a spark plug of an internal combustion engine and its H.T. lead with an optical transmitter in accordance with the invention arranged in the H.T. lead for converting the temperature of the spark plug, as measured by a thermocouple, into an optical signal, Fig.lA is a fragmentary detail of a modification in which the optical transmitter is connected to a branch from the H.T. lead, Fig.2 is a circuit diagram of the optical transmitter and its low pass input filter in accordance with the invention, and Fig.3 is a longitudinal section of the housing and casing for the optical transmitter and the low pass filter. Description of the preferred embodiment. A spark plug 10 for an internal combustion engine has a hot junction of a thermocouple embedded in or disposed on the tip of its insulator (not shown) adjacent the centre electrode 12 which co-operates with an earthed outer electrode 14. The H.T. distributor lead to the spark plug contains an optical transmitter 16 whose circuitry and construction will be described with reference to Figs. 2 and 3. An insulated cable 18 extends between the optical transmitter 16 and the spark plug 10 and is releasably attached thereto by a 3-pole snap-on connector 20 (shown diagrammatically) . The cable 18 contains the H.T. lead wire for the centre electrode 12 and two lead wires for the thermocouple. The H.T. lead wire contains or consists of a suppressor resistor for suppressing interference generated in the spark plug. The insulated cable 18 is surrounded by a metallic sheath which is electrically earthed to the spark plug housing. The lead 48 from the distributor is connected to the optical transmitter 16. In the modification of Fig.lA the insulated cable is split into two portions 18a and 18b between which is a T-connector 19. The lead 48 from the distributor is connected to the T-connector 19 so that the optical transmitter is not actually contained in the H.T. distributor lead but is connected thereto. The arrangement of the hot junction of the thermocouple in the tip of the spark plug insulator can be as described in DE-U-7001121 with the insulated lead wires from the hot junction extending through the centre hole in the insulator alongside the centre electrode rod. Because of the close proximity of the thermocouple junction to the spark and of its lead wires to the centre electrode, the lead wires pick up the high frequency oscillations from the spark and the lead wires must be at the H.T. potential of the centre electrode to avoid arcing. Thus, according to the preferred embodiment of the invention, the optical transmitter 16 has a low pass input filter and this filter, together with the optical transmitter, are disposed in separate compartments in a housing of soft ferromagnetic material. As shown diagrammatically in Fig.2 and construe ionwise in Fig.3, a composite inner housing 22 of soft ferromagnetic material is divided by a partition 24 to form a space 26 for the circuitry and power source of the optical transmitter 16 itself and a space 28 for the low pass input filter 30. The low pass filter 30 comprises inductances L1,L2 arranged respectively in the thermoelement lead wires 32,34, capacitors Cl and C2 connected in parallel between the lead wires 32,34, capacitors C3 and C4 connected in parallel between the lead wire 32 and the housing 22 and capacitors C5 and C6 connected between the lead wire 34 and the inner housing 22. The filter 30 is made as symmetrical as possible, both electrically and mechanically. Thus the capacitors Cl, C3 and C5 are of the same value, the capacitors C2, C4 and C6 are of the same value and the inductances LI and L2 are of the same value. The lead wires 32,34 pass from the space 28 to the space 26 through lead-through capacitors C7 and C8 in the soft ferromagnetic housing partition 24. The capacitors C7 and C8 are of equal capacity and are symmetrically disposed in the housing partition 24. The capacitors C3,C4,C5 and C6 are connected to a common junction 25 on the housing partition 24 between the lead-through capacitors C7 and C8. The junction 25 is at the same H.T. potential as the terminal 31 by which the H.T. lead wire in the cable 18 is connected to the one wall 45 of the inner housing 22, the terminal 31 being physically disposed between the terminals 33,35 for the thermoelement lead wires 32 and 34. The thermoelectric voltage between the lead wires 32,34 is used to control the frequency of a VCO 36 disposed in the space 26 together with its power source in the form of rechargeable batteries 38 and an LED 40 which is energised by a square wave output from the VCO 36. Preferably, the output frequency of the VCO is directly proportional to its control voltage, whereby the pulse frequency of the optical signal produced by the LED 40 is directly proportional to the temperature of the insulator tip of the spark plug, insofar as the thermoelement itself has a linear response. As shown in Fig.3, the soft ferromagnetic composite or combined inner housing 22 is cylindrical and the soft ferromagnetic partition 24 extends transversely of the housing axis. The inner housing 22 is at the H.T. potential by virtue of the H.T. lead wire being connected to the terminal 31. The inner housing 22 is surrounded by a two-piece insulator 42 which is itself surrounded by a cylindrical metallic outer housing or casing 44. The optical transmitter 16 is mounted on the engine block, whereby the metallic outer casing 44 is earthed. The insulator 42 has a low dielectric constant so that the capacitance between the outer casing 44 and the inner housing 22 is low. As shown in Figs. 1 and 3, the end wall 54 of the housing 22 opposite the end wall 45 is provided with a socket 46 for the H.T. distributor lead 48 and with a socket 50 for an optical conductor or light guide 52. In the case of the modification of Fig.lA, the socket 46 is omitted and the socket 50 can be centrally disposed on the end wall 54. The light guide 52 registers with the LED 40 or alternatively a light guide is provided within the housing 22 between the LED 40 and the socket 50. The optical conductor 52 leads, possibly together with other optical conductors appertaining to the remaining spark plugs of the internal combustion engine, to an optical receiver which may be of a conventional kind as described in DE-C-2512634 and which contains a light sensitive semiconductor element and a processor for processing its output to operate, for example, a digital display and/or an analog recording of the temperature of the tips of the spark plug insulators. Fig.2 shows how the battery 38 may be re-charged by disconnecting the cable 18 from the spark plug and connecting a suitable battery charger to the terminals 33,35. When the cable 18 is disconnected from the spark plug, the terminals 33,35 are no longer connected to one another and a capacitor C9 is charged via high resistors R1,R2 and R3. The base of a transistor Tr which feeds the battery power to the VCO is connected to the junction between the resistors R2 and R3. When the capacitor C9 is charged, the transistor Tr is switched off, so disconnecting the VCO from the battery 38. When the battery charger is connected to the terminals 33,35, the battery is charged via diodes Dl and D2. By way of example, good results can be obtained if LI - L2 - 1.2 mH, Cl « C3 = C5 « 22nF, C2 « = C4 = C6 - ljiF and C7 - C8 - 2 x 800 pF. C9 - lμF, Rl = 2.2 M-α, R2 = 562 KΛand R3 - 1 MΛ. The optical transmitter 16 may have an outer diameter of 54 mm and an overall length of 183 mm and the connecting cable may be 300 mm long. Advantages of the invention are the absence of external interference due to the symmetry of the low pass filter with respect to the inner housing, the separation of the spaces or chambers for the low pass filter and for the circuitry of the optical transmitter and the conducting of the measurement signals via lead-through capacitors and the screening of the transmitter by a housing of soft magnetic material. A further advantage is the lowering of the interference level by the symmetrical construction of the transmitter and of the lead to the outer screening and the conduction of the ignition voltage through an interference suppressor resistor.

Claims: CLAIMS 1. An optical transmitter for producing an optical signal indicative of the temperature of a component, such as a spark plug, operating under high tension, wherein a voltage dependent on the temperature to be measured, such as a thermoelectric voltage from a thermocouple hot junction in the component whose temperature is to be measured, is converted into an optical signal which is transmitted along an optical conductor (52) to a light sensor, characterized in that the lead wires (32,34) which transmit the temperature-dependent voltage are connected to the optical transmitter (16) via a low pass filter (30) which is contained within a housing (22) of ferromagnetic material, and in that the housing (22) is held at the said high tension at which the component, whose temperature is to be measured, stands and is itself surrounded by insulating material. 2. An optical transmitter according to claim 1, characterized in that the low pass filter (30) is electrically symmetrical with respect to the ferromagnetic housing (22) . 3. An optical transmitter according to claim 1 or 2, characterized in that the low pass filter (30) is mechanically symmetrical with respect to the ferromagnetic housing (22) . 4. An optical transmitter according to claim 1, 2 or 3, characterized in that the low pass filter (30) comprises at least one capacitor (C1,C2) between the lead wires (32,34) and at least one respective capacitor (C3-C6) between each lead wire (32,34) and the ferromagnetic housing (22) . 5. An optical transmitter according to any of claims 1 to 4, characterized in that the low pass filter (30) includes a respective inductance (Ll,L2) in each of the lead wires (32,34). 6. An optical transmitter according to any preceding claim, characterized in that the lead wires (32,34) to the optical transmitter (16) pass out of the ferro- magnetic housing (22) of the low pass filter (30) at lead-through capacitors (C7,C8). 7. An optical transmitter according to any preceding claim, characterized in that the insulating material (42) is surrounded by a metallic outer casing (44) held at earth potential. 8. An optical transmitter according to claim 7, characterized in that the metallic outer casing (44), at least where it surrounds the insulating material on the ferromagnetic housing of the low pass filter (30), is symmetrically constructed. 9. An optical transmitter according to any preceding claim, characterized in that the transmitter (36,40) itself, together with its own power source (38) , are disposed in a separate housing of ferromagnetic material and in that such separate housing is also held at the said high tension at which the component whose temperature is to be measured stands and is surrounded by insulating material. 10. An optical transmitter according to any preceding claim, characterized in that the H.T. conductor to said component and the lead wires (32,34) conducting the temperature dependent voltage from said component are surrounded by an earthed metallic sheath. 11. An optical transmitter according to any preceding claim, characterized in that the ferromagnetic housing or housings is or are incorporated in the high tension lead to said component, the high tension lead containing or comprising an interference suppressor resistance between the spark plug and the ferromagnetic housing of the low pass filter.

Inventors: LANG HERBERT, STEINKE LEO, LANG, HERBERT, STEINKE, LEO
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 1989
Main classification: G01K7
IPC codes: G01J5, G01K1, G01K7, G01K13, H01T13