Apparatus for measuring the intake air-flow rate of internal combustions engines

Abstract: In a drive processing circuit comprising a flow velo­ city measuring resistor (7) and a temperature compensating resistor (8) of a hot wire type air flow meter, a resistor (22) is inserted between a connecting point of two resis­ tors (17,18) and an inverting input terminal of an opera­ tional amplifier (15). A condenser (23) is inserted bet­ ween the inverting input terminal and the non-inverting input terminal of the amplifier (15). The resistance value of the inserted resistor (22) is set larger than the resistance value of the temperature compensating resistor 8). By means of the inserted resistor (22) and the inserted condenser (23) which act as a low pass filter, high frequency noises, which intrude through the tempera­ ture compensating resistor (8), are cut off.

Description: The present invention relates to an apparatus for measuring the intake air flow rate of internal combustion engines and, more particularly to an apparatus for use in an internal combustion engine for an automobile. Such an apparatus measures an intake air flow rate necessary for determining the fuel supply amount for an automotive internal combustion engine. The present invention relates to an apparatus for measuring the intake air flow rate of an internal combustion engine using a hot wire type air flow meter or a hot film type air flow meter for measuring an air flow rate by utilizing a cooling action of an air flow on a heat generating temperature dependent resistor disposed in an air flow passage. The hot wire type air flow meter or the hot film type air flow meter according to the present invention is suitably applicable to the measurement of an intake air flow rate of an automotive internal combustion engine. As a conventional intake air flow rate measuring apparatus for use in an internal combustion engine, there has been known a technique that an intake air flow rate measuring apparatus such as a hot wire type air flow meter has a drive processing circuit for heating a heat generating temperature dependent resistor and for processing various electric signals, for example, as disclosed in United States Patent No. 4400974. The conventional hot wire type air flow meter adopts a constant temperature type control circuit utilizing an operational amplifier as the drive processing circuit. The constant temperature type control circuit in the hot wire type air flow meter carries out a constant temperature control for the heat generating temperature dependent resistor. The constant temperature type control circuit constituting the drive processing circuit in the conventional hot wire type air flow meter has a following circuit structure. Namely, voltage is applied to a series resistor, which comprises a heat generating temperature dependent resistor and a current detecting resistor, and further the voltage, which is applied to the above stated series resistor, is adjusted by making as an input therefor with a partial voltage output by the series resistor. Besides, in an automotive internal combustion engine, the high frequency noises occur very often by the ignition spark and the discharge accompanying the electric supply to a distributor, and the radiation from the mobile ham transceiver is more serious. Concretely, in the conventional hot wire type air flow meter, since both a flow velocity measuring resistor constituting a heat generating temperature dependent resistor and a temperature compensating resistor constituting a temperature dependent resistor are exposed in an air flow passage, there is a phenomenon that high frequency noises intrude through an exposed portion, in which both resistors are exposed in the air flow passage, in the hot wire type air flow meter. Therefore, in case that the high frequency noises intrude through the drive processing circuit in the hot wire type air flow meter, since the operational amplifier constituting the drive processing circuit is made an error operation, and the voltage applied to the series resistor is made also an error operation, accordingly an error operation occurs in the drive processing circuit in the conventional hot wire type air flow meter. As a result, there existe a cow reliability of the partial voltage output by the series resistor in the drive processing circuit, in the conventional hot wire type air flow meter. The object of the present invention is to realize an apparatus for measuring the intake air flow rate of internal combustion engines wherein an intake air flow rate signal can be outputted accuratelly and an intrusion of high frequency noises in the drive processing circuit can be prevented. The object is solved according to the features of the independent claims. The dependent claims show advantageous embodiments and further developments of the invention. In accordance with the present invention, the apparatus for measuring the intake air flow rate of internal combustion engines comprises a flow velocity measuring resistor arranged in an air passage for supplying air into the internal combustion engine, a temperature compensating resistor arranged in the air passage for supplying air to the engine, a first low pass filter means connected electrically to the flow velocity measuring resistor and having an attenuation in regard to a high frequency noise; a second low pass filter means connected electrically to the temperature compensating resistor and having an attenuation in regard to a high frequency noise, an operational amplifier having an inverting input terminal and a non-inverting input terminal and being connected to the first low pass filter means and the second low pass filter means, and the operational amplifier compares an output signal from the first low pass filter means and an output signal from the second low pass filter means, and a current control means for controlling the current supplied to the flow velocity measuring resistor and the temperature compensating resistor in accordance with the output signal of the operational amplifier. According to the above stated structure of the present invention, the high frequency noises, which intrude through the flow velocity measuring resistor or the temperature compensating resistor in the drive processing circuit of the apparatus, can be cut-off by the low pass filter means having the attenuation against the high frequency noise, therefore the possibility of an error operation on the operational amplifier of the drive processing circuit can be eluminated. According to the present invention, even when the high frequency noises intrude through the flow velocity measuring resistor or the temperature compensating resistor in the drive processing circuit , it can be obtained in a practical use a remarkable effect that the intake air flow rate signal in the drive processing circuit can be outputted accurately. Examples of embodiments of the invention are described in detail below with reference to the following drawings : Fig.1 is a plane view showing one embodiment of the apparatus according to the invention, Fig. 2 is a cross-sectional view taking along II-II line in Fig. 1, Fig. 3 is a circuit diagram showing a drive processing circuit of one embodiment of the apparatus according to the invention, Fig. 4 is an essential part circuit diagram showing a part of a drive processing circuit of another embodiment of the apparatus according to the invention, Fig. 5 is an essential part circuit diagram showing a part of a drive processing circuit of a further embodiment of the apparatus according to the invention, and Fig. 6 is a circuit diagram showing a drive processing circuit of another embodiment of the apparatus according to the invention. One embodiment of the apparatus according to the invention will be explained in detail below referring to the drawings. A structure of an intake air passage with the apparatus according to the invention with a hot wire type air flow meter is shown in Fig. 1 and 2. An intake air passage 1 has an air suction passage 2 formed a venturi portion 4 therein, and the intake air passage 1 is connected to a throttle valve chamber at a lower end thereof. A fuel injector is mounted downstream of a throttle valve provided on the throttle valve chamber. Air sucked in an arrow marked direction in Fig. 2 passes through the venturi portion 4, and a part of the air passes through a branch air passage 3. Namely, the air entered from an air flow bypass portion 5 is sucked due to a negative pressure generated at an air flow confluence portion 6. The air flow bypass portion 5 is opened upstream of the venturi portion 4, and the air flow confluence portion 6 is opened downstream of the venturi portion 4, respectively. A flow velocity measuring resistor 7 and a temperature compensating resistor 8 are provided respectively in the branch air passage 3. Each of the flow velocity measuring resistor 7 and the temperature compensating resistor 8 is respectively a hot wire type temperature dependent resistor. The temperature compensating resistor 8 constitutes a part of a drive processing circuit in the apparatus according to the invention, which can have the fonction of a hot wire type air flow meter. A drive processing circuit module member is installed in a circuit mounting box 9. A drive processing circuit in the hot wire type air flow meter controls so as to heat the flow velocity measuring resistor 7, and further controls various electric signals. One embodiment of the drive processing circuit according to the invention has a structure stated below. In Fig. 3, a series resistor circuit comprising a resistor 10 and a resistor 11 is connected in parallel to the flow velocity measuring resistor 7. An intermediate connecting point between the resistor 10 and the resistor 11 is connected to an inverting input terminal of an operational amplifier 12 through a resistor 13. A current detecting resistor 14 is connected in series to the flow velocity measuring resistor 7. An intermediate connecting point between the flow velocity measuring resistor 7 and the current detecting resistor 14 is connected to a non-inverting input terminal of an operational amplifier 15 through a resistor 16. Further, the current detecting resistor 14 is connected to the ground terminal of the drive processing circuit. The inverting input terminal of the operational amplifier 15 is connected to an output terminal of the operational amplifier 15 through a resistor 18 and the temperature compensating resistor 8 via a resistor 22. The output terminal of the operational amplifier 15 is connected to a non-inverting input terminal of the operational amplifier 12 through a resistor 19. The inverting input terminal of the operational amplifier 12 and the non-inverting input terminal of the operational amplifier 12 are connected each other through a capacitor 20. The output terminal of the operational amplifier 12 is connected to the base of a control transistor 21 in which current from a battery having a voltage (V B ) is supplied to each of the respective resistors 7 and 10. The above stated drive processing circuit structure in the hot wire type air flow meter is a constant temperature type control utilizing the flow velocity measuring resistor 7 and the temperature compensating resistor 8 and carries out the constant temperature control for the flow velocity measuring resistor 7 and the temperature compensating resistor 8. The operation of the above stated drive processing circuit in the hot wire type air flow meter is explained, for example, in United Stated Patent No. 4297881. Next, the present invention has a following circuit constituting members in the drive processing circuit in addition to the above stated drive processing circuit in the conventional hot wire type air flow meter. Namely, in one embodiment of the present invention shown in Fig. 3, a resistor 22 is inserted between the connecting point of the resistor 17 and the resistor 18 and the inverting input terminal of the operational amplifier 15 in the drive processing circuit. Further, a capacitor 23 is inserted between the inverting input terminal of the operational amplifier 15 and the non-inverting input terminal of the operational amplifier 15 in the drive processing circuit. Herein, the resistant value of the resistor 22 is set larger than the compound resistant value of the resistor 18 and the temperature compensating resistor 8. In case even when the resistor 18 is not provided on the drive processing circuit, the resistant value of the resistor 22 is set larger than the resistant value of the temperature compensating resistor 8 itself. The flow velocity measuring resistor 7 and the temperature compensating resistor 8 have originally a low resistant value in this embodiment of the present invention. The drive processing circuit having the above stated resistor members fits right with the flow velocity measuring resistor 7 having a low resistant value or the temperature compensating resistor 8 having a low resistant value. Accordingly, in this embodiment of the present invention, when the temperature compensating resistor 8 has the low resistant value, a low pass filter having the resistor 22 and the capacitor 23 is formed in the drive processing circuit. The low pass filter comprising the resistor 22 and the condenser 23 has a predetermined cut-off frequency, which is determined by the resistor 22 and condenser 23 with a high frequency noise in regard to a normal mode input of the operational amplifier 15. Further, a capacitor 24 is inserted between the non-inverting input terminal of the operational amplifier 15 and the ground. The capacitor 24 is connected between an intermediate connecting point of the ground and the resistor 17 and a connecting point between the resistor 16 and the non-inverting input terminal side of the capacitor 23. Herein, in this embodiment of the present invention, the high frequency noises, which pass from the operational amplifier 15 through the flow velocity measuring resistor 7 and from the flow velocity measuring resistor 7 into the operational amplifier 15, are cut-off by the low pass filter comprising the resistance 16 and the condenser 24. Further, the high frequency noise, which intrude through the temperature compensating resistor 8, are cut-off the normal mode thereof by the low pass filter comprising the resistor 22 and the capacitor 23. Also, the high frequency noises, which pass through the operational amplifier 15 through the temperature compensating resistor 8, are cut-off in the common mode thereof by the resistor 16 and the capacitor 24. Further, since the resistor 22 and the capacitor 23 have a function so as to delay a negative feed-back input of the operational amplifier 15, it is possible to avoid the problems such as an oscillation in case that when the cut-off frequency, which is determined by the resistor 22 and the capacitor 23, is set larger than the cut-off frequency of the operational amplifier 15 and also when the cut-off frequency, which is determined by the resistor 22 and the capacitor 23, is set lower than the frequency of the high frequency noise, as shown in this embodiment of the invention. According to this embodiment of the invention, since the high frequency noises, which intrude through the flow velocity measuring resistor 7 and the temperature compensating resistor 8, are cut-off by the respective low pass filter, there is an effect that an intake air flow rate output signal or the partial output voltage (V OUT ) of the series resistor circuit in the drive processing circuit can be obtained accurately. Another embodiment of the invention will be explained referring to drawing. Fig. 4 is a modified embodiment of the invention in regard to the above stated embodiment shown in Fig. 3. Fig. 4 shows an essential part circuit diagram of the drive processing circuit in the hot wire type air flow meter, in which the respective same reference number indicates the same element shown in Fig. 3. In the modified embodiment shown in Fig. 4, an inductor 25 such as a coil is inserted between the connecting point of the resistor 17 and the resistor 18 and the inverting input terminal of the operational amplifier 15. The inductor 25 is a replacement body of the resistor 22 shown in Fig. 3. Further, in the modified embodiment shown in Fig. 4, an inductor 26 such as a coil is inserted between the resistor 16 and the condenser 24. Accordingly, by the provision of the inductor 25, the resistant value thereof is high only at the high frequency noises, therefore such high frequency noises, which intrude through the temperature compensating resistor 8, can be cut-off similarly to the above stated embodiment shown in Fig. 3. Also, by the provision of the inductor 26, the high frequency noises, which intrude through the flow velocity measuring resistor 7, can be cut-off similarly to the above stated embodiment shown in Fig. 3. In the modified embodiment of the invention shown in Fig. 4, the cut-off characteristic can be improved further by the function of the inductor 26. A further embodiment of the invention will be explained referring to drawing. Fig. 5 is a further modified embodiment of the invention in regard to the above stated embodiment shown in Fig. 4. Fig. 5 shows an essential part circuit diagram of the drive processing circuit in the hot wire type air flow meter, in which the respective same reference number indicates the same element shown in Fig. 4. In the further modified embodiment of the invention shown in Fig. 5, a resistor 27 is inserted between the inverting input terminal of the operational amplifier 15 and the non-inverting input terminal of the operational amplifier 15. The resistor 27 is a replacement body of the capacitor 23 shown in Fig. 4. Accordingly, by the provision of the resistor 27, the high frequency noises, which intrude through the temperature compensating resistor 8, can be cut-off similarly to the above stated embodiment shown in Fig. 4. A still further embodiment of the invention will be explained referring to drawing. Fig. 6 shows a circuit diagram in which the invention is adopted to another drive processing circuit in an air flow meter, and in which the respective same reference number indicates the same element shown in Fig. 3. The drive processing circuit in the air flow meter shown in Fig. 6 has another constant temperature type control circuit in the drive processing circuit in regard to the constant temperature type control circuit shown in Fig. 3. In Fig. 6, a bridge circuit is formed by the respective resistors which comprise the flow velocity measuring resistor 7, the temperature compensating resistor 8, the resistor 14, and a resistor 28. An intermediate connecting point of the bridge circuit is connected to the non-inverting input terminal of the operational amplifier 12 and the inverting input terminal of the operational amplifier 12. An inductor 29 such as a coil is connected to the inverting input terminal of the operational amplifier 12. An inductor 30 and a resistor 31 are connected to the non-inverting input terminal of the operational amplifier 12. A capacitor 32 is inserted between the inverting input terminal of the operational amplifier 12 and the non-inverting input terminal of the operational amplifier 12. Further, a capacitor 33 is connected between the connecting point of the resistor 14 and the resistor 28 and the intermediate connecting point of an inductor 30 and the capacitor 32. The detailed operation of this constant temperature type control circuit having the above stated bridge circuit in the drive processing circuit, except for the resistor 31, the two inductors 29 and 30, and the condenser 32, of the air flow meter is explained, for example, in United States Patent No. 4196622. In this embodiment of the invention, at the flow velocity measuring resistor 7 side, a low pass filter is formed by the resistor 31, the inductor 30 and the capacitor 33. And at the temperature compensating resistor 8 side, a low pass filter is formed by the inductor 29 and the capacitor 32. Accordingly, the high frequency noises, which intrude into the flow velocity measuring resistor 7, are cut-off by the low pass filter comprising the resistance 31, the inductor 30 and the capacitor 33. Also, the high frequency noises, which intrude through the temperature compensating resistor 8, are cut-off by the low pass filter comprising the inductor 29 and the capacitor 32. As stated above, in this embodiment of the present invention, since the high frequency noises, which intrude through the flow velocity measuring resistor 7 and the temperature compensating resistor 8, are cut-off by the respective low pass filter, accordingly there is an effect that the intake air flow rate output signal can be obtained accurately. As stated above, the invention is explained by various embodiments, to sum up, the low pass filter is positioned respectively in a front side of each input terminal of the operational amplifier so as to not apply the high frequency noises, which intrude through the flow velocity measuring resistor or the temperature compensating resistor, to each input terminal of the operational amplifier.

Claims: Apparatus for measuring the intake air flow rate of internal combustion engines, comprising a flow velocity measuring resistor (7) arranged in an air suction passage (3) for supplying air to the engine, a temperature compensating resistor (8) arranged in the air suction passage (3), a first low pass filter means connected electrically to the flow velocity measuring resistor (7) and having an attenuation characteristic to cut off a high frequency noise, comprising a first resistor (16) and a first capacitor (24), a second low pass filter means connected electrically to the temperature compensating resistor (8) and having an attenuation characteristic to cut off a high frequency noise, comprising a second resistor (22) and a second capacitor (23), a first operational amplifier (15) having an inverting input terminal connected to the second low pass filter means a non-inverting input terminal connected to the first low pass filter means, and an output connected to the temperature compensating resistor (8), the first operational amplifier comparing the output signal from the first low pass filter means and the output signal from the second low pass filter means, a current control means (10-13, 19-21) for controlling the current which flows to the flow velocity measuring resistor (7) and the temperature compensating resistor (8) in accordance with the output signal of the operational amplifier (15), said current control means comprising a second operational amplifier (12) the first low pass filter means being connected between the flow velocity measuring resistor (7) and the non-inverting input terminal of the first operational amplifier (15), the second low pass filter means being connected between the temperature compensating resistor (8) and the inverting input terminal of the first operational amplifier (15). Apparatus according to claim 1, characterized in that the first low pass filter means is connected between an intermediate connecting point of a series connection of said flow velocity measuring resistor (7) with a current detecting resistor (14) and the non-inverting input terminal of the first operational amplifier (15); and the non-inverting input terminal of the first operational amplifier (15) is connected to an intermediate point of the first resistor (16) and the first capacitor (24); and the second resistor (22) of the second low pass filter means is connected between the temperature compensating resistor (8) and the inverting input terminal of the first operational amplifier (15), and the second capacitor (23) of the second low pass filter means is connected between the inverting input terminal and the non-inverting input terminal of the first operational amplifier (15), and the intermediate point of the second capacitor (23) and the second resistor (22) is connected to the inverting input terminal of said first operational amplifier (15). Apparatus for measuring the intake air flow rate of internal combustion engines, comprising: a flow velocity measuring resistor (7) arranged in an air suction passage (3) for supplying air to the engine and connected electrically to a current control means (12, 21, 32), a temperature compensating resistor (8) arranged in the air suction passage (3) and connected electrically to the current control means (12, 21, 32), Said flow velocity measuring resistor (7) and said temperature compensating resistor (8) being arranged in respective branches of a bridge circuit (7, 8, 14, 18, 28) having a first end terminal (GND) and a second end terminal a first low pass filter means connected electrically to the flow velocity measuring resistor (7) and having a first coil (30) connected in series with a first capacitor (33), the intermediate point of the first coil (30) and the first capacitor (33) of the first low pass filter means being connected to the non-inverting input terminal of an operational amplifier (12), the first coil (30) of the first low pass filter means is connected to the flow velocity measuring resistor (7), and the first capacitor (33) of the first low pass filter means further being connected to the first end terminal (GND) , a second low pass filter means connected electrically to the temperature measuring resistor (8) and having a second coil (29) connected in series with a second capacitor (32), the intermediate point of the second capacitor (32) and the second coil (29) of the second low pass filter means is connected to the inverting input terminal of the operational amplifier (12), wherein the second capacitor (32) is connected between the inverting input terminal and the non-inverting input terminal of the operational amplifier (12), and wherein the current control means is for controlling the current which flows to the flow velocity measuring resistor (7) and the temperature compensating resistor (8) in accordance with the output signal of the operational amplifier (12). Apparatus according to claim 1 or 2, characterized in that the second operational amplifier (12) has an inverting input terminal and a non-inverting input terminal, the latter being connected to the output terminal of the first operational amplifier (15) through a resistor (19), a series resistor circuit (10, 11) is connected in parallel to the flow velocity measuring resistor (7), an intermediate connecting point of the series resistor circuit is connected to the inverting input terminal of the second operational amplifier (12), a current detecting resistor (14) is connected in series to the flow velocity measuring resistor (7), and the intermediate connecting point of the series connection of the flow velocity measuring resistor (7) with the current detecting resistor (14) is connected via the first low pass filter means to the non-inverting input terminal of the first operational amplifier (15), and a current control element (21) is provided in said current control means controlling the current which flows to the flow velocity measuring resistor (7) in accordance with the output signal of the second operational amplifier (12). Apparatus according to claim 4, characterized in that the first low pass filter means operates to cut off the high frequency noise which passes through the flow velocity measuring resistor (7), and the second low pass filter means having the second resistor and the second capacitor cut off a high frequency noise which intrudes through the temperature compensating resistor (8). Apparatus according to claim 3, characterized in that the first capacitor (33) is connected between the non-inverting input terminal of the operational amplifier (12) and the first end terminal (GND) of the bridge circuit, and the second end terminal of the bridge circuit is the connecting point of the flow velocity measuring resistor (7) with the temperature compensating resistor (8) and is connected with the current control means.

Inventors: AKAMATSU MASUO, SUZUKI MASAYOSHI, AKAMATSU, MASUO, SUZUKI, MASAYOSHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: G01F1
IPC codes: G01F1, A01N43, G01F5