SYMMETRICAL BRIDGE CIRCUIT FOR MEASURING MASS AIR FLOW

Abstract: A symmetrical bridge circuit having a sensing leg means and a compensation leg means for measuring mass air flow in the input of an internal combustion engine utilizes the same type of sensing and compensation elements (R₁,R₂) with the same temperature coefficients. The sensing leg means and the compensation leg means are compound circuits each having a conventional bridge leg (12,14) and a voltage divider leg (20,22). The conventional bridge leg outputs (16,18) are connected through voltage follower circuits (IA1A,IC1C) controlling the input of the voltage divider legs (20,22). The conventional leg (12) of the sensing leg means contains the sensing element (21) and the voltage output therefrom is a function of a first factor. The voltage divider leg (20) connected to the output of the voltage follower circuit(IC1A) further divides the voltage output by a second factor. The compensation element (R2) is connected in the voltage divider leg (22) of the compensation leg means. The voltage output of the conventional leg (14) of the compensation leg means is a function of a second factor and the voltage output from the voltage divider (22) is a function of the first factor. The outputs of the voltage dividers (20,22) control a power driver means (Q₁) supplying power to the bridge circuit. The output stage (IC1D) of the bridge circuit compares the voltage supplied by the power driver means with a fixed reference voltage to generate an analog signal representing the mass of the air flowing across the sensing element (R1). The symmetrical bridge circuit does not require compensating capacitors and special amplification circuits for the compensation legs.

Description: This invention relates to a circuit for the measurement of mass air flow in general and in particular for use in a mass air flow sensor to measure the mass air flow into an internal combustion engine. A circuit of this kind as described in DE 28 45 662 A has a structure as shown in Fig 1: A bridge circuit with bridge terminals B1, B2 is connected to a power supply through a power driver Q1. A sensing leg and a compensation leg, each leg including a voltage divider with a first resistor - R1, R2 - and an output resistor - R19, R21, are connected to the bridge terminals B1, B2. The first resistor R1 in the first voltage divider 12 is a sensing element located in the air flow and is responsive to the rate of the air flowing thereacross for modifying the output voltage across the output resistor R19 from which a sensing output voltage is derived. The first resistor R2 in the second voltage divider 22 is a compensation element located in the air flow and is responsive to the temperature of the air flowing thereacross for modifying the compensating output voltage across the output resistor R21. The resistors R1, R2 have positive temperature coefficients and are located in the air stream of an internal combustion engine, more particularly in the air flow meter, the resistor R1 being typically located upstream of the resistor R2. A control amplifier ICIB is electrically connected to receive the sensing output voltage and the compensation output voltage and is operable to control the conduction of the power driver Q1 for supplying power to the bridge. The resistor R1 is heated by the current flowing therethrough to a temperature determined by the bridge circuit and the resistor R2 is not heated by the circuit but is sensitive to the ambient temperature of the air flowing by the compensation element. The bridge circuit by controlling the power to the circuit maintains the temperature of the resistor R1 at a predetermined temperature above the temperature of the incoming air, thereby changing the bridge voltage which is an analog signal representing the mass of the air flowing across the resistors R1 and R2. But this is true only if the effect of changing air temperature on R1 is compensated by the effect of R2. Therefore, as both resistors are operated at the same bridge voltage, the matching of the temperature coefficients of the resistance material with the different values of resistance is required. It is the object of the invention to avoid such matching. The features of the invention are defined in claim 1. The invention will now be described with reference to the figures where Fig 2 shows the structure and Fig 3 an embodiment of the invention. In addition to Fig 1 a third voltage divider 14 including resistors R7 and R8 is connected to the bridge terminals. The values of these resistors define a second division factor with first resistor R7 equal to approximately nine times the value of the output resistor R8. The noninverting input of an amplifier ICIC, whose output is connected to the base lead of a transistor Q2, is connected to the junction 18 of the two resistors R7 and R8. The transistor Q2 is electrically connecting the second voltage divider 22, including the compensating resistor R2, a resistor R20 and the output resistor R21, to the bridge terminals B1, B2. The emitter lead is connected through a feedback resistor R9 to the inverting input of the amplifier ICIC. The noninverting input of an unity gain amplifier ICIA, is connected to the junction 16 of the resistor R1 and the resistor R19. The inverting input is directly connected to the output of the amplifier ICIA and to a fourth voltage divider 20 with a first resistor R3 and an output resistor R4 and an output 24. The amplifiers ICIA and ICIC are configured as voltage followers to reduce the electrical loading on the first and third voltage dividers 12, 14 respectively. The sensing output voltage at the junction 24 of the resistors R3 and R4 of the fourth divider 20 is a function of the second division factor of the third divider 14. The value of the first resistor R3 is nine times the value of the output resistor R4, which is the same relation as with first resistor R7 and output resistor R8 of the third voltage divider 14 defining the second division factor. The junction 24 of the two resistors R3 and R4 is electrically connected to the noninverting input of the control amplifier ICIB. As an additional feature, a resistor R23 is added to the noninverting input of the control amplifier ICIB to assure that the noninverting input is positive with respect to the inverting input so that the power transistor Q1 will be turned on and the circuit 10 will start when switching on the power supply. The junction 26 of the resistors R20 and R21 of the second divider 22 is electrically connected to the inverting input of the control amplifier ICIB. The relationship between the resistance values in this voltage divider 22 defines a first division factor N. In the preferred embodiment the total resistance in the upper portion of the voltage divider is R2 + R20. R2, by design, has a value equal to ten ohms at room temperature and the value of R20 is one-half the value of R2 or five ohms, R21 being equal to (R2 + R20) or fifteen ohms. For the bridge circuit to balance, the sensing output voltage on the noninverting input of the control amplifier ICIB must be equal to the compensating output voltage on its inverting input. The sensing output voltage at the junction 26 of the second divider 22 is equal to a function of the first and second division factors. In the first divider 12 the value of the resistor R19 is 1,5 times the resistance value of the sensing resistor R1 when unheated. As the power driver Q1 supplies more current to R1, the resistance of R1 increases until R1/R19=N= (R2 + R20)/R21. When the velocity of the air increases, the resistance of the sensing resistor R1 decreases as its temperature is reduced by heat conduction to the passing air. This increases the output voltage at the noninverting input of the amplifier ICIA. The output of this amplifier is supplied to the voltage divider 20 which furnishes the sensing output voltage to the noninverting input of the control amplifier ICIB. This causes the output of the control amplifier ICIB to increase thereby driving the power transistor Q1 to supply more power to the sensing resistor R1. This increases the resistance value of the sensing resistor R1 until the bridge balances. The output signal from the bridge circuit is supplied to an output amplifier ICID. The noninverting input of this amplifier is connected a voltage divider comprising two resistances R11 and R18 connected to the bridge terminals. This voltage divider functions as the gain control of the output amplifier ICID and as the bridge voltage changes the output voltage at the junction of the two resistances R11 and R18 changes accordingly. The inverting input of the output amplifier ICID is connected to a voltage divider comprising two resistances R12 and R17 connected to a regulated voltage. This voltage divider functions as the offset control of the output amplifier ICID. The output of the amplifier ICID is connected in a feedback circuit comprising resistances R15, R16 to its inverting input to set the gain of the amplifier. The sensing resistor R1 and the compensating resistor R2 are made from nickel and have the same temperature coefficient which is approximately +5420 parts per million per degree centigrade. Therefore a temperature increase of 75°C of the air, will increase the resistance of the sensing resistor R1 by 50% and thereby the voltage across R1. With this increase of air temperature, the resistance of compensation resistor R2 changes too, changing the first factor N, the ratio of the resistances in the second divider 22. Therefore the bridge remains balanced, keeping the voltage at the emitter of the power transistor Q1 and the bridge voltage constant for a given air flow. The voltage across the compensation resistor R2 is, in the preferred embodiment, approximately 1/12th of the voltage across the sensing resistor R1. The power delivered to R2 is 0.5 mW while the power delivered to R1 is about 75mW at very low air flow rate. The power ratio of the sensing to the compensation resistors is about 150:1. It is a further advantage of the invention that no amplification of the output of the compensation circuit is needed and therefore circuit stability is enhanced. As the phase lag is the same in both bridge legs, the need for compensating capacitors for keeping the circuit stable is avoided which otherwise would slow down the circuit operation. No adjustments of the sensing and compensation loops are needed. For fabrication purposes, a single, inexpensive integrated circuit element is used and thick film techniques may be used since the design depends on the ration of the resistors.

Claims: Circuit for measuring mass air flow comprising a power driver (Q1) connecting a bridge circuit with two bridge terminals (B1,B2) to a power supply; a control amplifier (ICIB) responsive to a sensing output voltage and a compensating output voltage for controlling the power driver (Q1) and thereby adjusting the bridge voltage between the bridge terminals (B1,B2); an output amplifier (ICID) responsive to the bridge voltage for generating a signal representing the mass air flow; wherein the bridge circuit has a first (12) and a second (22) voltage divider,each of them being electrically coupled to the bridge terminals and including a first resistor (R1; R2) and an output resistor (R19;R21) and wherein the first voltage divider (12,R1,R19) is including a sensor element as the first resistor (R1) and furnishing an output voltage across its output resistor (R19) from which the sensing output voltage is derived, the second voltage divider (22;R2,R21) is including a compensating element as the first resistor (R2) and furnishing the compensating output voltage across its output resistor (R21), the sensing resistor (R1) and the compensating resistor (R2) are located in the air flow and responsive to the air flow and the temperature of the air respectively; a third voltage divider (14) being connected to the bridge terminals and having a first resistor (R7) and an output resistor (R8),the relation of the resistances of which defining a division factor; the second voltage divider (22) being electrically coupled parallel to the output resistor (R8) of the third voltage divider (14); a fourth voltage divider (20,R3,R4) being electrically coupled parallel to the output resistor (R19) of the first voltage divider (12), and having a first resistor (R3) and an output resistor (R4), the relation of the resistances of these resistors being given by the division factor of the third voltage divider (14) and furnishing the sensing output voltage across its output resistor(R4). Circuit according to claim 1 wherein the sensing resistor (R1) and the compensating resistor (R2) have the same temperature coefficients. Circuit according to claim 2 wherein the sensing and the compensating resistor are fabricated from the same material. Circuit according to claim 2 wherein the ratio of the resistances of the output resistor (R19) and of the sensing resistor (R1) when unheated of the first voltage divider is 1,5. Circuit according to claim 1 additionally including voltage follower means (ICIA,ICIC) coupling the first voltage divider (12) to the fourth voltage divider (20) and the second voltage divider (22) to the third voltage divider (14). Circuit according to claim 5 additionally including a transistor (Q2) which is electrically coupling the second voltage divider (22) to the bridge terminals and which is controlled by said voltage follower means (ICIC) connected to the output (18) of the third voltage divider (14). Circuit according to claim 1 additionally including means (R23) electrically connected to one input of said control amplifier (ICIB) for causing the control amplifier to turn on said power driver (Q1) when the power supply to the circuit is initially switched on. Circuit according to claim 1 wherein said output amplifier (ICID) has one input electrically coupled to the output of a voltage divider (R11,R18) which is connected to the bridge terminals and has another input coupled to a fixed reference voltage.

Inventors: CARP, RALPH WOLF C/O ALLIED CORPORATION
Applicants: SIEMENS AKTIENGESELLSCHAFT
Application year: 1987
Main classification: G01F1
IPC codes: G01F1