Temperature compensating circuit

Abstract: A temperature compensating circuit having a third temperature sensor element (R R ) for detecting an ambient temperature disposed on the same substrate (10) on which first and second temperature sensor elements (R x , R y ) are located, and a voltage across the third sensor element, generated by supplying a current thereto, is applied to the first and second sensor elements. Therefore, a voltage applied to first and second sensor elements is automatically changed corresponding to change in ambient temperature, and accordingly a difference in temperature between first and second sensor elements can be precisely detected in a form of voltage signal.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a temperature compensating circuit, and more particularly to a circuit of this kind suitable in use for a flow sensor which is adapted to e.g. measure a temperature difference between two points on a chip and detect a flow quantity from the result of such measurement. Description of the Prior Art Figure 1 is a perspective view of a flow sensor of a type known as a microbridge flow sensor. The flow sensor as illustrated comprises a pair of temperature sensing elements or resistive elements R x , R y , a heater element H arranged between the resistive elements R x and R y and a base 10 supporting sensor elements R x , R y and heater element H. Base 10 is provided with a depression 20 formed on the upper surface thereof, as shown in Figure 1, such that the elements R x , R y , H are bridge thereacross. An arrow in the drawing indicates the direction of a flow. The heater element is heated to a relatively low temperature, e.g., at 60 °C. Also, sensor elements are thermally isolated from heater element so that a temperature at element RX on the upstream side of heater element is substantially equal to an ambient temperature T A °C. Figures 2 - 4 respectively show a conventional temperature compensating circuit including temperature sensor elements such as resistive elements R x , R y shown in Figure 1. In Figure 2, resistive elements R x , R y are serially coupled to constant current sources 1, 2, respectively, at points P1, P2, to form a bridge circuit. Differential amplifier 3 has its input terminals coupled to connecting points P1, P2 to receive a voltage signal Vz (=I(RX-RY)), where I represents a current flowing through the bridge circuit. Figure 3 shows a more simplified equivalent circuit which employs resistive elements R1, R2 in place of constant current sources 1,2. If R 1 , R 2 >> R x , R y is satisfied, current I shows a substantially constant value, so that the circuit of Figure 3 operates similarly to that of Figure 2. Figure 4 is another simplified circuit in which resistive elements R x , R y are connected in series and output Vz is extracted from connecting points P3 between them. Thus, output Vz is expressed by Vz = V(R x /(R x +R y )). Resistive elements R x R y are assumed to have a resistance temperature characteristic as shown in Figure 5. If a resistance value at 0 °C is R xo , a resistance value R x at a temperature T is expressed by R x = R xo (1+αT). Also, a resistance value R y at a temperature T is expressed, in the same manner, by R y = R yo (1αT). As described above, a temperature at resistive element R x at the upstream side is substantially equal to an ambient temperature T A °C because of a highly insulating material employed to insulate resistive element R x from the heater element. Therefore, when a fluid flow causes heat of the heater element to transfer such that the temperature of the downstream element R x is raised by ΔT D °C a temperature of resistive element RY at the downstream side of heater element H is expressed by T A +ΔT D °C. Under the condition as described above, output Vz supplied from the bridge circuit of Figure 1 to differential amplifier 3 is expressed as follows: Vz = I(R X - R Y ) = I [R XO {1 + α(T A + ΔT D )} - R YO (1 + αTA)] = I{R XO - R YO + α(R XO - R YO )T A + αR XO ΔT D } If ΔT D is assumed to be zero, in order to see a zero point fluctuation characteristic of output Vz, expression (1) is transformed to the following expression (2): Vz = I{R XO - R YO + α(R XO - R YO )T A } Further assuming that R XO is equal to R YO , Vz is zero. However, when R XO is not equal to R YO , e.g., due to variations in a manufacturing process, output Vz is determined by expression (2) and accordingly dependent on an ambient temperature T A . In Figure 3, if R 1 , R 2 >> R X , R Y stands, the same relationship as the circuit as shown in Figure 2 is established. In the circuit shown in Figure 4, output Vz is expressed by the following expression: Vz =V R XO {1 + α(T A + T D } R XO 1 + α T A + ΔT D + R YO 1 + αT A If ΔT D is zero, that is, if there is no flow, expression (3) is transformed to the following expression (4): Vz = V R XO R XO + R YO It can be seen from expression (4) that even if R XO is not equal to R YO , due to variations in production, voltage signal Vz is not influenced by ambient temperature T A since voltage Vz is calculated only by a ratio of resistance values R XO, R YO . However, it is generally difficult to make R XO, R YO equal to each other, the circuit arrangement of Figure 4 is more practical than that of Figure 2. Thus, in Figure 4, voltage signal Vz' at a reference point where temperature difference ΔT D between sensor elements R X , R Y is zero is expressed as (3) minus (4): Vz'=V R XO 1 + α(T A + T D R XO 1 + α T A + ΔT D + R YO 1 + αT A · V R XO R XO + R YO = R XO R YO αΔT D R XO + R YO 1 + αT A + R XO αΔT D R XO + R YO If change in ΔT D is small and if 1+αT A >> αΔT D stands, Vz' is expressed by the following approximation (6): Vz' ≈ V R XO R YO αΔT D R XO + R YO 1 + αT A As is apparent from approximation (6), voltage signal Vz' is influenced by ambient temperature T A such that, when α is a positive value, voltage signal Vz' becomes smaller as ambient temperature T A is higher. To reduce such influence of ambient temperature, diode D is inserted whose forward voltage represents a negative temperature sensor element R y , as shown in Figure 6. However, it is quite difficult to match the temperature coefficient of diode D with those of temperature sensor elements R x , R y . Also, temperature compensation is not precisely achieved e.g. by variations in forward voltage of diode D and difference in temperature between diode D and temperature sensor elements R x , R y which may occur if they are located separately. A microbridge fluid flow sensor is disclosed in former German Democratic Republic Patent Application NO. 209907 in which temperature sensitive elements mounted on a substrate form the arms of a bridge circuit with a heater being provided to heat fluid as it flows across the substrate between sensitive elements. The sensor includes an ambient temperature compensating circuit including a further temperature sensitive element but the compensating circuit lacks stability due inter alia to the remoteness of the compensating element from the bridge elements. United States Patent Number 4343 183 discloses an apparatus for measuring the flow of air through the air intake manifold of an internal combustion engine. A controller regulates current to a bridge circuit having a first temperature-dependent resistor which is disposed in the air flow within the manifold and whose resistance is a function of the current flow therethrough and the flow rate and temperature of the air. A second fast responding temperature-dependent resistor, which is disposed in the manifold and supplied with a very small current so that its resistance is essentially a function of the air temperature, provides an air temperature signal which is amplified and/or differentiated and supplied to the controller as a precise correction signal for the bridge diagonal voltage signal which is also an input signal to the controller, so that the current output of the controller is a measure of the air flow. This arrangement employs only two sensors and attempts to provide correction of the measurement relating to temperature of an air flow made by a single sensor to determine flow rate; it does not employ a heater. The present invention employs two sensors and a heater and a compensation circuit which includes a third temperature sensor for providing compensation for ambient temperature to the first two sensors to permit very accurate flow measurement. US-A-4566 320 discloses another flow sensing or differential pressure sensing device utilizing a pair of bridges. A first bridge operates a heating element and a temperature dependent resistor to in turn control an error integrator. The error feedback provides the voltage for both of the bridges thereby keeping the operation of the bridges compensated for ambient temperature variations. This controls the supply of current to a heating element dependant upon ambient temperature it does not maintain constant the current to the first and second sensors. OBJECTS AND SUMMARY OF THE INVENTION In view of the problem as mentioned above, it is an object of the present invention to provide a temperature compensating circuit which is capable of accurately eliminating influences of ambient temperature and producing a precise voltage signal indicative of a difference in temperature between two temperature sensor elements provided on a semiconductor substrate. According to the invention there is provided a fluid flow sensor comprising first and second temperature sensitive elements (Rx,Ry) formed on a semiconductor substrate (10) on opposite sides of a heating element in the direction of fluid flow so that fluid flow causes a differential heating of one of the temperature sensitive elements relative to the other, a circuit for measuring the fluid flow provided by the first and second temperature sensitive elements connected in series with a junction point between the first and second elements being connectible to a circuit for indicating fluid flow as a function of the voltage at the junction, characterised in an ambient temperature compensation circuit comprising means (4,R1) for providing a constant current and a third temperature sensitive element (R R ) formed on the semiconductor substrate and coupled with the constant current means such that a voltage developed across the third temperature sensitive element provides a voltage that is a function of the ambient temperature of the fluid which voltage controls, and maintains constant, a current supplied to the first and second temperature sensitive elements. The above and other objects and features of the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view showing an appearance of a flow sensor; Figures 2, 3 and 4 are schematic circuit diagrams respectively showing a prior art circuits; Figure 5 is a graph showing a resistance - temperature characteristic of a temperature sensor element; Figure 6 is a schematic circuit diagram showing a prior art temperature compensating circuit employing a diode; and Figures 7, 8 and 9 are schematic circuit diagrams respectively showing first, second and third embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will now be described with reference to Figure 7. In Figure 7, like parts corresponding to those in Figures 2 - 4 are designated the same reference numerals and detailed explanation thereof will be omitted. In a temperature compensating circuit of Figure 7, third temperature sensor element R R is disposed on the same substrate as sensor elements R x , R y . Constant current source 4 is coupled in series to third temperature sensor element R R to supply the circuit with constant current I. Amplifier 5 has its non-inverting input terminal coupled to a connecting point P 4 between source I and sensor element R R to be supplied with voltage V R and its output terminal coupled to an end of serially connected sensor elements R X , R Y to thereby form a voltage follower circuit. A voltage V R across sensor element R R is amplified by voltage follower circuit 5 to produce a voltage V R ′ which in turn is applied to temperature sensor elements R X , R Y , and consequently a voltage Vz is extracted from connecting point P 5 between resistive elements R X , R Y . Voltage V R ′ derived from amplifier 5 is expressed by the following expression (7): V R ′ = V R = IR R = IR RO (1 + αT A ) From this expression (7) and the foregoing expression (6), Vz′ is expressed as follows: Vz'= V R 'R XO R YO αΔT R XO + R YO 1 + αT A = IR RO 1 + αT A R XO R YO αΔT R XO + R YO 1 + αT A = IR RO R XO R YO αΔT R XO + R YO It will be understood from expression (8) that voltage Vz′ will not be influenced by ambient temperature T A . Further, a current I XY flowing sensor elements R X , R Y is expressed by the following expression (9): I XY = V R ' R X + R Y = IR R R X + R Y If the thickness of these elements varies by a ratio d with respect to a reference thickness, current I XY ′ through the sensor elements R X , R Y is: I XY '= IR R 1 + d R X 1 + d + R Y 1 + d = IR R R X + R Y As is apparent from the expression (9)′, current I XY ' is equal to current I XY , that is, the circuit is free from the influence of variation d in thickness which may occur in a manufacturing process. As described above, since third temperature sensor element R R is disposed on the same substrate surface as first and second sensor elements R X , R Y , a difference in temperature at the locations of sensor elements RR, RX is not present so that an error in temperature compensation can be largely reduced. Also, current through the sensor elements R X , R Y can be always maintained substantially constant even if these elements respectively have a different resistance value from each other. Figure 8 shows a second embodiment of the present invention where constant current source 4 in Figure 7 is replaced by resistor R 1 and feedback resistor R 2 , R 3 are connected to voltage follower circuit 5. Thus, output voltage V R ′ from voltage follower circuit is expressed by: V R '= V R R · R 3 R 1 · R 2 In this expression, if it is assumed that R 1 >> R R stands, current I through third sensor element R R is substantially constant, thereby making it possible to readily perform the same function as the embodiment shown in Figure 7. Figure 9 shows an arrangement for producing output Vz′ based on output voltage V R ′ of voltage follower circuit 5. Serially connected resistors R A , R B are connected to serially connected first and second temperature sensor elements R X , R Y to form a bridge circuit. Then, the inverting terminal of differential amplifier 7 is connected to a connecting point P 6 between resistors R A , R B while the non-inverting terminal of the same is connected to a connecting point P 7 between sensor resistive elements R X , R Y . In this arrangement, resistors RA, RB are selected so as to establish R Y ·R B = R X ·R A when temperature difference ΔT D between sensor elements R X , R Y is zero. As described above, the temperature compensating circuit according to the present invention has a third temperature sensor element for detecting an ambient temperature disposed on the same substrate on which first and second temperature sensor elements are located, and a voltage across the third sensor element, generated by supplying a current thereto, is applied to the first and second sensor elements. Therefore, a voltage applied to first and second sensor elements is automatically changed corresponding to change in ambient temperature, and accordingly a difference in temperature between first and second sensor elements can be precisely detected in a form of voltage signal.

Claims: A fluid flow sensor comprising first and second temperature sensitive elements (Rx,Ry) formed on a semiconductor substrate (10) on opposite sides of a heating element in the direction of fluid flow so that fluid flow causes a differential heating of one of the temperature sensitive elements relative to the other, a circuit for measuring the fluid flow provided by the first and second temperature sensitive elements connected in series with a junction point between the first and second elements being connectible to a circuit for indicating fluid flow as a function of the voltage at the junction, characterised in an ambient temperature compensation circuit comprising means (4,R1) for providing a constant current and a third temperature sensitive element (R R ) formed on the semiconductor substrate and coupled with the constant current means such that a voltage developed across the third temperature sensitive element provides a voltage that is a function of the ambient temperature of the fluid which voltage controls, and maintains constant, a current supplied to the first and second temperature sensitive elements. A fluid flow sensor as claimed in claim 1, characterised in that the constant current to the first and second temperature sensitive elements is provided by an amplifier (5), the voltage developed across the third temperature sensitive element being applied to an input of the amplifier and an output of the amplifier being connected to the first and second elements coupled in series. A fluid flow sensor as claimed in claim 1 or claim 2, characterised in that the means (4, R1) for providing the constant current through the first and second temperature sensitive elements includes a constant current source (4) connected in series with the third temperature sensitive element (R R ). A fluid flow sensor as claimed in claim 2, characterised in that the means (4,R1) for providing the constant current through the first and second temperature sensitive elements includes a resistor (R1) connected in series with the third temperature sensitive element (R R ) and the amplifier (5) is provided with a feedback resistor R3. A fluid flow sensor as claimed in any preceding claim, characterised in that the serially connected first and second temperature sensitive elements (R x , R y ) are connected across serially connected resistors (R A , R B ) to form a microbridge sensor and in that the junction point and the resistors (R A , R B ) are connected to the respective inputs of an electrical amplifier which in operation of the sensor provides an output dependant on the fluid flow between the temperature sensitive elements.

Inventors: HISANAGA TETSUO, HISANAGA, TETSUO
Applicants: YAMATAKE HONEYWELL CO LTD, YAMATAKE-HONEYWELL CO. LTD.
Application year: 1990
Main classification: G01F1
IPC codes: G01F1, G01D3, G01P1, G01P5