GB2327503A

GB2327503A - Device for measuring flow throughput of a flowing medium

Info

Publication number: GB2327503A
Application number: GB9815386A
Authority: GB
Inventors: Josef Kleinhans
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1997-07-17
Filing date: 1998-07-15
Publication date: 1999-01-27
Anticipated expiration: 2018-07-15
Also published as: GB2327503B; GB9815386D0; DE19730580A1; JPH1194620A

Abstract

A device for measuring the flow throughput of a flowing medium comprises a sensor (10), which comprises two resistance bridges (12, 13) and is exposed to the flowing medium. The resistors of the bridges (12, 13) are heated to an excess temperature by a heating resistor (Rh). One resistance bridge (12) produces the actual measurement signal (UM) and the other one produces a signal (UH) which is temperature-dependent and serves for regulation of the heating The two signals are converted into rectangular voltages with the keying ratios (TV(Tu)) and (TV(m) see figure 4). The mass of the flowing medium can be ascertained by comparison of the rectangular voltages in a phase detector with sign recognition as well as digital further processing with the aid of counters, whereby additional corrections or balances are possible.

Description

DEVICE FOR MEASURING FLOW THROUGHPUT OF A FLOWING MEDIUM The present invention relates to a device for measuring flow throughput of a flowing medium, for example the mass of air inducted by an internal combustion engine.

Sensors, and associated evaluating circuits, by means of which the throughput of a flowing medium can be ascertained are known from, for example, WO 97/18444. In the case of such sensors, which are suitable in particular to ascertain the mass of air inducted by an internal combustion engine, two temperature-dependent resistors are heated by a heating resistor to an excess temperature presettable relative to the medium. These resistors are arranged above and below the heating resistor with respect to the direction of flow of the medium, so that they are heated uniformly by this medium, but cooled to different extents by the flowing medium. The resulting measurement voltage is used for ascertaining the mass of the flowing medium. In order that the heating of the heating resistor can be checked reliably, further temperature-dependent resistors are used in a measurement bridge provided on the sensor element. By evaluation of the temperature of these resistors, an optimum heating regulation can be achieved. The known sensor comprises additional temperature-compensating circuit components which ensure that the measurement voltage to be evaluated is obtained free of errors. The signals of the known sensors are usually evaluated by an evaluating equipment, for example a control device in a vehicle, connected downstream thereof. The adaptation of the sensor to the downstream electronic system can in some circumstances lead to problems.

According to the present invention there is provided a measuring device for measuring the throughput of a flowing medium, with a sensor which can be exposed to the flowing medium and comprises two temperature-dependent resistance bridges, which are heatable to a presettable excess temperature by means of a heating resistor, wherein a first one of the bridges is a component of a heating regulation circuit and produces a temperature-dependent voltage at one diagonal and the second one of the bridges produces a measurement voltage at one diagonal in dependence on the mass of the flowing medium, characterised in that the two resistance bridges are connected in parallel with each other, that the voltage fed by the first resistance bridge to a heating regulator and the measurement voltage are transferred into rectangular voltages of equal frequency with the keying ratios and that the two keying ratios are counted out by means of a digital counter in order to ascertain the mass of the flowing medium.

Preferably, the second resistance bridge comprises at least two temperature-dependent resistors which, with reference to the direction of flow of the medium to be detected, are so arranged above and below the heating resistor that they are heated equally by this, but are cooled to different extents by the flowing medium, the measurement voltage being consequent on the temperature difference.

For preference, the formation of the keying ratios is effected by means of respective converting circuits, wherein one converting circuit performs a frequency synchronisation.

For frequency synchronisation, the keying ratio from the other converting circuit can be halved and the signal thus produced fed to said one converting circuit.

The rectangular signals with the keying ratios are preferably processed further by means of further digital components, which perform balancing functions, for example at least one additive basic balance and/or a multiplicative basic balance and/or a temperature balance.

For preference, the output signals of the digital components are superimposed on each other in an adding stage. The output signals of the adding stage can be fed to a counter with presetting, as well as the rectangular signal derived from the voltage dependent on the mass of the flowing medium, wherein the counter forms count values, which are superimposed on each other for the formation of an output signal serving as measure for the flowing medium.

Expediently, a phase-locked loop-circuit is present, which produces knock signals at an oscillator frequency, which signals serve as count signals.

For preference, a keying ratio converter, which delivers an output signal representing a measure of the ascertained mass of the flowing medium, is present at the output of the digital evaluating circuit.

A device embodying the invention may have the advantage by comparison with prior art devices that an optimum adaptation of an electronic analog system of the sensor to modern integrated switching circuit processes, which can exploit their advantages only by digital signal evaluation, can take place. A higher accuracy is thereby obtained in the end for the entire signal evaluation. These advantages can be achieved in that a sensor and an associated evaluating circuit are so modified that the measurement voltage as well as the so-called converting voltage, which can be derived from a compensation resistance arrangement and serve as a measure of the temperature, is converted into a rectangular voltage with the keying ratios and these keying ratios are evaluated with the aid of a phase-locked loop circuit in a phase detector. A particularly advantageous evaluation can be achieved by synchronisation of the oscillator frequency of the two keying ratios. It is particularly advantageous that the entire arrangement can be balanced by means of a programmed balance so that, for example, no sensitivities to light arise. Furthermore, it is advantageous that a coded signal can be formed for evaluation in the device, wherein this can be delivered in a flexible manner by way of an output. If required, the signal derived from the first bridge can be made available as an optional signal for further evaluation. It is particularly advantageous that the entire device is secure against interference as a consequence of the use of an electronic digital system.

An example of the invention will now be more particularly described by way of example with reference to the accompanying drawings, in which Fig. 1 is a schematic circuit diagram of a measuring device embodying the invention; Fig. 2 is a circuit diagram of one converting circuit in the device; Fig. 3 is a circuit diagram of another converting circuit in the device; and Figs. 4a) to g) are diagrams illustrating courses of signals produced in the device.

Referring now to the drawings, there is shown in Fig. 1 a flow measuring device comprising a hot-film air mass sensor 10 and a digital evaluating circuit 11. The sensor 10 comprises two resistance bridges 12 and 13, which at least partially comprise temperature-dependent resistors. The resistance bridge 12 represents the actual measurement bridge, the resistance bridge 13 being part of a heat-regulating circuit for a heating resistor Rh, which is a part of the sensor and heats the individual resistors to the desired measurement temperature. The two bridges 12 and 13 are connected in parallel with each other and are each connected by way of an inverter I between a reference voltage URef and ground. The sensor components are built up on a carrier, for example a substrate, and the sensor is exposed to the medium to be ascertained, for example the air flow in the induction duct of an internal combustion engine, in suitable manner.

The measurement bridge 12 is structured as a temperature difference bridge and comprises resistors Rabl, Rab2, Rau1, Rau2 and Rp. These resistors are temperaturedependent resistors which are heated to excess temperature by the heating resistor Rh.

The resistors Rabl and Rab2 are in that case arranged downstream of the heating resistor with respect to a given direction of flow of the medium to be detected whereas the resistors Raul and Rau2 are arranged upstream.

A measurement voltage UM, which arises across the bridge diagonal, is fed to a conversion circuit 14 of the digital evaluating circuit 11. The conversion circuit 14 is connected with the junction of the two resistors Rabl and Raul, with the junction of the resistors Rau2 and Rp and with the junction of the resistors Rp and Rab2 and forms a keying ratio TV(m).

The resistance bridge 13 associated with the heat-regulating circuit comprises resistors R1, R2, Rhf and Rlf. Rhf is in that case the heater temperature sensor and Rlf the air or media temperature sensor. The values of these resistors are dependent on temperature so that a voltage UH arises across the bridge diagonal of the bridge 13. This voltage is fed to the evaluating circuit 11, in particular it is fed to a heat regulator 15 as well as to a conversion circuit 16 with frequency synchronisation. The conversion circuit 16 furthermore increases the keying ratio TV(m) divided by 2 in a block T2.

The basic construction of the sensor 10 and its manner of function are described in WO 97/18444 and are therefore not explained in more detail. The evaluating circuit 11 evaluates the voltages UM and UH supplied by the sensor so that, as described in the following, a signal is obtained which corresponds directly with the mass m of the flowing medium.

The digital signal-evaluating circuit 11 comprises, apart from the mentioned conversion circuits 14 and 16 and the heating regulator 15, further digital components by which different balances can be performed. A component for performing a multiplicative basic balance with Tm is denoted by 17 and an additional basic balance Ta becomes possible by a block 18. An additive temperature balance Tu can be achieved by the digital component 19. The signals TV(m)*N/Tm, N/Ta and W(Tu)*N/Tu are fed to corresponding inputs Nm, Na and Nt of an adding stage 20. The signals produced in the balancing blocks are added in suitable manner in the adding stage 20.

The three digital balancing components 17, 18 and 19 are constructed in like manner.

They each comprise a block 21, 22 and 23 for the preliminary setting of Tm, Ta and Tu, blocks 24, 25 and 26 to which the preliminary setting and the signal respectively to be evaluated are fed, and counters 27, 28 and 29 which perform the respective multiplicative or additive correction.

The output signals of the adding stage 20 are fed to a further counter 30 with preliminary setting. A count value Ni, which is obtained from the keying ratio TV (m), is also fed to this counter 30. Moreover, the counter 30 is connected with a phase-locked loop circuit 31, which is also connected with the digital block 18 and the heating regulator 15. The counter 30 leads to the block 34, which is operated selectably in the circuit mode of parallel-in and serially-out or as a keying ratio converter. The block 34 can be operated, selectably by way of the switch 33, by an auxiliary frequency FAUX or by the frequency fvco produced in the phase-locked loop circuit. The output of the block 34 is connected with the output m OUT by way of a filter circuit with a resistor R and capacitor C. At this output m-OUT, a signal is derivable, which corresponds directly with the mass of the flowing medium.

Further inputs and outputs of the digital signalevaluating circuit are as follows: Ubal: terminal for the battery voltage, URef: terminal for a reference voltage, GND: earth terminal, PINT: PRIT terminal for program or pulse signal, DA: data input Tu-OUT: temperature output.

The output signal at the temperature output Tu-OUT is obtained from the keying ratio of the conversion circuit 16 with the aid of a signal converter 35 and is, for example, available directly as temperature value.

For and explanation of the function of the digital signal-evaluating circuit 11, the conversion circuits 14 and 16, by which the measurement voltage UM(m) is converted into a keying ratio TV(m) and the temperature-dependent voltage UH(Tu) is converted into a keying ratio TV(Tu), are initially described with the aid of Figs. 2 and 3.

The conversion circuit 14 and its connection to the measurement bridge 11 are illustrated in Fig. 2. The conversion circuit is connectible to the resistor Rp of the bridge 11 by way of two switches S1 and S2, wherein the voltages U1 and U2 are applied across the terminals.

A further connection leads from the circuit 14 to the junction of the resistors Rab1 and Rau2. A voltage UO lies at this junction. In detail, the circuit 14 comprises an integrator OP1, a comparator KP1 and an associated feedback circuit with resistors R4, R5 and R6 and a capacitor C4. A pull-up resistor Rpullupl, which is also connected with the switch S1, lies between the output of the comparator KP1 and a reference voltage terminal URef.

A flip-flop FF1 is connected to the output of the comparator KP1 and actuates the switch S2.

The conversion circuit 14, by which the measurement voltage UM is to be converted into a keying ratio TV(m), is an oscillating system. For the switching hysteresis of the comparator KP1 and on the assumption that the pull-up resistance is very much smaller than the resistances R5 and R6, the following equation applies: A = R5/R6 * URef.

When the signal at the output of the comparator KP1 is high, then for the integrator current IC4 = (U2 - Ul) /R4.

The time TH, for which the output of the comparator KP1 is high, is then computed according to the equation: IC4*TH = C4*A TH = (C4*AU) llC4 = R4*C4*AU/ (U2 - UO).

For the low time TL, there results in analogous manner: IC4 = (UO - U1) /R4 or IC4*TL = C4 + #U TL = C4*#U/IC4 = R4*C4*#U/ (U0-U1).

There then applies for the oscillation period that

1 1 U2 - U1 TH+TL = R4C4*#U ( ) = R4*C4*#U* U2 - U0 U0 - U1 (U2 - U0)(U0 - U1) For the oscillation frequency, there applies; (U2 - U0)(U0 - U1) f = * TH + TL R4 * C4 * #U U2 - U1 For the keying ratio, there applies: TH #U 1 (U2 - U0)(U0 - U1) U0 - U1 TV (m) = = R4 * C4 * * * = TH + TL U2 - U0 R4 * C4 * #U U2 - U1 U2 - U1 Since the voltages UO and U1 change in opposite sense with m, the keying ratio also changes in this case and is thus dependent on m and there applies: TV=f(r).

The conversion circuit, which is illustrated in Fig. 3, with frequency synchronisation corresponds largely with the circuit illustrated in Fig. 2. The individual components of this circuit are an integrator OP2, a comparator KP2 and a feedback circuit composed of resistors R10, R11 and R12 and a capacitor C10. A pull-up resistor Rpullup2 corresponds with the pull-up resistor according to Fig. 2. The corresponding switches are denoted by S3 and S4 and the flip-flop, which actuates the switches S3 and S4, has the reference FF2. The switches S3 and S4 can produce a connection for series connecting of the resistors R7, R8 and R9, which lie between the reference voltage URef and ground. A further connecting line of the conversion circuit 16 leads to the junction between the resistors R2 and Rlf. By way of this connecting line, the temperaturedependent voltage UT is fed to the non-inverting input of the operational amplifier OP2 and the inverting input of the comparator KP2.

By contrast to the circuit according to Fig. 2, the series connection of resistors R7, R8 and R9 is present is addition to the actual second bridge, whilst the series connection Rab2, Rp and Raul according to Fig. 2 is a component of the measurement bridge. The pull-up resistor in Fig. 3 leads to a phase detector 36 with sign recognition, which is connected by way of a block 37 with the output of the comparator KP2 and the pull-up resistor Rpullup2.

A further connection of the phase detector 36 leads by way of a block 38 to the output of the comparator KP1 of the circuit according to Fig. 2. The keying ratio TV (rn) is fed by way of this terminal, whilst the keying ratio TV(Tu) is feedable by way of the other terminal of the phase detector. The frequency of the signal fed to the blocks 37 and 38 is halved each time.

The production of the keying ratio TV(Tu) in the circuit of Fig. 3 occurs in like manner to the circuit of Fig. 2. The difference from the circuit according to Fig. 2 is that the frequency of the oscillating system or the oscillator is synchronised with the oscillating system or oscillator of the circuit of Fig. 2. If the oscillation period, the oscillation frequency and the keying ratio are forrned analogously for the circuit according to Fig. 2a, it can be recognised that the oscillator frequency is inversely proportional to the switching hysteresis, thus: f U.

Since AU = R11/R12*Ux, it is to be recognised that the frequency f is proportional to 1/Ux.

The frequency of the oscillator can be controlled by the voltage Ux, which occurs at the output of the phase detector 36 or at the pull-up resistor Rpullup2. This frequency control takes place in that the frequency of the keying ratio TV (rn) and the frequency of TV(Tu) are halved. In Fig. 3, this is represented by the blocks 37 and 38, which divide the supplied signal by the factor 2.

The signals, which arise after the frequency division and with the keying ratio of 50%, are fed to the phase detector with sign recognition and compared with each other. By the output signal of the phase detector with sign recognition, the frequency of the keying ratio TV(Tu) is regulated to become equal to the frequency of the keying ratio of TV(m).

Since both trequencies are equal, the two keying ratios TV(Tu) and TV(m) can be counted out with the same clock frequency in the overall circuit. This clock frequency is made available by the phase-locked loop circuit.

Together with the already described ditital components for the additive basic balance with Ta, a multiplicative basic balance with Tm and an additive temperature balance Tu, a corrected signal can be produced from the signals TV (n) and TV(Tu) or the associated current values NI in the counter 30, which corrected signal is delivered by way of the keying ratio converter 34 and the following resistance-capacitance filter as measurement value m for the mass of the flowing medium.

Some of the signals entered in Fig. 1 are illustrated as a function of time in Figs. 4a) to g).

Fig. 4a) shows the keying ratio TV (; n) and Fig. 4b) shows the keying ratio TV(Tu), wherein the frequency f TV (m) is equal to the frequency f TV(Tu). The lengths of the high and low phases of the two signals are, however, different.

The counter state Ni = TV(rh)*N is illustrated in Fig. 4c). Counting is always carried out from the edge change from low to high of the two signals according to Fig. 4a) or 4b). The counting ends when the signal according to Fig. 4a) passes from high to low. Resetting takes place before the beginning of counting. The maximum count value is Ni and the counting rate amounts to N.

The count value Nm = Ni/Tm is shown in Fig. 4d). The counting rate is N/Tm and the final value is always Nm.

Fig. 4e) shows the count value Na = N/Ta. The counting rate in that case amounts to N/Ta.

The counter state Nt = TV(Tu)*N/Tu is shown in Fig. 4f). The count rate amounts to N/Tu and the final value amounts to Nt.

Finally, the count value, which is obtained at the output of the counter 30 with preliminary setting is indicated in Fig. 49). The counter state is Nout = Ni + P, wherein P = Nm + Na + Nt. The final state of this counter amounts to Ni + P. The counter state Nout of the counter 30 can be represented by the following equation: N N N Nout=Ni+P+Ni+Nm+Na+Nt=TV(m)*N+TV(m) * + + * Tm TA Tu It is thus true for the output counter that:

@ut 1 1 TV(T = TV (m) * + + N Tm TA Tu The value for the mass of the flowing medium is ascertained from the counter content in block 32 (parallel-in and serially-out or keying ratio converter).

Claims

CLAIMS 1. A measuring device for measuring flow throughput of a flowing medium, comprising a sensor which is intended to be disposed in a flow path of the medium and which comprises a first heatable temperature-dependent resistance bridge for providing a temperature-dependent voltage at one bridge diagonal, a second heatable temperaturedependent resistance bridge connected in parallel with the first bridge and for providing a voltage dependent on the mass of the flowing medium at one bridge diagonal and resistance heating means for heating the bridges to an excess temperature, and a processing circuit which comprises heating regulating means for regulating the heating of the bridges in dependence on the temperature-dependent voltage, converting means for converting the voltages into rectangular voltage signals of equal frequency with respective keying ratios, and digital counting means for counting out the ratios to determine the mass of the flowing medium.
2. A device as claimed in claim 1, wherein the second bridge comprises at least two temperature-dependent resistors arranged so as to be disposed in use respectively above and below the heating resistance means with respect to a given direction of flow of the medium along the flow path and to be heated substantially equally to the heating means, but cooled differently by the medium flow, the voltage dependent on the mass of the flowing medium being indicative of the temperature difference of the at least two resistors..
3. A device as claimed in claim 1 or claim 2, the converting means comprising a respective converting circuit for producing each of the rectangular voltage signals, one of the circuits being arranged to synchronise the signal frequencies.
4. A device as claimed in claim 3, comprising means to halve the frequency of the rectangular voltage signal produced by the other converting circuit and to apply the signal with halved frequency to said one converting circuit for the frequency synchronisation.
5. A device as claimed in any one of the preceding claims, wherein the processing circuit comprises digital means for carrying out balancing functions in relation to the rectangular voltage signals.
6. A device as claimed in claim 5, wherein the balancing functions comprise at least one of an additive basic balancing, a multiplicative basic balancing and a temperature balancing.
7. A device as claimed in claim 6, comprising adding means for additive superimposition of balancing output signals of the digital means.
8. A device as claimed in claim 7, the counting means being operable in dependence on output signals of the adding means and on the rectangular voltage signal derived from the voltage dependent on the mass of the flowing medium to form count values which are superimposed to form an output signal acting as a measure of the mass of the flowing medium.
9. A device as claimed in any one of the preceding claims, comprising a phase-locked loop circuit for producing count signals for the counting means.
10. A device as claimed in any one of the preceding claims, wherein the processing circuit comprises a keying ratio converter controlled by the counting means to provide an output signal indicative of the mass of the flowing medium.
11. A measuring device substantially as hereinbefore described with reference to the accompanying drawings.