GB2309787A

GB2309787A - Measuring the throughput of a flowing medium

Info

Publication number: GB2309787A
Application number: GB9700977A
Authority: GB
Inventors: Wolfgang Kienzle; Josef Kleinhans; Hans Hecht; Gerhard Hueftle; Klaus Reymann; Uwe Konzlemann; Dirk Lamprecht
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1996-01-31
Filing date: 1997-01-17
Publication date: 1997-08-06
Anticipated expiration: 2017-01-17
Also published as: GB9700977D0; DE19603346A1; GB2309787B; JPH09218068A

Description

1 2309787 MEASURING MEANS FOR MEASURING THE THROUGHPUT OF A FLOWING MEDIUM

The present invention relates to measuring means for measuring the throughput of a flowing medium, for example the mass of air inducted by an internal combustion engine. 5 Sensors by which the throughput of a flowing medium can be ascertained are known from, for example, DE 05 43 24 040. In this case, the sensor is exposed to the flowing medium, for example the air flow in the induction duct of an internal combustion engine. The sensor usually comprises a heater which is heated by a regulated current to an excess temperature relative to the medium. The heater is associated with a heater temperature sensor as well as a temperature sensor which detects the temperature of the flowing medium. Two temperature-dependent resistors, which are heated uniformly by the heater, are disposed in the proximity of the heater. However, they are cooled to different extents by the flowing medium, since the resistor which is first in the direction of flow is cooled more strongly than the other resistor. The two resistors are components of a bridge circuit. The temperature difference resulting from the different degree of cooling results in a measurement voltage at a diagonal of the bridge, which voltage is evaluated in an evaluating circuit, for example a microcomputer, to ascertain the mass or flow rate of the flowing medium.

According to the present invention there is provided measuring means for measuring_ the throughput of a flowing medium, comprising a carrier, which is placeable in the flowing medium and on which a resistance arrangement is arranged, which is connected as a bridge with one diagonal thereof lying between a supply voltage and ground and comprises at least two temperature- dependent resistors which with respect to the direction of flow of the medium are arranged above and below at least one heating resistor, to which a heating voltage is applied so that they are heated uniformly by this but are cooled differently by the flowing medium and the measurement voltage, which arises at the other bridge diagonal due to the temperature difference, is applied to an evaluating equipment for ascertaining the throughput, characterised in that the heating voltage is substantially constant or constant section by section and the measurement voltage is evaluated in the evaluating equipment by means of at least one filed characteristic which produces a relationship between the measurement voltage and the throughput of the flowing medium.

Measuring means embodying the invention may have the advantage by comparison with known measuring arrangements that the circuit components usually needed for the temperature regulation of the heating circuit can be dispensed with without the required accuracy and reliability being impaired in the evaluation. Consequently, a noticeable cost reduction is possible for the same reliability. This advantage can be achieved by supplying a constant heating voltage or a heating voltage which is constant at least section by section, and any inadequacies possibly arising, for example too low a characteristic slope in a downstream evaluating circuit, can be corrected in that during the ascertaining of the throughput of the flowing medium account is taken of a characteristic or characteristics producing a relationship between the measurement voltage and the mass of the flowing medium.

It is particularly advantageous if the value of the heating voltage is influenced by changing its value in dependence on the mass of the flowing medium. Thus, a stepwise tracking is possible in advantageous manner. In the simplest case, tracking of the heating voltage is carried out by comparing the measurement voltage with a presettable threshold voltage and switching over to a higher heating voltage when this threshold is exceeded by the measurement voltage, a corresponding characteristic being switched over to for the evaluation at the same time. When the measurement voltage falls below the threshold voltage again, a return is made to the original state. Thus, the characteristic slopes necessary for an optimum evaluation can be achieved in advantageous manner.

More particularly, a change-over switch can be present, by way of which the heating voltage is fed to the heating resistor and the change- over switch is switchable by the evaluating equipment, such as a control device of an engine fitted with the measuring means, so that at least two different heating voltages can be applied to the heating resistor. The selection of the supplied heating voltage can then take place in dependence on the output voltage of the bridge and thereby in dependence on the intensity of the flowing medium. For preference, the output voltage is compared in the control device with at least one threshold voltage and, on the threshold voltage being exceeded, a drive control signal is applied by the control device to the change-over switch to switch this over from a lower to a higher voltage. The change-over switch can be switchable between several voltages such that a switching is initiated each time a threshold voltage is exceeded or fallen below and the lowest heating stage is selected for a low output voltage and thereby a small throughput of the flowing medium.

In an advantageous embodiment, the evaluation of the measurement voltage and the comparison of the measurement voltage with one or more threshold voltagesas well as any necessary switching-over is performed in a control device of an internal combustion engine fitted with the measuring means. The required characteristics are then filed in storage devices of the control device itself.

An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawing, the single figure of which is a schematic circuit diagram of measuring means embodying the invention.

Referring now to the drawings, there is shown measuring means for ascertaining the throughput of a flowing medium, the illustrated circuit being based on that disclosed in DE-OS 43 24 040, but with a different heating arrangement. The principal components of the measuring means are disposed on a carrier 10, for example a substrate, and exposed in suitable manner to a flowing medium to be measured, for example the air flow in the induction duct of an internal combustion engine.

A heating circuit of the measuring means comprises a resistor R-,, which is connected by way of a change-over switch UMS and a resistor R. to a terminal to which, for example, the battery voltage UB of the vehicle battery is applied. A Zener diode D1 and a capacitor Cl serve as protective circuit.

The switch UMS comprises at least one switching element, for example a transistor or similar, which is drivable by way of a terminal SC and a first switch setting connects the heating resistor RH by way of the resistor Rl to the terminal for the voltage UB, whilst in a second setting it connects the heating resistor RH by way of a resistor R2 with a terminal at which a selectable voltage Ul is present. In an extendEd version, the switch UMS comprises several terminals by way of each of which a suitable voltage can be applied to the heating resistor RH.

The actual measurement circuit is a resistance bridge circuit, which is denoted by DT, functioning as a temperature difference bridge (Tbridge circuit) with resistors RAB1, RAB2, RAU1, RAU2 and RP. These resistors are temperature-de pendent resistors which, as in the case of the heating resistor RH, are at excess temperature relative to the flowing medium. The resistors RAB1 and RAB2 are arranged downstream of the heating resistor with respect to the flow direction of the medium and the resistors RAU1 and RAU2 upstream of the heating resistor. They are heated uniformly by the heating resistor, but non-uniformly cooled by the flowing medium. In particular, the downstream resistors RAB1 and RAB2 are cooled less strongly, since the flowing medium has already heated up somewhat before it reaches them.

The resistance bridge of the bridge circuit DT is connected at one diagonal thereof to the output of an operational amplifier OP1, the inverting input of which is also connected with the output. A reference voltage UR is applied to the non-inverting input of the amplifier OP1 by way of a resistor R3. A resistor R4 is present between the resistor R3 and ground. The other side of the bridge diagonal is also connected to ground.

The voltage which arises across the other bridge diagonal is coupled out with the aid of a potentiometer P1, which is connected in parallel with the resistor RP. The wiper terminal of the potentiometer P1 is connected to the non-inverting input of an amplifier OP2, the inverting input of which is connected with the junction between the resistors RAB1 and RAM. The amplifier OP2 has a settable amplification. Digital amplification equalisation is carried out with the aid of a circuit block VA, which receives drive control signals from an external evaluating circuit by way of three terminals PR, DA and TA. The program for the regulation of amplification is fed by way of PR, DA being the data input and the pulse frequency being supplied by way of TA. The terminals can be connected with a control device SG.

The amplifier OP2 is also connected, by way of a voltage divider R5 and R6, with the output and with the inverting input of the amplifier OP1. The output of the amplifier OP2, at which the measurement voltage UM is derivable, is connected by way of a resistor R7 with the noninverting input of an operational amplifier OP3 and by way of a resistor R8 with ground. The inverting input of the operational amplifier OP3 is connected by way of a resistor R9 with the output of the amplifier OP1 and with the junction of the resistors RAB1 and RAU2. A resistor R10 is present between the output of the operational amplifier OP3 and its inverting input. A voltage UA, which is processed as an output voltage, is coupled out at the output of the operational amplifier OP3. The carrier 10 is connected to ground by way of a terminal GRD. The circuit part denoted by IC1 is, for example, an integrated switching circuit.

The resistor RH, thus the heating resistor, and the resistors RAU and RAB, which in the illustrated embodiment have the form of four separate resistors RAU1, RAU2, RAB1 and RAB2, are arranged on the carrier 10, for example a substrate or a membrane. The resistors RAU1, RAU2, RAB1 and RAB2 are each arranged laterally of the resistor RH, but in the immediate proximity thereof. The inflow of the medium is indicated in the figure by an arrow as well as the notation m.. It takes place in such a manner that the flow initially extends across the resistors RAU1 and RAU2, then over the resistor RH and subsequently over the resistors RAB1 and RAB2. The resistors first encountering the flow are cooled more strongly and the different cooling of the resistors is evaluated for ascertaining the flow rate or mass of the flowing medium.

When the heating resistor RH is operated with a fixed constant voltage UB, the measurement voltage Um arising at the bridge circuit and applied to the amplifier OP2 increases with increasing flow. The processing of the measurement voltage Um or of the prepared measurement voltage UM into the relevant mass of the flowing medium, for example air mass, is carried out in the control device subject to consideration of characteristics filed therein. These characteristics produce a relationship between t he measurement voltage UM and the mass of the flowing medium. The course of the characteristics is dependent on the voltage applied to the heating resistor. Since it is known to the control device which voltage is applied to the heating resistor, it can also select the characteristic which belongs to the heating voltage in use.

is Since the attainment of the characteristic slope, which is necessary for measurement purposes, can be problematic in some circumstances in the case of heating by means of a single presettable voltage UB, the heating voltage can, in one preferred embodiment, be varied in presettable manner. This variation is performed by the control device, wherein switching-over is carried out by a suitable drive control of the changeover switch UMS so that a voltage other than the voltage UB, for example the voltage U1, is fed to the heating resistor RH under predetermined conditions.

The determination of the change-over condition and thereby the choice of the suitable voltage value takes place in dependence on, for example, the mass of the flowing medium. For this purpose, the output voltage UM of the bridge circuit or the prepared voltage UA is compared with a presettable threshold value in a comparison stage of the control device SG. When the voltage UA exceeds the threshold value, the heating voltage, which is initially fixed at UB, is switched over to the higher voltage Ul. By switching-over of the heating voltage from a low to a higher voltage, the voltage arising across the measurement bridge also changes and thus also the voltages UM and UA. In that case the characteristic corresponding to the higher voltage must be selected for the correct computation of the mass of the flowing medium from the voltage UA. When the output voltage UA again falls below this threshold, the original state is returned to, the heating voltage thus being reduced to the voltage UB and the characteristic associated with the voltage UB being used in the evaluation.

By the afore-described switching-over of the heating voltage in dependence on the mass of the flowing medium, a tracking thus takes place of the heating voltage, which in the simplest case is effected by means of a preselected threshold for the measurement signal of the bridge circuit. If so desired, several switching thresholds can be introduced in order to achieve the optimum ratios of media flow to characteristic slope. Means must then be provided to enable the heating voltage to be varied in several steps. The heating voltage can then be increased by one step when the output voltage UA reaches or exceeds the next limit value.

Recognition of the threshold being exceeded or fallen below and initiation of the switching-over of the heating voltage from one value to another, as well as selection of the associated characteristic, take place in the control device SG itself.

With the described measuring means for ascertaining the throughput of a flowing medium, the heating voltage value can be low for small media flows and increased for higher media flows. Thus, a mode of operation is possible which reduces loading of the sensor, which is of particular advantage for sensors which ascertain the air mass inducted by an internal combustion engine. During starting of the engine and the heating- up phase associated therewith, a low heating voltage can initially be used. Thus, during the starting operation, the sensor can be protected and less electrical energy is needed during the starting operation, whereby the available electrical energy for other loads in the vehicle may be improved.

Claims

1. Measuring means for measuring the throughput of a flowing medium, comprising a carrier intended to be exposed to the medium flow, resistance means carried by the carrier and comprising a heating resistor connectible to a heating voltage and at least two temperature-de pendent resistors disposed respectively upstream and downstream of the heating resistor with respect to a predetermined direction of flow of the medium past the carrier and arranged in a bridge circuit connectible at one diagonal thereof between a supply voltage and ground, the temperature- dependent resistors being arranged to be heated to substantially the same extent by the heating resistor, but cooled to different extents by the medium flow so that a measurement voltage arises in the other diagonal of the bridge circuit as a result of the temperature difference of the temperature-dependent resistors, means to cause the heating voltage to be substantially constant or substantially constant section by section, and evaluating means for determining the medium throughput by evaluating the measurement voltage by means of at least one stored characteristic producing a relationship between the measurement voltage and the medium throughput.

2. Measuring means as claimed in claim 1, comprising switching means controllable by control means to cause a selectable one of a plurality of different values of the heating voltage to be applied to the heating resistor.

3. Measuring means as claimed in claim 2, the control means being arranged to control the switching means for selection of the heating voltage value in dependence on the measurement voltage.

4. Measuring means as claimed in claim 3, the control means being operable to compare an output voltage dependent on the measurement voltage with a threshold voltage and to control the switching means to change the heating voltage from a lower to a higher value in response to the output voltage exceeding the threshold voltage.

5. Measuring means as claimed in claim 3 or claim 4, the control means being operable to compare an output voltage dependent on the measurement voltage with a threshold voltage and to control the switching means to so change the heating voltage each time the output voltage exceeds or falls below the threshold voltage that a lowest heating voltage value is selected for a low output voltage.

6. Measuring means substanti al l y as hereinbefore described with reference to the accompanying drawing.

7. An internal combustion engine provided with measuring means as claimed in any one of the preceding claims, the carrier being arranged in an air induction duct of the engine and the evaluating means being arranged to determine the mass of air inducted by the engine.

8. An engine as claimed in claim 7, wherein the heating resistor of the measuring means is caused to be operated by the heating voltage at the lower or lowest of a plurality of values during a phase of heating up of the engine after starting.