JP2721400B2 - Thermal air flow meter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air flow meter of a type that detects a flow rate of intake air (referred to as air supply) of an internal combustion engine using a temperature resistance characteristic of a heating wire. More particularly, the present invention relates to a thermal air flow meter suitable for a gasoline engine for an automobile of an electronic fuel injection control system.

[Conventional technology]

For automobile engines, for example, gasoline engines,
In order to clear the strict exhaust gas regulations required for it,
An air-fuel ratio control method based on so-called air supply flow rate measurement, which measures an air supply flow rate and supplies an amount of fuel corresponding thereto, has been widely adopted. Air flow meters, so-called hot wire air flow sensors, have become widely used.

By the way, as described in Japanese Patent Publication No. 61-16026, Japanese Patent Application Laid-Open No. 61-283825, or Japanese Patent Application Laid-Open No. A flow detection unit D including a hot wire or the like, which is called a bridge type as shown in FIG. 16 or a non-bridge type as shown in FIG. 17, is provided, and a supply air flow Qa is obtained from a detection signal Va obtained therefrom. However, at this time, it is necessary to perform appropriate adjustment processing in order to absorb the variation in the characteristics of the hot-wire probe and the like and to keep it within a predetermined measurement error range. For this reason, signal processing is conventionally performed as shown in FIG. A circuit Q is provided, whereby an adjustment process as shown in FIG. 3 is performed to obtain a flow rate Qa.

In these circuits shown in FIGS. 2, 16, and 17, the flow rate detection unit D includes a hot wire probe 1 called a hot wire, a temperature probe 2 called a cold wire, and a balance for controlling a constant hot wire temperature. It is composed of resistance elements 3, 4 and the like which are elements for determining conditions.

The signal processing circuit Q includes an operational amplifier 5, a resistance element 6,
7, 8 and the like, and the above-described adjustment processing is performed by adjusting the resistance values of the resistance elements 7 and 8 formed of semi-fixed resistors. Specifically, the amplification factor is set by the resistance element 7, and The subtraction amounts are respectively adjusted by the resistance elements 8, whereby, as shown in FIG. 3, the detection signal is obtained under the predetermined two points of supply air flow rates Qa ₁ and Qa _2.
The adjustment is performed so that Va becomes the predetermined values Va ₁ and Va ₂ , respectively.

Note that in FIG. 3, represents the actual flow characteristics, and represents the detection characteristics.

[Problems to be solved by the invention]

The prior art described above has a problem that it is difficult to maintain a predetermined measurement accuracy required for application to control of an automobile engine or the like, because sufficient consideration is not given to widening the supply air flow rate measurement range. was there.

In other words, in the prior art, as described with reference to FIG. 3, the detection characteristics are different from the actual flow characteristics at two points, that is, a part where the supply air flow rate within the measurement range is relatively small and a part where the supply air flow rate is considerably large. Since it is only possible to calibrate the flow rate, it is not possible to guarantee the accuracy of flow rate detection in other parts, and as the flow rate measurement range becomes wider, the possibility of errors increasing increases. When the range is set, it is difficult to maintain a predetermined detection accuracy.

An object of the present invention is to provide a thermal air flow meter that can easily maintain a predetermined detection accuracy over a wide air supply flow rate measurement range and is applied to an automobile engine and that can always reliably control with high accuracy. To provide.

[Means for solving the problem]

In order to achieve the above object, the present invention provides a method for switching a characteristic of a signal processing unit used for calibrating a detection signal from a supply air flow rate detection unit, and for each level range of the detection signal, one of these characteristics. Is used to process the detection signal.

[Action]

Since the detection signal can be calibrated at two points each time the characteristics of the signal processing unit change, the detection signals can be calibrated at at least three points within the flow rate measurement range, and the flow rate measurement range is wide. Even after that, a decrease in measurement accuracy can be suppressed to a small extent, and high accuracy can be easily maintained.

〔Example〕

Hereinafter, the thermal air flow meter according to the present invention will be described in detail with reference to the illustrated embodiment. Prior to this, some examples will be described as reference examples after studying the implementation of the present invention.

First, FIG. 1 is a first reference example.
Q2 is a first and a second signal processing circuit, which receives the detection signal Va supplied from the air supply flow rate measuring unit D,
V _A and V _B are output, and are composed of operational amplifiers 8 and 9 and resistance elements 10 to 12 and 13 to 15, respectively.

Reference numeral 16 denotes an operation unit including a microcomputer and the like, and a signal processing circuit Q
1. It takes in the processing signals V _A and V _B output from Q2, and operates to give the supply air flow rate Qa by a predetermined arithmetic processing.

The air supply flow rate measurement unit D is of the non-bridge type described with reference to FIG.

The arithmetic unit 16 is for controlling the engine of the automobile. As shown in FIG. 4, the CPU 20, bus 21, timer 22, interrupt control unit 23, rotation counter 24, digital input port 2
5, analog input port 26, power backed up RAM
27, ROM28, output circuits 29, 30 etc., signal processing circuit Q
1, the processed signal is output from the Q2 V _A, V _B are taken through the analog input port 26 with the analog signal, such as a throttle valve opening sensor, the digital signal is a digital input port 25, such as from the crank angle sensor Ingested through. Apart from these, the signal from the rotation sensor is taken into the rotation number counter 24 and used as rotation number data.

When the car key switch 31 is turned on, the battery 32
Power is supplied to the CPU 20 and the like, a program stored in the ROM 28 is started, and the calculation result is supplied to the fuel injection valve and the ignition device as a predetermined control signal via the output circuits 29 and 30, and The engine is controlled.

Next, the calculation processing of the supply air flow rate Qa will be described with reference to the flowchart of FIG.

The processing of FIG. 5 is executed at predetermined intervals by, for example, a timer interrupt or the like. First, in processing 35, the processing signals V _A , output from the respective signal processing circuits Q1, Q2,
Performs reading of V _B, then in the determination processing 36, one of the processed signal V _B, is compared with a predetermined reference value V ₀ which is set in advance, the processed signal V _B of the level the level of the reference value V ₀ examines whether there above, first, when the result is Yes, the process signal V _B treated 37 as a detection signal Va, followed by FIG. 6, FIG. 7, any one of the table of Figure 8 using air supply flow rate Q from the detection signal Va by using the table portion TV _B about V _B therein
Search for and ask for a. Therefore, the result of the judgment processing 36 is No.
While going on to the detection signal Va of the processed signal V _A in process 39, by the subsequent process 40 which in turn is to to obtain the supply air flow rate Qa by the search of the table portion TV _A related Va.

At this time, usually, as requires less memory capacity required for the table creation, since relatively data small number of tables are used, for obtaining the supply flow rate Qa from the processed signals V _A or V _B , High accuracy can be obtained by interpolation calculation.

Therefore, in this reference example, using a 1 in FIG. 6 to the one Figure 8 table, and when the level of the processed signal V _B therein is less than the predetermined value V _0, when the above predetermined value V ₀ Then, the operation is performed so that the search is performed by changing the table characteristic for obtaining the supply air flow rate Qa from the detection signal Va, and after all, the detection signal Va
A plurality of signal processing characteristics are switched according to the level of Va, and the supply flow rate is detected.

Therefore, first, to describe the reference example to use the first of 6 Figure table, in this case, as it is clear from FIG. 1, in the region table portion TV _A of Figure 6 is retrieved, Processing signal V _A by first signal processing circuit Q1
In this case, the calibration of the detection characteristics with respect to the actual flow characteristics is performed by using the resistance elements 10, 11, 12
Adjustment on the other hand, while the table part TV
In the region _{where B} is searched, since the processed signal V _B according to the second signal processing circuit Q2 is being used, in turn, a calibration of the detection characteristics for actual flow characteristics, the resistance element 13,
It can be obtained by the adjustment of 14 and 15, and as a result, the whole range of the measurement range can be divided into two parts and partially calibrated.
Since adjustment and calibration can be performed at points, approximation of the detection characteristics to the actual flow characteristics can be sufficiently obtained, and the measurement accuracy can be kept high.

The reading accuracy of the air supply flow rate when the measurement result of the air supply flow rate Qa according to the reference example using the table of FIG. 6 is digitally converted by the A / D converter, using the number of bits of the A / D converter as a parameter. As shown in FIG.

As is clear from FIG. 9, the accuracy increases as the number of bits is increased. According to the reference example in the case of 10 bits, the error 2 % Accuracy could be maintained.

However, in the table of FIG. 6, each table portion T
V _A and TV _B are continuous, but as shown in FIG. 7, the relationship between the detection signal Va and the supply air flow rate Qa is two discontinuous lines, A table in which the signal change is large may be used.

In this reference example, first and second signal processing circuits are respectively provided.
According to Q1 and Q2, calibration in the low flow rate range and calibration in the high flow rate range are performed, and adjustment is performed at four points over the entire measurement range. Therefore, the table in FIG. According to the reference example used, as shown in FIG.
Even in the case of a bit, the accuracy can be further increased at a low flow rate, and an accuracy of 1% or less in error can be easily obtained. Note that, in FIG. 10, the characteristics indicated by the broken lines indicate the case of a continuous table. Further, the table at this time is not limited to a table including two discontinuous lines as illustrated, and may be a table including three or more lines.

Further, in the present invention, as shown in FIG. 8, the present invention may be implemented using a table composed of discontinuous lines such that a signal change with respect to the flow rate becomes large even in a high supply air flow rate range.

FIG. 11 shows the measurement accuracy characteristics according to the reference example using the table of FIG. 8 as described above. As is clear from this figure, it is understood that sufficient accuracy can be obtained from the low flow rate range to the high flow rate range. In this figure, the characteristics indicated by the broken lines are based on the embodiment using the table shown in FIG. Further, it is needless to say that the table shown in FIG. 8 is not limited to two discontinuous lines and may be a table having three or more discontinuous lines.

FIG. 12 shows another reference example of the present invention, in which 41 is an analog switch, 42 is a comparator,
Reads the outputs V and F of the analog switch 41 and the comparator 42, respectively, but the other configuration is the same as that of the reference example of FIG.

An analog switch 41 is processed signal V _B of the processed signal V _A and the second signal processing circuit Q2 of the first signal processing circuit Q1, it switched in accordance with the level of the output F of the comparator 42, the arithmetic unit 16 as the signal V It works to input to. At this time, when the signal F is at the low level, the analog switch 41 outputs the processed signal V _A as shown as signal V, the signal F is when it becomes high level, and outputs the processed signal V _B as the signal V Works like that.

Comparator 42 receives the processed signal V _B from the second signal processing circuit Q2, this and is compared with the predetermined reference value V ₀ which is set in advance, high when V _B ≧ V ₀ is satisfied It functions to output a signal F which becomes a level.

Therefore, it can be said that the reference example of FIG. 12 is obtained by replacing a part of the software processing by the arithmetic unit 16 in the reference example of FIG. 1 with hardware processing by the analog switch 41 and the comparator 42. The contents of the measurement processing of the supply air flow rate Qa are as shown in FIG.

That is, first, in step 43, the output signal V of the analog switch 41 and the output signal F of the comparator 42 are read, and in the next determination step 44, it is checked whether the level of the signal F is low or high. Then, one of the processes 45 and 46 is executed based on the result. Note that these processes 45 and 46 are the same as the processes 38 and 40 in FIG.

Therefore, in the reference example of FIG. 12, as in the reference example of FIG. 1, each of the tables of FIG. 6, FIG. 7, and FIG. Thus, the measurement accuracy described with reference to FIGS. 9, 10, and 11 can be obtained.

Next, FIG. 14 shows still another reference example of the present invention, in which 49 is a V / f (voltage / frequency) converter,
Others are the same as the reference example of FIG.

In this reference example, the output signal V of the analog switch 41
Is input to the V / f converter 49, where it is converted into a signal f having a frequency corresponding to the voltage of the signal V,
It is not affected by the voltage drop existing in the signal transmission line during this time or the potential fluctuation in the grounding section.
In addition, it has features such as resistance to electrical noise, and can provide high reliability.

Next, FIG. 15 shows an embodiment of the present invention, in which the signal processing circuit Q3
Is constituted by a broken line approximation function generation circuit. Therefore, in this embodiment, the operation is performed with the same characteristics as those in the case where the table described with reference to FIG. 6 is used. Further, in this embodiment, the bridge type described with reference to FIG. 16 is used as the flow rate detector D.

In FIG. 15, 50 is an operational amplifier, 51 and 52 are feedback resistance elements, 53 is a diode, 54 is a resistance element for adjusting an approximate characteristic switching point, and 55 and 56 are resistance elements for adjusting a subtraction amount.

When the output voltage of the operational amplifier 50 is lower than the voltage applied to the diode 53 from the constant voltage source via the resistance element 54, the diode 53 is conducting, so that the feedback resistance of the operational amplifier 50 is the resistance element. When the output voltage of the operational amplifier 50 becomes higher than the voltage applied to the diode 53, this diode 53
Is in a cutoff state, and at this time, the feedback resistance is only due to the resistance element 51.
The amplification factor of 50 changes between when the output voltage is lower than a predetermined value and when the output voltage is higher than the predetermined value, and operates as a broken line approximation function generation circuit as described above.

Therefore, these resistance elements 51, 52, 54, 55, and 56
, The detection characteristics are made to coincide with the actual flow characteristics at three points, as shown in FIG.
The approximation can be made by the broken line characteristics, and as described with reference to FIG. 9, the flow rate can be detected with a sufficiently small error, and high accuracy can be obtained.

In this embodiment, only one feedback circuit including the diode 53 is provided. Therefore, as shown in FIG. 6, the approximation is based on two types of broken line characteristics, and the number of adjustment points is three. Also, by further increasing the number of feedback circuits using diodes, calibration at four or more adjustment points can be enabled, and the measurement accuracy can be further improved.

〔The invention's effect〕

According to the present invention, since the calibration of the thermal air flow meter can be performed at at least three points in the measurement range, the measurement error can be sufficiently suppressed, and even if the measurement range is widened, high accuracy can be obtained. Therefore, it is possible to sufficiently cope with a wide range of the thermal air flow meter, and to sufficiently control the performance of a gasoline engine for an automobile, thereby sufficiently improving its performance.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a reference example of a thermal air flow meter according to the present invention, FIG. 2 is a circuit diagram showing a conventional example of a thermal air flow meter, FIG. FIG. 4 is a block diagram showing one reference example of the arithmetic unit, FIG. 5 is a flowchart for explaining the operation of one reference example of the present invention, and FIGS. 6, 7, and 8 are each reference figures of the present invention. 9 and 10, and FIG. 11 are explanatory diagrams of detection accuracy according to each of the reference examples and embodiments of the present invention, and FIG. 12 is another diagram of the present invention. FIG. 13 is a circuit diagram showing another embodiment of the present invention, FIG. 13 is a circuit diagram showing another embodiment of the present invention, and FIG. 15 is a circuit diagram showing an embodiment of the present invention. FIG. 16 is a circuit diagram showing an example of a bridge type thermal air flow meter, and FIG. 17 is an example of a non-bridge type thermal air flow meter. It is a circuit diagram. 1 ... hot wire probe, 2 ... temperature probe, 3, 4 ...
Resistive element, 8, 9 Operational amplifier, 10 to 15 Resistive element, 16 Operation unit, Q1 First signal processing circuit, Q2
A second signal processing circuit;

Claims (1)

(57) [Claims] A signal processing circuit for changing input / output characteristics to at least two different characteristics in accordance with the level of an input signal; In a thermal air flow meter of a type which performs an adjustment process and obtains an air flow signal, the signal processing circuit comprises: an operational amplifier having a non-inverting input terminal, an inverting input terminal, and an output terminal; and an inverting input terminal of the operational amplifier. A first feedback resistor connected between the first feedback resistor and the output terminal; a second feedback resistor having one terminal connected to the inverting input terminal of the operational amplifier; A diode connected between the other terminal and the output terminal such that the conduction direction is directed to the output terminal side, and a subtraction amount adjustment for applying a predetermined voltage to the inverting input terminal via a series resistance element. A resistance element for adjusting an approximate characteristic switching point for applying a predetermined voltage to a connection point between the second feedback resistance element and the diode via a series resistance element. A thermal air flow meter, wherein the detection signal is supplied to a non-inverting input terminal so that the air flow detection signal is obtained from an output terminal of the operational amplifier.