The present invention relates to a device for measuring an air flow rate using a heating resistor, for example, a heating resistor type air flow rate measuring device suitable for measuring an intake air flow of an internal combustion engine of a vehicle. More particularly, it relates to correcting measurement errors attributable to an intake air pulsation flow.
A heating resistor type air flow rate measuring device has been known as technology for measuring an air flow of an internal combustion engine. It makes use of a phenomenon that heat drawn from a heating resistor increases monotonically relative to an air flow rate. Since it can directly measure a mass flow rate, it is widely used as a flow meter in connection with vehicle fuel control.
In a heating resistor type air flow rate measuring device, the output voltage of a sensing circuit varies nonlinearly with respect to the air flow rate. In an intake pipe where a heating resistor is disposed, a pulsation flow is generated by opening and closing movements of intake and exhaust valves.
In such type of an air flow rate measuring device, a pulsation flow and output nonlinearity cause measurement errors with the flow rate measurement growing increasingly smaller than the actual flow rate as the pulsation flow grows larger as shown in
Delay in response of a heating resistor occurring when a pulsation flow is present also causes air flow measurement errors. If the output of the heating resistor is successively converted to flow rate values without involving delay in responding to true changes in flow speed caused by the pulsation flow, such measurement errors do not occur.
In reality, however, for the following reasons, it is difficult to eliminate delay in response of the output of a heating resistor. A heating resistor disposed in an intake pipe is required to be mechanically vibration-resistant while preventing deposition of fine dust coming through an air cleaner. To improve heating resistor reliability, heating wire to make up a heating resistor is wound around a bobbin-like part and the heating resistor is coated to enhance its mechanical strength. Such measures, however, cause the bobbin-like part and the coating material to be subjected to heating by the heating wire. This increases the heat capacity of a portion including the heating resistor. Consequently, due to a thermal delay of the heating resistor, measurement delays occur with respect to true intake air pulsations. Air flow rate values obtained by successively converting such delayed output voltage inevitably contain measurement errors.
To control an internal combustion engine, it is necessary to perform calibration by measuring measurement errors and determining correction values beforehand. An example of such calibration work is described in JP-A No. 105781/1996. According to JP-A No. 105781/1996, prior to shipment of a vehicle, correction values for correcting air flow measurement errors dependent on the engine revolution rate and accelerator opening are determined and mapped for the intake system of the vehicle. Namely, for conditions (engine revolution rates and accelerator opening degrees) which cause flow rate measurements to be smaller than actual air flow rates, correction values to be added to the flow rate measurements are determined and mapped. Similarly, for conditions (engine revolution rates and accelerator opening degrees) which cause flow rate measurements to be larger than actual air flow rates, correction values to be subtracted from the flow rate measurements are determined and mapped.
In the above method, a map like the one shown in
The pulsation flow is dependent on the length of an intake pipe. If, in the process of developing a vehicle, the length of the intake pipe is changed, the engine revolution rate at which flow pulsation becomes large also changes. Hence, it is necessary to perform calibration every time the length of the intake pipe is changed. Since the resonance frequency of a pipe line is dependent on the length of the pipe line, changing the length of the intake pipe or the position of a heating resistor in the intake pipe changes measurement errors of the heating resistor type air flow rate measuring device.
Also, when the atmospheric pressure or temperature changes while the engine is running, the resonance frequency of the pulsation changes thereby causing the relationship between the engine revolution rate, accelerator opening degree and intake air pulsation ratio to also change. Consequently, measurement errors also change, and it may occur that air flow measurement errors are not properly corrected.
For the above reasons, it is desirable that, in correcting pulsation-induced measurement errors of a heating resistor type air flow rate measuring device, changes made to the intake system of the engine and external factors such as the atmospheric pressure and temperature be taken into consideration.
JP-A No. 536320/2004 describes an error correction method devised by taking note of a phenomenon that an air mass sensor causes dynamic output errors dependent on a pulsation flow, particularly, due to resonance. In the method, a sensor output signal is supplied to a filter circuit and a correction circuit, then the correction circuit, using information supplied to the filter circuit, generates a corrected sensor signal. In concrete terms, a correction signal is generated by multiplying a difference signal, representing a difference between a sensor output signal and a filter output signal (that is, the sensor output signal having passed a low-pass filter), by a coefficient, and the correction signal is added to the sensor output signal.
An object of the present invention is to correct pulsation-induced measurement errors of a heating resistor type air flow rate measuring device. More particularly, the present invention aims at providing a measurement error correction method and an instrument which are not affected by changes made to the intake system of a vehicle and external factors such as the atmospheric pressure and temperature. Another object of the present invention is to make it possible to estimate air flow measurement errors without using a low-pass filter, unlike in the method described in JP-A No. 536320/2004.
Basically, according to the present invention, in an air flow measuring instrument which measures an air flow rate using a temperature-dependent heating resistor, an output from a sensing circuit including the heating resistor are converted to flow rate values by two or more methods, and a difference between the flow rate values obtained by the two or more methods is used to correct a measurement error.
For example, the output from the sensing circuit including the heating resistor is converted to flow rate values by two methods; based on a difference between the two flow rate values obtained by the two methods, a current measurement error of the air flow rate measuring device is determined (for example, by the heating resistor type air flow rate measuring device itself or by an engine control unit); and the measurement error thus determined is used for measurement error correction.
The inventors took note of a phenomenon that the difference between flow rate values determined, based on a sensor output of a sensing circuit of an heating resistor type air flow rate measuring device, using different flow rate measuring systems (that is, using different processes for converting the sensor output to flow rates) varies with the magnitude of measurement errors of the air flow rate measuring device. As a result, they devised a method to correct measurement errors based on a predetermined relationship between the difference and air flow measurement errors.
In other words, according to the present invention, an output of a sensing circuit including a heating resistor of an air flow measuring instrument is converted to flow rate values by two or more methods, and a measurement error of the air flow measuring instrument is corrected based on the difference between the flow rate values obtained by the two or more methods.
According to the present invention, preparing a map or a formula representing relationship between the difference between flow rate values obtained by plural methods (the difference represents a measurement error which varies depending on the state of a pulsation stream) and correction values for correcting measurement errors of an air flow measuring instrument makes it possible to correct measurement errors of the air flow measuring instrument. In this way, measurement error correction is not dependent on the intake system involved, so that, even if changes are made to the intake system, it is not necessary, unlike in conventional cases, to perform recalibration.
Also, even if the atmospheric pressure or temperature changes while an engine is running and, as a result, the pulsation resonance frequency changes causing the relationship between the engine revolution rate, accelerator opening degree and intake air pulsation ratio to also change, air flow rate measurement errors can be properly corrected.
A preferred mode for carrying out the present invention will be described with reference to an example of embodiment shown in drawings.
An embodiment of the present invention will be described in detail with reference to the accompanying drawings.
First, a principle of operation of the heating resistor type air flow rate measuring device will be described.
A sensing circuit (drive circuit) 100 of the heating resistor type air flow rate measuring device includes, broadly classified, a bridge circuit and a feedback circuit.
The bridge circuit includes a heating resistor 101 for measuring an intake air flow rate, a resistor 102 for intake air temperature compensation, and fixed resistors 103 and 104. The heating resistor 101 and the temperature compensation resistor 102 are each made of a temperature sensing resistor which is temperature-dependent. They are disposed in an intake pipe. The feedback circuit includes an operational amplifier 105 which inputs voltage between the heating resistor 101 and the fixed resistor 103 and between the temperature compensation resistor 102 and the fixed resistor 104, and a transistor 106 which controls current based on an output of the operational amplifier 105.
The bridge circuit and the feedback circuit are used to give feedback to the current flowing in the bridge circuit. That is, even if variations in air flow rate cause the quantity of heat drawn from the heating resistor 101 to change, the operational amplifier 105 and the transistor 106 work to apply a heating current Ih to the heating resistor 101 so as to maintain a constant temperature difference between the heating resistor 101 and the temperature compensation resistor 102. The heating current Ih is converted to voltage, and an output signal V2 dependent on the air flow rate is outputted. Namely, when the air flow is fast (flow rate is high), a large quantity of heat is drawn from the heating resistor 101, so that the heating current Ih is increased. When, on the other hand, the air flow is slow (flow rate is low), the quantity of heat drawn from the heating resistor 101 is small, so that the heating current Ih is decreased.
In the flow meter body, a sensing section 10 is disposed in a main air passage member 20 which makes up part of the intake pipe. The sensing section 10 has a subsidiary air passage member 14 and a housing member 1 for housing the sensing circuit (drive circuit). These members 1 and 14 are made of nonconductive material.
In the subsidiary air passage member 14, the heating resistor 101 for measuring an air flow rate and the air temperature compensation resistor 102 are supported via a conductive support member 5. The resistors 101 and 102 are electrically connected, via the support member 5, to a circuit board 2 disposed in the housing 1. The housing, circuit board, subsidiary air passage, heating resistor and temperature sensing resistor are united making up the heating resistor type air flow rate measuring device module.
A hole 25 is formed through a wall of the main air passage member 20. The subsidiary air passage member 14 is inserted from outside through the hole 25. The housing member 1 is fixed to the wall of the main air passage member 20 with screws 7. A seal material 6 is fitted between the subsidiary air passage member 14 and the main air passage member 20 to keep the intake pipe airtight.
A part denoted by the numeral 200 in
The aforementioned thermal delay in the heating resistor is further described in the following. To measure the air flow rate in the intake pipe, it is necessary to make arrangements for removing mechanical vibrations and preventing deposition of fine dust coming through an air cleaner. Hence, it is necessary, as shown in
To correct the measurement errors described above, the electronic circuit unit 200 shown in
Step S1 shown in
In one of the two methods, as shown in step S3, the output voltage is sampled at prescribed time intervals (the sampling interval time is required to be short, say 4 ms or less, or more preferably 1 to 2 ms, to enable extraction of a pulsation waveform), and the sampled output voltages are successively converted to flow rate values. The flow rate values thus obtained in a prescribed amount of time are averaged in step S4. The average value thus obtained is determined as a first flow rate value.
In the other of the two methods, as shown in step S5, an average value of the output voltage V2 during a prescribed amount of time is calculated in advance. The average output voltage value thus obtained is then converted, as shown in step S6, to a flow rate value, that is, a second flow rate value. In steps S6 and S3, voltage values are converted to flow rate values, for example, by using a map.
Subsequently, the difference between the first and second flow rate values (average flow rate values) obtained in steps S4 and S6 is calculated (step S7). Based on the difference value, a measurement error correction value is determined, for example, using a map (showing relationship between difference values and error values, as shown in
In
As shown in
In step S9, correction processing is performed using the correction value obtained in step S8. The correction processing includes, for example, adding the correction value to the flow rate value obtained in step S4. The air flow rate value thus corrected is finally outputted to an engine control unit (ECU) as an air flow rate signal for fuel injection control (step S10).
The electronic circuit unit 200 shown in
The above system is superior in that measurement errors are estimated using a difference between flow rate values calculated by the two methods and an actual waveform representing, for example, the pulsation amplitude ratio of the heating resistor type air flow rate measuring device, and that the estimated measurement errors are used to correct measurement values. The pulsation amplitude ratio does not increase unless the pulsation amplitude actually becomes larger.
According to the present invention, even in cases where the length of an intake pipe is changed in a vehicle development stage resulting in a change in natural frequency and thereby causing an area to be affected by a pulsation flow to be shifted, it is not necessary, unlike in conventional cases, to perform recalibration.
A technique has been known in which, as a measure to cope with minus errors of a heating resistor type air flow rate measuring device attributable to nonlinearity of its output voltage and generation of a pulsation flow, a heating resistor is disposed in a curved subsidiary air passage. As a measure against minus errors, the technique is effective to a certain extent, but there has been a problem with the technique that no measures are taken to cope with plus errors attributable to a backflow. According to the present invention, however, it is possible to correct measurement errors attributable to a backflow, too. The graph shown in
Pulsation-induced measurement errors of a heating resistor type air flow rate measuring device occur in a state where the engine revolution rate is relatively low and the air flow rate is also low. Therefore, it is considered that measurement correction values laid out on a one-dimensional map are good for correcting measurement values based on differences between flow rates obtained by the two methods. Depending on a vehicle (engine), however, measurement errors may occur in a state where the engine revolution rate is high and the air flow rate is also high. In such a case, measurement errors of a heating resistor type air flow rate measuring device may depend on the pulsation frequency. In cases where the measurement error dependency on the pulsation frequency cannot be ignored, three-dimensional maps, each for a flow rate, of measurement correction values are to be prepared with the engine revolution period (frequency) taken into account in addition to the differences between flow rates.
Intake air 67 taken in through an air cleaner 54 enters an engine cylinder 62 via a body 53 of a heating resistor type air flow rate measuring device, an intake duct 55, a throttle body 58, and an intake manifold 59 provided with an injector 60 to which fuel is fed. Gas 63 generated in the engine cylinder is discharged via an exhaust manifold 64.
Signals such as an air flow rate signal outputted from a circuit module 52 (equivalent to a sensing circuit 100 and the electronic circuit unit 200 shown in
Application of the present invention is not limited to cases where the above two conversion-to-flow-rate methods are used. The present invention can be applied to cases where other conversion-to-flow-rate methods are available to obtain a difference value similar to that obtained using the above two methods. Furthermore, in applying the present invention, measurement errors may be determined based on information obtained using more than two conversion-to-flow-rate methods.
Even though a main area of application of the present invention will be vehicle control, it can also be applied to cases where control is performed using a diesel engine for ships or power generators.
Number | Date | Country | Kind |
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2005-058757 | Mar 2005 | JP | national |