1. Technical Field
The present invention relates mainly to a flow rate detector of thermal type such as the one employed for detecting amount of intake air in vehicle engines, and more particularly to a method for improving precision in detection of a pulsating flow in vehicle engines.
2. Background Art
Generally in vehicle engines, a mixed gas of fuel and intake air is combusted in a combustion chamber of an engine, and a rotating power is generated utilizing a combustion pressure produced by the combustion. Therefore it is essential to precisely detect a flow rate of intake air in order to properly control injection amount at the time of combustion. For that purpose, various thermal type flow rate detectors for detecting a flow rate of intake air have been heretofore proposed, including a flow rate detector disclosed in the Japanese Patent Publication (unexamined) No.22563/1982.
Flow rate of intake air in vehicle engines varies depending on driving conditions of an engine. For example, when engine speed is constant, airflow-resistance in a throttle valve decreases, as opening of the throttle valve becomes larger. As a result, pressure in intake manifold increases thereby increasing a flow rate, and amplitude of pulsating flow becomes larger.
With respect to such pulsating flow, the relation between flow rate and output of detection is non-linear, and thermal response of an exothermic resistor is delayed. Therefore, when converting every detected output into pulsating flow using a thermal type flow rate detector, an average value of the detected flow rate becomes smaller than actual flow rate (this phenomenon is hereinafter referred to as leaning error). This leaning error augments as engine speed becomes higher and as pulsation amplitude becomes larger.
FIG. 7 is a characteristic diagram showing a relation between pressure P of intake manifold according to extent of opening of throttle valve and flow rate (average of flow rate signals) Qav of intake air.
Referring to FIG. 7, reference code Ca shows a curve of actual flow rate with increasing opening of a throttle valve under a constant engine speed. On the other hand, code Cb in FIG. 7 shows a curve of average flow rate detected by a conventional thermal flow rate detector. It is understood that a leaning error xcex94l is produced between the mentioned curves Ca and Cb. In case that a flow rate detecting characteristic appears as shown by the mentioned curve Cb, an identical flow rate may be detected under two loading conditions different from each other, which brings about a disadvantage of making it impossible to uniquely determine a flow rate.
In view of the foregoing disadvantage that a leaning error xcex94l is produced due to a lower detected flow rate than actual amount of intake air, a flow dividing type thermal flow rate detector of was proposed, as disclosed in the Japanese Patent Publication (unexamined) No.19510/1983 (hereinafter referred to as Prior Art 1).
In this Prior Art 1, a passage for divided flow, used as flow detecting tube in which an exothermic resistor is disposed, is formed longer than a passage for main flow, in order to increase inertia of air-flow in the mentioned passage for divided flow. As a result, an average flow rate through the passage for divided flow becomes larger than a flow rate of steady flow, and moreover pulsation amplitude of flow velocity in the passage for divided flow decreases. Therefore the mentioned detecting error due to delay in thermal response of exothermic resistor can be offset by properly establishing a ratio of inertia length of the passage for divided flow and the main passage.
Also, the Japanese Patent Publication (unexamined) No.205915/2000 discloses a thermal flow rate detector which compensates flow detecting error of a pulsating flow, using gain compensating means incorporated in a flow rate detector (hereinafter referred to as Prior Art 2).
This flow rate detector of Prior Art 2 comprises a flow computing circuit for computing a flow-detecting signal according to a flow rate on the basis a resistance value of a thermo-sensitive resistor disposed in a passage of an object fluid, and gain compensating means for amplifying a flow-detecting signal from the mentioned flow computing circuit, thereby compensating a gain lowered by a thermal time constant of a thermo-sensitive resistor.
FIG. 5 is a characteristic diagram showing a relation of gain and frequency characteristic of gain compensating means. Referring to FIG. 5, axis of ordinates stands for gain G of the gain compensating circuit, axis of abscissas stands for frequency f, and code To shows a curve of frequency characteristic of gain compensating means disclosed in the Prior Art 2.
Accordingly, in the flow rate detector of the Prior Art 2, when the pulsating flow frequency is below the first predetermined frequency f1B, the flow computing circuit outputs a flow-detecting signal of which value is substantially corresponding to the pulsating flow. At this time, the mentioned gain compensating means amplifies the flow-detecting signal by a predetermined constant gain G1.
On the other hand, when the pulsating flow frequency is higher than the first predetermined frequency f1B, the output of a flow-detecting signal from the flow computing circuit becomes lower due to thermal time constant of a thermo-sensitive resistor. The output of a flow-detecting signal becomes lower also due to non-linear relation between flow rate and flow-detecting signal. At this time, the gain compensating means amplifies flow-detecting signals by a gain G2 that corresponds to the frequency. As a result, the delay caused by thermal time constant of the thermo-sensitive resistor is compensated, and the gain compensating means outputs a waveform close to the variation in actual flow rate.
Further, when the pulsating flow frequency is higher than the second predetermined frequency f2B, the gain compensating means amplifies a flow-detecting signal outputted by flow computing circuit by a gain G3 ( greater than G1). As a result, a flow-detecting signal on the higher frequency side, which has little influence on the flow detection, is not amplified more than is required. As a result, a flow detecting error on the higher frequency side is not caused, and precision in flow detection is improved.
It is certain that the device according to the Prior Art 1 offsets a detecting error of a pulsating flow caused by delay in thermal response of an exothermic resistor. But a complicated structure is required to secure a necessary length of passage for divided flow, and the flow tends to be disorderly or turbulent in the proximity of the exothermic resistor. Moreover, there is a disadvantage that sufficient flow detection sensitivity cannot be secured because flow velocity in the proximity of the exothermic resistor for detecting flow rate becomes slower than the velocity of main flow.
Also, it is certain that gain compensating means according to the Prior Art 2 can compensate a decline of a flow-detecting signal caused by thermal time constant of the thermo-sensitive resistor. But a decline in flow-detecting signal still may take place depending on the waveform of pulsation or structure of thermo-sensitive resistor and flow detecting tube in which a thermo-sensitive resistor is disposed, in the pulsating flow having a frequency lower than f1B predetermined by thermal time constant of thermo-sensitive resistor. This may result in a large leaning error from actual flow rate. This point is now described in more detail.
As one of the important causes of a leaning error occurring even in the detection of a pulsating flow of a lower frequency than f1B predetermined by thermal time constant of thermo-sensitive resistor, there is a difference in flow rate between main tube and flow detecting tube, depending upon whether it is a pulsating flow or a steady flow. For example, in case that a flow detecting tube is contracted toward the outlet in order to improve the stability of flow, the area of outlet passage is smaller than the area of inlet passage. In the flow detecting tube of such contraction, a pulsating flow of high frequency generates swirling flows that are not formed from a steady flow, in the proximity of the outlet of flow detecting tube, thus making it difficult for the object fluid to flow into the flow detecting tube. As a result, a fluid loss of a pulsating flow in the flow detecting tube becomes larger than a fluid loss of a steady flow. Therefore an average flow rate of a pulsating flow in the flow detecting tube becomes lower than an average flow rate of a steady flow. This phenomenon wherein a pulsating flow performs different actions from a steady flow in a flow detecting tube is hereinafter defined as transient characteristic. In general, this transient characteristic is more prominent in a frequency range considerably lower than the frequency flB determined by thermal time constant of thermo-sensitive resistor.
On the other hand, as shown by the characteristic curve To in FIG. 5, the gain G1 of the gain compensating means according to the Prior Art 2 is a constant value with respect to a pulsating flow of lower frequency than the f1B predetermined by thermal time constant of thermo-sensitive resistor. Therefore, the effect of minimizing a leaning error of the mentioned gain compensating means is not sufficiently performed resulting in the leaning error. Consequently an accurate flow rate cannot be obtained.
The present invention was made to solve the above-discussed problems incidental to the prior arts, and has an object of providing a flow rate detector in which flow rate of an object fluid is detected by effectively reducing a leaning error even when the object fluid is pulsating, thereby improving detecting precision.
To accomplish the foregoing object, a flow rate detector according to the present invention is constituted as follows.
A first embodiment of the invention discloses a flow rate detector comprising:
flow detecting means for outputting a flow-detecting signal according to a flow rate of an object fluid utilizing heat transfer phenomenon to the object fluid from an exothermic resistor disposed in a fluid passage; and
gain compensating means for compensating a gain of the mentioned flow-detecting signal by amplifying the mentioned signal outputted by the mentioned flow detecting means;
in which the mentioned gain compensating means amplifies the mentioned flow-detecting signal with a substantially constant alternating current gain larger than a direct current gain at the time of zero frequency in a frequency range higher than a minimum pulsating frequency of the object fluid.
A second embodiment of the invention discloses a flow rate detector comprising:
temperature difference detecting means for detecting a difference between temperatures of thermo-sensitive resistors respectively disposed on upstream side and downstream side of an exothermic resistor disposed in a fluid passage; and
gain compensating means for compensating a gain of a flow-detecting signal by amplifying a temperature difference detecting signal from the mentioned temperature difference detecting means inputted as flow-detecting signal;
in which the mentioned gain compensating means amplifies the mentioned flow-detecting signal with a substantially constant alternating current gain larger than a direct current gain at the time of zero frequency in a frequency range higher than a minimum pulsating frequency of the object fluid.
A third embodiment of the invention discloses a flow rate detector comprising:
temperature detecting means for separately detecting temperatures of thermo-sensitive resistors respectively disposed on upstream side and downstream side of an exothermic resistor disposed in a fluid passage; and
flow rate computing means for computing a flow-detecting signal based on a temperature detecting signal outputted by the mentioned temperature detecting means;
in which the mentioned flow rate computing means consists of: first phase compensating means for amplifying and outputting a phase-advanced temperature detecting signal of an advanced phase corresponding to a temperature of a thermo-sensitive resistor disposed on upstream side of the exothermic resistor; second phase compensating means for amplifying and outputting a temperature detecting signal of an advanced phase corresponding to a temperature of a thermo-sensitive resistor disposed on downstream side of the exothermic resistor; and a differential amplifier for outputting as a flow-detecting signal a difference between output signals from the mentioned respective phase compensating means.
Preferably, in the second embodiment of the invention, both of the mentioned first and second phase compensating means consist of gain compensating means for amplifying the mentioned temperature detecting signal by a substantially constant alternating current gain larger than a direct current gain at the time of zero frequency, in the frequency range higher than the minimum pulsating frequency of the mentioned object fluid.
Preferably, in the flow rate detector according to the first embodiment of the invention, a flow-detecting signal is amplified by a constant gain larger than a gain of non-pulsating steady flow, in a predetermined frequency range in which an average value of a flow-detecting signal is lowered due to flow characteristics in the proximity of a flow detecting tube, flow detecting element or the like. Therefore, output amplitude of the gain compensating means becomes larger with the advance of phase. As a result, the average detected value of flow rate becomes larger due to non-linearity of flow characteristics, and a flow-detecting signal becomes close to actual average flow rate. Consequently, a leaning error in average flow-detecting signal is minimized even when an object fluid is pulsating, resulting in precise detection of flow rate of the object fluid.
Preferably, in the flow rate detector according to the second embodiment of the invention, since the aforementioned temperature difference detecting means detects a difference between temperatures of respective thermo-sensitive resistors disposed on upstream and downstream sides of an exothermic resistor, pulsating wave of a counter flow can also be detected. Also, a temperature difference detecting signal from the temperature difference detecting means is inputted to the gain compensating means as a flow-detecting signal, and the gain compensating means advances the phase of the mentioned signals, thus performing apparently quicker responses. Furthermore, a flow-detecting signal is amplified by a constant gain larger than a gain of non-pulsating steady flow, and therefore the average value of detected flow rate becomes larger and a flow-detecting signal close to actual average flow rate is attained. Consequently, in the same manner as in the first embodiment, a leaning error of an average flow-detecting signal is minimized even when an object fluid is pulsating, resulting in precise detection of flow rate of the object fluid.
In the flow rate detector according to the third embodiment of the invention, signal responses apparently become quicker by advancing the phase of respective temperature detecting signals of thermo-sensitive resistors disposed on upstream and downstream sides of an exothermic resistor. Furthermore, pulsating wave of a counter flow can also be detected. Therefore since a flow-detecting signal computed from the temperature difference of thermo-sensitive resistors is still closer to actual waveform of pulsation, the precision in flow detection is further improved.
In the flow rate detector according to the third embodiment of the invention, since the phase compensating means comprises the same gain compensating means as disclosed in the first embodiment, both of the advantages described in relation to the first embodiment are performed.