Suction air amount measuring device for an engine

Abstract: The content of the counter is latched at a timing of a pulse obtained by shaping the wave form of a received wave whose phase is modulated with Karman's vortex street, in order to demodulate the phase which is then converted into a binary value ; therefore, most of the signals can be processed at a level of a digital signal : that is, the apparatus cans be easily realized in the form of a digital IC, and a flow-rate measuring apparatus which is small and light in weight can be obtained at a low cost.

Description: Technical Field This invention relates to a suction air amount measuring device for an engine having a Karman's vortex street flow rate sensor for detecting a vortex signal in response to operating states of an engine and a noise reduction means for removing noise components of the sensors signal cause by the operation of a throttle valve device. Background of the Invention Various proposals have been made to a method of measuring a flow rate of a fluid from a frequency of Karman's vortex streets generated downstream of a vortex street generating member located in a conduit and are disclosed, for example, in Japanese Utility Model-Laid-Open No. 54-41665, Japanese Utility Model Laid-Open No. 57-160625 or Japanese patent application No. 54-123973. Japanese Utility Model Laid-Open No. 54-41665 discloses a method which makes use of the fact that ultrasonic signal waves transmitted across a duct are phase modulated by Karman's vortex streets generated in the fluid. Meanwhile, Japanese Utility Model Laid-Open No. 57-160625 makes use of the fact that the resistance of a thermistor located in a duct and heated with a constant current flow varies in response to Karman's vortex streets. In both of the conventional methods, the output electric signal is in the form of an analog signal which varies in response to Karman's vortex streets. Thus, where either one of the conventional devices is used, for example, as a suction air amount sensor for fuel injection control of an engine, normally the output electric signal is converted into binary digitized Karman's vortex street pulses for its application. A conventional method of converting an analog signal into such pulses is disclosed, for example, in Japanese Patent Publication No. 58-70131. By the way, when such a Karman's vortex street flow rate sensor is used as a suction air amount sensor for an engine, it will give rise to following problems as described in part in Japanese Patent Publication No. 58-70131. In particular, when a throttle valve is positioned at a high opening position, pulsations of air caused by an air sucking action of the engine are transmitted to a suction air path so that the output level of the flow rate sensor is varied significantly in response to such pulsations. To the contrary, when the throttle valve is positioned at a low opening position, so-called "wind whistles" are produced by suction air passing through the valve at a high speed and have an influence on the Karman's vortex street sensor in the suction air path so that the sensor develops an output in which high frequency noises are overlapped. When such an output signal is compared with a predetermined voltage on a voltage comparator in order to binary digitize the same to produce Karman's vortex street pulses as disclosed, for example, in above-mentioned Japanese Patent Publication No. 58-70131, a so-called "falling off" of pulses will occur upon high opening of the throttle valve, and on the contrary upon low opening of the valve, the output will involve pulses originating from overlapped high frequency noises. In JP-A-54-123973, there is a filter circuit for removing the noise components caused by the throttle valve, but this is not completely satisfactory. The present invention has been made to resolve such problems as described above, and it is an object of the present invention to provide a suction air amount measuring device for an engine which always outputs correct pulses of Karman's vortex street irrespective of an operating condition of the engine. By the way, as digital circuit techniques have been developed in recent years, associated devices such as, for example, microprocessors, gate arrays and memories have been remarkably reduced in price and improved in function. In view of such circumstances as described above, it is desired for a Karman's vortex street flow rate measuring instrument to be reduced in price, improved in function and reduced in size and weight by constituting signal processing circuitry thereof from digital circuits. However, the conventional techniques or means described above cannot suitably be formed from digital circuits. For example, if it is intended to form from a digital circuit a phase shifting circuit of the conventional technique which makes a principal part of a signal processing circuit, it gives rise to a problem that a register for a very large number of bits is required. The present invention has been made to also resolve such a problem as described just above, and it is another object of the present invention to provide a suction air amount measuring device wherein a signal processing system is employed which can be suitably formed from a digital circuit. Disclosure of the Invention A suction air amount measuring device for an engine according to the present invention comprises an ultrasonic wave generating vibrator and an ultrasonic wave receiving vibrator in an opposing relationship across a suction air path of said engine downstream of a vortex street generating member inserted in a fluid to be measured within said suction path, a resonance point detecting circuit for detecting whether or not said ultrasonic wave generating vibrator is excited at the resonance point thereof, a voltage controlled oscillator connected to be controlled by said resonance point detecting circuit, a counter for dividing output pulses of said voltage controlled oscillator by counting the pulses, a driving circuit for exciting said ultrasonic wave generating vibrator with the pulses divided by said counter, a latch circuit for fetching contents of said counter at a timing of each of pulses produced by wave form shaping an output of said ultrasonic wave receiving vibrator, and binary digitizing means for determining increased or decrease of said latch circuit. According to the present invention, since contents of a counter are latched at timings of pulses produced by waveform shaping received waves which are phase modulated by Karman's vortex streets in order to phase demodulate the received waves and then the phase is binary digitized, most part of a signal processing means can be processed at a digital signal level. Accordingly, an effect can be attained that a suction air amount measuring device is obtained which can be formed readily from a digital IC and is inexpensive, small in size and light in weight. Brief Description of the Drawings Fig. 1 is a block diagram showing an embodiment of flow rate measuring device of the present invention, and Figs. 2 to 5 are timing charts showing signals at various portions of the flow rate measuring device of Fig. 1. Best Mode of Carrying Out the Invention Now, an embodiment according to the present invention wherein a binary digitizing circuit is formed from digital circuits will be described in detail. Fig. 1 is a block diagram showing the embodiment. Referring to Fig. 1, reference numeral 1 denotes a suction air path which is a duct of a fluid to be measured. A vortex generating member 21 is located in the duct 1, and an ultrasonic wave generating vibrator 22 and an ultrasonic wave receiving vibrator 23 are located in an opposing relationship across the duct 1 downstream of the vortex street generating member 21. The ultrasonic wave generating vibrator 22 is excited via a driving circuit 10 by an output of a counter 9, and a resonance point detecting circuit 11 detects whether or not the ultrasonic wave generating vibrator 22 is excited at its resonance point. A voltage controlled oscillator 12 is controlled in response to an output of the resonance point detecting circuit 11, and an output of the voltage controlled oscillator 12 is counted and thus divided by the counter 9. The driving circuit 10 is driven by pulses obtained by division on the counter 9. Meanwhile, an output of the ultrasonic wave receiving vibrator 23 is delivered to a waveform shaping circuit 13 which thus produces waveform shaped pulses. At a timing of the rising edge of each of the pulses, a latch circuit 14 fetches a count value of the counter 9 and latches it therein. An output of the latch circuit 14 is delivered to a first low-pass digital filter 31. The first low-pass digital filter 31 is provided to remove high frequency noise components from the output of the latch circuit 14, and an output of the first low-pass digital filter 31 is forwarded to a second low-pass digital filter 32. The second low-pass digital filter 32 is provided to average the output of the first low-pass digital filter The outputs of the first and second low-pass digital filters 31, 32 are compared in magnitude with each other on a magnitude comparator 33, and an output of the magnitude comparator 33 is forwarded to a buffer 15. Now, operation will be described. Figs. 2 and 3 are timing charts showing signals at several portions of Fig. 1, and Fig. 2 illustrates a manner of counting of pulses by the counter 9 and a manner of phase demodulation by the latch circuit 14 while Fig. 3 illustrates a manner of binary digitization of a phase e demodulated by the magnitude comparator 33. It is to be noted that a relation between Figs. 2 and 3 is such that a demodulated signal e in Fig. 2 is a microscopic representation of a phase demodulated signal e in Fig. 3 . Referring to Fig. 1, the voltage controlled oscillator 12 is oscillating at, for example, about 20 MHz, and the frequency is controlled by the resonance point detecting circuit 11 such that the ultrasonic wave generating vibrator 22 may be excited at the resonance point thereof. Output pulses (the a of Fig. 2) of the voltage controlled oscillator 12 is counted in a recurring manner as shown by the b in Fig. 2 by the counter 9 which is formed, for example, as a binary up counter for 9 bits. Now, if the most significant bit is considered, it presents pulses which are obtained by dividing the output pulses a of the voltage controlled oscillator 12 by 512, that is, pulses of about 40 MHz (the c of Fig. 2). The ultrasonic wave generating vibrator 22 is excited via the driving circuit 10 by the pulses c . Ultrasonic waves generated by the ultrasonic wave generating vibrator 22 are first phase modulated by Karman's vortex streets within the duct 1 and then transmitted to the ultrasonic wave receiving vibrator 23. Then, an output of the ultrasonic wave receiving vibrator 23 is shaped by the waveform shaping circuit 13 in order to produce phase modulated pulses (the d of Fig. 2). Then, contents of the counter 9 are taken into the latch circuit 14 at a timing of the rising edge of each of the pulses d . Thus, the output of the latch circuit 14 (the e of Fig. 2) make a phase demodulated signal. In other words, since the counter 9 operates as a divider and also as a phase counter which divides a phase of one cycle of the transmitted wave c by 512 and counts them, if counter contents are taken in at a timing of the received wave d , then the output of the counter makes itself a phase demodulated signal e . After then, the phase demodulated signal e is delivered to the first low-pass digital filter 31 on which noise components are removed from the phase modulated signal e (a f of Fig. 2). In short, since not only Karman's vortex streets described above but also, for example, sound sources outside the duct and so on act to modulate the phase of ultrasonic waves and make noises in a demodulated signal, such noises must be removed by means of a filter. A demodulated signal f from which noises have been removed in this manner is subsequently averaged by the second low-pass digital filter 32 (a g of Fig. 3). Then, the two signals f , g obtained in such a manner as described above are compared in magnitude with each other by the magnitude comparator 33 in order to produce a binary digitized pulse signal h . The frequency of the pulses h corresponds to the frequency of the Karman's vortex streets. It is to be noted that all elements within a block of the processing circuit described above which is shown in broken lines in Fig. 1 process signals in the form of digital signals and accordingly can be readily formed into a digital IC at a low cost by a technique of, for example, gate arrays. By the way, since the embodiments of the present invention described above do not involve such fixation of an average value of the phase difference between the transmitting and receiving sides by a phase shifting circuit that is involved in a conventional technique, the phases of transmitted and received waves may sometimes have such a relation, for example, as seen in Fig. 4. In particular, in Fig. 4, such phase modulation is effected that timings of the rising edges of received pulses d are fluctuated before and after the count value b changes from the maximum count value 511 to the minimum count value 0. In this instance, the phase demodulated signal e presents a jump from the maximum valued 511 of the counter to the minimum value 1 as seen in Fig. 5 . However, counting of the counter is effected in a recurring manner when comparison in magnitude is made on the magnitude comparator 13. Thus, if it is determined that the maximum value 511 and the minimum value 0 are contiguous to each other, binary digitization can be effected in a similar manner to the case of Fig. 3 as shown by waveform curves f , g , h of Fig. 5 . Such determination can be readily made in digital processing. In other words, the embodiment of the present invention do not require a phase shifting circuit which is a principal part of a conventional processing circuit.

Claims: A suction air amount measuring device for an engine having a Karman's vortex street flow rate sensor for detecting a vortex signal in response to operating states of an engine and a noise reduction means for removing noise components of the sensors signal cause by the operation of a throttle valve, characterized in that said suction air amount measuring device comprises an ultrasonic wave generating vibrator (22) and an ultrasonic wave receiving vibrator (23) in an opposing relationship across a suction air path (1) of said engine downstream of a vortex street generating member (21) inserted in a fluid to be measured within said suction path, a resonance point detecting circuit (11) for detecting whether or not said ultrasonic wave generating vibrator is excited at the resonance point thereof, a voltage controlled oscillator (12) connected to be controlled by said resonance point detecting circuit, a counter (9) for dividing output pulses of said voltage controlled oscillator by counting the pulses, a driving circuit (10) for exciting said ultrasonic wave generating vibrator with the pulses divided by said counter, a latch circuit (14) for fetching contents of said counter at a timing of each of pulses produced by wave form shaping an output of said ultrasonic wave receiving vibrator, and binary digitizing means (33) for determining increase or decrease of said latch circuit.

Inventors: TSUTSUMI KAZUMICHI C O MITSUBI, TSUTSUMI, KAZUMICHI, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1987
Main classification: G01F1
IPC codes: G01F1, F02D41