JPS61283826A - Thermal type air flowmeter - Google Patents

Abstract

PURPOSE:To surely protect a thermosensitive element by a simple and small size construction by detecting an abnormal voltage of a heating power supply and providing a protective circuit for controlling an opening/closing element to be turned off whereby an abnormal voltage is prevented from being applied to a thermosensitive element portion. CONSTITUTION:A measuring apparatus comprising a thermosensitive element 12 and an air temperature measuring element 13 both provided in a suction pipe 11, resistor elements 14 and 15, a control transistor 17, comparator 18, an FF 19 and the like is added with a protective circuit 21 comprising an overvoltage detector circuit 32. A thermal type air flowmeter is constructed such that the power supply of a control circuit for the comparator 18, FF circuit 19 and the like is connected to the junction (a) of the detector circuit 32 and a resistor 31. Since, when a power supply voltage +B falls in an overvoltage, a current i1 flows to control a transistor 34 in the detecting circuit 32 and a control transistor 17, turning the transistors 34 on and 17 off, and therefore the bridge circuit comprising the thermosensitive element 12 is not supplied with the power supply voltage, the apparatus can be protected from the overvoltage.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is used to measure the intake air flow rate of an automobile engine, for example, and is particularly applicable to a thermal This invention relates to an air flow rate measuring device.

[Background Art] For example, when controlling an automobile engine electronically, the operating state of the engine is monitored, and engine operating controls such as fuel injection amount and ignition timing are executed based on the monitored data. It is something to do. In this case, there are various types of sensors that monitor the operating state of the engine, but in particular, there are sensors related to the amount of fuel injection.
There is a sensor that detects the amount of intake air.

Various types of air flow rate detection means are conceivable for use in such an engine control system, and for example, a thermal air flow rate measuring device is known.

This thermal air flow measuring device is equipped with a temperature sensing element having temperature-resistance characteristics that is controlled to generate heat by supplying heating power into the intake pipe, and the temperature state of this temperature sensing element is observed. The air flow rate flowing into the intake pipe is measured by this method.

That is, since the temperature sensing element is installed in the intake pipe and is exposed to the intake air flow, its heat dissipation characteristics are changed depending on the intake air flow rate. Specifically, when the intake air flow rate is large, there is a large air flow on the surface of the temperature sensing element, and when the same heating power is supplied, the temperature of this temperature sensing element is higher than when the air flow rate is small. !
The rate of temperature increase will be slower. Furthermore, in order to control the temperature of the temperature sensing element so as to maintain it at a specified constant temperature state, the heating power must be controlled to increase in proportion to the air flow rate.

When configuring such a thermal air flow measuring device, it is controlled to supply heating power to the temperature sensing element, so there is no possibility that an abnormally high voltage may be applied to the measuring device circuit. It is necessary to set up safeguards against such cases.

This protection means uses, for example, a Zener diode with a Zener voltage of several volts to monitor the voltage of the power supply circuit for heating power to the temperature sensing element, and is configured to lead the heating power circuit to the ground circuit when a normal voltage is generated. are doing.

However, since the current consumption in the temperature sensing element is relatively large in such a means, a Zener diode with a large capacity is required, which causes problems in its size and mounting means.

[Problems to be Solved by the Invention] This invention has been made in view of the above points, and even if an abnormally high voltage is applied to the measurement circuit section including the temperature sensing element, This is a protection circuit that has a small capacity and reliably prevents abnormal voltage from being applied to the temperature-sensing element, ensuring that the temperature-sensing element is protected. It is an object of the present invention to provide a thermal air flow rate measuring device that can be configured such that it can be effectively used, for example, as a means for measuring the intake air flow rate of an engine.

[Means for solving the problem J] That is, in the thermal air flow measuring device according to the present invention, the heating power supplied to the temperature sensing element is controlled to open and close by a switching element such as a control transistor. , the air flow rate measurement operation is executed, and the power supply voltage supplied to the switching element section is detected by a voltage detection means such as a Zener diode set in a circuit within the monolithic IC, and by this means. The switching element is turned off when an abnormal voltage is detected.

[Function] In the thermal air flow measuring device configured as described above, the switching element is turned on by a periodically generated start signal, and the heating power to the temperature sensing element is controlled to rise. When the temperature of the heating element rises to a specified temperature, the heating power is cut off to perform the measurement operation, and a measurement signal corresponding to the opening width when the heating power is supplied is output. become. During such measurement operations, if an abnormality occurs in the power supply voltage, the abnormal voltage will be detected by the protection circuit described above, and the switching element will be turned off and the temperature sensor will be turned off. The element is now protected from abnormal voltage. In this case, the protection circuit only needs to control the transistors that constitute the switching elements, so it can be constructed with a sufficiently small capacity.

[Embodiments of the invention] Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows the configuration of an air flow measuring device for measuring the intake air amount of an engine, in which a temperature sensing element 12 is installed inside an intake pipe 11 that guides the intake air flow to the engine. The temperature-sensitive element 12 is made of, for example, a platinum wire, and heat generation is controlled by supplying heating power, and has a temperature characteristic in which the resistance value changes in accordance with the temperature. Further, inside the intake pipe 11, an air temperature measuring element 13 having temperature characteristics similar to the temperature sensing element 12 described above is set. The temperature will be set accordingly.

Fixed resistance elements 14 and 15 are connected to these temperature sensing element 12 and air concentration measuring element 13, respectively, so that a bridge circuit is constructed together with these resistance elements 14 and 15. In this case, a resistor 1G for concentration compensation control is connected in series to the air temperature measuring element 13. A heating power source +B is connected to this bridge circuit via a control transistor 17.

In addition, a temperature sensing element 12 serving as an output terminal of the bridge circuit
and the connection point A1 between the resistor element 14 and the resistor element 15 and 1
The connection point B with 6 is connected to the input terminal of the comparator 18. This comparator 18 compares the potentials of points A and B and generates an output signal when the potential of point A is higher than the potential of point B.
The output signal from 8 resets and controls the flip-flop circuit 19. Specifically, the comparator 18 outputs the temperature of the temperature sensing element 12 in a state where the heating power is supplied to the temperature sensor 12 and the temperature rises until a temperature difference specified by the air humidity measured by the air temperature measuring element 13 is set. A signal will now be generated.

A periodically generated pulse-like start signal is supplied to the flip-flop circuit 19 as a set command signal. This start signal is generated, for example, in synchronization with the rotation of the engine. and,
The output signal of this flip-flop circuit 19 when set is taken out as an output signal via a buffer 20, and is also sent to the control transistor 17 via a protection circuit 21.
controls the base of the transistor 17 in this set state.
is turned on so that heating power is supplied to the bridge circuit including the temperature sensing element 12.

FIG. 2 shows a specific example of the protection circuit 21, in which the power source B is grounded via a resistor 31, an overvoltage detection circuit 32 consisting of a plurality of Zener diodes connected in series, and a resistor 33. It consists of

From the connection point a between the resistor 31 and the overvoltage detection circuit 32, the comparator 18 and the flip-flop circuit 19 are connected.
The voltage signal at the connection point between the overvoltage detection circuit M32 and the resistor 33 is supplied to the base of the transistor 34. This transistor 34 is connected in series to the base circuit of the control transistor 11, and in its on state controls the transistor 17 to turn off, and in its off state controls the transistor 17 to turn on. In this case, the output signal from the flip-flop circuit 19 is also inverted by the inverter 35 and supplied to the base circuit of the transistor 34, so that the control transistor 11 is turned on when the flip-flop circuit 19 is set.

That is, when the power supply voltage +B is in a normal potential state, the Zener diode constituting the overvoltage detection circuit 32 is in a non-conducting state, so the voltage 1i1 is in a "0" state, and the control circuit section is Current 12 begins to flow and normal measurement operations are performed. On the other hand, when the power supply element B is in an abnormal voltage state, the above-mentioned current 1
1 flows, and a power higher than the clamped voltage is not supplied to the control circuit portion. Further, the transistor 34 is controlled to be in an on state by the current 11 flowing.
The control transistor 17 is controlled off, and the temperature sensing element 12
Power is no longer supplied to the bridge circuit, including the bridge circuit, and it is protected from overvoltage.

The voltage value of the heating power supplied from the control transistor 11 to the bridge circuit is compared with a reference voltage value from a reference voltage power source 23 in an OP amplifier 22. The base of the control transistor 11 is controlled by the output signal from the OP amplifier 22, so that the voltage of the heating power supplied to the bridge circuit, particularly the temperature sensing element 12, is set as a reference.

That is, in the thermal air flow measuring device configured as described above, when the flip-flop circuit 19 is set by a start signal generated in synchronization with the rotation of the engine, the control transistor 11 is turned on. heating power set at a constant voltage is supplied to the temperature sensing element 12. And this temperature sensing element 12
generates heat of 11JI, and its temperature rises at a rate inversely proportional to the flow rate of air flowing into the intake pipe 11.

In this way, when the temperature of the temperature sensing element 12 is raised by the heating power and the temperature rises until a specified temperature difference is set with respect to the air temperature measured by the air temperature measuring element 13, the comparator 18 An output signal is generated to reset the flip-flop circuit 19. Therefore, in response to the reset of the flip-flop circuit 19, the control transistor 17 is controlled to be turned off, and the heating power to the temperature sensing element 12 is controlled to be cut off.

In such an air flow measurement operation, the temperature of the temperature sensing element 12 to which heating power is supplied increases at a rate inversely proportional to the intake air flow rate flowing into the intake pipe 11.
The time width from the rise of the heating power to the rise of the output signal from the comparator 18 until the heating power is controlled to be cut off is proportional to the intake air flow rate. Therefore, the pulse-like signal from the set terminal Q of the flip-flop circuit 19 corresponding to the set and reset operations becomes an air flow rate measurement signal that expresses the measured air flow rate in terms of pulse time width.

When performing such a measurement operation, the power supply voltage +B
is in a normal state, the voltage value is detected by the overvoltage detection circuit 32.
Since the current is below the Zener voltage of the Zener diode constituting the
The Ill transistor 17 is now controlled only by the output signal of the flip-flop circuit 19, and a normal air flow measurement operation is performed. On the other hand, the power supply voltage +
When B exceeds the Zener voltage and becomes high, current 11 begins to flow, transistor 34 is turned on, control transistor 17 is turned off, and the bridge circuit including temperature sensing element 12 is protected from overvoltage. It becomes like this. In this case, the overvoltage detection circuit 3 is also included in the control circuit section.
A limited power supply is supplied with the clamp voltage No. 2, and protection is provided from abnormal voltage.

[Effects of the Invention] As described above, in the thermal air flow rate measuring device according to the present invention, the temperature sensing element portion, which becomes the sensor that performs the measurement operation when the power supply voltage becomes abnormal, is protected from overvoltage. Become more effectively protected.

In this case, since the current to be controlled can be made sufficiently small, this protection circuit can be configured with a Zener diode built into a monolithic IC, which is extremely small. Moreover, it is easy to implement, and can be effectively used, for example, as a means for measuring the intake air flow rate of an automobile engine.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram illustrating a thermal air flow rate measuring device according to an embodiment of the present invention, and FIG. 2 is a circuit diagram showing in detail a protection circuit portion used in the above embodiment. DESCRIPTION OF SYMBOLS 11... Intake pipe, 12... Temperature sensing element, 13... Air temperature measuring element, 11... Control transistor, 18...
...Comparator, 19...Flip-flop circuit,
21... Protection circuit, 32... Overvoltage detection circuit (Zener diode circuit).

Claims (1)

[Scope of Claims] A temperature sensing element whose heat generation is controlled by heating power having a temperature-resistance characteristic set in an air flow to be measured; and heating supplied to the temperature sensing element. a switching element for controlling opening/closing of electric power; a means for turning on the switching element in accordance with a periodically generated start signal to control rising of heating power to the temperature sensing element; means for controlling the opening/closing element to turn off and controlling the heating power to be cut off when the temperature rises to a specified temperature state; and means for generating an air flow measurement signal representing a time width of supply of the heating power to the temperature sensing element. and a protection circuit that detects abnormal voltage of the heating power source and turns off the switching element.