JPH0654252B2 - Switching control type thermal flow sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a thermal type flow sensor for measuring an intake air amount of an engine and the like.

[Conventional technology]

The thermal type flow sensor uses the heat transfer phenomenon from a heating element consisting of a temperature sensitive resistor arranged in the intake air to the intake air, and the detection circuit uses a constant temperature measurement method with excellent responsiveness. Things are commonly used. In the constant temperature measurement method, the bridge circuit and the differential amplifier are configured so that the temperature of the heating element is always higher than the intake air temperature by a constant temperature.Since the power loss of the transistor that supplies the current to the bridge circuit is large, An intermittent control method has been proposed in which duty control is performed to reduce power loss and a similar temperature measurement method is obtained.

A conventional example of this intermittent control system is shown in FIGS. 2 and 5.
In FIG. 2, a temperature sensing element 2 for heat generation and a temperature sensing element 3 for detecting intake air temperature are provided in the intake pipe 1, and the temperature sensing elements 2 and 3 are made of, for example, platinum or tungsten whose resistance value changes with temperature. , A thin wire made of nickel or the like, or a thin film resistance element. These temperature sensitive elements 2 and 3 and the fixed resistors 4 to 6 form a Wheatstone bridge, and the connection point between the fixed resistor 6 and the temperature sensitive element 2 and the fixed resistors 4 and 5 are connected.
The connection point is input to the differential amplifier circuit to amplify the difference between the unbalanced voltage of the bridge and the voltage of the DC unbalanced power supply 17.
The differential amplifier circuit includes fixed resistors 7 to 9 and a differential amplifier 10. The output voltage of the differential amplifier circuit is converted into a pulse signal by the pulse width conversion circuit 12. The triangular wave generator 11 serves as a signal source for pulse width conversion, and the pulse width conversion circuit 1
At 2, the pulse signal is converted into a pulse signal having a duty ratio proportional to the output of the differential amplifier circuit, and the switching transistor 13 is on / off controlled by this pulse signal. The electric power of the DC power supply 18 which is intermittent by the transistor 13 is made continuous by the smoothing circuit including the inductor 15 and the capacitor 16 and the diode 14, and supplies the current to the bridge circuit. Each resistance value is set so that the bridge is balanced when the temperature of the heat-generating temperature sensor 2 is higher than the intake air temperature by a certain temperature.
A constant temperature circuit can be realized by configuring the feedback circuit as described above.

Next, the operation of the heat-sensitive flow rate sensor having the above configuration will be described. Now, assuming that the air flow rate in the intake pipe 1 has increased,
The temperature of the heat-generating temperature sensitive element 2 decreases, and the resistance value decreases. Along with this, the potential at the connection point between the heat-generating temperature sensitive element 2 and the fixed resistor 6 decreases, and the output voltage of the differential amplifier 10 increases. Further, the duty ratio of the digital output of the pulse width conversion circuit 12 changes, and the high time of the voltage increases. Therefore, the ON time of the transistor 13 increases, the heating current to the bridge smoothed by the smoothing circuit also increases, and the temperature of the heat-generating temperature sensitive element 2 is prevented from decreasing. As a result, the temperature of the heat-generating temperature sensitive element 2 is kept constant.

At this time, the duty ratio D of the switching signal of the switching transistor 13 is Becomes Here, I is the heating current, Vin is the voltage of the DC power supply 18, Rh is the resistance value of the heat-generating temperature sensing element 2, and R6 is the resistance value of the fixed resistor 6.

On the other hand, the relationship between the heating current I and the air flow rate Q during constant temperature control is Becomes However, A and B are constants. Therefore, the duty D is Becomes Since Rh and R6 are constant, the duty D is a function of Q, assuming that the power supply voltage Vin does not change.
Therefore, the flow rate Q can be obtained by measuring the off time or the off time of the control signal of the switching transistor 13. Moreover, since the output signal of the sensor is digital, there is an advantage that no interface with the microprocessor is required.

Next, the effect of the DC unbalanced power supply 17 will be described. When this DC unbalanced voltage is taken into consideration, the resistance value Rh of the temperature sensing element 2 for heat generation is given by the following equation.

Where Rk is the resistance value of the temperature sensor 3 for detecting the intake air temperature, R
4 and R5 represent the resistance values of the fixed resistors 4 and 5. Since the DC unbalanced power source 17 is provided, Rh has a resistance value larger by ΔRh than the value for balancing the bridge. This △ Rh
Let ΔE be the voltage of the DC unbalanced power supply 17, Becomes However, A is a DC differential gain. This DC differential gain A is And R8 and R9 represent the resistance values of the resistors 8 and 9. As is clear from the above equation, the resistance value of the heat-generating temperature-sensitive element 2 depends on the heating current I, that is, changes with the flow rate, and the larger the flow rate, the smaller Rh. Then, this change becomes larger as the differential gain becomes smaller. In order to realize an ideal constant temperature circuit, the larger the DC differential gain, the better.

FIG. 3 shows a frequency characteristic 22 of the differential amplifier circuit, which shows a flat frequency characteristic up to the resonance frequency of the smoothing circuit. 2
Reference numeral 1 denotes the open loop characteristic of the differential amplifier 10. or,
Reference numeral 24 in FIG. 4 shows a frequency characteristic of the sensor output with respect to a minute flow rate change when the average flow rate Q is constant. In this characteristic 24, a peak is seen near the resonance frequency, which is due to the resonance frequency of the smoothing circuit, and the differential gain of the differential amplifier circuit is too large at the resonance frequency with a phase delay exceeding 180 deg. . This peak becomes larger and unstable as the average flow rate increases and as the DC differential gain increases. For this reason, the DC differential gain cannot be set large, and an ideal constant temperature circuit cannot be realized. Further, as the flow rate increases, the resonance frequency response is exhibited, so that the measurable flow rate range becomes narrow and the maximum measurable flow rate range is limited to about 50 g / sec. Since this flow rate is about one-third of the maximum intake air volume of a 2000cc naturally aspirated engine, it was not possible to cover the entire flow rate range.

FIG. 5 shows another example of the conventional switching control type thermal flow sensor, and the differential amplifier circuit includes a differential amplifier 10 and resistors 7 to 7.
9 and 19, the difference between the voltage of the DC unbalanced power source composed of the constant current circuit 20 and the fixed resistor 19 and the output of the bridge circuit is amplified. Other configurations are the same as those of the conventional example shown in FIG.

The operation of the above configuration is the same as in the case of FIG. 2, and the DC unbalanced voltage is necessary for adjusting the AC characteristics of the sensor. FIG. 7 shows the frequency characteristics of the sensor when the differential gain is constant,
The average flow rate Q is constant (2 g / s in the (a) diagram, 10 in the (b) diagram.
The frequency characteristic of the sensor output with respect to the minute flow rate change at 0 g / s) is shown. As is clear from the figure, the larger the flow rate Q, the wider the frequency band, and when the flow rate is small as in the case of (a), a peak appears around the resonance frequency of the smoothing circuit, and the DC unbalance voltage ΔE The larger is, the larger the peak is. On the other hand, as shown in (b), average flow rate Q
Is larger, the smaller the DC unbalanced voltage ΔE, the more resonant characteristics. Therefore, if ΔE is reduced so that the system does not become resonant in the small flow rate range, it becomes resonant in the large flow rate range and the maximum measurable flow rate range is limited to about 50 g / sec. Couldn't cover.

[Problems to be Solved by the Invention]

The conventional switching control type thermal flow sensor is configured as described above, and exhibits a resonating frequency response as the flow rate increases, and the relationship between the DC unbalance voltage and the frequency characteristics is small when the flow rate is large. Because it depends on the case
The measured flow rate range has become narrow.

The present invention has been made to solve the above problems, and an object thereof is to obtain a switch type controlled thermal flow sensor having an optimum frequency response over a wide flow range.

[Means for Solving the Problems]

In the switching control type thermal flow sensor according to the first aspect of the present invention, a differential delay circuit for amplifying the output of the bridge circuit is provided with a phase delay compensating circuit for compensating for the phase delay.

A second aspect of the invention provides a means for increasing the DC unbalanced voltage applied to the differential amplifier circuit as the intake flow rate increases.

[Work]

In the first aspect of the invention, the differential amplifier circuit compensates the phase delay to reduce the AC differential gain while maintaining the DC differential gain, and the gain margin at the resonance frequency of the smoothing circuit in which the phase is abruptly delayed. Becomes larger, and the frequency response is adjusted to be slightly damped.

In the second invention, the DC unbalanced voltage applied to the differential amplifier circuit increases with the flow rate. Therefore, the resonance phenomenon is suppressed in the entire flow rate range.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. First
The drawing shows a first embodiment of the present invention, in which a series circuit of a resistor 21 and a capacitor 22 is connected in parallel with a feedback resistor 9. The resistance value of the resistor 21 is set smaller than that of the resistor 9. Other configurations are the same as those in FIG.

In the above configuration, the resistors 9 and 21 and the capacitor 22 constitute a phase delay compensation circuit. Therefore, the frequency characteristic of the AC differential gain of the differential amplifier circuit can be adjusted by appropriately selecting the values of the resistor 18 and the capacitor 19.
As shown in FIG. 3, the DC response gain is higher than the conventional one, and the high frequency gain is a low frequency response characteristic. Therefore, the frequency response characteristic to the minute flow rate fluctuation as a sensor is 2 in FIG.
5, the frequency characteristic having the optimum attenuation characteristic can be obtained by reducing the AC gain of the differential amplifier circuit.

FIG. 6 shows a second embodiment of the present invention, in which a voltage-current converter 28 outputs a current proportional to the voltage across the fixed resistor 6, and its output is supplied between the resistors 8 and 19. The fixed resistors 6 and 19 are set to be sufficiently smaller than the fixed resistors 8 and 9. Further, the output current of the voltage / current alternator 28 is set to be sufficiently smaller than the heating current flowing through the resistor 6 and larger than the current flowing through the resistors 8 and 9. At this time, the output current of the voltage-current converter 28 is the resistance 1
It flows through 9 and 6 to the ground. Therefore, the DC gain of the differential amplifier circuit is determined by the resistance values of the fixed resistors 8 and 9,
The DC unbalanced voltage is determined by the output current of the voltage-current converter 28 and the fixed resistor 19. The flow rate detection, constant temperature control, and influence of the DC unbalanced voltage on the frequency response are basically the same as the conventional ones. As a result, since the DC unbalanced voltage is proportional to the heating current, it is small in the small flow rate range and increases as the flow rate increases. Therefore, as shown in FIG. 7, the optimum frequency response is obtained in the entire flow rate range.

Although the voltage across the resistor 6 is selected as the input voltage of the voltage-current converter 28 in the above-described second embodiment, for example, the connection point of the temperature sensitive elements 2 and 3 that exhibits a monotonically increasing characteristic according to the flow rate. The same effect can be obtained even if the voltage is selected.

〔The invention's effect〕

As described above, according to the first aspect of the present invention, by reducing the AC gain of the differential amplifier circuit, the frequency characteristic of the sensor, which was resonant, is attenuated, and the measurement flow rate range is expanded. Moreover, the DC differential gain can be increased more than before, an ideal constant temperature circuit was realized, and the measurement accuracy was improved.

According to the second aspect of the invention, the DC unbalance voltage that affects the frequency response of the sensor is changed depending on the flow rate, so that the optimum frequency response characteristic is obtained in the entire flow rate range.
Moreover, the measurable flow rate range has been expanded.

[Brief description of drawings]

FIG. 1 is a block diagram of a flow sensor according to a first embodiment of the present invention, FIG. 2 is a block diagram of a conventional flow sensor, and FIG. 3 is a differential amplification of the flow sensor shown in FIGS. 1 and 2. FIG. 4 is a frequency characteristic diagram of the circuit, FIG. 4 is a frequency response characteristic diagram of the flow sensor shown in FIGS. 1 and 2, FIG. 5 is a configuration diagram of another conventional flow sensor, and FIG. 6 is a second diagram of the present invention. FIGS. 7 (a) and 7 (b) are frequency response characteristic diagrams of the flow rate sensor shown in FIGS. 5 and 6, respectively. 1 ... Intake pipe, 2, 3 ... Temperature sensing element, 4-9, 19, 2
6 ... Fixed resistance, 10 ... Differential amplifier, 11 ... Triangular wave generator, 12 ... Pulse width conversion circuit, 13 ... Switching transistor, 14 ... Diode, 15 ... Inductor, 16, 27 ... Capacitor , 17 ... DC unbalanced power supply, 18 ... DC power supply, 28 ... Voltage-current converter. The same reference numerals in the drawings indicate the same or corresponding parts.

Claims (2)

[Claims] 1. A temperature-sensitive element which is arranged in intake air and has a characteristic that its resistance value changes with temperature, a bridge circuit composed of this temperature-sensitive element and a fixed resistor, and an output of the bridge circuit is amplified. A differential amplifier circuit having a phase delay compensating circuit for compensating the phase delay, a control pulse generating circuit for generating a pulse having a duty ratio corresponding to the output of the differential amplifier circuit, and a power amplifier and a bridge circuit are provided. , A switching control characterized by comprising an intermittent control circuit for controlling an energization time corresponding to the duty ratio, and a smoothing circuit for making a discontinuous output of the intermittent control circuit continuous and supplying a current to a bridge circuit. Type thermal flow sensor. 2. A temperature sensing element, which is arranged in intake air and has a characteristic that its resistance value changes with temperature, a bridge circuit composed of this temperature sensing element and a fixed resistor, and increases with an increase in intake flow rate. DC unbalanced voltage generating means for generating a DC unbalanced voltage, a differential amplifier circuit for amplifying a difference between the output of the bridge circuit and the DC unbalanced voltage, and a pulse having a duty ratio corresponding to the output of the differential amplifier circuit A control pulse generation circuit that generates a power supply, a power supply and a bridge circuit that are provided between the power supply and the bridge circuit, and control the energization time according to the duty ratio, A switching control type thermal flow sensor, which is provided with a smoothing circuit for supplying a current to the circuit.