Air flow meters in general we don't discuss much. But as an flow meter it is an indispensable presence. Today we take everyone to introduce its classification and characteristics. According to the characteristics of the air flow sensor, the fuel control system is divided into the L-type control that directly measures the intake air amount and the D-type control that directly measures the intake air amount according to the measurement method of the intake air amount (according to the intake manifold negative pressure and the engine). The rotational speed indirectly measures the intake air amount. In the microcomputer ROM of the D-type control mode, the intake air amount in various states, which is based on the engine speed and the pressure in the intake pipe, is stored in advance, and the microcomputer operates according to the measured each. In the state of the intake pressure and speed, refer to the amount of intake air stored in the ROM, can calculate the fuel amount L-type control used in the air flow meter and the general industrial flow sensor is basically the same, but it can adapt to the harsh environment of the car, but the step The requirement for a sharp change in the flow rate that occurs at the throttle and the requirement for high-precision detection in the uneven airflow caused by the shape of the intake manifold before and after the sensor.
First, the structure of the valve type air flow sensor
The valve type air flow sensor is installed on the gasoline engine and is installed between the air filter and the throttle valve. Its function is to detect the intake air amount of the engine, and convert the detection result into an electric signal, and then input it into the microcomputer. The sensor is composed of an air flow meter and a potentiometer.
First look at the working process of the air flow sensor. The air sucked by the air cleaner rushes toward the shutter, and the shutter is stopped at a position where the intake air amount is balanced with the return spring, that is, the opening degree of the shutter is proportional to the intake air amount. The rotating shaft of the shutter is also equipped with a potentiometer, and the sliding arm of the potentiometer rotates synchronously with the shutter, and the opening of the measuring piece is converted into an electric signal by the voltage drop of the sliding resistor, and then input into the control circuit.
2. The Karman vortex air flow sensor has developed a small and lightweight air flow sensor in order to overcome the shortcomings of the valve type air flow sensor, that is, under the premise of ensuring measurement accuracy, expanding the measurement range, and eliminating the sliding contact. Karman vortex air flow sensor. The Karman vortex is a physical phenomenon. The detection method of the vortex and the electronic control circuit are not related to the detection accuracy. The passage area of ​​the air and the size change of the vortex generating column determine the detection accuracy. Moreover, since the output of such a sensor is an electronic signal (frequency), the AD converter can be omitted when a signal is input to the control circuit of the system. Therefore, in essence, the Karman vortex air flow sensor is a signal suitable for microcomputer processing. This kind of sensor has the following three advantages: high test accuracy, output of linear signal, simple signal processing; long-term use, performance does not change; because it is to detect volume flow, it does not need to correct temperature and atmospheric pressure.
Detection principle: When the overhead wires in the field are blown by the wind, they will make a sound. The higher the wind speed, the higher the sound frequency. This is because the airflow flows through the wires and forms a vortex. This phenomenon occurs in fluids such as liquids and gases. This phenomenon can be used to make a scroll flow sensor. Two columns of vortexes are formed after the column is placed in the pipe, and the flow rate can be measured according to the frequency at which the vortex appears. Because the vortex is in two parallel rows, and the left and right alternately appear, similar to the street lamps on both sides of the street, so there is a vortex street. Because this phenomenon was first discovered by Kaman, it is also called Kaman vortex. Ultrasonic Karman vortex air flow sensor. Ultrasonic air flow sensor is equipped with two intake channels, main channel and bypass channel. Part of it is set on the main channel. The purpose of setting the bypass channel is to be able to adjust the flow of the main channel so that the detection characteristics of the main channel are ideal. That is to say, for an engine with a different exhaust volume, by changing the cross-sectional size of the air flow sensor channel, a variety of air flow sensors can be used to cover a plurality of engines. The triangular prism on the main channel and several vortex magnifying plates form a Karman vortex generator. On both sides of the Karman vortex, oppositely, an ultrasonic transmitter and an ultrasonic receiver belonging to the electronic detecting device are disposed, and the two components can also be classified into the sensor, and the electrical signals generated by the two electronic sensors are transmitted through the air. The control circuit (hybrid integrated circuit) of the flow sensor is shaped, amplified, and converted into an ideal waveform, and then input into the microcomputer. In order to utilize the ultrasonic inspection vortex, a sound absorbing material is adhered to the inner wall of the vortex passage for the purpose of preventing irregular reflection of the ultrasonic wave.
The pressure change detection type Karman scroll air flow sensor vortex alternates from both ends of the vortex generator, so that the vortex generator is alternately generated at both ends, so the pressure at both ends of the vortex generator is also alternately changed. The change of pressure is guided into the mirror cavity through the pressure guiding hole on the tapered side of the vortex generator. The mirror in the mirror cavity is tensioned by a very thin tension band. Therefore, the tension band is applied. Distortion and vibration occur, and in addition, the tension is applied to the tension band by the leaf spring, whereby it is difficult to influence the pressure change other than the vibration and the swirl pressure, so that stable torsion and vibration can be obtained.
The pressure formed by the vortex appears through the pressure guiding hole into the mirror cavity, which is synchronized with the pressure change in the reflecting cavity, and the mirror forms a torsion and vibration on the tension band. The mirrors are very lightweight and operate even at low flow rates and very little pressure changes. In the upper part of the mirror, a light sensor composed of a light-emitting diode and a phototransistor is arranged correspondingly. When the light emitted by the diode is reflected by the mirror and is incident on the phototransistor, it becomes a current and is output through the waveform circuit.
Characteristics of the compressed air flowmeter sensor: When the throttle is closed from full to open within 30 seconds, that is, the response characteristic of the sensor is as shown in the figure, the curve under the figure is the F/V converted card. The output characteristics of the vortex air flow sensor, the curve on the graph is the opening characteristic of the throttle. It can be seen from the figure that the air flow sensor can accurately reflect the air flow rate change (1~45 milliseconds). Relationship with frequency: There is a linear relationship between the air flow rate and the vortex frequency over a very wide flow range.
3. Ultrasonic Kalman Vortex Air Flow Sensor with Differential Pressure Sensor The characteristics of the Karman vortex air flow sensor are high precision, long life and high reliability. However, a high-performance engine that further reduces fuel consumption and increases output power also requires an extended detection range of intake air, but the old-style ultrasonic Kalman vortex air flow sensor will overmodulate in high-flow areas. The impact of this factor is that this sensor has the disadvantage of insufficient metering range. To this end, an air flow sensor with a differential pressure sensor has been developed.
1. A vortex generator with low pressure loss: The function of the vortex generator is to form a stable vortex over the entire flow range.
2. Pipe structure with less pressure loss
3. Measuring small vortex pressure 4. Air flow sensor with differential pressure sensor
4. Structure of hot wire air flow sensor The hot wire as the heating element is made of platinum wire with a diameter of 70um. It is tensioned inside the pipe and designed to be 120 degrees higher than the intake air temperature. There is also an air temperature compensation resistor in the temperature sensor. It is formed from a platinum film printed on an alumina ceramic substrate, which is placed in the pipe together with precision resistors. In order to prevent the performance of the dust attached to the hot wire from deteriorating, a dust burning circuit is provided. When the ignition switch is placed in the off position, the hot wire is heated to 1000 degrees, 1 second under certain conditions to burn off the dust. And other attachments. Since the platinum wire is used as the heating element, the responsiveness is good.
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