Sensor analysis of automotive electronics: several common automotive sensors

In order to ensure the normal driving of the car, people convert various operating conditions information such as vehicle speed, temperature of various media, engine operating conditions, etc. into electrical signals to the car controller (such as computers, etc.), so that the engine is in In the best working state, these signals are collectively called car sensors. One of the characteristics of the development of modern automotive technology is that more and more components are electronically controlled.

The role of automotive sensors is very large. Once a sensor fails, the corresponding device will not work properly or even not work. In the past, automotive sensors were used solely on engines, but now they have been extended to chassis, bodywork, and lighting electrical systems, including measuring temperature, pressure, flow, position, gas concentration, speed, brightness, dry humidity, distance, etc. Automotive sensors.

Vehicle speed sensor

Figure 1 shows the traditional speedometer, which is mechanical. A typical mechanical odometer is connected to a flexible shaft. There is a steel cable in the flexible shaft. The other end of the flexible shaft is connected to a gear of the transmission. The rotation of the gear drives the rotation of the steel cable. The steel cable drives a magnet in the odometer cover ring. The cover ring is connected to the pointer and the pointer is set to zero position through the hairspring. The speed of the magnet's rotation speed causes the change in the size of the magnetic field line. The balance is broken and the pointer is driven.

The unit of the tachometer is 1 / min & TImes; 1000, which shows how many thousand revolutions per minute the engine rotates. The tachometer can intuitively display the speed of the engine under various operating conditions. The driver can always know the operation of the engine, and cooperate with the transmission gear and throttle position to keep it in the best working state, which is helpful for reducing fuel consumption and extending the engine life. benefit.

In addition to mechanical tachometers, more electronic tachometers are now available, with pointer and liquid crystal digital display. There is a digital integrated circuit in the table. It calculates the voltage pulses delivered by the ignition coil and drives the pointer to move or digital display after calculation. (figure 2).

There is also a tachometer that obtains the pulse signal from the generator and transmits it to the tachometer circuit to display the speed value, but the value is not very accurate due to factors such as generator belt slippage.

Temperature Sensor

The temperature sensor is mainly used to detect the engine temperature, intake gas temperature, cooling water temperature, fuel temperature and catalytic temperature. There are three main types of temperature sensors, wire wound resistance type, thermistor type and thermocouple resistance type. The three types of sensors have their own characteristics, and their applications are slightly different. Wirewound resistance temperature sensor has high accuracy, but poor response characteristics; thermistor temperature sensor has high sensitivity and good response characteristics, but poor linearity, adapts to low temperature; thermocouple resistance temperature sensor has high accuracy and measures temperature Wide range, but it needs to be used together with amplifier and cold junction processing.

Take the water temperature sensor shown in Figure 3 as an example, its interior is a semiconductor thermistor, the lower the temperature, the greater the resistance; conversely, the smaller the resistance, installed on the water jacket of the engine block or cylinder head, and cooling water Direct contact to measure the temperature of engine cooling water. The electronic control unit measures the temperature of the engine cooling water based on this change. The lower the temperature, the greater the resistance; otherwise, the smaller the resistance. Based on this change, the electronic control unit measures the temperature of the engine cooling water as a correction signal for fuel injection and ignition timing. Simply put, the user can know the current running state of the car, stop or move, or how long the exercise is based on the temperature of the engine water temperature.

In addition to the above resistance temperature sensors, bimetallic temperature sensors are also widely used in automotive electronics. The bimetallic temperature sensor is a temperature sensor made by using the temperature characteristics of the bimetallic sheet, such as a cold start timer switch, which is composed of a housing, a bimetallic sheet, a heating coil, and contacts.

The cold start timer switch is a "temperature-time" switch, which is installed at the engine water outlet with a screw connection to detect the engine water temperature. There is a normally closed contact inside this switch, and its movable arm is made of bimetallic sheet. The bimetallic sheet is formed by bonding two metal sheets with different thermal expansion coefficients. As the temperature changes, the two metal sheets produce a difference in thermal expansion. When the temperature is low, the bimetallic sheet maintains its original state; the temperature rises to a certain extent At this time, the bimetallic sheet is bent toward the side of the metal sheet with a low expansion coefficient. There are two heating coils around the bimetallic sheet. After heating, the bimetallic sheet can be bent to break the contact. After the engine is warmed up to a certain temperature, the originally closed contact should be open.

When the engine is cold, the time switch contact is closed. When cold starting, the ignition switch is in the starting position, and current flows through the starting power supply terminal. The current is formed by the battery through the ignition switch, the cold start nozzle, the bimetallic piece of the time switch, the contacts, and the ground, so that the cold start nozzle sprays oil. At the same time, there is also current flowing through the heating coil of the time switch. The two heating coils heat the bimetallic sheet. When it bends to open the contacts, the cold start injector stops the fuel injection.

After starting, the start switch is turned off, the ignition switch is turned from the starting position to the off position, and no current flows through the cold start fuel injector to stop the fuel injection. At the same time, both heating coils are powered off, but at this time the engine is already in the warm-up state, and its water temperature rises enough to bend the bimetal, and the contacts are disconnected, which means that when the engine is running normally, the cold The contacts of the start injector time switch remain open, and the cold start injector does not inject oil.

Automobile manufacturers all stipulate the operating temperature of the cold start injector time switch according to their own design, such as Nissan series is below 18 ℃, Toyota series is below 35 ℃. The lower the water temperature, the longer the working time of the cold start injector. The injection time of the cold start injector depends on how long the injection valve is open.