The car computer is an electronic device that automatically processes the input signals of various sensors according to a predetermined program and then outputs the signals to the actuator to control the operation of the car.
Classification of car computers
At present, automotive computers have been widely used, such as body computers, engine computers, transmission computers and ABS computers. Although the number and types of computers on different models are different, the general development trend is to use a main computer to process the input signals of most sensors, and use some smaller electronic control units to control other systems.
Composition of car computer
The main part of a car computer is a single chip microcomputer, which is a circuit board that integrates a microprocessor (CPU), memory, and input and output interfaces. The microprocessor is the core component of the single-chip microcomputer. The microprocessor converts the input analog signal into a digital signal, and performs comparison processing based on the stored reference data to calculate the output value. The output signal undergoes power amplification to control the actuator, such as the fuel injector And relays. With the increasing computing power and memory capacity of single-chip computers, the functions of automotive computers are also increasing.
Working process of car computer
(1) Signal filtering and amplification
The input circuit receives input signals from sensors and other devices, and filters and amplifies the signals. The purpose of the input signal amplification is to increase the signal to a level that can be recognized by the car computer. Some sensors, such as oxygen sensors, generate a low voltage signal less than 1V, which can only generate a very small current. Such a signal is sent to the computer. The microprocessor must be amplified before. This amplification is done by the amplifier circuit in the input chip of the computer.
(2) Analog-to-digital (A / D) conversion
Because many sensors produce analog signals, and the microprocessor processes digital signals, it is necessary to convert the analog signals to digital signals. This work is done by the analog-to-digital converter in the computer input chip. The analog-to-digital converter continuously scans the analog input signal of the sensor at fixed time intervals, assigns a fixed value to the analog signal, and then converts this fixed value into a binary code. In some automotive computers, the input processing chip and the microprocessor are integrated.
(3) The microprocessor calculates the signal that has been preprocessed and sends the processed data to the output circuit. The output circuit amplifies the digital signal, and some of it also needs to be restored to an analog signal to drive the actuator to work.
With the increase in the level of automotive electronics and automation, there will be more and more automotive computers, which will inevitably lead to increasingly complex body harnesses. In order to realize the rapid transmission of information between multiple car computers, simplify the circuit and reduce costs, the communication network technology should be connected between car computers to form a network system. For example, the transmission needs to be coordinated with the engine to automatically shift gears according to factors such as vehicle speed, engine speed, and power load. Therefore, the transmission computer needs to obtain signals such as throttle position sensor, vehicle speed sensor, water temperature sensor, and engine speed sensor. Information transfer between computer and engine computer, this work is usually done by CAN bus.
Car computer features
(1) Cars need to drive on different roads and climatic conditions, and the working environment of car computers is poor, often subject to vibration and changes in temperature and humidity. The power supply voltage of the car computer varies greatly, and it is also interfered by electromagnetic waves inside and outside the car. Therefore, the car computer needs high reliability and durability to the environment.
(2) The car computer must be sufficiently intelligent, have self-diagnosis and detection capabilities, be able to find faults in the system in time, and store fault codes to inform maintenance personnel where the fault may exist to facilitate maintenance. For example, airbags must be opened in a timely, correct, and rapid manner at critical moments, but most of the time the airbags are on standby, so the airbag computer must have self-checking capabilities to continuously confirm whether the airbag system is working properly.
(3) With a few exceptions, all automotive computers use a 5V power supply to drive their sensors. In the electronics industry, the 5V voltage is almost universal as a standard for transmitting information. This voltage is high enough for transmission reliability, but low enough for the safety of computer chips, and the use of computer industry standard voltages will make electronic parts manufacturing specifications and low cost for car manufacturers.
Car computer maintenance
The internal circuit of the car computer can be divided into two parts, namely the conventional circuit including the input, output and conversion circuit and the microprocessor. Conventional circuits mostly use general-purpose electronic components, which can be repaired if damaged. In the actual use process, the faults of the car computer mostly occur in the conventional circuit. If you want to repair a car computer, you must first determine that it is a computer failure, so as not to blindly repair it, causing unnecessary waste of time and causing other circuit failures.
(1) Determine whether the computer is damaged
The usual method to determine the damage of the computer is that the relevant sensor signals can be input into the computer normally, but the computer cannot correctly output the control signal to drive the actuator. Although this sentence is simple, it requires a lot of detailed and detailed basic inspection work. For example, if the engine fails to start, after checking, it is determined that there is no frequency voltage on the injector plug during starting. After checking that the related circuits are normal and the start signal can be input to the engine computer normally, but the computer does not output the drive signal to the injector, so that the engine computer can be concluded Internal failure.
(2) Look for damaged components according to the circuit
According to the circuit diagram or the actual line direction, find the corresponding computer terminal connected to the fuel injector, and then use the digital multimeter's on and off block to start from the determined computer terminal and search along the computer's printed circuit until a certain transistor is found. This is because computers usually use high-power transistors to amplify the execution signal to drive the actuator, so most of the causes of such failures are caused by a short circuit of the transistor that acts as a switch.
(3) Measuring transistor
Determine the 3 poles of the transistor. The pin corresponding to the printed line is the collector of the triode, and the thinner printed line next to it is the base. The confirmation method is to insert the multi-plug of the engine computer, start the engine, use the voltage block of the multimeter to connect to the printed line to be confirmed, and display 5V as the base. Test the triode with a multimeter. If the forward and reverse resistances of the collector (c) and base (b) are found to be infinite, it means that the triode has been disconnected; if the resistance between the collector (c) and the emitter (e) is found to be zero , It means that the triode has been broken down. In addition, you need to measure other transistors and diodes connected near the transistor.
(4) Determine the replacement transistor
There are several methods to determine the model of the transistor: â‘ Model. Check the model on the triode, and determine the matching domestic triode through the triode correspondence table. â‘¡ Resistance. The base of the triode is generally connected with resistors. The resistance of the base should be similar to that of the original triode, and the resistance of different colors is different. Because the base of the transistor is controlled by the size of the current and the voltage value of the computer is fixed, it is necessary to use a resistor to control the current. If the current is too large, the transistor will be burned, and if the current is too small, it cannot be triggered. â‘¢Measurement. Use the diode measuring block of the multimeter to measure the properties of the triode. According to the characteristics of the transistor, only one pin should be unidirectional with respect to the other two pins. With this property, it can be determined to be a triode. Only a pair of pins with unidirectional conduction is a field effect tube. The pin where the foot is on is the base of the transistor.
(5) Solder the replacement transistor to the circuit board
When welding, pay attention to the minimum amount of solder to avoid overheating. After the welding is completed, use a multimeter to measure that the pins should not be connected to each other.
(6) Test maintenance effect
Connect the computer board to the wiring harness of the car body under bare conditions, start the engine and check whether the corresponding function is normal. At the same time, touch the transistor with your hand. Some heat is normal. If you are hot, there will be problems. Observe whether the fault lamp is on and conduct a road test with a certain mileage.
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