With the rapid development of the economy, more and more users own cars, and the number of unskilled drivers has increased greatly. The promotion of automatic transmissions for automobiles is important for improving the economy, safety, comfort and reducing exhaust emissions of automobiles. The impact of it makes driving easier and safer. At present, automatic transmissions are widely used in foreign cars. The AG4 hydraulic automatic transmission is the main transmission device used in the Volkswagen series. It uses the vehicle speed and throttle opening as the input signal of the electronic control unit (ECU). After being processed by the electronic control unit, it is output to the solenoid valve, using the solenoid valve. The hydraulic circuit is controlled, and the shift valve of each gear is operated by controlling the on/off of the oil passage, thereby completing the shift control. Its superiority is to eliminate the complicated shifting of the manual transmission and the frequent operation of the pedal clutch pedal. By stepping on the accelerator pedal, the automatic shifting can be skillfully realized, making driving simple and labor-saving. The electronic control system of the hydraulic automatic transmission makes the car automatically shift gears, and the switching speed is soft and stable, so it feels comfortable to ride and drive.
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The ECU uses the new 16-bit microcontroller MC9S12DP256 developed by Motorola for automotive electronics . Its main features: integrated 256KB flash memory on-chip, large capacity, fast read and write speed, application of phase-locked loop technology to improve the electromagnetic compatibility of the system, and background development mode (BDM) makes the user's development and design work more concise High efficiency. The background development mode includes resource access and operation control, and cooperates with the instruction listing and the endpoint logic. The ECU designed by the user can be debugged through the single-line interface BKGD, which is less invasive than the previous background debugging system, and its friendly The user debugging interface helps developers to write source programs online in real time, then compile, connect, and finally download to the target system to debug the operation and finally complete the development process.
Control system composition
The electronic control system is mainly composed of control unit, sensor and switch components.
The control unit is the core of the automatic transmission electronic control system. It measures the operating parameters such as throttle opening, vehicle speed and transmission oil temperature measured by various sensors installed on the engine and automatic transmission, and the current status of various switches. The signal is calculated, compared and analyzed, and its internally set control program is called to issue commands to the various actuators to activate the corresponding solenoid valve to achieve control of the transmission.
The spool box is bolted to the bottom of the transmission housing with seven solenoid valves N88-N94. The solenoid valve is controlled by the automatic transmission control unit and is divided into two types: on-off valve and regulating valve.
â—† On-off valve: Solenoid valves N88, N89, N90, N92, N94, its function is to control the solenoid valve to open or close an oil passage through the automatic transmission control unit, so that the transmission can be changed into a certain gear position.
â—† Regulating valve: solenoid valves N91 and N93. The solenoid valve N91 regulates the lock-up clutch pressure; the solenoid valve N93 regulates the main oil passage pressure, that is, the pressure of the multi-plate clutch and brake.
The transmission oil temperature sensor is mounted on a flat transmission line of a spool valve that is immersed in the automatic transmission oil. The transmission oil temperature sensor is a negative temperature coefficient resistor, that is, as the temperature increases, its resistance decreases. When the automatic transmission oil temperature reaches the maximum value of 150 ° C, the lock-up clutch is engaged. When the torque converter is unloaded, the automatic transmission oil temperature starts to cool. If the temperature does not drop, the automatic transmission control unit lowers the transmission.
The multi-function switch is mounted on the rear of the transmission housing and is controlled by the shift lever zipper. The function of the multi-function switch is to transmit the information of the pole position to the automatic transmission control unit; to control the switch of the backup lamp; to stop the starter from engaging in the running state, and to lock the shift lever.
The transmission speed sensor is mounted on the left side of the top of the transmission housing and accepts the speed of the large sun gear in the planetary gear mechanism. The automatic transmission control unit uses the speed of the large sun gear to accurately determine the shift timing and control the operation of the multi-plate clutch. During the shifting process, the output torque of the engine is reduced by retarding the ignition advance angle.
The vehicle speed sensor is mounted on the right side of the top of the transmission housing and receives vehicle speed information via a pulse wheel on the drive bevel gear. The main function of the vehicle speed sensor is as an input parameter for shift control and torque converter blocking control.
The shift lever lock solenoid valve is mounted on the shift lever and is connected to the ignition system to lock the pole position. When the brake pedal is depressed, the pole position lock is released and the shift lever can be pushed into the other pole position.
The downshift switch is integrated with the accelerator pedal zipper and mounted on the diaphragm of the engine compartment. When the accelerator pedal is pressed beyond the throttle full open position, the downshift switch starts to work. The function of the downshift switch is that when the switch is depressed, the transmission is immediately forced to shift into the adjacent low gear, for example, from 4 to 3; when the switch is depressed, the air conditioner will be cut off for 8 seconds to increase the output power. .
The brake light switch is mounted on the brake pedal bracket. The automatic transmission control unit determines whether the vehicle is in a braking state by the brake light switch signal. The function of the brake light switch is that when the vehicle is stationary, the shift lever can only move out of the P or N position when the brake pedal is depressed; the control unit uses the brake switch signal to lock the shift lever.
ECU hardware design
Overall hardware structure
The hydraulic automatic transmission electronic control unit is a typical multi-input and multi-output system. It knows the running state of the car through the input signals such as the throttle opening and the vehicle speed, and then judges the shifting time according to the shifting rule of the memory, and controls the solenoid valve. The output signal causes the corresponding actuators such as the clutch and the brake to operate, and realizes various control of the automatic shifting. Its hardware structure consists of the minimum system, power module, input signal channel, output signal channel and CAN bus communication module, as shown in Figure 1.
smallest system
The central processing unit is the core component of the ECU. This system uses the Motorola MC9S12DP256 chip as the central processing unit. As the world's largest supplier of automotive electronic semiconductor devices, Motorola's microcontrollers are widely used in automotive electronic control units. The MC9S12DP256 is widely used for its powerful functions and excellent performance.
The MC9S12DP256 microcontroller is a mid-range chip in a high-speed, high-performance 5.0V Flash memory product based on a 16-bit HCS12 core and 0.25μm microelectronics technology. Its high performance-price ratio makes it ideal for use in some mid- to high-end automotive electronic control systems. The MC9S12DP256 has a clock speed of up to 25MHz, and also integrates many standard modules on the chip, including two asynchronous serial communication ports SCI, three synchronous serial communication ports SPI, eight channels of input capture/output comparison timers, and two 10-bit 8 Channel A/D conversion module, 1 8-channel pulse width modulation module, 49 independent digital I/O ports (20 of which have external interrupt and wake-up functions), 5 CAN modules compatible with CAN2.0A/B protocol, and The internal IC bus module has 256KB of flash EEPROM, 12KB of RAM, and 4KB of EEPROM.
The MC9S12DP256 microcontroller has three main features:
â—† On-chip integrated 256KB flash memory
In recent years, with the maturity of the application of flash memory in the microcontroller chip, the development and application of the microcontroller has ushered in a new leap. Flash is a non-volatile storage medium that reads its contents as easily as RAM, but writes it faster than EPROM. At the same time, the content in Flash can remain unchanged after the system is powered down. The main advantages of Flash are simple structure, high integration density, and low cost. Because Flash can be erased locally, and the number of writes and erases can reach tens of thousands of times, the development of the microcontroller no longer requires expensive simulators.
â—† Application of phase-locked loop technology improves system electromagnetic compatibility
In a microcontroller application system that does not use a phase-locked loop in the past, the crystal oscillator circuit becomes a large interference source due to its relatively high operating frequency (usually several megahertz to several tens of megahertz). This problem is designed for the system. The layout of the circuit board has brought a lot of inconvenience. The MC9S12DP256 microcontroller's clock generation system subtly uses the phase-locked loop technology, so it can generate several megabytes of system clock through software programming under the external tens of kilohertz external crystal oscillator, thus reducing external radiation interference and improving The stability of the system.
â—† Simple background development mode (BDM) further reduces development costs and makes field development and system upgrades easier
Due to the powerful features and rich on-chip resources of the MC9S12DP256, there is no need to extend more interfaces. For the smallest system, it also includes crystal oscillator circuit, reset circuit and BDM debug interface. The system uses a 16MHz external crystal oscillator; the reset circuit uses a microprocessor power supply monitoring chip MAX708, which can simultaneously output an active-high and active-low reset signal. The reset signal can be input by VCC voltage, manual reset, or independent. The comparator is triggered.
Power module
A reasonable power supply module design can provide a strong guarantee for the work of the ECU. This system requires two power supply voltage levels of +12V and +5V, while the vehicle battery only provides +12V voltage, so the power module uses three-terminal voltage regulator 7805 and 7812 to achieve the system requirements, as shown in Figure 2. The battery +12V voltage is input to the ECU45 pin and the ECU23 pin respectively to input 7805 and 7812, and the 5805 generates +5V to supply the minimum system and other +5V chips, and the 7812 regulated +12V gives the solenoid valve driver chip, solenoid valve, etc. powered by.
Analog input channel
The analog input signal has a total of 2 channels, which are the throttle opening signal and the transmission oil temperature signal. For the throttle opening signal, it is sent by the engine control unit to the automatic transmission control unit via the CAN bus. Since we do not have the application layer protocol of its CAN bus, it is not available directly from the automatic transmission harness interface. Throttle opening signal. Therefore, instead of the throttle opening signal, a displacement sensor is installed under the accelerator pedal to obtain an accelerator pedal displacement signal.
Because the input impedance of the MC9S12DP256 chip A/D module is very high, the leakage current of the analog input pin is only 100nA. When the input voltage is 2.5V, it is equivalent to the input resistance of 25MΩ, and the reference voltage can be selected between 0~5V. The externally measured signal with a full-scale range below 5V can be directly measured without buffering or an amplifier.
Therefore, for the analog input channel of the system, no special design is needed, and the two analog signals are directly introduced into the PAD00 and PAD01 ports of the minimum system.
Pulse input channel
There are two channels of pulse input signals, which are the vehicle speed signal and the transmission speed signal. For the input pulse amount, the design of the input channel adopts the processing method of filtering and shaping first. The circuit principle is shown in Figure 3. The comparator LM339 has a reference level of 2.5V, which is also the supply voltage of the sensor, which is obtained by dividing the power supply module VCC. Before the input signal is input to the comparator, it must first be shunted to reduce the current entering the comparator, as shown by R4 in the figure. The filtering uses RC low-pass filters, such as C4 and R3 in the figure, to filter out high-frequency noise interference. D2 and D4 are clamp diodes that fix the potential of the LM339's input pin 5 to 2.5±0.7V. Finally, enter LM339 for comparison, output the regular square wave, enter the input capture ports PT0 and PT1 of MC9S12DP256, and complete the input of pulse quantity.
Digital input channel
The digital input signal has a total of 12 channels, which are 4 multi-function switch signals, 5 solenoid valve feedback signals, parking/neutral signals, brake indicator switch signals and low-range switch signals. For digital inputs, opto-isolators are used for signal isolation and amplitude conversion.
Output channel design
The ECU controls the corresponding solenoid valve and the like through the output signal to complete the various control of the automatic shifting. The system designs digital output channels and pulse output channels according to different types of output signals.
Digital output channel
The digital output signal has a total of 6 channels, which are the 5-way shift solenoid valve signal and the shift lever lock solenoid valve signal. For the digital output signal, when driving the solenoid valve, the Darlington tube integrated chip ULN2803 is used for power amplification, and the drive solenoid valve is output in two parallel manners.
Pulse output channel design
The pulse quantity output signal has a total of 2 channels, which are the lock control solenoid valve signal and the main oil passage pressure regulating valve signal. For these two pulse outputs, different pulse output channels are designed.
The lockout control solenoid signal output channel uses the same design as the digital output channel, except for the signal from the smallest system. The blocking control signal is sent by the PP0 port of the minimum system PWM module.
The signal output of the main oil pressure control valve is realized by means of a controllable power supply, as shown in FIG. The minimum system PWM module PP1 port sends a control signal, and the power supply with the maximum amplitude of 12V through the controllable power supply BTS621, and the power supply that changes with the control signal is added to both ends of the main oil pressure control solenoid valve.
Hardware anti-jamming design
Due to the complicated working conditions and the harsh working environment during the operation of the automobile, the electronic control unit must not only withstand the vibration and impact caused by the bad road surface, but also withstand the electromagnetic interference of the automobile itself and the outside world. Therefore, a series of measures against interference and reliability must be taken during the system design process.
Anti-power interference
The battery power supply used in the car is 12V, and the internal resistance is small. It is an ideal power supply, but the voltage will still fluctuate within a certain range during actual operation. When designing the circuit, the 5V components inside the ECU are powered by the three-terminal regulator tube 7805 of the regulated output, which can ensure the reliable operation of the ECU. The solenoid valve works also with the 7812 power supply with regulated output. Since the power of the main oil passage pressure regulating solenoid valve is large, in order to avoid affecting other shifting solenoid valves when the solenoid valve is working, the main throttle pressure regulating valve of the road is powered by a separate set of 7812, so that the system works stably. Do not interfere with each other. In addition, a decoupling capacitor is provided between the power supply of each component and the ground, which can effectively ensure the stability of the power supply and reduce the mutual coupling of the power supply.
anti-I/O channel interference
The system adopts a filter circuit in the pulse input channel, and adopts photoelectric isolation technology in the digital input channel, the digital output channel and the CAN bus communication module. The use of opto-isolators can cut off the electrical connection between the minimum system and the I/O channels and external circuits, effectively preventing interference intrusion. The interference source generally has a large voltage amplitude, but the energy is small, and only a weak current can be formed, and the light-emitting diode of the photoelectric input terminal operates under a current state, and the interference source is suppressed because it cannot supply sufficient current, thus The opto-isolator can effectively suppress the interference of spikes and various noises, thereby greatly improving the signal-to-noise ratio on the I/O channel.
Printed circuit board anti-jamming design
Electromagnetic compatibility design in printed circuit boards is especially important. Electromagnetic compatibility refers to the ability of an electronic device to function properly in a variety of electromagnetic environments. The purpose of the electromagnetic compatibility design is to enable the electronic device to suppress various external interferences, enable the electronic device to work normally in a specific electromagnetic environment, and at the same time reduce electromagnetic interference to other electronic devices. Therefore, first of all, reasonable wiring is required. When designing the wiring, try to avoid parallel lines with long distances. Pull the distance between the lines and the lines as much as possible. The signal lines and the ground and power lines should not cross as much as possible. Double panels are used, which are routed laterally on one side of the printed board, and longitudinally on the other side, and then connected at the intersection with metal through holes. Second, choose a reasonable wire width. Since the transient interference generated by the transient current on the printed line is mainly caused by the inductance of the printed conductor, the inductance of the printed conductor should be minimized. The inductance of a printed conductor is proportional to its length and inversely proportional to its width, so short and thick conductors are advantageous for suppressing interference.
Grounding design is also an important aspect of controlling interference in printed circuit board design. In the ground design, care should be taken to separate the digital ground from the simulated ground and try to bold the ground. If the grounding wire is very thin, the grounding potential changes with the current, causing the signal level of the electronic device to be unstable.
software design
The main control flow of the system software is to collect the current vehicle driving state and the driver's intention including the current pole position, gear position, vehicle speed, throttle opening, engine speed and other signals, and judge and select the reasonable shifting rules (economic, dynamic and snow). Shift law), output control amount, control shift solenoid valve and torque converter lock. The program automatically starts running after each power-on, and the ECU first collects peripheral signals. According to the position of the shift lever which represents the driver's will, the corresponding processing is performed, and the parking, reverse and neutral processing are respectively performed in the P, R and N positions. If it is in the forward gear position, the corresponding mode judgment is made. If it is snow or sports mode, it will enter the corresponding shifting rule module, otherwise it will enter the economic mode. After entering this mode, it will enter different shifting rules according to the two factors of throttle opening and vehicle speed. The shifting law is shown in Figure 6. The two-parameter control law of vehicle speed and throttle opening is adopted.
At present, automatic shifting technology has been widely used in mid- to high-end cars, and the electronic control unit (ECU) is the control core of the vehicle to achieve automatic shifting, which will directly affect the economy and power of the vehicle. The automatic shift control system described in this paper utilizes Motorola's new high-performance single-chip MC9S12DP256, which not only realizes the automatic shifting process, but also adopts advanced control algorithms to make the shifting safe, comfortable and stable. The control unit has been tested several times and is working well, providing a basis for further application to more models.
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