The air suspension mainly has a passive suspension and a controllable electronic suspension. The passive suspension suppresses and reduces the dynamic load and vibration of the car body and the wheel to a certain extent, ensuring the driving safety and the ride comfort of the vehicle. However, since the stiffness and damping coefficient of the passive suspension are generally selected empirically, it is optimal only in certain environments, and once the load, road condition, speed and other factors change, the passive suspension cannot be automatically adjusted accordingly, and it cannot be adjusted manually. . To overcome this drawback, an Electronic Air Suspension System (ECAS) was created. ECAS is currently the most advanced car suspension system. It can automatically adjust the suspension stiffness, body height, air consumption, and has the advantages of quick response, easy installation and simple operation, along with changes in road conditions, load and speed. Therefore, the controllable electronic suspension has become a hot topic in the field of automotive electronics, and it has broad prospects for development.
1 Composition and principle of ECASThe electronically controlled air suspension system consists of an electronic control unit (ECU), a height sensor, an air spring, a speed sensor, a shock absorber, and a vehicle height lifting control keyboard. The ECU detects the height of the vehicle body in real time through the height sensor, indirectly obtains the vertical acceleration of the vehicle body, and detects the speed of the vehicle through the speed sensor. The ECU maintains several indicator heights and three levels of adjustable damping values, which are consistent with spring comfort, driving safety and application specifications. The vehicle speed is automatically executed by the ECU under different driving conditions, and the height and damping value can be manually controlled by the driver. By comparing the height sensor detection result with the indicator height, if the height difference exceeds a certain tolerance range, the solenoid valve will be excited, and the actual height will be adjusted to the indicator height by charging and discharging gas. The damper damping force is a total of three gears. The damper is controlled according to the vehicle's ascending speed and acceleration, and the corresponding damping force is executed to meet the requirements of the ride comfort and the ride comfort. The structure of the electronically controlled air suspension is shown in Figure 1.
ECAS is mainly composed of 6 large functional modules, which are central processing unit, signal input module (ie sensor signal), signal output module (ie control output), operation interface module, power module, other modules (external memory, RS485 communication). , the system upgrades the expansion port).
2.1 MC9S08GB60
The microcontroller is a core component of the ECU. It handles a large number of input and output signals frequently, and it requires high precision and real-time control. This design uses the enhanced 8-bit automotive microcontroller from Freescale, Inc. - MC9S08GB60 microcontroller. The device has 64K flash and 4K E2PROM, highly integrated four serial communication ports (SCI1, SCI2, SPI, I2C), up to 8 timers (PWM), 8-channel 10-bit A/D conversion module .
2.2 Signal Sensing Input Module
The module consists mainly of 3 height sensors and 1 speed sensor. The body height sensor is equivalent to the series resistance of the inductor. The equivalent inductance corresponds to about 20mH at 0° corner, about 8mH at -45° rotation angle, and about 35mH at +45° rotation angle. The equivalent resistance is 120Ω. To this end, an LC three-point oscillating circuit is designed to detect the signal from the vehicle height sensor. A sine wave generator is designed, which is composed of TL082 components and peripheral circuits. The frequency of the sine wave changes continuously with the change of the equivalent inductance of the height sensor. Then, a square wave whose frequency changes with the inductance is outputted by the comparator, and is input to the input capture port of the MCU through triode amplification and optocoupler isolation. The MCU detects the transmitted signal from the height sensor by detecting this changing frequency. The circuit is shown in Figure 2. The detection of the speed sensor signal is also achieved by detecting the frequency. The principle is similar to the height sensing input circuit.
2.3 signal control output module
The ECU uses the PWM mode output to control the opening of the solenoid valve, and outputs a control signal according to the deviation between the current actual height and the expected adjustment height. The ECU calculates the adjustment pulse length of the solenoid valve. If the amount of height to be adjusted is large, the ECU will give a long pulse because there is no danger of overshoot. At the same time, the fast rising speed will reduce the pulse length, so that the vehicle can be accurately controlled. The height adjustment speed greatly avoids the high overshoot and oscillation adjustment. For the solenoid valve drive, the design uses the NUD3124 relay driver chip produced by ON Semiconductor. The high reverse avalanche energy capacity (350mJ) of the NUD3124 (car version) device controls most relays used in automotive applications. The control signal is isolated by optocoupler and output to the NUD3124 driver chip. The NUD3124 drives the solenoid valve and adds a diode protection circuit to the output of the NUD3124.
2.4 Power Module, Operator Interface Module and Other Extended Function Modules
The ECAS system has two main voltage sources, one is a 24V voltage source and the other is a 3V voltage source. The 3V voltage source is divided into a digital voltage source and an analog voltage source. The 24V power supply is taken out by the vehicle's own power supply, then filtered by the π-type filter, and then stabilized by the Zener diode. After passing through a filter circuit, a stable 24V voltage source is finally obtained. The 3V voltage source is similar, except that an isolation resistor is required between the digital power supply and the analog power supply to prevent crosstalk.
The operation interface is mainly keyboard input and LED display. When the driver wants to manually control the damping and vehicle height, he can input its operation through the keyboard, and then the corresponding LED lights up to display its input. The keyboard input is filtered, optocoupler is isolated and IC106 filtered and protected, and finally sent to the ECU, and then the ECU output control drives the corresponding LED to illuminate. Other modules mainly include interfaces for future upgrades, as well as RS485 communication, large-capacity memory, and so on. The large-capacity memory adopts ATMAL's AT24C1024, which is connected to the MCU through PTC2/SDA and PTC3/SCL; RS485 can be connected by a typical connection, and the chip adopts max3485; other unused pins are led out through the slot for future upgrade. Use.
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