Design of automotive electronically controlled air suspension system based on MC9S08GB60 single chip microcomputer

Abstract: An automotive electronically controlled air suspension system with Freescale MC9S08GB60 microcontroller as the control core is designed. The hardware circuit system and specific circuit design are emphasized, and the key points of software design are introduced. Through bench tests in the laboratory, it is verified that this system effectively improves the three important indicators of suspension dynamic travel, wheel dynamic load and vehicle body vertical acceleration compared with the passive suspension system, and improves the comfort of the vehicle while achieving vehicle body height adjustment control. The circuit structure is simple, the stability is good, and it has practical application value.

0 Introduction

Air suspension mainly includes passive suspension and controllable electronic suspension. Passive suspension can suppress and reduce the dynamic load and vibration of the vehicle body and wheels to a certain extent, ensuring the driving safety and ride comfort of the vehicle. However, since the stiffness and damping coefficient of passive suspension are generally selected according to experience, they are only optimal under specific circumstances. Once the load, road conditions, speed and other factors change, the passive suspension cannot be adjusted automatically, let alone manually. In order to overcome this defect, the electronic air suspension system (ECAS) was created. ECAS is the most advanced automobile suspension system at present. It can automatically adjust the suspension stiffness and vehicle height according to the changing factors such as road conditions, load, speed, etc., reduce air consumption, and has the advantages of quick response, easy installation, and simple operation. Therefore, controllable electronic suspension has become a hot topic in the field of automotive electronics research, and it has broad development prospects.

1 Composition and Principle of ECAS

The electronically controlled air suspension system consists of an electronic control unit (ECU), height sensor, air spring, speed sensor, shock absorber, vehicle height control keyboard, etc. The ECU detects the vehicle height in real time through the height sensor, indirectly obtains the vertical acceleration of the vehicle body, and detects the vehicle speed through the speed sensor. The ECU stores several index heights and three-level adjustable damping values, and the index height is consistent with the comfort of the spring, 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 also be manually controlled by the driver. By comparing the height sensor detection result and the index height, if the height difference exceeds a certain tolerance range, the solenoid valve will be stimulated, and the actual height will be adjusted to the index height by charging and discharging air. The shock absorber damping force has three levels. The shock absorber is controlled according to the vehicle body rising speed and acceleration, and the corresponding damping force is executed to meet the requirements of vehicle driving smoothness and ride comfort. The structure of the electronically controlled air suspension is shown in Figure 1.

2 Design of ECAS system functional modules

ECAS is mainly composed of 6 functional modules, namely central processing unit, signal input module (i.e. sensor signal), signal output module (i.e. output of control quantity), operation interface module, power module, and other modules (external memory, RS485 communication, system upgrade expansion port).

2.1 MC9S08GB60

The microcontroller is the core component of the ECU. It often processes a large number of input and output signals, and it must achieve high-precision and real-time control. This design uses the enhanced 8-bit automotive microcontroller MC9S08GB60 from Freescale, USA. The microcontroller has 64K flash and 4K E2PROM, and is highly integrated with four serial communication ports (SCI1, SCI2, SPI, I2C), up to 8 timers (PWM), and 8-channel 10-bit A/D conversion modules.

2.2 Signal sensor input module

The module is mainly composed of 3 height sensors and 1 speed sensor. The equivalent inductance of the vehicle height sensor is connected to the series resistor. The equivalent inductance corresponds to about 20mH at a 0° angle, about 8mH at a -45° angle, and about 35mH at a +45° angle. The equivalent resistance is 120Ω. For this purpose, an LC three-point oscillation circuit is designed to detect the signal from the vehicle height sensor, that is, a sine wave generator is designed, which is composed of TL082 components and peripheral circuits. The frequency of the sine wave changes with the change of the equivalent inductance of the height sensor, and then a square wave with a frequency that changes with the inductance is output through the comparator. After transistor amplification and optocoupler isolation, it is input to the input capture port of the MCU. The MCU detects the 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 its frequency. The principle is similar to the height sensor input circuit.

2.3 Signal control output module

ECU uses PWM to output the control of the opening of the solenoid valve, and outputs the control signal according to the deviation between the current actual height and the expected adjustment height. ECU calculates the adjustment pulse length of the solenoid valve. If the height to be adjusted is large and there is no danger of overshoot, ECU will give a long pulse. At the same time, the fast rise speed will reduce the pulse length, so that the height adjustment speed of the vehicle can be accurately controlled, which greatly avoids the overshoot and oscillation adjustment of the height. For the drive of the solenoid valve, this design uses the NUD3124 relay driver chip produced by ON Semiconductor. The high reverse avalanche energy capacity (350mJ) of the NUD3124 (automotive version) device can control most relays used in automotive applications. The control signal is output to the NUD3124 driver chip after optical coupling isolation, and the solenoid valve is driven by NUD3124, and a diode protection circuit is added to the output end of NUD3124.

2.4 Power module, operation interface module and other extended function modules

The ECAS system mainly has two voltage sources, one is 24V voltage source and the other is 3V voltage source. The 3V voltage source is divided into digital voltage source and analog voltage source. The 24V power supply is derived from the vehicle's own power supply, then filtered by a π-type filter, stabilized by a voltage regulator, and finally passed through a filter circuit to obtain a stable 24V voltage source. The 3V voltage source is similar to this, except that an isolation resistor must be added 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 the operation through the keyboard, and then the corresponding LED lights up to display the input. The keyboard input is filtered, optically isolated, and IC106 filtered and protected, and finally sent to the ECU, and then the ECU output controls the corresponding LED to light up. Other modules mainly include interfaces for future upgrades, RS485 communication, and large-capacity storage. The large-capacity storage uses AT24C1024 from ATMAL, which is connected to the microcontroller through PTC2/SDA and PTC3/SCL; RS485 can be connected in a typical way, and the chip uses max3485; other unused pins are led out through slots for future upgrades.

3. Software Design of Automobile ECAS

The application software of the electronic control unit for air suspension (ECAS) consists of a system initialization module, a manual and automatic height adjustment module, a signal acquisition module, a keyboard response module, an output control module, etc. The main program is a loop body, which is responsible for adjusting the vehicle height and damping. The vehicle height signal is converted into a square wave signal with a certain duty cycle by the sensor, and then compared with the preset calibration height in the microprocessor, and a control signal is output. When the calibration height is about to be reached, the duty cycle of the output signal is reduced to prevent overcharging. The specific main program block diagram is shown in Figure 3.

4 Experiment and result analysis

This design conducted a two-degree-of-freedom 1/4 vehicle air suspension test. By comparing and analyzing the electronically controlled air suspension and passive air suspension under the same road excitation at a certain frequency, different suspension dynamic travels, vehicle dynamic loads and vertical accelerations were obtained to verify the feasibility of this design [4] and whether this design has achieved the purpose of improving vehicle driving smoothness and ride comfort. This provides technical reserves and test methods for the next step of converting scientific research results into automotive electronic products.

This test system uses the 8800 CNC hydraulic servo vibration test system of INSTRON Company, USA, air springs, shock absorbers, controllers designed in this paper, acceleration sensors, vehicle height sensors, speed sensors, Wavebook signal collectors, computers, etc. The test principle is shown in Figure 4. Two more sensors are added to the test system, namely acceleration sensors and pressure sensors. These two sensors are added to measure the vertical acceleration of the spring and the dynamic load of the tire. During this test, the excitation signal uses a white noise random input signal that simulates a Class B road and a speed of 50km/h. The test time is 30s, the sampling interval is 0.01s, and the working height of the air spring is 275mm. The vertical vibration acceleration of the spring mass before and after the air suspension plus the controller, the suspension dynamic travel and the dynamic load of the tire are collected respectively. The experimental results are shown in Figure 5. Through this experiment, we can see that the electronically controlled air suspension system designed in this paper has obvious improvements over the passive suspension in terms of suspension dynamic travel, vehicle dynamic load and vertical acceleration. The root mean square value of the vertical vibration acceleration of the sprung mass has decreased by 12.89%, indicating that the controller designed in this paper has effectively improved the driving smoothness of the wheels and obtained better suspension characteristics, which has practical application value!