“With the rapid development of automotive electronics, the number of Electronic control units in modern cars has gradually increased. These electronic control units can be roughly divided into power transmission control (such as engine control and transmission control), chassis control (such as automobile anti-lock brake system ABS). ) And three types of body control. Among them, the body control system is mainly used to improve the convenience of driving and the comfort of riding. The system covers a wide range, including light control, door control, seat control, climate (air conditioning) control, dashboard Display, etc.This article will introduce a design method for a distributed body control system, which can provide electrical power to car lights, wipers and chassis.
With the rapid development of automotive electronics, the number of electronic control units in modern cars has gradually increased. These electronic control units can be roughly divided into power transmission control (such as engine control and transmission control), chassis control (such as automobile anti-lock brake system ABS). ) And three types of body control. Among them, the body control system is mainly used to improve the convenience of driving and the comfort of riding. The system covers a wide range, including light control, door control, seat control, climate (air conditioning) control, dashboard display, etc. This article will introduce a design method of a distributed body control system, which can perform distributed control on nodes such as car lights, wipers and chassis part solenoid valves.
The main functions of the control system are as follows:
●Control all lights on the car.
●Control the wiper to work at low speed, high speed and intermittently.
●Control the solenoid valves connected with power take-off, all-wheel drive, inter-wheel and inter-axle differential.
●Real-time response to the actions of the cab control switch: in accordance with the SAE (Society of Automobility Engineers) standard, the real-time response time is between 10ms and 100ms.
●Light self-check function: After the car is powered on, all lights are turned on for 5ms and then turned off to check whether all lights are working properly.
●Fault diagnosis and location capability: The intelligent power switch has over-current, over-temperature, over-voltage protection and open-circuit detection functions. The MCU can determine which load is faulty by detecting the status of the state pins of each channel of the intelligent power switch.
2. System structure
The system uses a distributed structure. Because there are many control objects in the body control system and the positions are scattered, if the point-to-point centralized control method is adopted, a large number of connection cables are required between the control module and the controlled object, which will inevitably cause complicated wiring in the car, difficult manufacturing and installation, and failures. Hidden dangers. The distributed system structure can design the control module according to the position of the control object, thereby shortening the distance between the control object and the control module. The modules communicate through the LIN bus. This method requires only one wire as the communication wire, so that a total of three wires are added to the ground wire and the power wire, which simplifies the wiring and system structure, increases the flexibility of the system, and is convenient Add or subtract nodes to the ground. In addition, the distributed system structure can also reduce the quiescent current of the single board and increase the stability of the single board. Figure 1 shows the system structure diagram.
According to the functional requirements and structural characteristics of the system, the control system is divided into a main control module and three sub-modules (front module, chassis module and rear module). The main control module is located in the cab of the car. It mainly detects the state of the control switches in the cab, and implements the corresponding control strategy according to the state of these control switches, and then sends the control commands to the sub-modules, and checks the work of the electrical equipment at the same time Status, if there is a fault, an alarm will be displayed. The front module is located at the front of the car and mainly controls the electrical equipment at the front of the car, including the front lights (high beam, low beam, fog lights, left and right front turn signals), wipers, fans, heating, and emergency alarms. , The work of the horn, etc. The chassis module is located on the chassis of the car and mainly controls the operation of the solenoid valves connected to the power take-off, all-wheel drive, inter-wheel and inter-axle differential. The rear module is located at the rear of the car, and mainly controls the work of the electrical equipment at the rear of the car, including the tail lights, brake lights, and left and right rear turn signals.
The functions of the main control module and sub-modules are shown in Figure 2. Among them, the voltage adjustment unit can convert the 24V voltage in the car into a 5V voltage and supply it to 5V chips such as single-chip microcomputers and power chips.
The microprocessor control unit (MCU) in the main module is realized by Mo-torola’s MC68HC908GZ16, which is mainly used to collect the input switch status to complete the corresponding control strategy. At the same time, the control command is sent to each sub-module through the serial port, and According to the load status fed back by each sub-module, judge whether there is a fault, and if there is a fault, an alarm will be displayed. Figure 3 shows the hardware schematic diagram of the main module.
The microprocessor control unit in each sub-module adopts MC68HC908GR8, which is mainly used to complete the load driving, and collect the working status of the load and send it to the main control module.
The task of the switch state detection unit is to convert the parallel data into serial data and send it to the MCU after the 24V switch state voltage is converted into 5V, which can greatly reduce the number of pins of the MCU.
The power drive unit is composed of a power chip and a serial/parallel conversion chip. After the power chip replaces the traditional relay, many automobile loads cannot be directly driven by the MCU or low-current interface device, but the power chip can be controlled by the MCU to output a large current to drive each Kind of load. Can each sub-module select different power chips according to the load? This design selects MC33286 and MC33888 to drive resistive loads, and MC33289 to drive inductive loads. After MCU serially outputs the control command through the I/O port, it will be sent to the power switch through the serial/parallel chip.
The fault display unit uses three light-emitting diodes to indicate three sub-modules. If one of the sub-modules fails, the corresponding light-emitting diode will light up and the buzzer will alarm. At this time, the sub-module uses light-emitting diodes to indicate that the load status display driven by the sub-module will also respond, so that it can be very intuitive to see which load on the sub-module has open circuit, over-current, over-temperature, over-voltage and other faults .
3. System communication
This system adopts LIN bus communication mode. The LIN bus is based on the universal SCI/UART interface, and its cost is lower than CAN communication. LIN can be used as an auxiliary to the CAN communication network. The LIN bus is mainly used in the control of low-speed systems that do not require the performance, bandwidth and complexity of CAN, such as switch-type loads or position-based systems (including car rearview mirrors, car locks, car seats, car windows, etc.). Therefore, LIN is more conducive to the realization of a distributed control system that connects the car to the CAN network.
The characteristics of the LIN bus are as follows:
●Using low-cost single-line 12V data transmission, line drive and receiving characteristics meet the improved ISO 9141 single-line standard;
●The transmission rate can reach 20kbit/s;
●Using a single-master/multi-slave structure, no bus arbitration is required, and the master node controls the bus access;
●Based on the universal UART/SCI hardware interface, almost all microcontrollers have the necessary hardware for LIN;
●The slave node does not need a crystal or ceramic oscillator to achieve self-synchronization, thereby reducing the hardware cost of the slave node;
●The signal transmission waiting time in the worst case can be fully guaranteed, so bus access conflicts can be avoided.
The communication sequence of this system is controlled by the main control module, and the LIN data frame is defined by the data content, not the destination address of the data. This definition allows multiple nodes to receive the same information, and data can be exchanged in multiple ways. Data can be sent from the master node to one or more slave nodes, or through the slave node to the master node or other slave nodes. Therefore, the communication between slave nodes does not need to pass through the master node, and the master node can broadcast information to all nodes in the network. The data communication in the system is mainly that the main module sends control commands to the three sub-modules and the three sub-modules feedback fault status data to the main module. This system defines four data types, one is the data received by each of the three modules, the second is the data received at the same time by the front and rear modules, the third is the data received by the three modules at the same time, and the fourth is The data sent by each of the three modules independently. The communication process can refer to the system software process shown in Figure 4.
4. System anti-interference design
Since the electromagnetic interference on the car is relatively severe, the anti-interference ability of the system is required to be higher, and the anti-interference design of the system is carried out in both hardware and software. In the hardware, the optical isolation circuit is used in the communication line? The power supply also adopts DC-DC isolation. A decoupling capacitor is designed between the power supply and the ground to filter out the high frequency noise from the power supply. In the layout of the printed circuit board, the digital circuit and the power drive circuit are reasonably separated, which can minimize the signal coupling between each other. The watchdog technology is adopted in the software, thereby increasing the reliability of the system.
This design scheme can realize the basic functions of a distributed body control system, and has a simple structure and low cost. In addition, the design idea is also applicable to the control of cars and medium-sized vehicles.