Application design of automobile power carrier bus system

At present, there are many existing automotive bus standards, among which the CAN bus and J1850 are widely used. These buses all use special data harnesses, and the requirements of the ECU units of the car for data transmission are inconsistent, so it is necessary to arrange several different data networks in the car at the same time. In addition, the implementation of these data buses requires the arrangement of special data communication harnesses, which increases the harnesses, manufacturing costs and maintenance difficulties in the car, and brings unstable factors to the data transmission in the car. This article introduces a new automotive bus data transmission method - automotive power carrier bus data communication technology, which can realize data transmission and sharing between various ECU modules in the car without increasing the wiring harness in the car.

1 Analysis of System Bus Communication Channel Model

Like the low-voltage grid carrier communication system, the load of the automotive power line carrier system is also complex and time-varying. Various types of electrical appliances are sometimes connected and sometimes disconnected, which makes the impedance characteristics of the wires have great volatility. The transfer function of the system changes at any time with the change of load, and it is a time-varying system. The time-varying communication channel model shown in Figure 1 can be used to describe the automotive power line carrier bus system. In the figure, except for the noise interference represented as an additive random interference process, the other parts of the system are represented by the corresponding frequency response function. The transfer function and noise model in the communication system can be obtained by measurement or theoretical analysis. This system model comprehensively summarizes the important characteristics that must be considered when designing a communication system.

Figure 1. Time-varying communication channel model of automotive PLC system

To further study the characteristics of the automotive power line carrier system, it is necessary to determine the parameters of the transmission characteristics in the channel model. This paper uses the vector network analyzer 8712ES produced by Aglient, USA, and its structural block diagram is shown in Figure 2. For an automotive system, it is actually impossible to fully and accurately describe the parameters of the vehicle network. Here, the experimental method is used to study its channel model. The experimental method regards the system as a black box. It is not necessary to know the structure inside the box accurately. As long as the transfer function describing the signal transmission characteristics of this black box is obtained through experiments, the system can be described.

Figure 2 Network analyzer structure diagram

For the automotive power line carrier communication channel, its frequency response is a slowly changing random process. This random process can be regarded as the output of a white noise with a variance of σ2 passing through a causal stable filter. By correctly selecting the coefficients of this filter, this random process can be expressed with finite parameters. The obtained data is sent to the computer for processing and the variance change begins to slow down. The system frequency response can be determined, which is expressed as 3 coefficients and 1 white noise variance:

Click here to view the image in a new window. According to the statistical characteristics of the model coefficients, the frequency response of the communication channel should be the output of the white noise random process after passing through the filter composed of the AR model coefficients. The frequency response of the channel can be generated by programming a computer. Here, it is assumed that the coefficients are independent Gaussian random variables. The simulation results are shown in Figure 3.

Figure 3 Simulation results of automotive power line carrier communication channel response

Based on a large number of experimental measurements, this paper studies the transmission characteristics of the automotive power line carrier communication channel in the 500 kHz to 10 MHz frequency band, and establishes a third-order autoregressive model of the channel amplitude-frequency characteristics using a random signal processing method. The following conclusions are obtained:

① The automotive power line carrier communication channel does not have the multi-aperture transmission problem that is usually encountered in low-voltage power line carrier communication channels.

② The automotive power line carrier communication channel is time-varying. In the frequency domain, this time-varying property only occurs in the frequency range below 5 MHz. When the frequency is higher than 5 MHz, the time-varying property is not obvious.

2 System Design

Figure 4 Schematic diagram of the network topology structure of automotive carrier communication

According to the analysis of the system bus communication channel model, the automotive power carrier bus adopts the automotive carrier communication standard and protocol; at the same time, combined with the actual data transmission rate requirements of various electrical appliances in the car, an automotive harness carrier communication network with different data rates is established. The high-speed carrier communication network connects the modules in the car that require high data transmission rates, while the electrical modules that do not require high rates use low-speed communication networks. In this way, all electrical appliances in the car can be connected together through a few power harnesses to form several subsystems. These subsystems share information through network connectors (gateways) to achieve coordinated actions of various electrical modules in the car and realize intelligent control of the car. Figure 4 is a schematic diagram of the network topology structure of automotive carrier communication. In this topological automotive carrier communication system, the connection between each electrical module and the carrier communication module adopts the new automotive carrier communication bus standard.

The car carrier communication system in this design adopts a master-slave structure, and the overall network structure is distributed in a tree shape. The system includes a master control module and multiple slave control modules. From the perspective of network topology, the entire communication system is composed of a master control module, a car power harness, and a slave control module. In the system, the car power harness also serves as a communication channel. Figure 5 shows the connection between the master control module connected to the carrier communication system in the car and loads such as electric doors and windows and electric chairs. Control information is transmitted between various control modules through the car power harness.

Figure 5 Load connection principle of automotive carrier communication system

3 Implementation of system control unit

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The intelligent automobile lighting control system using carrier communication technology includes a master control module and a slave control module. Figure 6 is a block diagram of the system control unit connection. As can be seen from the figure, except for the external interface, there is not much difference between the master control module and the slave control module. They both include a CPU module, a modulation and demodulation module, and a coupling module. These are all necessary units for carrier communication. The specific implementation of these modules will be discussed in detail below.

Figure 6 System control unit connection diagram

Each control unit system uses PIC series microcontrollers, the main control unit uses PIC16F877, and the slave control unit uses PIC16F873. PIC (Peripheral Interface Controller) is a series of microcontrollers launched by Microchip Corporation of the United States.

3.1 Specific implementation of the main control unit

The following mainly introduces the specific implementation of the CPU in the system based on the specific application of the CPU module in the main control unit. Figure 7 shows the control connection circuit of the CPU module of the main control unit.

The main control module has no specific load control requirements. According to different functions, it can be divided into two parts: internal system and external system. In terms of the external system, it mainly plays the role of information exchange with the outside of the system, including the human-machine interface and the CAN bus module. Through this part, the system can receive commands sent from the outside, and can also send the status information of each unit module of the system to the external system. In terms of the internal system, the task of the main control unit is to convert the external commands into specific control content, send them to each slave control unit in the system, and receive the status information sent by each slave control unit, and manage and control the operation of the entire system.

Figure 7 Main control unit CPU module control connection circuit

The main control module realizes the external system tasks through its interface with the external system. As shown in Figure 6, the system-extended CAN bus interface is used to exchange information with other subsystems in the car to achieve information sharing among the subsystems in the car, so that they can work together to achieve intelligent control of the car. The keyboard module extended by the main control module is used to receive control commands from the operator.

The communication of information within the bus system is achieved through the carrier coupling module. According to the above analysis, the signal coupling module of the system includes two parts: sending and receiving. Separating the sending and receiving coupling outputs can avoid confusion in information transmission within the system.

In addition, the main control unit also extends a system status information display module, which displays the operating status of each slave control unit in the system through a series of LEDs. Since the main control unit has many interfaces, a CPU with 33 input/output pins is used as the main control unit control CPU.

For each slave control unit, since they do not need to communicate with modules outside the system and do not need too many external interfaces, the PIC16F873 microcontroller with fewer 21 input/output pins is selected as the slave control unit to control the CPU in order to save costs. The slave control unit includes a CPU module, a carrier communication module, and a power electronics module to control the load.

3.2 Implementation of FSK modulation and demodulation

In the carrier communication system, the signal coming out of the CPU module is a binary data sequence that has been coded and processed. The signal modulation and demodulation process used in this system is divided into two layers, namely FSK modulation and demodulation and spread spectrum modulation and demodulation.

In this system, the FSK modulation strategy is implemented by using two integrated chips: one as a modulation chip and the other as a demodulation chip. The FSK modulation of the system is implemented by using a voltage-controlled crystal oscillator (VCO) chip. This chip generates a sine wave, and the frequency of its output signal is proportional to the DC voltage applied to the chip. By changing the given voltage of the VCO chip, the frequency of its output AC signal waveform can be changed, and then this AC sine wave is used for FSK modulation. The specific circuit is shown in Figure 8. As can be seen from the figure, the system uses a signal generator chip XR2206. This chip is a multi-purpose voltage-controlled crystal oscillator, which is particularly suitable for FSK signal modulation. The chip only needs a few peripheral devices to work properly. The resistors at pins 7 and 8 of the chip and the capacitor at pin 6 together determine the signal frequency of the device at logic "1" and logic "0". The voltage divider circuit at pin 3 is used to shape the output sine signal of the chip. (The external circuit diagram at pins 3 and 6 is not drawn.)

Figure 8 FSK signal modulation circuit

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The demodulation of FSK signal is realized by a phase-locked loop integrated chip. The phase-locked loop keeps the input signal waveform frequency locked. When the frequency of the input signal waveform changes, the phase-locked loop will generate an error flag signal, prompting the phase-locked loop to change the locking frequency to re-match the frequency of the input signal. By carefully adjusting the chip circuit, the locking frequency is consistent with the intermediate frequency of the two frequencies of logic "1" and logic "0". The specific demodulation circuit is shown in Figure 9. As can be seen from the figure, the system performs FSK demodulation operation through the chip RC2211N. According to the above analysis, the chip works based on a phase-locked loop topology principle. The important external components in the circuit include the external components of pins 8 and pin 13. The parameters of these components set the intermediate frequency, attenuation coefficient and gain of the phase-locked loop. According to the design of the system, after the signal is FSK modulated, it will be sent to the frequency hopping spread spectrum modulation module to perform frequency hopping spread spectrum modulation on the signal.

Figure 9 Signal FSK demodulation circuit

3.3 Implementation of Frequency Hopping Spread Spectrum Signal Modulation and Demodulation

Combined with the electromagnetic environment in the car and the characteristics of the carrier channel of the car power harness, the system adopts the frequency hopping spread spectrum modulation method. Regardless of whether the frequency hopping spread spectrum system is slow or fast, the general input modulation signal is the modulated digital signal s(t), and its carrier generally uses the intermediate frequency band, and then enters the "frequency converter" (multiplier) of the frequency hopping system, and is "mixed" with another radio frequency that randomly changes its frequency value provided by the "frequency synthesizer" controlled by the PN code as a carrier, and then the bandpass filter outputs the transmission signal to form the transmission module of the spread spectrum modulation system. At the receiving end, a process opposite to this is carried out. Signal modulation is used to improve the performance of the basic communication system under strong interference conditions, so that the system can identify and avoid frequency bands with strong interference.

Figure 10 Frequency hopping spread spectrum signal modulation circuit

Frequency hopping spread spectrum signal modulation is implemented using an integrated chip, as shown in Figure 10. Specific process: The high-frequency spread spectrum carrier signal provided by the voltage-controlled chip MAX8038 is sent to the integrated chip MC1496, which completes the amplitude modulation operation of the carrier signal and the FSK modulation signal. The chip MC1496 is a multiplier that works in the suppressed carrier amplitude modulation mode. In the suppressed carrier amplitude modulation mode, the carrier frequency is not transmitted, so that greater transmission efficiency can be obtained. The high-frequency carrier signal generation chip MAX8038 is a voltage-controlled signal generator with a signal frequency of 10 kHz to 20 MHz. The demodulation principle of the frequency hopping spread spectrum signal is similar to the modulation process. The modulated high-frequency spread spectrum signal is sent to the MC 14% multiplier chip, and multiplied with the carrier signal of the same frequency in the previous process for amplitude demodulation operation, and the demodulated signal of the frequency hopping spread spectrum signal can be obtained.

4 Bus system communication performance test

In order to evaluate the performance of the system, the experiment tested the bit error rate of data transmission received and sent by each control port of the system at different data transmission rates.

The experimental test is carried out in the order of testing the main control unit first and then testing each slave control unit at a fixed data transmission rate. The experiment can realize the test requirements by programming the functions of the corresponding buttons. For example, if it is necessary to test the bit error rate of the signals received by each slave control unit when the main control unit sends a signal, you can directly press the pre-set button to make the main control unit in the system send data until the control button is pressed again. The data sent in the experiment is set to a cycle from 00H to FFH, so that the receiving end can know whether the data is sent correctly by comparing the received data value with the pre-set value. If the received data is not equal to the pre-set data, the error count will be increased by 1. In the experiment, the number of bytes sent each time is set to 5,000 times, so that the performance of the system can be evaluated more accurately and some accidental factors can be eliminated. The specific experimental data are listed in Table 1.

Table 1 Experimental data

Experiments show that it is feasible to use the car wiring harness for power harness carrier communication in the car. This technology can improve the intelligence level of the car on the basis of reducing the wiring harness used in the car. Power harness carrier technology has great application prospects in data transmission in the car.

References

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