introduction
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At present, network technology is a new technology in the development of automotive electronics. It is not only a technology to solve the problem of line complexity and wire harness increase in automotive electronics, but also its communication and resource sharing capabilities become a basis for new electronic and computer technology applications in the car, and is the support of the information and control system on the vehicle.
Automotive electronic networks can be divided into control-oriented networks (CON) and information-oriented networks (ION). According to the speed of network information transmission, the Society of Automotive Engineers (SAE) divides the network into three categories: A, B, and C. Class A is a low-speed network, the baud rate is below 9600bps, and the baud rate is below the 125kbps medium-speed network class B, and above 125kbps is the high-speed network class C. The wheel speed (ie the linear speed of the wheel rotating around the axle) sensor (referred to as the wheel speed sensor) signal can be shared by the engine control module, the anti-lock braking system (ABS) control module and the instrument control module, so that the vehicle is in the process of braking The anti-lock brake control module and the engine control module are jointly controlled to achieve the best braking performance. Although the ABS system has been widely used in developed countries, the method of wheel speed signal processing is protected by special circuits and chips in the form of hardware and software as part of the electronic controller (ECU) of the ABS system. In the domestic processing of the wheel speed signal, there is a problem that the threshold value of the wheel speed recognition is too high (the vehicle speed cannot be correctly measured when the vehicle speed is lower than 10 km/h).
The author uses the developed drum speed sensor test bench to test. According to the signal characteristics generated by the wheel speed sensor, the signal processing circuit of the automobile wheel speed sensor based on CAN bus is designed, and the signal is collected and quantified by the single chip microcomputer. The results show that the designed wheel speed sensor system has the advantages of low wheel speed measurement threshold (vehicle speed up to 3km/h), reliable operation and strong anti-interference ability. At the same time, it can be used as the measuring point of CAN bus LAN to realize the digitization of sensor signals. , networked transmission.
Wheel speed sensor
Since the magnetoelectric sensor works stably and reliably, it is hardly affected by environmental factors such as temperature and dust. Therefore, the variable speed reluctance electromagnetic sensor is widely used in the wheel speed sensor currently used in automobiles. The variable reluctance wheel speed sensor consists of a stator and a rotor. The stator includes an induction coil and a magnetic head (a magnetic level formed by a permanent magnet). The rotor can be in the form of a ring gear or a gear. The magnetic head is fixed on the magnetic pole bracket, the bracket is fixed on the long shaft, the ring gear is integrally connected through the hub and the brake hub, and the long shaft passes through the wheel and cooperates with the inner bearing.
The rotational speed of the rotor is proportional to the angular velocity of the wheel. The drum drives the wheel to rotate, and the tooth tip and the gap between the teeth of the sensor rotor alternately approach and leave the magnetic pole, so that the magnetic field in the stator induction coil changes periodically, and an alternating sine wave signal is induced in the coil. The test bench controls the wheel to operate in various operating conditions and measures the sensor output signal. The experimental results show that the signal generated by the variable reluctance wheel speed sensor has the following characteristics:
(1) The signal generated by the sensor is a sinusoidal signal close to zero mean;
(2) The amplitude of the sine wave signal is affected by the air gap interval (the air gap between the head and the ring gear, generally about 1.0 mm is optimal) and the wheel speed. The smaller the air gap interval, the higher the wheel speed, and the larger the amplitude of the sine wave signal;
(3) The frequency of the sine wave signal is affected by the number of teeth of the ring gear and the wheel speed. It is the number of teeth passing through the head coil per second, which is equal to the number of teeth of the ring gear multiplied by the wheel speed per second.
The test simulates the front wheel of the BJ212 model, which simulates the vehicle speed with the drum speed. When the rotational speed of the control drum is 3km/h, the amplitude of the sine wave signal generated by the 88-tooth sensor is about 1V, and its frequency is 31Hz; when the rotational speed of the control drum is 100km/h, the amplitude of the sine wave signal generated by the sensor The value is approximately 7V and its frequency is 1037 Hz. Due to the influence of burrs and other environmental factors generated by gear processing, the actual signal is a superposition of a certain frequency component interference signal in the above signal.
Wheel speed signal detection
Each sinusoidal signal outputted by the wheel speed sensor is shaped and shaped to generate a square wave signal, and the following circuit can be processed by the following method: (1) directly send the T0 count of the single chip microcomputer, and use T1 as a timer. Read the value of T0 in each T1 timing time, and calculate the wheel speed; (2) Convert the square wave signal to F/V first, and then convert the wheel speed by MCU A/D; (3) The wave signal is sent to the external interrupt/INT0 pin of the MCU, and it is set to the edge trigger mode. The T1 is used as the timer square wave signal for periodic measurement, and the wheel speed is calculated. The first method has a large wheel speed error measured at low speeds. Assuming that the wheel speed is constant, the value of T0 is read once every T1 timing time, and the value read during T1i and T1i+1 time sometimes differs by 1 when the position of the head and the tooth tip is read. When the wheel speed is low, The value of T0 in the T1 timing time is small, so the relative error is large, and the threshold value of the wheel speed identification is too high. The second method improves measurement accuracy at low speeds but increases the cost of hardware F/V and A/D converter chips. The third method can effectively improve the measurement accuracy at low speed without increasing hardware expenditure.
Wheel speed sensor system hardware
The hardware of the wheel speed sensor system is centered on the 80C31 single-chip microcomputer (external expansion 8kRAM and 8kEPROM). The peripheral circuits have circuits such as signal processing circuits, bus control, and bus interfaces.
After the wheel speed sensor generates a signal, it is filtered, shaped, and optically isolated, and sent to the /INT0 input pin of the 80C31. T1 is used as a timer to periodically measure the pulse signal. SJA1000, 82C250 form the control and interface circuit with CAN bus. In the design process of the wheel speed sensor, full consideration is given to its anti-jamming and stability. The input/output terminals of the MCU are optically isolated, and the watchdog timer (MAX813) is used for time-out reset to ensure reliable operation of the system.
Signal processing circuit
According to the signal characteristics of the wheel speed sensor, the processing circuit is composed of a limiter circuit, a filter circuit and a comparison shaping circuit.
The limiter circuit limits the amplitude of the positive half cycle of the wheel speed sensor output signal Vi to 5V or less, and the output of the negative half cycle to -0.6V. The filter circuit is designed as an active low-pass filter with feedback. The cut-off frequency is 2075Hz (designed at the maximum speed of 200km/h, the frequency corresponding to the sensor output signal), and Q=0.707 is selected. A certain comparison voltage is set in the comparison shaping circuit, and the square wave signal is output compared with the filter output signal. The amplitude of the LM311N output square wave is 10V. After R5 and R6 are divided, the square wave signal with the amplitude of 5V is sent to the optical isolator.
Bus communication circuit
The bus interface circuit includes a sensor and CAN bus interface and a dashboard node and a CAN bus interface. The transmission of data, control commands and status information between the sensor and the node is implemented by the bus interface circuit. The use of a bus interface facilitates the formation of a vehicle network topology of a bus network. It has the characteristics of simple structure, low cost and high reliability.
The interface between the sensor and the CAN bus is based on the CAN controller SJA1000, and the interface between the sensor and the physical bus is realized by the 82C250. All functions of the CAN bus physical layer and the data link layer are performed by the communication controller SJA1000. It has two working modes, BasicCAN (82C200 compatible mode) and PeliCAN (extended feature). It adopts multi-master structure and has interfaces with various types of microprocessors.
The pin function and electrical characteristics of the SJA1000 are fully compatible with the 82C200, which has stronger error diagnosis and processing functions than the 82C200. It has a programmed clock output, a programmable transfer rate (up to 1Mbps), a programmable output driver configuration, a configurable bus interface, and bus access priority with identification code information. The controller is easy to use, inexpensive, and has a working temperature range of -40 to 125 ° C. It is especially suitable for use in automotive and industrial environments.
The 82C250 acts as an interface between the CAN bus controller and the physical bus and is designed for high-speed transmission of information (up to 1 Mbps). It provides differential transmit capability to the CAN controller and differential transmit capability to the bus, fully compatible with the ISO11898 standard. In the sports environment, it has anti-transient, anti-radio and anti-electromagnetic interference performance, and the internal current limiting circuit has the function of protecting the transmission output stage when the circuit is short-circuited. The chip is characterized by the input level of the Rs (8) pin, which can work in three modes: (1) high speed mode (Vrs < 0.3Vcc); (2) slope mode (0.4Vcc 0.75Vcc) . When the chip is operating at high speed, the transmit and output transistors are simply turned on and off as quickly as possible. The slopes that limit the rise and fall are not measured. Shielded cables are used to avoid RF interference. When the chip is operating in a slope mode, the bus can be unshielded twisted pair or parallel. The limit on the slope of the rise and fall depends on the value of the Rs pin-to-ground connection resistance and is proportional to the current at the Rs pin.
The signal levels of the SJA1000 and 82C250 are TTL compatible and can be directly interfaced. However, in order to improve reliability and anti-jamming performance, in the design of smart sensors, they are optically isolated. The RD, WR, ALE, and INT of the SJA1000 are connected to the RD, WR, ALE, and INT0 pins of the 80C31, respectively. The P0.0 to P0.7 of the 80C31 is connected to the AD0 to AD7 of the SJA1000, and the 80C31 and SJA1000 are powered by a unified 5V power supply. Approximately 0.5Vcc of holding potential is provided to the RX1 pin of the SJA1000. The CANC of the 82C250 and the CANL are connected to the 120Ω matching resistor and then connected to the physical bus. The Rs pin is grounded and the high speed mode is selected. The transmission medium uses shielded wires to improve the anti-jamming capability of the bus interface.
test results
First test the signal processing circuit. The sine wave generated by the XD5-1 signal generator is used to simulate the sensor signal input circuit, and the input and output waveforms are observed by the dual trace oscilloscope. When the input signal is above 0.6V, the circuit outputs a square wave and no signal is lost. The frequency is from 20 to 2075 Hz. Similarly, there is no signal loss in the test. When the signal is less than 0.6V, there is no square wave output, that is, noise below 0.6V cannot enter the microcomputer system. The threshold of the minimum signal can be changed by adjusting the resistance of R2 and R3 in the circuit. The sensor signal was tested on a drum sensor test rig.
The radius of the front wheel of the BJ212 model is 0.375m, and the ring gear of the magnetic induction sensor is 88 teeth. The difference between the measured value of the speed measurement system and the reading value of the speedometer is due to the error of the speedometer. The speed is from 3 to 200km/h, and the corresponding frequency is from 31 to 2075Hz. The designed speed measurement system completely covers this speed range. When using a non-contact infrared speedometer, the error is within 0.3%, which proves the rationality of the sensor and signal processing circuit. Information transmission test with the dashboard node: The sensor speed measurement system is consistent with the receiving and transmitting signals of the instrument panel node; the data format of the transmitted and received signals is consistent with the set 11-bit data format.
in conclusion
The wheel speed sensor based on CAN bus fully utilizes the potential of magnetic induction sensor, and has the advantages of low threshold value (3km/h) for vehicle speed recognition, high measurement accuracy, practicability and anti-interference, and reliable operation. It is used in the sports environment and is easy to form a network with other measurement and control nodes to realize networked transmission of sensor data.
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