introduction
At present, network technology is a new technology developed in the field of automotive electronics. It is not only a technology to solve the problem of complicated circuits and increased wiring harnesses in automotive electronics, but also its communication and resource sharing capabilities become a basis for the application of new electronic and computer technologies in vehicles, and it is the support of information and control systems in vehicles.
Automotive electronic networks can be divided into control-oriented networks (CON) and information-oriented networks (ION) according to their functions. According to the speed of network information transmission, the American Society of Automotive Engineers (SAE) divides the network into A, B, and C categories. Class A is a low-speed network with a baud rate below 9600 bps, and then baud rate below 125 kbps is a medium-speed network category B, and above 125 kbps is a high-speed network category C. The signal of the wheel speed (that is, the linear speed of the wheel rotating around the wheel axis) sensor (referred to as the wheel speed sensor) can be shared by the engine control module, the anti-lock brake system (ABS) control module, and the instrument control module, so that the vehicle is in the braking process , Anti-lock brake control module and engine control module are jointly controlled to achieve the best braking efficiency. Although the ABS system has been widely used in developed countries, the method of wheel speed signal processing is made in the form of hardware and software as part of the electronic controller (ECU) of the ABS system to make special circuits and chips for protection. In the domestic processing of wheel speed signals, there is a problem that the threshold for wheel speed identification is too high (the vehicle speed cannot be accurately measured when the vehicle speed is less than 10km / h).
The author used the developed drum wheel speed sensor test bench to test, and designed the signal processing circuit of automobile wheel speed sensor based on CAN bus according to the characteristics of the signal produced by the wheel speed sensor, and used a single chip microcomputer to collect and quantify this signal. 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 a measuring point for CAN bus LAN to realize the digitization of sensor signals , Network transmission.
Wheel speed sensor
Since the magnetoelectric sensor works stably and reliably, it is hardly affected by environmental factors such as temperature and dust, so the wheel speed sensor currently used in automobiles widely uses variable reluctance electromagnetic sensors. Variable reluctance wheel speed sensor is composed of stator and rotor. The stator includes an induction coil and a magnetic head (a magnetic level composed of permanent magnets). The rotor can be in the form of ring gear or gear. The magnetic head is fixed on the magnetic pole bracket, the bracket is fixed on the long shaft, the ring gear is connected as a whole through the wheel hub and the brake hub, and the long shaft passes through the wheel and cooperates with the internal bearing.
The speed of the rotor is proportional to the angular velocity of the wheels. The drum drives the wheels to rotate, and the tooth tip and the gap between the teeth of the sensor alternately approach and leave the magnetic poles, causing the magnetic field in the stator induction coil to change periodically, and an AC sine wave signal is induced in the coil. Control the test bench to run the wheels in various working conditions and measure the output signal of the sensor. 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 sine wave 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 magnetic head and the ring gear, generally about 1.0mm is the most ideal) and the wheel speed. The smaller the air gap interval, the higher the wheel speed and the greater 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 speed of the wheel, which is the number of teeth passing through the head coil per second, which is equal to the number of teeth of the ring gear times the wheel speed per second.
The test simulates the front wheel of the BJ212 model, using the drum speed to simulate the vehicle speed. When the control drum speed 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 control drum speed is 100km / h, the amplitude of the sine wave signal generated by the sensor The value is about 7V and its frequency is 1037Hz. Due to the influence of burrs and other environmental factors caused by gear processing, the actual signal is an interference signal with a certain frequency component superimposed on the above signal.
Wheel speed signal detection
Each sine wave signal output from the wheel speed sensor is conditioned and shaped to produce a square wave signal. The subsequent circuit can process the square wave signal in the following ways: (1) Send the T0 count directly to the microcontroller and use T1 as the timer. Read the count value of T0 within the timing time of each T1, and calculate the wheel speed; (2) Convert the square wave signal to F / V first, and then obtain the wheel speed by the A / D conversion of the single chip microcomputer; (3) The wave signal is sent to the external interrupt / INT0 pin of the single-chip microcomputer, and it is set to the edge trigger mode. T1 is used as the timer to measure the square wave signal, and the wheel speed is calculated. The first method has a large wheel speed error measured at low speed. Assuming that the wheel speed is constant, the T0 count value is read once every T1 timing time, and the value is read within T1i and T1i + 1 time. Due to the positional relationship between the head and the tooth tip during reading, sometimes the difference is 1, when the wheel speed is low, The count value of T0 in the T1 timing time is small, so the relative error is large, resulting in the threshold value of the wheel speed recognition being too high. The second method can improve the measurement accuracy at low speed, but increases the cost of hardware F / V and A / D conversion chips. The third method can effectively improve the measurement accuracy at low speed without increasing the hardware cost.
Wheel speed sensor system hardware
The hardware of the wheel speed sensor system is based on the 80C31 single-chip microcomputer (external expansion of 8kRAM and 8kEPROM). The peripheral circuit has circuits such as signal processing circuit, bus control and bus interface.
After the signal generated by the wheel speed sensor is filtered, shaped, and photoelectrically isolated, it is sent to the / INT0 input pin of 80C31. T1 is used as a timer for periodic measurement of pulse signals. SJA1000, 82C250 make up the control and interface circuit with CAN bus. In the design process of the wheel speed sensor, the anti-interference and stability are fully considered. The input / output terminals of the single-chip microcomputer are all photoelectrically isolated, and the watchdog timer (MAX813) is used for timeout reset to ensure the 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 output signal Vi of the wheel speed sensor to less than 5V, and the negative half cycle makes its output to -0.6V. The filter circuit is designed as an active low-pass filter with feedback, and its cut-off frequency is 2075Hz (designed according to the maximum vehicle speed of 200km / h, and the frequency corresponding to the sensor output signal), select Q = 0.707. 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 square wave output by the LM311N is 10V, and the square wave signal with the amplitude of 5V after R5 and R6 is divided and sent to the photoelectric isolator.
Bus communication circuit
The bus interface circuit includes a sensor and a CAN bus interface and an instrument panel node and a CAN bus interface. Through the bus interface circuit to achieve the transmission of data, control instructions and status information between sensors and nodes. It is easy to form a bus local area network topology using a bus interface. 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 through 82C250. All functions of the CAN bus physical layer and data link layer are completed by the communication controller SJA1000. It has two working modes: BasicCAN (82C200 compatible mode) and PeliCAN (extended features). It uses a multi-master structure and has interfaces to various types of microprocessors.
The pin functions and electrical characteristics of SJA1000 are fully compatible with 82C200, and have stronger error diagnosis and processing functions than 82C200. It has a programmable clock output, a programmable transmission rate (up to 1Mbps), a programmable output driver configuration, a configurable bus interface, and a bus access priority defined with identification code information. The controller is easy to use, cheap, and has a working ambient temperature range (-40 to 125 ° C), which is especially suitable for automotive and industrial environments.
82C250, as the interface between the CAN bus controller and the physical bus, is designed for high-speed transmission of information (up to 1Mbps) for automobiles. It provides the differential receiving function for the CAN controller and the differential sending capability for the bus, and is fully compatible with the ISO11898 standard. In the sports environment, it has anti-transient, anti-RF and anti-electromagnetic interference performance. The internal current limiting circuit has the function of protecting the transmission output stage when the circuit is short-circuited. The characteristic of the chip is that through the design of the input level of the Rs (No. 8) pin, it can work in three working modes: (1) high-speed mode (Vrs <0.3Vcc); (2) slope mode (0.4Vcc0.75Vcc) . When the chip works in high-speed mode, the transmit and output transistors are simply turned on and off as quickly as possible, without measuring the slopes that limit the rise and fall, and use shielded cables to avoid radio frequency interference. When the chip works in slope mode, the bus can use unshielded twisted pair or parallel lines. The limitation on the rising and falling slope depends on the connection resistance value of the Rs pin to ground and is proportional to the current of the Rs pin.
The signal levels of SJA1000 and 82C250 are compatible with TTL and can be directly interfaced. However, in order to improve reliability and anti-interference performance, in the design of smart sensors, they are separated by photoelectricity. The RD, WR, ALE, and INT of SJA1000 are connected to the RD, WR, ALE, and INT0 pins of 80C31 respectively. 80C31's P0.0 ～ P0.7 interface with SJA1000's AD0 ～ AD7, 80C31 and SJA1000 use a unified 5V power supply. Provide a sustain potential of approximately 0.5Vcc to the RX1 pin of SJA1000. The CANC of 82C250 and CANL are connected in parallel with 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 wire to improve the anti-interference ability of the bus interface.
test results
Test the signal processing circuit first. Use the sine wave generated by the XD5-1 signal generator to simulate the sensor signal input circuit, and observe the input and output waveforms with a dual trace oscilloscope. When the input signal is above 0.6V peak, the circuit outputs a square wave without signal loss. The frequency is from 20 to 2075 Hz. Similarly, no signal is lost during the test. When the signal is less than 0.6V, there is no square wave output, that is, the noise less than 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. Test the sensor signal on the drum sensor test stand.
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 displayed value of the speed measurement system in the table and the reading value of the speedometer is due to the error of the speedometer. The vehicle speed is from 3 to 200km / h and the corresponding frequency is from 31 to 2075Hz. The designed speed measurement system completely covers this vehicle speed range. When tested with a non-contact infrared speedometer, the error is within 0.3%, proving 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 sending signals of the dashboard node; the data format of the sent 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 the magnetic induction sensor. It has the advantages of low threshold for vehicle speed recognition (3km / h), high measurement accuracy, practicality and strong anti-interference, reliable operation, etc. It is suitable for automotive It is used in sports environment, and it is easy to form a network with other measurement and control nodes to realize the network transmission of sensor data.
The Transient Voltage Suppressor (TVS) is a high-performance protection device in the form of a diode. When the two poles of a Tvs Diode are subjected to a reverse transient high-energy shock, it can change the high impedance between the two poles to a low impedance at a speed of the order of 10 minus 12 powers, absorbing up to several kilowatts of surge power. The voltage between the two poles is clamped to a predetermined value, effectively protecting the precision components in the electronic circuit from various surge pulses.
Transient Voltage Suppressor
Transient Voltage Suppressor,Tvs Diode,Tvs Diode Ethernet,Tvs Diode For Automotive
Dongguan Agertech Technology Co., Ltd. , https://www.agertechcomponents.com