At present, network technology is a new technology in the field of automotive electronics. It is not only a technology to solve the problems of complex lines and increased wiring harnesses in automotive electronization, but also its communication and resource sharing ability has become a basis for the application of new electronic and computer technology in vehicles and the support of vehicle information and control system.
Automotive electronic network can be divided into control oriented network (CON) and information transmission oriented network (ion). According to the transmission speed of network information, SAE divides the network into three categories: A, B and C. Class A is a low-speed network, and the baud rate is below 9600bps. Then, the baud rate below 125kbps is class B of medium-speed network, and the baud rate above 125kbps is class C of high-speed network. The wheel speed (i.e. the linear speed of the wheel rotating around the axle) sensor (hereinafter referred to as the wheel speed sensor) signal can be shared by the engine control module, anti lock braking system (ABS) control module and instrument control module, so that the anti lock braking control module and engine control module can jointly control the vehicle during braking to achieve the best braking efficiency. Although the ABS system has been widely used in developed countries, the wheel speed signal processing method is made into special circuits and chips in the form of hardware and software as a part of the electronic controller (ECU) of the ABS system. The threshold value of wheel speed identification is too high (the wheel speed cannot be measured correctly when the vehicle speed is lower than 10km / h).
According to the signal characteristics generated by the wheel speed sensor, the signal processing circuit of automobile wheel speed sensor based on CAN bus is designed, and the signal is collected and quantified by single chip microcomputer. The results show that the designed wheel speed sensor system has the advantages of low wheel speed measurement threshold (speed up to 3km / h), reliable operation and strong anti-interference ability. At the same time, it can be used as the measurement point of CAN bus LAN to realize the digital and networked transmission of sensor signal.
Wheel speed sensor
Due to the stable and reliable operation of magnetoelectric sensors, which are almost unaffected by environmental factors such as temperature and dust, variable reluctance electromagnetic sensors are widely used in automobile wheel speed sensors. Variable reluctance wheel speed sensor consists of stator and rotor. The stator consists of induction coil and magnetic head (magnetic stage composed of permanent magnet). The rotor can be in the form of ring gear or gear. The rotor in the form of gear is shown in Figure 1 (a). The magnetic head is fixed on the magnetic pole support, the support is fixed on the long shaft, the gear ring is connected as a whole through the wheel hub and brake hub, and the long shaft passes through the wheel and fits with the internal bearing, as shown in Figure 1 (b).
The rotational speed of the rotor is directly proportional to the angular speed of the wheel. The drum drives the wheel to rotate, and the gap between the tooth tops and teeth of the sensor rotor alternately approaches and leaves the magnetic pole, so that the magnetic field in the stator induction coil changes periodically and the AC sine wave signal is induced in the coil. Control the test bench to make the wheels run under 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 sine wave signal is affected by air gap interval (the air gap between magnetic head and gear ring is generally about 1.0mm, which is the most ideal) and wheel speed. The smaller the air gap interval, the higher the wheel speed and the greater the amplitude of sine wave signal;
(3) The frequency of sine wave signal is affected by the number of teeth of ring gear and wheel speed. It is the number of teeth passing through the head coil per second, that is, it is equal to the number of teeth of ring gear multiplied by the wheel speed per second. The signal generated by the variable reluctance wheel speed sensor is shown in Figure 2.
The test simulates the front wheel of BJ212 model, and the speed is simulated by drum speed. When the rotating 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 speed of the control drum is 100km / h, the amplitude of the sine wave signal generated by the sensor is about 7V and its frequency is 1037hz. Due to the influence of burr and other environmental factors caused by gear machining, the actual signal is the interference signal superimposed with a certain frequency component in the above signal, as shown in Fig. 2 (b).
Detection of wheel speed signal
Each sine wave signal output by the wheel speed sensor is conditioned and shaped to produce a square wave signal. The processing of square wave signal in the subsequent circuit can be carried out in the following ways: (1) directly send it to the t0 counter of the single chip microcomputer and use T1 as the timer. Read out the recorded value of t0 in each T1 timing time, and calculate the wheel speed; (2) The square wave signal is first f / V converted, and then the wheel speed is obtained by single chip microcomputer A / D conversion; (3) The square wave signal is sent to the external interrupt / INT0 pin of the single chip microcomputer, which is set as the edge trigger mode. T1 is used as a timer to periodically measure the square wave signal, and the wheel speed is calculated. The wheel speed error measured by the first method is large at low speed. Assuming that the wheel speed remains unchanged, the recorded value of t0 is read once every T1 timing time, and the value is read in T1i and T1i + 1 time. Because the position relationship between the magnetic head and the tooth top sometimes differs by 1 during reading, when the wheel speed is low, the recorded value of t0 in T1 timing time is small, so the relative error is large, resulting in high threshold value of wheel speed identification. 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 hardware expenditure.
Wheel speed sensor system hardware
The hardware of wheel speed sensor system takes 80C31 single chip microcomputer as the core (external expansion 8kram and 8keprom). Peripheral circuits include signal processing circuit, bus control and bus interface circuit. Its structure block diagram is shown in Figure 3.
After filtering, shaping and photoelectric isolation, the signal generated by the wheel speed sensor is sent to the / INT0 input pin of 80C31. T1 is used as a timer to measure the cycle of the pulse signal. SJA1000 and 82C250 constitute the control and interface circuit with CAN bus. In the design process of wheel speed sensor, its anti-interference and stability are fully considered. The input / output terminals of single chip microcomputer are photoelectric 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 wheel speed sensor, the processing circuit is composed of amplitude limiting circuit, filter circuit and comparison shaping circuit, as shown in Figure 4.
The amplitude limiting circuit limits the amplitude of the positive half cycle of the wheel speed sensor output signal VI below 5V, and the negative half cycle makes its output -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 speed of 200km / h and 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 to output a square wave signal compared with the filter output signal. The amplitude of lm311n output square wave is 10V. After R5 and R6 partial voltage, the square wave signal with amplitude of 5V is sent to the photoelectric isolator.
Bus communication circuit
The bus interface circuit includes the interface between sensor and can bus and the interface between instrument panel node and can bus. The data, control instructions and status information between sensors and nodes are transmitted through bus interface circuit. It is easy to form the vehicle LAN topology of bus network by using bus interface. The utility model has the advantages of simple structure, low cost and high reliability.
The interface between sensor and can bus takes can controller SJA1000 as the core, and realizes the interface between sensor and physical bus through 82C250. All functions of CAN bus physical layer and data connection layer are completed by communication controller SJA1000. It has two working modes: basiccan (82c200 compatibility mode) and Pelican (extended characteristics). It adopts multi master structure and has grounding interfaces connected with various types of microprocessors.
The pin function and electrical characteristics of SJA1000 are fully compatible with 82c200, and have stronger error diagnosis and processing functions than 82c200. It has programmable clock output, programmable transmission rate (up to 1Mbps), programmable output driver configuration, configurable bus interface, and defining bus access priority with identification code information. The controller has the advantages of convenient use, low price and working environment temperature range (- 40 ~ 125 ℃), which is especially suitable for automobile and industrial environment.
As the interface between CAN bus controller and physical bus, 82C250 is designed for high-speed transmission of information (up to 1Mbps). It provides differential receiving function for CAN controller and differential sending capability for bus, which is fully compatible with iso11898 standard. In the moving environment, it has the performance of anti transient, anti RF and anti electromagnetic interference. The internal current limiting circuit has the function of protecting the transmission output stage in case of circuit short circuit. The feature of the chip is that through the design of the input level of RS (No. 8) pin, it can work in three working modes: (1) high speed mode (VRS “0.3vcc); (2) slope mode (0.4vccrs” 0.6vcc); (3) standby mode (VRS “0.75vcc) When the chip works in high-speed mode, the transmitting output transistor is simply turned on and off as soon as possible without measuring the slope limiting the rise and fall, and shielded cables are used to avoid RF interference. When the chip works in slope mode, the bus can use unshielded twisted pair or parallel lines. The limit on the rise and fall slope depends on the connection resistance value from RS pin to ground It is proportional to the current of RS pin.
The signal level of SJA1000 and 82C250 is compatible with TTL and can be directly interfaced. However, in order to improve the reliability and anti-interference performance, photoelectric isolation is used between them in the design of intelligent sensors. Rd, WR, ale and int of SJA1000 are respectively connected with RD, WR, ale and INT0 pins of 80C31. P0.0 ~ p0.7 of 80C31 is connected with AD0 ~ Ad7 of SJA1000. 80C31 and SJA1000 are powered by unified 5V power supply. Provide the rx1 pin of SJA1000 with a maintenance potential of about 0.5Vcc. Connect 120 Ω matching resistance between canh and canl of 82C250 in parallel and then connect to physical bus. RS pin is grounded. Select high-speed mode. The transmission medium adopts shielded wire to improve the anti-interference ability of bus interface.
Test the signal processing circuit first. The sine wave generated by xd5-1 signal generator is used to simulate the sensor signal input circuit, and the input and output waveform is observed by double trace oscilloscope. When the peak value of the input signal is above 0.6V, the circuit outputs square wave and no signal is lost. The frequency is from 20 to 2075hz. 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, the noise less than 0.6V cannot enter the microcomputer system. The threshold value of the minimum signal can be changed by adjusting the resistance values of R2 and R3 in the circuit. Test the sensor signal on the drum sensor test-bed. The test results are shown in Table 1.
The radius of the front wheel of BJ212 model is 0.375m, and the ring gear of magnetic induction sensor is 88 teeth. The difference between the displayed value of the speed measuring system and the reading value of the speedometer in the table is due to the error of the speedometer. The speed ranges from 3 ~ 200km / h and the corresponding frequency ranges from 31 ~ 2075hz. The designed speed measurement system fully covers this speed range. When the non-contact infrared tachometer is used to test, the error is within 0.3%, which proves the rationality of the sensor and signal processing circuit. Information transmission test with the instrument panel node: the received and transmitted signals of the sensor speed measurement system are consistent with those of the instrument panel node; The data format of the transmitted and received signals is consistent with the set 11 bit data format.
The wheel speed sensor based on CAN bus gives full play to the potential of magnetic induction sensor. It has the advantages of low threshold value of vehicle speed recognition (3km / h), high measurement accuracy, strong practicability and anti-interference, reliable operation and so on. It is suitable for use in automobile sports environment, and is easy to form a network with other measurement and control nodes to realize the networked transmission of sensor data.