The peripheral circuit of the wheel speed sensor, built around the 80C31 microcontroller, includes a signal processing unit, bus control logic, and a bus interface. Once the wheel speed sensor generates a signal, it undergoes filtering, shaping, and optical isolation before being sent to the /INT0 input pin of the 80C31. Timer T1 is used to periodically measure the pulse signal, ensuring accurate timing for data acquisition.
To interface with the CAN bus, the SJA1000 and 82C250 are used to form the control and communication interface. Throughout the design process, great emphasis was placed on anti-jamming and system stability. The MCU's input and output terminals are optically isolated, and a watchdog timer (MAX813) is employed to perform time-out resets, guaranteeing reliable system operation even under unexpected conditions.
Signal Processing Circuit Design
Based on the characteristics of the wheel speed sensor signal, the processing circuit consists of three main components: a limiting circuit, a filtering circuit, and a comparison-shaping circuit, as illustrated in Figure 4.
The limiting circuit ensures that the positive half-cycle of the sensor output voltage Vi is limited to 5V or less, while the negative half-cycle is clamped to -0.6V. The filter circuit is designed as an active low-pass filter with feedback, featuring a cutoff frequency of 2075Hz—calculated based on the maximum speed of 200km/h. A quality factor (Q) of 0.707 is selected for optimal performance.
The comparison-shaping circuit sets a reference voltage, allowing it to compare the filtered signal and generate a square wave output. The LM311N provides a 10V amplitude square wave, which is then divided by resistors R5 and R6 to reduce the amplitude to 5V. This 5V square wave is then fed into the optical isolator, ensuring electrical isolation between the sensor and the microcontroller, enhancing overall system safety and reliability.
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