Signal processing circuit diagram of automotive sensor

Filter 18.432M

The wheel speed sensor's peripheral circuit is built around the 80C31 microcontroller, incorporating signal processing, bus control, and a bus interface. Once the sensor generates a signal, it undergoes filtering, shaping, and optical isolation before being sent to the /INT0 pin of the 80C31. Timer T1 is used to periodically measure the pulse signal, ensuring accurate timing and data acquisition. The SJA1000 and 82C250 work together to form a control and interface circuit for the CAN bus, enabling efficient communication within the system. Throughout the design process, special attention was given to anti-jamming and stability. The MCU's input and output terminals are optically isolated, and a watchdog timer (MAX813) is implemented to handle time-out resets, ensuring the system operates reliably even under adverse conditions. The signal processing circuit is designed based on the characteristics of the wheel speed sensor's output. It includes 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’s output voltage (Vi) is limited to 5V or less, while the negative half-cycle is clamped to -0.6V. The filter circuit is an active low-pass filter with feedback, set to a cutoff frequency of 2075Hz—determined by the maximum speed of 200km/h. A Q value of 0.707 is selected for optimal performance. The comparison-shaping circuit sets a reference voltage to compare with the filtered signal, generating a square wave output. The LM311N produces a square wave with a peak-to-peak voltage of 10V. This signal is then divided by R5 and R6 to reduce the amplitude to 5V, which is subsequently sent to the optical isolator for safe transmission. This comprehensive design ensures that the wheel speed sensor operates efficiently, accurately, and reliably in real-world environments.