Signal processing circuit design for automotive sensors

The peripheral circuits of the wheel speed sensor hardware structure with 80C31 microcontroller as the core include signal processing circuits, bus control and bus interface circuits.

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 to measure the period of the pulse signal. SJA1000 and 82C250 form the control and interface circuit with the CAN bus. During the design process of the wheel speed sensor, its anti-interference and stability were fully considered. The input/output terminals of the microcontroller were photoelectrically isolated, and a watchdog timer (MAX813) was used for timeout reset to ensure reliable operation of the system.

Signal processing circuit design

According to the signal characteristics of the wheel speed sensor, the processing circuit consists of a limiting circuit, a filter circuit and a comparison shaping circuit, as shown in Figure 4.

The limiting circuit limits the amplitude of the positive half cycle of the wheel speed sensor output signal Vi to 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 cutoff frequency is 2075Hz ( Designed based on the maximum vehicle speed of 200km/h, the frequency corresponding to the sensor output signal), select Q=0.707. Set a certain comparison voltage in the comparison shaping circuit, and compare it with the filter output signal to output a square wave signal. The amplitude of the square wave output by LM311N is 10V. After being divided by R5 and R6, a square wave signal with an amplitude of 5V is obtained and sent to the optoelectronic isolator.