Interface technology circuit analysis of sensors used in ESP

ESP is the abbreviation of English Electronic Stability Program, which is translated into Chinese as "electronic stability program". It is a general term for a system or program designed to improve a vehicle's handling performance while effectively preventing the car from losing control when it reaches its dynamic limits. It helps the vehicle maintain dynamic balance by analyzing the vehicle's driving status information from various sensors, and then issuing correction instructions to ABS and ASR. ESP can maintain the best stability of the vehicle under various conditions, and the effect is more obvious in the case of oversteer or understeer.

This article introduces the circuit analysis of sensors and interface technology in ESP:

Circuit principle

Steering wheel angle sensor interface

The output of the steering wheel angle sensor is a quadrature encoded pulse. Quadrature-encoded pulses consist of two pulse trains with varying frequency and a fixed phase offset of one-quarter period (90°), as shown in Figure 1. By detecting the phase relationship between the two signals, it can be judged as clockwise and counterclockwise, and the signals can be counted up/down accordingly to obtain the current cumulative count value, that is, the absolute turning angle of the steering wheel, and the rate of change of the turning angle. That is, the angular velocity can be measured through the signal frequency. In addition, the steering wheel angle sensor has a zero output signal. When the steering wheel is in the middle position, the signal outputs 0V, otherwise it outputs 5V. Through this signal, the absolute turning angle can be calibrated online.

Figure 1 Steering wheel angle sensor pulse sequence waveform

The interface circuit between C164CI and steering wheel angle sensor is shown in Figure 2. There is a built-in incremental coding orthogonal decoder on the chip. The decoder uses the two pins of timer 3 (T3IN, T3EUD) as the input of orthogonal pulses. After the relevant registers are correctly set, the data register of timer 3 The value is proportional to the steering wheel angle, so the steering angle can be easily calculated. The steering wheel angle sensor used in this article corresponds to 44 pulses per revolution. If the data register of timer 3 is set to T3, the absolute rotation angle is.

Figure 2 Steering wheel angle sensor interface circuit

By performing a differential operation, the rotation angle change rate can be obtained. The microcontroller sends the calculated parameters to the ECU through CAN.

Wheel speed sensor interface

According to the signal characteristics of the wheel speed sensor introduced in the previous part, the interface circuit is designed as shown in Figure 3.

Figure 3 Wheel speed sensor interface circuit

The circuit uses two-stage filtering and shaping to ensure that the wheel speed signal will not be lost at extremely low speeds and to avoid signal interference caused by suspension vibration. In the figure, resistor R2 introduces the first-level hysteresis comparison, and 74HC14 is used to introduce the second-level hysteresis comparison.

Yaw rate, longitudinal/lateral acceleration sensor

The installation positions of the yaw angular velocity and longitudinal/lateral acceleration sensors are basically the same, and the outputs are all analog quantities of 0V-5V. Since the signal fluctuation characteristics caused by car bumps are consistent, they are packaged in the same module. The hardware interface is shown in Figure 4, which implements hardware analog pre-filtering to suppress high-frequency noise components in the analog signals from the sensor and prevent aliasing during the sampling process.

By adjusting the parameters of each resistor-capacitor component in Figure 4, you can set the filter cutoff frequency and delay size. During the operation of the car, when driving on a better road, the delay should be as small as possible because the signal is better, but when driving on a bumpy road, it is hoped that the filtering effect will be better. However, once the frequency characteristics of the hardware filter are designed, they cannot be modified in real time, so the digital filtering link needs to be designed in software. Commonly used digital filters include Wiener filters, Kalman filters, linear predictors, adaptive filters, etc. Here we choose the first-order low-pass filter with small calculation amount and good real-time performance.

Figure 4 Yaw angular velocity, longitudinal/lateral acceleration sensor interface circuit

This article discusses the structural characteristics and signal characteristics of commonly used sensors in ESP systems, and designs the signal processing interfaces of each sensor, including hardware interface circuits and software processing solutions. An integrated module containing yaw angular velocity and longitudinal/lateral acceleration sensors is designed to transmit data to the ECU through the CAN bus, which has good anti-interference and reliability.