Sensors and interface technology for automotive ESP

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Sensors and interface technology for automotive ESP [Copy link]

Author: Yu Liangyao, Wu Kaihui, Song Jian, State Key Laboratory of Automotive Safety and Energy Conservation, Tsinghua University

Readings: 70

Citations: 0

Published on: 2006-08-09 07:47

Source: Electronics World`

Abstract: This paper analyzes the characteristics of common sensors in the automotive electronic stability program system, designs the hardware interface and software interface of common sensors, and implements a solution for integrating sensors. This solution transmits data with the electronic control unit via the CAN bus and has the characteristics of good anti-interference performance and high reliability.
Keywords: Electronic Stability Program; Sensor; CAN bus

Introduction

ESP (Electronic Stability Program) is a landmark invention of automobile electronic control. Different research and development institutions have different names for this system. For example, Bosch called it Vehicle Dynamics Control (VDC) in the early days, and now Bosch and Mercedes-Benz call it ESP; Toyota calls it Vehicle Stability Control System (VSC), Vehicle Stability Assist System (VSA) or Electronic Stability Control System (ESC); BMW calls it Dynamic Stability Control System (DSC). Although the names are different, they all add a lateral stability controller to the traditional vehicle dynamics control system, such as ABS and TCS, to control the distribution and amplitude of lateral and longitudinal forces in order to control the dynamic motion mode of the car under any road conditions, thereby improving the dynamic performance of the car under various working conditions, such as braking, sliding, driving, etc. ESP has been mass-produced abroad, but it is still in the research stage in China. There is still a lot of work to be done to achieve industrialization.

Figure 1 shows the schematic diagram of the structure of the automotive ESP. Its electronic components mainly include the electronic control unit (ECU), steering wheel sensor, longitudinal acceleration sensor, lateral acceleration sensor, yaw rate sensor, wheel speed sensor, etc. As an important electronic control system to ensure driving safety, the normal operation of each sensor of ESP is the basis for effective control. This paper introduces the characteristics of commonly used sensors in ESP, designs the sensor hardware interface and software interface, and verifies them in real vehicle tests.

Figure 1 Schematic diagram of ESP composition

Introduction to commonly used sensors in ESP

As shown in Figures 1 and 2, the commonly used sensors in ESP are as follows.

Figure 2 ESP commonly used sensors

Steering wheel angle sensor

ESP identifies the driver's operating intention by calculating the size of the steering wheel angle and the rate of change of the angle. The steering wheel angle sensor converts the steering wheel angle into a signal that can represent the driver's desired driving direction. The steering wheel angle is generally determined based on photoelectric coding. The encoder installed on the steering column contains the encoded information such as the direction of rotation and angle. The information on this encoder is scanned by a proximity photoelectric coupler. After the ignition switch is turned on and the steering wheel angle sensor turns a certain angle, the processor can determine the current absolute steering wheel angle through a pulse sequence. The communication between the steering wheel angle sensor and the ECU is generally completed through the CAN bus.

Yaw angular velocity sensor

The yaw angular velocity sensor detects the deflection of the car along the vertical axis. The size of the deflection represents the stability of the car. If the deflection angular velocity reaches a threshold, it means that the car has a dangerous condition of skidding or tail swinging, and the ESP control is triggered. When the car deflects around the vertical axis, the vibration plane of the micro-tuning fork in the sensor changes, and the yaw angular velocity is calculated by the change in the output signal.

Longitudinal/lateral acceleration sensor

The acceleration sensors in ESP include longitudinal acceleration sensors along the direction of the car's advance and lateral acceleration sensors perpendicular to the direction of advance. The basic principle is the same, but they are installed at a 90° angle. ESP generally uses micromechanical acceleration sensors. Inside the sensor, a small piece of dense material is connected to a movable cantilever, which can reflect the size of the longitudinal/lateral acceleration of the car. Its output is about 2.5V when static. Positive acceleration corresponds to positive voltage change, and negative acceleration corresponds to negative voltage change. Every 1.0-1.4V corresponds to 1g acceleration change. The specific parameters vary depending on the sensor.

Wheel speed sensor

When detecting wheel speed signals on a car, the most commonly used sensor is an electromagnetic induction sensor. The general practice is to install the sensor on the non-rotating part of the wheel assembly (such as the steering knuckle or the axle head), opposite to the gear ring made of magnetic conductive material that rotates with the wheel. When the ring gear rotates relative to the sensor, the sensor is stimulated by the change of magnetic resistance, and the frequency of this alternating voltage is proportional to the wheel speed. The ECU uses a special signal processing circuit to convert the sensor signal into a square wave of the same frequency, and then calculates the wheel speed by measuring the frequency or period of the square wave.

In the original ESP system, the longitudinal/lateral acceleration sensor and the yaw rate sensor were implemented separately. Now, the sensor cluster mode is basically used, and these three sensors are designed as one, communicating with the ECU through the CAN bus. Figure 3 shows the sensor assembly produced by SIMENS VDO and BEI.

Figure 3 Sensor Cluster

In order to add new ESP functions and better control the stability system of the whole vehicle, such as hill hold control (HHC) and drive by wire (SbW), Bosch proposed a modular HW and SW concept and developed the third generation of highly flexible and low-cost chronic sensor cluster DRS MM3.x.

Design of ESP Common Sensor Interface

The block diagram of the design made in this paper is shown in Figure 4. In the figure, the steering wheel angle sensor signal is processed by the microcontroller and sent to the ECU through the CAN bus (B in Figure 4); the yaw rate sensor and the longitudinal/lateral sensor are designed in the same module because of their similar signal characteristics and installation positions (A in Figure 4); because ESP has high real-time requirements for the wheel speed sensor signal, it is directly sent to the ECU after signal conditioning (C in Figure 4). In Figure 4 A and B, the microprocessor is required to process the signal and transmit the data through the CAN bus. This paper uses Infineon's SAK-C164CI. The chip is designed for automotive applications and has built-in AD converter, input signal capture, and orthogonal decoder. It has fast computing speed and is very suitable for ESP sensor signal processing.

Figure 4 Sensor connection block diagram

Steering wheel angle sensor

interface The output of the steering wheel angle sensor is an orthogonal coded pulse. The orthogonal coded pulse contains two pulse sequences with a variable frequency and a fixed phase offset of one quarter cycle (90°), as shown in Figure 5. By detecting the phase relationship of the two signals, it can be determined as clockwise and counterclockwise, and the signal is added/subtracted accordingly to obtain the current count accumulation value, that is, the absolute angle of the steering wheel, and the rate of change of the angle, that is, the angular velocity, can be measured by the signal frequency. In addition, the steering wheel angle sensor has a zero position output signal. When the steering wheel is in the middle position, the signal outputs 0V, otherwise it outputs 5V. Through this signal, the absolute angle can be calibrated online.

Figure 5 Steering wheel angle sensor pulse sequence waveform

The interface circuit between C164CI and steering wheel angle sensor is shown in Figure 6. The chip has a built-in incremental encoding quadrature decoder, which uses two pins of timer 3 (T3IN, T3EUD) as the input of the quadrature pulse. After correctly setting the relevant registers, the value of the data register of timer 3 is proportional to the steering wheel angle, so the angle can be easily calculated. The steering wheel angle sensor used in this article corresponds to 44 pulses per turn. If the data register of timer 3 is T3, the absolute angle is:

(1)

Figure 6 Steering wheel angle sensor interface circuit

By performing differential operation on equation (1), the rate of change of the turning angle can be obtained. The microcontroller sends the calculated parameters to the ECU via CAN.

The wheel speed sensor interface

is designed based on the wheel speed sensor signal characteristics introduced in the previous section. The interface circuit is shown in Figure 7.

Figure 7 Wheel speed sensor interface circuit The

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

yaw rate, longitudinal/lateral acceleration sensors

and yaw rate, 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 the bumps of the car are consistent, they are encapsulated in the same module. Its hardware interface is shown in Figure 8, which implements hardware analog pre-filtering to suppress the high-frequency noise components in the analog signal from the sensor and prevent aliasing during the sampling process. The op amp uses LMX324 with full swing output.

By adjusting the parameters of each resistor and capacitor in Figure 8, the filter cutoff frequency and delay size can be set. During the operation of the car, when driving on a good road, the delay should be as small as possible due to the good signal, while when driving on a bumpy road, the filtering effect is expected to be good. However, since the frequency characteristics of hardware filtering cannot be modified in real time once they are designed, it is necessary to design a digital filtering link in the software. Common digital filters include Wiener filter, Kalman filter, linear predictor, adaptive filter, etc. Here, a first-order low-pass filter with small calculation amount and good real-time performance is selected, and the transfer function is:

Figure 8 Interface circuit of yaw rate, longitudinal/lateral acceleration sensor

(2)

Where t is the time constant and s is the Laplace operator.

The digitalized calculation formula is:
(3)
Where is the current AD sampling value, yn is the filtered value, yn-1 is the last filtered value, and k is the filter coefficient. By adjusting, the size of t can be set. The relationship is:

, where 0<k<256

. In order to facilitate shift calculation, k is rounded to 256.

The selection of k depends on the current road condition, which is identified by the original signal before digital filtering. The microcontroller packages the filtered signal, the original signal, the value of k, and the road identification result and sends them to the ECU via the CAN bus. Figures 9a and 9b are a set of comparison curves of the longitudinal acceleration sensor collected in the bumpy road test.

Figure 9a Data curve before digital filtering Figure 9b Data curve after digital filtering

Conclusion

This paper discusses the structural characteristics and signal characteristics of commonly used sensors in ESP systems, and designs the signal processing interface of each sensor, including hardware interface circuits and software processing solutions. An integrated module containing yaw rate and longitudinal/lateral acceleration sensors is designed, which transmits data with ECU through CAN bus and has good anti-interference and reliability. The design of this paper has been verified in real vehicle tests.

References:
1. Anton T. Van Zanten, Rainer Erhardt, Klaus Landesfeind and Georg Pfaff, VDC systems Development and Perspective, SAE Paper 980235
2. Anton T. Van Zanten, Robert Bosch GmbH, Evolution of Electronic Control System for Improving the Vehicle Dynamic Behavior, AVEC'02, 20024481
3. Dongshin Kim, Kwangil Kim, Woogab Lee, Development of Mando ESP, SAE TECHNICAL PAPER SERIES, (SP-1781) 2003-01-0101
4. Chen Zaifeng, Song Jian, Yu Liangyao. Wheel speed sensor signal processing for automotive anti-lock brake system. Automotive Engineering. 2000(22),4:282-285.