Research on a new vehicle shock absorption detection method based on digital signal processing

frozenviolet

Research on a new vehicle shock absorption detection method based on digital signal processing [Copy link]

Abstract: A new vehicle-mounted shock absorption detection method based on digital signal processing is introduced. This method uses the acceleration signal detected by the automobile tire pressure monitoring system (TPMS) (the system automatically provides detection data for the shock absorption system while monitoring tire pressure) and extracts the vibration acceleration value using digital signal processing methods. The theoretical derivation of this method and its software simulation example are given.
　　Keywords: Automobile tire pressure monitoring system (TPMS); digital signal processing (DSP); FFT; DFT; filtering

　　This method works by using the wheel acceleration signal detected by the TPMS (Tire Pressure Monitoring System). While monitoring tire pressure, the TPMS system uses its built-in acceleration sensor to provide data for the shock absorption detection system.

1. Automobile Tire Pressure Monitoring System (TPMS)
　　With the development of integrated circuits and the widespread application of microprocessors, automotive electronic products have developed rapidly. At present, automotive electronic products can be roughly classified into the following categories: automotive engine electronic control system, automotive transmission and driving control system, automotive safety and fault diagnosis system, automotive information display system, automotive multiplex bus, automotive environmental protection electronic products, electric vehicles, etc. There are many types, and it can be said that it has formed its own independent automotive electronics industry. TPMS is the main automobile safety alarm device in the automobile safety and fault diagnosis system. Its function is to detect and display tire pressure/temperature/acceleration signals in real time, and generate an alarm signal when the pressure is abnormal. TPMS helps to improve the service life of tires and vehicle driving safety.
　　 TPMS consists of several wireless digital sensors (lower computers) installed inside the tire (vacuum tire) and a host system with wireless transceiver circuits. Its structure is shown in Figure 1. The master-slave asynchronous wireless serial communication method is adopted between the upper computer and each lower computer.
　　The lower computer is responsible for detecting the internal information of the tire, the upper computer displays the information, and generates an alarm signal when the pressure and temperature reach a dangerous value. The communication between the lower computer and the upper computer is completed by RF high-frequency signal. The lower detection device is installed on the edge of the tire hub as shown in Figure 2, the wheel edge point 1, it can detect pressure, temperature, acceleration signals; the upper computer display interface is installed on the instrument panel in the cab.

2 Design purpose and function of this method
　　Currently, most of the cars equipped with TPMS are mid- to high-end cars, and many of these cars have the function of automatically adjusting the characteristics of the shock absorber according to the road conditions. The existing road condition detection method is based on the acceleration sensor installed on the car body. When the car is driving under different road conditions, the control system adjusts the shock absorber of the car accordingly according to the vibration conditions of the car body, making driving safer and more comfortable.
　　This method works based on the wheel hub edge acceleration signal detected by TPMS, and uses digital signal processing methods to separate the vibration acceleration value of the wheel caused by road fluctuations.
　　The advantage of this method is that the wheel acceleration signal is not filtered by the suspension device, so it reacts more directly and sensitively to the road conditions. At the same time, by comparing the wheel vibration acceleration with the body vibration acceleration, the performance of the suspension device can be evaluated and the failure of the suspension system can be located in time.

3 Theoretical derivation
　　The sensor is located at the edge of the wheel hub and its motion acceleration model is as follows:
3.1 Motion model of a point on the wheel edge when driving on an ideal flat road
　　The motion model of a point on the edge of the wheel when driving on an ideal flat road is shown in Figure 3.

　　Assume that the wheel radius is R, and it rolls along the plane without slipping. The velocity of the center point C is V _c =v, the acceleration is α, and the direction is along the y-axis. Assume that the directions of the x-axis and y-axis are i and j, and take C as the base point. The total acceleration of point P is:
　　
　　The original total acceleration of point P is synthesized by the linear acceleration of the wheel itself in the forward direction, the centripetal acceleration of the wheel edge point and the gravity acceleration of the sensor itself. The reason why only the first two items are considered here is that the gravity acceleration of the sensor itself is a constant. After Fourier transform, the energy is concentrated in the region with zero frequency, and the vibration acceleration signal to be extracted is a rapidly changing quantity, and its energy cannot be concentrated in this region.
　　Discretization processing:
　　Assume that the wheel angular velocity is Ω0, the wheel simulation angular frequency is Ω=Ω ₀ /2π, and the sampling period is T, then the digital angular frequency ω ₀ =2πΩT=Ω ₀ T.
　　 Assume that v and α are constants in one sampling time.
　　After discretization of equation (2), we get:
　　
　　
　　Therefore, it can be explained that |α _P ² (n)| has only 3 pulses in its frequency domain expansion. This equation (4) is called the frequency domain expansion equation. These 3 pulses are:
　　
　　In physical terms, it is directly related to the wheel speed.
3.2 Extracting vibration acceleration values under road vibration conditions
Assuming that the car is driving on a normal road, the difficulty of the lower computer detection is that the sampling period T must change with the angular velocity: (2) When the speed is too low, T is too large, resulting in system energy waste. 　　Solution: T is divided into several levels according to the Ω ₀ value. 　　Assume a criterion for whether the sampling is correct or not: If the system frequency domain is expanded into a single or 3 pulses, the sampling is correct. 3.3 Program flowchart　　The system generates a program that can automatically call the sampling period T in a loop. The initial value of T can be set to the T value of the last correct sampling as needed, which can reduce the number of loops. If the sampling is wrong, continue to call the minimum value of T, and loop in sequence until the last value. If all are wrong, the sampling fails this time. The program flowchart is shown in Figure 4.　　
　
　　
　　
　　

4 Simulation Analysis
4.1 Description of Simulation Conditions
　　The sampling frequency is 2π times the highest frequency;
　　T value distribution:
　　when v is 0-18 km/h, T = 200 ms;
　　when v is 18-36 km/h, T = 100 ms;
　　when v is 36-72 km/h, T = 50 ms;
　　when v is 72-144 km/h, T = 25 ms.
　　Assume R = 0.3 m, and the vibration acceleration is a random noise signal with an amplitude of 10V.
4.2 Simulation results
　　(1) Simulation analysis of various v values under uniform speed. Due to space limitations, this paper only uses the minimum and maximum speed values to illustrate the problem, as shown in Figure 5.

　　It can be seen from Figure 5 that when the car is driving at a constant speed, the vibration acceleration signal extracted by this method is very effective and will not be affected by the driving speed of the car. (2) ^When
　　α=0.5 m/s2 , the simulation analysis of each v value is shown in Figure 6.

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　　As the vehicle speed increases, the effect will become a little worse, but it is acceptable for the shock absorbers.

5 Conclusion
　　 This method not only extracts the vibration signal, but also separates the wheel rotation signal. If the detection reliability can be improved, it can even replace the speed detection sensor of the automobile ABS system.
　　At present, TPMS obtains power from the wireless feeding device installed on the car fender. When the wheel rotates, the sensor needs to repeatedly approach the charging coil to accumulate enough working energy. Therefore, the measurement is intermittent. At present, it cannot replace the ABS speed detection sensor with high real-time requirements. To realize this function, the lower computer must generate electricity autonomously or find other efficient energy transmission methods. Technical solutions in this regard are currently being studied.

ylgxiao

:L Why is there no program for software design? :Q