Research on Comprehensive Test Bench for Automobile Rubber Bushings

With the continuous development of science and technology, people have put forward higher and higher requirements for the ride comfort and handling stability of automobiles. The parameters of the automobile suspension system directly affect the smoothness of the vehicle, and the spring is the core of almost all the dynamic performance of the suspension system. Due to the advantages of rubber materials themselves, such as light weight, impact resistance and easy absorption of high-frequency vibration, coupled with the improvement of rubber and metal bonding technology, the metal rubber spring formed by the combination of metal and rubber, namely the rubber bushing, is widely used in the guide device and support device in the automobile suspension. Therefore, it is necessary to study the influence of the stiffness of the rubber bushing on the elastic kinematic law of the suspension. This bushing is generally composed of 1 or 2 rigid sleeves bonded and filled with rubber bodies in the middle. When in use, the outer cylinder is fixed, and the inner cylinder deforms accordingly with the movement of the wheel. Its deformation response characteristics directly affect the kinematic and dynamic performance of the suspension.

The service life of rubber elastic elements can generally reach 16 years. Due to the lack of necessary testing equipment, it is impossible to test the main parameters such as the stiffness of the elastic elements. At the same time, there is a lack of relevant test data, which results in the replacement of rubber elastic elements during the frame repair period (5 years), which greatly increases the maintenance cost, and some rubber elastic elements are mainly imported. Therefore, China Automotive Technology and Research Center has developed a comprehensive test bench for Zhejiang Wanxiang System Co., Ltd. and Hefei Axle Co., Ltd., which is suitable for the stiffness measurement and life fatigue test of various rubber elastic components (see Figure 1). The test bench is mainly composed of a platform, a hydraulic system, an electric servo system, an electronic control and measurement system, and uses Advantech industrial computer and acquisition card as the test bench control center. It can complete the static stiffness, dynamic stiffness and fatigue life test of the rubber bushing in the X, Y, and Z linear directions, as well as the static stiffness and fatigue life test in the rotation direction. It can also complete the pull-off force test of the inner tube of the rubber bushing. The control and test of the test process are automatically completed by the computer. The software is processed through a specific program. The control accuracy and measurement accuracy of the system can reach less than 1%. The human-computer interaction interface is friendly, and the test data is accurate and reliable, fully meeting the MSA requirements.

Figure 1 Comprehensive test bench for automotive rubber bushings

The overall structure of

the test bench is shown in Figure 2. The test bench is divided into two channels: hydraulic and servo. The hydraulic channel consists of an upper fixed beam, a middle moving beam and a column to form a specimen clamping frame, which is located above the workbench. The hydraulic cylinder and the servo valve are located below the workbench. The force sensor is relatively fixed to the moving beam. The displacement sensor is a hysteresis telescopic type. The sensor body is fixed to the bottom of the hydraulic cylinder, and the magnetic ring is fixed to the hydraulic piston. The magnetic signal changes with the movement of the hydraulic cylinder to measure the displacement signal. The rotation channel is a crank-connecting rod structure. The servo motor system is located below the table. The torque sensor, angle sensor and specimen fixture are located above the table. The angle sensor shaft is connected to the specimen, and the shell is connected to the torque sensor flange through a bracket to measure the relative angle of the inner sleeve and the outer sleeve when the specimen is twisted. The crank eccentric wheel is engraved with scales, each scale corresponds to an angle, and the swing angle of the specimen is adjusted by adjusting the eccentricity.

Figure 2 Overall structure of the test bench

The maximum load of the hydraulic channel is 20kN, and the stroke is ±20mm. The maximum frequency of the dynamic stiffness test is 25Hz, the amplitude is ±2mm, and the maximum frequency of the fatigue test is 5Hz. The maximum swing angle of the rotating channel is ±18°, the maximum measured torque is ±500N.m, and the maximum operating frequency is 3.6Hz. The capacity of

the hydraulic system

hydraulic pump station is 100L, the rated pressure is 21MPa, and the rated flow is 100L/min. The pump station motor adopts ABB products, Italian ATOS overflow valve and imported plunger pump, and is equipped with auxiliary equipment such as liquid level meter, thermometer, oil separator and filter. The oil temperature and oil pressure are equipped with digital display instruments, and the oil temperature has an over-temperature alarm unloading protection function. The oil pressure can be adjusted steplessly from 0 to 21MPa through a digital potentiometer. The

measurement and

control scheme adopted by the measurement and control system test bench is: using Advantech industrial computer as the main control machine, and measuring and controlling the test system through a data acquisition card. The industrial computer sends the action command to the hydraulic servo control card through the analog output card. The control card outputs the valve signal after PID adjustment based on the deviation between the feedback signal of the sensor and the command signal, and controls the hydraulic cylinder to produce the corresponding action. The industrial computer processes the displacement and load signals collected by the data acquisition card to obtain the force-displacement characteristic curve of the specimen.

When the servo motor is driven, the industrial computer controls the servo driver through the MPC-08 servo control card to make the drive motor produce corresponding rotation, and at the same time measures the torque and angle signals, and obtains the torque-angle characteristic curve after processing. [page]

1. System hardware

The system hardware composition is shown in Figure 3, which can be divided into two parts: the hydraulic measurement and control system and the servo motor measurement and control system.

Figure 3 Hardware composition of computer measurement and control system

(1) Hydraulic measurement and control system:

The hydraulic measurement and control system is mainly composed of displacement sensor, load sensor, signal amplifier ADAM3016, 818L data acquisition card, hydraulic servo control card, high-speed analog output card PCI1721, switch input and output card PCLD782/785 and servo valve.

(2) Servo motor measurement and control system:

The servo motor measurement and control system is mainly composed of angular displacement sensor, torque sensor, signal amplifier ADAM3016, 818L data acquisition card, 32-bit four-axis stepper/digital servo control card MPC-08, switch input and output card PCLD782/785 and servo controller.

2. System software

The system software mainly includes the following modules: system initialization module, sensor calibration and switch detection module, channel manual debugging module, test module, database query module and test report output module. The initialization module is mainly used to load the board hardware driver, set the board initial state, test parameter initial value, etc. The calibration module mainly completes the calibration of sensor signals and command output signals, and adopts the multi-point interpolation method to effectively solve the signal nonlinearity problem. The manual debugging module mainly adjusts the initial position or initial load of each test channel to meet the needs of different initial installation positions and loads of the test piece. The test module is the core part of the entire measurement and control system. It converts the parameters input by the user into a series of control instructions, calls the access module of the corresponding hardware, and drives the test device and the execution device to complete the corresponding test functions. After the test, the test results can be stored in the database, and the test records can be queried in the database management module. Considering the running speed, scalability and maintainability, the system software is written in VB6.0.

(1) DMA high-speed signal acquisition

Under normal circumstances, when the acquisition frequency is more than 100 times the signal frequency, the acquisition signal is not distorted (that is, the signal has at least 100 points per waveform). Since the dynamic stiffness test loading waveform frequency is relatively high (20Hz), the ordinary software trigger acquisition method cannot meet the requirements of this frequency signal acquisition. Therefore, the measurement and control system adopts DMA acquisition mode, and the maximum acquisition frequency can reach 40kHz. The data is read from the DMA buffer periodically through the Timer control, which not only meets the requirements of high-speed signal acquisition, but also ensures the consistency of the time of each sampling point.

(2) DMA high-speed waveform output

When the number of voltage points of the signal waveform driving the hydraulic servo valve is small, the hydraulic cylinder will crawl or move unsmoothly. Since the minimum timing cycle in the VB6.0 environment is about 30ms, there are only more than 30 points per second when using timing output, which is seriously distorted for signals with a frequency of more than 1Hz. Therefore, the system uses the PCI-1721 high-speed analog output card, which supports DMA high-speed output function and has a maximum output rate of 10MHz. When each waveform outputs 1000 points, it can fully output a 100Hz waveform signal, which is sufficient to meet the requirements of the test bench. The DMA output function of the card can return the number of output waveforms. This function can be used to calculate the time required for the test and the remaining time. When conducting a 200h fatigue test assessment, the time error is no more than 5s.

(3) Command correction

Since the sensor output signal has temperature drift, its zero point will also change after a certain number of uses. In addition, the fluctuation of the sensor power supply voltage and the difference in system oil pressure will affect the measurement and control accuracy of the measurement and control system. In order to improve the control accuracy of the system, we corrected the control command in the software. In the dynamic stiffness test, a preload test was first carried out according to the set waveform and frequency before the test, and the command was adjusted according to the measured test results. Then, the formal test was carried out according to the adjusted command, eliminating the influence of factors such as the measurement system, control system and the stiffness difference of different specimens on the control accuracy, greatly improving the control accuracy. In the fatigue test, the command waveform was adjusted online in real time according to the error between the sensor feedback signal and the test requirement signal, so that the control accuracy of the system was less than 0.5%.

Conclusion

The system software was developed based on VB6.0 under the WINDOWS 2000 system, with a friendly human-computer interaction interface and simple operation.

After the sensor signal was calibrated with multi-point linear calibration, the system measurement accuracy can be guaranteed to be above 0.5%. After the software control adopts instruction correction, the control accuracy is greatly improved. When the load range of the 100kN load sensor is above 3kN, the control error can be guaranteed to be within 0.5%.

The application of the test bench provides the necessary test data for the localization of rubber components. The test bench has good versatility and can meet the performance test of various rubber elastic components, and has broad market prospects.