9 months ago

Essais & Simulations n°104-105

  • Text
  • Virtual
  • Imulations
  • Ssais
  • Octobre
  • Mesure
  • Novembre
  • Simulation
  • Vibration
  • Contraintes
  • Essais
Dossier : Essais virtuels


Essais virtuels shape, and are just a high degree of difference in the logarithmic coordinates, indicating that there is only a constant factor of them. After checking the test record, the gains are different in the different levels, so the high degrees of the curves are different. All the transfer functions in the 5~10Hz are slowly climbing, in the 60~70Hz near the peak, which is the characteristics of power amplifier system itself. The shaker The shaker can convert the electrical energy into the mechanical energy. It consists of the moving part, the magnetic circuit system, the support spring and so on. The working principle is: in a constant magnetic field, an alternating electromagnetic force is generated by the alternating current, which stimulates the movement component of the vibration. The electromagnetic force is proportional to the magnetic intensity, the alternating current and the effective length of the coil. The transfer function of the shaker is the ratio of the control sensor’s signal and the current signal from the power amplifier. Figure 3 shows the transfer functions of the shaker in a satellite centre tube test and a satellite test. When the objects of the vibration test are not same, the transfer functions are different. For the same object, the transfer functions of the different levels are same. The filter and the amplifier DR DR Figure 2: the transfer functions of the power amplifier. Figure 3: the transfer functions of the shaker. Usually the signals of the control sensors are weak and there are many noises in these signals. The filter and the amplifier upgrade the signals before the signals are fed into the controller. The transfer function of the filter and the amplifier is the ratio of the signal to the controller and the control sensor’s signal. Figure 4 shows the transfer functions of the filter and the amplifier in a satellite centre tube test and a satellite test. As shown from the two graphs, the transfer functions equal the constant 1. The vibration controller The closed-loop control system of the satellite vibration test is composed of the digital vibration controller, shaker and DR Figure 4: the transfer functions of the filter and amplifier. the other equipments. It adjusts the magnitude of the output voltage signal to meet the test condition. Tests generally use the control methods: an average of 4 points. The equation of the average is: (1) Where: A i (f) is the acceleration amplitude of the point i. A I (f) is the average acceleration amplitude of the 4 points. The point I is a virtual control point which isn’t existing. Firstly, the digital vibration controller calculates the average acceleration amplitude of the 4 points’ accelerations by the Eq. (1), compares the average with the reference acceleration and computes the error. Secondly, the new drive signal’s amplitude is calculated by the system transfer function, compression speed, scan rate and other parameters. Finally, the sine drive signal is generated by the discrete sine wave generating mechanism and the D/A digital-analogy conversion device. Summary According to the facility and the transfer of signal, the system is divided into 5 subsystems: the vibration control system, power amplifier, shaker, acceleration transducer, the filter and amplifier. In E SSAIS & S IMULATIONS ● OCTOBRE, NOVEMBRE, DÉCEMBRE 2010 ● PAGE 22

Essais virtuels addition to the shaker, the other four subsystems handle the electronic signals: current, voltage. The shaker converts the current to the electromagnetic force, which is a forced response analysis in the virtual vibration test system. Design the virtual vibration test system Overall scheme In the virtual vibration test system, all the analysis is carried out in the frequency domain. The subsystems associated with the electronic signals are simulated in MATLAB and the selfprogrammed software. The forced response analysis of the shaker is applied in MSC.Patran/MSC.Nastran and LMS.Virtual.Lab. DR Figure 5: the flow chart of the shaker modelling. For the power amplifier, the filter and amplifier, the control sensors, the transfer functions are between the frequency and the ratio of output and input. The table look-up method is used in MATLAB. Next, the simulation methods for the shaker and the vibration control system are focused. Simulation method of the Shaker The forced response analysis is very important in the shaker’s simulation. The finite element model (FEM) is established for the shaker by stepping up method. The shaker has been separated into several substructures. The FEM is established and the model test is made for every substructure. The FEM is modified by the experimental data. The shaker’s FEM is constructed via the modified substructures. This method includes four steps: 1) Build the sub-structure primary model; 2) Do the vibration tests of the sub-structures; 3) Analyze the relativity of the test result and the calculation result; 4) Modify the universal sub-structure model. The simulation of the shaker test has two steps in approach. The first step is to predigest the structure and build the FEA model to ensure that the numerical value of the model is right. The second step is basis the fact experiment data to modify the FEA model in order to make the FEA model accord with the dynamic characteristic of the reality structure and get the true analysis result of the model. Figure 5 is the flow chart of modelling the shaker and satellite. Figure 2 is the detailed flow chart of simulation testing of the table-control vibration. Model testing data is a criterion of the mathematic model improvement, and the FRF of the vibration test can be used to check whether the model is right or not. But the FRF error will directly infect the result of model refinement, and influence the precision of the simulation analysis. Model improvement usually includes two steps, one is the model diagnosis and the other is the model updating. About the model diagnosis, firstly we should study the modal shape relativity of calculating and testing and use the model shapes and energy distribution to diagnose the regions of leading error and the parameters that should be modified. Then we can adjust the local nodes and elements of the error area to form the new analysis model and ascertain these parameters value. We should make the number of the updated parameters fewer, or else the process is very complex and difficult. Simulation method of the vibration controller The vibration controller is the core of the virtual vibration test system. It is a signal source which can auto-adjust the output signal frequency and amplitude of vibration parameters. The controller can be divided into sine and random vibration controller. Figure 6 shows the flow chart of the sinusoidal vibration control system. Firstly, all the parameters are set include the test level, the frequency range, the compression speed and so on. Secondly, a smaller drive signal is created. According to the system transfer functions, the response is calculated, and the drive signal is modified until the response equal the start level. The process is: The initial frequency is f 0 . The start level is α T . The compression speed is α. The initial voltage of the drive signal is U 0 . The transfer function is H(f). So the response is: (2) Calculate the error between the response and the start level: (3) The next response is: (4) E SSAIS & S IMULATIONS ● OCTOBRE, NOVEMBRE, DÉCEMBRE 2010 ● PAGE 23

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