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Recent content by hechengli

  1. hechengli

    vibration recorders for transmission lines

    does anyone know about the currently available vibration recorder for tranmission lines? Pavica, Sefag, scolar and hilda? I heard only pavica and sefag are available now days. I would appreciate if anyone let me know their websites. I know pavica already. Thanks a lot!
  2. hechengli

    Bearing vibration signs - fundamental and harmonics frequencies

    so, what are your questions? if you want to experiment, you have to mearsure the response of the system to some kind of input, say, impuse input. then doing fft, you can find the magnitude of frequency response.
  3. hechengli

    piezoactuators and amplifier?

    Hi guys I want to buy some piezoactuators and amplifiers, does somebody happen to know which company is better in these products? thanks!
  4. hechengli

    variance of a zero mean white noise after go throught a LTI system

    I figure it out, that is pretty easy when you know how to do it. Thanks, guys!
  5. hechengli

    variance of a zero mean white noise after go throught a LTI system

    if we have a zero mean, white noise sequence x[n] with a variance sigma^2, if it is fed into a LTI system with impulse response h[n]=(0.6)^n, what is the variance of the output, how to calculate it? convolution or Z transform? thanks
  6. hechengli

    the unit after taking FFT transform

    what is the unit of the data after taking FFT transform? say, if we have a group of acceleration data, usually the acceleration is in m/s^2 or g, what is the unit after the FFT transform and take the magnitude? many people just say it is Frequency Response Function(FRF) and label Magnitude for...
  7. hechengli

    harmonic forcing under natural frequency

    thanks again! what I want to see is the transient response of the system under the harmonic force at its natural frequency. Not the steady state. When I do experiment, I found the magnitude goes from zero to steady state, so I am kind of interested in find the exact solution for a damped...
  8. hechengli

    harmonic forcing under natural frequency

    thanks for the reply! what I want is the analytical solution to the harmonic forcing at the natural frequency, find x(t)=F(t,c,m,k), which can be drawn against the time t,and the amplitude of x will go from zero to a certain value, state state. is that easy, I haven't find any book have the...
  9. hechengli

    harmonic forcing under natural frequency

    for a second order differential equation,( mass-spring-damping) system, if we have harmonic force acting on the system, we can find the solution x(t), but how about if the frequency of harmonic force is the natural frequency? there will be resonant. we know it won't go to infinity because of...
  10. hechengli

    Hi, I have to do frequency respons

    I think, frequency response analysis is, give one sinusoidal input, find the output, usually, for linear system, the frequency is same as the input, find the gain of magnitude and phase shift. I think there are many experiment equiment can do that. the find the natural frequency of the...
  11. hechengli

    how to find the derivative of the eigenfunction?

    for point moment input to the structural system, usually we can use generalized moment as the input to the system, which means we need find the derivative of the eigenfuntion. how can we find the derivative of the eigenfunction if the structure is 3 dimensional? just use diff(eigenfunction)...
  12. hechengli

    Running M-file from Simulink

    use matlab funciton block in simulink,
  13. hechengli

    how to get acceleration after Ansys harmonic analysis run

    after running the harmonic analysis in Ansys, the output is usually displacement against frequency, how can I get acceleration against frequency? Thanks
  14. hechengli

    how to model the base acceleration activation

    if I have a cantilever beam, with one side fixed and the the other side free, and there are acceleration excitation from the fixed side, the whole beam vibration with the base. could some one tell me how to model this in Ansys? the "fixed side" can't be fixed, coz there is...
  15. hechengli

    state space model of a structure

    for a state space model of a structure, say, a cantilever beam. xdot=Ax+Bu y=Cx+Du how to deal with the situation if the force u, maybe a moment, is not in the same direction of displacement x any ideas, suggestions, Thanks!
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