Suppose I want to find the FRF- Frequency Force Function of a cantilever beam. I am using 10g accelerometer which gives me the acceleration values of the beam after impact. Now I want to get the force function.
Do I need to have an impact hammer to measure my force of impact or can I get the force from the accelerometer it self.
My accelerometer has 3 channels x,y,z.
Yes, you need to independently measure the force to be able to construct the FRF
It might be possible to try the following (open to corrections)
1. Obtain the acceleration vs time from computed response
of a model of the beam ..(hopefully a linear response..).else number
of outputs ..
2. Use FFT to obtain frequency response of above viz 1. and plot Force
vs Freq .
3. Compare with measured result...
4. For improved results perhaps Kalman filter applied to m/s error and
statistical result for perturbed input around estimated operating point may be
useful..
Cheers
When the system you are dealing with is known, the FRF can be directly calculated from mass, stiffness and damping matrices, according to the classical formulation of the dynamic problem. Whereas if the system is unknown, the FRF can be computed from vibration measurements, but this implies not only the measurement of the response, but also the measurement of the excitation force. I hope it to be useful.
In case modeling is not possible ..it is still interesting to figure a method
of finding the force function from the response measured by the accelerometer.
It might be possible to attach a known simple system to the end of the
cantilever (perhaps a spring-mass system ) and with responses from both systems
it is probable that the force function can be worked out..
Cheers
One application of a vibrator motor is to generate a sinusoidally varying
force ( a vibrator motor has symmetrical eccentric weights attached to the shaft )
which generates a good sinusoidal Force with known amplitude.Frequency can be varied with an inverter. So with known applied force and measured output
response .. we obtain the frequency response of the system as we vary
the frequency. This should work whether the system is known or unknown.
Cheers
Since you are using only a 3-channel accelerometer, I suppose the cantilever beam is not that big. In this case, making use of an impact hammer is the best and simplest solution. Instead, if you need to generate a force in a certain frequency-bandwidth in order to capture only determined modes of your system, then you can a vibrator motor.
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