We present two methods for exciting the bridge for measurements. One uses a shaker and the other an impulse-force hammer.
The shaker is placed in contact with the bridge with as little load mass from the shaker. A signal, typically white noise is input to the shaker. Both the force signal from the shaker and accelerometer signals are recorded to compute the driving-point admittance.
With the impulse-force hammer, an individual strikes the bridge with a short, single instance to excite the bridge. The impulse-force hammer outputs the force signal applied to the testing body, and the accelerometer signal measures the movement of the bridge. With both signals, similar to the shaker and accelerometer method, the driving-point admittance is computed.
There are benefits and disadvantages to both excitation methods. Since transfer function computation is statistical in nature, in that the transfer function is computed using the cross power spectral density, and since the signal used to drive the shaker is white noise, the measured signals can be recorded indefinitely. Therefore, the resulting transfer function is less susceptible to experimental noise. However, the drawback to the shaker is its hefty mass relative to the impulse-force hammer. Obtaining an accurate response at higher frequencies is more difficult with a shaker.
The benefit of the force-impulse hammer is that the mass of the hammer is not as much of an issue as the shaker in exciting the body of the instrument. Therefore, the high-frequency components of the resulting transfer function are more accurate. However, the drawback with the force-hammer is in its difficulty in reproducing the exact same measurement. It is difficult if not impossible to strike the body of the instrument with the same force by hand. Furthermore, the resulting signals are short compared to what can be obtained when a shaker is used.
