The only feasible alternative for exciting large structures is inertial excitation. This method generates a reaction excitation force within a structure by controlled acceleration of a mass. This technique allows a structure to be dynamically excited without back-up fixturing. Figure 1 shows one configuration for generating this reaction force. The mass is accelerated by a servo controlled hydraulic cylinder to generate the output force.
Inertial excitation has been used in testing laboratories by constructing make-shift systems of actuators and masses or by using available systems of counter-rotating weights. The make-shift systems are useful for one specific test, but do not offer the flexibility for general field or laboratory testing. The counter-rotating weight system also has limited flexibility because it is impossible to control the output force over varying frequencies (the output force is actually a function of the rotational speed or frequency.)
Two inertial exciters have been developed to meet the need for a general purpose testing tool. Both use a combination of accelerating mass and a servo controlled hydraulic actuator to generate the inertial force. Both are portable and are designed to be used conveniently on many types of structures.
The only real limitation to inertial excitation is that the mass of the test structure be much larger than that of the exciter heads. (Loading can seriously effect data taken on structures where the exciter head mass is significant.) Some general applications are outlined below:
- Building Excitation
The units can be used to perform noise and vibration studies on building structures. Artificial excitation can be used to simulate inputs from various types of mechanical equipment (i.e. pumps and motors, compressors, large machine tools). Data taken can dictate equipment isolation requirements and structural changes to reduce vibration levels outside the equipment areas.
- Seismic Analysis
the low frequency capability of the exciter heads can be used to simulate seismic (earthquake) inputs to structures such as bridges. The heads require a minimum of fixturing and make field testing relatively easy to accomplish. In the past it has been virtually impossible to field test such structures.
- Ship Hull Excitation
Large plate structures, such as ship hulls, can be studied under field conditions. The plate bending modes can be found and maximum stress points determined under dynamic conditions.
- Foundation Testing
Large mounting structures, such as those used for steam turbines and generators, can have dynamics that adversely affect operation of the units (i.e. cause bearing or turbine blade failure). This can occur if the mounting structure has a resonance near the operating speed of the units. On-site studies of these structures, using inertial excitation, can pinpoint problems before costly failures occur.
A Typical Seismic Analysis Application
One specific application outlining exactly how inertial excitation can be used is the seismic analysis of an electrical transmission tower. In an earthquake environment, the most critical inputs to a tower structure are in the horizontal direction. If the tower structure is designed with a resonant frequency within the frequency spectrum of the earthquake environment, large motions can occur at the line connection insulators. These large motions can cause mounting failures and downed power lines.
In order to determine if a tower design is susceptible to this type of failure a transfer function analysis between the tower base and the connection points is performed. A small inertial excitation system simulates an earthquake input at the tower base. This system proves ideal because of its portability and the ease of fixturing to the tower.
The required input characteristics are determined from typical earthquake data. * An earthquake of magnitude 6.0 or higher on the Richter scale can produce horizontal ground accelerations of up to 0.6 g in a frequency range of 0.5 to 40 Hz. This is within the capability of the inertial excitation system.