1) INTRODUCTION: Vibration testing is the process of applying a controlled amount of vibration to a test specimen, usually for the purposes of establishing reliability or testing to destruction. In practice the test article is securely mounted on a shaker table or actuator, which may be operated by electro-dynamic or hydraulic force; typically hydraulic force is used at very low frequencies because of the large displacements involved, and electro-dynamic force is used where higher frequencies are involved.

An electro-dynamic shaker is a linear motor: a moving coil in a fixed magnetic field that is the same principle used in the construction of a loud-speaker. The magnetic field is generated either by permanent magnets or a DC current in a field coil, and audio power is provided by an amplifier of suitable rating, typically requiring about 10 watts of audio/ pound force generated.

Some type of signal source is necessary to drive the amplifier, and an accelerometer is needed to measure the vibration response of the test article. Accelerometers are referred to as "Integrated", meaning they have a built-in amplifier and need a current source for power, or "Charge" type, requiring an external charge-converter to make a usable signal from their output. If the test article is large, then the response may vary across its surface and multiple accelerometers may be used and the outputs averaged.

The overall response curve is usually VERY non-uniform due to the response curves of the amplifier and shaker, and mechanical resonances of the shaker, test article, and mounting fixtures. To cure this, a controller us used to servo the actual measured response to the desired response curve. Controllers may be rack-mounted analog instruments or digital computer-based products.

The signal source usually attempts to simulate the real-world environment that the test article will operate in. Two methodologies are commonly used: Swept-sine and Random testing. In the Swept-sine approach the frequency is swept back and forth with amplitudes corresponding to the desired test levels. In Random testing the frequency spectrum of a noise source is shaped to represent the environment that the article will operate in. An additional test approach is Classical shock testing where the article is subjected to one or more high level shock pulses; this is similar to a one time drop-test that might occur in shipping. In all three approaches the test level can be increased until destruction occurs, thus establishing the safety margins.

2) SHAKERS: Vibration may be accomplished with hydraulic actuators or high-level acoustics for specialized testing, but only electro-dynamic type will be discussed here. All are variations on the common audio speaker design, i.e., they are a coil moving linearly in a magnetic field; instead of a speaker cone attached to the coil, a mounting plate is attached to which test articles are mounted. There are very tiny ones for accelerometer testing, but the most popular size is around 100 force-pounds. A shaker is rated at the maximum force it can apply to the total moving mass, which is the sum of the shaker moving parts, the fixture and the test specimen; a shaker could apply a 100# force at 100 G’s to a 1# total mass or 10 G’s to a 10# mass. Remember force = mass times acceleration. A 100# system is well suited for testing circuit cards and small devices up to a few pounds, as tests are frequently specified up to 10 G’s or so test level.