Measurement Testing

Mjr · Site Admin Joined: 13 Jul 2005 Posts: 294

Electro-dynamic shakers and vibration-test systems are necessary to comply with myriad vibration-test requirements – many of which are government-mandated. Sine, Random, and Shock testing are considered the three basic vibration test environments and are often available in the standard ratings for many shakers and vibration test systems. Vibration test systems may be rated in "peak force" for sine and shock, and in "rms force" for random. When comparing and contrasting one manufacturere’s shaker systems with other manufacturer's systems, it is very important to look at more than equivalent overall force rating. Carefully note the individual sine, random and shock rating curves to ensure the selected system will meet the overall desired performance.

Sine Ratings
Most industry standard sinusoidal vibration test specs are within the 5 - 2,000 Hz frequency range. So a particular manufacturer’s systems are rated for full performance within this range.

Further, a certain manufacturer’s systems may be operated at lower and higher frequencies but typically at reduced ratings. Some customized or highly-specialized systems may be fully rated below 5 Hz and above 2,000 Hz.

Sine performance is defined by an envelope of maximum displacement, velocity, and acceleration as a function of frequency for a given amplifier/shaker and test load weight combination. The sine performance is bounded by the system maximum displacement at low frequencies (typically two inches), maximum velocity at mid-band frequencies (typically 70-80 inches/sec) and maximum acceleration (for various product load weights) at the high frequencies.

Random Ratings
Most industry-standard random vibration test specs are within the 2,000 Hz frequency range. Random testing performance depends greatly upon the selected power spectral density (PSD) shape, frequency range, amplitude and test load weight and test load structural characteristics. There are almost an infinite number of these types of combinations.

To ensure uniformity, your chosen vibration test facility or system manufacturer should utilizes a common set of test conditions. These may include a flat power spectral density (g2/Hz) from 20 to 2,000 Hz and a heavy, non-resonant load (commonly 3 to 4x armature weight). All of the manufacturer’s systems should be able to be operated at lower and higher frequencies and with non-flat PSD but normally at reduced ratings.

Other shaker manufacturers sometimes use the less-demanding ISO 5344 PSD shape for random rating, which is flat from 100 to 2,000 Hz but rolls off sharply below 100 Hz to 20 Hz. The better random rating technique provides consistency and ensures high overall system performance.

Shock Ratings
Shock testing utilizing electro-dynamic shaker systems instead of drop testers and hydraulic shakers, offers the benefits of superb accuracy, repeatability, and convenience of common test fixtures.

The higher displacements and velocities available with newer model electrodynamic shaker systems permit even a wider range of shock pulses.

The Shock performance depends upon several different variables including pulse shape (half-sine, trapezoid, sawtooth, etc.), pulse peak amplitude, pulse duration, test load weight and load dynamics. The pulse shape, amplitude and duration parameters actually determine the velocity and displacement requirements for the specified pulse.

There are an infinite number of shock pulse shapes. However, half-sine and sawtooth shapes are very common examples. Well-engineered shaker systems are typically rated with these common pulses and are graphed in terms of peak acceleration amplitude vs. test load weight. It’s easy to determine shock system test capability from these curves. Note that higher peak amplitudes are achievable with sawtooth vs. half-sine pulses and higher amplitudes are generally achievable for narrow pulses with light loads.

Some other shaker manufacturers may offer somewhat misleading or very-limited shock-performance specs or information. Commonly, this is defined as a crude force number of 3x random force rating or 2x sine force rating. This generic statement may theoretically be valid for some very-limited shock transient pulses, but it is completely inadequate because it doesn’t take into consideration the shock pulse shape or duration. The shock rating of 3x random rating is a gross oversimplification derived from generating 3 sigma peaks (3x rms) in a random test. Also, the crude force number (3x random rating) frequently refers to a theoretical "shaker" limit only, but not the specific amplifier/shaker system combination. Some manufacturers require an optional matching transformer to achieve even moderate shock performance.

Bottom line: your shaker or vibration system’s shock rating performance curves should be based on real test data, not theoretical mathematical computations.

It’s very important to look at the shock-pulse curves (peak acceleration vs. test load weight) to judge the real capability of the vibration or shaker system. Only when the pulse type, pulse width, peak amplitude and payload weight are carefully and concurrently evaluated can the actual maximum system shock performance be fairly assessed.

Pre- and Post-Compensation Pulses are Paramont to Shock Performance!

Compensation Pulses
Shock pulse capability also relies on the pre- and post-compensation pulses. These pulses are generated by the vibration controller to utilize both the upper and lower half of the shaker stroke (from center) and to start and end the pulse at the shaker center position. The pre and post compensation pulse shape, duration and amplitude are crucial to optimize the main pulse shock capability without adding significant amplitude and frequency components. The A particular vibration controller, for example, may automatically create these special compensation pulses to maximize the amplifier/shaker shock performance. Other controllers often can’t generate optimum pre and post compensation pulses and, hence, can’t guarantee full shock capability for a specific amplifier/shaker system.