BY BRÜEL & KJÆR
Direct loading of a piezoelectric accelerometer's output, even by relatively high impedance loads, can greatly reduce the accelerometer's sensitivity as well as limit its frequency response. To minimize these effects the accelerometer output signal is fed through a preamplifier which converts to a much lower impedance, suitable for connection to the relatively low input impedance of measuring and analyzing instrumentation (1).
In the CCLD accelerometers, the preamplifier is built-in, so no external unit is needed, but an input furnished with the ability to provide the power for the unit is needed. Today this is a very common feature. In addition to the function of impedance conversion, most preamplifiers offer additional facilities for conditioning the signal.
(2) A calibrated variable gain facility to amplify the signal to a suitable level for input to other instruments;
(3) A secondary gain adjustment to "normalize" awkward" transducer sensitivities;
(4) Integrators to convert the acceleration proportional output from accelerometers to either velocity or displacement signals;
(5) Various filters to limit the upper and lower frequency response to avoid interference from electrical noise, or signals outside the linear portion of the accelerometer frequency range;
(6) Other facilities, such as overload indicator, reference oscillator, and battery condition indicator is also often included;
Measuring InstrumentationLEARN MORE
A portable, general-purpose vibration meter or analyzer will usually be the most convenient and cost-effective measuring instrument to use for machine monitoring and simple vibration assessment tasks.
Very simple instruments exist giving only simple results like total value from 10 to 1000 Hz. However, with modern digital techniques, the cost of adding more features have diminished, so the natural choice is an analyzer with one or two channels and many capabilities of analysis and link to PC. They also include the possibility to add new software.
For more complex tasks the ultimate in operating convenience and analysis speed is obtained with a real-time analyzer, where many parallel frequency bands are evaluated almost instantaneously and shown on a continuously updated display. Systems with more than 1000 channels are used for large structures.