Measurement Types

Vibration data is usually acquired by attaching one or more vibration sensors to the surface of a machine or structure.

• Acceleration response is typically measured using an accelerometer

• Velocity is measured using a Laser vibrometer

• Machine shaft displacement is measured using a Proximity probe

While the machine or structure is vibrating, a sampled time waveform is acquired from each sensor using a multi-channel data acquisition system, FFT FFT is an acronym for Fast Fourier Transform. The FFT algorithm transforms a uniformly sampled time domain signal into its corresponding Digital Fourier Transform (DFT). The Inverse FFT transforms the DFT back into its original time domain signal. The FFT in ME'scope transfroms any number of samples, not just powers of 2. analyzer, data recorder, or portable data collector.  Further signal processing is then performed on the acquired time waveforms, and different types of measurement functions are calculated.  In order to display a shape in animation, a set of measurements must be acquired at all points and in all directions where shape values are desired.  Typical sets of measurements used to obtain ODS's and mode shapes are;

• Simultaneously acquired multi-channel time waveforms

• Frequency Response Functions (FRFs).  An FRF An FRF (Frequency Response Function) is a cross-channel frequency domain function that defines the dynamic properties of a structure between an excitation Force and the Response caused by that force. An FRF is defined as the ratio (Response DFT / Force DFT). The FRF is a special case of a Transfer Function, where the response is the numerator (Output) and the force is the denominator (Input). is the Fourier spectrum A Fourier spectrum is also called the Discrete Fourier Transform, or DFT. of a response divided by the Fourier spectrum of the force that caused the response.

• Transmissibility's.  A Transmissibility is the Fourier Spectrum of a response divided by the Fourier Spectrum of a (fixed) reference response.

• Cross spectra. A Cross spectrum A cross-channel function that is calculated by multiplying the Fourier spectrum of a waveform by the complex conjugate of the Fourier spectrum of another waveform. is the Fourier Spectrum of a response multiplied by the complex conjugate of the Fourier Spectrum of a fixed reference response.

• ODS FRFs.  An ODS FRF is the Auto Spectrum of a response combined with the phase between the response and a fixed reference response.