Master-Class

Eddy Current Proximity Probe Measures Displacement (mostly according to API 670) Dynamic range: 500:1 Frequency range: DC -10kHz (Theoretical) DC -2000Hz (Practical) Advantage Non contacting No moving parts, no wear Works to DC limitation Variations in magnetic properties of shaft and geometric irregularities of shaft give erroneous signal components. Local calibration necessary A low dynamic range limits practical frequency range as displacement is relatively small at high frequencies.
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Accelerometer Measures acceleration Dynamic range: 10 8 : 1 (160 dB) Advantage limitation Self generating High impedance output No moving parts, no wear No true DC response Rugged Very large dynamic range Wide frequency range Compact, often low weight High stability Can be mounted with any orientation
Type of Bandpass Filter Two types of band pass filters are used for frequency analysis. 1. Constant Bandwidth filters where the bandwidth is constant and independent of the centre frequency of the filter. 2. Constant Percentage Bandwidth (CPB) filters, where the bandwidth is specified as a certain percentage of the centre frequency i.e. The bandwidth is increasing for an increase in centre frequency. CPB filters are some times called Relative Bandwidth filters.
Velocity Pickup Measures velocity Induced voltage e proportional to: Magnetic field B, length of windings l and relative velocity v Dynamic range: 1000:1 Advantage Self generating Low impedance Limitation Moving parts prone to wear Large size Sensitive to orientation Sensitive to magnetic fields High lower limiting frequency (>app. 10 Hz) as it operates above resonance Friction against the motion of the Moving Element will cause reduced output signal.