Calculator Size Vibration Meter Reads • Acceleration, Velocity, Displacement Autoranging Digital Readout • Rechargeable Battery
The VT1 encapsulates the means of measuring vibration displacement, velocity, acceleration into a calculator format instrument incorporating rechargeable battery power source for total portability plus autoranging digital display of level over three decades.
Signal source is a fixed sensitivity piezo-electric accelerometer a variety of transducers and accessories to facilitate VT1 operation are available separately, described under the P.V.A.
Operation comprises simply applying the transducer to the test object, selecting the parameter required, and noting the reading, always presupposing that the data is within the VT1’s bandwidth/dynamic range window.
Application areas are confined to those giving rise to (quasi) steady state readings such as may occur in monitoring rotating machinery, vibratory plant, etc… associated with continuous industrial processes. In addition national standards set benchmarks relating to vibration both from a reliability and environmental standpoint. The VT1 is useful in areas where periodic wideband vibration data helps generate an overview of the dynamics of plant and machinery conditions, where continuous monitoring and/or spectral analysis is not warranted.
Implementation
The VT1 schematic is shown in Fig.1. The analogue section generates simultaneous displacement, velocity and acceleration (DVA) functions, selectable for routing to the rms AC/DC converter and thence to the display section. The autoranger maintains the displayed function within bounds giving 1% worst case resolution over a 66dB dynamic range of input.
Application Notes
Fixed transducer scaling (25pC/g nom.) plus autoranging reduces operational procedure to parameter (DVA) selection, minimising error.
Measurement units are displayed along with the level. That for displacement is µM peak-peak sine and, being scaled from an rms average, is subject to form factor error, but compensated by the harmonic weighting (1/ω2) of double integration implicit in acceleration/displacement conversion.
Magnetic mountings, probes, degrade the frequency response. Transducers are directional, Qt ≡ g x cosθ, the axial error between the vibration vector and transducer axis, having implications in the application of probes.
Autoranger update time and AC/DC converter averaging time, interacting with the dynamics of the physical signal, may produce large display perturbations ; resolvable perhaps by manual averaging or slowing the update/averaging parameters.
Peak analogue signal level of 4.5V imposes a worst case, full scale display reading, crest factor Vpeak/Vrms of 2.25, equivalent to a 20% duty cycle rectangular wave signal. This level of crest factor introduces <1% additional conversion error.
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