Download Applied Vibration Measurement, Analysis and Control Lab

AMERICAN UNIVERSITY IN CAIRO
MECHANICAL ENGINEERING DEPARTMENT
MENG 475: Applied Vibration Measurement, Analysis and Control
Lab 1: Vibration Transducers & Signal Analysis
Objective
The objective of this lab is to measure vibrations using different types of vibration sensors and to analyze
their signals in the time and frequency domains. The transducers used are: accelerometer, velocity probe,
and displacement sensor. A brief description of the theory of operation, advantages and limitations of each
type is given. You will then be instructed to use these instruments for vibration measurements in a lab setup.
The measurements will be recorded to be discussed in a written report.
Equipment
 Function generator
 Power amplifier
 Electrodynamic shaker
 Multi-channel data acquisition platform
 VIBROPORT general vibration measurement system
 Accelerometer
 Velometer
 Proximity probe
Background
The essentials elements of measurement:
Physical
behavior





Sensor
Transducer
Signal
conditioner
Display unit or data
acquisition system
Data analysis
Sensor: device which detects and responds to physical quantity to be measured
Transducer: converts quantity to be measured to an analog signal that can be manipulated (ex: Volt)
Signal conditioner: amplify, filter, integrate, differentiate, etc.
DAQ: data acquisition hardware, software, calibration, processing, displaying
Data analysis: extracting useful information from measurements
Vibration Transducers
1. Accelerometer: an instrument that measures the acceleration of a vibrating body.
The accelerometer uses a piezoelectric transducer.
Piezoelectric materials (e.g. quartz) generate an electrical
charge when subjected to deformation or mechanical stress.
The charge disappears when the loading is removed. A
small mass is preloaded against a piezoelectric crystal.
When the base vibrates, acceleration of the mass generates a
force on the piezoelectric crystal, which produces a
proportional electric charge. A charge amplifier is then
required to amplify this output. Velocity and displacement
can be obtained by integration.
A basic accelerometer measures the acceleration in one
direction. Triaxial accelerometers exist to measure
accelerations in 3 perpendicular directions simultaneously.
Piezoelectric accelerometer
Advantages
 Compactness
 Ruggedness
 High sensitivity
 Good response at high frequencies
 Can be used at high temperatures
Disadvantages
 Represents a dynamic load on the vibrating structure.
Therefore there is a minimum vibrating mass constraint.
 Accelerometer output requires signal conditioning
(charge amplification)
 Contact measurement pickup
 Cannot be used at very low frequencies
Fastening:
1. The magnet can be used on a clan plain ferromagnetic surface.
2. The threaded pin/stud is screwed into the mounting area (clean and plain) and secured. The
accelerometer is screwed onto the threaded pin
3. The sensor probe can be used when other methods are not possible. The sensor probe should be kept
perpendicular to the machine surface. The contact force and angle should be kept constant.
4. An internal threaded part can be bonded onto the surface using an adhesive. The screw can be
removed upon completion of the measurement
Using a threaded pin would be best because it can withstand higher dynamic forces.
2. Velometer or velocity meter: an instrument that measures the velocity of a vibrating body
When an electrical conductor (coil winding) is moved in
a magnetic field, an emf is induced in the conductor.
This emf is proportional to the velocity of the coil
relative to the magnetic field. The magnetic field may be
produced by a permanent magnet or an electromagnet.
Velocity pick-up
Laboratory Schenck velocity pickup:
It is used in the lower frequency range. The upper frequency limit is 2kHz. Minimum vibrating mass: 3 kg.
Advantages
 Measures relative velocity
 Sensor output does not require any
signal conditioning
 Relatively cheap
Disadvantages
 Heavy and bulky; therefore dynamically loads the
vibrating structure which imposes a constraint on the
minimum vibrating mass
 Life time is shortened by wear since the pickup contains
parts which are moving relative to each other
 Contact measurement pickup.
3.Vibrometer or displacement meter: an instrument that measures the displacement of a vibrating body.
Proximity probe: used for non-contacting displacement
measurement (shaft vibration and shaft displacement). It operates
in accordance with the eddy current principle. The displacement
transducer coil, extension cable, and circuit elements of the
oscillator form an oscillating circuit. The transducer produces a
magnetic field around the coil. If an electrically conducting
material is present within the range of this magnetic field, eddy
currents will be generated within this surface and they will
attenuate the oscillator circuit. This attenuation is converted into a
gap proportional to the output signal by the oscillator (note that the
larger the gap, the weaker the eddy currents generated, the smaller
the attenuation, and the higher the voltage output across the
elements of the oscillator circuit.)
Eddy current probe
The gap (displacement) and gap voltage are linearly related over a certain range (0.2mm to 2.2mm for the
proximity probe available in the laboratory). Typical frequency response allows measurements of vibrations
with frequencies up to 2kHz. The output of the proximity probe is fed into the VIBROPORT, which is a
general vibration measurement device that performs the necessary signal conditioning.
Advantages
 Non contact probe
 No dynamic load on the vibrating mass,
therefore no minimum limit for the vibrating
mass
 Measures relative displacement (could be
disadvantageous)
Disadvantages
 Free space and minimum distance limitations
from electrically conducting surfaces other
than the vibrating surfaces
 Minimum shaft diameter (25mm for available
proximity probe)
 Measures relative displacement (could be
advantageous)
 Requires additional electronics and additional
signal conditioning
Experimental Setup
Charge
amplifier
Conditioning
circuit
Spectrum
analyzer
P A
Shaker
V
Power
amplifier
Function
generator
Function Generator: used to generate an electrical signal of the desired frequency and amplitude. The
available model generates sine, square and triangle wave functions from 1mHz to 19.99 MHz, with
amplitudes up to 10V.
Power Amplifier: used to amplify or attenuate the signal generated by the function generator. It can supply
a large current, while the function generator has little power.
Electrodynamic Shaker: Used to provide the mechanical excitation.
Multi-channel data acquisition platform: used to view the output signal in the frequency and time
domain.
Required
1. Start by feeding a signal from the function generator to the analyzer. Analyze the spectrum of
different waveforms (sine, square and triangular). Comment on the harmonics.
2. Drive the shaker at different frequencies and observe the outputs of the three sensors used
(accelerometer, velocity sensor and proximity probe). Tabulate your results as shown below.
Knowing that the sensitivity of the proximity sensor is 8 mV/m, determine the sensitivity of the
velometer and accelerometer.
Function generator
Frequency [Hz]
Amplitude [V]
Channel 1:
Proximity probe
reading [V]
Channel 2:
Velometer reading
[V]
Channel 3:
Accelerometer
reading [V]