The art of instrumentation 09 2005
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The Art of
Instrumentation &
Vibration Analysis
Back to the Basics –
Forward to the Future
Our Objective…
•
The objective of Condition
Monitoring is to provide information
that will keep machinery operating
longer at the least overall cost.
– What it is NOT:
•
•
•
Establish new measured point records
Means to show analytical brilliance
The answer to every problem!
Back to the Basics…
•
Vibration
– Simple Harmonic Motion
•
•
Oscillation about a Reference Point
Modeled Mathematically as…
x(t ) = X sin ωt
Back to the Basics…
Period, T
Unit Circle
RMS
0
0 to Peak
Peak-to-Peak
Back to the Basics…
•
Basic Signal
Attributes
– Static
•
•
Slowly
Changing
Temperature
•
Basic Signal
Attributes
–
Dynamic
•
•
Sensor must
respond in fractions
of a Second
Vibration,
Amperage,
Pressure
Back to the Basics…
•
Dynamic Signal
Fundamentals
–
–
–
–
Amplitude
Frequency
Timing
Shape
•
Timing,
or
Amplitude
Frequency
Signal
Shape
–Phase
Proportional by
Determined
Waveform
to
theSimple
severity ofby
reciprocal
of
– •Represented
vibratory
Period
motion
•the
time
delay
Complex
CPS
or two
Hzas
– •between
Pattern
Expressed
Recognition
••signals
RPM
Peak to Peak
Orders
– ••Leading
Zero to Peak
RMS
– •Lagging
Peak and RMS Comparison
Relationships of Acceleration,
Velocity and Displacement
The Big Picture
Sensor(s)
Cables
Data Acquisition
& Storage
Signal Conditioning
Communications
Remote
Analysis and
Diagnostics
Displacement Sensors
•
Elements
– Probe, matched extension cable, Driver
Displacement Sensors
•
How it Works:
The tip of the probe contains an
encapsulated wire coil which radiates the
driver's high frequency as a magnetic field.
When a conductive surface comes into close
proximity to the probe tip, eddy currents are
generated on the target surface decreasing the
magnetic field strength, leading to a decrease
in the driver's DC output. This DC output is
usually 200mV/mil or in a similar range.
Displacement Sensors
•
Pro’s and Con’s
– Pro’s
•
•
Measures Displacement
Rugged
– Con’s
•
•
•
Limited Frequency Range (0-1000Hz)
Susceptible to electrical or mechanical runout
Installation Issues
Velocity Sensors
•
Pro’s and Con’s
– Pro’s
•
•
Measures Velocity
Easier Installation than Displacement
– Con’s
•
•
•
Limited Frequency Range (0-1000Hz)
Susceptible to Calibration Problems
Large Size
•
Acceleration Sensors
Pro’s and Con’s
– Pro’s
•
•
•
•
Measures Accel.
Small Size
Easily Installed
Large Frequency Range (1-10,000 Hz)
– Con’s
•
•
Measures Acceleration (requires Integration to Vel.)
Susceptible to Shock & Requires Power
Machine Speed Sensors
•
•
•
•
•
Displacement Probes
Active or Passive Magnetic Probes
Optical Permanent
Stroboscopes
Laser Tach
Voltage or Current?
•
Current Output Accelerometers
– 4-20 mA Output
•
•
Proportional to Dynamic Signal and/or Overall
Voltage Output Accelerometers
– Preferred in U.S.
– Generally 100mV per g Sensitivity
AC and DC Signal Components
•
Signals have both AC and DC
– AC considered the “Dynamic” Signal
– DC is the “Static” Signal
•
•
Displacement Probes – Set “Gap” for DC
Accelerometers – “Bias” voltage is DC
AC and DC Signal
Components
• How AC and DC
work together:
– AC signal “rides” the
DC bias (VB)
•
Affects the Dynamic
Range of the
Sensor.
Power Circuit for Accelerometers
“Strips off”
DC
Voltage
•
Grounds
A Potential Problem
Source
–
Ground Loops
• Caused when two or
more grounds are at
different potentials
• Sensors should be
grounded only at the
sensor, not the
monitoring rack!
Sensor Cables
•
Coaxial with BNC Connectors
– Long Coaxial can become antennas!
•
Twisted, Shielded Pair
– Teflon Shield – ground at only one end!
Sensor Cables
•
Driving Long Cables
–
–
Under 90 feet, cable capacitance no problem –
Cable Capacitance spec’d in Pico-farads per
foot of cable length
Over 90 feet or so, CCD must supply enough
current to charge the cable as well as the sensor
amplifier.
•
May result in amplifier output voltage becoming “Slew
Rate Limited”
Sensor Cables
•
Output of Sinusoid looks like this:
•
What’s Happening?
–
–
The + part of the signal is
being limited by the current
available to drive the cable
capacitance.
In the – part of the sin wave,
the op-amp must “sink” the
current being discharged by
the cable capacitance.
Sensor Cables
•
Practical Effect:
–
–
–
Signal distortion produces
harmonics
May lead to vibration signals
being misinterpreted.
To calculate the maximum
frequency for a length of cable:
Signal Conditioning
•
•
•
•
•
Gain
Integration (Hardware)
AC/DC Coupling
Anti-Aliasing Filter(s)
Sample and Hold Circuit
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