振动分析和仪器的艺术达芬奇

– 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
“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!
• 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.
Signal Conditioning
Data Acquisition & Storage
Communications
Remote Analysis and Diagnostics
Displacement Sensors
• Elements
– Probe, matched extension cable, Driver
优秀精品课件文档资料
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.
Signal Gain Circuit
• X1 and X10 are Common
– Gain is simply amplification of a Signal – Careful – Should kno ADC input range first!
– 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
– Simple Harmonic Motion
• Oscillation about a Reference Point • Modeled Mathematically as…
x(t)Xsint
Back to the Basics…
Unit Circle
Period, T
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
AC/DC Coupling
• Normally, Systems are AC coupled
– Means that there is a DC blocking Capacitor that only allows AC signal through to the system
• MAARS Innovation
Sensor Cables
• Coaxial with BNC Connectors
– Long Coaxial can become antennas!
• Twisted, Shielded Pair
– Teflon Shield – ground at only one end!
Sensor Cables
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
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Peak and RMS Comparison
Relationships of Acceleration, Velocity and Displacement
The Big Picture
Sensor(s)
Cables
Machine Speed Sensors
• Displacement Probes • Active or Passive Magnetic Probes • Optical Permanent • Stroboscopes • Laser Tach
Voltage or Current?
• Current Output Accelerometers
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.
– 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
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
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
• 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
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
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• 100mV/g accel; +-5V input range = +-50 g’s • Can “Clip” Signal
Signal Integration
• Best to Integrate as close to signal source as possible
– Reduces noise
• Vibration, Amperage, Pressure
Back to the Basics…
• Dynamic Signal Fundamentals
– Amplitude – Frequency – Timing – Shape
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– What it is NOT:
• Establish new measured point records • Means to show analytical brilliance • The answer to every problem!
Back to the Basics…
• Vibration
• 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.
– DC Switch that allows AC and DC to work on the same data channel without contaminating phase
– Allows use of same channel to record data for shaft centerline (DC) and Transient data (AC)