11.1 Sensors and transducers for vibration measurement

Vibration sensors are crucial tools for measuring and analyzing mechanical system vibrations. From accelerometers to velocity and displacement sensors, these devices capture different aspects of vibration, helping engineers understand and control system behavior.

Selecting the right sensor involves considering frequency range, amplitude, and environmental factors. Understanding sensor principles and specifications like sensitivity, frequency response, and dynamic range is key to accurate vibration measurement and analysis in mechanical systems.

Vibration Sensors: Types and Transducers

Accelerometers and Their Mechanisms

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• Vibration sensors categorized into three main types measure different aspects of vibration (accelerometers, velocity sensors, displacement sensors)
• Piezoelectric accelerometers generate electrical charge proportional to applied acceleration using the piezoelectric effect
• Capacitive accelerometers detect acceleration by measuring changes in capacitance due to internal mass movement
• Piezoresistive accelerometers measure acceleration-induced stress on a suspended mass using strain gauges
• Accelerometers operate based on Newton's Second Law of Motion where force on a proof mass relates proportionally to its acceleration

Velocity and Displacement Sensors

• Velocity sensors (electromagnetic velocity pickups) measure rate of change of displacement in vibrating structures
• Displacement sensors directly measure physical movement of vibrating objects (eddy current probes, linear variable differential transformers (LVDTs))
• Laser Doppler vibrometers use Doppler effect to measure velocity and displacement of vibrating surfaces without contact
• Velocity sensors typically use a magnet moving through a coil to generate voltage proportional to velocity based on Faraday's law
• Eddy current displacement sensors detect changes in magnetic field caused by eddy currents induced in a conductive target
• LVDTs measure displacement using principle of mutual inductance between a primary coil and two secondary coils

Working Principles of Vibration Sensors

Accelerometer Principles

• Piezoelectric accelerometers generate electrical charge when stressed, converted to voltage output proportional to acceleration
• Capacitive accelerometers measure capacitance change between fixed plates and movable plate attached to proof mass
• Piezoresistive accelerometers use strain gauges to measure stress induced by acceleration on suspended mass
• Resonance frequency limits upper frequency of accurate measurement for accelerometers

Velocity and Displacement Sensor Principles

• Electromagnetic velocity sensors generate voltage proportional to velocity based on Faraday's law of electromagnetic induction
• Eddy current displacement sensors detect changes in magnetic field caused by induced eddy currents in conductive target
• LVDTs measure displacement using mutual inductance between primary and secondary coils
• Laser Doppler vibrometers measure Doppler shift in reflected laser beam to determine vibrating surface velocity

Sensor Selection for Vibration Measurement

Frequency and Amplitude Considerations

• Frequency range of interest critical for sensor selection (accelerometers for high-frequency up to several kHz, velocity sensors for mid-frequency ~1 Hz to 1 kHz, displacement sensors for low-frequency below 100 Hz)
• Amplitude range of expected vibration must ensure accurate measurement without saturation or insufficient sensitivity
• Dynamic range defines ratio between largest and smallest measurable vibration amplitudes (expressed in decibels)

Environmental and Physical Factors

• Environmental factors influence sensor selection (temperature, humidity, electromagnetic interference)
• Physical constraints of measurement setup impact sensor choice (size limitations, mounting options)
• Required accuracy and resolution of measurement should match selected sensor specifications
• Temperature range and temperature sensitivity specify operational limits and performance variation with temperature

Vibration Sensor Specifications and Characteristics

Sensitivity and Frequency Response

• Sensitivity expressed in different units for each sensor type (mV/g for accelerometers, mV/(in/s) for velocity sensors, mV/mil for displacement sensors)
• Frequency response describes usable frequency range of sensor (specified as deviation from reference sensitivity, ±5% or ±3 dB)
• Resonance frequency important for accelerometers as it limits upper frequency of accurate measurement

Additional Performance Metrics

• Transverse sensitivity indicates sensor's susceptibility to vibrations perpendicular to primary sensing axis
• Linearity describes consistency of sensor's output corresponding to input across measurement range (expressed as percentage of full scale)
• Dynamic range defines ratio between largest and smallest measurable vibration amplitudes (expressed in decibels)
• Temperature range and temperature sensitivity specify operational limits and performance variation with temperature