3.3 Basic vibration measurements
3.3.1 Types of vibration transducer
on springs. Surrounding this mass is a damping fl uid, usually oil. A coil of wire is attached to the outer case as shown in Fig. 3.2.
In operation, the case is held against and moves with the vibrating object, while the internal mass remains stationary suspended on the springs. The relative motion between the permanent magnet and the coil generates a voltage that is proportional to the velocity of the motion, hence it is a voltage pickup.
Velocity transducers provide a direct measure of vibration velocity (SI unit – ms−1, although mms−1 is commonly used). The main points regarding velocity transducers are:
• They are often moving coil devices – an electric coil is suspended around a permanent magnet that is kept static.
• They are designed to have a low natural frequency (typically a few Hz), chosen to be much lower than the anticipated excitation frequency range (typically 10–1000Hz).
• They are usually large and fairly heavy, but robust.
• They cannot be used on lightweight structures due to the mass loading effect – they change the vibration characteristics of the component whose vibration they are trying to measure.
• Multi-axis transducers are available. These usually stand on their own base – often an adjustable tripod is provided to cater for use on sloping or uneven surfaces.
Springs Coil
Permanent magnet Damping fluid
Isolating washer Vibrating
machine surface
3.2 Velocity transducer.
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• They are simple – low-cost voltage amplifi cation is commonly used.
Battery usage is relatively effi cient and the transducer and its signal conditioning are often housed in a single rugged package.
The most common acceleration transducer is the piezoelectric acceler- ometer (Fig. 3.3). It consists of a quartz crystal (or barium titanate – a manufactured quartz) with a mass bolted on top. In operation, the acceler- ometer case is held against the vibrating object and the mass wants to stay stationary in space. With the mass stationary and the case moving with the vibration, the crystal stack gets compressed and relaxed. The piezoelectric crystals generate a charge output that is going positive and negative as the crystals are alternately compressed tighter and relaxed about the preload.
The charge output is proportional to force, therefore acceleration according to Newton’s Second Law; hence the name accelerometer.
The accelerometer is self-generating, but the signal output has such high impedance that it is not usable by most analysis equipment. The output impedance of the accelerometer must be matched to the impedance of the readout instrument for accepting the signal from the accelerometer. The output signal from the accelerometer must be converted to a low- impedance signal by special electronics. This electronic circuitry can be outside or inside the accelerometer. There are two types of accelerometer that you can explore with the following links:
Springs Connector (case ground)
Internal electronics
Piezoelectric crystal stack
Isolating washer Vibrating
machine surface Seismic mass
Bolt compressing crystals
3.3 Piezoelectric accelerometer.
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• Voltage type
• Charge type.
The charge-mode accelerometer must have a charge amplifi er nearby. This provides the proper impedance matching. The short piece of cable between the accelerometer and the charge amplifi er is critical. It must be a low-noise cable and its length cannot be changed. The accelerometer, cable and charge amplifi er must be calibrated as a unit and not changed.
The voltage-mode accelerometer has the impedance-matching electron- ics built inside. It needs no charge amplifi er, and the cable length is not critical. All it needs is a low-cost power supply to power the internal elec- tronics. Some readout instrument manufacturers provide built-in power supplies to power their electronics.
It is important to use charge amplifi ers with charge accelerometers and power supplies with voltage accelerometers. When the electronics are built into the accelerometer, this simplifi es their use. The voltage acceler- ometers are labelled ICP (integrating circuit piezoelectric) by some manu- facturers. Accelerometers provide a direct measure of vibration acceleration (SI unit – ms−2, or g, where 1g = 9.81ms−2).
The main points regarding accelerometers are:
• They are designed to have a natural frequency well above the antici- pated excitation frequency range.
• They offer a wide frequency range from 0.2Hz to 10kHz for better than 5% linearity.
• They are usually small in size and light in weight (can be less than 1 gram).
• For high-temperature applications the charge mode system should be used.
• The accelerometer is sensitive to vibration frequencies much higher than the proximity probe or the velocity pickup.
• A wide dynamic range (160dB) and good linearity means they can detect very small vibrations and not be damaged by very large vibrations in a wide frequency range.
• The accelerometer is the only transducer that can accurately detect pressure wave vibrations in high frequencies.
Other transducers that are used today for vibration measurement are the strain gauge, piezo-resistive accelerometer, piezo-fi lm/piezo-patch, etc.