Biomedical Engineering Reference
In-Depth Information
industrial accelerometers manufactured of a Piezo-ceramic material sandwiched
between a seismic mass and the structure base. The seismic mass and Piezo-
ceramic create a simple mass/spring system with a very high natural frequency.
The piezoelectric sensor is versatile, reliable, and the most popular vibration
sensor for machinery monitoring. Capacitive accelerometers are popular due to
low-cost production, high sensitivity, good dc-response and noise-performance,
low power dissipation, and a simple structure.
Displacement Sensor
Displacement sensors measure the distance where an object moves and they can
also be used to measure object height and width. There are mainly two types of
displacement sensors: Contact type and not contact type. Non-contact sensors are
based on various technologies including electric field, electromagnetic field, and
light/laser (capacitive type, inductive type). Displacement sensor can provide
contactless linear displacement measurement of absolute position in hydraulic and
pneumatic cylinders and due to this reason they offer wide application in indus-
tries. Displacement sensors are widely used not only to measure the distance of a
moving object but also it can be embedded in other sensors or transducer devices
to measure pressure or level or flow.
Eddy current probes are non-contact sensors primarily used to measure shaft
vibration, shaft/rotor position, and clearance. Also referred to as displacement
probes, eddy current probes are typically applied on machines utilizing
sleeve/journal bearings. They have excellent frequency response with no lower
frequency limit and can also be used to provide a trigger input for phase-related
measurements [ 11 ].
Different types of mechanical sensors can be considered such as [ 7 ]: Potenti-
ometer type (linear, Rotary), Capacitive and inductive type, Variable reluctance
type (LVDT), Strain gauges. Figure 2.12 shows various types of displacement
sensors. Table 2.9 summarizes the displacement sensors comparison by detection
methods.
LVDT-Linear Variable Differential Transformer (Positional Sensor)
LVDT is linear position sensor, which can measure movements as small as a
few millionths of an inch up to several inches, and also capable of measuring
positions up to ±20 in. (±0.5 m). Figure 2.13 shows the components of a typical
LVDT. The transformer's internal structure consists of a primary winding centered
between a pair of identically wound secondary windings, symmetrically spaced
about the primary. The coils are wound on a one-piece hollow form of thermally
stable glass reinforced polymer, encapsulated against moisture, wrapped in a high
permeability magnetic shield, and then secured in cylindrical stainless steel
housing. This coil assembly is usually the stationary element of the position
sensor. The moving element of an LVDT is a separate tubular armature of mag-
netically permeable material called the core, which is free to move axially within
the coil's hollow bore, and mechanically coupled to the object whose position is
being measured. This bore is typically large enough to provide substantial radial
clearance between the core and bore, with no physical contact between it and the
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