Page 355 - Mechatronics with Experiments

October 9, 2014 8:1
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JWST499-Cetinkunt
JWST499-c06
SENSORS 341 254mm×178mm
Oscillator

V= V sinω t
p r r

Primary
coil
X

Core FIGURE 6.11: Linear variable
differential transformer (LVDT) and
Secondary its operating principles. An
coils
oscillator circuit generates the
excitation signal for the primary
V= kVx winding. The demodulator circuit
s o p
removes the high frequency signal
V out
De- content and obtains the magnitude
modulator Filter of the induced voltage, which is
related to the core position.

windings changes in relation to the position. Hence, we have a well-defined relationship
between the induced voltage and the position. In LVDTs, both windings are stationary, and
a rotor core made of a material with high magnetic permeability couples the two windings
electromagnetically. In rotary LVDTs (resolvers and syncros), the primary winding is
located on the rotor, and the secondary winding on the stator. Either the rotor winding or
the stator windings can be excited externally by a known voltage, and the induced voltage
on the other winding is measured which is related to the position. The operating principles
of a LVDT, resolver, and syncro are shown in Figures 6.11–6.16. Notice that the syncro is
just a three-phase stator version of the resolver.
The LVDT is an absolute position sensor. On power-up, the sensor can tell the
position of the magnetic core relative to the neutral position. The LVDT’s primary winding
is excited by a sinusoidal voltage signal. The induced voltage on the secondary windings has
the same frequency except that the magnitude of the voltage is a function of the position of
the magnetic core. In other words, the displacement modulates the magnitude of the induced
voltage. As the core displacement increases from the center, the magnitude of the voltage
differential between the two stator windings increases. The core material must have a large
magnetic permeability compared to air, such as iron–nickel alloy. A non-magnetic stainless

FIGURE 6.12: Pictures of LVDTs (left) and a resolver (right).