Download An Angular velocity Transducers: Inductive Transducer: Inductive

An Angular velocity Transducers:
Inductive Transducer:
Inductive transducers may be either the self-generating or the
passive type. The self-generating type utilizes the basic electrical
generator principle that when there is relative motion between a
conductor and magnetic field, a voltage is induced in e conductor
(generator action). This relative motion between field and conductor
is supplied by chances in the measurand.
Fig (7) Tachometer with a permanent magnet stator.
(The device is a do generator.)
– A tachometer is an inductive transducer that directly
converts speed or velocity into an electrical signal. In
one technique, the object whose angular velocity is to
be measured is directly coupled to the rotor of a do
generator, as shown in Fig. (7). The coupling turns
the rotating armature between the poles of a
permanent magnet, thereby inducing a voltage in the
windings of the rotor. The voltage developed may
approach 10 mV per revolution per minute (rpm) and
can be fed directly into a dc voltmeter calibrated in
rpm units. Alternatively, the rotating armature may be
simply a permanent magnet.
The coils are wound around the fixed poles as »
shown in Fig. (8). This configuration provides
an alternating signal, which has certain
advantages
over a dc voltage in that noise and ripple »
signals can be-filtered more readily before
further signal amplification.
One obvious application of this transducer is »
in the field of frequency determination, when
the tachometer is attached directly to the
frequency generator. For more exact
frequency determination, a digital counting
system is used.
• Fig (8) Tachometer with a permanent magnet rotor.
•
•
•
Flow velocities are also measured by inductive transducers. This method finds
extensive use in systems that cannot be opened to the atmosphere. The
measurement of liquids containing suspended solids such as sewage and the
feed to paper mills presented considerable problems until the advent of the
electromagnetic flow meter. As its name implies, it can be used to measure the
flow of any flowing material that is electrically conductive. The meter can be
regarded as a section of pipe that is lined with an insulating material. Two
saddle coils are arranged opposite each other, and electrodes diametrically
opposed are arranged flush with the inside of the lining. If the coils are
energized. the moving liquid, as a length of conductor, cuts the lines of force
and generates an electromotive force that is picked up by the electrodes. By
suitable circuitry and amplification, an electrical signal proportionate to flow
can be obtained.
Figure (9) gives a general idea of the construction of such a meter. The
principle on which the meter operates is that of the do generator. The generator
rotor is replaced by the pipe between two magnetic poles. As the fluid flows
through the magnetic field, an electromotive force is induced in it and can be
picked up by the electrodes. This can be expressed mathematically as
E = BlV
–
–
–
–
–
–
where
E =emf, volts
B = field strength Webbers per square meter (teslas)
I =conductor length, meters
V = velocity of conductor, meters per second
In the case of the electromagnetic flow meter, the flowing
liquid represents the conductor and the internal diameter
corresponds to the length /. If the field strength B is
maintained constant the only variable is v, hence, the
electromotive force is proportional to the velocity.
– Fig (9) Inductive input transducer for measuring flow.
• The electromagnetic flow meter has the advantage of
causing no drop in the pressure of the fluid and having a
very large range. It is not suitable for low velocities, the
smallest range possible being 2 ft/sec for full scale or 1
ft/sec with the sacrifice of some accuracy. It is limited to
the measurement of fluids having a conductivity in
excess of 0.1 siemens/m. Of great importance is the fact
that the readings are unaffected by variations in
viscosity, density. temperature, pressure, or conductivity.
The electrodes must, of course, be kept clean, and this
can present a problem in the measurement of sewage.
In which grease, a poor electrical conductor, may be
present. (For certain designs for measuring a very small
amount of flow, the conductivity of the liquid may have to
be as high as 0.5 siemens /m.)
» EXAMPLE
» Given an inductive input transducer Fig (9) for flow
measurement, the diameter of the conduit is 0.0125
m. If a flow velocity of 10 m/sec produces a magnetic
field strength of 0.2 T, find the electromotive force
generated.Solution
» E=Blv = (0.2 W/m2) (0.0125 m) (10 m/sec)
=
0.025 V