Transcript capacitive transducer

CAPACITIVE TRANSDUCER
A capacitive pressure transducer consists of an airtight housing in
which a metallic diaphragm is placed. At the inner side, there is a
fixed plate. In between the diaphragm and the fixed plate, the
dielectric medium is placed.
When the pressure is applied to the diaphragm, it moves toward
the fixed plate resulting a change in the capacitance. This
capacitance is calibrated into voltage proportional to the applied
pressure
Advantages:
As these transducers have high input impedance, the loading effect is
minimum.
They require small force for operation hence is useful for small displacement,
pressure measurements and requires small power.
These transducers have good frequency response. They are less affected by
stray magnetic fields.
Advantages:
•These transducers require good quality isolation so as to protect the
transducer (metal plates) from stray capacitance.
•Guard rings are necessary so as to minimize the stray electric fields.
•Sometimes frequency response may be affected by the loading effects from
connecting links and cables.
•The performance may be affected by parameters like dust, temperature,
moisture, vibration and so forth.
•They require complex circuit arrangements like bridge, amplifier, etc. for
measurement purpose.
Piezoelectric transducers::
“When
a mechanical force is applied to a piezoelectric material (crystal), the
dimensions of material gets changed resulting in the generation of electric charge
or electric potential across surface of the crystal.” This principle is used in
piezoelectric transducers for the measurement of pressure, force and
acceleration, i.e. mechanical input and electrical output
. In the reverse manner, if electric charge or electric potential is applied across
the surface of metal, it results the change in dimensions of the crystal due to
deformation.
The piezoelectric materials used are classified as follows:
•Natural like quartz crystal, Rochelle salt
•Synthetic like lithium sulphate, barium titanante, ethylene diamine tartrate and
polymer films can also be used as piezoelectric materials.
For piezoelectric effect, the crystal should have natural asymmetrical charge
distribution. Because of this asymmetric charge distribution, the lattice
deformation takes place. The lattice deformation is nothing but relative
displacement of positive and negative (opposite) charge within the lattice.
If the asymmetric charge distribution is not available in some material (synthetic),
ferroelectric ceramics then such materials get polarized artificially by applying
strong electric field. This electric field produces asymmetric lattice.
On the surface of piezoelectric crystal, metal electrodes are connected. From
these electrodes, the metallic wires or leads are taken out for deriving electric
charge. Mechanical deformation of a piezoelectric crystal takes place after
applying the mechanical force. This deformation of a piezoelectric crystal has the
following modes: thickness expansion, transverse expansion, thickness shear and
face shear. These modes depend upon the shape and orientation of the crystal.
Where, d is the crystal charge sensitivity constant for the given crystal in C/N
F is the force applied to the crystal in N.
The charge sensitivity is defined as the charge generated per unit force
applied to the crystal. Due to the applied force F, the thickness of the crystal
gets changed (mechanical deformation).
Where, A is the area of crystal WL in m2
t is the thickness of the crystal in m
Y is the Young’s modulus for the material of the crystal in N/m2
W is the width of crystal in m
L is the length of the crystal in m
Due to the charge developed at electrodes the output voltage Volt gets
increased.
Equivalent circuit of piezoelectric transducer:
Fig 7.15 (b) shows an Equivalent circuit of piezoelectric transducer. The voltage
Volt is developed due to the generation of charges which is given by
In such type of transducer, the charge amplifiers are used.
The features of piezoelectric transducer are as follows:
•More stability
•Not affected by temperature variations and humidity
•Maximum output
•Useful for dynamic parameter measurements
•Quartz crystal has good stability, so it is used as a frequency
generator oscillator.
Applications:
Piezoelectric transducers have applications in vibration pick up,
accelerometer, pressure sensor, sound pressure and so on.
STRAIN GAUGES
Strain gauges is defined as an elongation or compression per unit
area. Strain gauges work on the principle of piezoresistivity. If a
metal wire or conductor is stretched or compressed it resistances
changes because of the changes in length, resistivity and the cross
sectional area this effect is called piezoresistive effect.
Suppose the dimension of the wire as follows:
A is the uniform cross sectional area of the wire in m2
L is the length of the wire in m
ρ is the resistivity in Ω-m
R is the resistance in Ω
If the conductor is stretched or compressed (strained) the above
mention parameter change as follows:
∆A is the change in cross sectional area
∆L is the change in length of the wire
∆ρ is the change in resistivity
∆R is the change in resistance
The change in resistance is due to following factors
i. Per unit change in the length ∆L/L
ii. Per unit change in the area ∆A/A
iii. per unit change in the resistivity ∆ρ/ρ
If the change in resistivity of the material due to strain is neglected the
gauge factor is
CLASSIFICATION OF STRAIN GAUGE
BONDED WIRE STRAIN GAUGES
A grid or fine wire of resistance is cemented to the base (sheet)
paper or sheet of Teflon or Bakelite. A particular resistance wire is
covered on to protect it in mechanical damage and faithfully handle
the compression and elongation (stress) .the classification of strain
gauges vary with application .strain gauge should have high gauge
factor ,low resistance temperature coefficient, no hysteresis and
resistance should be linear section of strain
Different type of strain gauges are: linear, rossette, torque helical etc
BIMETALLIC THERMOMETER
The concept of differential expansion of bonded strips of two
metals. In certain metal, the volume changes with temperature and
the coefficient of change is not the same for all metals
WORKING PRINCIPLE
The two different metal strips are joined or welded together and
heated, the resultant strip having lower expansion rate gets banned
, the deflection of the strips is directly proportional to the square of
length and temperature is inversely proportional to the thickness of
the metal.
As a name indicates, a bimetallic thermometer consist of two
different metal strips according to the change rise of temperature
,the length of the metals gets change with respect to the change in
thermal expansion this expansion result the bending of bimetallic
strip. The side having low coefficient of thermal expansion as shown
in figure
The deflection of the metal strip is applied to the pointer sliding over scale
which is proportional to the temperature variations. The deflection or
movement produced by the bimetallic strip is small, Also if the size of strip is
small, deflection is small. For large deflection size of the strip should be large to
avoid this metal strip element is wounded in spiral and helical shape .As outer
part of the spiral strips, a pointer is attached which moves over the calibrated
scale . As temperature increases, the spiral strip wind up and the pointer
deflects over the calibrated scale in clock wise direction as shown in figure. The
complete assembly is placed in the case
ADVANTAGES:
It is easy to install.
Its maintenance is not complex.
It is mechanically rigid and tough.
It has low cost.
It has wide operating temperature range.
BOURDON TUBE:
These are the primary pressure sensing elements used for measurement of
medium and high pressures. Bourdon tubes are elastic members and convert the
pressure into mechanical displacements. An important aspect is that Bourdon
tube is a primary sensing element, which senses pressure and converts it into
mechanical displacement. This displacement can be converted into electrical
signals using secondary transducers. Materials used for construction of Bourdon
tube phosphor bronze, beryllium copper ,monel, Ni-Span C and alloy steel.
Pressure range is 100 to 690 MPa.
Working PRINCIPLE
When the pressure which is to be measured, is applied to the pressure
sensitive element ,it reacts and resulting into mechanical displacement.
Mechanical displacement is proportional to applied pressure. This mechanical
displacement is converted into electrical signals by using electrical
transducers. From this discussion, it is noted that the electrical signal is
proportional to the mechanical displacement and hence proportional to the
applied pressure.
There are various types of bourdon tubes according to shape or form, namely
C-type , helical, and twisted.
C-BOURDON TUBE:
It is made by winding an elliptical flattened tube to form a segment of a circle
having an arc of 250 degree. This tube has two ends, out of which one is sealed.
The other end is open for applying the process pressure and fixed at the socket
as shown in fig 7.28(a)
When the pressure is applied at the open end, the tube tends to straighten out
because the internal and outer radii of the bourden tube are different. So the
tube takes different areas for the pressure. The non-linear pressure is converted
into linear motion or displacement by means of a pointer and calibrated
scale(deflection) arrangement. Necessary link, lever gear and pinion attachment
is provided to the deflection system. thus the applied pressure is measured by
means of deflection over linear scale.
SPIRAL BOURDON TUBE:
Spiral bourdon tube is made by winding the elliptical flattened tube into spiral
form of 2 or 3 windings around the same axis as shown in fig 7.28 (b).one end of
the tube is sealed and the other is open
When a pressure is applied to a spiral bourdon tube. It tends to uncoil and
produces the deflection and displacement at the free end. This displacement
is used for indication transmission. The produced displacement is applied to
the pointer and scale arrangement. The displacement is more, so there is no
need of magnification elements i.e. mechanical amplification. Due to this the
gear and pinion arrangement is eliminated while increases the accuracy.
Spiral bourdon tube has following advantage:
1. It has more accuracy.
2. There is no friction because there is no gear and pinion arrangement.
3. It has higher sensitivity because there is no friction and loss of motion.
HELICAL BOURDON TUBE:
Figure 7.28(c) shows a helical bourdon tube. By winding a elliptical flattened
tube
into a helical (helix) form, this tube is manufactured. This tube provides high
movement of the free end. Mechanical movement or displacement of helical
bourdon tube more than that of spiral bourdon tube.
A shaft is connected at the centre of the helical winding and a pointer is attached
at the sealed end using proper connecting link . As the applied pressure changes,
the helical winding provides a circular displacement, which is indicated by the
pointer connected to the shaft.
Helical bourdon tube has following advantage:
1. It provides satisfactory operation over continuous fluctuating pressure
signals.
2. It has higher capacity to handle the over range pressures.
3. There is no necessity of mechanical amplification gear and pinion
arrangement.
Pressure range of a bourdon tube depends upon diameter of the coil, thickness of
the tube, proportional material used and number of helical coil/tube. For lowpressure measurements, coils are used whereas for high pressure measurements,
16 to 20 coils can be used. Bourdon tube depends upon the diameter of tube,
design quality, materials used calibration arrangement. It is +-0.5 to +-2%.