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Capacitive
transducer
We know that :
C=kЄ° (A/d)
Where :
K=dielectric constant
Є° =8.854 *10^-12
D=distance between the plates
A=the area over lapping plate
Hence the area is different
Since , the capacitance is inversely proportional to the
spacing between the two plates , any variation causes a
corresponding variation in the capacitance .
A force apply to the diaphragm (DF):
That functions as one plate of a simple capacitor changes the
distance between (DF), and the static plate .
The result of change in c is usually measured with an oscilloscope
circuit .
The transducer as a part of the oscilloscope circuit cause a change
in the frequency of the oscillator .
This change in frequency is a measure of the magnitude of the
applied force .
The capacitive transducers has an excellent is a measure of the
magnitude of the applied force
The capacitive transducer has an excellent frequency response it’s
disadvantages are sensitivity to temperature variation and possibility
of erratic distorted .
Digital meters :
The digital voltmeter displays measurement of AC or dc voltages as
discrete values , instead of a pointer deflection , which reduces error
of interpolation of a (Dvm ) and analog device .
Some specification of (Dvm) “when you need to use” are :
1- input voltage range
2- accuracy as high as + or - 0.005%
3- stability
4- resolution or calibration
So (dvm) are specified as :
A)
B)
C)
D)
Ramp type Dvm
integrating Dvm
continues balance Dvm
Successive approximation
1- ramp type Dvm :
The Dvm is based on the measurement of time that
takes for a linear ramp voltage to rise from zero
volt to the level of the input voltage .
(or the decrees from the level of the input voltage
to zero volt )
The resultant time is measured with an electronic
time interval counter , where this count is
displayed as a number of digits , on the Dvm tube .
As the gate open on oscillator
generate clock pulses a counter
will totalize the number of pulses
passed through the gate which
indicates the magnitude of the
input voltage .
There is also a sample rate – multi
vibrate , which can be adjusted by
front panel , controlling rate from
few cycles per second , to as high
as 1000 ‘cycle per sec’ and grater .
It’s also either provides an initiating Pulses for the next ramp voltage , or
generate a rest pulse which returns all the DCU : (Decade Counting Unit ) to
their zero state .
Signal analyzer :
DC input voltage
The first instrument to measure
any set of spectrum content of
signal was a device called
“harmonic distortion analyzer.
These early distortion analyzer
measured total harmonic
distortion , this is a problem ,
because it didn’t indicate which
harmonic was responsible for the
measured part of the signal .
New analyzer called wave analyzer could separate harmonics distortions and
evaluate each one .
A periodic signal may consist of a sum of elemental sinusoidal .
A plot of the Fourier coefficients as a function of the
frequency index , displays the signal spectrum .
The word “spectrum” implies that the variable (k)
corresponds some how to the frequency .
Ak
One can find the spectrum from
the signal ‘decomposition’ .
The signal from the spectrum
known as composition .
k
A wave analysis is an instrument designed to measure the
relative amplitude of signal frequency component in
complex or distorted wave forms .
The instrument acts as a frequency acts as a frequency
selection meter which is tuned to the frequency of one
signal components while rejecting all other components .
The wave to be analyzed in terms of it’s separate frequency
component is applied to an input attenuator .
The bandwidth (BW) of the
instrument is very narrow and
typically 1% of the .
selected frequency .
The wave analyzer is applied
industrially in the field of reduction
of sound and vibration generated by
machines and appliances .
The source of noise or vibration
generated by a machine must first be
identified before it can be
eliminated .
A fine spectrum analysis with wave analyzer will show varies
discrete frequencies that can be related to motion within the
machine .
Relative response in (dB)
0
-20
-40
-60
-80
0.3
0.5
1
2
Normalized frequency
Attenuation characteristic
3