Capacitive transducer

We know that : C=kЄ° (A/d)

Where: K=dielectric constant Є° =8.854 *10^-12D=distance between the plates A=the area over lapping plate

Since , the capacitance is inversely proportional to the spacing between the two plates , any

variation causes a corresponding variation in the capacitance .

Hence the area is different

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 ) Ramp type Dvm

B ) integrating Dvm C ) continues balance Dvm D ) 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:

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 .

DC input voltage

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.

One can find the spectrum from the signal

‘decomposition. ’The signal from the spectrum known as composition .

A k

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.

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Relative response in (dB)

0

0.3 0.5 1 2 3

Normalized frequency

Attenuation characteristic