TransducerBasics

Urvish Soni

Transducer

• A transducer is a device which converts the energy from one form to another form. This energy may be mechanical, electrical, optical or thermal.

• These are widely used in measurement work because not all quantities that need to be measured can be displayed as easily as others.

Basic requirements of transducers

• The main function of a transducer is to respond only for the measurement under specified limits for which it is designed.

• It is, therefore, necessary to know the relationship between the input and output quantities and it should be fixed.

• Transducers should meet the following basic requirements

Basic Requirements Of a Transducer

Ruggedness. It should be capable of withstanding overload and somesafety arrangement should be provided for overload protection.

Linearity. Its input-output characteristics should be linear and itshould produce these characteristics in symmetrical way.

Repeatability. It should reproduce same output signal when the sameinput signal is applied again and again under fixed environmentalconditions e.g. temperature, pressure, humidity etc.

• High Output Signal Quality. The quality of output signal should begood i.e. the ratio of the signal to the noise should be high and theamplitude of the output signal should be enough.

• High Reliability and Stability. It should give minimum error inmeasurement for temperature variations, vibrations and othervarious changes in surroundings.

• Good Dynamic Response. Its output should be faithful to input whentaken as a function of time. The effect is analyzed as the frequencyresponse.

• No Hysteretic. It should not give any hysteretic during measurementwhile input signal is varied from its low value to high value and vice-versa.

• Residual Deformation. There should be no deformation on removal oflocal after long period of application.

Classification Of Transducers

• The transducers may be classified in various ways such as on the basis of electrical principles involved, methods of application, methods of energy conversion used, nature of output signal etc.

Primary and Secondary Transducers

• Primary and Secondary Transducers: Transducers, on the basis of methods of applications, may be classified into primary and secondary transducers.

• When the input signal is directly sensed by the transducer and physical phenomenon is converted into the electrical form directly then such a transducer is called the primary transducer.

Primary

• For example a thermistor used for the measurement of temperature fall in this category.

• The thermistor senses the temperature directly and causes the change in resistance with the change in temperature.

Secondary

• When the input signal is sensed first by some detector or sensor and then its output being of some form other than input signals is given as input to a transducer for conversion into electrical form, then such a transducer falls in the category of secondary transducers.

• For example, in case of pressure measurement, bourdon tube is aprimary sensor which converts pressure first into displacement, andthen the displacement is converted into an output voltage by an LVDT.In this case LVDT is secondary transducer.

Active and Passive Transducers.

• Transducers, on the basis of methods of energy conversion used,may be classified into active and passive transducers.

• Self-generating type transducers i.e. the transducers, which developtheir output the form of electrical voltage or current without anyauxiliary source, are called the active transducers.

Passive one!

• Transducers, in which electrical parameters i.e. resistance, inductance or capacitance changes with the change in input signal, are called the passive transducers.

• These transducers require external power source for energy conversion. In such transducer electrical parameters i.e. resistance, inductance or capacitance causes a change in voltages current or frequency of the external power source.

Analog and Digital Transducers

• Transducers, on the basis of nature of output signal, may be classified into analog and digital transducers.

• Analog transducer converts input signal into output signal, which is a continuous function of time such as thermistor, strain gauge, LVDT, thermo-couple etc.

• Digital transducer converts input signal into the output signal of the form of pulse e.g. it gives discrete output.

Static characteristic

• Accuracy

• Precision

• Error

• Linearity

• Reproducibility

• Repeatability

• threshold

• dead zone

• Hysteresis

• Creep

• Span

• Range

Dynamic characteristic (fidelity)

• Speed of response

• Lag

• Dynamic error

Accuracy

Definition : The accuracy of an instrument is a measure

of how close the measured value of the instrument is close

to the true value.

The accuracy of an instrument may be expressed in one of the

following ways:-

a) In terms of the measured variable itself

b) In terms of span of the instrument

c) In terms of percent of upper-range value

d) In terms of actual output reading

Static Error : It is the difference between the actual value indicated by

the instrument and the true value of the quantity being measured.

Static Error = Instrument reading – True value

Precision

Definition : It is the ability of a measuring instrument to

give identical responses for repeated applications of the

same value of the measured quantity under the same

conditions of use.

Precision is sometimes stratified into:-

a) Repeatability : It is the degree of closeness with which a quantity

can be repeatedly measured by using the same instrument and

operator, and repeating during a short time period.

b) Reproducibility : It is the extent of variation arising in the measured

value of a quantity using the same measurement process among

different instruments and operators, and over longer time periods.

Drift : The gradual shift in the indication or record of the instrument

over an extended period of time, during which the true value of the

variable does not change is referred to as drift.

Different kinds of drifts are as follows:-

a) The entire instrument calibration may gradually shift by the same

amount. This is called as zero drift that can be easily corrected by

shifting the pointer position.

b) In case of span drift, calibrations from lowest value to the highest

value get shifted by proportional amount. Hence, higher

calibrations get shifted more than the lower calibrations.

c) In some instruments, only a certain portion of the calibration gets

shifted while the remaining portion of the scale remains

unaffected.

Difference between Accuracy and Precision

Sensitivity

Definition : It is the response of an instrument denoted by

a change in the output reading corresponding to a change

in the value of the input variable. (This relationship may

be linear or non-linear).

An instrument with a large sensitivity denotes even a

slight change in the input.

LinearityDefinition : It is the relationship between the change in output

corresponding to the change in input variable, when they are in direct

proportion.

Most instruments are specified to function over a particular range

and the instruments can be said to be linear when incremental

changes in the input and output are constant over the specified range.

Resolution

Definition : It is defined as the smallest input increment

change that gives some small but definite numerical

change in the output.

Threshold

Definition : If the instrument input is very gradually

increased from zero, there will be a minimum value

required to give a detectable output change. This

minimum value defines the threshold of the instrument.

Dead Zone

Definition : This is the range of different input values

over which there is no change in the output value.

This is produced by friction, backlash or hysteresis in

the instrument.

• Accuracy :- the accuracy of a measurement indicates the nearness value to the actual value of quantity.

• Precision :- it is the measure the the degree to which successive measurement differ from each other. By this we can get fixed value of variable.

• Error :- It is the algebraic different between the actual value and measured value.

• Repeatability :- it is the defined as the variation of the scale readings. It is random in nature. Repeatability is measure of closeness with which a given input can be measure over and over again.

• Reproducibility :- it is defined as the degree of closeness by which a given value can be repeatedly measured.

• Sensitivity :- it is the ratio of Change in output of an instruments to the change input.The sensitivity of an instruments should be as high as possible.

• Linearity :- Accuracy and linearity is closely related to each other. it is defined as the ability of an instruments to reproduce its input linearly.

• Dead Zone :- it’s largest change in input quantity for which there is no output.

• Threshold :- If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument.

• Span – It can be defined as the range of an instrument from the minimum to maximum scale value. In the case of a thermometer, its scale goes from −40°C to 100°C. Thus its span is 140°C. As said before accuracy is defined as a percentage of span. It is actually a deviation from true expressed as a percentage of the span.

• Range – It can be defined as the measure of the instrument between the lowest and highest readings it can measure. A thermometer has a scale from −40°C to 100°C. Thus the range varies from −40°C to 100°C.

• Drift – Drift is the change in the reading of an instrument of a fixed variable with time.

• Creep- The mechanism of creep depends on temperature and stress. It’s a deformation of instrument

Hysteresis

It can be defined as the different readings taken down when an instrument approaches a signal from opposite directions.

Dynamic

• Speed of response :– its defined as the when we changed in input value and how much rapidly we get the change in output value by instruments. by this we can get the system is how much fast and active.

• Measuring lag:- It is the retardation or delay in the response of a measurement system to changes in the measured quantity.

• Dynamic error:- It is the difference between the true value of the quantity changing with time & the value indicated by the measurement system if no static error is assumed.