8/10/2019 Transducers (lecture notes)

1/13

Lecturer: Jomo N. Gill

MEASUREMENTS ANDINSTRUMENTATION

ELE 4101

LECTURE IX

8/10/2019 Transducers (lecture notes)

2/13

CONCEPTS TO BE EXAMINED

2

- Transducers

General classes of transducers

Variable parameter analog transducers

Resistive transduction

8/10/2019 Transducers (lecture notes)

3/13

TRANSDUCERS

All physical quantities measured with electronicinstruments are first converted into signalvoltages, which can in turn be amplified or

otherwise made into suitable form for recording orindication. As used in instrumentation, the termtransducer denotes the devices used to make thisconversion.

3

Transducers use almost all knownelectromechanical principles to convert themeasured quantities into their electrical analogs.

8/10/2019 Transducers (lecture notes)

4/13

GENERAL CLASSES OF

TRANSDUCERS

Transducers are classified into four main groups:

- self-generating analog- variable parameter analog

- frequency or pulse generating

- digital

4

8/10/2019 Transducers (lecture notes)

5/13

contd

Analog transducers produce an electrical outputsignal that is a proportional continuousmeasurement of the input signal variations.

5

Pulse rate transducers produce voltage pulseswhose frequency or random pulse generation rateis proportional to the input parameter variations.

Digital transducers produce a unique codedvoltage or current form for each discrete valuesensed.

8/10/2019 Transducers (lecture notes)

6/13

VARIABLE PARAMETER

ANALOG TRANSDUCERS

These produce an output which is a proportional

fraction of an original resistance, capacitance orinductance.

6

The changes in resistance, inductance or

capacitance caused by the physical phenomenonbeing measured, are translated into correspondingvoltage or current effects. These are thenconverted into an output signal.

8/10/2019 Transducers (lecture notes)

7/13

contd

The conversion requires auxiliary circuitry. TheWheatstone bridge and its variations are

commonly used.

7

8/10/2019 Transducers (lecture notes)

8/13

RESISTIVE TRANSDUCTION

The variation of resistance is one of the mostcommon transduction principles. The resistance ofa resistive element can be varied by severalmethods, including:

8

- sliding a wire along a rheostat

- applying mechanical stress

- varying light intensity to a photosensitivematerial

- changing its temperature

8/10/2019 Transducers (lecture notes)

9/13

contd

Resistive transduction is used by:

- displacement transducers

- strain gauges- resistive temperature devices (RTDs)

- thermistors

- photoconductive devices

9

8/10/2019 Transducers (lecture notes)

10/13

contd

10

8/10/2019 Transducers (lecture notes)

11/13

POTENTIOMETERS

The potentiometer or slidewire resistor relies onrotation or mechanical displacement of a sliding

contact to determine the value of the resistanceoutput. They are used in the direct measurementof linear and angular displacement and motion.

11

Potentiometers also find application as the sensingelement in many transducers designed to measurepressure, acceleration, force, and liquid level.

8/10/2019 Transducers (lecture notes)

12/13

contd

The resistive element consists of either woundwire, or a ribbon of deposited carbon film,platinum, or other compound.

12

Inside a potentiometer Schematic diagrams

8/10/2019 Transducers (lecture notes)

13/13

VARIOUS TYPES OF

POTENTIOMETERS

13