CHAPTER 5

Intermediate Elements

Introduction

• The output signal of any transducer needs to be modified by elements known as intermediate elements, so that it can be displayed or recorded with convenience. These include:

1. Amplifiers for amplifying the transducer output.

2. Compensating devices. (not covered)

3. Differentiating/integrating elements.(not covered)

4. Filters for filtering out unwanted signal.

5. A-D/D-A converters.

6. Data transmission elements.

5.1 Amplifiers• Amplifiers increase the magnitude of the signal from a

transducer so that it can be conveniently displayed/recorded.

• These may be of mechanical, hydraulic, pneumatic, optical or electrical/electronic types, depending on the type of transducer.

• Attenuators are used in cases where reduction in the magnitude of the signal from the transducers is needed.

5.1.1 Mechanical Amplifying Element• Simple in operation, rugged type and inexpensive.• Eg. Huggenburger extensometer, Bourdon pressure

gauge.• Disadvantage: friction and stiction effects, backlash

errors, environmental temperature changes, and inertial effects due to relatively higher mass.

Fig. 5.1 A typical mechanical amplifier (a lever type device)

5.1.2 Hydraulic Amplifying Element• Wide range of applications in the form of hydraulic

actuators in the control elements used in the automobile hydraulic brakes and hydraulic steering systems.

• Advantage: compactness of a specified force.• Disadvantage: possible leakages and problems in dusty

environments.

5.1.3 Pneumatic Amplifying Element• Pneumatic transducer (flapper nozzle type) was described

in Chap.4, for converting mechanical displacement Xi to pressure P2.

• Used in industrial environment where compressed air is easily available.

Fig. 5.3 Pneumatic relay as amplifier

5.1.4 Optical Amplifying Element• Common application is in the taut suspension type of the

optical type galvanometer which is highly sensitive.• Cannot be used in dynamic measurements due inertia

effects of the mirror mass.

Fig. 5.4 A typical lamp and scale arrangement

5.1.5 Electrical Amplifying Element• Commonly transistor based or employ suitable integrated

circuits or both.• The following are characteristics of an ideal amplifier:

1. Infinite input impedance

2. Zero output impedance

3. Large gain

4. Zero output for zero input

5. Ability to filter spurious inputs

6. Excellent frequency response

Fig. 5.5 A typical electrical amplifying element

5.4 Filters • Filters is used to remove unwanted signals from the

desired transducer signal. • 5.4.1 Classification of Filters• Filters may be classified according to Fig. 5.24, where the

amplitude ratio of output of the filter to the input is plotted against frequency.

Fig. 5.24 Various types of filters

5.5 A-D and D-A Converters5.5.1 A-D Converters

• Simplest type is of potentiometric type, which employs comparison elements that are in the form of semiconductor relays/magnetic cores.

Fig. 5.30 Potentiometric type A-D Converter

5.5.1 A-D Converters (cont.)• Based on a similar principle, a successive approximation

type A-D converter, employing a D-A converter is commonly used.

Fig. 5.31 Successive approximation type A-D converter

5.5.1 A-D Converters (cont.)• Another type of A-D converter, based on different principle

is counting type A-D converter, shown in Fig. 5.32 and dual slope integrating type converter is shown in Fig. 5.34(a)

Fig. 5.32 Counting type A-D converter Fig. 5.34(a) Dual slope integrating type A-D converter

5.5.2 D-A Converters• Whenever digital signals have

to be converted to analog, the switching defining the digital signal value has to be changed to an equivalent voltage.

5.7 Data Transmission Elements• When the measured variables have to be transmitted over

long distances from the measuring points to a location for display/recording of data, data transmission elements are employed. Two categories are:

1. Land-line/cable type – transmitted by wires/pipes. Applied in process plants, power generating stations. Includes electrical, pneumatic and position type elements.

2. Radio-frequency (RF) type – transmitted by radio waves. Used in aerospace systems.

5.7.1 Electrical-Type Data Transmission Elements

• The input measured variable (motion signal) is made to change an electrical quantity, the effect of which is transmitted by wires to the receiving ends, for record/display.

Fig. 5.36 Data transmission by change of electric quantity

5.7.2 Pneumatic-Type Data Transmission Elements• Land-line type, uses the flapper-nozzle arrangement.• Examples include pneumatic flow transmitter and force-

balance type pneumatic transmitter.

5.7.3 Position-Type Data Transmission Elements

• Motion signal (rotation of a pointer) is transmitted over long distances, by use of synchros.

5.7.4 Radio-Frequency Transmission Elements

• Uses radio-frequency waves for data transmission and no wires/cables required.

• In large systems, a number of input signals may be transmitted by such units.

Fig. 5.41 RF telemetry for data transmission