Page 804 - Basic Electrical Engineering

Input voltage to the motor is changed automatically according o the
difference between the measured speed and the required speed. Amplifier, A
is used to amplify the signal.

Precision, accuracy, sensitivity, etc., are considered the most important
attributes of any measurement system. The more precise the instrument is, the

more expensive it becomes.
A measuring instrument provides information about the magnitude of a

certain variable quantity. In its simplest form it is a single unit which
provides readings on a graduated scale according to the magnitude of the

unknown quantity applied to it.

11.2 ANALOG AND DIGITAL INSTRUMENTS

An analog instrument provides output (measured value) continuously as the

quantity being measured changes. A deflection-type instrument as shown in
Fig. 11.3 is an example of an analog instrument. Here, the pressure of the

liquid inside a container is measured by recording the deflection of a pointer
on a calibrated scale as shown.

As the pressure of liquid changes due to heating, the pointer moves over a
graduated (calibrated) scale slowly and continuously indicating the pressure
of the liquid.

A digital instrument provides output readings in discrete steps only. For
example a digital instrument used for measurement of speed will read speed

in discrete numbers and not as fraction of a revolution. Digital instruments
have come into wide use due to the rapid growth of computers in control
operations of systems and production processes. Instruments whose output is

in digital form is suitable in computer-based control operations as the
instrument can be directly interfaced with the computer. However, analog

instruments can also be interfaced with computers by an A to D (analog to
digital) converter which will convert the analog output signal into digital

form for the computer to read and further process. Where control operations
must be fast as also the money spent on the instrumentation system has to be

kept low, digital instruments are preferred over analog instruments alongwith
A to D converters.