Page 812 - Basic Electrical Engineering

of inputs repeat cyclically. The output of a measurement system may be
distorted, and hence may not be a true reflection of the input in terms of
magnitude and phase relationship. The difference between the true value

(actual value) of the quantity which is varying with time and the value
indicated by the measurement system is called the dynamic error of

measurement. The rapidity of response of a measurement system is its speed
of response. The delay or lag in response of a measurement system to the

changes in the input quantity being measured is called measuring lag.
In dynamic analysis, we determine the characteristics like measuring lag,

speed of response, dynamic error, etc.
For the purpose of analysis of a system, equations representing the system
are written first. System response is studied by applying different kinds of

test signals (like step input signal, ramp input signal, etc.) either in time
domain or in frequency domain. In time-domain analysis a time-varying test

input signal is applied to the system and the output behaviour of the
measurement system is studied as a function of time. In frequency response

analysis, the behaviour of the system is studied by applying sinusoidal input,
as a function of frequency.

11.7 CLASSIFICATION OF THE INSTRUMENT SYSTEM

Measuring instruments can be classified into separate categories on the basis
of different criteria. These classifications are useful in knowing the

characteristics of instruments and their selection for a particular use.
Instruments are classified as primary or absolute instruments, secondary or
derived instruments, null-type and deflection-type instruments, indicating

instruments, integrating instruments, recording instruments, analog and
digital instruments, monitoring and control instruments, electromechanical

and electronic instruments, etc. These are discussed in brief as follows.

11.7.1 Active and Passive Instruments

In active instruments, the measurement output is entirely produced by the

quantity being measured. In passive instruments, the quantity being measured