To choose the instrument, most suited to a particular measurement application, we have to know the system characteristics and to have a clear understanding of all the parameters involved in defining the characteristics of the measurement device..
The performance characteristics of instruments and measurement systems can be divided into two distinct categories:
i)Static characteristics
ii)Dynamic characteristics
In this article we will be discussed about Dynamic characteristics . Refer the following URL for the static characteristics http://instrmentationtechnics.blogspot.in/2016/09/static-characteristics-of-instrument_15.html
The performance characteristics of instruments and measurement systems can be divided into two distinct categories:
i)Static characteristics
ii)Dynamic characteristics
In this article we will be discussed about Dynamic characteristics . Refer the following URL for the static characteristics http://instrmentationtechnics.blogspot.in/2016/09/static-characteristics-of-instrument_15.html
ii)Dynamic characteristics:
Instruments rarely respond to the instantaneous changes in the measured variables.Their response is slow or sluggish due to mass, thermal capacitance, electrical capacitance, inductance etc. sometimes, even the instrument has to wait for some time till, the response occurs. These type of instruments are normally used for the measurement of quantities that fluctuate with time.
The behavior of such a system, where as the input varies from instant to instant, the output also varies from instant to instant is called as dynamic response of the system. Hence, the dynamic behavior of the system is also important as the static behavior.
The dynamic inputs are of two types:
a) Transient
b) Steady state periodic
a) Transient:
Transient response is defined as that part of the response which goes to zero as the time becomes large.
b) Steady state periodic:
The steady state response is the response that has a definite periodic cycle.
The variations in the input, that are used practically to achieve dynamic behaviour are:
Step input:-
The input is subjected to a finite and instantaneous change. E.g.: closing of switch.
Ramp input:-
The input linearly changes with respect to time.
Parabolic input:-
The input varies to the square of time. This represents constant acceleration.
Sinusoidal input:-
The input changes in accordance with a sinusoidal function of constant amplitude.
The various static characteristics are:
i) Speed of response
ii) Measuring lag
iii) Fidelity
iv) Dynamic error
v) Bandwidth
vi) Time constant
vii) Settling time
i) Speed of response:
It refers to its ability to respond to sudden changes of amplitude of input signal. It is usually specified as the time taken by the system to come close to steady state conditions, for a step input function. Hence the speed of response is evaluated from the knowledge of the system performance under transient conditions and terms such as time constant, measurement lag, settling time and dead time dynamic range are used to convey the response of the variety of systems, encountered in practice.
(or simply)
It is defined as the rapidity with which a measurement system responds to changes in the measured quantity.
ii) Measuring lag:
It is the retardation or delay in the response of a measurement system to changes in the measured quantity. The measuring lags are of two types:
a) Retardation type: In this case the response of the measurement system begins immediately after the change in measured quantity has occurred.
b) Time delay lag: In this case the response of the measurement system begins after a dead time after the application of the input.
iii)Fidelity:
It is defined as the degree to which a measurement system indicates changes in the measurand quantity without dynamic error.
iv) Dynamic error:
It is the difference of true value of the quantity changing with the time the value indicated by the instrument provided static error is zero. Total dynamic error is the phase difference between input and output of the measurement system.
(or simply)
It is defined as the degree to which a measurement system is capable of faithfully reproducing the changes in input, without any dynamic error.
v) Bandwidth:
It is the range of frequencies for which its dynamic sensitivity is satisfactory.For measuring systems, the dynamic sensitivity is required to be within 2% of its statics sensitivity.
vi) Time constant:
It is associated with the behaviour of a first order system and is defined as the time taken by the system to reach 0.632 times its final output signal amplitude. System having small time constant attains its final output amplitude earlier than the one with larger time constant and therefore, has higher speed of response.
vii) Settling time:
It is the time required by the instrument or measurement system to settle down to its final steady state position after the application of the input. Fo portable instruments, it is the time taken by the pointer to come to rest within - 0.3% to +0.3% of its final scale length while for panel type instruments, it is the time taken by the pointer to come to rest within -1% to +1% of its final scale length. Smaller settling time indicates highest speed of response.Settling time is also dependent on the system parameters and varies with the condition under which the system operates.
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