Characteristics of Measurement Systems Flashcards
Acquiring a measurement:
o Sense
o Acquire
o Analyse
o Express
8 Properties of sensors:
o Sensitivity o Resolution o Repeatability o Reproducibility o Dynamic Range o Non-linearity o Hysteresis o Response Time
o Repeatability
§ A measure of the closeness in agreement between successive
measurements (usually quantified by the standard deviation of the
measurements)
o Reproducibility
§ Repeatability assessed by a different user under different operating
conditions
o Dynamic Range
§ Ratio of a measurement range to resolution
5%)
o Non-linearity
§ Maximum deviation from linearity of input:output relationship
o Hysteresis
§ Condition when input:output relationship changes depending on
direction or range of the input variable
o Response Time
§ Time taken for the output of a sensor to reach within a certain range
of a new steady state (usually within
o Sensitivity
§ Ratio of output signal to input quantity
o Resolution
§ Minimum change in input quantity resolvable at the output
o Peak to Peak
§ Difference between max and min
o DC component
§ Mean of the signal
o Root-mean-square amplitude
§ Square root of the mean of the squared signal
o Relationship between AC, DC and RMS
§ AC Component
• The RMS of a signal that has had the DC component removed
is called the “AC” component of the signal, and is equivalent
to the standard deviation of the signal
§ The DC and AC components contribute to the total RMS of the signal
Power
o Total power
§ Time average of the square of the signal
x,power = x(t) ^2
§ Unit of power is (unit of x)2
o Thermal Noise
§ Caused by random thermal agitation in conductors
V= sqrt(m4kRT. f
§ Power spectrum: Uniformly distributed (“white”)
o 1/f Noise
§ Many origins, including fluctuating conductivity due to imperfect
contact between two materials
§ Power spectrum: 1/f
Shot Noise
§ Caused by statistical fluctuations in the movement of charge
§ Power spectrum: uniform
o Popcorn Noise
§ Caused by manufacturing defects, usually a metallic impurity, in the
junction of semiconductor device
§ Power density: 1/f^n, where n is usually 2
- Interference
and
o Methods of controlling:
o Interference is a kind of noise caused by events outside the measurement system. § Shielding § Grounding § Balancing § Filtering
Addition of Noise
v total ^2 =v1 ^2 + v2 ^2 + 2 gamma v1.v2
§ Where gamma is correlation coefficient; and = 0 if noise is uncorrelated
Accuracy
o Refers to the closeness of the result of a measurement to the “true value”
of the measurand.
o A qualitative concept and should not be used in a quantitative sense.
o Uncertainty
§ A parameter that characterizes the dispersion of the values that
could be reasonable associated with the measurand
§ Often quantified by the standard error in the mean:
u= omega/ root(n)
omega =stdv
Uncertainty in sensor measurements:
ux= sqrt( (u,y/y)^2 + u,s/s)^2) *X )
