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SISTEM INTRUMENTASITEL 303
LECTURE 2: PERFORMANCE OF STATIC CHARACTERISTICS IN MEASUREMENT
AND INSTRUMENTATION
Mohammad Iqbal
Teknik Elektro, Universitas Muria Kudus
2
Contents
Definition of static Requirement of static characteristics
preservation Static calibration Passive & active sensors
3
Definition of Static
The steady state relationship between input and output of an instrument
Measurement of quantities that are constant or vary quite slowly with respect to time.
It does not involve differential equations. All the static performance characteristics are
obtained by one form or another via a process called static calibration.
4
Contents
Definition of static
Requirement of static characteristics preservation
Static calibration Passive & active sensors
5
Requirement of static characteristics preservation
Precise & Accurate– Measurements that are close to each other
are precise – Measurements that are close to the correct
value are accurate– Measurements can be:
• Precise but inaccurate• Neither precise nor accurate • Precise and accurate
6
Examples
(a) Low precision,
low accuracy
(b) Precise not accurate
(c) Precise and accurate
Three industrial robots were programmed to place componentsat a particular point on a table. The target point was the centerof a circle shown below. The results are:
static characteristics
Accuracy– a measure of how close the output reading of the
instrument is to the correct value
7
02040
20 40
Ideal device
Accuracy at % of FS
Value of measurand
Out
put o
f de
vice
static characteristics
Precision– a term that describes an instrument’s degree of
freedom from random errors– If a large number of readings are taken of the
same quantity by a high precision instrument, then the spread of readings will be very small
JIM07 - FKM - UTeM 8
static characteristics
9
LinearityIt is highly desirable that the measurement system has alinier relationship between input and output means thatthe change in output is proportional to the change in the value of the measurandDeviation from true linearity is called linearity error
10
static characteristics
Linearity– The input and output relationship of a linear transducer can
be represented by the following equation:
– where y is the output of transducer, x is the input of transducer, m is the slope of curve (transfer function), c is the offset.
– Often, the straight line approach is used for certain range of operation for a non-linear system.
y = mx + c
11
static characteristics
Hysteresis – Hysteresis results in predictable error. May be due
to internal friction, freeplay or looseness in the mechanism of an instrument.
– Also in electrical phenomena (relation between the output voltage and the input field current in a d.c. generator) - the effect is due to magnetic hysteresis of the iron in the field coils.
– The transfer functions differ with the increase and decrease of inputs as shown in the following figure.
12
Gen
eral
ized
gr
aph
of
ou
tpu
t/in
pu
t re
lati
onsh
ip
wh
ere
hys
tere
sis
is p
rese
nt.
(F
rom
P. H
. Syd
enh
am,
Han
dboo
k of
M
easu
rem
ent
Sci
ence
, V
ol.
2,
Ch
ich
este
r, U
.K.,
Joh
n W
iley
& S
ons,
198
3)
Sensitivity
Sensitivity is the ratio of change in magnitude of the output to the change in magnitude of the measurand
– Sensitivity=(output)/(input)
13
Sensitivity and Non Linearity
14
15
Contents
Definition of static Requirement of static characteristics
preservation
Static calibration Passive & active sensors
16
Static Calibration
Imagine a situation in which all inputs (desired, interfering or modifying) except one are kept at some constant values.
The one input under study is varied over some range of constant values which causes the output(s) to vary over some range of constant values.
The input/output relationships developed in this way comprise a static calibration valid under the stated constant conditions of all other inputs.
The procedure may be repeated for other inputs for overall instrument static behaviour
Ultimate objective is to define measurement accuracy
17
Contents
Definition of static Requirement of static characteristics
preservation Static calibration
Passive & active sensors
18
Passive & Active Sensors
Sensors are often transducer in that they are devices that convert input energy of one form into output energy of another form.
Categorisation of sensors: depends on how they interact with the environment they are measuring– Passive sensors: they do not add energy as part of the
measurement process but may remove energy in their operation. E.g. thermocouple converts a physical temperature into a voltage signal
– Active sensors: they add energy to the measurement environment as part of the measurement process. E.g. in a radar or sonar system, the distance to some object is measured by actively sending out a radio (radar) or acoustic (sonar) wave to reflect off some object and measure its range from the sensor
19
EXERCISE
What is the meaning of the following words:– Measurand– Physical quantity– Data– Parameter– Transducer– Actuator
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ANSWER TO EXERCISE
– Measurand: Physical quantity being measured
– Physical quantity: Variable such as pressure, temperature, mass, length, etc
– Data: Information obtained from the instrumentation/measurement system as a result of the measurements made of the physical quantities
– Parameter: Physical quantity within defined (numeric) limits.
– Transducer: A device that converts one form of energy to another
– Actuator: Electronic transducer that converts electrical energy into mechanical energy
21
Next Lecture
Analysis on Experimental Data
End of Lecture 2