Lecture 2 _Basic_Definitions_Concepts.pdf

Introduction to Engineering
Experimentation, Third Edition
M. Ghommem, Spring 2023, 10:21 AM -- 1--
Spring 2023
Basic Definitions and Concepts
Engineering Measurements
(MCE 311)
Dr. Mehdi Ghommem

The sense
➢ To sense: to perceive, to become aware of the
magnitude of a certain quantity.
➢ The human body has many highly developed senses
which allows us to perceive the world around us.
Sight, smell, touch, hearing, taste.
➢ The human senses are limited in their capacity to obtain
and store precise mechanical quantities. For this reason
we have developed “measuring devices”.

➢ Sensor: A device for detecting, or measuring
physical phenomena
➢ Transducer: A device that converts a quantity
from one form to another (The output is
usually an electrical signal)
Tool of measurement

The process or the act of measurement consists of
obtaining a quantitative comparison between a
predefined standard and a measurand.
Measurand
Input
Standard
Result
Readout
Measurement Process

➢ The word measurand is used to designate the
particular physical parameter being observed and
quantified; that is, the input quantity to the measuring
process. The act of measurement produces a result.
➢ The result has a meaning only when it is compared
with other results obtained by the same device using
inputs of a known value.
➢ There are two types of comparison: Direct
comparison and Indirect comparison.
Measurement Process

Direct Comparison:
➢ Involves a direct comparison with a standard.
➢ Used when the accuracy required is within the
limits of the human senses.
➢ Ex: - Ruler or tape measure
- Tuning a musical instrument

Indirect Comparison:
➢ Takes use of some form of transducing device which
converts a basic form of input into an analogous form and
presents the output as a known function of the original
input
➢ The output is easily understood by human senses.
➢ Ex: Human senses are not designed to detect the strain in
a metal bar as a force is being applied. A measuring
instrument known as a strain gauge can be used in
conjunction with a digital multimeter to provide a digital
representation of the strain that our senses understand.

Process
Inputs Output
Controllable
variables
Perturbations
(assumptions)
…
…
v1 v2 vn
p1 p2 pn
x y
Measurement Process

Most measuring systems fall within the framework of a
general arrangement consisting of three phases or
stages:
Stage 1: A detection-transduction, or sensor-
transducer, stage.
Stage 2: An intermediate stage, the signal-
conditioning stage.
Stage 3: A terminating, or readout recording, stage.
Measurement Process

M. Ghommem, Spring 2023, 10:21 AM -- 10-
-
1 2
3
Measurement Process

-
Stage 1: Must detect the measurand while being insensitive
to every other input. Unwanted inputs are called noise or
drift.
Stage 2: Modifies the information from the first stage so that
it is acceptable to the third, or terminating stage. It may
also perform basic operations such as filtering, noise
removal, integration or differentiation.
Stage 3: Provides the information sought in a form
comprehensible to one of the human senses or to a
controller.
If the output is intended for human recognition, it is almost
always presented as a relative displacement or a digital
readout.
Measurement Process

-
Example
Stage 1
Piston/Cylinder: Pressure to force
Fall: Force to displacement
Stage 2 No signal conditioning
Stage 3 Readout: Scale and index

-
Example
Identify the generalized stages in the instrument shown

-
Types of sensors
➢ Passive sensors: They are sensors in which the
output energy is supplied by the input signal. They do
not require an external source of energy to work.
Diaphragm pressure gauge

-
➢ Active sensors: They are sensors that require an
auxiliary source of energy to function.

-
➢ Deflection Type devices: The measured quantity
produces some physical effect that engenders a
similar but opposing effect in some part of the
instrument

-
➢ Null-Type Devices Attempts to maintain deflection at zero
by the suitable application of an effect opposing the one
generated by the measured quantity.
Advantages:
• More sensitive
• More accurate
• No calibration
Disadvantage:
• Poor for dynamic inputs

-
➢ Analog sensors: They are sensors that produce an
analog output signal. A signal is said to be analog if it
has values at every instant. It is a continuous signal. An
analog sensor produces a continuously varying output
value over its range of measurement
http://www.robotplatform.com/knowledge/sensors/robot_sensors.html

-
➢ Digital sensors: They are sensors that produce a digital
(discrete) signal. A discrete signal is a signal that is
known only at discrete points in time. Digital sensors
have only two states often called "on" and "off."
http://www.robotplatform.com/knowledge/sensors/robot_sensors.html

-
Analog vs. digital signal

-
Accelerometer
(analog sensor)
Optical encoder
(digital sensor)
Position sensors Output
Examples of analog and digital sensors

-
Pressure sensors
Analog sensor Digital sensor
• Digital sensors can be easily interfaced with a digital computer.
• Analog sensors need an Analog-to-Digital Converter (ADC or
A/D) if we need to interface with a digital computer.

-
Types of Input Quantities
Static input: Constant in time.
Dynamic input: Varying in time.
a. Steady-state periodic
b. Non-repetitive or transient
i. Single pulse or aperiodic
ii. Continuing or random

-
Terms used in measurements
✓ Error = true value –measured value
✓ Absolute Error = absolute of the (true-
measured)
✓ Relative Error = (absolute error)/(true
value)
✓ Accuracy = 100 –Relative Error [%]

-
Terms used in measurements
✓ Systematic Error = consistent, repeatable
error
✓ Random Error = caused by a lack of
repeatability
✓ Random Error = reading – average of
readings
✓ Systematic Error = average of readings –
true value

-
⚫ The calibration of a measuring device
involves feeding known magnitudes of the
input quantity into the sensor-transducer and
observing (recording) the system’s output
behavior.
⚫ This exercise results in the production of a
calibration curve which allows the user to
determine the value of a measurand using
the output of the measuring device.
Sensor calibration:

-
Static calibration:
When the calibration procedure is applied to
static signals.
Dynamic calibration:
The calibration procedure is applied to time-
dependent signals. Many different approaches
are possible.
Sensitivity:
It is the slope of the calibration curve

-
Example of a calibration curve

-
➢ If the output is exactly proportional to the input (output =
constant x input), then a single simultaneous observation of
the input and the output will determine the constant of
proportionality or the sensitivity of the sensor. This is called
a single-point calibration.
Single Point Calibration
Displacement = 2 x Pressure
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0.00 1.00 2.00 3.00 4.00 5.00 6.00
Pressure [MPa]
Displacement
[mm]
Single
Observation

-
More often, however, Multipoint calibration is used and a
number of different input values are applied. Multipoint
calibration is needed when the output is not simply
proportional (i.e. non-linear), and, more generally,
improves the accuracy of the calibration.
Multipoint Calibration
Displacement = 2.07 x Pressure - 0.019
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0.00 1.00 2.00 3.00 4.00 5.00 6.00
Pressure [MPa]
Displacement
[mm]
Multiple
Observations

-
Non linear calibration curve

Lecture 2 _Basic_Definitions_Concepts.pdf

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