asg 6 data acquisition detection

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6.2 Electromechanical limit switches
6.3 Inductive proximity detectors
6 - Data acquisition:
detection
v Slow break contact (C Fig
.5)
This operating mode features:
-
non-distinct action and release points,
- mobile contact speed equal or proportional to the control device
speed (which should be no less than 0.1m/s = 6m/min). Below this,
the contacts open too slowly, which is not good for the electrical
performance of the contact (risk of an arc maintained for too long),
- an opening distance also dependent on the control device stroke.
The design of these contacts sets them naturally in positive opening
operation mode: the push-button acts directly on the mobile contacts.
6.3 Inductive proximity detectors
The physical principles of these detectors imply that they only work on
metal substances.
b Principle
The sensitive component is an inductive circuit (L inductance coil). This
circuit is linked to a C capacitor to form a circuit resonating at frequency
Fo usually ranging from 100kHz to 1MHz.
An electronic circuit maintains the oscillations of the system based on the
formula below:
These oscillations create an alternating magnetic field in front of the coil.
A metal shield set in the field is the seat of eddy currents which induce an
extra load and alter the oscillation conditions
(C Fig.6).
The presence of a metal object in front of the detector lowers the quality
factor of the resonant circuit.
Case 1, no metal shield:
Reminder:
Case 2, with metal shield:
Detection is done by measuring variation in the quality factor (approx. 3%
to 20% of the detection threshold).
The appr
oach of the metal shield causes the quality factor to drop and
ther
eby a dr
op in the oscillation range.
The detection distance depends on the nature of the metal to detect.
134
A Fig. 5 Positions of a slow break contact
A Fig. 6 Operating principle of an inductive
detector

6.3 Inductive proximity detectors
6 - Data acquisition:
detection
b Description of an inductive detector (C Fig.7)
Transducer: this consists of a stranded copper coil (Litz wire) inside a half
ferrite pot which directs the line of force to the front of the detector.
Oscillator: there are many kinds of oscillators, including the fixed negative
r
esistance oscillator –R, equal in absolute value to the parallel resistance
pR of the circuit oscillating at the rated range:
- if the object to detect is beyond the rated range,
l
Rp
l
>
l
-R
l
, oscillation
is maintained,
- otherwise, if the object to detect is within the rated range,
l
Rp
l
<
l
-R
l
,
oscillation is no longer maintained and the oscillator is locked.
Shaping stage: this consists of a peak detector monitor
ed by a two-
threshold comparator (Trigger) to prevent untimely switching when the
object to detect nears the rated range. It creates what is known as
detector hysteresis
(C Fig.7bis).
Power input and output stages: this powers the detector over wide voltage
ranges (10VDC to 264VAC). The output stage controls loads of 0.2A in
DC to 0.5A in AC, with or without short-circuit protection.
b Inductive detection influence quantities
Inductive detection devices are particularly affected by certain factors,
including:
- detection distance,
- this depends on the extent of the detection surface,
- rated range (on mild steel) varies from 0.8mm (detector of ø 4)
to 60mm (detector of 80 x 80),
- hysteresis: differential travel (2 to 10% of Sn) to prevent switching
bounce,
- frequency with which objects pass in front of the detector, called
switching (maximum current 5kHz).
b Specific functions
• Detectors protected against magnetic fields generated by welding
machines.
• Detectors with analogue output.
• Detectors with a correction factor of 1* where the detection distance is
independent of the ferrous or non-ferrous metal detected.
• Detectors to select ferrous and non-ferrous metals.
• Detectors to control rotation: these under-speed detectors react to the
frequency of metal objects.
• Detectors for explosive atmospheres (NAMUR standards).
*When the object to detect is not made of steel, the detection distance of the detector
should be proportional to the correction factor of the substance the object is made of.
D
Mat
X
= D
Steel
x K
Mat X
Typical correction factor values (KMat X) are:
- Steel = 1 -
- Stainless steel = 0.7
- Brass = 0.4
- Aluminium = 0.3
- Copper = 0.2
Example: D
Stainless
= D
Steel
x 0.7
135
6
A Fig. 7 Diagram of an inductive detector
A Fig. 7bis Detector hysteresis

6.4 Capacitive proximity detectors
6 - Data acquisition:
detection
6.4 Capacitive proximity detectors
This technology is used to detect all types of conductive and isolating
substances such as glass, oil, wood, plastic, etc.
b Principle
The sensitive surface of the detector constitutes the armature of a
capacitor.
A sinusoidal voltage is applied to this surface to create an alternating
electric field in front of the detector.
Given that this voltage is factored in relation to a reference potential (such
as an earth), a second armature is constituted by an electrode linked to
the r
eference potential (such as a machine housing).
The electrodes facing each other constitute a capacitor with a capacity of:
where
ε
0
= 8,854187.10
-12
F/m permittivity of vacuum and ε
r
relative
permittivity of substance between the 2 electrodes.
Case 1: No object between electrodes (C Fig.8)
Case 2: Isolating substance between electrodes (C Fig.9)
=> (ε
r
= 4)
In this case, the earth electrode could be, e.g. the metal belt of a
conveyor.
When mean
ε
r
exceeds 1 in the presence of an object, C increases.
Measurement of the increase in the value of C is used to detect the
presence of the isolating object.
Case 3: Pr
esence of a conductive object between electrodes
(C Fig
.10)
where
ε
r
1 (air) =>
The presence of a metal object also causes the value of C to increase.
b Types of capacitive detectors
v Capacitive detectors with no earth electrode
These work directly on the principle described above.
A path to an earth (reference potential) is required for detection.
They are used to detect conductive substances (metal, water) at great
distances.
Typical application: Detection of conductive substances through an
isolating substance
(C Fig.11).
136
A Fig. 8 No object between electrodes
A Fig.9 Presence of an isolating object between
electrodes
A Fig. 10 Presence of a conductive object
between electrodes
A Fig. 11 Detection of water in a glass or plastic
recipient

6.4 Capacitive proximity detectors
6 - Data acquisition:
detection
v Capacitive detectors with earth electrode
It is not always possible to find a path to an earth. This is so when the
empty isolating container described above has to be detected.
The solution is to incorporate an earth electrode into the detection
surface.
This creates an electric field independent of an earth path
(C Fig.12).
Application: detection of all substances.
Ability to detect isolating or conducting substances behind an isolating
barrier, e.g.: cereals in a cardboard box.
b Influence quantities of a capacitive detector
The sensitivity of capacitive detectors, accor
ding to the above-mentioned
basic equation, depends on the object–sensor distance and the object’s
substance.
v Detection distance
This is related to the dielectric constant or relative permittivity of the object’s
substance.
To detect a wide variety of substances, capacitive sensors usually have a
potentiometer to adjust their sensitivity.
v Substances
The table (C Fig.13) gives the dielectric constants of a number of
substances.
137
6
Substance ε
r
Acetone 19.5
Air 1.000264
Ammonia 15-25
Ethanol 24
Flour 2.5-3
Glass 3.7-10
Glycerine 47
Mica 5.7-6.7
Paper 1.6-2.6
Nylon 4-5
Petroleum 2.0-2.2
Silicone varnish 2.8-3.3
Polypropylene 2.0-2.2
Por
celain
5-7
Dried milk 3.5-4
Salt
6
Sugar 3.0
Water 80
Dry wood 2-6
Green wood 10-30
A Fig
.
13
Dielectric constants of a number of
substances
A Fig. 12 Principle of a capacitive detector with
earth electrode

6.5 Photoelectric detectors
6 - Data acquisition:
detection
6.5 Photoelectric detectors
These work on a principle suiting them to the detection of all types of
object, be they opaque, reflective or virtually transparent. They are also
used for human detection (door or safety barrier opening).
b Principle (C Fig
.14)
A light-emitting diode (LED) emits luminous pulses, usually in the close
infrared spectrum (850 to 950nm).
The light is received or otherwise by a photodiode or phototransistor
according to whether the object to detect is present or not.
The photoelectric current created is amplified and compared to a
r
eference threshold to give discrete information.
b Detection system
v Through-beam (C Fig.14bis)
The emitter and receiver are in separate housings.
The emitter, a LED in the cell of a converging lens, creates a parallel light
beam.
The receiver, a photodiode (or phototransistor) in the cell of a converging
lens, supplies a current proportional to the energy received.
The system issues discrete information depending on the presence or
absence of an object in the beam.
Advantage: The detection distance (range) can be long (up to 50m or
more); it depends on the lens and hence detector size.
Disadvantages: 2 separate housings and therefore 2 separate power
supplies.
Alignment for detection distances exceeding 10m can be problematic.
v Reflex systems
There are two so-called Reflex systems: standard and polarised.
• Standard reflex (C Fig.15)
The light beam is usually in the close infrared spectrum (850 to 950nm).
Advantages: the emitter and receiver are in the same housing (a single
power supply). The detection distance (range) is still long, though less
than the through-beam (up to 20m).
Disadvantage: a reflective object (window, car body, etc.) may be
interpr
eted as a r
eflector and not detected.
• Polarised reflex (C Fig.16)
The light beam used is usually in the red range (660 nm).
The emitted radiation is vertically polarised by a linear polarising filter
. The
reflector changes the state of light polarisation, so part of the radiation
r
etur
ned has a horizontal component. The r
eceiving linear polarising filter
lets this component through and the light reaches the receiver.
Unlike the reflector, a reflective object (mirror, sheet metal, glazing) does
not alter the state of polarisation so the light it reflects cannot reach the
receiving polariser
(C Fig.17).
Advantage: this type of detector overcomes the drawback of the
standar
d r
eflex.
Disadvantages: this detector is mor
e expensive and its detection
distances are shorter:
IR reflex >15m
Polarised reflex > 8m
138
A Fig. 14 Principle of a photoelectric detector
A Fig.15 Principle of photoelectric reflex
detection
A Fig. 16 Principle of polarised photoelectric
reflex detection
A Fig. 17 Polarised reflex system: principle of
non-detection of reflecting objects
A Fig
. 14bis
Through-beam detection

6.5 Photoelectric detectors
6 - Data acquisition:
detection
v Direct reflection (on the object)
• Standard direct reflection (C Fig.18)
This system is based on the reflection of the object to detect.
Advantage: no need for a reflector.
Disadvantages: the detection distance is very short (up to 2m). It also
varies with the colour of the object to “see” and the background behind it
(at a given setting, the distance is greater for a white object than a grey or
black one); a background which is lighter than the object to detect can
make detection impossible.
• Direct reflection with background suppression (C Fig.19)
This detection system uses triangulation.
The detection distance (up to 2m) does not depend on the reflectivity of
the object but on its position, so a light object is detected at the same
distance as a dark one and a background beyond the detection range will
be ignor
ed.
v Optic fibres
• Principle
The principle of light wave propagation in fibre optics is based on total
internal reflection.
Internal reflection is total when a light ray passes from one medium to
another with a lower refractive index. The light is reflected in totality
(C Fig. 20) with no loss when the angle of incidence of the light ray is
greater than the critical angle [
θ
c
].
Total internal reflection is governed by two factors: the refraction index of
each medium and the critical angle.
These factors are related by the following equation:
If we know the refractive indexes of the two interface substances, the
critical angle is easy to calculate.
Physics defines the refractive index of a substance as the ratio of the
speed of light in a vacuum (c) to its speed in the substance (v).
The index of air is considered as equal to that of a vacuum 1, since the
speed of light in air is almost equal to that in a vacuum.
Ther
e ar
e two types of optic fibr
es: multimode and single-mode.
• There are two types of optic fibres: multimode and single-mode
(C Fig
.21)
- Multimode
These are fibres where the diameter of the core, which conducts light, is
l
arge compared to the wavelength used (
φ 9 to 125
µm, L
o
= 0.5 to 1 mm).
T
wo types of pr
opagation ar
e used in these fibr
es: step index and graded
index.
- Single-mode
By contrast, these fibr
es have a very small diameter in comparison to the
wavelength used (
φ <= 1 µm, L
o
= usualy 1.5 µm). They use step-index
propagation. They are mostly used for telecommunication.
This explanation illustrates the care that has to be taken with these fibres
when, for example, they ar
e pulled (r
educed tensile str
ength and moderate
radii of curvature, according to manufacturers’ specifications).
Multimode optical fibres are the most widely used in industry, as they have
the advantage of being electr
omagnetically r
obust (ECM – Electr
oMagnetic
Compatibility) and easy to implement.
139
6
A Fig. 18 Principle of standard direct
photoelectric detection
A Fig. 19 Principle of direct photoelectric
detection with background suppression
A Fig. 20 Principle of light wave propagation in
fibre optics
A Fig. 21 Types of optic fibr
es

6.5 Photoelectric detectors
6.6 Ultrasonic detectors
6 - Data acquisition:
detection
140
• Detector technology
The optic fibr
es are positioned in front of the emitting LED and in front of
the r
eceiving photodiode or phototransistor
(C Fig
.22)
.
This arrangement is used to:
- position electronic components away from the monitoring point,
-
operate in confined areas or at high temperature,
- detect very small objects (of around 1mm),
- depending on the configuration of the fibre ends, operate in through-
beam or proximity mode,
Note that extreme care must be taken with the connections between the
emitting LED or receiving phototransistor and the optic fibre to minimise
light signal losses.
b Influence quantities in detection by photoelectric systems
A number of factors can influence the performance of these detection
systems.
Some have been mentioned already:
- distance (detector-object),
-
type of object to detect (diffusing, reflective or transparent substance,
colour and size),
- environment (light conditions, background, etc.).
6.6 Ultrasonic detectors
b Principle
Ultrasonic waves are produced electrically with an electroacoustic
transducer (piezoelectric effect) supplied with electrical energy which it
converted into mechanical vibrations by piezoelectricity or
magnetostriction phenomena
(C Fig. 23).
The principle involves measuring the time it takes for the acoustic wave to
pr
opagate between the sensor and the target.
The speed of propagation is 340m/s in air at 20°C, e.g. for 1m the measuring
time is about 3ms.
This time is measured by the counter built in a microcontroller.
The advantage of ultrasonic sensors is that they can work over long distances
(up to 10m) and, above all, detect any object which r
eflects sound, r
egar
dless
of its shape or colour
.
b Application (C Fig.24)
Excited by the high-voltage generator, the transducer (emitter-receiver),
generates a pulsed ultrasonic wave (100 to 500kHz, depending on the
product) which travels through the ambient air at the speed of sound.
As soon as the wave meets an object, a r
eflected wave (echo) returns to the
transducer. A microprocessor analyses the incoming signal and measures
the time interval between the emitted signal and the echo.
By comparing it with preset or ascertained times, it determines and monitors
the status of the outputs. If we know the speed at which sound is
propagated, we can calculate a distance using the following formula:
D = T.Vs/2 where
D: distance between detector and object,
T: time elapsed between mission and reception of the wave,
Ss: speed of sound (300m /s).
The output stage monitors a static switch (PNP or NPN transistor)
corresponding to an opening or closing contact, or provides an analogue
signal (current or voltage) directly or inversely proportional to the measured
distance of the object.
A Fig. 22 Principle of an optic fibre detector
A Fig
.23
Principle of an electroacoustic
transducer
A Fig. 24 Principle of an ultrasonic detector

6.6 Ultrasonic detectors
6 - Data acquisition:
detection
b Specific featur
es of ultrasonic sensors
v Definitions (C Fig
.25)
Blind zone: zone between the sensing face of the detector and the
minimum range where no object can be reliably detected. It is impossible
to detect objects correctly in this zone.
Objects should never be allowed thr
ough the blind zone when the detector
is operating as this could make the outputs unstable.
Detection zone: the area within which the detector is sensitive.
Depending on the model, this zone can be adjustable or fixed with an
or
dinary push button.
Influence quantities: ultrasonic detectors are especially suitable for
detecting hard objects with a flat surface perpendicular to the detection
axis.
However, there are a number of factors that can disrupt ultrasonic
detector operation:
- Sudden strong draughts can accelerate or divert the acoustic wave
emitted by the object (part ejected by air jet).
-
Steep temperature gradients in the detection field. When an object
gives off a lot of heat, this creates differing temperature zones which
alter the wave propagation time and prevent reliable detection.
- Sound-absorbing materials. Materials such as cotton, cloth and
rubber absorb sound; the ‘’reflex’’ detection mode is advised for
products made of these.
- The angle between the front of the target object and the detector’s
reference axis. When this angle is other than 90°, the wave is not
reflected in the detector axis and the working range is reduced.
The greater the distance between the object and the detector, the more
apparent this effect is. Beyond ± 10°, detection becomes impossible.
- The shape of the object to detect. Owing to the above-mentioned
factor, very angular objects are more difficult to detect.
v Operating mode (C Fig.26)
• Diffuse mode: a single detector emits the sound wave and senses it
after it has been reflected by an object.
In this case, it is the object that reflects the wave.
• Reflex mode: a single detector emits the sound wave and receives it
after r
eflection by a reflector, so the detector is permanently active. In this
case, the reflector is a flat, rigid part, such as a part of the machine. The
object is detected when the wave is br
oken. This mode is especially
suited to detecting absorbent substances or angular objects.
• Through-beam mode: the through-beam system consists of two separate
products, an ultrasonic emitter and a receiver, set opposite each other.
b Advantages of ultrasonic detection
- No physical contact with the object, so no wear and ability to detect
fragile or fr
eshly-painted objects.
- Any substance, regardless of its colour, can be detected at the same
range with no adjustment or correction factor.
- Static devices: no moving parts inside the detector, so its lifetime is
unaffected by the number of operating cycles.
- Good resistance to industrial environments: vibration- and impact-
r
esistant devices, devices r
esistant to damp and dusty envir
onments.
-
Lear
ning function by pr
essing a button to define the working detection
field. The minimum and maximum ranges are learnt (very accurate
suppression of background and foreground to ± 6mm).
141
6
A Fig. 25 Working limits of an ultrasonic detector
A Fig
.
26
Uses of ultrasonic detection. a/ In
proximity or diffuse mode, b/ In r
eflex
mode

6.7 RFID -Radio Frequency IDentification- detection
6 - Data acquisition:
detection
6.7 RFID -Radio Frequency IDentification- detection
This section describes devices that use a radio frequency signal to store
and use data in electronic tags.
b Overview
Radio Frequency IDentification (RFID) is a fairly recent automatic identification
technology designed for applications requiring the tracking of objects or
persons (traceability, access control, sorting, storage).
It works on the principle of linking each object to a remotely accessible
read/write storage capacity.
The data ar
e stored in a memory accessed via a simple radio frequency
link requiring no contact or field of vision, at a distance ranging from a
few cm to several metres. This memory takes the form of an electronic
tag, otherwise known as a transponder (TRANSmitter + resPONDER),
containing an electronic circuit and an antenna.
b Operating principles
A RFID system consists of the following components (C Fig.27 and 28):
- An electr
onic tag,
- A read/write station (or RFID reader).
v The reader
Modulates the amplitude of the field radiated by its antenna to transmit
read or write commands to the tag processing logic. Simultaneously, the
electromagnetic field generated by its antenna powers the electronic
circuit in the tag.
v Tag
This feeds back its information to the reader antenna by modulating its own
consumption. The reader reception circuit detects the modulation and
converts it into digital signals
(C Fig.29).
b Description of components
v Electr
onic tags
Electronic tags consist of three main components inside a casing.
• Antenna (C Fig.30):
This must be adjusted to the frequency of the carrier and so can take
several forms:
- coil of copper wire, with or without a ferrite core (channelling of field
lines), or etched on a flexible or rigid printed circuit, or printed (with
conductive ink) for fr
equencies of less than 20MHz;
- dipole etched onto a printed circuit, or printed (with conductive ink) for
very high frequencies (>800MHz).
142
A Fig. 28 View of components in a RFID system
(Telemecanique Inductel system)
A Fig. 30 Inside of an RFID tag
A Fig
.
29
Operation of a RFID system
A Fig. 27 Layout of a RFID system

6.7 RFID -Radio Frequency IDentification- detection
6 - Data acquisition:
detection
• Logical processing circuit
This acts as an interface between the commands r
eceived by the antenna
and the memory
.
Its complexity depends on the application and can range from simple
shaping to the use of a microcontroller (e.g. payment cards secured by
encryption algorithms).
• Memory
Several types of memory are used to store data in electronic tags (C Fig.31).
“Active” tags contain a battery to power their electronic components. This
configuration increases the dialogue distance between the tag and the antenna
but requires regular replacement of the battery.
v Casing
Casings have been designed for each type of application to group and
protect the three active components of a tag:
(C Fig.32a)
- credit card in badge format to control human access,
- adhesive support for identification of library books,
- glass tube, for identification of pets (injected under the skin with a
syringe),
- plastic “buttons”, for identification of clothing and laundry,
-
label for mail tracking.
There are many other formats, including: key ring, plastic “nails” to
identify wooden pallets, shockproof and chemical-resistant casings for
industrial applications (surface tr
eatment, fur
naces, etc.)
(C Fig
.32b)
.
v Stations
A station (C Fig
.33a)
acts as an interface between the contr
ol system (PLC,
computer, etc.) and the electronic tag via an appropriate communication
port (RS232, RS485, Ethernet, etc.).
It can also include a number of auxiliary functions suited to the particular
application:
-
discr
ete inputs/outputs,
- local processing for standalone operation,
- control of several antennas,
- detection with built-in antenna for a compact system
(C Fig.33b).
143
6
A Fig. 31 Storage capacities range from a few bytes to several dozen kilobytes
A Fig. 32 a et b a - RFID formats designed for
different uses
b - RFID industrial
(Telemecanique Inductel)
A Fig. 33a Diagram of a RFID reader
A Fig. 33b Photo of a RFID reader (Telemecanique
Inductel Station)
Type Advantages Disadvantages
ROM • Good resistance to high temperatures • Read only
• Inexpensive
EEPROM • No battery or backup battery • Fairly long read/write access time
• Number of write operations limited to 100,000 cycles per byte
RAM • Fast data access • Need for backup battery built into tag
• High capacity
FeRAM • Fast data access • Number of write operations limited to 10
12
(ferroelectric)
• No battery or backup battery
• High capacity
a b