Introduction to Technical Mathematics

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Transcript Introduction to Technical Mathematics

03 - Actuators and Sensors
Introduction to Actuators and Sensors
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The intent of this presentation is to present enough information to provide the reader with a
fundamental knowledge of actuators and sensors used within Michelin and to better understand
basic system and equipment operations.
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Actuators and Sensors
This Presentation consists of six modules. The following is a brief description of the information
presented in each module.
Module 1 – Pushbuttons
Module 2 – Limit Switches
Module 3 – Inductive Proximity Detectors
Module 4 – Ultrasonic Proximity Detectors
Module 5 – Capacitive Proximity Detectors
Module 6 – Photo Detectors
Appendix – Terms and Definitions
Introduction to Actuators and Sensors
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Module 1: Push Buttons
Module 1
Push Buttons
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Module 1: Push Buttons
Pushbutton Control Stations
A pushbutton station is a device that can provide control of a motor with the pressing of the appropriate
pushbutton. With pushbutton stations a motor can be controlled from any one of several locations while
only using one magnetic starter. The start, forward, reverse, fast, slow and stop operations of a motor
may be controlled by pushbuttons.
Auxiliary pushbuttons are used to control the motor from remote points. The auxiliary control may be a
simple stop or start pushbutton or the same controls as the main station. Industrial pushbuttons are
used in control circuits to actuate magnetic contactors or remote-operated controllers which handle
power circuits.
Pushbuttons are pilot devices and must be ruggedly constructed to withstand operator and mechanical
abuse. They are frequently exposed to oils, coolants, chemicals, dusts, and various foreign elements.
Heavy electrical loads are sometimes carried by the contacts of the pushbutton because of the high
inrush current drawn by larger contactors. Pushbuttons are classified as standard-duty or heavy-duty,
according to their current handling capabilities.
Heavy-duty oil tight, and multi-light-control oil-tight are classified according to their ability to withstand
these conditions. The terms standard-duty and heavy-duty describe only the contact capacity of the
pushbutton in its making and breaking ability. These designations have no reference to the service
conditions or frequency of operation.
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Module 1: Push Buttons
Standard Duty Pushbuttons
Most systems use standard duty pushbutton stations to control A-C or D-C starters. General-purpose
pushbutton stations are designed for operating flexibility and ease of wiring. Padlocking attachments are
sometimes added to the pushbutton stations. These padlocking attachments are added to lock the
"stop" button in the depressed position.
Single Contact Ratings
Standard-duty units have contacts rated for pilot duty, typically as follows:
AC
3.0 Amperes, 120 volts
1.5 Amperes, 240 volts
0.75 Ampere, 480 volts
0.6 Ampere, 600 volts
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Module 1: Push Buttons
Heavy Duty Pushbuttons
Heavy-duty pushbutton stations are found in many industrial applications. They have approximately
twice the current rating of the standard-duty station. They come with any combination of pushbuttons,
selector switches, jogging buttons, and pilot lights. Pushbuttons are available with flush, extended, or
mushroom heads. They may have either momentary or maintained contacts. Heavy-duty pushbutton
units have double-break contacts rated for pilot duty, typically as follows:
AC
6 Amps, 120 Volts
3 Amps, 240 Volts
1.5 Amps, 480 Volts
1.2 Amps, 600 Volts
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DC
1.1 Amps, 120 Volts
0.55 Amps, 230 Volts
0.2 Amps, 550 Volts
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Module 1: Push Buttons
Pushbutton Station Descriptions
General-purpose stations have steel bases and steel wraparound covers that fit tightly to exclude dirt
and dust, as illustrated below.
Watertight and dust tight stations have a stainless steel enclosure with a gasket between the cover
and base. Water tight, corrosion-resistant stations are intended for applications where they are subject
to corrosive fumes or liquids. The enclosure is made of nonmetallic material, and synthetic rubber boots
protects the pushbuttons. This is illustrated below.
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Module 1: Push Buttons
Pushbutton Station Descriptions
Pushbutton stations for hazardous locations are suitable for hazardous gas and hazardous dust
locations. A machined surface is provided between the cover and base.
Oil tight pushbutton control stations are used wherever there is presence of oil, coolant, and other noncorrosive industrial liquids. These control stations may be the surface mounted, flush mounted or
pendant types. These stations have an enclosure that is sealed with a neoprene gasket. One or more
neoprene washers tightly seal individual oil tight units.
All these control units may include momentary or maintained contact pushbuttons. The contacts are
usually the double-break type. These contact blocks can be assembled in numerous contact
arrangements.
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Module 1: Push Buttons
Pushbutton Station Descriptions
Single and double circuit constructed contact blocks are frequently used in any combination of "normally
open" or "normally closed" circuits. Double circuit blocks have an individually operated contact plunger
for each circuit made from high strength phenolic to resist cracking and warping. Several contact blocks
can be stacked behind the pushbutton operator, as illustrated below.
Each unit usually consists of two parts, the operator and the contact block. The bodies of the operators
are die-castings, the operating buttons are molded plastic, and the locking rings are usually aluminum.
One or more synthetic rubber washers provide the oil tight seal between the panel and operator.
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Module 1: Push Buttons
Pushbutton Station Descriptions
Accessories
Some types of pushbutton stations may be supplied with a padlocking attachment that locks the stop
button in the depressed position. Some pushbutton stations may have pilot lights on them that indicate
their particular function. Pushbutton stations usually use mushroom heads as an E-stop button.
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Module 1: Push Buttons
Pushbutton Station Descriptions
Pushbutton Operators
Different types of pushbutton and cylinder lock operators are used. Some of these are flush button,
extended button, mushroom head, and jumbo mushroom head. The buttons are a solid color usually
made of molded plastic. The cylinder lock type pushbutton can be locked in various positions and the
key can be removed from the lock. A keyhole "dust cover" prevents dirt and other foreign matter from
entering. Key operators may be used with identical locks, dissimilar locks, or with dissimilar locks having
a master key system.
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Module 1: Push Buttons
Pushbutton Station Descriptions
Pushbutton Schematic Symbols
Pushbutton operators come in various types and configurations. Below the common types are
illustrated:
Stop Pushbutton
Stop Pushbutton with multiple contacts
Emergency Stop
Emergency Stop with multiple contacts
Start Pushbutton
Start Pushbutton with multiple contacts
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Module 1: Push Buttons
Selector Switch Operators
Selector Switches can be obtained in two, three, four, and etc. position types. Cams in the operator
sleeve actuate the contact blocks. These selector switches can provide maintained or spring return
contact operation. Many circuit combinations can be made since there is a number of different operating
cams available and a variety of contact blocks.
Standard Lever
Extended Lever
Selector Switch Schematic Symbols
Selector switch operators come in various types and configurations. Below the common types are
illustrated:
Selector switch 2 position 2 contacts
Selector switch 3 position 3 contacts
2
1
2
1
x
3
x
x
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Module 1: Push Buttons
Key Operated Selector Switch
Key operated selector switches work just like normal selector switches plus a key locking feature to
prevent unauthorized change or accidental tripping from a selected position. These switches also
provide maintained or spring return momentary cam action on the contact plungers.
Key Operator
Key Operated Selector Switch Schematic Symbols
1
2
2
1
x
3
Key operated Selector switch 2 position 2 contacts
x
x
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Module 1: Push Buttons
Illuminated Pushbuttons
These devices combine the functions of both a pushbutton operator and an indicating light in a single
unit and require only half the space of two separate units. The light unit may be either a resistor type or
an encapsulated transformer. These units are usually designed to give illumination from all angles. The
contact blocks can still be used for circuit control as before. A rubber diaphragm in the operator, a
rubber lens gasket, and a rubber panel gasket can provide oil tight assembly.
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Module 1: Push Buttons
Pushbutton Safety
1. Pushbuttons are pilot devices and must be ruggedly constructed to withstand operator and
mechanical abuse. They are frequently exposed to oils, coolants, chemicals, dust and various foreign
elements. This is why all pushbuttons, selector switches, indicating lights, and control stations shall be
of the oil tight type.
2. All control stations shall be mounted in a reasonably clean and dry location.
3. Pushbuttons shall be mounted on a surface that is not less than 45 degrees from the horizontal plane.
This is to help prevent dust or dirt from falling into them that could cause sticking.
4. All start pushbuttons shall be mounted above or to the left of their associated stop pushbuttons.
5. Stop pushbuttons shall be red in color, and the red color shall not be used to identify pushbuttons
having other functions. They shall be the unguarded types, so they can be activated easily with the palm
of a hand or other parts of the body.
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Module 1: Push Buttons
Pushbutton Safety
6. Start pushbuttons shall be of the fully guarded type, so they cannot be activated accidentally. There
should be a definite intent to push the button before the machine can start.
7. Pushbuttons shall retain their color identification throughout their life. The buttons should also never
be painted to change their function.
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Module 1: Push Buttons
End of Module 1
Push Buttons
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Module 2: Limit Switches
Module 2
Limit Switches
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Module 2: Limit Switches
Limit switches are devices used for interlocking a mechanical motion or position with an electrical circuit.
The selection of a limit switch involves both electrical application requirements and mechanical
placement. Limit switches are normally in the control circuit and can start a machine in motion or stop its
motion. They can also restrict the travel of a machine between two points or possibly initiate a separate
action at a pre-determined point of machine travel. Limit switches are also often used in safety devices
for the protection of both personnel and equipment.
Types of Limit Switches
The function performed by limit switches is similar no matter how complex the design of the electrical
circuit. The main differences are the mechanical design and the means by which the switch is actuated.
Actuators
An actuator is the external linkage that detects the mechanical movement of the machine. This actuates
an internal mechanism that results in contacts opening or closing.
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Module 2: Limit Switches
Examples of Actuators
Roller Lever
Forked Lever
Adjustable Length
Roller Lever
Rod Lever
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Spring Rod
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Module 2: Limit Switches
Limit Switch Schematic Symbols
Normally open limit switch
(N.O.)
Normally open held closed limit switch
(N.O.H.C.)
Normally closed limit switch
(N.C.)
Normally closed held open limit switch
(N.C.H.O.)
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Module 2: Limit Switches
Common Detector Symbols
Pressure Switch N.O.
Pressure Switch N.C.
Float Switch N.O.
Float Switch N.C.
Flow Switch N.O.
Flow Switch N.C.
Thermal Switch N.O.
Thermal Switch N.C.
Foot Switch N.O.
Foot Switch N.C.
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Module 2: Limit Switches
End of Module 2
Limit Switches
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Module 3: Inductive Proximity detectors
Module 3
Inductive Proximity Detectors
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Technical Mathematics
Module 3: Inductive Proximity Detectors
Inductive Proximity Detectors
These devices can fulfill the same functions as the mechanical limit switch contacts, and have the
following advantages:
- Rapid speed of operation
- Insensitive to humid or damp surroundings
Principles of Operation
N.O.
N.C.
Inductive proximity sensors are designed to operate by generating an electromagnetic field and
detecting the eddy current losses generated when ferrous and nonferrous metal target objects enter the
field.
The sensor consists of a coil on a ferrite core, an oscillator, a trigger-signal level detector and an output
circuit.
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Module 3: Inductive Proximity Detectors
As a metal object advances into the field, eddy currents are induced in the target. The
result is a loss of energy and a smaller amplitude of oscillation.
The detector circuit then recognizes a specific change in amplitude and generates a signal
which will turn the solid-state output “ON” or “OFF.”
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Module 3: Inductive Proximity Detectors
The active face of an inductive proximity switch is the surface where a high-frequency
electro-magnetic field emerges.
A standard target is a mild steel square, one mm thick, with side lengths equal to the
diameter of the active face or three times the nominal switching distance, whichever is
greater.
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Module 3: Inductive Proximity Detectors
To determine the sensing distance for materials other than the standard mild steel, a correction factor is
used. The composition of the target has a large
effect on sensing distance of inductive
proximity sensors. If a target constructed
from one of the materials listed is used,
multiply the nominal sensing distance by
the correction factor listed in order to
determine the nominal sensing distance for that target.
Note that ferrous-selective sensors will not detect brass, aluminum or copper, while nonferrous selective
sensors will not detect steel or ferrous-type stainless steels.
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Module 3: Inductive Proximity Detectors
The difference between the On and Off points is called hysteresis or differential travel. The amount of
target travel required for release after operation must be accounted for when selecting target and sensor
locations.
Hysteresis is needed to help prevent chattering (turning on and off rapidly) when the sensor is subjected
to shock and vibration or when the target is stationary at the nominal sensing distance.
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Module 3: Inductive Proximity Detectors
The switching frequency is the maximum speed at which a sensor will deliver discrete individual pulses as
the target enters and leaves the sensing field. This value is always dependent on target size, distance
from sensing face, speed of target and switch type.
This indicates the maximum possible number of switching operations per second. Sensors can be
connected in series with a load. For proper operation, the load voltage must be less than or equal to the
minimum supply voltage minus the voltage drops across the series connected proximity sensors.
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Module 3: Inductive Proximity Detectors
Sensors can be connected in parallel to energize a load. To determine the maximum allowable number of
sensors for an application, for an application, the sum of the
maximum leakage current of the sensors connected in parallel must be less than the maximum OFF-state
current of the load device.
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Module 3: Inductive Proximity Detectors
TTL Wiring
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Module 3: Inductive Proximity Detectors
Wiring Configurations
Cable Style Proximity Switch
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Module 3: Inductive Proximity Detectors
Wiring Configurations
Micro Style Proximity Switch
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Module 3: Inductive Proximity Detectors
Wiring Configurations
Pico Style Proximity Switch
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Module 3: Inductive Proximity Detectors
Wiring Configurations
AC Limit Style Proximity Switch
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Module 3: Inductive Proximity Detectors
Various Styles of Proximity Detectors
Miniature Flat Pack Series
Power and Gripper Applications
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Module 3: Inductive Proximity Detectors
Various Styles of Proximity Detectors
Cylinder PositionSeries
Ring and Slot Applications
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Module 3: Inductive Proximity Detectors
This concludes
Module 3
Inductive Proximity Detectors
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Module 4: Ultrasonic Proximity Detectors
Module 4
Ultrasonic Proximity Detectors
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Module 4: Ultrasonic Proximity Detectors
Ultrasonic sensors operate by emitting and receiving high-frequency sound waves. The frequency is usually
in the order of 200 kHz, which is too high for the human ear to hear.
There are two basic modes of operation: opposed mode and diffuse
(echo) mode.
In opposed mode, one sensor emits the sound wave and another, mounted
opposite the emitter, receives the sound wave.
In diffuse mode, the same sensor emits the sound wave and then listens for the echo that bounces off an
object.
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Module 4: Ultrasonic Proximity Detectors
The sensing range is the distance within which the ultrasonic sensor will detect a target under fluctuations of
temperature and voltage.
Ultrasonic sensors have an inherent blind zone located at the sensing face. The size of the blind zone
depends on the frequency of the transducer. Objects located within the blind spot can not be reliably
detected.
Certain characteristic of targets must be considered when using ultrasonic sensors. These include target
shape, material, temperature, size and positioning. Soft materials such as fabric or foam rubber are difficult
to detect by diffuse ultrasonic technology because they are not sound-reflective.
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Module 4: Ultrasonic Proximity Detectors
Various Types of Ultrasonic Proximity Detectors by Allen Bradley
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Module 4: Ultrasonic Proximity Detectors
For normal operation do not connect the control pin. Hold and synchronize features can be used for special
applications.
To inhibit sensor operation and hold the output to its present state connect the control pin (2) to 0V DC. The
sensor will not transmit or receive ultrasonic pulses until this voltage is removed from the control pin.
Switching output models will be latched and analog output models will hold their value during this period.
To synchronize the transmission of ultrasonic pulses between several sensors connect the control pins
together. This feature reduces the potential for sensor crosstalk between models that are mounted in close
proximity to one another.
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Module 4: Ultrasonic Proximity Detectors
Typical Beam Pattern
Analog Output Styles
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Module 4: Ultrasonic Proximity Detectors
Discrete and Analog Output Styles
Introduction to Actuators and Sensors
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Module 4: Ultrasonic Proximity Detectors
This Concludes
Module 4
Ultrasonic Proximity Detectors
Introduction to Actuators and Sensors
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Module 5: Capacitive Proximity Detectors
Module 5
Capacitive Proximity Detectors
Introduction to Actuators and Sensors
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Module 5: Capacitive Proximity Detectors
Capacitive proximity sensors are designed to operate by generating an electrostatic field and detecting
changes in this field caused when a target approaches the sensing face. The sensor’s internal workings
consist of a capacitive probe, an oscillator, a signal rectifier, a filter circuit and an output circuit.
In the absence of a target, the oscillator is inactive. As a target approaches, it raises the capacitance of the
probe system. When the capacitance reaches a specified threshold, the oscillator is activated which triggers
the output circuit to change between “on” and “off.”
The capacitance of the probe system is determined by the target’s size, dielectric constant and distance from
the probe. The larger the size and dielectric constant of a target, the more it increases capacitance. The
shorter the distance between target and probe, the more the target increases capacitance.
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Module 5: Capacitive Proximity Detectors
The standard target for capacitive sensors is the same as for inductive proximity sensors. The target is
grounded per IEC test standards. However, a target in a typical application does not need to be grounded to
achieve reliable sensing. For a given target size, correction factors for capacitive sensors are determined by
a property of the target material called the dielectric constant. Materials with higher dielectric constant values
are easier to sense than those with lower values.
Each capacitive sensor can be classified as having either a shielded or unshielded construction. Shielded
sensors are constructed with a metal band surrounding the probe. This helps to direct the electrostatic field to
the front of the sensor and results in a more concentrated field.
Shielded construction allows the sensor to be mounted flush in surrounding material without causing false
trigger. Shielded capacitive proximity sensors are best suited for sensing materials with low dielectric
constants (difficult to sense) as a result of their highly concentrated electrostatic fields. This allows them to
detect targets that unshielded sensors cannot.
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Module 5: Capacitive Proximity Detectors
Unshielded sensors do not have a metal band surrounding the probe and hence have a less concentrated
electrostatic field. Many unshielded models are equipped with compensation probes, which provide increased
stability for the sensor.
Unshielded capacitive sensors are also more suitable than shielded types for use with plastic sensor wells,
an accessory designed for liquid level applications. The well is mounted through a hole in a tank and the
sensor is slipped into the well’s receptacle. The sensor detects the liquid in the tank through the wall of the
sensor well.
The electrostatic field of an unshielded sensor is less concentrated than that of a shielded model. This
makes them well suited for detecting high dielectric constant (easy to sense) materials or for differentiating
between materials with high and low constants.
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Module 5: Capacitive Proximity Detectors
Various Types of Capacitive Proximity Detectors
Introduction to Actuators and Sensors
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Module 5
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Appendix: Terms and Deffinitions
Axial Approach: The approach of the target with its center maintained on the reference axis.
Complementary Outputs: (N.O. & N.C.) A proximity sensor that features both normally open and normally
closed outputs, which can be used simultaneously.
Correction Factors: Suggested multiplication factors taking into account variations in the target material
composition. When figuring actual sensing distance this factor should be multiplied with the nominal sensing
distance.
Current Consumption: The current consumed by the proximity switch when the output device is in the off
condition.
Differential Travel: See Hysteresis.
Dual Output: Sensor which has two outputs which may be complementary or may be of a single type (i.e.
two normally open or two normally closed).
Effective Operating Distance: (Sr) The operating distance of an individual proximity switch measured at
stated temperature, voltage, and mounting condition.
False Pulse: An undesired change in the state of the output of the proximity switch that lasts for more than
two milliseconds.
Flush Mounting: A shielded proximity sensor which can be flush mounted in metal up to the plane of the
active sensing face.
Free Zone: The area around the proximity switch which must be kept free from any damping material.
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Appendix: Terms and Deffinitions
Hysteresis: The difference, in percentage (%), of the nominal sensing distance between the operate (switch
on) and release point (switch off) when the target is moving away from the sensors active face. Without
sufficient hysteresis a proximity sensor will “chatter” (continuously switch on and off) when there is significant
vibration applied to the target or sensor.
Isolation Voltage: Maximum rated voltage between isolated outputs or input and output.
Lateral Approach: The approach of the target perpendicular to the reference axis.
Leakage Current: Current which flows through the output when the output is in an “off” condition or deenergized. This current is necessary to supply power to the electronics of the sensor.
LED: Light Emitting Diode used to indicate sensor status.
Maximum Load Current: The maximum current level at which the proximity sensor can be continuously
operated.
Maximum Inrush Current: The maximum current level at which the proximity sensor can be operated for a
short period of time.
Minimum Load Current: The minimum amount of current required by the sensor to maintain reliable
operation.
Sensing Distance: The distance at which an approaching target activates (changes state of) the proximity
output.
Normally Closed: Output opens when an object is detected in the active switching area.
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Appendix: Terms and Deffinitions
Normally Open: Output closes when an object is detected in the active switching area.
NPN: The sensor switches the load to the negative terminal. The load should be connected between the
sensor output and positive terminal.
Operating Distance, Rated: The operating distance specified by the manufacturer and used as a reference
value. Also known as nominal sensing distance.
PNP: The sensor switches the load to the positive terminal. The load should be connected between the
sensor output and negative terminal.
Programmable Output: (N.O. or N.C.) Output which can be changed from N.O. to N.C. or N.C. to N.O. by
way of a switch or jumper wire. Also known as selectable output.
Repeatability: The variation of the effective operating distance measured at room temperature and constant
supply voltage. It is expressed as a percentage of the sensing distance.
Residual Voltage: The voltage across the sensor output while energized and carrying maximum load
current.
Response Time: See Switching Frequency.
Reverse Polarity Protection: Proximity sensors which are protected against a reversal in voltage polarity.
Ripple: The variance between peak-to-peak values in DC voltage. It is expressed in percentage of rated
voltage.
Sensing Range: The rated operating distance.
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Appendix: Terms and Deffinitions
Shielded: Sensor which can be flush mounted in metal up to the plane of the active sensing face.
Short Circuit Protection: (SCP) Sensor protected from damage when a shorted condition exists for an
indefinite or defined period of time.
Sinking: See NPN.
Sourcing: See PNP.
Switching Frequency: The maximum number of times per second the sensor can change state (ON and
OFF) usually expressed in Hertz (Hz). As measured in DIN EN 50010.
Target: Object which activates the sensor.
Three-Wire Proximity Switch: An AC or DC proximity sensor with three leads, two of which supply power
and a third that switches the load.
Two-Wire Proximity Switch: A proximity sensor which switches a load connected in series to the power
supply. Power for the proximity switch is obtained through the load at all times.
Voltage Drop: The maximum voltage drop across a conducting sensor.
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Module 6: Photo Detectors
Module 6
Photo Detectors
Introduction to Actuators and Sensors
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Module 6: Photo Detectors
Photo Detectors
Introduction
Photoelectric sensors are used in many applications and industries to provide accurate detection of
objects without physical contact. In its most basic form, a photoelectric sensor can be thought of as a
“limit switch-like” device, where the mechanical actuator or lever arm function is replaced by a beam of
light. Photoelectric sensors operate by sensing a change in the amount of light that is either reflected or
blocked by an object to be detected (target). The change in light could be the result of the presence or
absence of the target, or as the result in a change of the size, shape, reflectivity or color of a target. A
photoelectric sensor can be used in applications to sense targets at distances from less than 5 mm (0.2
in.) to over 250m (820 ft). Successful sensing with a photoelectric sensor requires that the object to be
detected (target) causes a sufficient change of light level detected by the sensor and that the user has a
clear understanding of the sensing requirements. The following must be clearly understood:
The sensing requirements,
The sensing environment, and
The capabilities and limitations of the photoelectric sensor.
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Module 6: Photo Detectors
Photo Detectors
Basic Concepts and Components
There are four basic components to any photoelectric sensor:
Light source
Light detector
Lenses
Output switching device
Light Source
A light emitting diode (LED) is a solid-state semiconductor that emits light when current is applied. The
figure above shows the construction of an LED. LEDs are made to emit specific wavelengths or colors
of light. Infrared, visible red, green, and blue LEDs are used as the light source (emitter) in most
photoelectric sensors. Different LED colors offer different desirable characteristics. Infrared LEDs are
the most efficient; they generate the most light and the least heat of any LED color. Infrared LEDs are
used in sensors where maximum light output is required for an extended sensing range. In many
applications, a visible beam of light is desirable to aid setup or confirm sensor operation. Visible red is
most efficient for this requirement.
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Module 6: Photo Detectors
Photo Detectors
Visible red, blue, and yellow LEDs are also used in special applications where specific colors or color
contrasts must be detected. These LEDs are also used as status indicators on photoelectric sensors.
LEDs are rugged and reliable components, making them ideal for use in photoelectric sensors. They
operate over a wide temperature range and are very resistant to damage from shock and vibration.
Light Detector
A photo-detector is the component used to detect the light source. A photodiode or phototransistor is a
robust solid-state component that provides a change in conducted current depending on the amount of
light detected. Photo-detectors are more sensitive to certain wavelengths of light. The spectral response
of a photo-detector determines its sensitivity to different wavelengths in the light spectrum.
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Module 6: Photo Detectors
Photo Detectors
Lens
LEDs typically emit light and photo-detectors are sensitive to light over a broad area. Lenses are used
with LED light sources and photo-detectors to narrow this area. As the area is narrowed, the range of the
LED or photo-detector increases. As a result, lenses also increase the sensing distance of photoelectric
sensors. The light beam from an LED and lens combination is typically conical in shape. The area of the
cone increases with distance.
Output Device
Once a sufficient change of light level is detected, the photoelectric sensor switches an output device to
provide an interface to machine logic. Many types of discrete and variable (analog) outputs are available,
each with particular strengths and weaknesses.
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Module 6: Photo Detectors
Photo Detectors
Photoelectric Sensing Modes
Different methods of sensing are referred to as sensing modes. There are three basic types:
Transmitted beam
Retro-reflective
Diffuse
While many applications can be handled by any of these sensing modes, each offers specific strengths
and weaknesses to consider. These strengths and weaknesses are summarized on the next page in
Table 1.
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Module 6: Photo Detectors
Photo Detectors
Light/Dark Operate Output
The terms “light operate mode” and “dark operate mode” are used to describe the action of a sensor
output when a target is present or absent.
A light operate mode output is ON (energized, logic level one) when the receiver can “see” sufficient light from
the light source.
For transmitted beam and retro-reflective sensing, a light operate output is ON when the target is absent and
light can travel from the light source to the receiver.
For diffuse sensing (all types), the output is ON when the target is present and reflecting light from the light
source to the receiver.
A dark operate output is ON (energized, logic level one) when the receiver cannot “see” the light from
the light source.
For transmitted beam and retro-reflective sensing, a dark operate output is ON when the target is present and
light from the light source is blocked and cannot reach the receiver.
For diffuse sensing (all types), a dark operate output is ON when the target is absent.
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Module 6: Photo Detectors
Photo Detectors
Introduction to Actuators and Sensors
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Module 6: Photo Detectors
Photo Detectors
Transmitted Beam
In this mode the light source and receiver are contained in separate housings. These two units are
positioned opposite each other so that the light from the light source shines directly on the receiver.
Targets must break (block) the beam between light source and receiver. Transmitted beam sensors
provide the longest sensing distances and the highest level of operating margin.
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Module 6: Photo Detectors
Photo Detectors
Retro-reflective
Retro-reflective is the most popular sensing mode. A retro-reflective sensor contains both the light
source and receiver, in one package. The light beam emitted by the light source is reflected by a special
reflective object and detected by the receiver. The target is detected when it breaks this light beam.
Special reflectors or reflective tapes are used for retro-reflective sensing. Unlike mirrors or other flat
reflective surfaces, these reflective objects do not have to be aligned perfectly perpendicular to the
sensor. Misalignment of a reflector or reflective tape of up to 15 degrees will typically not significantly
reduce the margin of the sensing system
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Module 6: Photo Detectors
Examples of Photo Detectors
.
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Module 6: Photo Detectors
Examples of Photo Detectors
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Module 6: Photo Detectors
Module 6
Photo Detectors
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