Download The Basics of Photoelectric Controls

The Basics of
Photoelectric Controls
Table of Contents
Theory of Operation
1
Modes of Detection
3
Fiber Optics
5
Excess Gain
6
Contrast
9
Photoelectric Control Logic
9
Output Devices and Switch Logic
Logic Functions Using Switch Logic
Appendix
12
14
Inside back cover
Theory of Operation
A photoelectric sensor is a type of switch. It is a switch that is
turned off and on by the presence or absence of received light.
The inherent advantages of such a "non-contact" switch have
resulted in widespread application throughout the industrial
world. Some of these advantages include:
• The presence or absence of an object can be detected without
direct contact.
• There are no mechanical parts or levers to wear out.
• The switch can be mounted a great distance from the object to
be detected.
In its basic form a photoelectric control is the simplest of
electronic circuits. The basic components consist of a power
supply, light source, a photo detector, and an output device.
The development of light emitting diodes (LED) introduced a
solid state alternative to incandescent lamps as source
elements in photoe-lectric controls.
In 1970 Opcon introduced the first modulated LED
photoelectric control system suitable for use in sawmills, steel
factories, and other harsh environments.
Light Emitting Diodes - a Powerful Light Source
Today, LED's are recognized by almost everyone. Since their
development in the early 1960's, they have found their way into
many common products used everyday, including wrist
watches, calculators, and counters.
LED's are solid state devices that function just the opposite
from photo detectors. When current is applied to them, LED's
emit light energy. LED's offer several advantages over
incandescent bulbs when applied to photoelectric controls.
They can be rapidly turned on and off, are extremely small,
consume very little power, and have an extremely long life
(100,000 hours continuous). Also, since LED's are solid state
devices they are much more immune to vibration than
incandescent bulbs.
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Fig. 1 A simple photoelectric control
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Key to this system is the photo detector. It is composed of a
silicon semiconductor material that conducts current whenever
it sees light. Photoelectric controls make use of this property to
control output devices such as mechanical relays, triacs, and
transistors. These, in turn, are used to control machinery.
Early Photoelectric Controls Used in Industry
Early industrial photoelectric controls used focused light from
incandescent bulbs to activate a cadmium sulfide photocell.
Since they were not modulated, ambient light such as arc
welders, sun light, or fluorescent light could easily "false trigger"
these devices. Also, the delicate filaments in the incandescent
bulbs had a relatively short life span, and did not hold up well
under high vibration and the kind of shock loads normally found
in an industrial environment. Switching speed was also limited
by the slow response of the photo cell to light/dark changes
Fig. 3
LED's emit light energy over a narrow wavelength. In
comparing the relative efficiency of various light sources, one
can see that infrared gallium arsenide (I.R.) LED's emit energy
only at 940 nanometers. (See figure 4 below.) Because this
wavelength is at the peak of a sili-con photodiode's response,
maximum energy transfer between source and detector is
achieved.
A silicon photo detector's sensitivity to light energy also peaks
in the infrared light spectrum. This contributes to the high
efficiency and long range possible when used in conjunction
with I.R. LED's.
Fig. 4 Comparing various light sources
Fig. 2 An early incandescent photoelectric
control with cadmium sulfide photocell
detector
Visible LED's are sometimes used as light sources in
photoelectric controls. Because the beam is visible to the
naked eye, the principle advantage of visible LED's is ease
of alignment. Visible beam photoelectric controls usually
have lower optical performance than controls using I.R.
LED's.
Solid State Technology
Brings Many Benefits to the End User
The application of light emitting diodes and silicon photo
detectors in photoelectric controls has resulted in many benefits
to the user of these systems. These include:
A Complete Modulated Photoelectric Control
A complete photoelectric control is made up of 8 basic
components. They include:
• Long life
• Small sensor size
• Immunity to vibration
• Long sensing distances
• Immunity to ambient light
Modulated Photoelectric Controls
A modulated photoelectric control operates on a principle
similar to that of a radio station and radio receiver. The receiver
is tuned to accept the desired radio frequency and reject other
possible interfering modulation frequencies from other radio
sources. The photoelectric sensor must be able to distinguish
its own source light and ignore other light interference.
Fig. 5 A radio transmitter and receiver are similar
to a modulated LED source and detector system.
Two advantages of a modulated beam sensor system are:
• Increased immunity to false triggering from ambient light (i.e.
sunlight, fluorescent and incandescent lamps)
• Longer sensing ranges.
Modulation gives the detector the ability to distinguish between
pulsed LED light and ambient light. This makes it possible to
increase the detector's input signal sensitivity. Also, because
the source LED is being energized for only a fraction of the
modulation time period, normal operating drive current levels
can be increased providing a high light output without
exceeding the allowable heat dissipation of the device thereby
increasing its useful life.
EXAMPLE: An LED rated for a maximum drive current of 100
mA continuous can, by modulating the current "on" 50% of the
time and "off" 50% of the time (50% duty cycle), increase the
drive current to 200 mA without damage to the LED.
Fig. 7 Modulated LED photoelectric sensor block diagram
The Power Supply provides regulated DC voltage and current
to the sensor circuitry. It may be built internally into the
control, or it may be externally connected.
The Modulator is a pulse generator which cycles the Source
Current Amplifier and Source LED at the desired frequency
and duty cycle.
The Photo Detector is either a photodiode or phototransistor
device selected for maximum sensitivity at the source LEDs
emitted light wavelength. The detector generates a small
amount of current when it sees light. This is connected to the
Detector Amplifier which blocks current generated by
background light and provides amplification of the detected
signal to a usable level for the photoelectric control.
The Demodulator comes in two types: Synchronous and
Asynchronous.
A Synchronous Demodulator is connected to the output of the
Modulator so it is energized to "look" whenever the source is
turned on. Synchronizing the modulator and demodulator
creates a shutter effect that makes the sensor virtually immune
to any ambient light conditions. Due to the fact that the
modulator and demodulator are electronically tied together, this
scheme is utilized mainly in self-contained reflex and diffuse
proximity sensors where the source and detector are in the
same housing. This system is also used in our 70 Series control
units with the 70 Series remote optic sensors since the
modulator and demodulator are contained in the same housing.
The second type of demodulation system is called an Asynchronous Demodulator. These are designed to respond to
the narrow pulse created by the modulated source signal, and
have no direct connection to the modulator. This system is ideal
for use in self-contained through-beam sensor systems where
the optics are separated by long distances. Excellent ambient
light immunity is also a feature with this method of
demodulation.
Lastly, the Output device is the active part of the control for the
end user. Depending on the application and the switching
requirements of the load, this device may be a relay, solid state
triac, FET or transistor device. In some cases the device is built
into the control or is available as a replaceable module.
Fig. 6 Modulation provides more light energy.
Modes of Detection
Optical sensor elements can be packaged and positioned in a
variety of ways according to application requirements.
Sensing modes consist of two basic detection principles:
1.
2.
Thru-beam
Reflection (diffuse reflection, reflex detection)
Thru-Beam Detection
Features—Thru-beam detection
• Longest optical range
• Highest possible signal strength (excess gain)
• Greatest light/dark contrast ratio
• Surface reflectivity and color have little effect
• Best trip point repeatability
Limitations—Thru-beam detection
• Two components to wire across detection zone
• Alignment can be difficult
Reflex Detection
Fig. 8
A thru-beam sensor consists of separate source and detector
elements. They are aligned facing each other across an area
which the target object crosses. Detection occurs when an
object blocks the entire effective beam. The effective beam area
is defined as the column of light that travels in a straight line
between lenses. In non-lensed versions, it is the light traveling
from source chip to detector chip.
Reflex photoelectric controls position the source and detector
parallel to each other on the same side of the object to be
detected. The light source is directed at a retroreflector (a
specially designed device that returns most light it receives
back in the same direction from which it came). An example of
a retroreflector is a bicycle reflector. The detector receives the
light returned from the retroreflector. An output occurs when an
object breaks the beam between the photoelectric sensor and
the retroreflector.
Because the light must travel in 2 directions (to the
retroreflector and back to the control), reflex controls will
typically not sense as far, nor will they have the optical
performance of a thru-beam control. Reflex controls do offer a
powerful sensing system that does not require electrical wire
to be run to both sides of the sensing area. This combination
of high sensing power and ease of mounting has made reflex
sensors the most popular choice of all sensing modes.
Features—Reflex detection
• Long range
• High light/dark contrast ratio
• Easy installation and alignment
Limitations—Reflex detection
• Can be false triggered by shiny surfaced objects
Fig. 9 Thru-beam sensor effective beam pattern
Fig. 10 If the object to be detected is smaller than the effective
beam diameter, apertures over the lenses may be required.
Because light from the source is transmitted directly to the
photo-detector, thru-beam sensors offer the longest range,
highest possible signal strength and greatest light/dark
contrast ratio.
Fig. 11 Reflex controls provide a powerful sensing system with both
source and detector mounted on the same side of the sensing
region.
Polarized Reflex Detection
Polarized reflex sensors use a polarizing filter over the source
and detector that "conditions" the light from the source such
that the photoelectric control only sees light returned from the
retroreflector. A polarized reflex sensor is used in applications
where shiny surfaces such as metal or shrink wrapped boxes
may false trigger the control.
Polarized reflex sensing is achieved by combining some
unique properties of polarizers and retroreflectors. These
properties are:
• Polarizers pass light that is aligned along only one plane.
• Corner cube retroreflectors depolarize light as it travels through
the plastic face of the retroreflector.
Light emitted from the source is aligned by a polarizer. When
this light reflects off the retroreflector, it is depolarized. The
returning light passes through another polarizing filter in front of
the detector. The detector's polarizer is mounted 90° to the
source's polarizer. Only the light which has been rotated by the
corner cube retroreflector can pass through the detector's
polarizer. Light that bounces off other shiny objects, and has not
been rotated 90°, cannot pass through the detector's polarizer,
and will not trigger the control.
Optical detection is affected by the type, texture and
composition of the target object's surface.
Other considerations when selecting a proximity sensor for an
application are: the amount of contamination in the
environment and the distance to the nearest background
surface.
Fig. 13 Optical proximity sensors detect light bounced
off the target object.
Focused proximity sensors are a special type of proximity
sensor where the source and detector are focused to a point in
front of the sensor. Focused proximity sensors can detect
extremely small objects, or look into holes or cavities in special
applications. Background ob-jects will not false trigger a
focused proximity sensor since they are "cross-eyed" and
cannot see past a certain point.
Fig. 14 Focused proximity sensors focus the source and detector on
a small spot to detect small objects, or look into holes or cavities.
Fig. 12
Note that polarized reflex sensors will not work with reflective
tape utilizing glass bead reflective surfaces. Also, shiny objects
wrapped with clear plastic shrink-wrap will potentially false
trigger a polarized reflex control since under certain conditions
these act like a corner cube reflector.
Features—Polarized reflex detection
• Will not be confused by surface reflections from target objects.
• High light /dark contrast ratio.
• Simple installation and alignment. Wire only one side of sensing
zone.
Limitations—Polarized reflex detection
• Operating range is half that of non-polarized sensors since much
of the signal is lost using a polarizing filter.
• The sensor can be fooled by shiny objects wrapped with shrink
wrap material.
Proximity (Diffuse Reflective) Detection
Proximity detection is similar to reflex detection in that the
source and detector elements are mounted on the same side of
the object being detected. The difference is that the optics are
positioned so that the source and detector's field of view cross.
Light from the source bounces off the target back onto the
detector when it enters the area of coincidence (detection zone).
The detection zone can be controlled or shaped by the angle,
separation, power and beam patterns of the source and
detector.
Features—Proximity detection
• Simple installation and alignment. Wire only one side of sensing
zone.
• Can detect differences in surface reflectivity.
Limitations—Proximity detection
• Limited sensing range.
• Light / dark contrast and sensing range are object surface
reflectivity dependent.
Background Rejection (Perfect Prox®) Detection
This detection scheme is really a special type of diffuse
reflective sensor. It combines extremely high sensing power
with a sharp optical cut-off. This allows the sensor to reliably
detect targets regardless of color, reflectance, contrast or
surface shape, while ignoring objects just outside of the target
range.
This method uses two different photo-detectors. For the
Perfect Prox unit with a six-inch range, the near detector has
a range of 0 to 24 inches. Its far detector has a range of 6 to
24 inches.
Objects closer than six inches are detected only by the near
detector. Objects between 6 and 24 inches are detected by
both detectors.
If the near signal is stronger than the far signal, the sensor
output is ON. If the far signal is stronger or equal to the near
signal, the output is OFF. The result is a sensor with a high
excess gain for six inches, followed by a sharp cutoff.
Features—Background Rejection (Perfect Prox) detection
• Improved background rejection.
• Detects light and dark objects at very similar ranges.
Fig. 15 Comparison of Perfect Prox and Diffuse gain curves.
Fig. 16 Beam pattern for a Perfect Prox sensor
Fiber Optics
Fiber optics are bundles of thin plastic or glass fibers that
operate on a principle discovered in 1854 by John Tyndahl.
When Tyndahl shined a beam of light through a stream of water,
rather than emerge straight from the stream of water as might
be expected, the light tended to bend with the water as it arced
towards the floor. Tyndahl discovered that the light was
transmitted along the stream of water. The light rays inside the
stream bounced off the internal walls of water and were thereby
contained inside. This principle has come to be known as "total
internal reflection."
the fibers from excessive flexing as well as the environment.
Benefits of Fiber Optic Sensing
The addition of fiber optics to photoelectric sensing has
greatly expanded the application of these devices. Because
they are small in diameter and flexible, fiber optics can bend
and twist into tiny places formerly inaccessible to bulky
electronic devices.
Fiber optics operate in the same sensing modes as standard
photoelectric controls: thru-beam, proximity and reflex. Sensing
tips come in a variety of sizes and shapes to fit special
application requirements.
Fig. 17 Fiber optics use the principle of total internal
reflec-tion to bend light.
Industry has since discovered that the principle of total internal
reflection also applies to small diameter glass and plastic fibers,
and this has lead to a rapid growth of applications throughout
the industry. For example, fiber optics are used to optically
transmit data in the communications field, and to transmit
images or light in medicine and industry. Photoelectric controls
use fiber optics to bend the light from the LED source and
return it to the detector so sensors can be placed in locations
where common photoelectric sensors could not be ap-plied.
Fiber optics typically are used to solve difficult sensing problems
such as detecting small objects or objects in inaccessible
places. Since they contain no electronics, fiber optics can also
operate at much higher temperatures than other types of
sensors. Glass fiber optics are availa-ble that operate up to
900° F continuously. Plastic fiber cables can run many feet into
environments requiring intrinsically safe sensing operations.
Fiber optics also operate well in environments with high
vibration.
Fiber optics do have a few drawbacks. First, they have a limited
sensing distance. Typical sensing distance in proximity mode is
3 inches, 15 inches for thru-beam mode. Second, they are
typically more expensive than other photoelectric controls.
Finally, because of their small sens-ing area, fiber optics can be
easily "fooled" by a small drop of water or dirt over the sensing
surface.
Features—Fiber optics
• Permits sensing in confined spaces.
• Ability to bend around corners.
• No electronics at sensing point.
• Operate at high temperature (glass).
• Total immunity from electrical noise and interference.
• Plastic fibers can be cut to desired length.
Fig. 18 Fiber optics used with photoelectric controls are
covered with outer sheaths of PVC plastic or stainless steel
armored cable.
Fiber optics used with photoelectric controls are made up of a
large number of individual glass or plastic fibers which are
sheathed in suit-able material for protection. The fiber optics
used with Eaton photoelectric controls are covered by either
PVC plastic, or stainless steel armored jackets. Both protect
Limitations—Fiber optics
• Short sensing distance (unless lensed).
• Expensive (glass).
• Can be fooled by moisture or dirt on the sensing surface in diffuse
proximity mode.
Excess Gain
The ability of a photoelectric control to detect an object at a
given distance is measured in units of "excess gain." Excess
gain is a measure of energy available between the source and
detector to overcome signal loss due to dirt or contamination in
the environment. Excess gain is the single most important factor
that must be considered when choosing a photoelectric control
for a particular application.
By definition, excess gain is the ratio of the amount of light the
detector sees to the minimum amount of light required to trip
the sensor. This ratio is depicted graphically for all photoelectric
sensors. In figure 18, excess gain is plotted along the vertical
(or Y) axis, starting at 1, the minimum amount of light required
to trigger the detector. Every point above 1 represents an
amount of light more than that required to trigger the
photoelectric control. This is the "excess gain" that the detector
sees.
The boxes will pass about two to five inches from the sensor
as they move along the conveyor at the sensing location.
Given a choice between the two proximity sensors whose
excess gain curves appear in figure 20, which photoelectric
control should be selected for this application?
Fig. 21 Excess gain for two proximity controls.
If the decision were based solely on specified range, unit
number 1 would be selected. However, if this unit were
installed in this application, it might fail after a short time in
operation. Over time, contaminants from the environment
would settle on the lens, decreasing the amount of light the
sensor is seeing. Eventually, enough lens contamination would
occur that the photoelectric control would not have enough
excess gain to overcome the signal loss created by the coating,
and the application would fail.
Fig. 19 Typical excess gain curves for different detection modes.
When this excess gain is plotted against distance, the result
is an excess gain curve. For example, in figure 18, the excess
gain for the proximity sensor at 2 inches is 100, or 100 times
the minimum amount of light required to trigger the
photoelectric control.
The Importance of Excess Gain
Often, the standard of comparison when making a choice
between different photoelectrics is range. More important to
most applications is the amount of sensing "punch" or the
excess gain that is available within the detecting region. For a
typical application, the higher the excess gain within the
sensing region, the more likely the application will work. It is this
power or "punch," that will determine whether the photoelectric
control will continue to operate despite the buildup of dirt on the
lens, or the presence of contamination in the air.
For example, figure 19 shows an application detecting
boxes on a conveyor in a filthy industrial environment.
A better choice for this application would be unit 2. Unit 2
delivers much more excess gain in the operating region
required for this application, and will therefore work much more
successfully than unit 1. Do not be taken in by maximum range
specifications. ALWAYS LOOK FOR MAXIMUM EXCESS
GAIN AT THE RANGE WHERE THE DETECTION IS TO
OCCUR!
Excess Gain and Modes of Detection
Thru-Beam Performance
Excess gain for thru-beam photoelectric controls is the
simplest to measure since it is almost exclusively a function of
the separation between the source and detector. The excess
gain at a given distance can be determined by the following
formula:
Excess Gain = (Maximum Range/Operating Range)2
For example, the excess gain at 10 feet for a thru-beam
control with a maximum range of 100 feet would be:
(100/10)2 = 100.
Plotted as shown:
Fig. 20 Box detection
Fig. 22 Excess gain curve for a thru-beam control rated for 100
feet.
The excess gain a thru-beam sensor pair will ultimately have
depends on how well aligned the sensors are, the separation
between them, the sensitivity of the Detector Amplifier and the
optical beam pattern the sensors exhibit. Every lens generates
a different pattern. These patterns are determined by the size,
shape, material and quality of the lens, the size and intensity or
sensitivity of the source or detector chip and the focal length
between chip and lens. A relationship that clarifies the
understanding of various patterns is the comparison of
radiation patterns to shotgun patterns.
Fig. 24 Excess gain curves for long range, short
range and focused proximity photoelectric controls.
Compare the excess gain curves in figure 23. The short range
sensor delivers high excess gain over a short sensing
distance and then drops off rapidly. This is due to the fact that
the source beam and the detector's field of view converge
over a short distance from the lenses, so the energy present in
the area of coincidence is very high. This makes detecting
small or difficult to sense surfaces possible. It also provides
you with the ability to ignore objects or surfaces in the near
background.
Fig. 23 Comparison of beam patterns to shotgun patterns.
The long range sensor's source beam and detectors field of
view are positioned close together on the same axis. This
results in maximum convolvement of the source beam and
detector field of view out to the maximum range of the sensor.
Excess gain peaks several inches out from the sensor then
drops off slowly over distance. Optical detection up to 10 feet on
a white surface is possible. A clear field of view several feet
from the sensor is necessary to prevent latch up or false
triggering.
A shotgun barrel's length and choke can be seen as the
counterpart of focal length. Similarly, barrel gauge is
comparable to chip size and shell load is comparable to
radiant power and sensitivity.
If you had a box of shotgun shells in which all the shells were
exactly the same, you could get different performance from
each shell by putting each in a different style shotgun. If you
fired one from a sawed off shotgun, the resulting pattern would
be very broad with little appreciable range. If you placed
another shell in a regular barreled shotgun, the pattern would
be narrower but with much greater range. A third shell could be
fired from a shotgun with a choke on it and the pattern would be
narrower and the range longer.
(See the Eaton’s Sensing Solutions catalog for a
breakdown of sensing fields for specific models)
Diffuse Proximity Performance
Because excess gain in proximity units is dependent on more
variables than a thru-beam system, the graphic plot is more
complex. Since nearly every proximity sensor has a different
combination of lenses and beam angles, nearly every excess
gain curve for proximity sensors is different. The excess gain
for a proximity sensor is expressed as a ratio between the
amount of light received to the amount of light required to
trigger the sensor.
Excess Gain = Light Received /Minimum Light Required
Published excess gain curves for proximity sensors are
determined using a large Kodak #R-27 90% reflectance test
card. The following figures are typical of proximity excess
gain curves.
Fig. 25 Short range proximity sensors provide high
excess gain at close range without triggering on near
background objects. Long range sensors detect objects
up to 10 feet away.
In a focused spot proximity sensor the source and detector are
positioned behind the lens in order to focus the energy to a
point. The source beam and detector field of view converge at
the focus point forming a sensing zone about 0.128 inches or
smaller in diameter. The excess gain is extremely high at this
focused point and drops off very fast on either side of the
sensing zone. A focused proximity sensor is excellent for
sensing into holes or cavities, for detecting very small objects,
or level detection between two surfaces.
Retroreflectors and Reflex Performance
Reflex sensor range and excess gain are dependent on reflector
quality. Two types of retroreflector target materials are available,
corner cube and embedded glass bead reflectors.
Fig. 26 On a focused spot sensor, the energy is focused to
a point out from the lenses, thus forming a detection zone
which will be blind at any spot other than the point of focus.
Reflex Performance
Excess gain curves for reflex sensors are similar in appearance
to proximity sensors. In this case, excess gain and range are
related to the light returned from a retroreflective target. Other
than the actual retroreflector type used, the maximum operating
range is also dependent on lens geometry and detector
amplifier gain. Reflex sensors feature ranges up to 75 feet to a 3
inch retroreflector. As shown in Figure 26 the effective beam is
defined as the actual size of the retroreflector surface. The
entire reflector must be blocked by the target object before the
sensor will recognize a beam blockage and switch its output.
Fig. 28 Corner cube and glass bead retroreflectors.
Corner cube retroreflectors provide the highest signal return to
the sensor. Cube style reflectors exhibit 2000 to 3000 times the
reflectivity of white paper. When a ray of light strikes one of the
three adjoining sides (A) arranged at right angles to each
other, the ray is reflected to the second side (B) and then to a
third (C) then back to its source in a direction parallel to its
original course. Thousands of these cube shapes are molded
into a rugged plastic reflector or vinyl material. The fact that the
light returned from a corner cube surface is depolarized with
respect to the received light makes this the only retroreflec-tor
to use with polarized reflex sensors.
Target size affects the maximum sensing range.
Fig. 27 Beam pattern for a reflex sensor.
Fig. 29 Reflector areas affect on reflex performance.
Glass bead retroreflectors are available in tape form for use in
dispensers for package coding on conveyors and in sheet form
which the user can cut to size. The bead style surface is
typically rated at 200 to 900 times the reflectivity of white paper.
Glass bead type retroreflectors cannot be used with polarized
reflex sensors.
Contrast
Contrast measures the ability of a photoelectric control to detect
an object. It is expressed as a ratio between excess gain under
light conditions and excess gain under dark conditions.
Contrast Ratio = Excess Gain under light conditions /
Excess Gain under dark conditions
When applying photoelectric controls, the sensing mode that
provides the greatest contrast ratio should be selected. For
reliable operation, a ratio of 10:1 is recommended.
Contrast and Sensing Modes
The contrast a thru-beam or reflex sensor perceives is affected by:
detect a semi-transparent plastic bottle moving through the
sensing zone. Given that the excess gain at that range equals
100, and the bottle blocks 5% of the light energy and passes
95% of the light, the contrast ratio would be approximately 1
(100/95). This does not meet the 10:1 ratio recommended, and
indeed the application would not work. The thru-beam pair is
simply too powerful for this application. Note that the high
excess gain provided by this thru-beam sensor pair does not
offer any advantage in this application.
A focused proximity sensor with the excess gain shown on the
chart in figure 30, and positioned 3 to 4 inches from the bottle
could provide the high contrast required, and provide
excellent back-ground rejection. This product would work
better in this application than the thru-beam pair.
• The light transmissivity of an object or surface.
• The size of an object in relation to the effective beam size.
The contrast a proximity sensor perceives is related to the
amount of light an object or surface reflects back to the sensor.
This reflectivity is affected by:
• How far the object or surface is from the sensor.
• Color or material of the object or surface.
• Size of the object or surface.
The ideal application provides infinite contrast ratio of the
detection event. This is the case, for example, when 100% of
the beam is blocked in reflex or thru-beam sensing modes, or
when nothing is present in the case of proximity sensing modes.
Understanding the contrast ratio becomes critical when this
situation does not exist, such as detecting semi transparent
objects, or when sensing extremely small objects.
For example, a thru-beam sensor is positioned 10 inches apart to
In the detection zone 3 to
4 inches from the sensor,
the excess gain is 20 to
100. The background will
have no affect on the
reliability because
excess gain = 0.
Fig. 30
Photoelectric Control Logic
Photoelectric control logic can be divided into two categories.
First, are devices that "condition" the signal between the
detector and the output device. Typically, these are logic
modules that are mounted inside the photoelectric control and
include timing or counting functions. Second, is switch logic.
Output devices from two or more controls can be wired in
series or parallel providing an output to the load only when the
correct combination of controls is energized.
Timing Functions
Timing functions provide a natural extension to the simple
sensor by altering the raw sensor signal to make it more useful
for controlling local action in response to sensed events.
Timing functions "condition" the detection event causing the
output signal from the sensor to be stretched, shortened or
displaced in time.
Light Operate Versus Dark Operate
In most applications, photoelectric controls generate an output
whenever an object is detected. This occurs in one of two
modes. If the control generates an output when the photo
detector sees light, the control is said to be working in the
"Light Operate" mode. If the control generates an output when
the photo detector does not see light, the control is said to be
working in the "Dark Operate" mode. Light /dark operation is
normally selected by a switch mounted inside the control. This
is most useful when the sensor is equipped with a single pole
output device.
Off Delay Logic
Off delay logic holds the output on for a predetermined period of
time after an object is no longer detected. The output is turned
on as soon as the object is detected. Off delay ensures that the
output will not drop out despite short periods of signal loss. If an
object is once again detected before the output times out, the
output will remain on. Delay off logic is useful in marginal
applications susceptible to periodic signal loss such as web
detections.
Fig. 31 Demonstration of light operation versus dark
opera-tion.
When a photoelectric control is operating without a logic
function, an output is generated for the length of time an object
is detected. This can be expressed in the following diagram.
Fig. 34 Off delay logic.
On/Off Delay Logic
Fig. 32 Diagram showing object detected versus output.
On/off delay logic combines on and off delay so the output will
be generated only after the object has been detected for a
predetermined period of time, and will drop out only after the
object has no longer been detected for a predetermined period
of time. Combining on and off delay "smooths" the output of the
photoelectric control for applications such as jam detection, fill
level detection and edge guide.
One-Shot Logic
On Delay Logic
On delay logic allows the output signal to turn on only after the
object has been detected for a predetermined period of time.
The output will turn off immediately after the object is no longer
detected. This logic is useful if a sensor must avoid false
interruptions from small objects, but detect a large or slow
moving object. On delay is useful in bin fill detection, or jam
detection since it will not false trigger on the normal flow of
objects going past.
Fig. 35 On/off delay logic
One-shot logic generates an output of predetermined length
no matter how long an object is detected. A one shot can be
programmed to trigger on the leading or trailing edge of the
object detected. A standard one-shot must time out before it
can be retriggered. One-shot logic is useful in applications that
require an output of specified length, such as an air valve
actuating a kicker on a conveyor line.
Fig. 33 On delay logic.
Fig. 36 One-Shot Logic
Retriggerable One-Shot Logic
Underspeed/Overspeed Detection Logic
Just like a standard one-shot, a retriggerable one-shot
generates an output of predetermined length whenever an
object is detected. A retriggerable one-shot will restart each
time an object is detected and will remain triggered as long as
a stream of objects are detected before the one-shot times out.
A retriggerable one-shot is useful in detecting underspeed
conditions in conveyor lines.
This detection logic is capable of detecting overspeed
conditions as well as underspeed conditions. An underspeed /
overspeed detector counts a predetermined number of objects
within a specified length of time. If the system operates either at
a higher or lower rate, an output is generated.
.
Fig. 40 Overspeed/underspeed detection logic.
Fig. 37 Retriggerable one-shot logic.
Delayed One-Shot
Delayed one-shot logic combines on delay and one-shot logic.
In this function the one-shot feature is delayed for a
predetermined period of time after an object has been detected.
A delayed one-shot is useful in applications where the
photoelectric control cannot be mounted exactly at the site
where the action caused by the output device is taking place.
Applications include a spray paint booth (the control cannot be
mounted inside the booth), high temperature ovens or drying
bins.
Fig. 38 Delayed one-shot logic.
Underspeed Detection Logic
Underspeed detection logic operates identically to a
retriggerable one-shot in that it detects speeds that fall below a
certain predetermined level. In addition to this feature, the
underspeed detector has a built in latch feature that shuts the
system completely down when the speed slows to the
predetermined level. This prevents the one-shot from
retriggering once it times out, thereby eliminating erratic
switching while the motor is winding down.
Fig. 39 Underspeed detection logic
Output Devices and Switch Logic
A photoelectric control actively interfaces to the outside world
through an output switching device. The load to be energized
may be a solenoid or relay coil, counter module, or input card
to a programmable controller. Depending on the current
requirement, AC or DC operating voltage, and switching speed,
an appropriate output device must be selected for best long
term performance. Photoelectric con-trols are available with
built-in solid state AC, DC or AC /DC switches, as well as with
sockets for replaceable output modules for quick repair and
system flexibility.
Types of Output Devices and Their Symbols
Transistor devices (DC switch)
Fig. 42
Transistors are solid state DC switching devices. They are
most commonly used in low voltage DC powered photoelectric
sensors as the output switch. Two types of transistors are
commonly used depending on the switching function. The NPN
current sink provides a contact closure to DC common and the
PNP current source provides a contact closure to the DC
positive rail. A transistor can be thought of as a single pole
switch which must be operated within its voltage and maximum
current ratings. Any short on the load will immediately destroy a
transistor switch.
Relay Devices
Fig. 41
A relay is a mechanical switch which is available with a variety
of contact configurations. Relays can handle large load currents
at high voltages allowing them to directly interface with motors,
large solenoids, brakes and ejectors. They can switch either AC
or DC loads. Contact life depends on the load current, and
frequency of operation. Relays are subject to contact wear and
contact resistance build-up. Also, because of contact bounce
they can produce erratic results with coun-ters and
programmable controller inputs unless the input is filtered.
Being mechanical, they can add 10 to 25 mS to a
photoelectric's response time.
Since relays are most familiar to factory personnel, and
because they provide multiple contacts, relays are the most
common output device used with photoelectric sensors.
Fig. 43 A transistor can be looked at as a single pole switch.
Switching inductive loads creates voltage spikes many times
the control voltage level which would exceed the maximum
voltage rating of the transistor. Peak voltage clamps such as
zener diodes or transorbs are utilized to protect the output
device. Transistor outputs are typically rated to switch loads of
250 mA at 30 VDC maximum.
Features—Mechanical relays
• Switch high currents /voltages
• Multiple contacts
• Switch AC/DC voltages
• Tolerant of momentary short circuits and large inrush currents
Limitations—Mechanical relays
• Slow response time (10-25 mS)
• Contact and mechanical wear
• Contact bounce
• Affected by shock and vibration
Fig. 44 Action of a Zener diode used to clamp
inductive spikes for transistor protection
Features—Transistor switches
• Virtually instantaneous response time
• Low off state leakage and voltage drop
• Infinite life when operated within rated current/voltage
• Not affected by shock/vibration
• Interface direct to TTL and CMOS circuits
Limitations—Transistor switches
• Low current handling
• Cannot tolerate large inrush currents (unless clamped)
• Destroyed by short circuit
Triac Devices
Fig. 47 Use of R.C. snubber networks to reduce triac false
triggering by inductive
spikes. Features—Triac switches
• Fast response time (8.33 mS)
• Tolerant of large inrush currents
• Direct interface to counters and programmable controllers
• Infinite life when operated within rated current/voltage
• Not affected by shock/vibration
Fig. 45 Optically isolated triac switch
A triac is a solid state device designed to control AC current.
Triac switches turn "ON" in less than a microsecond when its
gate (control leg) is energized and shuts "OFF" at zero crossing
of the AC power cycle. Because a triac is a solid state device, it
is not subject to the mechanical limitations of a relay such as
contact bounce, pitting and corrosion of contacts, or shock and
vibration sensitivity. Switching response time is limited only to
the time it takes the 60 Hz AC power to go through one-half
cycle.
Fig. 46 Triac switches can be turned "ON" at any
point in the AC power cycle, "OFF" only at zero
crossing.
As long as the triac is used within its rated maximum current
and voltage specifications, life expectancy is virtually infinite.
Triac devices used with photoelectric sensors generally are
rated for 2 A loads or less.
Triacs do have limitations. Like a transistor, shorting the load will
destroy a triac. Inductive loads directly connected to the triac or
large voltage spikes from other sources can false trigger a triac
device. To reduce the effect of these spikes a snubber circuit
composed of a resis-tor and capacitor in series is connected
across the device. Depending on the maximum load expected
to be switched, an appropriate snubbing network to protect the
triac must be used. The snubbing network contributes to the
"OFF STATE" leakage the load would see. A triac rated for 1 A
loads may have 5 mA of "OFF STATE" leakage. This leakage
must be taken into account switching loads requiring little
current such as inputs to PLC's. In "ON STATE" triacs exhibit
about 1.7 VRMS voltage drop.
Limitations—Triac switches
• Can be false triggered by large induced currents
• Snubber network contributes "OFF STATE" leakage
• Destroyed by short circuits
Bilateral FET Device (AC/DC Switch)
Fig. 48
The FET (Field Effect Transistor) is slated to be the solid state
switch of the future because of its near ideal operating
characteristics. The voltage applied to the gate controls the
conduction resistance between the source and drain. In the
"OFF STATE," source to drain resistance is typically hundreds
of megohms and only about 1 ohm when "ON." FET switches
exhibit no "OFF STATE" leakage and, being resistive devices,
do not develop the fixed voltage drop across its terminals like
other solid state switch devices. Unlike a triac switch, switching
occurs immediately. FET devices are independent of voltage or
current phase. FET switches can be configured in circuits which
will control AC or DC voltages and will not generate switch
induced line noise like relay and triac switches. FET switches
cannot tolerate line spikes or large inrush currents. The device
must be protected by using a voltage spike clamping device
such as a transorb. It shunts voltage spikes which exceed the
conduction threshold voltage of the transorb and dissipates the
energy as heat.
Features—Bilateral FET switches
• Switch AC or DC voltages
• Low "ON STATE" voltage drop
• Extremely fast response time
• Infinite life when operated within voltage/current ratings
• Interface direct into TTL and CMOS circuits
• Does not self-generate line noise
Limitations—Bilateral FET switches
• Cannot tolerate large inrush currents
• Can be destroyed by line spikes (if not clamped)
Two-wire switch device (AC/DC)
A two-wire photoelectric sensor is composed of 3 main
components: the photoelectric sensing head, the power switch
base, and the wiring receptacle. The power base assembly is a
standard mechanical limit switch style body which houses the
sensor power supply, output switch circuit, and socket for plugin logic modules. As the name implies, it requires only 2
connections just like the standard mechanical limit switch. All
electronics are encapsulated in epoxy for electrical insulation of
components and to provide vibration and shock resistance.
A two-wire electronic switch approximates a mechanical switch
with the circuit shown above. When the switch is open the
resistor provides a leakage path around the switch to power the
photoelectric circuitry. This creates an "OFF STATE" leakage of
about 1.7 mA. When the switch is closed the zener diode
regulator maintains enough voltage to power the circuitry. The
diode bridge converts AC load current to DC for powering the
sensor. Two-wire switches "steal" their operat-ing power from
the load circuit. This means there will be some leak-age current
when the switch is off and about 7 to 9 volts dropped across the
switch when it is on. Leakage current must be about 1.7 mA or
less to ensure compatibility with programmable controller inputs.
Because very little power is available to operate the control
(including LED status indicators, source, detector, switch
circuitry, and logic module), large lens surfaces are needed to
provide reasonable optical performance and make up for the
lack of available operating current. A two-wire photoelectric
control compromises optical performance for low "OFF STATE"
leakage.
The voltage drop across the switch is cumulative when more
than one switch is wired in series with a load. The "OFF
STATE" leakage current is cumulative when more than one
switch is connected in parallel with a load.
Features-2-wire switch
• Two-wire connection (low wiring cost)
• Familiar wiring and rugged package
• Switch AC or DC loads
• Low leakage current in "OFF STATE"
• Short circuit protection and EMI /RFI immune models available
Limitations-2-wire switch
• Reduced optical performance compared to 3 and 4 wire style sensors
• High "ON STATE" voltage drop across the switch
Fig. 49 Two-wire switch circuit
Logic Function Using Switch Devices
The output devices from two or more photoelectric sensors can
be wired together in series or parallel to perform logic functions.
It is important to keep in mind when dealing with output device
logic that an "ON" condition may represent either object
presence or absence. The user has a choice, through selection
of light operate or dark operate outputs or a light/dark switch in
the control.
Parallel ("OR" Function)
The term "OR" in binary logic defines the resultant output as
being ON if one "OR" more of the inputs is on.
The user should also be aware of the possible side effects of
these connections dependent on the type of switch used. These
side effects include: excessive voltage drop in series connected
switches and excessive leakage current in parallel connected
switch devices. The load being switched is a determining factor
at which point the above effects will interfere with proper
operation. Output switches that exhibit the above effects are:
triac devices ("OFF STATE" leakage current), and two-wire
devices ("OFF STATE" leakage and "ON STATE" voltage drop).
Fig. 50 "OR" function truth table.
The "OR" function is accomplished by connecting switches in
parallel. The diagram shows normally open relay contacts. If
switch A, B, "OR" C closes, the load will be energized. The
switches shown could be any optically isolated solid state
switch having both its terminals available for connection
including: (A) isolated NPN transistor, (B) isolated bilateral
FET, and (C) isolated triac device. "OR" logic functions can
also be accomplished using 3-wire type sensors.
Fig. 51 Symbols of optically isolated solid state switches.
10 to 30 VDC powered sensors employing transistor output
switches normally have one leg of the switch connected to DC
common (the emitter on NPN transistors), or the positive rail
(the emitter on PNP transistors). In this case, NPN current sink
outputs may only switch loads in parallel to circuit common
and PNP current source outputs may only switch in parallel to
the positive DC rail.
Fig. 53 “OR" circuit utilizing solid state AC switches and the
affect of summed leakage currents.
Summed leakage currents equal: (1.7 mA)(3) = 5.1 mA total
"OFF STATE" leakage delivered to the load. If the load is
effected by the total leakage applied, a shunt resistor can be
connected across the load to reduce the leakage seen by
the load. This problem is only encountered when switching
programmable controller inputs or other high impedance
inputs.
Example application utilizing parallel "OR" logic
Two thru-beam sensor systems are positioned to monitor the
possible presence of intruders into a building. The output
devices from the photoelectric sensors are connected in parallel
with each other with normally open contacts as long as the
beams are complete. The sensors are set for dark energize.
The load to be energized is a solenoid that will release the latch
on a cage full of hungry guard dogs.
Series "AND" logic/gating functions
The term "AND" in binary logic defines the resultant output as
being ON only when all inputs are ON. This is accomplished
by connecting switches in series with the load. This type of
circuit is best suited for the application of isolated switch
devices not tied to power supply rails with one of their
connections, such as a three-wire switch.
Fig. 52 Current sink and current source “OR” circuits with
output devices tied internally to one side of the DC supply
line.
Two, three and four-wire AC switches can be connected in
an "OR" circuit configuration. Proper wiring techniques
recommend only switching the hot side of the line in AC
circuits.
When parallel connecting triac switches or two-wire
photoelectric sensors, attention should be paid to how much
"OFF STATE" leakage current the load will see. The leakage
current is summed in parallel connections as shown in the twowire "OR" circuit, Fig. 54.
Fig. 54 "AND" function truth table with example series circuit.
When using solid state switches for this function the voltage
drop across each switch will reduce the power the load will
receive. This is mainly a problem only when utilizing two-wire
photoelectrics because of the significant voltage drop (7 to 9 V)
they exhibit. Depending on the minimum amount of voltage a
load will require to operate properly will determine how many
two-wire switches or voltage drops may be connected in series.
Fig. 55 Some solid state switches can reduce the
amount of voltage the load will ultimately see.
"OFF STATE" leakage currents are not cumulative in a series
circuit.
The idle state of the switch contacts must be set depending
on the sensing mode by programming the light/dark energize
switch in the photoelectric sensor.
"AND" Gating function
The logical "AND" configuration is commonly used to perform
gating functions. We can use the gating function to perform
inspection of fill levels, object placement or presence. The
gate sensor usually will employ a one-shot logic module set to
trigger on detection of an edge (light/dark, or dark/light). The
one-shot will allow a short period of time for the inspection
sensor to determine whether the object is OK. If not, the circuit
will be completed signaling an alarm, firing a sole-noid for
rejection of the object, or shutting down the machine.
Example gating application
A short range proximity sensor is positioned near the neck of a
jar. The sensor will be set to energize when the jar is detected.
A one-shot logic module inside the proximity sensor is set to
trigger on the dark to light transition for a short period of time.
The proximity gating sensor's output device is connected in
series with a thru-beam sensor switch that is set in light
energize mode. The sensors interface to a programmable
controller which monitors the inspection system input. Jars with
labels will never complete the series circuit so the
programmable controller will ignore them. If a jar without a label
is detected, the series circuit will be energized for the gate
sensors one-shot period signaling the programmable controller
to reject the jar at the reject location.
This manual has been prepared for use by personnel, licensees and
customers of Eaton Corporation. The information contained herein is the
property of Eaton, and may not be copied or reproduced in whole or in
part, without prior written approval.
Eaton reserves the right to make changes, without notice, in the
specifications and materials contained herein and shall not be
responsible for any damages, direct or consequential, caused by reliance
on the materials presented.
Fig. 56 Using the gating function to inspect jars for
presence of labels. System rejects non-labeled items.
Any information and/or application example, including circuitry and/or wiring
diagrams, programming, operation and/or use shown and/or described in
this manual are intended solely to illustrate the operating principles of the
product. The presentation of an example of use shown and/or described
herein does not guarantee nor imply such example will perform in a
particular environment when converted to practice. Eaton does not assume
responsibility or liability for actual use based on the examples illustrated
and/or described herein. No patent liability is assumed by Eaton with
respect to use of any applications, information, circuitry, diagrams,
equipment or programs shown and/or described herein.
Appendix
Speed Conversion Table
feet/minute inches/minute inches/second seconds/inch
0.5
6
0.1
10
1
12
0.2
5
2
24
0.4
2.500
3
36
0.6
1.666
4
48
0.8
1.250
5
60
1.0
1.000
6
72
1.2
0.833
7
84
1.4
0.714
8
96
1.6
0.625
9
108
1.8
0.555
10
120
2.0
0.500
11
132
2.2
0.435
12
144
2.4
0.417
13
156
2.6
0.385
14
168
2.8
0.358
15
180
3.0
0.333
16
192
3.2
0.313
17
204
3.4
0.294
18
216
3.6
0.278
19
228
3.8
0.263
20
240
4.0
0.250
21
252
4.2
0.238
22
264
4.4
0.227
23
276
4.6
0.217
24
288
4.8
0.208
25
300
5
0.200
30
360
6
0.167
40
480
8
0.125
50
600
10
0.100
feet/minute inches/minute inches/second seconds/inch
60
720
12
0.083
70
840
14
0.071
80
960
16
0.063
90
1080
18
0.056
100
1200
20
0.050
125
1500
25
0.040
150
1800
30
0.033
175
2100
35
0.029
200
2400
40
0.025
225
2700
45
0.022
250
3000
50
0.20
275
3300
55
0.018
300
3600
60
0.016
325
3900
65
0.015
350
4200
70
0.014
375
4500
75
0.013
400
4800
80
0.012
450
5400
90
0.011
500
6000
100
0.010
600
7200
120
0.008
700
8400
140
0.007
800
9600
160
0.006
900
10800
180
0.0055
1000
12000
200
0.005
1250
15000
250
0.004
1665
19980
333
0.003
2500
30000
500
0.002
5000
60000
1000
0.001
For more
information,
call 1-800-426-9184