Chapter 7
Photoelectric Sensing
Photoelectric Sensing
• Photoelectric Sensing detects target with
optoelectronic device that
– Reflects light back to sensor
– Has light interrupted
– Has light partially absorbed
• Can classify sensors by
Classification by Sensing
Classification by Output Circuit
Photoelectric Sensing Modes
Photoelectric Sensing Modes
• Opposed mode
– Positions opto-emitter and opto-detector opposite
each other and aims emitter directly at detector
• Retroflective mode or reflex mode
– Retrorefletctive sensor contains both photo-emitting
and photo-detecting device.
– Detectors sense reflected beams off surface of object
moving past which breaks the reflected beam
• Proximity mode
– Uses photosensor detecting presence of object
directly in front of it. Senses own reflected energy off
sensed object. Creates own light beam.
• Light Curtain
– Light produced by a line or predefined array of
emitting and receiving photoelectric sensors
which detect objects anywhere in the defined
sensing area.
• Polarizing Filter
– Allows retroreflective sensor’s emitter to send
a horizontally polarized wave.
– Receiver accepts only a vertically polarized
wave.
– Can sense solid objects (not transparent)
– Not affected by shape, color or surface
material
– Filter in front of receiving sensor prevents light
received directly from emitter, so only reflected
light is received.
• Detecting transparent objects
– Use a reflector
– No polarizing filter
– Retroreflective sensing mode
Reflective Sensing Mode
Narrow sensing area
More precise than ordinary
diffuse reflective sensors.
Detects object in area where
emitting light and receiving
light cross each other.
Background and surrounding
objects don’t affect this sensor.
Reflective Sensing Mode
Emits light to object and detects it
by sensing difference of reflecting
angle.
Detects small objects with great
accuracy.
Detects object at certain distance
by returning light at an incident
angle which hits a lens and
photodiode.
Uses optical lamp to detect color by
distinguishing between the
difference in reflection between the
object sensed and its background
color.
Has two receiving sensors.
Detects object by looking at
difference of light intensities of
the two sensors.
• Photocells
– Appropriate for good sensitivity to visible light
• Photodiodes
– Best for tasks needing very fast response time
– Has linear response to light over several
orders of magnitudes.
• Modulation Types
– Pulse Modulation
• Most photoelectric sensors use pulse modulation
methods for emitting light.
• Turn on and off
• Has high switching speed
• Can distinguish between emitted light and ambient
light
• Works at long range.
Intensity
Time
• Unmodulated Light
– Few sensors use this type
– Use in color mark sensor
– Use for high speed fiber optic sensors
– Has constant intensity (unmodulated)
– High speed response times
– Slight susceptibility to ambient light
Intensity
Time
Advantages
• Main advantages of Lasers
– Have just one wavelength
– Superior in directivity
– High energy density
– Easily interface with other lasers
• Semiconductor Lasers
– Most common for industrial and
manufacturing applications
– Class 1
– Safest category
– No safety precautions necessary
Precautions
• Always ground the sensor’s frame.
• A capacitor ground the sensor and
improves noise resistance.
• Don’t use laser sensors near steam.
• Don’t immerse laser in water.
• Types of Photoelectric Sensor Delays
– On-delay
• Eliminates output signal transient detections
• Used to detect long objects
• Used to detect jamming of conveyer or assembly line.
– Off-delay
• Prolongs output signal for small amount of time
• Used if connecting device has slow response time
– One-shot
• Makes all sensor outputs have same period
• Allows other devices to detect signal
• Ways to reduce proximity photosensor’s
sensitivity to specular (shiny or reflective)
surfaces.
– Use proximity sensor with no lens.
• Eliminates collimated sensing patterns due to the
lens
• Photometers
– Measure visible incident light
– Measures irradiance (flux/area)
• Radiometer
– Measures non-visible light
– IR and UV
Applications
Sense if part conforms to standards.
Detect if all the resistors have been taped together correctly.
Can detect wires as small as 0.1 mm in diameter.
Impervious to supply voltage variations and temperature changes.
High sensitivity to color.
CIE curve corrected.