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.