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Photoelectric ( Opto-electronic) proximity sensors
A photoelectric proximity sensor uses a light-beam generator, a photo-detector, a special amplifier, and a microprocessor. The light beam reflects from an object and is picked up by the photo-detector. The light beam is modulated at a specific frequency, and the detector has a frequency-sensitive amplifier that responds only to light modulated at that frequency. This prevents false imaging that might otherwise be caused by lamps or sunlight.
Photoelectric sensors offer non-contact sensing of almost any substance or object up to a range of 10 meters. Photoelectric sensors consist of a light source (usually an LED, light emitting diode, in either infrared or visible light spectrum) and a detector (photodiode). Due to the high intensity infra-red energy beam, these sensors have major advantages over other opto-electronic systems when employed in dusty environments. With their focused beam and long range, opto-electronic sensors are increasingly used in applications where other sensing techniques are lacking in sensing distance or accuracy.
Photoelectric sensors are available in a variety of modes including:
1. Infrared Proximity (Diffuse Reflective)
Proximity type photoelectric sensors detect the light reflected by the target itself. Proximity photoelectric sensors are preferable for general purpose sensing applications, particularly where the detected object is only accessible from one direction.
2. Transmitted Beam (Thru-beam)
Transmitted beam photoelectric sensors use separate infrared transmitters and receivers. Objects passing between the two parts interrupt the infrared beam, causing the receiver to output a signal.
3. Retro-reflective (Reflex)
Retro-reflective photoelectric sensors operate by sensing the light beam that is reflected back from a target reflector. As with thru beam models, objects which interrupt the beam activate an electronic output.
4. Polarized Retro-reflective (Polarized Reflex)
Polarized retro-reflective sensors work like normal retro-reflective sensors but use a polarizing filter in front of the transmitter and receiver optics. These filters are designed so that shiny objects are reliably detected.
5. Fibre Optic
Fibre optic sensors use fibre optic cable to conduct light from the LED to the sensing area, and another cable to return light from the sensing area to the receiver. By using fibre optic cables, the electronics can be protected from hostile environments such as temperature extremes and harsh chemicals. Fibre optics also allow sensing in extremely confined spaces.
6. Background Rejection
STI's background rejection sensors use a special arrangement of two sensing zones: the near-field zone is where objects can be detected, the far-field zone is where objects cannot be detected. There is an extremely sharp cut-off between these zones. The cut-off range is adjustable. These sensors are ideal for applications where background objects need to be ignored.
An acoustic proximity sensor works on the same principle as sonar. A pulsed signal, having a frequency somewhat above the range of human hearing, is generated by an oscillator. This signal is fed to a transducer that emits ultrasound pulses at various frequencies in a coded sequence. These pulses reflect from nearby objects and are returned to another transducer, which converts the ultrasound back into high-frequency pulses. The return pulses are amplified and sent to the controller. The delay between the transmitted and received pulses is timed, and this will give an indication of the distance to the obstruction. The pulse coding prevents errors that might otherwise occur because of confusion between adjacent pulses.
