Photoelectric sensors represent perhaps the largest variety of problem solving choices in the industrial sensor market. Today ’s photoelectric technology has advanced to the point where it is common to find a sensor that will detect a target less than 1 mm in diameter while other units have a sensing range up to 60 m. These factors make them extremely adaptable in an endless array of applications. Although many configurations are available including laser-based and fiber optic sensors, all photoelectric sensors consist of a few of basic components. Each contains an emitter, which is a light source such as an LED
(light emitting diode) or laser diode, a photodiode or phototransistor receiver to detect the light source, as well as the supporting electronics designed to amplify the signal relayed from the receiver.
Probably the easiest way to describe the photoelectric operating principal is: the emitter, also referred to as the sender, transmits a beam of light either visible or infrared, which in some fashion is directed to and detected by the receiver. Although many housings and designs are available they all seem to default to the basic operating principal.
Just as the basic operating principal is the same for all photoelectric families, so is identifying their output. “Dark-On” and ”“Light-On” refers to output of the sensor in relation to when the light source is hitting the receiver. If an output is present while no light is received, this would be called a “Dark On ” output. In reverse, if the output is ON while the receiver is detecting the light from the emitter, the sensor would have a “Light-On ” output. Either way, a Light On or Dark On output needs to be selected prior to purchasing the sensor unless it is user adjustable. In this case it can be decided upon during installation by either flipping a switch or wiring the sensor accordingly.
The method in which light is emitted and delivered to the receiver is the way to categorize the different photoelectric configurations. The most reliable style of photoelectric sensing is the through beam sensor. This technology separates the emitter and receiver into separate housings. The emitter provides a constant beam of light to the receiver and detection occurs when an object passing between the two breaks the beam. Even though it is usually the most reliable, it often is the least popular due to installation difficulties and cost. This is because two separate pieces (the emitter and receiver) must be purchased, wired and installed. Difficulties often arise in the installation and alignment of two pieces in two opposing locations, which may be quite a distance apart.
Through beam photoelectric sensors typically offer the longest sensing distance of photoelectric sensors. For example, units are available with a 25 m and more sensing range. Long range is especially common on newly developed photoelectric sensors such as models containing a laser diode as the emitter. Laser diodes are used to increase sensing accuracy and detect smaller objects These units are capable of transmitting a well-collimated beam with little diffusion over the sensing ranges as long as 60 m. Even over these long distances, some through beam laser sensors are capable of detecting an object 3 mm in diameter, while objects as small as .01 mm can be sensed at closer ranges. However, while precision increases with laser sensors the speed of response for laser and non-laser through beam sensors typically remain the same, around 500 Hz. An added bonus to through beam photoelectric sensors is their ability to effectively sense an object in the presence of a reasonable amount of airborne contaminants such as dirt. Yet if contaminants start to build up
directly on the emitter or receiver, the sensor does exhibit a higher probability of false triggering. To prevent false triggering from build up on the sensor face, some manufacturers incorporate an alarm output into the sensor ’s circuitry. This feature monitors the amount of light arriving on the receiver. If the amount light decreases to a certain level without a target in place, the sensor sends a warning out by means of a built in LED and/or an output wire.
A very familiar application of a through beam photoelectric sensor can be found is right in your home. Quite often, a garage door opener has a through beam photoelectric sensor mounted near the floor, across the width of the door. This sensor is making sure nothing is in the path of the door when it is closing. A more industrial application for a through beam photoelectric is detecting objects on a conveyor. An object will be detected anyplace on a conveyor running between the emitter and receiver as long as there is a gap between the objects and the sensors light does not “burn through ” the object. This is more a figurative term than literal. It refers to an object that is thin or light in color and allows the light emitted from the emitter to penetrate the target so the receiver never detects the object.
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