What Is Photoelectric Sensor ?

What Is Photoelectric Sensor ?

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.

Dubai Sensor

 
Diffuse sensors operate under a somewhat different style than retros and through-beams although the operating principle remains the same: diffuse photoelectrics actually use the target as the “reflector”, such that detection occurs upon reflection of the light off the object back onto the receiver as opposed to an interruption of the beam. The emitter sends out a beam of light. Most often it is a pulsed infrared, visible red or laser beam, which is reflected by the target when it enters the detectable area. The beam is diffused off of the target in all directions. Part of the beam will actually return back to the receiver inside of the same housing in which the sensor originally emitted it from. Detection occurs and the output will either turn on or off (depending upon if it is Light On or Dark On) when sufficient light is reflected to the receiver. This can be commonly witnessed in airport washrooms, where a diffuse photo will detect your hands as they are placed under the faucet and the attending output will turn the water on. In this application, your hands act as the reflector.

Due to the operating principle of using the target as the reflector, diffuse photoelectrics are often at the mercy of target material and surface properties; a non-reflective target such as matte-black paper will have a significantly decreased sensing range as compared to a bright white target. But, what seems as a drawback on the surface can actually be a benefit in practice. Because diffuse sensors are somewhat color dependant, certain versions are suitable for distinguishing dark and light targets in applications that require sorting by contrast or quality control. Specialty versions of diffuse sensors are even capable of detecting different colors. Also, with only the sensor itself to mount, installation of diffuse sensors is usually simpler than for through-beams and retros.
Deviations of sensing distances and false triggers when reflective backgrounds are present led to the development of other diffuse sensors. These new developments, allow the diffuse sensor to “see ” an object while simultaneously ignoring any objects behind it.In the simplest of terms, the sensor is looking out at specific point in the foreground and ignoring anything beyond that point. There are two ways in which this function is achieved, the first and most common is using fixed-field technology. In this technology, the emitter sends out a beam of light like a standard diffuse photoelectric sensor. In turn, the light is received by two receivers and a comparator then evaluates how the light is received. One receiver is focused on the “sweet spot ” or desired sensing location and the other on the background or long range. If the comparator finds the long-range receiver is detecting a higher intensity of reflected light, than the amount on the focused receiver, the output will not turn on. Only when the intensity of light on the focused receiver is above the long-range receiver will an output occur.

Adjustable sensing distance versions are also available. The receiver element in an adjustable-field sensor is accomplished by the use of an array of receivers and a potentiometer to electrically adjust the sensing distance.
Fixed-field and adjustable-field photoelectric sensors operate optimally at their preset “sweet spot ”. They allow for the recognition of small parts and a tight drop-off between the sensed target and cutoff point. They also offer an improvement over a standard diffuse sensors ’ difficulty in sensing different color targets. However, target material surface qualities, such a high gloss, can produce various results. In addition, highly reflective objects outside of the sensing area tend to
send enough light intensity back to the receivers for the output to trigger, especially when the receivers are electrically adjusted.
To combat these limitations, a technology known commonly as true background suppression by triangulation was developed. True background suppression sensors emit a beam of light exactly like a standard diffuse, but unlike fixed-field sensors, which rely on light intensity, background suppression units rely completely on the angle at which the beam returns to the sensor.
To accomplish this, background suppression sensors employ two or more receivers accompanied by a
focusing lens. The receivers remain in a fixed position, while the lens is mechanically adjusted to change the angle of received light. .This configuration allows for an extremely steep cutoff between target and background, sometimes as small as .1 mm. Also, this is a more stable method when reflective backgrounds are present, or large target color variations are an issue: reflectivity and color affect the intensity of reflected light, not the angles of refraction used by triangulation-based background suppression photos.