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Photoelectric Sensor Overview (continued)
The photoelectric family with the next longest sensing distance is called retro-reflective sensors, commonly referred to as a “retro”. Retro--reflective sensors operate similarly to through-beams without being able to reach the same sensing
distances. Certain units may still be used in applications needing ranges of up to 10 m. The similarity between retro-reflective and though beam photoelectrics is that there is a constant beam that needs to be broken in order for an output to occur. But, instead of having a separate housing for the emitter and receiver, they are both located in the
same housing, facing the same general direction. The emitter produces a laser, infrared or visible light and projects the beam towards a specially designed reflector, which returns the beam, back to the receiver. .Detection occurs when the light path is
broken or otherwise interfered with. If the output occurs when the beam is broken, the sensor would be considered a dark-on photo. A reason one would specify a retro-reflective sensor over a through beam is because only one location needs to be wired for installation. The opposing side simply requires installation of the reflector. This could result in a big cost savings in both parts and time. However, objects that are very shiny or highly reflective like a mirror, a can, or small juice box wrapped in clear plastic can provide a challenge to a retro-reflective photoelectric. These targets may reflect enough light to “trick ” the sensor: because ample light is reflected from the object, the receiver may not recognize that the beam has been interrupted and the sensor does not identify that the target has passed. Some manufacturers have addressed this problem with a polarization filter, which allows only light reflected a specially designed reflector to be received, and not erroneous reflections from the target. 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.
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