Photoelectric Sensors Photoelectric Sensors
As the manufacturing world becomes more and more automated, industrial sensors have become the key to increasing both productivity and safety.
Industrial sensors are the eyes and ears of the new factory floor, and they come in all sizes, shapes, and technologies. The most common technologies are inductive, capacitive, photoelectric, magnetic, and ultrasonic. Each technology has unique strengths and weaknesses, so the requirements of the application itself will determine what technology should be used. This article is focused on photoelectric sensors and defines what they are, their advantages and some basic modes of operation.
Photoelectric sensors are readily present in everyday life. They help safely control the opening and closing of garage doors, turn on sink faucets with the wave of a hand, control elevators, open the doors at the grocery store, detect the winning car at racing events, and so much more.
A photoelectric sensor is a device that detects a change in light intensity. Typically, this means either non-detection or detection of the sensor’s emitted light source. The type of light and method by which the target is detected varies depending on the sensor.
Photoelectric sensors are made up of a light source (LED), a receiver (phototransistor), a signal converter, and an amplifier. The phototransistor analyzes incoming light, verifies that it is from the LED, and appropriately triggers an output.
Photoelectric sensors offer many advantages when compared to other technologies. Sensing ranges for photoelectric sensors far surpass the inductive, capacitive, magnetic, and ultrasonic technologies. Their small size versus sensing range and a unique variety of housings makes them a perfect fit for almost any application. Finally, with continual advances in technology, photoelectric sensors are price competitive with other sensing technologies.
A photoelectric sensor, or photo eye, is an equipment used to discover the distance, absence, or presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. They are largely used in industrial manufacturing.
A photoelectric sensor, or photo eye, is an equipment used to discover the distance, absence, or presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. They are largely used in industrial manufacturing. There are three different useful types: opposed (through beam), retro-reflective, and proximity-sensing (diffused).
Through-Beam Sensors Most accurate Longest sensing range Very reliable
Reflective Sensors Only slightly less accurate than through-beam Sensing range better than diffuse Very reliable
Diffuse Sensors Only install at one point Cost less than through-beam or reflective
A photoelectric sensor emits a light beam (visible or infrared) from its light-emitting element.A reflective-type photoelectric sensor is used to detect the light beam reflected from the target.A thrubeam type sensor is used to measure the change in light quantity caused by the target crossing the optical axis.
Photoelectric sensors satisfy applications found in many industries, such as material handling, packaging, food processing, and transportation.
Background SuppressionOur Background Suppression Photoelectric Sensors are designed for applications requiring the sensor to see a target very close to a reflective background. This background suppression is particularly effective when the target and background have similar reflectivity (for example, light returned to the sensor from the target is roughly equal to the light reflecting from the background) or when dark targets are to be sensed against a lighter, more reflective background.
Clear Object Detection photoelectric sensors address the three biggest challenges to the sensor: contrast, shape variations and wet environments.
Color & Contrast Photoelectric Sensors. From true RGB color recognition sensors designed for industrial use to sensors to detect registration marks and a wide range of background colors, including difficult pastels, we make color/contrast sensors to meet your applications needs.
Sensor, including diffuse sensor reflective sensor, through-beam sensor, retro-reflective sensor, and distance sensor settable sensors
Photoelectric Sensors detect objects, changes in surface conditions, and other items through a variety of optical properties.A Photoelectric Sensor consists primarily of an Emitter for emitting light and a Receiver for receiving light. When emitted light is interrupted or reflected by the sensing object, it changes the amount of light that arrives at the Receiver. The Receiver detects this change and converts it to an electrical output. The light source for the majority of Photoelectric Sensors is infrared or visible light (generally red, or green/blue for identifying colors)
1. Long Sensing DistanceA Through-beam Sensor, for example, can detect objects more than 10 m away. This is impossible with magnetic, ultrasonic, or other sensing methods.2. Virtually No Sensing Object RestrictionsThese Sensors operate on the principle that an object interrupts or reflects light, so they are not limited like Proximity Sensors to detecting metal objects. This means they can be used to detect virtually any object, including glass, plastic, wood, and liquid.3. Fast Response TimeThe response time is extremely fast because light travels at high speed and the Sensor performs no mechanical operations because all circuits are comprised of electronic components.4. High ResolutionThe incredibly high resolution achieved with these Sensors derives from advanced design technologies that yielded a very small spot beam and a unique optical system for receiving light. These developments enable detecting very small objects, as well as precise position detection.5. Non-contact SensingThere is little chance of damaging sensing objects or Sensors because objects can be detected without physical contact.This ensures years of Sensor service.6. Color IdentificationThe rate at which an object reflects or absorbs light depends on both the wavelength of the emitted light and the color of the object. This property can be used to detect colors.7. Easy AdjustmentPositioning the beam on an object is simple with models that emit visible light because the beam is visible.
Sensing ModesPhotoelectric sensors provide three primary methods of target detection: diffused, retro-reflective and thru-beam, with variations of each.
Diffused ModeIn diffused mode sensing, sometimes called proximity mode, the transmitter and receiver are in the same housing. Light from the transmitter strikes the target, which reflects light at arbitrary angles. Some of the reflected light returns to the receiver, and the target is detected. Because much of the transmitted energy is lost due to the targets angle and ability to reflect light, diffused mode results in shorter sensing ranges than is attainable with retro-reflective and thru-beam modes.
The advantage is that a secondary device, such as a reflector or a separate receiver, is not required. Factors affecting diffused mode sensing range include the target’s color, size, and finish because these directly affect its reflectivity and therefore its ability to reflect light back to the sensor’s receiver
Diffused Convergent Beam ModeConvergent beam mode is a more efficient method of diffused mode sensing. In convergent beam mode, the transmitter lens is focused to an exact point in front of the sensor, and the receiver lens is focused to the same point. The sensing range is fixed and defined as the focus point. The sensor is then able to detect an object at this focal point, plus or minus some distance, known as the “sensing window”. Objects in front of or behind this sensing window are ignored. The sensing window is dependent on the target’s reflectivity and the sensitivity adjustment. Because all of the emitted energy is focused to a single point, a high amount of excess gain is available, which enables the sensor to easily detect narrow or low reflectivity targets.
Diffused Mode with Background SuppressionDiffused mode sensing with background suppression detects targets only up to a certain “cut-off” distance, but ignores objects beyond the distance. This mode also minimizes sensitivity to a target’s color among the diffused mode variations. One main advantage of diffused mode with background suppression is the ability to ignore a background object that may be incorrectly identified as a target by a standard diffused mode photoelectric sensor.
Diffused mode with background suppression can operate at a fixed distance or at a variable distance. Background suppression can be accomplished technically in two ways, either mechanically or electronically.
Diffused Mode with Mechanical Background SuppressionFor mechanical background suppression, there are two receiving elements in the photoelectric sensor, one of which receives light from the target and the other receives light from the background. When the reflected light at the target receiver is greater than that at the background receiver, the target is detected and the output is activated. When the reflected light at the background receiver is greater than that at the target receiver, the target is not detected and the output does not change state. The focal point can be mechanically adjusted for variable distance sensors.
Diffused Mode with Electronic Background SuppressionWith electronic background suppression, a Position Sensitive Device (PSD) is used inside the sensor instead of mechanical parts. The transmitter emits a light beam, which is reflected back to two different points on the PSD from both the target and the background material. The sensor evaluates the light striking these two points on the PSD and compares this signal to the pre-set value to determine whether the output changes state.
Retro-reflective modeRetro-reflective mode is the second primary mode of photoelectric sensing. As with diffused mode sensing, the transmitter and receiver are in the same housing, but a reflector is used to reflect the light from the transmitter back to the receiver. The target is detected when it blocks the beam from the photoelectric sensor to the reflector. Retro-reflective mode typically allows longer sensing ranges than diffused mode due to the increased efficiency of the reflector compared with the reflectivity of most targets. The target color and finish do not affect the sensing range in retro-reflective mode as they do with diffused mode.
Retro-reflective mode photoelectric sensors are available with or without polarization filters. A polarization filter only allows light at a certain phase angle back to the receiver, which allows the sensor to see a shiny object as a target and not incorrectly as a reflector. This is because light reflected from the reflectors shifts the phase of the light, whereas light reflected from a shiny target does not. A polarized retro-reflective photoelectric sensor must be used with a corner-cube reflector, which is a type of reflector with the ability to accurately return the light energy, on a parallel axis, back to the receiver. Polarized retro-reflective sensors are recommended for any application with reflective targets
Non-polarized retro-reflective photoelectric sensors usually allow longer sensing ranges than polarized versions, but can falsely identify a shiny target as a reflector.
Retro-reflective mode for clear object detectionDetecting clear objects can be achieved with a retro-reflective mode for clear object detection photoelectric sensor. These sensors utilize a low hysteresis circuit to detect small changes in light that commonly occur when sensing clear objects. The clear object mode sensor uses polarized filters on both the sensor transmitter and receiver to reduce false responses caused by reflections from the target.
Retro-reflective mode with foreground suppressionRetro-reflective sensors with foreground suppression will not falsely identify glossy targets as the reflector when they are within a certain distance, or dead zone. This mode is suited for detecting shrink-wrapped pallets, as a standard retro-reflective mode sensor can mistake the glossy covering for a reflector and not change state. Optical apertures in front of the transmitter and receiver elements in the sensor housing produce a zone to eliminate erroneous detection of reflective material.
Thru-beam modeThru-beam mode—also called opposed mode– is the third and final primary method of detection for photoelectric sensors. This mode uses two separate housings, one for the transmitter and one for the receiver. The light from the transmitter is aimed at the receiver and when a target breaks this light beam, the output on the receiver is activated. This mode is the most efficient of the three, and allows the longest possible sensing ranges for photoelectric sensors.
Thru-beam mode sensors are available in a variety of styles. The most common includes one transmitter housing, one receiver housing, and one light beam between the two housings. Another type is “slot” or “fork” photoelectric sensors that incorporate both transmitter and receiver into one housing, with no alignment required. Light grids are arrays of many different transmitters in one housing and many different receivers in another housing, which, when aimed at each other, create a virtual “sheet” of light beams.
Fiber optic sensingFiber sensors guide the light from the transmitter through either plastic or glass cables called fiber optic cables. In applications involving small targets or unfavorable conditions, fiber optic cables may be the optimum solution. Fiber optic cables allow either diffused mode or thru-beam mode sensing.
Glass fiber optic cables are constructed from tiny strands of glass that are bundled together inside an application-specific sheath. Glass fiber optic cables are typically more rugged than plastic versions, more efficient in light transmission resulting in longer sensing ranges, and work well with both visible red and infrared light.
Plastic fiber optic cables are manufactured from a light conductive plastic monofilament material and are housed in a protective PVC jacket. Plastic fiber optic cables are typically more flexible and cost-effective than glass versions, can be cut to length, and work only with visible light.
SIDEBAR/BOXApplication Specific Photoelectric SensorsIn addition to the standard modes of operation for photoelectric sensors, several application specific sensors also exist. These sensors are used to solve many non-traditional photoelectric applications, such as detecting changes in a target’s color, porous targets, and invisible markings on products.
Examples of application specific sensors include:
Color – Color sensors are available in a wide variety of styles and options. The most basic color sensors are single channel units, which can be programmed to detect a single color. More advanced units can detect up to ten or more unique colors and allow multiple shades to be programmed on the same channel. Typical applications include quality control where different colors are marked on the product, as a stage of production is complete. Another possible application would be to program multiple shades of a color on the same channel. These colors could indicate the manufacturers acceptable range of color variance for a finished product in a dyeing or injection molding application.
Contrast – Contrast sensors are used to detect a difference in two colors or media. The sensor is first taught two different conditions. Next, it evaluates the current conditions, and if the current target’s reflected light is closer to the first condition the output will remain off. If the current target’s reflected light is closer to the second condition, the output will change state. A typical application for contrast sensing is registration mark detection before cutting or converting paper in the packaging industry.
Luminescence – Luminescence sensors are used to detect inks, greases, glues, paints, chalks and other materials with luminescent properties. Marks on irregular backgrounds and clear or invisible markings are easily sensed using an ultraviolet light source. Typical applications for luminescence sensors are detecting the clear tamper-proof seals on medicine bottles or detecting a defective product that has been marked with chalk (i.e. a knot in a piece of wood).
Light grids – Light grids are used to create a grid or sheet of light. There are many variations, sizes and applications for light grids. Miniature, high-resolution light grids can be used for small parts counting. Larger grids can be used to ensure part ejection from a press before the next press cycle. Safety light grids are used to create a safe “perimeter” around a machine so that operators are protected from potentially dangerous parts of the machine.
Passive infrared – Passive infrared sensors are used to detect movement of an object within a defined sensing area or zone. The term passive is used because the sensor does not emit any light, but instead detects infrared emissions from an object with a temperature that is different than the surrounding environment. A typical application for passive infrared sensors is controlling automatic doors or lights.
Zone scanners – Much like passive infrared sensors, area scanners are used to detect the presence or movement of an object within a defined sensing area or zone. The main difference is that active infrared sensors emit light and are able to detect movement of an object in the area when the temperature of the target cannot be determined. A typical application could be detecting vehicles approaching an overhead door in a warehouse since neither the temperature of the vehicle or the environment could be determined.
Laser photoelectric sensors – Lasers are sometimes used as sensor light sources. Laser photoelectric sensors are available in thru-beam, diffuse scan, and diffuse scan with background suppression versions. Lasers provide high intensity visible light, which enables simple assembly and adjustment. Laser technology makes it possible to detect extremely small objects at a distance.Fiber optics photoelectric sensors – These sensors use an emitter, receiver, and a flexible cable that is full of tiny fibers meant to transmit light. When glass fibers are used, the emitter source is infrared light. When plastic fibers are used, the emitter source is visible light. Fiber optics can be adapted to thru-beam, retro-reflective scan, or diffuse scan sensors. Fiber optics works best for small sensing areas or small objects.Remote photoelectric sensors – These are used for remote sensing and contain only the optical components of a sensor.Working PrinciplePhotoelectric sensors provide three basic methods of target detection:
Diffused mode or proximity-sensing mode – In this sensing mode, the transmitter and receiver are placed in the same housing. Light from the transmitter strikes the target, which reflects the light at arbitrary angles. Some of the reflected light returns to the receiver, and the target is detected. Other variations to this mode include diffused convergent beam mode, diffused mode with background suppression, and diffused mode with mechanical background suppression.Retro-reflective mode – Like in the previous mode, the transmitter and receiver are in the same housing. A reflector is used to reflect the light from the transmitter back to the receiver. The target is detected when it blocks the beam from the photoelectric sensor to the reflector. Other variations to this mode include retro-reflective mode for clear object detection, retro-reflective mode with foreground suppression.Thru-beam mode or opposed mode – The transmitter and the receiver are placed in two separate housings. The light from the transmitter strikes the receiver and when a target breaks this light beam, the output on the receiver is activated. This mode is considered as the most efficient of the three modes because it allows the longest possible sensing ranges for photoelectric sensors.
There are two operating modes followed by photoelectric sensors:
Dark Operate (DO) – Here the load is energized when light from the emitter is absent from the receiver.Light Operate (LO) – Here the load is energized when light from the emitter reaches the receiver.ApplicationsThe following are the key applications of photoelectric sensors:
Detect changes in a target’s color, contrast and luminescenceDetect porous targets, and invisible markings on productsDetect the presence or movement of an object within a defined sensing area or zoneDetect the level of contents in a hopperCheck products passage in a rinsing processLocate the position of an automated storage and retrieval systemDetect the presence/absence of contents in a bottle/ milk cartonCheck the passage of cars on a conveyerCheck the seating of workpieces for an NC processorCheck positioning of cars in final assembly lineCheck passage of engine blockVerify fill level of coffee in cansCount bottles moving on a high speed conveyorDetect missing labels on bottlesEnsure safely control at the opening and closing of garage doorsEnable turning on sink faucets with the wave of a handControl elevators, and open the doors at the grocery storeDetect the winning car at racing events.Photoelectric Sensors – Applications
Accurately Detects Automobiles in Parking Lots
Detects Black Tires at Long Distance
Detects Presence of Transparent Films
Detects Transparent Bottles
Warning Alarms for Cranes
Detects Chip Components on the Tapes of Taping Machines
Detects Wafer Cassette Racks
Confirms Wafer Cassette Mounting
Counts Clear Plastic Cups in High Speed
Distinguish the Front and Back Side of Nuts
Detects Objects through Narrow Spaces
Inspects End-polished Components
Detects Bad Marks
Remotely Detects Presence of Glass Substrates
Detects Protrusion of Large Cargo with a 60-m Sensing Distance
Accurately Detects Small-diameter Pins
Prevents Non-woven Textiles from Snaking
Detects Broken Drills in Environments with Sprayed Oil Mist
Detects Glass Substrates in Vacuum Chambers
Detects Degradation of UV Lamps Used for UV Sterilization in Food Production
Detects Degradation of UV Lamps Used for UV Adhesion and Hardening at the Dicing Stage
Detects Degradation of UV Lamps Used for UV Hardening of High-precision Resin Components
Detects Degradation of UV Lamps Used for UV Cleaning of LCD Glass
Detects Degradation of UV Lamps Used for Drying in UV Printing
Monitors Degradation of UV Lamps Used for Bonding LCD Substrates with UV Adhesion
Detects Minute Amounts of Silver Paste
Detects Positioning of Infusion Pack Caps
Detects Raised Shrink-wrap FilmPrecisely Detects and Counts PET Bottles
Detects Transparent LCD Substrate Mapping
Picking Sensor Implements and Inspects Component Extraction Confirmation