Photoelectric sensing: Choosing the correct technology and mode for your application
March 11, 2008
Photoelectric sensors, named because they rely on the photoelectric effect of semi-conductor material described by Albert Einstein in 1905, have been used in industrial manufacturing processes since the late 1940’s for detection, inspection, and automation of machine controlled processes. These sensors can be thought of as the “eyes” of an automated factory or process line. For instance, a photoelectric sensor may detect when a part is in position for the next manufacturing step, verify that a web is properly tensioned, or count parts at a high rate to ensure the correct amount of product is being processed. Since they were first developed, advances in circuit technology and engineering have resulted in smaller, faster, and less expensive sensors, which provide a cost-effective solution to a wide variety of applications worldwide. To allow the most efficient sensing, several sensor types and modes are available.
The technology used by photoelectric sensors consists of an emitter, the sensor’s light source, which is typically a Light Emitting Diode (LED) or a red laser and a receiver, or the electrical component, which exhibits the photoelectric effect. When photons strike the component, electrons are produced and a small electrical current is generated. Photoelectric sensors also contain a detection circuit, which responds to this current. Another critical part is the optical component, which projects the emitted light into the sensing zone and collects the received light onto the receiver. In order for the sensor to be used in an industrial setting, a power supply circuit is also required to accept a DC voltage that powers the sensor as well as an output circuit that turns on and off depending on the amount of light striking the receiver. Photoelectric sensors are grouped into three broad sensor types depending on how the components are assembled: self-contained, remote electronics, and fiber optic amplifier types.
Self-contained sensors, the most commonly used sensor type, combine all of the optics and electronics within a mechanically robust sensor housing that can be simply set up by mounting the sensor and connecting the power supply and output wiring. Self-contained sensors are available in many shapes and sizes to accommodate space constraints and environmental challenges that are often present in industrial manufacturing process plants. In general, larger sensors offer longer operating ranges due to the increased size of the lenses, but improvements in optical design and receiver electronics of small sensors have made this less of an issue. Self-contained sensors are available in all of the sensing modes, which will be described later, and should be considered the most reliable and robust sensor platform.
Photoelectric sensors with remote electronics decrease the size of the sensor installed at the application by separating the optical components, the emitter and receiver electronics, from the detection circuitry, power supply and output circuitry. The two parts of the sensor are connected by cables which convey electrical signals between them, allowing the optical components to be very small and easily embedded into a machine, while the wiring and hook-up connections are easily accessible in a wiring junction box on the outside of the machine. The disadvantage of remote electronic sensors is the additional installation time, as the installer must mount the sensors, route the connecting wires and mount the remote electronics housing. Plus, the small optical components also limit the maximum operating range of the sensor.
Like remote electronic sensors, fiber optic amplifiers separate the electronics from the sensing location, allowing the same advantages. The main difference is that all of the electronics are contained in a single housing with only the light being conveyed to and from the sensing area by plastic or glass fiber optic assemblies. The use of plastic and glass fiber optical assemblies provides excellent optical performance in small confined spaces. Additionally, glass fibers provide high temperature resistance in sensing areas ranging from 250º to 480 ºC.
Once a sensor type is selected according to the application’s requirements, the most reliable sensing mode should be chosen. The user should first consider opposed-mode sensing—the sensing mode used for applications such as sensing residential garage doors to ensure that no one is standing under the door when it closes. In this beam-blocking mode, the sensor’s emitter and receiver are housed in two separate units, with the emitter placed opposite the receiver so that the target object is detected when it breaks the effective beam. The object breaking the beam must be larger than the specified “effective beam”, defined by the size of the receiving sensor’s lens diameter, although this can be reduced by external mechanical apertures to detect very small object when necessary. Since the light beam travels directly from the emitter to the receiver, opposed-mode sensing offers the highest level of sensing energy to overcome possible contamination in the environment or sensor misalignment from shock or vibration. Opposed-mode sensing also offers reliable object detection even with challenging shiny or dark objects. As long as the object is opaque to the emitted light, it will cast a shadow on the receiver when the object passes between the emitter and receiver. Any light reflected off of the object is directed back towards the emitter, never reaching the receiver. Although ranges of less than 1 m are most common, opposed-mode sensing can be reliably accomplished over distances of up to 200 m in both indoor and outdoor environments.
Another type of beam-breaking sensing mode, the retroreflective mode, offers a high level of sensing reliability while requiring only one sensor to be installed and wired for applications where sensing or electrical connections are only available from one side. Retroreflective modes contain both the emitter and receiver in a single housing, compared to the opposed-mode’s two separate sensors. In this mode, a retroreflector similar to a bicycle reflector is placed on the opposite side of the object, returning the emitted light directly back to the sensor. The target object passes between the sensor and the retroreflector, breaking the light path between the emitter, reflector, and receiver. Although it can be more convenient, dirt build-up on the retroreflector and the sensor lens cause this method to lose excess gain, or the light falling on the receiver above the amount required to operate the sensor’s amplifier, twice as fast as opposed-mode sensors. Retroreflective sensors also should not be used for detecting small objects or for precise positioning control, since creating a small effective beam with this method is challenging.
A disadvantage to using retroreflective sensors is their susceptibility to false-proxing, the undesirable reflection of the sensing beam from the target object. A shiny object that breaks the sensor beam may reflect enough light back to trick the sensor and pass through the beam undetected. To combat this, many retroreflective sensors include cross-polarizing filters that effectively block the light reflected directly from the object, yet allow the light returned from the reflector to hit the receiver. Sensors with polarizing filters offer a maximum sensing range of 1 to 8 m, while non-polarized modes accurately sense objects that that don’t have shiny surfaces over a range of 1.5 to 12 m.
Some applications are best solved with a sensing mode that does not rely on the targeted object breaking the beam. Similar to retroreflective mode sensors, diffuse-mode sensors contain both the emitter and receiver circuitry in the same sensor housing; however, in diffuse-mode, the light path is created when the object bounces the light beam from the emitter back to the receiver. These sensors have a relatively short range of 2 meters, but are practical for thousands of real world applications. Diffuse-mode sensors are easy to set up; the installer simply aims the sensor at the object being detected and adjusts the sensor until a reliable on and off switching is established.
Since diffuse-mode sensors rely on the amount of light reflected back, changes in the object’s color, distance, and background can cause problems resulting in missed detections or false outputs. As a result, diffuse-mode sensors must be mounted in a way that the sensor sees the object in a controlled fashion. A special category of diffuse-mode sensors, background suppression sensors, feature a special optical design to ensure that the sensor sees the object within a defined sensing range of up to 2 m, but ignores light returned from any object beyond the range. Background suppression sensing is not available with the fiber optic amplifier sensor types.
Fiber optic amplifiers offer many unique sensing features made possible by the design freedom of the fiber optic assemblies. One noteworthy application is liquid detection. Special optical designs utilizing chemically resistant fluoropolymers can be used to detect the presence or absence of liquid in contact with the fiber end tip. This can be used as a liquid level switch to monitor a day tank. It is also widely used to detect an unwanted leak in a storage tank or piping system. The fiber optic end tip is placed in a normally dry containment pan or tray. If the system develops a leak, the sensor output will switch when the fiber optic end comes in contact with the accumulating liquid. This switched output can alert operators of the problem and prompt them to take corrective measures.
Leak detection is merely one of a wide variety of applications that the latest advancements in photoelectric technology have allowed photoelectric sensors to solve. Knowing your sensing requirements such as the available space, environmental factors, the object’s size and reflective properties as well as required sensing range will allow you to select the best sensor type and mode for your application.