Common Photoelectric Sensing Modes and How to Choose
February 19, 2018
Sensing mode is one of the most important criteria when selecting a photoelectric sensor. The best sensing mode for your application will reliably detect your object without being confused by factors in the environment. The following article explains the 3 most common sensing modes and typical applications for each.
Opposed-Mode (Through-Beam) Sensing
In opposed-mode sensing, also known as through-beam sensing, the sensor's emitter and receiver are housed in two separate units. The emitter is placed opposite the receiver so that the light beam goes directly from the emitter to the receiver.
The opposed mode should be used whenever possible because it is the most reliable sensing mode. This is because light passes directly from the emitter to the receiver. An object is detected when it breaks the effective beam, which is the column of light directly between the emitter’s lens and the receiver’s lens. The diameter of the effective beam is the same diameter as the lenses of the emitter and receiver. An object must be the diameter of the effective beam or larger to be reliably detected.
Because the light beam goes directly from the emitter to the receiver and doesn’t have to reflect off of anything, opposed-mode sensors have very high excess gain. Opposed-mode sensing offers much higher excess gain than any other mode of sensing, making it ideal in dusty, smoky, foggy, misty, or oily environments.
It doesn't matter how shiny or dark your object is, or even what color. The object physically passes between the emitter and receiver and is detected when it blocks the beam of light. Therefore, variables such as surface reflectivity, color, and finish don't affect opposed-mode sensing.
If you need a sensor to accurately count parts, then an opposed-mode sensor is perfect. Due to the well-defined effective beam, opposed-mode sensors are most reliable for accurate parts counting, as long as diameter of the effective beam is no larger than the part.
If your object doesn’t completely block the effective beam, however, it runs the risk of not being detected. You can compensate for this by using apertures, lenses, or fiber optics to shape the effective beam to match your part’s profile.
In addition, some opposed-mode sensors can "burn through" less opaque materials like paper, cloth, or plastics. In these cases, you can often lessen the signal strength by adding apertures, or by intentionally misaligning the emitter and the receiver.
Opposed-mode sensing should be avoided for the detection of translucent or transparent objects, since clear objects do not block the effective beam.
Opposed-mode sensing is great for applications that need:
- Long range sensing
- Sensing through heavy dirt, dust, mist, condensation, oil, film, etc.
- Precise position sensing
Unlike an opposed-mode sensor, a retroreflective sensor contains both the emitter and receiver elements in a single unit. The effective beam is established between the emitter, a retroreflector, and the receiver. As with an opposed-mode sensor, an object is sensed when it interrupts or "breaks" the effective beam.
Most retroreflectors are made up of many small corner-cube prisms. A light beam enters a corner cube prism through its hypotenuse face and is reflected from the three surfaces. In this way, the retroreflector returns the light beam to its source. Most corner-cube retroreflectors resemble bicycle reflectors and are molded using clear acrylic plastic, manufactured in various sizes, shapes, and colors.
If an opposed-mode sensor is not an option, a retroreflective-mode sensor may be a good choice. For example, a retroreflective-mode sensor offers a convenient alternative to opposed-mode if electrical connections are only possible on one side.
Retroreflective-mode sensors offer relatively long ranges. Like opposed-mode sensing, retroreflective sensing is also a beam-break mode, so objects can often be detected regardless of their reflectivity. For this reason, the retroreflective mode is also a reliable sensing mode, even if the target’s color or finish is inconsistent.
Retroreflective-mode sensors lose excess gain twice as fast as opposed-mode sensors due to dirt build-up on both the retroreflector and the sensor lenses. This is because the light travels through four windows, out through the emitter window and retroreflector window, then back through the reflector window and the receiver window.
There is also much less available excess gain in a retroreflective mode sensing beam, due to the inefficiencies of the retroreflector and because the light must travel twice as far to reach the receiver, as compared to the opposed mode.
In addition, it can be difficult to create a small effective beam with a retroreflective mode sensor, so avoid using this mode for detecting small objects or for precise positioning control. Banner does offer a few select retroreflective sensors that have an effective beam of less than 1 inch. Ask about them!
Clear objects can also be a challenge for most retroreflective sensors. In retroreflective mode, an object must interrupt the beam to be detected. However, some retroreflective sensors like the QS18 COD offer polarized coaxial optics that are made specifically for effectively detecting clear objects..
The optics of a good quality retroreflective sensor are designed and assembled with great care to minimize "proxing.” Proxing is when an object with a shiny surface perfectly parallel to the light beam returns enough light to the sensor to mimic the light coming back from the reflector and causes the object to not be detected. Unpolarized retroreflective sensors may be susceptible to proxing. When using retroreflective sensors with objects that may be shiny, remove the risk of proxing by:
- Using a polarized retroreflective sensor, which contains polarizing filters on the emitter and receiver
- Angling the sensor relative to the object to avoid direct
Most retroreflective sensors are designed for long-range sensing, and suffer a "blind spot" when the reflector is mounted too close to the sensor. Check the excess gain curve of your retroreflective mode sensor to see where the "blind spot" occurs.
Opposed-mode sensing is great for applications that need:
- Long range sensing
- High speed detection
- Detection of objects larger than 1 inch
- Clear object detection using polarized clear object detection sensors
Diffuse-Mode Sensing (Proximity)
Diffuse-mode sensing is the most common type of proximity sensing. In diffuse mode sensing, light emitted from the sensor strikes the surface of the object to be detected and is diffused, sending some light back to the receiver element of the sensor. With a diffuse-mode sensor, the object is detected when it "makes" the beam. That is, the object reflects some of the sensor’s transmitted light energy back to the sensor.
Most diffuse-mode sensors use lenses to focus the emitted light rays and to gather in more light. These lenses also help to extend the range of diffuse-mode sensors.
Diffuse-mode sensors have only one item to be mounted: the sensor itself. This is ideal for situations where a sensor can be mounted only on one side of the target. Diffuse-mode sensors are very easy and convenient and are often used when opposed or retroreflective-mode sensors aren't practical.
The response of a diffuse-mode sensor is dramatically influenced by the surface reflectivity of the object to be sensed. The reflectivity of an object directly affects the distance at which it can be reliably sensed by a diffuse-mode sensor. While it depends on the sensor size and power, generally diffuse sensors have shorter ranges than opposed-mode or retroreflective-mode sensors. Many diffuse sensors require the target to be less than 1 meter away. Smaller objects and curved shiny objects can also be harder to detect because less light is reflected back to the sensor.
Most diffuse mode sensors lose their excess gain very rapidly as dirt and moisture accumulates on their lenses. In some instances, dirt build-up on the sensor lens can unintentionally channel the light beam from the emitter directly to the receiver, so that the sensor will act as if an object is constantly in front of it.