Photoelectric Sensor Applications

As the manufacturing world becomes more integrated with automated technology, it’s important to understand how this technology can help you. Industrial automation can make the lives of those working on the production floor easier and help increase a company’s productivity. One type of those device helping workers is known as a photoelectric sensor.

The most common mode on photoelectric sensors is the proximity mode (or diffuse), where the light source from the sensor is being reflected off the target and returned to the sensor’s receiver. This return of light to the sensor confirms the presence of the target. This proximity mode allows sensing from one side of the target, but the strength of the return light depends on the distance, color, and incident angle to the target.  When those factors can be controlled, the proximity mode is easy to install and can provide reliable sensing. Depending on the physical spacing between sender and receiver in the proximity mode, this will impact the mode’s sensitivity to the accumulation of dust and dirt and its survival in that challenging environment. 

Another popular mode on photoelectric sensors is the through-beam (or opposed) mode. With this, one way or another, the sensor’s sender and receiver are displaced, and the target passes in-between them.  When the target passes between the sender and receiver, the light from the sender is blocked from the receiver, and this absence of light at the receiver confirms the target's presence.

One disadvantage of the through-beam mode is that one-half of the sensor needs to be physically attached on the other side of the target, and where a fiber-optic through-beam is not used, both components of the through-beam system need to be powered, requiring wiring to each component.  Transparent and some translucent targets may not provide sufficient attenuation of the light beam for the sensor to reliably detect their presence.

Our third mode option is Retro-Reflective; this uses a reflector on one side of the target, and the sensor with its sender/receiver on the other side. When a target passes between the reflector and the sensor, the light beam path is blocked, and this absence of light at the receiver confirms the target's presence. This then sends the predetermined information to the controller. The advantage of using this mode, is the independence of target color, incident angle to the target, and distance to the target.

The disadvantage of the Retro-Reflective mode is that the reflector may experience gradual accumulation of dust, dirt, residue, or moisture with a corresponding attenuation of light returned to the sensor, reduced sensing range, and subsequently, loss of effectiveness.  Depending on the installation, the reflector may easily become misaligned or damaged.  Because of the specifics, some applications may not permit the installation of a reflector.  Transparent and some translucent targets may not provide sufficient attenuation of the light beam for the sensor to reliably detect their presence.

Some applications are space-constrained, or simply need better control of the light beam.  Applications of this nature are often best served by using fiber-optic light guides with the photoelectric sensor.  The fiber-optics are a conduit for the light beam and are available in different apertures and geometric shapes.  Fiber-optics, when chosen appropriately, may change the way the sensor responds to the target and performs.  The geometric shape of the fiber can be chosen to favor the sensing task.  Fiber-optics can be selected to survive in higher temperatures than the sensors or in wash-down applications, where the sensor is not rated for that exposure.

In general, most applications for photoelectric sensing should be relatively clean environments.  If the application is dusty or dirty, special considerations need to be addressed for the sensor to remain a reliable means of sensing, such as periodic sensor lens and/or reflector cleaning.

Compared to other sensing technologies, photoelectric sensing has one the fastest response times, due in part to the fast speed that light - VS - Ultrasonics, traveling at the slower speed of sound, have inherently slower response times. 

The choice of LED light source for the photoelectric sensor is important.  IR (infrared) is non-visible and can pass through some target materials, and can more easily pass through a reasonable accumulation of dust and dirt. Both properties can be used as an advantage for some applications.  IR LED sources usually offer the longest sensing range, compared to other visible LED sources in the same product family from the same manufacturer. Infrared red light is the optimal LED source when attempting to sense darker-colored targets within the proximity mode.

Since IR will normally pass through ­­­transparent targets such as glass, plastics, and some translucent targets with minimal attenuation. It is not a recommended solution for sensing such targets in either the retro-reflective or through-beam modes. IR is an excellent source for sensing these targets in the proximity mode.

Having visible LED light sources give the advantage of being able to see where the light beam strikes the target in the proximity mode. But, this mode and amount of light returned to the receiver are very dependent on the color of the target. Darker colors absorb more of a visible light source so that the amount of reflected light and its subsequent sensing range is significantly reduced.  The lesser light returned from darker targets could be an advantage, if trying to distinguish between lighter targets and darker targets.

We hope this blog was insightful and encourage you to contact us to see how our sensors can help your automation and control process.