Which Photoelectric Sensor Should I Be Using?

There are many variations within the category of photoelectric sensors, so how do you select the best sensor for your application? Below, I will discuss the benefits of different types of photoelectric sensors and sensing modes.

Through Beam

Through beam sensors consist of an emitter and a receiver. The emitter produces a beam of light, while the receiver identifies whether that light is present or not. So, when an object breaks the beam, an output is triggered by the receiver. Some of the advantages of using the simple through beam technology is that, unlike some of the other photoelectric sensors, it doesn’t matter the color, texture or transparency of your target.

Retroreflective

What if you would like to have a through beam sensor, but don’t have enough room for two sensor heads in your application? Retroreflective sensors have an emitter and receiver within one housing and use a high-quality reflector to reflect the light beam back to the sensor head. This allows for easy connection of just one sensor head, but it doesn’t have the range of your typical through beam sensor. When using these types of sensors, you must factor in how small or reflective your target material is. If you are trying to sense a highly reflective material, then the light reflected back to the receiver could cause the sensor to think an object is present. If you are having these problems, but still want to use a retroreflective sensor, then you should consider versions with a polarizing lens. These lenses make the sensors insensitive to interference with shiny, reflective material.

Fork

Fork sensors include the transmitter and receiver in one housing, and they are already aligned. This saves time and energy during set up. Fork sensors are fantastic for small component and detail detection.

Diffuse

If you don’t have room for a sensor head on each side of your application or even a reflector, or you have had trouble with the alignment of a retroreflective sensor, a diffuse sensor may be a good choice. Diffuse sensors use technology to be able reflect light off the material and back to the sensor. This eliminates the need for a second device or reflector. This significantly reduces set up. You can simply place your target material in front of the sensor and teach it to that point. Once your object reaches that point, the light will be reflected back to the sensor, producing the output. While they are simpler to install, they also have a shorter range compared to through beam sensors and may be affected by your material’s color or the reflectivity or your background… Unless, you have a diffuse sensor with background suppression.

Background Suppression

Diffuse sensors have an emitter and receiver in one housing. In diffuse sensors with background suppression, the emitter and receiver are at a fixed angle so that they intersect at the position of your target material. This will help narrow the operating area (area in which your target material will be entering) and not let reflective material in the background have an influence in your detection.

Conclusion

Photoelectric sensors are simple to use when you need non-contact detection of a material’s presence, color, distance, size or shape, and with their various types, housing and sizes, you can find one that is ideal for your application.

Fork Sensors – The Ideal Through-Beam

Through-beam sensors are a true time proven solution to many photoelectric applications.  These sensors can detect anything regardless of color, texture or reflectivity, all that needs to happen is the light beam needs to be blocked.  Add an optional aperture and you can detect even the smallest of parts.  With the various light sources available and you can detect small parts (with a laser light) or blast through the harshest of environments with an infrared light source.  These sensors come in several housings or styles for instance tubular (as small as 8mm), block, fiber optic and the fork style sensor.

Through-beam sensors are used in applications that require sensing ranges from 2 millimeters to 100 meters and in some cases longer.  Since these sensors require a light emitter and a receiver that are in separate housings, you have to mount and wire each component separately.  Once mounted you have the task of aligning the receiver to the emitter, which could be a tedious task.  Just imagine trying to line up the devices that use an infrared light source with a working range of 50 meters or even 150 millimeters.

forksensorFork sensors, also referred to, as c slot or u slot, are the ideal through-beam sensor.  First, they are self-contained in one housing so there is no need to align the emitter and receiver.  This is important because in the harsh environments if the sensor is bumped or jarred the receiver and emitter stays aligned.  Secondly, the housings are typically metal offering an extremely robust sensor.  Third, since the sensor is integrated into one housing installation is much easier, one part to mount and only one wiring connection.  These sensors are available from 5mm to 220 mm wide openings.

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Timing is everything – Which light is the right light?

Shortly after posting my last blog, Which light is the right light, I had a customer call with a problem in a machining cell.  They are using a self-contained through-beam sensor, in the form of a fork sensor, with a red light source. They required a small light spot to detect a tool.  As in most machining centers, there is a lot of coolant flying around in the cell and a fine mist in the air.  When water based coolants dry, they separate and leave a white film on surfaces, including photoelectric lenses.  This customer had to shut down their cell and clean off the lens at least once per shift, which was costing them production, time, and money because of false signals.

As we spoke on the phone, I suggested that they use the infrared version because we can burn through the contamination in the environment, in this case the film left behind from the coolant.  The customer wanted some sort of idea of how much residue we could burn through so I did some simple testing and sent him the following pictures.  Picture 1 is a heavy dusting, of all things, coffee creamer.  Picture 2 is a nice dollop of grease from a grease gun and picture 3 is a film of hand cream.

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