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Photoelectric Sensors in the Packaging, Food, and Beverage Industry

The PFB industry requires the highest standards of quality and productivity when it comes to both their products and their equipment. In order to keep up with the rising demands to produce high quality parts quickly, many in the industry have incorporated photoelectric sensors into their lines. With their durable designs, accurate measurements and fast data output speeds, it is easy to see why. Combine the sensors’ benefits with the clean and well-lit environment of a PFB plant, and it begins to feel like this product was made specifically for the industry. There are many variants of photoelectric sensors, but the main categories are: through beam, diffuse, and retro-reflective sensors.

Through Beam

Through beam sensors come in many different shapes and sizes but the core idea stays the same. An emitter shoots LED red, red laser, infrared, or LED infrared light across an open area toward a receiver. If the receiver detects the light, the sensor determines nothing is present. If the light is not detected, this means an object has obstructed the light.

Applications:

Object detection during production
Detecting liquid in transparent bottles
Detecting, counting, and packaging tablets

Diffuse Beam

Diffuse beam sensors operate a little differently in that the emitter and receiver are in the same housing, often very closely to one another. With this sensor, the light beam is emitted out, the light bounces off a surface, and the light returns to the receiver. The major takeaway with the diffuse beam sensor is that the object being detected is also being used as the reflecting surface.

Applications:

Label detection
Monitoring the diameter of film
Verifying stack height on pallet

Retro-Reflective Beam

Retro-reflective sensors are similar to diffuse beam sensors in that the emitter and receiver are also contained within the same housing. But this sensor requires an additional component — a reflector. This sensor doesn’t use the object itself to reflect the light but instead uses a specified reflector that polarizes the light, eliminating the potential for false positive readings. Retro-reflective sensors are a strong alternative to through beam when there isn’t room for two separate sensor heads.

Applications:

Transparent film detection
Detection of shrink-wrapped pallets
Detecting any reflective target

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.

Photoelectric Methods of Operation

Photoelectric sensors vary in their operating principles and can be used in a variety of ways, depending on the application. They can be used to detect whether an object is present, determine its position, measure level, and more. With so many types, it can be hard to narrow down the right sensor for your application while accounting for any environmental conditions. Below will give a brief overview of the different operating principles used in photoelectric sensors and where they can be best used.

Diffuse

Diffuse sensors are the most basic type of photoelectric sensor as they only require the sensor and the object being detected. The sensor has a built-in emitter and receiver, so as light is sent out from the emitter and reaches an object, the light will then bounce off the object and enter the receiver. This sends a discrete signal that an object is within the sensing range. Due to the reflectivity being target-dependent, diffuse sensors have the shortest range of the three main discrete operating principles. Background suppression sensors work under the same principle but can be taught to ignore objects in the background using triangulation to ensure any light beyond a certain angle does not trigger an output. While diffuse sensors can be affected by the color of the target object, the use of a background suppression sensor can limit the effect color has on reliability. Foreground suppression sensors work in the same manner as background suppression but will ignore anything in the foreground of the taught distance.

diffuse

Retro-reflective

Retro-reflective sensors also have the emitter and receiver in a single housing but require a reflector or reflective tape be mounted opposite the sensor for it to be triggered by the received light. As an object passes in front of the reflector, the sensor no longer receives the light back, thus triggering an output. Due to the nature of the reflector, these sensors can operate over much larger distances than a diffuse sensor. These sensors come with non-polarized or polarizing filters. The polarizing filter allows for the sensor to detect shiny objects and not see it as a reflector and prevents any stray ambient light from triggering the sensor.

retroreflective

Through-beam

Through-beam sensors have a separate body for the emitter and receiver and are placed opposite each other. The output is triggered once the beam has been broken. Due to the separate emitter and receiver, the sensor can operate at the longest range of the aforementioned types. At these long ranges and depending on the light type used, the emitter and receiver can be troublesome to set up compared to the diffuse and retro-reflective.

throughbeam

Distance

The previous three types of photoelectric sensors give discrete outputs stating whether an object is present or not. With photoelectric distance sensors, you can get a continuous readout on the position of the object being measured. There are two main ways the distance of the object is measured, time of flight, which calculates how long it takes the light to return to the receiver, and triangulation, which uses the angle of the incoming reflected light to determine distance. Triangulation is the more accurate option, but time of flight can be more cost-effective while still providing good accuracy.

Light type and environment

With each operating principle, there are three light types used in photoelectric sensors: red light, laser red light, and infrared. Depending on the environmental conditions and application, certain light types will fare better than others. Red light is the standard light type and can be used in most applications. Laser red light is used for more precise detection as it has a smaller light spot. Infrared is used in lower-visibility environments as it can pass through more dirt and dust than the other two types. Although infrared can work better in these dirtier environments, photoelectric sensors should mainly be used where build-up is less likely. Mounting should also be considered as these sensors are usually not as heavy duty as some proximity switches and break/fail more easily.

As you can see, photoelectric sensors have many different methods of operation and flexibility with light type to help in a wide range of applications. When considering using these sensors, it is important to account for the environmental conditions surrounding the sensor, as well as mounting restrictions/positioning, when choosing which is right for your application.

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Imagine the Perfect Photoelectric Sensor

Photoelectric sensors have been around for a long time and have made huge advancements in technology since the 1970’s. We have gone from incandescent bulbs to modulated LED’s in red light, infrared and laser outputs. Today we have multiple sensing modes like through-beam, diffuse, background suppression, retroreflective, luminescence, distance measuring and the list goes on and on. The outputs of the sensors have made leaps from relays to PNP, NPN, PNP/NPN, analog, push/pull, triac, to having timers and counters and now they can communicate on networks.

The ability of the sensor to communicate on a network such as IO-Link is now enabling sensors to be smarter and provide more and more information. The information provided can tell us the health of the sensor, for example, whether it needs re-alignment to provide us better diagnostics information to make troubleshooting faster thus reducing downtimes. In addition, we can now distribute I/O over longer distances and configure just the right amount of IO in the required space on the machine reducing installation time.

IO-Link networks enable quick error free replacement of sensors that have failed or have been damaged. If a sensor fails, the network has the ability to download the operating parameters to the sensor without the need of a programming device.

With all of these advancements in sensor technology why do we still have different sensors for each sensing mode? Why can’t we have one sensor with one part number that would be completely configurable?

BOS21M_Infographic_112917

Just think of the possibilities of a single part number that could be configured for any of the basic sensing modes of through-beam, retroreflective, background suppression and diffuse. To be able to go from 30 or more part numbers to one part would save OEM’s end users a tremendous amount of money in spares. To be able to change the sensing mode on the fly and download the required parameters for a changing process or format change. Even the ability to teach the sensing switch points on the fly, change the hysteresis, have variable counter and time delays. Just imagine the ability to get more advanced diagnostics like stress level (I would like that myself), lifetime, operating hours, LED power and so much more.

Obviously we could not have one sensor part number with all of the different light sources but to have a sensor with a light source that could be completely configurable would be phenomenal. Just think of the applications. Just think outside the box. Just imagine the possibilities. Let us know what your thoughts are.

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Photoelectric Output Operate Modes and Output Types

Photoelectric sensors are used in a wide variety of applications that you encounter every day. They are offered in numerous housing styles that provide long distance non-contact detection of many different types of objects or targets. Being used in such a variety of applications, there are several outputs offered to make integration to control systems easy and depending on the sensing mode when the output is activated in the presence of the target.

Diffuse Diffuse sensors depend on the amount of light reflected back to the receiver to actuate the output. Therefore, Light-on (normally open) operate refers to the switching of the output when the amount of light striking the receiver is sufficient, object is present. Likewise, Dark-on (normally closed) operate would refer to the target being absent or no light being reflected back to the receiver.

Retroreflective and through-beam sensors are similar in the fact they depend on the target interrupting the light beam being reflected back to the receiver. When an object interrupts the light beam, preventing the light from reaching the receiver, the output will energize which is referred to as Dark-on (normally open) operate switching mode or normally open. Light-on (normally closed) operate switching mode or normally closed output in a reflex sensor is true when the object is not blocking the light beam.

Outputs from photoelectric sensors are typically either digital or analog. Digital outputs are on or off and are usually three wire PNP (sourcing output) or NPN (sinking outputs). The exception to this is a relay output that provides a dry or isolated contact requiring voltage being applied to one pole.

Analog outputs provide a dynamic or continuous output that varies either a voltage (0-10 volt) or current (4-20mA) throughout the sensing range. Voltage outputs are easier to integrate into control systems and typically have more interface options. The downside to a voltage output is it should not be ran more than 50 feet. Current outputs can be ran very long lengths without worry of electrical noise. As additional advantage of the analog output is that it has built in diagnostics, at its minimum there will always be some current at the input unless the device completely fails or the wire is damaged.

Some specialty photoelectric sensors will provide a serial or network communication output for communications to higher level devices. Depending on the network, IO Link, for instance, additional diagnostics can be provided or even parameterization of the sensors.
Interested in learning more about photoelectrics basics? You can also request a copy of the new Photoelectric Handbook.

Photoelectric Basics – Light On or Dark On

Recently I was asked if light on and dark on for a photoelectric sensor was the same as normally open and normally closed. The short answer is yes, but I think it justifies more of an explanation. In the world of proximity sensors, capacitive sensors, and mechanical switches when the target is present the output changes state and turns on or turns off; there is no ambiguity.

With photoelectric sensors, instead of normally open or normally closed we refer to light-on operate or dark-on operate because we are referring to the presence or absence of light at the sensor’s receiver. The output of a light-on operate sensor is on (enabled, high, true) when there is sufficient light on the receiver of the sensor. Conversely, the output of a dark-on operate sensor is on when the light source is blocked or no light can reach the receiver.

There are three modes of operation with photoelectrics: diffuse, retro-reflective, and through-beam; and the sensing mode determines if the sensor is normally light-on or dark-on. Retro-reflective and through-beam sensors function as light-on operate sensors because under normal operating conditions there is a reflector or a light emitter providing a light beam back to the sensor receiver. If no object is blocking the light beam to receiver the output is on, normally closed. If the target or object is in between the reflector or emitter then the light beam can no longer reach the receiver causing the output to turn off.

Since the diffuse mode of operation requires the target or object to reflect the light source back to the receiver, it functions as a dark-on operate, normally open. If no object or target is placed in front of the sensor, no light will be reflected back to the receiver. When the object is present, the output changes state from normally open to closed.

The chart below should help to summarize the above:

Shedding the Light on Diffuse Mode Photoelectric Sensors

Photoelectric sensors have solved numerous industrial applications over the years. There are basically three different sensing modes. The first is diffuse or reflex mode, next is retro reflective, which requires a reflector, and the third is through beam, transmitted or opposed. These field devices provide an excellent means of detecting target at a distance without contacting the object. All of the sensing modes are based on the sensor’s ability to detect a change in light reaching the sensor’s receiver. In this posting, we will review the diffuse or reflex photoelectric sensor.

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