Using photoelectric sensors on the production line

Like humans, automated machinery isn’t perfect – and little mistakes can add up to a whole lot of cost and headache.

By Greg Knutson, Banner Engineering November 1, 2008

Like humans, automated machinery isn’t perfect — and little mistakes can add up to a whole lot of cost and headache. Keeping a close eye on your production and catching rejects before they get to packaging and shipping is key, and photoelectric sensors using the latest technology can be a cost-effective, convenient solution for product inspection and verification — right on the production line.

Photoelectric sensors have been used in automation for more than 60 years for these reasons. This kind of sensor is essentially an optical control designed to detect visible or invisible light and respond to changes in light intensity. From confirming an assembly component’s location on the line to counting the number of products as they pass, photoelectric sensors have provided manufacturers with essential operational data. However, users quickly found that these sensors couldn’t perform every task at the same accuracy and speed.

Photoelectrics now deliver big performance in smaller packages, thanks to electronic miniaturization. Not only are today’s compact photoelectric sensors simpler to mount and implement in a wider range of applications, they are often available at a lower cost. While these benefits are significant, some of the greatest advantages are best demonstrated by taking a closer look at sensor design itself to see how today’s incarnations of photoelectric technology far surpass those of the past.

Seeing liquid clearly in bottling applications

Photoelectric sensing isn’t exactly new to the bottling industry. In fact, it’s often used to help verify liquid presence and level in many of these applications. However, with the increased popularity of the bottled water industry, some of these sensors have fallen short. In a clear bottle/clear water sensing applications, the low-contrast nature of the target makes it difficult for the sensor to distinguish between the bottle and water, making presence and level detection particularly challenging.

Other beverage industry applications can be troublesome as well. In order for a photoelectric sensor to correctly detect the contents of a colored or partially opaque bottle, it needs sufficient excess gain. Excess gain is extra sensing energy; it’s a measurement of the sensing light energy available that is over and above the minimum amount required to operate the sensor’s amplifier and trigger the sensor. This excess gain is what keeps a sensor’s signal from being attenuated from contaminants such as dirt or dust in the environment. In this case, if the sensor used in a frosted or colored container application has too little excess gain, the signal is attenuated by the bottle, delivering a false positive. On the other hand, if the beam has too much excess gain, it will “burn” through the bottle and the water contained, delivering a false negative reading.

Solving these applications required a new sensing beam. When a 1,450 nm infrared sensing beam is used, water can absorb about 1,000 times more energy than at visible wavelengths. The result is a high-contrast application, with the clear bottle and clear water appearing differently to the sensor. When paired with just enough excess gain, the sensing beam “burns” through a clear bottle but is absorbed by the clear water for accurate liquid detection.

The same design is just as effective in the latter example, where a photoelectric sensor with high excess gain is used to detect liquid or some liquid-containing substances in a frosted or colored bottle, bag or container. As mentioned earlier, without the specialized beam, this high excess gain would cause the beam to “burn” through both the container and its contents. But in this case, the water-containing content still acts as a light-blocker, ensuring accurate results at a fraction of the price of alternative sensing systems. This method can be used to detect anything with high enough water content as well as ordinary objects with sufficient opacity — anything from fruit juice in a labeled plastic bottle to bagels in a colored bag.

Far-sighted sensing for small components

Detecting a small flange on a moving object located meters away can be challenging for a photoelectric sensor. However, sensors often cannot be placed directly next to the target object due to manufacturing conditions such as a lack of mounting space available, EMI/RFI interference or the presence of weld spatter and sparks.

One solution is to keep the sensor mounted at a safe distance while ensuring the sensing beam has the range required to reach the target. Numerous long-range sensors are available, but one of the most convenient solutions for this kind of application is a fixed-field sensor with a visible red laser sensing beam. This sensor detects objects within a defined sensing field, ignoring objects located just beyond the sensing field cutoff. Its background suppression makes the sensor especially convenient in areas where manufacturing conditions prohibit the use of emitter/receiver pairs and reflectors. The laser is narrower than LED alternatives, making it ideal for small object detection, and the visible red sensing beam allows easy alignment with the target object despite the increased distance.

Identifying rejects while resisting the elements

For many of today’s photoelectric sensors, water and chemicals no longer provide the threat they once did. In these applications, sensors often had to be treated with special care such as covering or removing them altogether until the cleaning cycle was complete. Several techniques have now ensured sensors can withstand these challenging conditions, as well as extreme heat, dust and grime.

Sealing plays an important role in preventing moisture ingress from causing a sensor to fail. Many photoelectric sensors feature encapsulated electronics and are rated IP67, 68 or even 69K to withstand some of the most challenging conditions.

The highest first-digit possible in IP ratings is six. So these ratings indicate that a sensor rated IP67, 68 or 69K will be completely protected from dust, dirt and grime common in manufacturing environments. The second digit refers more specifically to the sensor’s ability to withstand moisture ingress. An IP67 rating ensures a sensor can withstand temporary immersion (for about 30 minutes in up to 1 meter of water), and IP68 means the sensor is protected under longer periods of immersion under pressure. An IP69K-rated sensor is specifically designed to resist high-pressure spray common in sanitary applications — specifically, withstanding sprays up to 1,200 psi, which would otherwise destroy a sensor.

Another robust solution for demanding industrial environments involves a photoelectric sensor affixed with a liquid-tight perfluoroalkoxy (PFA) outer shell — or “jacket.” PFA is a type of fluoropolymer that provides exceptional resistance to acids such as those present in caustic manufacturing environments, as well as the harsh cleaning chemicals often used in washdown. This jacket is fitted onto a photoelectric sensor to protect the sensor from corrosion and moisture ingress.

In extreme temperatures, a photoelectric sensor’s electronics are at high risk of damage, leading to sensor failure. For high temperature environments, remote sensors — which use glass fiber optics and remote amplifiers — are often used. The glass fibers can resist some of the most hostile environments, including temperatures up to 480 C (900 F), corrosive materials, extreme moisture and high levels of electrical noise, shock and vibration. While the fiber optics can enter directly into the harsh sensing environment, the amplifier is mounted remotely — typically in a control cabinet safely out of harm’s way. This arrangement is also ideal for explosive or other hazardous areas.

The IP ratings mentioned earlier indicate when a sensor can resist airborne contaminants, moisture and high-pressure spray in plant applications. In particularly harsh, dirty environments, even sensors with high IP ratings can be negatively affected by a combination of dust, smoke or fog in the sensing path and oil, grease or soot build-up on the face of a sensor. These factors can reduce a sensing system’s reliability by attenuating the signal strength. To overcome these environmental challenges, sensors with high excess gain can be used, since this extra sensing energy allows the beam to burn through contaminants and reliably detect the target. Opposed-mode sensors usually work best in these types of applications.

Using UV light for challenging inspections

The first example illustrated how a photoelectric sensor with a 1,450 nm infrared sensing beam can detect clear water within a clear bottle. But what if you need to detect a clear material or identify a clear material on another surface?

For some clear materials, luminescence sensors can provide a solution by using UV light to detect luminophores, a chemical compound that, when it reacts, results in cold light (as opposed to incandescence, which is light caused by heat) that is invisible to the human eye. When the UV light projects on a luminescent material such as some adhesives and petroleum-based products, the light “excites” electrons in the luminophores, causing them to react and emit visible light. The sensor then generates an output upon detecting this glow.

Typical applications for luminescence sensors include lumber optimization or detecting tamper-evident seals and leaking oils, as well as fluorescent product markings where the background color may cause a contrast change that would challenge traditional sensors. These sensors can detect luminescence inherent in a material, or it can sense luminophores that have been added to a material in process to make it luminescent.

These and many more photoelectric sensors have been optimized for some of today’s greatest product inspection and verification challenges. By delivering high performance at a lower cost, they’ll help you maintain your production line — and your bottom line.

A photoelectric sensor with a 1450 nm infrared sensing beam, which is inherently absorbed by water, enables simple discrimination between a clear plastic bottle and the clear water it contains for accurate presence and level detection.

When a photoelectric sensor is fitted with a liquid-tighter outer shell or “jacket” made of PFA, it is protected from moisture ingress and corrosion caused by harsh acids and other chemicals in washdown environments.

Glass fiber optics are ideal for high-temperature applications — unlike their plastic counterparts, glass fibers will not melt in extreme temperatures up to 480°C (900°F). For explosive or other hazardous environments, a remote sensor featuring glass fiber optics is placed within the sensing environment, while the sensor’s amplifier is located away from the sensing area.

Luminescence sensors use UV light to detect luminophores, which are invisible to the human eye, in materials such as some adhesives and petroleum-based products. In some applications, luminophores can be added to the target object to help detect product markings, as depicted in the image above for lumber optimization.

Author Information

Greg Knutson is a senior applications engineer at Banner Engineering, a manufacturer of photoelectric controls, machine safety systems and wireless networks. In his role, Knutson assists customers in the selection of appropriate products for their individual applications. Knutson has 17 years of applications experience in the factory automation industry, including 13 years at Banner.