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Considerations for Selecting a Photo Sensor

8/4/20 | Tom Hopkins, Rexel Technical Consultant

Photo sensors are simple presence sensing devices. To ensure successful application, you should give some thought to the physics of light before using one. Let’s look at some considerations for selecting a photo sensor.

#1. Limitations

Before selecting a photo sensor, you need a general understanding of how they work. For example, how does a diffuse sensor work? It sends light out to bounce off an object. The light comes back to the sensor and triggers an output. This sensor type is very popular, arguably the most common photo sensor in industrial applications. They work well most of the time, but it is important to understand the limitations of a diffuse sensor.

CONSIDER OBJECT COLOR

For example, say you have been sensing cardboard boxes on a conveyor with a diffuse sensor. The application has been working for years just fine, then the marketing department decided to change the box color from its original light tan to a very dark color. Suddenly your boxes are passing by without being seen. What happened?

Back to physics. A dark color absorbs more light, so less light is reflected to the sensor. You might be able to turn up the gain and see the darker box without a problem. But what if the sensor is already at full power, and what about the possibility that marketing may make a change again? Why not set up a solution that will work every time, no matter what? You have other things to be worried about than what marketing might do in the future.

BEST PRACTICES FOR SELECTING A PHOTO SENSOR

For a robust solution, it is always best to break a light beam. That could be a two-device solution, where you have a light source and a receiver (we call this transmitted beam). Or you can send light from a single device to a reflector (we call this retro-reflective). With either solution, an object that gets between the two components will block the light and be sensed.

APPLICATION EXAMPLE - USING THE DIFFUSE SENSOR’S COLOR SENSITIVITY TO YOUR ADVANTAGE

One of my customer’s plants produces printed material of various types. They make small booklets out of newsprint type paper. The individual pages are printed and cut, and then those pages are assembled to make small booklets. While the assembly process of the booklets is automated, the machine loading is done by hand.

They were having issues with operators not paying attention and loading the wrong items in the wrong locations, producing hundreds if not thousands of useless booklets. What we did was have them add a black line to the edge of the pages. When the pages are stacked it makes a long registration mark on the side of the stack. Using a small diffuse laser spot sensor, the white paper would trigger the sensor, but the small black line would not. Using reverse logic and the location of the registration mark, we were able to determine if the correct stack was placed in the right feeder position.

CONSIDER REFLECTIVE BACKGROUNDS

When selecting a photo sensor, consider an application with a shiny background. Perhaps you are shooting a diffuse sensor across a conveyor that has shiny aluminum or stainless sides as part of its construction. A reflective surface in the background can potentially send back more light than the object you are trying to detect. This could be an exercise in frustration when setting up the sensor. Yes, you could angle the sensor to reduce the reflection off the background, but is that the most robust solution?

SOLUTION OPTIONS FOR A REFLECTIVE BACKGROUND

Again, if you’re selecting a photo sensor, the most robust solution is breaking a beam. But if you cannot set up a beam to break, below are two diffuse sensor options.

Background-suppression technology. Background suppression allows you to set a limitation to the distance at which the sensor will trigger, no matter the background. Interestingly, background suppression sensors are also immune to the variable of object color talked about above. A background suppression sensor will detect an object at the same distance no matter the color shade.
Background-reflection technology. This type of diffuse sensor uses the background as a reflector. Any object between the sensor and the background will trigger an output. This can be very helpful if you need to use a single device but want to make sure the background is not a trigger concern. This could be effective if you need to look down on a shiny conveyor belt, for example.

CONSIDERATIONS AROUND RETRO-REFLECTIVE SENSORS AND SHINY OBJECTS

It’s important to consider the material you’re working with when selecting a photo sensor. As you will recall, a retro-reflective sensor bounces light off a reflector. With this type of sensor, you are creating a beam to break, which is always the most robust way to detect an object. However, a retro-reflective sensor can be fooled by a shiny object. The best way to describe this issue is to tell a story.

APPLICATION EXAMPLE – SMALL CHANGES CAN CREATE FAILURES

I have an original equipment manufacturer (OEM) that often works with conveyors. They took my advice and were using retro-reflective sensors as a robust solution. One day, I got a call that product was getting by the sensor without triggering. The product was a five-gallon bucket being prepared for filling. The issue was intermittent, and they were perplexed. The job was a duplication of a project they had built before, so why was there an issue now?

My first question was, “What has changed?” The answer was, “Nothing really. Just this time the bucket is metal with a paper wrapper, instead of an opaque plastic bucket.” Of course, this metal bucket would stop the light beam just like the original bucket, so what were we missing? As we got into it, I discovered there was a metal seam that was not covered by the wrapper. Periodically this shiny metal seam was lining up with the sensor. That shiny surface behaved as if it was the reflector. As such, the bucket would sail by the sensor without a trigger.

POLARIZED LIGHT

The solution was to use a polarized retro-reflective version. What is polarized light and why did that help? Think about polarized sunglasses. With them on, you can see through the surface of the lake water when you are fishing. This is because light reflecting off a smooth surface becomes polarized. Your sunglasses are designed to filter out that polarized light.

With a polarized sensor, the light reflecting off the smooth surface of the shiny object is simply ignored. There is a bit more to it, but suffice it to say, a polarized sensor will not be fooled by reflective objects.

#2. Dusty Environments

The environment should always factor into selecting a photo sensor. Obviously, a dirty environment is not the best for the transmission of light. But if it is only slightly dusty and the sensors can be cleaned off periodically, it may be a fine option. That said, how can you count on the maintenance personnel to clean off your critical sensors and avoid an expensive crash?

CONSIDER USING SMART SENSORS

Newer DC sensors often come with built-in intelligence. A smart sensor is one that typically only switches discrete information BUT has the capability to give you analog info as well. Imagine being able to tell whether the sensor is dirty, or overheating, or close to failure?! Additionally, when replacing a sensor, the PLC would immediately detect a new device and download the settings of the old one. No need for a skilled technician to set it up.

You can stock plain vanilla discrete sensors and turn on this intelligence for critical applications when needed. This technology is handled through the IO-Link protocol. For more information see the link at the end of this blog.

Key Takeaways

For a robust photo sensor solution, break a light beam whenever you can. Meaning, use transmitted-beam or retro-reflective technology.
If you do not have space for an opposing receiver or reflector to create the above, consider background-suppression or background-reflection technology.
When using retro-reflective sensors, always use polarized sensors so shiny objects will not fool the sensor.
Consider smart sensors for critical applications. Know you are about to have an expensive crash before it happens.

HELPFUL LINKS

The following resources can help you make an informed decision when selecting a photo sensor:

We Can Help

Do you need help selecting the right photo sensor for your applications? We can help! Our Automation Specialists are here to help you evaluate your materials, environment, and more to ensure successful implementation. Contact us today!