How Are Photoelectric Sensors Used In Automation?

Industrial environments are subject to extremes in temperature, radiation, electrical or chemical hazards, and contaminants like dust and moisture. This can cause photoelectric sensors to register false positives, leading to breakdowns and assembly line incidents.

Therefore, you must select the right type of sensor for your industry. Photoelectric sensors come in a variety of types, and each has pros and cons. But how are photoelectric sensors used in automation, and which type is best for the industrial environment?

Below, we examined the most useful types of sensors for automation industries to help you choose the right type for your business.

What Are Photoelectric Sensors?

Photoelectric sensors are popular in all industries and have been used on assembly lines in automation for decades. They detect the presence or absence of an object through light. To do this, they emit a light beam (either visible or infrared).

They are widely used in the automated industry for a very good reason. Compared to ultrasonic technologies that use sound, photoelectric sensing has faster response times. This is due to the faster speed of light than sound. And faster response times mean higher productivity rates.

However, to ensure that your photo-electric sensors work at their optimal capacity, regular maintenance is required. Photoelectric sensing works best in sterile environments, so periodic sensor lens and/or reflector cleaning is essential. This is especially true of industrial settings.

Photoelectric sensing can utilize different modes of operation. We examine the top sensing modes, below.

Sensing Modes

There are three main sensing modes for photoelectric sensors. Each has distinct advantages and disadvantages. But these are mostly influenced by the working conditions surrounding them.

Let’s take a closer look at the top three sensing modes, how they function, and how suitable they are for the automation industry.

Diffused

A diffuse photoelectric sensor is also known as an optical proximity sensor. In the proximity mode, the light source, and the receiver, are housed in the same device. The sensor’s light source reflects off the target and returns to the sensor’s receiver.

This return of light to the sensor confirms the target’s presence. The strength of the return light depends on the distance and incident angle to the target, and even the target’s color. The proximity mode is easy to install and can provide reliable sensing where those factors can be controlled.

However, the distance between the sender and receiver in this sensing mode makes it sensitive to dust and dirt. Therefore, it is not suitable for harsh industrial environments. However, in the food processing industry, where high levels of hygiene are mandatory, this can work very well.

Retro-Reflective

The retro-reflective sensing mode entails using the sensor with its sender/receiver on one side, and a reflector on the side of the target.  When the target passes between the reflector and the sensor, the light beam’s path is blocked.

As with the diffused mode, this absence of light at the receiver confirms the target’s presence. However, unlike the proximity mode, the success of the retro-reflective mode is not dependent on distance or incident angle to the target, nor its color.

Unfortunately, the retro-reflective mode is not without disadvantages. The gradual accumulation of dust or moisture on the sensor may cause a reduced sensing range, and eventually render it ineffective. Depending on the harshness of the environment, it is also subject to possible damage.  

Thru-beam

A thru-beam sensor measures the change in light quantity when a target crosses the optical axis. To put it more simply, the sensor’s sender and receiver are placed in opposition to each other.

The target passes in between them, and when it does so, the light from the sender is blocked from the receiver. This absence of light confirms the target’s presence. The disadvantage is that transparent or translucent targets are also not always reliably detected.

Also, one-half of the sensor needs to be physically on the other side of the target. Unless a fiber-optic thru-beam is used, both components need to be powered. This need for wiring up both the sender and receiver may be infeasible on your particular workstation.

Your Choice Of Light Source Matters Too

Your choice of LED light source for your photoelectric sensor is important.

• With visible LED light sources, you can see where the light beam strikes the target in a proximity sensing mode.

• With IR (infrared) light sources, the light beam is invisible and can pass through certain target materials. It is also less affected by dust particles. This makes it ideal for the industrial environment.

Conclusion

When selecting photoelectric sensors for your automated work environment, assess the conditions before making your choice. Different sensing modes are suitable for particular environments.

The wrong choice will inevitably cause malfunction, which could result in delays at best, and, at worst, breakdowns of equipment or workplace accidents.