Self-contained through-beam sensors

Figure 1: Self-contained laser through-beam sensors have superior excess gain characteristics over such sensing modes as retroreflective and diffuse. Not only are laser through-beams very accurate, their high excess gain enables them to maintain their accuracy in the type of harsh environments that would degrade that of most conventional sensors.

Through-beams also have a tightly controlled, small sensing area called the effective beam, which is defined by the size of the emitter and receiver lenses — the smaller the lens, the smaller the effective beam. Because the beam has such a small diameter, very small targets or variations of targets can break it. This combination of excess gain and small effective beam increases the utility and accuracy of through-beam sensors. Moreover, these sensors are immune to variations in target colour, reflectivity or surface condition.

The advantages of self-contained through-beams

There are two significant drawbacks to traditional through-beam sensors: their two-piece architecture and their need for accurate, stable alignment (see Figure 2). A conventional through-beam system requires a separate emitter and receiver. These systems may be difficult to install because of space or configuration requirements and the need for additional complex wiring. Beyond that, to achieve excess gain, the system must be properly aligned and stay in alignment over time regardless of vibration, incidental operator interaction and random impacts.

Figure 2: Traditional through-beam sensor using fibre optics involves comparatively complex set-up, including tricky calibration, multiple cables and multiple components. This makes for longer set-up time and a less robust overall configuration during heavy, vibration-prone production cycles.

This class of sensor provides significant advantages to the production line. Because the sensors are always in alignment, set-up time is vastly reduced. If they are dislodged during the production, they can be quickly repositioned. Because they are self-contained, they eliminate the need for additional wiring and fragile fibre-optic cabling.

Self-contained through-beam sensor features include:

• Highly visible red emission for easy set-up
• Infrared emission for dirty environments
• Single-piece housings with pre-aligned optics for quick installation
• Rugged and rigid single-piece metal housings for stable, accurate readings over long periods of production time
• High-resolution capability that can sense very small parts or very small part variation
• High switching frequency capability to handle rapid production rates.

Figure 3: The self-contained through-beam shown is easy to install. The sensor’s rigid construction means that its emitter and receiver are always in alignment and it needs only one cable and no auxiliary electronics. If it’s a laser version, it can see through dirty environments, and the one-piece housing conserves space.

L-shaped through-beams

The latest example of this fast-expanding technology is the new L-shaped self-contained through-beam sensor. Customers needed a sensor that can handle identification and quality verification duties while installed within impact-rich environments that would destroy conventional sensors or unacceptably degrade sensor output. The resulting L-shaped design can be used in a variety of applications where a fork style would be too large or inappropriately configured, or where the sensor would be exposed to sensor-destroying contact or products moving in multiple directions. Because of its angle design, this sensor can be tucked away out of danger and still get the job done (see Figure 4). Its rugged housing, incorporating potted electronics, can withstand punishment that would destroy or misalign conventional sensors.

Laser through-beams

Another step forward is the self-contained laser through-beam sensor. Laser emission is required for applications that demand higher resolution. Unlike through-beams that use standard visible red emission, this style substitutes laser for the more diffuse visible red beam (visible red being the more economical version for most applications).

Figure 4: L-shaped sensors are ideal for detecting cases or other square objects on a conveying system. It can be used to see the leading edge of a case or package to initiate glue control, open flap detection or other gating operations for package inspection.

Dynamic optical windows — a specialised through-beam sensor

Dynamic optical windows are closed through-beam photoelectric sensors that detect an object’s movement as it passes through its square or rectangular loop, as shown in Figure 5. The ability to detect movement makes these sensors perfect for counting small parts ejecting from dies or machines in a random pattern or attitude. Rather than simply interrupting a static beam as with self-contained through-beam sensors, the entire part must enter and exit completely through the inner perimeter of the dynamic optical window for the sensor to recognise or count the part. The sensor will not detect motionless objects within its sensing field. For example, a motionless clear plastic transport tube will be ignored but parts passing through it will be detected.

Self-contained through-beams — the bottom line

Today, no manufacturer can allow less than 100% in-spec parts to come off the assembly line. The control device with the highest leverage on product quality is the sensor. Almost any sensor can be used for error proofing, given a high level of application expertise on the factory floor.

Figure 5: In this example, a dynamic optical window error proofs a progressive die program by making sure each finished part exits the die correctly. This also simultaneously protects the die from double hits by making sure each part is off the die and the press before the next hit.