Photoelectric Sensor Essentials

IDEC Corporation, Sunnyvale, CA

Photoelectric (PE) sensors represent a discrete sensor technology widely used throughout industry. Also called photoelectric switches or photoeyes, they use the presence or absence of light to provide an on/off output to supervisory automation and monitoring systems (Figure 1). Although the technology has been in use for many years, there are many device configurations and some recent advancements worthy of note.

Other sensors may use physical touch, or non-contact physical properties such as sound or electromagnetics, to detect objects. However, PE sensors are often the better choice for sensing manufacturing products such as boxes or bottles, or even parts of a machine itself, because of these leading advantages:

• Non-contacting: Avoiding contact means the sensor won’t be damaged, and the target can’t be marred.

• Wide range: Sensors are available for ranges from very close to very far, and targets do not need to be perfectly aligned for detection.

• Variable targets: Almost any opaque material can be sensed, and specialized versions are available for other types of materials.

PE sensor technology operates by detecting the light reflected from an object or obscured by an object. An emitter generates the light, and a receiver detects the presence or absence of light. These devices use long-life solid-state elements to generate the light, and because the light pattern spreads out once it leaves the emitter, there are several common types of light sources, depending on the application:

• Red LED: This general-purpose light source works well for many applications, and it produces a visible spot or beam making it easy to align.

• Infrared LED: This source is efficient and less susceptible to nuisance triggers from dust, but it is not visible to users, making it more difficult to align.

• Red laser: Lasers provide a tight beam diameter which works at long ranges, but they are more expensive, can be triggered by small contaminants in the path, and must be installed such that they don’t affect the eyes of workers in the area.

In addition to light sources, users must evaluate four major operating categories of PE sensors to determine which is best for their application (Figure 2):

• Through-beam: Requires installation of a separate emitter and receiver module spanning the target path. These work well for detecting products moving through a machine or conveyor.

• Polarized retro-reflective: The emitter and receiver are combined into a single device and point at a passive reflector across the target path. They are easier to install than through-beam, and even work with reflective targets.

• Diffuse-reflective: These are similar to retro-reflective, but no reflector is needed. They provide good detection of known targets over a short distance, but they can be affected by target color and reflectivity.

• Background suppression: This type uses one emitter and two receivers in a single device, and it is tuned to recognize a standard background at a close and fixed distance, so the device can reliably detect even small targets.

Beyond these characteristics, designers will need to consider the device form factor, response time, wiring methods, and voltage/signaling needs. Small devices with short response times, flexible configuration options, selectable operating modes, and other nice-to-have features such as laser-marked QR codes for accessing instructions via a mobile device provide the best usability for most applications.

PE sensors often provide the best discrete detection price/performance ratio for a wide variety of applications including machinery, material handling systems, processing/manufacturing, and many others.

This article was written by Bruce Fink, Product Marketing Manager at IDEC Corporation (Sunnyvale, CA). For more information, visit here  .

---