Whether you are selecting a self-contained (amplified) or board-level photoelectric sensor, you will undoubtedly be confronted with the ever-popular photoelectric sensor decision of choosing your desired sensing type. If you are unfamiliar with the various sensing methods, this can quickly become a confusing endeavor.
Photoelectric Sensors: Overview and Terminology
The initial challenge you will encounter is the lingo. You will find that across the industry, suppliers tend to use a variety of words to ultimately describe the same thing – the first being photoelectric sensors themselves. You will find that a device consisting of a light transmitter and receiver used to detect the presence or absence of an object with a beam of light can be described as the following: photoelectric sensor, photo eye, photo interrupter or even a photomicrosensor. Sometimes the word “switch” is used instead of “sensor”. You should also be aware of the term “proximity sensor” which could indicate either a specific type of photoelectric sensor, or a metal-detecting inductive proximity sensor which is an entirely different type of technology. Realize that inductive proximity sensors are a great solution for ferrous materials at close range. However, photoelectric sensors are the ideal alternative when you require a bit more range or of course when your detection target is non-ferrous.
Photoelectric Sensors Types
Now that you know you’re on the right track in selecting photoelectric sensor technology, let’s explore the two most common types of sensing methods: through beam and diffuse. Both designs detect the presence or absence of an object without any physical or electrical contact. This is often preferred because the sensor does not impede the movement of your detected object.The main difference between a through beam and diffuse type sensor is the location of the emitter and receiver. A through beam sensor (aka thru-beam, transmissive, interrupter, slotted, beam break, or opposed) has the emitter and receiver designed to face each other. See Figure 1. Alternatively, the diffuse style sensor (aka reflective, diffused, proximity) locates the emitter and photo detector adjacent to each other. See Figure 2.
Diffuse Sensor: Applications and Overview
An example of this style is the compact GP2S60 from Sharp with a 0.5mm detecting distance. With no object present, the diffuse sensor’s receiver sees mostly darkness. When an object passes by, light bounces off of the object and returns to the receiver, triggering detection. (Please note that with many photoelectric sensors you can decide if you want your discrete output to be sent when an object is present or absent (light ON or dark ON). We’ll save those details for another day and just discuss the most common set-ups today).
Through Beam Sensor: Applications and Overview
The through beam design means that the receiver is aligned with the light emitter and is waiting for an object to pass in between the two, triggering detection because the object is preventing the light beam from getting to the receiver. A great example of this design is your garage door sensor system located about 6 inches off of the ground on either side of your garage door. The purpose of detection is of course to prevent the garage door from closing on you, your Tesla, Billy’s bicycle or even the beloved Rufus, your next door neighbor’s dog who loves to roam the neighborhood. Though advantageous over long ranges, a through beam design doesn’t have to involve completely separate packaging. The “u-shape” or “forked” through beam design involves the same general concept of requiring the beam to be blocked from getting to the receiver, but it comes in one compact unit. A benefit to this setup is that the emitter and receiver are guaranteed to always be perfectly aligned. Examples of this are the through hole mount OPB620 sensor from Optek Technology with a 4.83 mm wide sloth and the EE-SX771 self-contained sensor From Omron with a 5 mm wide slot.
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Overall though beam designs are considered the most reliable solution because they are unaffected by object surface properties such as texture, gloss, angle and color. A typical rule of thumb is to select a through beam setup if at all possible to guarantee accuracy. However some downsides to this choice are installation and cost. Separate emitter and receiver units requires you to install and power at two points on the system and also typically increases your price.
Through Beam vs Diffuse Sensor: Primary Differences
Unlike the through beam method that will detect any opaque object that blocks enough light, the diffuse method relies on the reflective properties of the detection target itself. Because of the dependency on the material color, finish, and surface properties, diffuse sensors are considered less reliable than their through beam counterparts. Some examples of challenging detection would be if you have varying dark and light colors and varying matte or gloss finishes. Navy blue and black return significantly less light than white or yellow so the navy and black objects must be closer to the sensor in order to be detected. Your gloss finish should return more light to the receiver, but what if that object is at an angle and reflects all of that light elsewhere? Don’t even get me started on transparent or translucent targets. They are their own monster. However with all things considered, diffuse sensors are a great option. If you do your homework and determine that your object is stably detected with the diffuse sensor, you can enjoy the benefit of a single install location and typically a lower cost solution.
Photoelectric Sensor Applications: Where are Photoelectric Sensors Used?
Beyond use in industrial automation applications, you can find photoelectric sensors everywhere – from sensing money and product in a vending machine to train ticketing machines to that automated book drop at the local library. Photoelectric sensors are probably a lot closer to home than you realize. At work you can find them detecting paper in the printer and automatic stapler, detecting disk presence in your computer and detecting your physical presence near a door. At home they are detecting disk presence in your DVD player, louver and fan motor rotation in your AC/heater, and can even be found in some smoke detectors.
After improving your photoelectric sensor vocabulary and learning the basic pros and cons of through beam and diffuse style sensors, you can now select with confidence when adding a photoelectric sensor to your next project.