Four Proximity Sensors PK, Who can Win?

There are various common proximity sensor systems, each with its own set of operating conditions and benefits for assessing detection, distance, and proximity. This article presents an overview of four different tiny, fixed embedded system solutions, as well as their basic operating principles, to assist engineers in deciding which one to use based on their design needs.

Proximity sensors allow you to sense the existence and distance of items without having to touch them. The sensor sends out an electromagnetic field, light, or ultrasonic waves that are reflected by or pass through the item before returning to the sensor,  Due to the lack of mechanical elements, proximity sensors have a substantial advantage over typical limit switches in terms of durability and longevity.

When looking for the best proximity sensor technology for a certain application, cost, range, size, refresh rate or delay, and material impacts, as well as what factors are most relevant to the design, are all factors to consider.

Ⅰ. Ultrasonic proximity sensors

Ultrasonic proximity sensors, as the name implies, emit ultrasonic sound pulses called "chirps" that are used to detect the presence of objects and to determine distances between them. They are made up of a transmitter and a receiver, and their function is based on the echolocation concept (Figure 1).

Figure. 1 How Ultrasonic Sensors Work

The sensor can measure the distance to an object by measuring the time it takes for a chirp to reflect off a surface and return, also known as "time of flight" (ToF). Although the transmitter and receiver are usually located next to each other, echolocation can still be used if the transmitter and receiver are separated. Ultrasonic transceivers are devices that integrate transmit and receive functionalities into a single package in some instances.

The color and transparency of objects have no effect on ultrasonic sensor readings since sound is employed instead of electromagnetic radiation. They also have the advantage of not emitting light, making them ideal for both dark and bright areas. Sound waves spread out over time and distance like ripples on water, and depending on the application, this expansion of the detecting region, or field of view (FoV), can be a gain or a drawback. Ultrasonic proximity sensors, on the other hand, can provide a cost-effective, adaptable, and safe solution with high levels of precision, relatively high refresh rates, and the ability to broadcast hundreds of chirps per second.

One of the most significant drawbacks of ultrasonic sensors is that variations in air temperature can impact the speed of sound waves, limiting measuring accuracy. However, by measuring the temperature of the air between the transmitter and receiver and modifying the calculations as necessary, it can be balanced. Another constraint is that ultrasonic sensors cannot be used in a vacuum since there is no air to transmit sound in a vacuum. Soft materials also don't reflect sound as well as hard surfaces, reducing precision. Finally, while ultrasonic sensor technology is comparable to sonar in concept, it cannot be used underwater.

Ⅱ. Photoelectric sensors

For detecting the presence of things, photoelectric sensors are a viable option. They are usually infrared-based, and common uses include garage door sensors and people counting in stores, but they can also be used in a variety of other industrial settings.

Photosensors can be used in a variety of ways (Figure 2).

In a through-beam system, the emitter emits a beam on one side of the item, and the detector detects the beam on the other side. If the beam is broken, there is anything in the way.

The emitter and detector are placed on one side of the object, while the mirror is placed on the other.

The diffuse reflection type, like the mirror reflection type, has the emitter and detector on the same side, but without a mirror, any detected item would reflect the produced light back. This isn't an accurate technique to estimate distance.

Figure. 2 Photoelectric Sensors – Through-Beam, Retro-Reflective, and Diffuse 

The photosensors can be configured as through-beam or retro-reflective if the application requires a longer sensing range and lower latency. However, they necessitate meticulous installation and alignment, making system installation difficult in a crowded area. Smaller objects are better detected with diffuse-type implementations, which can also be mobile detectors.

Photoelectric sensors may be utilized in unclean conditions, such as those found in industrial settings, and they have a longer lifespan than other options due to the lack of moving parts. The sensor's performance may be maintained as long as the lens is protected and kept clean. While they can identify most items, clear and shiny surfaces, as well as water, can cause complications. Other drawbacks include the inability to calculate precise distances and, depending on the light source, the inability to distinguish specific colored objects (such as red objects when using infrared).

Ⅲ. Laser rangefinder

Laser ranging (LRF), which was previously an expensive choice, has recently become a more viable solution in many applications. High-power sensors work in the same way as ultrasonic sensors, but instead of sound waves, they use a laser beam.

Due to the ultra-fast propagation speed of photons, determining  ToF accurately is difficult. Techniques like interferometry can assist preserve accuracy while lowering costs in this case (Figure 3). Due to the utilization of electromagnetic beams, laser rangefinder sensors frequently have exceptionally long measuring ranges (up to thousands of feet) and extremely fast response times.

Figure. 3 Laser Rangefinder Sensor Implementation Using Interferometry

These sensors have ultra-low latency and ultra-long range capabilities, but they also have drawbacks. Lasers need a lot of power, so they're not suited for battery-powered or portable applications, and there are also worries about eye health. Another factor to consider is that  FoVs are narrow, and they, like photosensors, do not perform well on water or glass. Despite the fact that the cost of this technology has fallen, it remains one of the more expensive solutions.

Ⅳ. Inductive sensors

Inductive sensors have been around for a long time, but they are currently becoming more popular. However, because they employ a magnetic field to identify objects, they only work on metallic items, unlike other proximity-sensing technologies (Figure 4). Metal detectors are an example of typical use.

Figure. 4 How an Inductive Sensor Works

The detection range varies depending on how the sensor is configured. Counting the rotation of gear by sensing the presence of gear teeth near the sensor could be a short-range application. Longer-range uses could include counting automobiles using inductive sensors embedded in the road surface, or even ultra-long-range detection of space plasma.

Inductive sensors are employed as proximity sensors in shorter-range applications and can give incredibly high refresh rates since they are based on the principle of sensing variations in electromagnetic fields. On ferrous materials like iron and steel, this sensor operates better.

Inductive sensors are a cost-effective alternative for a variety of applications. However, the restrictions of the materials they can sense, as well as the fact that they are susceptible to numerous types of interference, must be taken into mind.

Ⅴ. Summary

Ultrasonic sensors are frequently the best overall technology when considering all proximity sensing application issues (Figure 5). They are inexpensive, can detect the presence of objects, precisely compute distances, and are simple to operate, all of which are obvious advantages.

Figure. 5 Comparison of Four Proximity Sensor Technologies