Proximity Sensors: Reviewing the Different Technologies

Most commonly utilized as a no-touch method to provide either simple object detection or precise distance measurement to an object, there are now many technologies that fall under the proximity sensor hierarchy, each offering different operating principles, strengths, and drawbacks. 

However, with such a variety of options available, how does an engineer choose the technology best suited for their design? 

To aid designers in this process, this article will discuss four of the most popular proximity sensor technologies that would realistically fit in portable or small fixed embedded systems and are suitable for moderate ranges of detection from a few inches up to tens of feet:

• Ultrasonic
• Photoelectric
• Laser rangefinder
• Inductive sensors

Capacitive and Hall effect sensors are two other popular proximity sensor technologies that will not be considered here due to their typically limited use in very close-range detection scenarios.

Before delving into each of the four technologies highlighted above, it is important to note that no proximity sensor technology will offer a one-size-fits-all solution for every application and intended use. There are many factors to consider when selecting a proximity sensor technology such as cost, detection range, package size, refresh rate, and effect of materials. 

Understanding where each technology falls on the spectrum of these different factors and which are the most crucial to the end application will be key to making the right selection.

Ultrasonic Technology

Ultrasonic sensors generate ultrasonic pulses of sound and measure the time it takes for that pulse to bounce off an object and return. They can be used to calculate the distance to said object or simply detect its presence. 

An ultrasonic sensor implementation can use either individual transmitter and receiver modules—wherein the transmitter emits a chirp and the receiver detects it—or the transmit and receive functions can be combined into a single module known as an ultrasonic transceiver. In implementations where separate transmitter and receiver modules are used, they are typically positioned as close together as possible for the greatest accuracy.

Figure 1. General implementation of ultrasonic technology

Due to their simple design, ultrasonic sensors are a low-cost option with a number of advantages that make them well suited for a wide array of applications. Capable of sending out hundreds of pulses per second, ultrasonic sensors are accurate with a high refresh rate. 

Because ultrasonic sensors are based on sound rather than electromagnetic waves, the color and transparency of objects, as well as operation in light or dark environments, have no impact on accuracy or function. In addition, as sound waves spread over time, their detection area increases, which can be a strength or weakness based on the design needs.

Although sound is not impacted by light or darkness, the speed of sound is impacted by changes in air temperature. Any dramatic changes in this temperature can greatly affect the accuracy of ultrasonic sensors. This can be offset by measuring the temperature in order to update any calculations, but this is still a limitation of the technology. 

These sound waves can also be limited by soft or absorbent materials that do not allow sound to bounce off as efficiently. Lastly, ultrasonic sensors are not intended for underwater use and their dependence on sound waves means that they are non-functional in a vacuum where there is no medium for sound transmission. CUI Devices’ blog, The Basics of Ultrasonic Sensors, covers this technology in further detail.

Photoelectric Technology

Most effective for absence or presence detection, photoelectric sensors are commonly recognized for their use in garage door sensors or occupant counting in stores, among other industrial, residential, and commercial applications. With no moving parts, photoelectric sensors generally have long product life cycles. They are able to sense most materials, but transparent objects or water could lead to issues. 

They offer several different implementations: through-beam, retroreflective, and diffuse-reflective. 

The through-beam implementation (Figure 2) is what one might recognize as the garage door sensor mentioned above with a transmitter and receiver placed opposite from one another. Any break in the beam between these two points indicates to the sensor the presence of an object. 

Figure 2. Through-beam implementation

Retroreflective (Figure 3) places the transmitter and receiver next to each other with a retroreflector placed opposite that reflects the beam from transmitter to receiver. 

Figure 3. Retroreflective implementation

Diffuse-reflective (Figure 4) operates similar to retroreflective, but rather than bouncing beams off a reflector, it bounces the beam off any nearby object, much like ultrasonic sensors. However, this implementation does not have the ability to calculate distance.

Figure 4. Diffuse-reflective implementation

 

The different implementations also have their advantages as through-beam and retroreflective offer long detection ranges and quick response times, while diffuse-reflective is good at detecting small objects. Photoelectric sensors are also a robust solution commonly found in industrial environments so long as the lens remains free from contaminants. With that being said, distance calculation is a virtually nonexistent capability of photoelectric sensors and object color as well as reflectivity can cause issues. 

The various photoelectric implementations also require careful mounting and alignment, which can lead to additional challenges in complex systems.

Laser Rangefinder Technology

Utilizing electromagnetic beams rather than sound waves, laser rangefinder sensors operate on similar principles as ultrasonic sensors. While this technology has become more economically viable in recent years, it is still a much more expensive option compared to ultrasonic and other technologies. 

Laser rangefinder technology does have an extremely long detection range upwards of hundreds or thousands of feet, along with fast response times. Due to the speed of light being much faster than the speed of sound, time of flight measurements can be a challenge for laser rangefinder sensors. This is where implementations like interferometry can be utilized to lower cost and improve accuracy. 

Figure 5. Typical laser rangefinder interferometry setup

As mentioned earlier, laser range finding is by far the most expensive technology discussed in this article, making it less feasible for many engineers’ bill of materials. The lasers used in this sensor technology also draw a lot of power, limiting its use in portable applications, while also exposing users to potential eye safety risks. 

Depending on the intended application, a laser’s relatively focused sensing area and lack of dispersion could be seen as an advantage or limitation. Laser rangefinders also do not perform well when dealing with water or glass.

Inductive Technology

Although based on an older operating principle, inductive sensors have recently gained more widespread use. Unlike the other three technologies discussed thus far, however, inductive technology is only suitable for metallic objects. 

Inductive sensors operate by detecting changes in its magnetic field as metallic objects come within its detection range. This is the basic operating principle of any metal detector.

Figure 6. Inductive sensors are used to detect metal objects

Outside of the common metal detector, inductive sensors have a wide detection range typically in the realm of millimeters to meters. This could include close-range applications such as counting gear rotations or longer-range implementations like vehicle detection on roadways. 

They perform best with ferrous materials (i.e., iron and steel), but can still detect non-magnetic objects with a reduced detection range. Inductive sensors also boast extremely fast refresh rates, simple operation, and flexibility in terms of their detection range. However, they are ultimately limited by what they can sense and are prone to interference from a variety of sources.

Conclusion

There are many factors to consider when it comes to selecting a proximity sensor technology. Understanding the benefits and tradeoffs of the different technologies discussed in this article can make this selection process easier. 

Table 1. Matrix comparison of the covered proximity sensors by cost, range, size, refresh rate, and effect of material.

Although each technology has its most appropriate uses, ultrasonic sensors are often a good overall choice due to their low cost, ability to detect both presence and distance, and typically straightforward implementation. This is why ultrasonic sensors are found in such a wide array of designs while continuing to find new uses and applications. 

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