As new and older technologies become smaller, cheaper, and consume less power, the number of options available for sensing is constantly increasing. Proximity sensors are no exception to this expansion, with various sensors operating on completely different principles. While having multiple options can be beneficial, how do engineers determine which sensor technology should be used for detection, distance, and proximity applications?
What is a proximity sensor?
Proximity sensors are a non-contact method that provides simple "present/absent" logic or accurately measures the exact distance to an object. The term proximity sensor is important because they come in a wide range of sizes and detection distances. This article will focus on popular proximity sensors best suited for portable or small fixed embedded systems. These technologies include ultrasonic, photoelectric, laser rangefinders, and inductive sensors, which are well-suited for medium detection ranges from a few inches to tens of feet. Capacitive and Hall effect sensors are also efficient proximity sensors, but they are best suited for detection at very close distances and are not considered.
Proximity sensor design considerations
Even disregarding cost, no proximity sensor can perform all potential tasks better than others. Therefore, when reviewing the ideal proximity sensor technology for a specific application, many attributes must be considered and their importance weighted.
1. Cost: Few projects can ignore the cost of their components. Proximity sensors may account for only a small portion of the total budget or consume a large portion of it.
2. Scope: Although the scope of specific products may vary, proximity sensor technology has general limitations on the distances and ranges they can detect.
3. Size: Size is very important for embedded designs because proximity sensors can range in size from a grain of rice to large items that a person cannot carry.
4. Refresh rate: Most proximity sensors operate by transmitting signals and detecting return signals, which imposes a physical limitation on their update frequency, known as the refresh rate.
5. Material effects: Some sensors behave differently on hard surfaces and fibrous surfaces, while others behave differently depending on the color of the object.
What is an ultrasonic proximity sensor?
Ultrasonic sensors use ultrasonic pulses of sound waves to detect the presence of objects, or, through additional processing, to detect the distance to objects. They work using a transmitter and receiver, and the principle of echolocation. By emitting a chirp and measuring the time it takes for the chirp to reflect off a surface and return, an ultrasonic sensor can measure the distance to an object. Although often shown in configurations where the transmitter and receiver are as close to each other as possible, these principles still apply when they are separated. There are also ultrasonic transceivers that integrate the transmitting and receiving functions into a single package.
Ultrasonic detection is highly accurate and boasts a very high refresh rate, capable of emitting tens or hundreds of pulses or chirps per second. Because it is based on sound rather than electromagnetic waves, the color and transparency of an object have no effect on the reading. This same characteristic also means they do not require or generate light, making them ideal for environments with natural darkness or where darkness is desired. Sound waves also propagate over time, increasing their detection area—a benefit or a disadvantage depending on the application. Due to their simple design, they are also very inexpensive, versatile, and safe.
However, ultrasonic sensors have their unique drawbacks. The sensor has two parts, a transmitter and a receiver, which can appear as a single unit or as a separate device. Since the speed of sound varies with temperature, any drastic temperature fluctuations will affect accuracy. However, this may be offset by temperature measurements to update calculations. Soft materials also affect accuracy because sound waves reflect poorly on these absorbing surfaces. While the concept may be very similar to sonar, ultrasonic sensors are not designed for underwater use. Finally, their dependence on sound makes them completely useless in a vacuum, as there is no medium for sound propagation. For more information on the basic operation and implementation of ultrasonic sensors, please click here.
Photoelectric proximity sensor
Photoelectric sensors are highly effective for detecting absences or presences. While ideal for many industrial applications, they are commonly used in residential and commercial environments, such as garage door sensors or people counting in stores. In terms of implementation, optical sensors can be provided in several variations. Through-beam sensors implement a transmitter on one side and a detector on the other, with detection occurring due to the interruption of the beam. Retroreflective sensors reflect the beam from the transmitter back to the detector when the transmitter and detector are in the same location. Finally, diffuse sensors arrange the transmitter and detector together, but the emitted light will be reflected from any nearby surface, similar to how ultrasonic sensors work.
Due to the lack of moving parts, photoelectric sensors typically have a long lifespan and can sense most materials, although transparent materials and water may cause problems. Through-beam and retroreflective setups offer long sensing ranges and very fast response times. Diffuse-reflective setups can detect small objects and can also act as motion detectors. They can withstand the dirty environments common in industrial applications, provided the lens is not contaminated. However, their ability to calculate distance to objects is very limited, and the color and reflectivity of objects can be problematic. System installation in busy environments can be complex due to the need to mount and align the through beam and retroreflector.
Laser rangefinder sensor
Laser ranging is a technology that has only recently become economically feasible in many applications. It works on the same principle as ultrasonic sensors, but uses electromagnetic beams instead of sound waves. Because of the high speed of light, calculating the time of flight requires considerable accuracy; therefore, other methods, such as interferometry, are sometimes used to reduce costs while maintaining accuracy. These sensors typically have extremely long ranges, reaching hundreds or thousands of feet, and can have extremely fast response times depending on the option.
Although the price of this technology has decreased, it remains one of the most expensive options, costing several orders of magnitude more than the technologies discussed earlier. The increased power required to operate the laser has drawbacks, including limited lifespan for portable applications and potential eye safety issues. For better or worse, as a laser, while there is some dispersion over distance, the sensing area is still relatively small. Finally, the technology is not suitable for water or glass, further limiting its applicability.
Inductive sensor
Despite being based on an older concept, inductive sensors have become increasingly popular in recent years. Unlike other technologies on this list, inductive sensors only work with metallic objects. Just as a rotating magnet in a coil is the basis for generating electricity, an inductive sensor is used to generate a magnetic field and then detect changes in that field as a metallic object passes by. This is the foundation of any metal detector.
Depending on their configuration, their detection range can be very small; applications can calculate gear rotation by detecting the presence of gear teeth near the sensor. For longer distances, inductive sensors can be embedded in roads to detect vehicles traveling on them, or, in extreme cases, optimized to detect space plasma. However, as electronic proximity sensors, inductive sensors typically operate in the millimeter to meter range. Due to their operating principle, they perform better on ferrous metals such as steel, but have a smaller detection range on non-magnetic metals. Because they are based on changes in electromagnetic fields, they typically have extremely fast refresh rates.
Inductive sensors are very flexible in their range and applications, and are conceptually very easy to operate. This simplicity produces relatively inexpensive sensors, but it also highly limits their sensing range and makes them susceptible to interference from various sources.
In summary
There are many different options for proximity sensors. This blog covers some of the more popular mid-to-long-range technologies on the market. When considering all the costs and deployment challenges, ultrasonic sensors are often the best overall solution. This is because they are inexpensive, can detect presence and distance, and are easy to use. Due to these advantages, ultrasonic sensors are ubiquitous and can be found in a variety of home and industrial environments.
