Tutorial: Understanding ultrasonic sensors
01 January 2022
Figure 1. Basic ultrasonic transmitter/receiver arrangement. (image source: CUI Devices)
The longevity & continuing popularity of ultrasonic sensors can be attributed to the fact that they are inexpensive, highly adaptable & can be used in a wide variety of applications. Their adaptability has meant that more recently, they have also found uses in newer technologies such as autonomous vehicles, industrial drones & robotic equipment.
The full version of this article was originally featured in the January 2022 issue of EPDT magazine [read the digital issue] and the Digi-Key Article Library. And sign up to receive your own copy each month.
Here, Jeff Smoot, Vice President of Engineering at electronic components manufacturer, CUI Devices, on behalf of electronic component distributor, Digi-Key Electronics explains the principles of operation of ultrasonic sensors, considers their advantages and disadvantages, and reviews some of their most common applications...
How do they work?
The basic operation of an ultrasonic sensor is analogous to how bats use echolocation to find insects while in flight. A transmitter emits a short burst of high-frequency sound waves called a ‘chirp’, containing frequencies between 23 kHz and 40 kHz. When this pulse of sound hits an object, some of the sound waves are reflected back to the receiver. By measuring the length of time between when the sensor transmits and receives the ultrasonic signal, the distance to the object can be calculated using the following equation:
d = 0.5 * t * c
d = distance (metres)
t = time between transmission & reception (seconds)
c = speed of sound (343 metres per second)
Note that d is the measured time for the sound pulse to travel in both directions – this must therefore be multiplied by 0.5 to calculate the duration of travel in one direction, which ultimately equals the distance to the object.
The simplest ultrasonic sensors are configured to have the transmitter and receiver located adjacent to each other (Figure 1). This arrangement maximises the amount of sound travelling in a straight line from the transmitter, while reflecting in a straight line back to the receiver, thereby helping to reduce measurement errors.
Ultrasonic transceivers combine a transmitter and receiver in a single enclosure. This further improves measurement accuracy (by minimising the distance between them), while having the added benefit of reducing board space.
When calculating the distance to an object based on the readings from a sensor, several factors must be considered. Sound naturally travels in all directions (vertically and laterally), so the further the pulse of sound travels from the transmitter, the greater the opportunity it has to spread out over a wider area – much like how a beam of light spreads out from a flashlight.
It is for this reason that ultrasonic sensors are not specified for a standard area of detection; instead, they are specified for either beam angle or beam width. Some manufacturers specify sensor beams from the transmitter by full-angle deviation, while others specify by straight-line deviation. When making comparisons between sensors from different manufacturers, it is important to be aware of how they specify sensor beam angle...
Based on mature and well-understood physical principles, their relative simplicity and versatility, combined with low cost, have allowed ultrasonic sensors to stand the test of time. Commonly used for distance measurement and presence detection in a variety of consumer and industrial applications, ultrasonic sensors have also shown that they will continue to find uses in newer and ever more challenging applications well into the future.
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