Updated: May 28th, 2018

IoT ultrasonic sensors are designed for non-contact detection of solid and liquid objects. These sensors are used for a wide variety of functions from monitoring the level of water in a tank to fluid identification/concentration, to detecting object proximity. Ultrasonic sensors have become indispensable for IoT delivery and are widely used for building smart, connected products. Understand more about the different types of sensors, how they work, and their applications for everything from smart car reversal systems to smart waste bins.

How Ultrasonic Sensors Work

Ultrasonic transducers convert ultrasound waves to electrical signals and vice versa. These devices work on a principle similar to that used by transducers in radar and sonar systems, which evaluate the attributes of the target object by processing the echo signals from radio or sound waves, respectively. Ultrasonic sensors consist of two parts: a transmitter and receiver, which create a transducer that converts ultrasound waves into electrical signals (A/C) or vice versa. The transceiver vibrates and creates an ultrasonic wave that is transmitted and travels until it hits an object and is reflected back to the receiver. The interval between the signal being sent and received is typically referred to as time-of-flight (ToF) and depends on the distance the ultrasonic wave travels until it is reflected. The basic equation: time is equal to distance divided by speed, can be used to measure fluid level, fluid identification/concentration, and distance.

Ultrasonic Transducer Technology

The transceiver vibrates and creates an ultrasonic wave using piezoelectric transducers or capacitive transducer technologies. Piezoelectric crystals change size and shape when a voltage is applied: A/C voltage makes them oscillate at the same frequency and produce ultrasonic sound. Capacitive transducers use electrostatic fields between a conductive diaphragm and a backing plate. A single ultrasonic transducer can both generate and receive a signal, but the two functions are often separated in order to optimize the performance of each task.

IoT Ultrasonic sensors - working principle
Ultrasonic working principle Image credit: stab-iitb.org

Proximity Sensor

A proximity sensor is a sensor that detects the presence of nearby objects without requiring physical contact. Ultrasonic sensors are typically used as a proximity sensor, setting a threshold distance which can determine whether an object is an obstacle. This kind of proximity sensor is commonly used in the robotics industry.

Distance Measurement

Ultrasonic sensors are ideal tools for measuring distance without requiring contact with the object and are an efficient method of precisely measuring small distances. As the distance from the object is proportional to the time interval between transmitting and receiving signals, a simple analysis of this data can reveal changes in the sensor’s distance to the object.        

Liquid Level

There is a wide range of applications for liquid level sensors, each with its specific characteristics. The most typical application involves measurement of the tank liquid level of certain liquids, where the tank height is known. In this case, sound waves hit the surface of the liquid, and the sound echo signals are reflected back towards the sensor. The time that it takes for the sound wave to return to the sensor is directly proportional to the distance between the piezoelectric sensor and the liquid in the tank. This time period is measured by the sensor which is then used to calculate the level of liquid in the tank. The speed of sound waves can sometimes be affected by the variations in temperature and the sensor design should accommodate these variations.

Fluid Identification/Concentration

Fluid identification/concentration involves changes in the propagation velocity of sounds between different liquids. The sensor measures the ToF of a known distance e.g., tank width, and the micro-controller calculates the fluid speed of sound. This value can be compared to the values in a look-up-table, which is used to identify the liquid.       


IoT Ultrasonic sensors - motor applications
Reverse alert for motor applications Image credit: arduino-info.wikispaces.com

Advantages of Ultrasonic Sensors

  • Ultrasonic sensors produce ultrasonic frequencies that humans cannot hear, making them ideal for use in environments that require low noise levels.
  • An ultrasonic sensor response is not dependent upon the surface color or optical reflectivity of the object e.g., a glass plate or a shiny aluminum plate.
  • These sensors don’t require much electricity, are simple in design, and are relatively inexpensive.
  • Ultrasonic sensors with digital (On/Off) outputs have excellent repeat sensing accuracy. It is possible to ignore immediate background objects, even at long sensing distances because switching hysteresis  (the physical property value lags behind changes in the causation effect) is relatively low.

Disadvantages of Ultrasonic Sensors

  • Ultrasonic sensors have a minimum sensing distance.
  • Changes in the environment, such as temperature, pressure, humidity, air turbulence, and airborne particles affect ultrasonic responses.
  • Targets with low density, such as foam and cloth, tend to absorb sound energy and these materials may be difficult to sense at long ranges.
  • Ultrasonic sensors must be in the direct line of sight of the surface of the object in order to receive an adequate sound echo. Additionally, the reliability of these sensors requires a minimum object surface area.
  • Smooth surfaces reflect sound waves more efficiently than rough surfaces.

With such a wide range of applications and uses for ultrasonic sensors, it’s not surprising that they have become indispensable IoT components, used to create connected products and systems.