PIR and ultrasonic sensors: What's the difference and how do they work? | Engineers' Insight | Avnet Abacus

PIR and ultrasonic sensors: What's the difference and how do they work?

By Alessandro Mastellari | October 21, 2019

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There are two primary technologies used in detecting the presence of people, pyro-electric / passive infra-red (PIR) and ultrasonic. Alessandro Mastellari takes a look at how they differ and how they work.

Our environment and our society are very rapidly becoming more connected and, as a result, smarter. This connected technology is changing and enhancing the way that we live and work, bringing greater productivity and efficiency to our homes and workplaces.

At the heart of this revolution is the Internet of Things (IoT) – a network of connected devices that already comprises billions of devices, and shows no signs of slowing as new devices and applications arrive incessantly.

While the applications for this smart connected technology are many and varied – and growing, some of the most established and most common are in the area of building energy management (BEM) and security. With energy costs rising and environmental consideration high on almost every agenda these days, making good use of energy is an increasingly important topic.

Along with the ability to manage efficient use of energy, most smart building systems provide a high level of automation which delivers convenience to building occupants. However, to be truly useful, these systems need the ability to detect the presence (or absence) of humans. With this ability, they can deliver automated ‘on demand’ energy management, turning lights on and controlling the environment when it is needed due to people being present.

There are two primary technologies used in detecting the presence of people, pyro-electric / passive infra-red (PIR) and ultrasonic. Both work quite differently and have different benefits, depending on the application.

All objects with a temperature above absolute zero emit heat energy in the form of radiation – this is known as Wien’s Law. PIR sensors work on the basis of detecting changes in this infrared (IR) radiation thereby detecting the presence of a human – or any other warm, moving object.

A block diagram of an IoT connected human sensor using a PIR as the main sensing element

PIR sensors typically have two slots in them, each of which is capable of detecting IR radiation. As a warm object (human) passes in front of one detector, a positive pulse is generated and, as it moves past the second detector window, a negative pulse is generated. This represents movement and, using relatively simple analogue electronics based upon a pair of operational amplifiers, a signal is generated to signify the presence of a moving warm body. Often an optical IR filter is fitted to the front of the sensor to limit the wavelengths to those of interest – the IR energy from a human body is around 10µm, for example.

The area that a PIR detector is able to cover is a function of its placement in the room as well as the lens that is typically fitted over the sensing element. In many cases this will be a Fresnel type lens (often made from semi-opaque plastic) that concentrates IR radiation from a wide area on to the sensor. Read this Engineer's Insight post for more on PIR sensors and how they are now being adapted to new applications.

An alternative approach is to use ultrasonic transducers to detect people present within a building. This uses sound waves at a frequency higher than humans are able to hear – typically in the range 30kHz to 10MHz. Transducers comprise a pair of devices, one is a transmitter and one is a receiver. A sound pulse is sent out at a given frequency and, as it bounces off objects in its path, it is reflected and captured by the receiver. In an empty room, the reflections will come from the opposite wall and the time taken for the reflection to be received will be proportional to the distance between the transducer and the wall. When a human enters the room then the pulses will reflect from them and, as they are closer than the wall, the time taken to receive the reflections will be less.

A diagram of a room showing how an ultrasonic sensor can be used to detect movement

Ultrasonic sensors can be used for room-level sensing and
individual robotic appliances

These types of systems are known as ‘Time of Flight’ (ToF) systems as, for a given medium such as air, the sound waves travel at a constant speed – meaning that the object distance can be detected by knowing the time taken for the sound pulse to be received.

Both PIR and ultrasonic detection can be used in standalone systems and ‘connected’ (IoT) systems, principally to detect the presence of humans – but there are other applications as well.

Lighting control is a key part of many BEM systems that sense the presence of people and control lighting so that it is only on when needed. PIR sensors can be used for this, but they do require the person to be moving. On the other hand, ultrasonic sensors can scan an empty room and then know when one or more people are present. As part of a more sophisticated BEM, this information can be used to control and automate heating, ventilation and air conditioning (HVAC) systems to save energy and minimise environmental impact.

PIR sensors are very commonly used in security or intrusion detection systems in both domestic and commercial applications. As they are placed away from the potential point of entry (door or window) they will detect any intruder before they are able to reach the sensor and tamper with it.

Connected robots are becoming more popular in both domestic and commercial applications. In the home, simple robots can vacuum rooms unattended while similar technology is increasingly being used for robotic lawn mowers. Objects in the path of these robots are an issue and ultrasonic transducers are commonly used to detect these obstructions and change the direction of travel.

In industrial applications such as large factories and warehouses, robots are used to move goods from one place to another. While these automated guided vehicles (AGVs) may be intended to run in defined paths that should be free of objects, almost all AGVs use ultrasonic transducers to detect other AGVs and objects that have inadvertently been left in their path.

The applications of this non-contact distance sensing technology are many and varied. For example, it is common to embed ultrasonic sensors in tanks to measure the level of liquids. This is equally applicable to domestic heating oil tanks as it is to large tanks of chemicals in process industries. In the connected world of the IoT, the level information could be used to drive a system that manages auto-replenishment.

This post only scratches the surface when it comes to the capabilities of PIR and ultrasonic, and there are a number of important factors to consider in the design process. For a technical deep-dive on how to approach sensor selection when designing motion detection systems, register for our latest webinar with Murata. Alternatively, get in touch with one of our technical specialists by clicking the Ask an Expert button.