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Reliable occupancy sensing needs finer motion detection

We know that energy usage will double in the next 20 years. We recognise that more than 40% of energy usage is down to domestic and commercial buildings. We understand that occupancy sensors could save more than 75% of the energy we use for lighting and HVAC.

So what on earth could hinder more widespread adoption of energy-saving building automation systems?

The answer sadly lies in the well-known limitations of incumbent occupancy sensor technology. High rates of false alarms or nuisance tripping, dead spots and inability to detect stationary targets are real turn-offs. To many, the existing occupancy sensor technologies simply aren’t fit for purpose.

Passive infrared (PIR) sensors will always detect what we term ‘major motion,’ such as a person walking for more than a metre. Ultrasonic sensors are more sensitive, and will also detect what is defined as ‘minor motion’, a hand movement for example. But is this simplistic, 2-level classification of motion sensing sufficient? We don’t think so.

A new classification: ‘finer motion’

The truth is whether we’re at work or at home, we’re simply not always mobile. Let’s face it our lives have become, and will become, more sedentary. We know this. When we’re sat still catching up on our social media or watching TV, at a desk typing our emails or lying in bed asleep, we are present, but we’re not involved in major or minor motion.

We are though in motion. As long as we’re breathing we’re exhibiting movement, and occupancy sensors need to be able to detect this, if smart, energy-saving building automation systems are to operate reliably and effectively. There needs to be another level of motion classification and sensor technology needs to be able to detect it.

The English language alas doesn’t make it that easy in coming up with a third level of motion classification. What generic description sits comfortably with ‘major’ and ‘minor’ and yet suits the smallest perceptible movement of a beating heart or a breathing chest?

Words like ‘lesser’, ‘inferior’, or ‘minimal’ make it sound rather unimportant, ‘lower’, ‘smaller’ or ‘miniature’ seem to trivialise the movement, ‘slighter’ just doesn’t sound right...

For the purposes of this article then we’ll call it ‘finer’. So ‘finer motion’ sits below ‘minor motion’. We need our occupancy sensors to be able to detect ‘finer motion’ just as readily as they do major and minor motion. Only then can we introduce building automation systems that reliably detect occupants.

The problem with sensor technology

And that’s the problem in a nutshell, since PIR and ultrasonic sensor based systems will only detect major and minor motion and haven’t got the sensitivity or resolution to detect finer motion, they really can’t faithfully and reliably detect human presence.

Used as the standard sensing solution in many intruder alarms, PIR sensors detect major human motion by sensing mainly lateral movement of heat, though they do depend on having a clear line of sight. It will not detect minor or finer motion. To their advantage of course, as purely passive devices, PIR sensors don’t consume any energy.

Ultrasonic sensors on the other hand will cope with obstacles, will detect both major and minor human motion and also pick up motion directly towards a sensor. They will not though sense the finer human motion as we’ve defined it. Since these types of sensors need to constantly emit high intensity ultrasonic waves, there is also a significant power implication.

So why not radar?

Although not as widespread in use, Doppler radar based occupancy sensors do of course address many of the limitations of both PIR and ultrasonic occupancy sensors. And indeed the base technology has been used to monitor finer physiological movements for some 50 years.

The problem has been however that microwave solutions using the Doppler Effect have been ‘continuous wave’ in nature, meaning they transmit a known frequency and receive the bounce backs from reflecting objects all of the time. This again needs not an insignificant amount of power, and mostly analog designs have been costly to deploy.

What’s more, while radar signals will certainly pass through clothing materials and are more than sensitive enough to detect repetitive cardiopulmonary movements, they only provide phase information and just can’t measure absolute distance.

As a result, even continuous wave radar based occupancy sensors can’t distinguish between simultaneous finer human movements and deliver the absolute reliability that occupancy sensing demands.

A solution using UWB impulse radar?

In arriving at an occupancy sensor capable of reliably detecting major, minor and finer human motion, Novelda decided to focus its attentions on a radar-based solution, and set itself the target of resolving the inherent distance measurement, power and BOM cost deficiencies.

Instead of continuous wave operation, ultra-wideband (UWB) impulse radar techniques for emitting and sampling signal pulses were proven to achieve highly accurate absolute distance measurements, determined by the time differences between transmitted and received pulses.

And by taking what is essentially a spread spectrum approach and using digital signal processing to recover the return signal, UWB radar can operate at much lower power levels than conventional radar.

This then overcomes the potential consumer concern of not wanting a rather high-power radar sitting within human reach. To give it some context, the technique allows radar operation at power levels less than 1/1000th that of a regular Bluetooth headset.

The spread spectrum nature of UWB also means it can coexist with other RF systems without causing, or being affected by, interference. While UWB operates at very low power levels, DSP techniques can reliably extract the signal from noise, a bit like ADSL delivering broadband connectivity from ordinary phone lines.

Radar occupancy sensing is ‘out-of-the-box’

So Novelda’s X4M300 occupancy sensor, illustrated in Figure 1, integrates all of the components (and more) required by commercial occupancy sensing products, including lighting and HVAC control, security and elderly care.

It comprises the XeThru X4 radar SoC, PCB antennas, a microcontroller, glue logic, PSU, USB interface, GPIO and LED status indicators. Its BOM cost is low, its physical size is small, its operating temperature seriously industrial. It’s ready to go.

Able to deliver sub-mm sensing accuracy at distances up to 10m, and with a total reach of 30m means the module provides the reliable major, minor and finer motion detection capability that building automation systems really need from occupancy sensors.

More than that, the X4M300 presence sensor module single-handedly ‘debunks’ radar. It takes care of the radar, the antenna, the PCB and signal processing algorithm design. There’s a complete hardware and software development kit too (the X4M03 – check it out). The hard work is done.

So radar is no longer the preserve of complex and costly, high-end markets, it's ready for 100% reliable human occupancy detection – be our movements major, minor or somewhat finer.

X4M300 Presence Sensor
Figure 1. Novelda’s UWB impulse radar occupancy sensor.

Read more about the X4M300 presence sensor