As new buildings and factories are being built and/or retrofitted, mmWave sensors can add improved and new capabilities not previously possible

BY
KEEGAN GARCIA, Marketing Manager
Texas
Instruments mmWave Sensors Business
www.ti.com

Engineers
and architects designing new office buildings and factories are increasingly
seeking improved sensors to implement a variety of energy-saving safety and
security features. These sensors are used to detect the movement and location
of people in office buildings as well as robots or other machines in factories.
Multiple sensor types have been used with some success in earlier designs, but
designers are now just discovering millimeter-wave (mmWave) sensors. Low-cost mmWave
single-chip radars can act as sensors and offer many benefits not possible with
other sensors. As new buildings and factories are being built and/or
retrofitted, mmWave sensors can add improved and new capabilities not
previously possible.

Primary applications
The
main value of detecting and tracking people is energy conservation. In
buildings with many workers, the location and density of people varies widely
over time. It makes sense to adjust the heating and air conditioning and
lighting based upon human occupancy. If people are present, the optimum amount
of heating or cooling and lighting can be provided to maintain productivity. If
no humans are present, cooling and heating can be greatly reduced and lighting
turned off, resulting in major energy savings.

Security
systems can also be enhanced by automated surveillance. People location,
counting, and tracking are the essential data needed to protect some spaces and
to ensure that doors and gates are controlled to fit a desired security
protocol.

Safety
is another use of proximity detection, especially in factories where moving
robots and other machinery can pose a threat to humans. Motion sensing with
appropriate warnings can reduce accidents significantly. Other uses include
traffic monitoring at intersections and parking lots as well as level sensing
of liquids or solids in tanks.

The
success of all of these applications depends upon the reliability and accuracy
of people detection and tracking. The mmWave sensors can provide just that.

Sensor survey
Delivering
optimum heating, air conditioning, and lighting to an office building or
factory can result in major operational budget savings. For example, changing
the temperature and turning off the lights if no occupants are in a room
provide significant energy savings. Energy Star states that the average
commercial building wastes 30% of the energy that it consumes if it does not
account for the presence of people. The key to achieving lower costs is
providing just the right amount of environmental conditions at the right time.

Building
and factory automation systems rely upon sensors of all types to monitor a wide
range of physical characteristics. Some examples include temperature, humidity,
and ambient light sensors as well as smoke detectors. Other sensors are
magnetic detectors for window and door monitoring and detectors for carbon
monoxide and other toxic gases or dust and pollen. All of these sensors send
signals as feedback to central control systems that operate heating and air
conditioning (HVAC) and lighting systems. Employee comfort and productivity
increases with proper temperature and lighting conditions.

The
essential factor is the monitoring process that determines how many people and where
the people are in a building. Special sensors are needed to detect and count
people. Proximity sensors or detectors are used for this purpose.

One
popular proximity detector is the passive infrared (PIR) sensor. It detects
body heat and sends a signal to a controller for action. Simple PIR detectors like
the one in Fig. 1 have a single broad coverage area with
no discrimination ability other than coarse motion. The more sophisticated IR detectors
use multiple IR lasers or LEDs to illuminate an area to create a 3D scene that
can be captured by IR detectors. The digitized scene is then analyzed in
software to determine a person count.

Fig. 1: A basic passive infrared
detector like this one covers a large area but cannot distinguish detail.

Similar
options are available with ultrasonic sensors or video cameras. Ultrasonic detectors
paint an area with high-frequency sound in the 40-kHz to 80-kHz range and
recover the reflections that can be analyzed in software for people detection. Video
cameras are becoming more ubiquitous and can perform a similar function with
machine vision software but encounter concerns over privacy.

All
of the people-counting sensors are useful in some way but often do not provide
the accuracy in determining the number of people present and other finer details.
People motion is also difficult to capture. More accurate proximity sensors are
needed for some applications.

Radar sensors
You
probably think of radar as that technology used in airplane and ship navigation
and safety or as a component in the newer advanced driver assistance systems (ADAS)
used in vehicles for adaptive cruise control and automatic braking. Yet radar
is now proving to be a superior sensor type in counting people and determining
their location and movement thanks to the integration of intelligent processing
and smaller form-factor packaging. A radar sensor could easily pinpoint the
number of people occupying a conference room like the example shown in Fig. 2. Low-cost CMOS
single-chip radars are now available and being applied to this application.

0918_Special_Smart-Building_Fig-2

Fig. 2: A typical business space like
a conference room can be covered by one radar sensor and can accurately count
the number of occupants at the table.

One
type of radar device is the frequency-modulated continuous wave (FMCW) type
operating in the 76-GHz to 81-GHz range. The best way to think of a single-chip
radar system is to view it as just another type of sensor. As it turns out, mmWave
radars are unusually good at proximity detection of objects, motion sensing, and
measuring distance to the objects detected.

Recall
that radar is used to measure distance, direction (angle), and speed. Police
speed radars and baseball speed guns are good examples. An on-chip transmitter (TX)
radiates a signal that is then reflected from a remote object and returns to a
receiver (RX) also on-chip. The transmitted FMCW signal is a linear increasing frequency
change for a short duration called a chirp (see Fig. 3).
The chirps are repeated at a desired rate.

0918_Special_Smart-Building_Fig-3

Fig. 3: An FMCW chirp signal has a frequency that linearly increases over a short duration of time.

Fig.
4
shows a simplified radar transceiver. The
frequency of the return signal is mixed at the receiver (RX) with the
transmitted frequency to produce an intermediate frequency (IF) that is
digitized. Using this data with on-chip processors measures the time of transit
and computes the distance given the known speed of radio waves. Location and
angle (azimuth) can also be detected by using highly directional antennas. FM
radar also lets you measure motion and speed. The result is a very flexible and
programmable sensor that makes a very accurate people-sensing device.

0918_Special_Smart-Building_Fig-4

Fig. 4: A simplified representation of
a single-chip radar transceiver.

Some
of the key benefits of radar compared with other proximity sensors are:

  • They
    see in the dark whereas video cameras do not. They are not affected by smoke or
    dust in the air or bright sunlight.
  • Their
    high frequency provides greater precision in resolving closely spaced objects.
    They can determine location and direction of motion. Multiple antennas form
    phased arrays with beamforming to help improve the people-counting accuracy.
  • They
    measure fine degrees of motion and speed. Tests of PIR sensors versus mmWave
    sensors prove that the radar sensors can pick up on fine motions such as a human
    breathing.
  • They
    can detect humans and motion through walls. Tests have shown that radar sensors
    can easily penetrate sheetrock or plastic walls and deliver an accurate count. However,
    two layers of dry wall or plywood do greatly attenuate the radar signal, making
    detection difficult.

While the mmWave radar sensors perform well in
people counting, keep in mind that a great deal of the credit for detection and
discrimination of objects goes to the processing software. Sophisticated
algorithms and DSP microcontrollers are required to get the best results.