As robotics technologies advance, so do complementary sensor technologies. Like our five senses, combining different sensing technologies offers the best results when deploying robotic systems into changing and uncontrolled environments.
Some of the very popular sensor technologies being used in robots include force and torque sensors, touch sensors, 1D/2D infrared (IR) range finders, 3D time-of-flight light detection and ranging (LIDAR) sensors, inertial measurement units (IMUs), cameras, GPS and others.
Among them is a relatively new technology in robotic sensing, namely the millimeter-wave (mmWave) radar sensor, a special class of radar technology using short-wavelength in the electromagnetic spectrum.
mmWave radars transmit electromagnetic wave signals with a wavelength that is in the millimeter range. The objects in their path reflect the signal to be captured by the radar system to determine the objects’ range, velocity, and angle.
A mmWave system that operates at 76–81 GHz (with a corresponding wavelength of about 4 mm) can detect movements that are as small as a fraction of a millimeter.
Another important advantage of mmWave sensors over vision and LIDAR-based sensors is their immunity to environmental conditions like rain, dust, smoke, fog, or frost. They are lightweight and three times smaller and half the weight of miniature LIDAR range finders. Additionally, mmWave sensors enable the accurate measurement of the distance of objects in their field of view and the relative velocities of any obstacles.
mmWave sensors can also work in complete darkness or in the glare of direct sunlight. The size of their components, such as the antennas required to process mmWave signals, is small. Another advantage of short wavelengths is of high accuracy.
Interestingly, the millimeter waves were once considered unﬁt for practical use in radar due to the absence of suitable means of generation, reception, channelization, and transmission of electromagnetic waves in the millimeter range. The sensors were initially expensive and large and required multiple discrete components. Besides, the laws of millimeter-wave propagation in the nonhomogeneous atmosphere were not studied enough.
Today, millimeter waves are increasingly used in car radar, cloud radar, radar, and radiometry for concealed weapon detection(CWD), high-speed wireless access, ultra-high-speed wireless local area networks (WLAN), and other means of communications, including radar-based communication systems.
Since mmWave sensors use millimeter-wave frequency, they have several advantages:
- It can penetrate various materials such as drywall, plastic, clothes, etc.
- mmWave sensors are not sensitive to various environmental conditions like direct sunlight, shadows, or light reflections off of the water.
- mmWave technology can detect glass walls, partitions, and furnishings where light-based sensing solutions could fail.
- mmWave technology is highly directional: It can produce a compact beam with 1º angular accuracy. It has accuracy in the mm range at 60-64 GHz and 76-81 GHz.
- The size of components such as the antenna required to process mmWave signals is small.
- Light-like: It can be focused and directed using standard optical procedures.
- mmWave sensors are less mechanically complex and thus reduce manufacturing alignment and error calibration processes.
- mmWave sensors have high integration, competitive cost, high resolution, ultra-fast response, and reliable performance.
Let’s now look at some of the applications of mmWave sensors in robotics.
- Detecting glass walls: Modern buildings have extensive use of glass walls and partitions. This makes it necessary for service robots and autonomous vacuum or mop systems to sense these surfaces to prevent collisions. Glass has proved difficult to detect using the camera or IR-based sensors. But mmWave sensors can easily detect the presence of glass walls as well as materials behind them.
- Measuring ground speed: mmWave sensors can supply accurate odometry information for autonomous robots that traverse over loose gravel, dirt, or wet areas, where other low-cost speed measuring approaches may fail when the wheels slip on surfaces. A ground-speed mmWave radar sensor enables to compensate for velocity-measurement errors for uneven terrain variables that can result in sensor pitch, yaw, and roll and introduce a rotational velocity component.
- Safety guards at robotic workstations: Historically, a safety curtain or keep-out zone has been used around a robot to ensure the physical separation of robots from humans. Today, sensors make it possible for a virtual safety curtain to separate robotic operations from unplanned human interaction and avoid a robot-to-robot collision. All vision-based safety systems require controlled lighting, which increases energy consumption, generates heat, and requires maintenance, such as frequent cleaning of lenses in dusty manufacturing environments like textile or carpeting. With a wide field of view and long detection range, mmWave sensors are robust at detecting objects regardless of lighting, humidity, smoke, and dust on the factory floor.
- Mapping and navigation: Another widespread application of mmWave radar sensor is mapping and navigation. mmWave radar can accurately map obstacles in a room and use the free space identified for autonomous operation.