August 03, 2019 by Gaber Intihar
A Time-of-Flight (ToF) sensor works by sending out a signal (usually ultrasound or light) and waiting for it to bounce back from an obstacle and return to the sensor. The distance to the object can then be calculated from the time it took for the signal to return.
Advances in the field of MEMS (microelectromechanical systems) technology have enabled the miniaturisation of such ToF sensors.
TDK recently launched the first commercially-available sensors of this type: they promise accurate position tracking at just a fraction of the size and power consumption of traditional ToF sensors. The tech giant has unveiled two models—the CH-101 and the CH-201—which have a maximum operating range of 100 and 500 cm respectively.
The engineers at TDK were able to fit a piezoelectric MEMS ultrasonic transducer and a digital signal processor onto a single, low-powered application-specific integrated circuit (ASIC). The end result is a 3.5 mm x 3.5 mm package that they dubbed a ‘sonar on a silicon chip’. It includes all of the components needed for making ToF measurements.
The sensors support always-on operation and can consume as little as 8 µA of current when working at 1 sample per second (the maximum sampling rate is 100/second). This makes them very energy efficient, which opens up a wide variety of new applications for ToF sensors.
Sensing
In spite of such a reduction in size and energy consumption, these sensors retain high accuracy: the measurement error of the CH-201 is as small as 0.35 mm for an object 120 cm away. On top of this, both models support a 180° field-of-view, which means that a single sensor can provide room-scale sensing.
Lighting
Ultrasonic ToF sensors have a handful of important advantages over other types of ToF sensors: they can operate in all lighting conditions (including direct sunlight), aren’t affected by ambient noise, can detect objects of any size and colour (even if they’re transparent), and also maintain users’ eye safety (as no lasers are involved).
Power Supply
To make their sensors easy to incorporate into existing systems, the engineers at TDK designed their sensors to operate on a 1.8 V power supply and use an inter-integrated circuit (I²C) interface. It’s also possible to control multiple sensors with a single microcontroller, which allows for complicated multi-sensor setups.
These features open the doors for new use cases. The small size and power consumption make these sensors ideal for use in smartphones, wearables, and smart-home products.
For example, the optical proximity sensor of a smartphone could easily be replaced with an ultrasonic sensor that can be housed under the screen.
VR and AR
Since these sensors can accurately track objects in an entire room, they’re ideal for use in virtual reality and augmented reality applications. Installing these sensors in drones, robot vacuums—or even cars—can enable the technology in question to accurately and reliably keep track of their surroundings and avoid any obstacles in their path.
Beyond such consumer applications, these sensors have also received a lot of interest in the more industrial areas, including of course the IoT.
TDK will produce many more MEMS-based sensors in the future. But for now, only the CH-101 is available to consumers worldwide—as the CH-201 is currently shipping only to select customers.
