Learn about the different ways companies use 3D time-of-flight sensor technology to create products for facial recognition, smart cars, and manufacturing.
Time-of-flight (ToF) sensors measure linear distance to a target object. Although they can be implemented in a number of ways, they all depend on an unchanging physical constant: the speed of light.
How does ToF technology work?
In essence, a wave pulse is aimed at the target and the time it takes to return is measured. Time is measured for the round trip, but we are only concerned with the outward journey, so it is divided in half. Now we have speed as the speed of light and time. Since distance is equal to speed times time, distance can be easily calculated.
There are two common variations on this theme:
- The amplitude of the light wave is modulated by a known frequency and the phase shift is measured.
- The light wave is sent as a 50% duty cycle square wave pulse and the amount of reflected light is measured over a specified period of time.
Of course, this is just a brief overview of the technology. To implement these sensors more effectively in designs, it is a good idea to learn more about time-of-flight technology.
Flight detection time today
ToF technology is available in many different forms. There are chips that contain the actual optical transmitters and receivers on board, and others that work with external cameras.
Because some of the apps are mobile and some are installed, power consumption may or may not be a critical factor.
Facial recognition will require much more resolution than an application to indicate whether a person is occupying an exclusion zone by mistake (or not!). A machine vision manufacturing application may require formidable resolution, but it will only deal with what happens a centimeter or two away.
Let’s see how companies use ToF detection in their products.
TI ToF Sensor and Controller
The OPT8241 ToF sensor, together with TI’s OPT9221 ToF controller, forms a two-chip solution to create a 3D camera.
This is how the sensor and controller divide the workload.
The OPT8241 Time of Flight (ToF) sensor offers a quarter video graphics matrix resolution (320 x 240) at data frame rates of up to 150 frames per second (600 readings per second). Timing generator programmability offers the ability to optimize various depth sensing performance metrics, including device power consumption, signal-to-noise ratio, and environmental cancellation. The unit is available in an optimized optical package (COG-78), 8,757mm × 7,859mm × 0.7mm.
The OPT9221 Time of Flight Controller (TFC) is a companion device for the OPT8241 3D ToF Sensor. Your job is to calculate the depth data from the raw data from the digitized sensor. When working with the OPT8241, the unit collects information that will be used to implement filters and masks and to dynamically control overall system settings for expected performance. This device comes in a 256-pin, 9mm × 9mm NFBGA package.
Figure 1. TI’s OPT8241ToF sensor and OPT9221ToF controller form a two-chip solution for creating a 3D camera. Image courtesy of Texas Instruments (PDF).
In its literature, TI suggests three likely applications for the duo:
- Presence detection for industrial security. Is the object in a danger zone a human, a robot, or perhaps a vehicle?
- Counting and locating people
- Location and identification of people
Espros 3D ToF Imaging Chip
The EPC660 from Espros is a 3D-TOF imaging chip that works with the company’s DME 660 distance measuring camera. It is a system-on-chip (SOC) camera system that communicates through a 12-bit parallel video interface. Only a few additional components are needed to integrate a 3D camera into mobile devices.
Figure 2. The EPC660 from Espros. Image courtesy of Espros.
The unit can solve target distances with a resolution in the range of millimeters, even if they are up to 100 meters away. Up to 1000 TOF images can be taken per second.
The EPC660 will find use in applications including:
- Detection and counting of people.
- Machine safety
- Automobile collision avoidance system
- Pedestrian detection and breakout systems
- Gesture control
- Body size measurement
- Human-machine interface
AMS ToF Sensor
AMS’s TMF8701 Time of Flight Sensor comes in a small 2.2mm x 3.6mm x 1.0mm package. Its operating wavelength is 940 nanometers. It is a VCSEL (Vertical Cavity Surface Emitting Laser) device, a technology that has many advantages compared to horizontally oriented devices.
Figure 3. TMF8701 ToF Sensor. Image courtesy of AMS.
Explore series form Artilux
The methodology used by many manufacturers works at wavelengths of less than 1000 nanometers (one micrometer). 850 nanometers and 940 nanometers are typical options. The problems here are twofold:
Human vision ranges from approximately 380 to 740 nanometers, and the retina can absorb energy at 850 nanometers and 940 nanometers. Therefore, systems operating at these frequencies present serious risks to human vision.
The sun emits much of its energy in the submicron range, so any measurement system that works in this range will suffer from overwhelming solar interference.
For these reasons, manufacturers want to move away from nearly visible wavelengths, going from 940 nanometers to areas of 1000 nanometers and beyond. The problem has been being able to reliably detect reflected signals at these higher wavelengths.
The new technology exploited by the Explore series of devices integrates GeSi as the light-absorbing material on a CMOS silicon wafer. This new methodology effectively extends the viable wavelength limit to 1550 nm. The company has combined GeSi methods with modulation frequencies of 300 MHz and above. The result is greater precision, improved performance in sunlight, and a reduced risk of eye damage.
Artilux has developed this technology in cooperation with TSMC. The first full-spectrum 3D ToF image sensors in the new Explore series, with multiple resolutions and ecosystem partners, will be announced in the first quarter of 2020.
What’s next in ToF technology?
Time-of-flight technology, while not new, is attracting a lot of interest lately, particularly from smartphone makers. Big improvements in facial recognition for high-end devices and availability further down the food chain seem to be in the offing.
There are many rumors, but few concrete specifications. Expect to see great partnerships between smartphone vendors and ToF camera and sensor chip producers in the very near future.
