Time-of-flight (ToF) sensors measure the linear distance to a targeted object. Although they can be implemented in a variety of ways, they all depend on one unalterable 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. The time is measured for the trip there and back, but we’re only concerned with the one-way trip so it’s divided in half. We now have velocity as the speed of light and time. Since distance is equal to velocity multiplied by time, distance can be readily 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 out as a 50% duty cycle square-wave pulse and the amount of light reflected back is measured over a given time period.
Of course, this is only a brief overview of the technology. To most effectively implement these sensors into designs, it’s a good idea to learn more about time of flight technology.
Time of Flight Sensing Today
ToF technology is available in many different guises. There are chips that contain the actual optical transmitters and receivers on board, and others that work with external cameras.
Because some of the applications are mobile and some sit in place, power consumption may or may not be a critical factor.
Facial recognition will require far more resolution than will an application that serves to indicate if a person is erroneously (or otherwise!) occupying an exclusion zone. A machine vision manufacturing application might require formidable resolution but will only concern itself with what’s happening a centimeter or two away.
Let’s look at how companies are using ToF sensing in their products.
TI's ToF Sensor and Controller
Here’s how the sensor and controller divide the workload.
The OPT8241 time-of-flight (ToF) sensor offers quarter video graphics array (320 x 240) resolution at data frame rates up to 150 frames per second (600 readouts per second). The programmability of the timing generator offers the capability to optimize for various depth-sensing performance metrics including device power consumption, signal-to-noise ratio, and ambient cancellation. The unit is available in an Optimized Optical Package (COG-78), 8.757 mm × 7.859 mm × 0.7 mm.
The OPT9221 time-of-flight controller (TFC) is a companion device for the OPT8241 3D ToF sensor. Its job is to compute the depth data from the raw digitized sensor data. Working with the OPT8241, the unit gleans information to be used to implement filters and masks and to dynamically control the overall system configuration for the intended performance. This device comes in a 256-Pin, 9-mm × 9-mm 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 their literature, TI suggests three likely applications for the duo:
- Presence Detection for Industrial Safety. Is the object in a danger zone a human, a robot or perhaps a vehicle?
- People Counting and Locating
- People Locating and Identification
Espros' 3D ToF Imager Chip
The EPC660 from Espros is a 3D-TOF imager chip that works with the company’s DME 660 distance measurement camera. It is a system-on-chip (SOC) camera system that communicates via 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 resolve target distances with resolution in the millimeter range even if they are up to 100 meters away. Up to 1000 TOF images can be taken each second.
The EPC660 will find use in applications including:
- People detection and counting
- Machine safety
- Car collision avoidance system
- Pedestrian detection and breaking systems
- Gesture control
- Body size measurement
- Man-Machine interface
ToF Sensor from AMS
The TMF8701 time-of-flight sensor from AMS comes in a tiny 2.2mm x 3.6mm x 1.0 mm package. Its operating wavelength is 940 nanometer. It is a VCSEL (Vertical Cavity Surface Emitting Laser) device, a technology that enjoys many advantages when compared horizontally oriented devices.
Figure 3. TMF8701 ToF sensor. Image courtesy of AMS.
The 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 choices. The problems here are twofold:
Human vision extends from about 380 to 740 nanometers, and the retina can absorb energy at 850 nanometers and 940 nanometers. Thus, systems operating at these frequencies present grave risks to human vision
The sun emits much of its energy at the sub-micrometer range, so any measurement system operating at this range will suffer from overpowering solar interference.
For these reasons, manufacturers want to move away from the near-visible wavelengths, way past 940 nanometers into the areas of 1000 nanometers and more. The problem has been to be 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 absorption material on a CMOS silicon wafer. This new methodology effectively extends the workable wavelength limit up to as far as 1550nm. The company has combined GeSi methods with modulation frequencies of 300 MHz and higher. The result is higher accuracy, improved performance in sunlight and a reduction of eye damage risk.
Artilux has developed this technology in cooperation with TSMC. The first wide spectrum 3D ToF image sensors of the new Explore Series, with multiple resolutions and ecosystem partners, will be announced in Q1 2020.
What's Next in ToF Technology?
Time of flight technology, while not new, is attracting much interest of late, particularly from smartphone manufacturers. Big improvements to 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 hard specifications. Expect to see major alliances between the smartphone vendors and producers of both ToF cameras and sensor chips in the very near future.