Engineer-to-engineer: understanding time-of-flight sensing

Revolutionizing industries: Unveiling the game-changing applications of direct time-of-flight (dToF) sensors in the industrial, mobile, and consumer markets

In our blog post, Latest optical sensing technology lights the way ahead for new robot designs we gave an overview of advanced 3D sensing solutions, multi-channel spectral sensors and projection lighting systems enable robots to ‘see’ the world around them, and use light to communicate with people. Now, let us take an engineer’s perspective and drill down into considerations and tools for evaluating multi-zone direct time-of-flight sensing.

High accuracy and scalable multi-zone 1D-dToF proximity sensing is often selected for its design flexibility, highest redundancy and most competitive pricing. This enables advanced obstacle detection and environment awareness for a range of up to 5 meters, while simple determination of first (closest) and second (next closest) objects require less compute-intense detection algorithms. 

For example, the compact TMF882x from ams OSRAM offers complete multi-zone sensing functionality in a single modular device for applications like laser detect auto-focus (LDAF), home service robots, user presence detection, contactless switching and gesture recognition.


White paper: Understanding time-of-flight sensing

As a leading supplier of ranging sensors which enable the camera auto-focus function in smartphones, ams OSRAM has built up deep expertise in the implementation of time-of-flight sensing that it is transferring to other industrial applications like robotics based on significant market interest.

In the new White Paper, Understanding time-of-flight sensing, I explore in detail how accurate ranging sensors in small, surface-mount packages are spreading from mobile to industrial and consumer applications. Written from an engineer’s perspective, the topics include:

  • The types of ToF sensor and their mode of operation
  • The sweet spot for direct time-of-flight sensor applications
  • Using a dToF sensor in the application: the role of the sensor module algorithm histogram
  • Using a dToF sensor in the application: 

o    optical configuration
o    optical design considerations

  • How to start developing a dToF sensor-based ranging system

You can register here to download your copy of the white paper.


Supporting design engineers to overcome the challenges of their optical designs

The principle of operation of a dToF sensor is simple. However, implementing this principle in practice entails complex hardware and software components, to ensure that the sensor maintains accurate performance even in adverse operating conditions.

The internal VCSEL (vertical cavity surface-emitting laser) emits multiple pulses of 940nm infrared light. Reflections from each pulse can be individually detected by the internal high-sensitivity SPAD (single proton avalanche diode) detector. The time measured by the time-to-digital converter from each received pulse is plotted in a histogram which provides a robust method for the sensor module’s algorithm to determine the degree of confidence which applies to each object detection event.

Importantly, in a single-zone dToF sensor, the closest object detected within the fixed field of view will produce peaks in the histogram. In comparison, the multi-zone dToF sensors from ams OSRAM gives the user the flexibility to adjust the field of view to the requirements of the application.

Furthermore, as an optical device, a dToF sensor requires careful consideration of the properties of the cover glass which protects the sensor from damage. The considerations which apply to the cover window include: shape, angle relative to the sensor, thickness and air gap, IR transmissivity, contamination and calibration.

While the cover window’s primary purpose is physical protection of the sensor, it also has an optical effect on the beams of light emitted and received by the sensor. The first principle determining the design of the cover window should be to distort or attenuate the light path to the least possible extent to maintain the highest possible range and accuracy.


How to start developing a dToF sensor-based ranging system

After reading the white paper to gain understanding of the basic principles of a dToF sensor and the important considerations, first-time users learn best by active evaluation of a dToF sensor and its integration into a prototype design.

The design engineer is substantially supported in this effort by the wide range of products tools and resources available from ams OSRAM. This includes multiple evaluation kits that provide a ready-made environment for evaluating different configurations of the sensor settings, plus different optical assemblies. There are lots more guidelines and details in the white paper.

In addition, as a quick-start guide, I’ve produced the first of a series of engineer-to-engineer videos where – in under 10 minutes – I walk design engineers through using TMF882X-SHIELD boards on the Arduino® UNO form factor development platforms for quick evaluation of the TMF8820, TMF8821 & TMF8828 multi-zone dToF sensors. Featuring a small (20mm x 12mm) sensor breakaway board, this kit can be easily integrated into custom, prototype hardware. Cover glass samples and air gap spacers are provided enabling evaluation of the system for optimal optical performance. Watch the video here.

Learn more about the ams OSRAM dToF sensors and short-range proximity sensors.