Time-of-Flight Systems for Distance Measurement and Object Detection

Distance measurement and object detection play an important role in many fields, including factory automation, robotic applications, and logistics. In particular, in safety applications, objects or people at a certain distance need to be detected and responded to. For example, a robotic arm may need to stop operating immediately once a worker enters a danger zone.

Therefore, time of flight (ToF) is becoming increasingly important. With ToF technology, light is emitted from a modulated light source (such as a laser) and the light beam is captured by a sensor or camera after reflecting off one or more objects. Therefore, the distance can be determined by the time delay ∆T between the emission and reception of the emitted light. The time delay is proportional to twice the distance between the camera and the object (round trip). Therefore, the distance can be estimated as the depth d = (c × ∆t)/2, where c is the speed of light. ToF cameras output 2D data along with the required depth information.

ToF allows an entire image to be recorded at once. No line-by-line scanning is required, nor is relative motion between the sensor and the object being observed. ToF is often classified as LIDAR (Light Detection and Ranging), but it is actually a flash LIDAR-based approach, not scanning LIDAR.

There are basically two different approaches to measuring the time of flight of a light pulse using ToF: pulsed mode of operation based on charge coupled device (CCD) technology and continuous wave (CW) mode of operation.

In pulse mode, the time elapsed between the transmission and reception of a light pulse is measured, while in CW mode, the phase shift between the transmission and reception of a modulated light pulse is determined. Both modes of operation have their own advantages and disadvantages. Pulse mode is more tolerant to ambient light and is therefore more advantageous for outdoor applications, as the technology typically relies on high-energy light pulses emitted in a short time with a short integration window. CW mode may be easier to implement, as the light source does not have to be short and has rising/falling edges. However, if the accuracy requirements become more stringent, a higher frequency modulated signal will be required, which may be difficult to achieve.

The existing pixel size allows the chip to have a high resolution, which supports not only distance measurement but also object and gesture recognition. The measuring distance ranges from a few centimeters (<10 cm) to several meters (<15 m).

Unfortunately, not all objects are equally detectable. The object's condition, reflectivity and speed can affect the measurement.

Figure 1. Time-of-flight measurement principle

Figure 2. ToF system functional block diagram

Measurements can also be distorted by environmental factors such as fog or strong sunlight. Ambient light rejection helps resolve distortion issues caused by strong sunlight.

Semiconductor manufacturers such as ADI provide complete 3D ToF systems to support the rapid implementation of 3D ToF solutions. They integrate data processing, laser driving, power management, and software/firmware into an electronic control unit. Other components include a transmitter that emits a frequency-modulated light signal and a detector that records the reflected signal. The block diagram is shown in Figure 2.

Components such as analog front ends (AFEs) with integrated depth calculation capabilities will be very helpful in building such systems. The ADDI9036 provides this capability. It is a complete CCD ToF signal processor with an integrated laser diode driver, a 12-bit ADC, and a high-precision clock generator to generate timing for the CCD and laser. The ADDI9036 is responsible for processing the raw image data from the VGA CCD sensor to generate depth/pixel data.

ADI also works with design partners to jointly provide finished modules and development platforms. These evaluation systems can be used to develop specific customer algorithms. Finished modules and platforms help accelerate development, which is particularly important in time-sensitive business areas such as industrial and automotive engineering.

References

3D Imaging with ADI Time-of-Flight Technology. Analog Devices, Inc., 2020.

About the Author

Thomas Brand joined Analog Devices in Munich, Germany in 2015 while still completing his master's degree. After graduation, he joined Analog Devices as a trainee. In 2017, he became a field applications engineer. Thomas supports large industrial customers in Central Europe and specializes in the field of Industrial Ethernet. He graduated in electrical engineering at the University of Cooperative Education in Mosbach, Germany, and then obtained a master's degree in international sales at the University of Applied Sciences in Constance, Germany. Contact: thomas.brand@analog.com.