Author: Thomas brand, senior field application engineer, ADI company

Distance measurement and target detection play an important role in many fields, including factory automation, robot application and logistics. Especially in the field of security applications, it is necessary to detect and respond to objects or people at a specific distance. For example, once a worker enters a hazardous area, the manipulator may need to stop operation immediately.

Therefore, time of flight (TOF) becomes more and more important. Using TOF technology, light is emitted from a modulated light source (such as a laser). The light beam is reflected by one or more objects and captured by a sensor or camera. Therefore, by the time delay between transmitting light and receiving transmitted light? 6? 2T to determine the distance. The time delay is proportional to twice the distance (round trip) between the camera and the object. Therefore, the distance can be estimated as depth d = (c) × Δ t) / 2, where C is the speed of light. The TOF camera outputs 2D data and the required depth information.

TOF allows the entire image to be recorded at once. There is no need for progressive scanning or relative motion between the sensor and the observed object. TOF is usually classified as lidar (light detection and ranging), but it is actually a method based on flash lidar rather than scanning lidar.

There are basically two different methods to measure the flight time of optical pulse by TOF: pulse operation mode based on charge coupled device (CCD) technology and continuous wave (CW) operation mode.

The elapsed time between the transmission and reception of the optical pulse is measured in the pulse mode, and the phase shift between the transmission and reception of the modulated optical pulse is measured in the CW mode. Both modes of operation have their own advantages and disadvantages. Pulse mode is more resistant to ambient light, so it is more conducive to outdoor applications, because the technology usually relies on high-energy light pulses emitted by short integrated windows in a very short time. CW mode may be easier to implement because the light source does not have to be short and has rising / falling edges. However, if the accuracy requirements become more stringent, higher frequency modulation signals will be required, which may be difficult to achieve.

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

Unfortunately, not all objects can be detected equally. The condition, reflectivity and velocity of the object will affect the measurement results.

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Figure 1. Principle of time of flight measurement

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Figure 2. Functional block diagram of TOF system

The measurement results may also be distorted by environmental factors such as fog or strong sunlight. Ambient light suppression helps solve the distortion problem 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 drive, power management and software / firmware into an electronic control unit. Other components include a transmitter that transmits FM optical signals and a detector that records reflected signals. The block diagram is shown in Figure 2.

Components such as analog front end (AFE) with integrated depth computing function will be very helpful to build such a system. Addi9036 provides this function. It is a complete CCD TOF signal processor with integrated laser diode driver, 12 bit ADC and high-precision clock generator for generating timing for CCD and laser. Addi9036 is responsible for processing the original image data from VGA CCD sensor to generate depth / pixel data.

ADI also works with design partners to 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 critical business areas such as industry and automotive engineering.

reference material

3D imaging is realized by using ADI time of flight technology. ADI, 2020.

Introduction to the author

Thomas brand joined ADI in Munich, Germany in 2015 when he was still studying for a master’s degree. After graduation, he participated in the trainee program of ADI company. In 2017, he became a field application engineer. Thomas supports large industrial customers in Central Europe and focuses on the field of industrial Ethernet. He graduated from the Joint Education University in Mosbach, Germany, majoring in electrical engineering, and then obtained a master’s degree in international sales from the University of Applied Sciences in Konstanz, Germany.

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