With the maturity of LiDAR (Light Detection and Ranging) technology, the price of products has become relatively more reasonable, and it has been widely used in fields related to navigation, range-finding, collision detection and 3D mapping. This article will show you the product features and applications of SiPM Direct Time of Flight (dToF) LiDAR introduced by onsemi.
Traditional depth sensing technology has lower precision and shorter distance
Today, many applications in the market require accurate depth sensing measurement, including industrial, consumer, automotive, and other relevant products. There are many different methods for depth sensing, including stereo triangulation, phase detection pixels and structured light measurement using standard CMOS image sensors.
The method of stereo triangulation is to triangulate the received light from two different cameras to obtain distance detection. By comparing the position differences of the objects between the images taken by the cameras, the distance between the cameras and the objects can be calculated. The advantage of this method is that it adopts a passive method and standard image sensor, while the disadvantage is that it needs two cameras. The maximum detection distance depends on the distance between cameras, and it is highly dependent on the lighting conditions, and it needs the cost of computational. It is suitable for low-cost depth cameras and indoor short-distance applications.
If you want to use a single camera to get the distance from the points of the scene, you must use the phase-detection pixel technology. The image sensor can calculate the phase difference of the received light by pairs of pixels with light shields at different pixel level positions or using multiple photodiodes under the same microlens to calculate the depth. This method has the advantages of the passive method and standard image sensor, but the disadvantages are poor depth resolution, high dependence on lighting conditions, computational cost and short measurement distance, which is suitable for auto-focusing of smartphones.
Another way of structured light is to use a camera with a traditional CMOS image sensor to analyze the received infrared light pattern, and use distortion to calculate the depth in the scene. The distorted pattern can be used to obtain the 3D shape of the object. Its advantage is that it is suitable for short-distance measurement, but its disadvantage is that it is use active method and sensitive to ambient light. The depth error will increase with the distance, which is not suitable for long distance and is mainly used for face recognition.
LiDAR technology provides high precision and long distance
The LiDAR technology introduced in this article can work under all illumination conditions because of its high depth and angular resolution and active method using an infrared light transmitter and receiver so that it can provide depth sensing superior to the above alternative methods. LiDAR is widely deployed in many different markets, and is suitable for various applications and use cases, including augmented reality and virtual reality (AR/VR) applications in automobile, industry, robot and consumer.
Generally, LiDAR adopts the direct time of flight (dToF) measurement technology, which calculates the time delay between the transmitted signal and its echo(es). The other method is the indirect time of flight (iToF), both of which can use a pulse or continuous modulation.
iToF integrates many light pulses in a limited number of bins, usually two, by using the modulation of image sensor pixels, and the timing of the return pulses is determined by measuring the charges in different bins. iToF is suitable for short-distance depth sensing applications and is used indoor and in environments where the sensor is not exposed to direct sunlight.
dToF is suitable for short-range and long-range depth sensing applications. It provides a faster acquisition rate and the ability to measure multiple echoes and detects multiple objects in the return path. This method can be realized by a single measurement or multiple measurements accumulated for each reading.
High-sensitivity sensors are very important to the performance of the LiDAR system. Typical sensors used in dToF LiDAR system include linear mode detectors such as PIN diode and avalanche photodiodes (APD). These legacy detectors are being rapidly replaced by high-performance sensors, including silicon photomultipliers (SiPM), SiPM array and single-photon avalanche photodiode (SPAD) array. These sensors are manufactured by the CMOS process and can provide tight uniformity between part−to−part, low voltage operation and very high gain characteristics, which is quite ideal for mass production of LiDAR with low cost and high performance.
SiPM dToF LiDAR provides accurate range-finding with low power
In the Internet of Things (IoT), automotive and industrial applications, there are more and more range-finding and sensing applications that are expected to benefit from low power consumption and high performance SiPM technology. Especially LiDAR applications using near-infrared (NIR) wavelengths that are safe for human eyes, such as automotive ADAS (Advanced Driver Assistance Systems), 3D depth maps, mobile, consumer and industrial range-finding.
In order to take advantage of the high gain and bandwidth of the SiPM sensor, dToF technology can be used to provide accurate range-finding with the lowest power budget. When the photons return, the signal chains of the LiDAR system can digitize the detected laser echo(es) using an analog-to-digital converters (ADC) or time-to-digital converters (TDC). The ADC-based system allows full pulse digitization, which provides additional information about the target, such as reflectivity inferred from the pulse shape. However, the TDC-based method has cost and power advantages because the discrimination circuit is relatively simple to implement, and this method is compatible with narrow pulse width lasers. This means that a higher peak power can be used for each pulse without affecting the eye safety limits.
Cost-optimized industrial and commercial single-point range finder development kit
SECO-RANGEFINDER-GEVK introduced by onsemi is a SiPM dToF LiDAR platform, which is a complete single-point range finding development kit for cost-optimized industrial and commercial applications. This kit is based on the latest NIR SiPM (RB series) from onsemi. It integrates all the essential subsystems required for the application, including laser and reference circuit (Tx), receiving circuit (Rx), power management system and core FPGA and UART communication.
SiPM is a single photosensitive, high performance, solid-state sensor, which is composed of closely−packed SPAD sensors and the summed array of integrated quench resistors, so that it has high gain (~1x106), high detection efficiency (> 50%) and fast timing (sub-nanosecond rise time). The kit has been certified by FDA, and includes a multifunctional GUI, which can comprehensively evaluate the range finder performance and adjust the system variables, such as the buffer number of pulses or the bias voltage of SiPM-RB photomultiplier.
SECO-RANGEFINDER-GEVK supports detectable range larger than 0.11 m to 23 m, has an out-of-the-box operation, user-friendly GUI and adjustable system variables, can optimize system cost, has built-in TDC, is based on FPGA(ice3), has a bin width of about 85ps, can perform automatic TDC calibration (FPGA reference clock), supports different power supply options including USB(5V) and PMOD connector (3.3V), and adopts RB series SiPM detector. This is a 905nm laser diode transmitter with BK7 plano-convex lens coated with 650-1050nm, which can maximize the range finder. 905±5nm band pass optical filter (FWHM: 30±5nm) for RX can obtain the highest sensitivity in the selected spectrum, and meets the laser safety standards IEC/EN 60825-1:2014, 21 CFR 1040.10 and 1040.11, except for the deviations pursuant to Laser Notice No.56, the high sensitivity of onsemi SiPM allows the use of low-power lasers to improve eye safety.
SECO-RANGEFINDER-GEVK provides software adjustable settings suitable for various industrial and IoT applications and FPGA-based TDC, readout, communication interface and control of two regulated bias supplies. onsemi has created a software model that can accurately determine the system performance under various input conditions. A hardware range-finding platform with SiPM sensors can help developers get familiar with the underlying technology and all necessary building blocks.
In addition, this kit helps to evaluate the core components of the system, such as SiPM sensors. Through the test points provided on the hardware and various functions and configurable parameters on the GUI, developers will be able to evaluate the kit in a quick and user-friendly manner and accelerate the application speed. In addition, the kit also supports an extensible system with Bluetooth® development kit (BDK-GEVK) and other sensors and actuators, which can be applied to indoor navigation and range-finding, collision detection, 3D mapping and other fields.
Conclusion
SiPM dToF LiDAR features high gain and bandwidth, providing accurate range-finding with the lowest power budget, and meeting the range-finding needs of industrial and commercial applications. onsemi's SiPM dToF LiDAR development platform provides a complete combination of software and hardware, which can speed up the development of products and is worthy of deep understanding and adoption by manufacturers who intend to invest in range-finding applications.