Overview of a Smart Optical Time-of-Flight Sensor Technology
With billions of sensors already being deployed in all types of devices and applications, the sensor revolution is well underway. Through the Internet of Things (IoT) and the Internet of Everything (IoE), smart sensors have the potential to significantly help our societies resolve countless global challenges. In order to do so in a timely manner, major advancements are required in the development of ultra-efficient sensors.
Sensors are very closely linked to the IoT, as its core function is to collect valuable data. Fundamentally, what makes a smart sensor "smart" is its onboard signal/data-processing capabilities. Through the IoT, large amounts of quality, sensor-based data can be collected at any time and from anywhere, and it can then be transmitted over a network in real time. This provides enhanced awareness of our immediate or remote environment, bringing forth opportunities for faster and better decision-making, as well as gains in efficiency and productivity.
The new generation of ultra-efficient smart sensors requires key characteristics such as small size, low cost, low power consumption, low bandwidth consumption, and high reliability. And this is precisely what Leddar® technology has achieved.Based on almost a decade of relentless research, this unique solution breaks the mold for what's possible in detection and ranging applications, providing a smarter alternative to inefficient sensing solutions and setting the stage for large scale deployments.
Leddar Technology Overview
Leddar® (acronym for light-emitting diode detection and ranging) is a patented sensing technology developed by LeddarTech, a successful spin-off of Canada's leading optics and photonics research institute, Institut national d'optique (INO). The main innovation behind this new approach lies in the superior signal processing that drives every Leddar sensor. Combined with the use of visible or infrared LEDs to perform time-of-flight measurements, Leddar technology provides continuous, rapid and accurate detection and ranging, without any moving parts.
What Makes It Better?
Contrary to collimated emitters (lasers), the Leddar sensor's LEDs and emitter optics are used to create a diffuse beam covering a wider area of interest. The receiver collects the backscatter of the reflected light from objects in the beam and, using full-waveform analysis, detects the presence of objects in each segment of the beam, measuring the distance of the detected objects (based on the time taken by the light to return to the sensor). Accumulation and oversampling techniques are used to maximize range, accuracy and precision.
Leddar technology also allows for a high level of versatility, and a wide range of optics options are available for the sensor modules, providing a variety of beam patterns for different needs. For example, in response to current needs, the technology is presently being offered in the form of two main adaptable platforms: 1) a small and cost-effective single-element module, which provides a narrow, yet diffuse beam that is particularly suitable for applications like level sensing, proximity detection, etc., and a 2) multi-element module, which offers a much wider beam and provides lateral discrimination abilities, suitable for applications that might traditionally use laser scanners, or multiple sensors.
This unique sensing technology presents multiple advantages. The use of a diffuse light beam increases the detection robustness of specular surfaces. Another benefit is its high performance in harsh weather conditions such as rain or snow. Aligning the sensor is also easier, which results in fast and simple installation. Additionally, the multi-element receiver provides detection and ranging for multiple segments of the beam without the need to scan (no moving parts). This makes for a more compact, reliable and rugged assembly, all of which translate to an extended service life.
Leddar sensors use LEDs to generate very short light pulses, typically 100,000 pulses per second. The time-of-flight (ToF) principle essentially consists in measuring the time taken by a light pulse to travel from the sensor to a remote object and to return to the sensor. The range R of the detected object is deduced from the measured full round-trip time T of the light pulse using the simple relation R = c T / 2 n, where c is the speed of light in vacuum and n denotes the refractive index of the medium in which the light pulse propagates.
Depending on the characteristics of the target's surface, the light pulse is either absorbed, totally reflected, or reflected diffusely. This causes different irradiances of the echo pulse at the receiver, which are measured by the Leddar sensor. This measured irradiance depends on the distance measured by the ToF principle and the angle of incidence that can be determined by imaging-collecting optics that focus the reflected beam on the sensor’s photodetectors. A 16-element photodetector is typically used in Leddar® sensors (shown in Figure 1).
Beam Pattern for Multi-Element Option
The multiple-element photodetector has a rectangular sensing area. The purpose of the emission optics of a Leddar sensor is to direct as much of the emitted light from one or more LEDs into a pattern that best fits the photodetector geometry. The purpose of the reception optics is to collect the backscatter of light from objects in that beam onto the photodetector. The combined emission and reception optics solution can be designed to obtain different beam widths. Currently, optics options with beam widths of approximately 9°, 18°, 24°, 34°, 45° and 95° are available on the multi-element option (the single-element option comes with a narrow 3° beam). Figure 2 illustrates a simulated emission beam pattern of a Leddar® sensor with an overlay of the matching segments provided by the reception optics corresponding to the photodetector elements.
How Does It Work?
The LED source is pulsed at a rate of approximately 100,000 pulses per second. The light pulses propagate through the detection area and reflected light is captured by the optics and the photodetector. The sensor signal is amplified, and the signal acquisition is synchronized to the pulses.
An oversampling scheme using multiple light pulses is implemented to improve the resolution of the acquired signal. Typical oversampling values are 4 or 8 which produces a digitized signal with an increased number of samples for improved accuracy and precision. In addition to oversampling, an accumulation process is accomplished in order to improve the signal to noise ratio. The oversampling value and number of accumulations influence the detection/measurement, the range, accuracy and precision of the measurements. The performance of the sensor can thus be optimized with these parameters to meet the requirements of the application.
Detection and Distance Measurement
The detection and distance measurement is performed by the sensor's processor, using the acquired signals (one per photodetector element). The signals consist of a series of values representing light amplitude at incremental distances from the sensor. The number of samples in the signal is chosen according to maximum range required.
The amplitude of each sample is an indicator of the quantity of light reflected back from a given object at that specific distance. The amplitude depends on distance, size, reflectivity and angle of the object with respect to the sensor.
An object will be detected by the sensor if a light pulse above a predetermined threshold is found. The threshold at which a peak in the trace is interpreted as the presence of an object depends on the signal-to-noise ratio. LeddarTech determines the default threshold level for each sensor based on the signal-to-noise ratio. A threshold table is applied in the detection processing of the traces, and a threshold offset parameter is provided on most sensors in order to adjust this threshold table. The offset can be set to increase or decrease the sensitivity of the sensor. This can be used to ignore the presence of objects returning weak signals or to maximize detection of such objects and filter false detections in the application software.
Another setting available on Leddar sensors is the LED intensity. LED intensity control can be set to manual mode or automatic mode. In manual control mode, the setting is adjusted by the operator to best fit the application. In automatic control mode, the sensor will dynamically adjust the LED intensity based on the amplitude of the signal for objects detected in the sensor beam. With this control, a sensor model can be used for a wide range of applications with different range requirements and also be used in applications where objects can both be very close or far from the sensor.
Figure 3 illustrates a typical trace signal for one segment. In this example, the sensor is collecting light reflected back by two separate objects. Full waveform analysis performed by Leddar sensors provides the capacity to detect multiple objects in the same segment. This is possible if the closer object is smaller than the illuminated area for that segment. The beam can then illuminate another object that isn’t completely "shadowed" by the closer object.
To provide a complete view of the entire detection area, the multi-element sensor software generates a combined graphical representation of all segments, as shown in Figure 4 below. The wide beam contains multiple pulse signals like the one illustrated in Figure 3, creating several segments side by side and different detection zones. The pulsed light sent over such a wide area captures the entire signal to detect multiple objects, not only over a certain distance, but also laterally, enabling lateral positioning.
Benefits of Leddar Technology
Leddar sensors provide three key benefits compared to competing products: a high range-to-power ratio, target detection in low-visibility conditions and the ability to resolve multiple targets.
When compared to other detection technologies such as laser scanners, radar, video, thermal imaging, ultrasonic and passive infrared, Leddar excels on the widest range of performance criteria, due to its ruggedness, rapid data acquisition, 16 independent segments (multi-element platform) and simultaneous acquisition capabilities. Optimized for detection and ranging up to 100 m or more, Leddar can efficiently and cost-effectively serve multiple industries.
In addition, this technology is available in various module forms, with or without enclosure. As an opto-electronic technology, it can easily be adapted to almost any final application.
Of particular benefit to developers and integrators is the technology's unmatched cost/performance ratio, its ability to detect multiple objects in each segment and lateral discrimination (multi-element platform), its long detection range with low-power LEDs, real-time object-tracking capabilities, detection in adverse weather conditions, and ocular safety.
Integrating with Leddar Sensor Modules
At the heart of all Leddar sensors lies the LeddarCore with patented Leddar Light Processing, providing an ultra-low-power sensor core that can easily fit into a small footprint. Because of their compact size and flexible interfaces, Leddar sensing modules can be easily integrated into any system, allowing developers and integrators to use this advanced method in their own products.
The modules were designed to facilitate both mechanical and software integration. Namely, the included software development kit provides sample code for developers to quickly integrate sensor data into their application software. There is also a choice of popular communication interface options (e.g., USB, RS-485 and UART), and additional headers are also available for custom expansion. As for the receiver, one can choose from several beam options, ranging from 3° to 95°.
To encourage developers to see how well the technology works and how easily it is integrated, a low-cost evaluation kit is readily available for trial purposes.
This innovative core technology is giving rise to the creation of a completely new generation of ultra-efficient smart sensors, greatly changing the way detection and ranging capabilities are integrated into a wide range of industries and applications. Not only can it make these functions more accurate, more reliable and more robust, but it can also make the entire solution more cost-effective, which will be vital in the deployment of profitable high-volume applications. By providing these important characteristics, Leddar is poised to become a key enabler in the smart sensor revolution.
For more information on this patented* technology, visit leddartech.com.
Leddar® is a registered trademark of LeddarTech Inc. Leddar® technology is covered by one or more of the following U.S. patents: 7635854, 7640122, 7855376, 7895007, 7917320, 8159660, 8242476, 8310655, 8319949, 8436748 and 8619241 or international equivalents. Other patents pending.