The Short-Range, High-Accuracy Compact Pulsed Laser Ranging System

Hongbin Ma

Yuan Luo

Yan He

Shiguang Pan

Lihong Ren

Jianhua Shang

Submission received: 20 January 2022 / Revised: 2 March 2022 / Accepted: 8 March 2022 / Published: 10 March 2022

Abstract

A short-range, compact, real-time pulsed laser rangefinder is constructed based on pulsed time-of-flight (ToF) method. In order to reduce timing discrimination error and achieve high ranging accuracy, gray-value distance correction and temperature correction are proposed, and are realized with a field programmable gate array (FPGA) in a real-time application. The ranging performances—such as the maximum ranging distance, the range standard deviation, and the ranging accuracy—are theoretically calculated and experimentally studied. By means of these proposed correction methods, the verification experimental results show that the achievable effective ranging distance can be up to 8.08 m with a ranging accuracy of less than ±11 mm. The improved performance shows that the designed laser rangefinder can satisfy on-line ranging applications with high precision, fast ranging speed, small size, and low implementation cost, and thus has potential in the areas of robotics, manufacturing, and autonomous navigation.

1. Introduction

Since the first laser rangefinder with a ruby laser was developed in 1961, laser raging has experienced ongoing development and has since become a mature and well-known active optical distance measuring technology. With the cost reduction of system composition and promising potential technical advances, laser ranging technology is playing a growing role in various applications [1]. Atmospheric studies and surface topography mapping are still mainstream applications of laser ranging [2,3]. In view of the ability to sense surrounding objects in a real-time and high-performance way, laser ranging is bringing about significant changes in modern industry.

Nowadays, the rapid developments in autonomous driving, unmanned aerial vehicles, robotics, and automotive industries have put forward higher requirements for 3-D imaging obstacle avoidance systems [4]. For this purpose, using laser ranging techniques to obtain target distances in different azimuths by controlling the scanning angle of the spatial beam is a tried-and-true solution [5]. For example, the Defense Advanced Research Projects Agency (DARPA) of the U.S. launched ‘the Sweeper’, a small laser radar specially designed for autonomous driving. VLS-128 developed by the Velodyne Company is one of the latest products used for obstacle avoidance in driverless automation with a detection range of 300 m and a minimum angular resolution of 0.11° [6]. With regard to the obstacle avoidance system, there are mainly three parts—those are the beam scanning control, ranging, and imaging. Among them, laser ranging performances—including ranging accuracy, ranging efficiency, and ranging data upload rate—are particularly important [7,8].

The working principles of the laser rangefinder can be divided into time measuring and phase measuring. The type of existing laser rangefinders can be sorted into triangulation mode, pulsed laser mode, frequency-modulated continuous-wave (FMCW) mode, correlation mode, time-correlated single-photon counting (TCSPC) mode, dual-comb mode, and frequency comb interferometry, among others [5]. Each laser ranging method has its merits and shortcomings. The laser rangefinder based on FMCW is realized by a linear modulation of the frequency of a single mode laser and is able to provide the simultaneous measurement of distance and velocity according to the round trip time and the Doppler shift respectively [9,10,11]. By employing the coherent detection, the FMCW laser rangefinder has better ranging precision. However, there is a trade-off between the ranging precision and the cost and complexity of a system (the modulation, optical layout, signal processing, etc.). Related to the TCSPC laser rangefinder, it operates at the ultimate limit of detection sensitivity and can offer a large measurement range at the expense of greater time requirements for statistical analysis [12,13,14]. Meanwhile, the expensive single-photon detector used in the TCSPC laser rangefinder increases the system cost.

Generally speaking, the most common implementation of laser ranging is based on the time-of-flight (ToF). The extremely narrow laser pulse width, excellent laser pulse repetition rate, and high collimation accuracy mean it can measure distance at a higher speed even with a single measurement [15,16]. Most recently, the expanding applications of robotics, manufacturing, and other autonomous navigation technologies also promote higher requirements for short-range rangefinder with high ranging accuracy, high ranging efficiency, high ranging data upload rate, lower cost, and less power consumption [17,18]. The short-range pulsed ToF ranging method is one preferred solution. It presents the advantages of fast response time, high repetition frequency, no continuous modulation, simple system structure, and low power consumption [19]. Furthermore, with the help of a field-programmable gate array (FPGA), the system design will become more simple and cheaper. However, for the time discrimination process of the short-range pulsed ToF rangefinder, the optical detectors with gain, different reflectivity of different targets, and different distances for back reflection, and so on will give rise to timing discrimination errors of the echo laser pulse. As a result, the distance measurement will be inaccurate [20].

In recent years, the system configuration, photodetectors, signal processing method, etc. have been investigated aiming to improve the performance of short-range pulsed ToF laser range finder, and there are a great number of related research reports [21,22,23,24]. The interference from the ambient light is an important factor limiting the actual ranging performance. To reduce the interference from sky background noise and increase the SNR, Ahmed Hassebo et al. used simulations and experiments to optimize the receiver aperture shapes, locations, and sizes in the short range below 2 km [25]. In order to measure the same distance with a lower laser power, the light trapping structures of photodetectors have been studied and the photodetectors with a diameter of 30 μm and a quantum efficiency of more than 55% are developed for short ranges measurement [26]. The lateral propagation of input light into thin layers of silicon and germanium on silicon is improved; correspondingly, the interaction between the detected laser signal and the photodiode are enhanced. Nonetheless, the design of system hardware circuit and the processing of echo signal acquisition data are optimized [27,28]. The signal processing is a necessary and effective academic and technical method to explore better ranging performance.

As previously mentioned, measuring the time interval between transmitted laser pulse and the received laser pulse accurately is a significant guarantee to achieve accurate distance detection for the pulsed ToF laser rangefinder. There is no doubt that the timing discrimination error limits the ranging accuracy. For this reason, a differential hysteresis timing discrimination method is reported in detail [29]. Compared with the traditional leading-edge timing discrimination, this timing discriminator circuit that uses fewer components has better single-shot ranging accuracy. In addition, with the most frequently used detectors in the pulsed ToF ranging system, the avalanche photodiode detector (APD) working in the linear mode and the single-photon detector triggering on its first received photon, Silvano Donati and his co-authors analyze the influence of the timing error as well as the time-walk systematic error, and set up the evaluation method of ranging precision [30]. Their corresponding theory is meaningful for calculating the theoretical ranging precision of the distance measuring instruments, in particular the pulsed ToF laser rangefinder. For nonlinear time walk error, the related compensation method on the basis of double threshold correction is studied. Through setting two threshold points of the rising edge of echo signal pulse and establishing the relationship between these two points and the error, the walk error can be greatly reduced to 0.337 ns [31]. With respect to the improvement of dynamic ranging accuracy, dual-channel timing discrimination by combining constant-threshold timing discrimination method with peak timing discrimination method is proposed [32]. Consequently, the accurate echo timing discrimination is realized, and the improved ranging accuracy of short-range laser rangefinder is within ±30 mm.

In this paper, which aims to improve the ranging accuracy in short-range laser ranging, a compact real-time pulsed ToF laser rangefinder combined with the gray-value distance correction and temperature correction is developed. First, considering the ranging application requirements—such as ranging performances, size, weight, and cost—the laser ranging system design principle and implementation are introduced in detail. Second, in view of the variations of detected target reflectance, photodetector responsibility, and optical propagation properties, the intensity and saturation of echo laser pulse changes, and the rising and falling edges of the echo laser pulse drifts. This broadening of the echo laser pulse seriously restricts the ranging accuracy. Therefore, gray-value distance correction for the timing discrimination is proposed, and the experiments were carried out to verify this method. At a distance of 2355 mm, the results show that the improved distance difference is 16 mm, the standard deviation is up to 4.60 mm, and the ranging accuracy is within ±9 mm. Furthermore, the temperature correction is also adopted to optimize the ranging performances. The specific implementation process and the related experimental results are included. Third, taking a black foam plate with a reflectance of about 0.1 as the detected target, the real-time ranging verification experiments are conducted with the help of gray-value distance correction and temperature correction realized with FPGA. The maximum achievable distance reaches 8.08 m and the corresponding ranging accuracy can be done within ±11 mm. Finally, the conclusion is presented.

2. Measurement Principle

Pulsed ToF method calculates the distance by measuring the time interval between the transmitted laser pulse and the received echo pulse [33]. The calculation formula of detected target distance is

where S is the target distance, c is the speed of light in the air, and t is the round-trip flight time of laser pulse. For the actual pulsed laser rangefinder, t is calculated by recording the number of internal clock counters during the period from the transmission of optical pulse to the receiving of echo signal. As shown in Figure 1, if the internal clock frequency is f , the period is τ , and the count data from the time of laser emission to the echo signal entering the detector counter is n , then equation 1 can be modified as follows,

To sum up, the accuracy of time t is related to the internal clock frequency f (1/ τ ). The accuracy of t then affects the ranging accuracy (Δ S ). The ranging accuracy (Δ S ) can be calculated by,