Maximizing precision over extended unambiguous range for TOF range imaging systems

The maximum unambiguous range for time-of-flight range imaging systems is inversely proportional to the chosen modulation frequency. However, increasing the unambiguous range by decreasing the modulation frequency will generally also degrade the range measurement precision. We describe a technique that significantly extends the range of a time-of-flight imaging system without compromising range precision. This is achieved by employing two modulation frequencies simultaneously. The chosen frequencies can be a combination of high and low frequency, or two similarly high frequencies. In this paper we present experimental results comparing single frequency; dual high and low frequency; and dual high frequency operation and demonstrate that range precision need not be appreciably compromised to achieve an extended unambiguous range

Heterodyne range imaging in real-time

A versatile full-field range imaging system has previously been constructed. This system is configurable in software to produce either high precision or fast acquisition range images. Indicatively a 10 second exposure has been shown to produce a range image of sub-millimeter precision, whilst video frame rate (30 fps) acquisition provides for centimetre precision. Currently the acquisition time of the system is to a large degree constrained by the off-line processing of the frames by an external computer. This paper presents an alternative to the off-line PC image processing utilising an Altera Stratix II FPGA. Processing rates up to 30 frames per second have been achieved with the added advantage that many of the previous systempsilas existing digital electronics can also be accommodated, providing for an even more compact and flexible system

Development and characterisation of an easily configurable range imaging system

Range imaging is becoming a popular tool for many applications, with several commercial variants now available. These systems find numerous real world applications such as interactive gaming and the automotive industry. This paper describes the development of a range imaging system employing the PMD-19 k sensor from PMD technologies. One specific advantage of our system is that it is extremely customisable in terms of modulation patterns to act as a platform for further research into time-of-flight range imaging. Experimental results are presented giving an indication of the precision and accuracy of the system, and how modifying certain operating parameters can improve system performance

Multiple frequency range imaging to remove measurement ambiguity

Range imaging systems use a specialised sensor to capture an image where object distance (range) is measured for every pixel using time-of-flight. The scene is illuminated with an amplitude modulated light source, and the phase of the modulation envelope of the reflected light is measured to determine flight time, hence object distance for each pixel. As the modulation waveform is cyclic, an ambiguity problem exists if the phase shift exceeds 2π radians. To overcome this problem we demonstrate a method that superposes two different modulation frequencies within a single capture. This technique reduces the associated overhead compared with performing two sequential measurements, allowing the system to retain high range measurement precision at rapid acquisition rates. A method is also provided to avoid interference from aliased harmonics during sampling, which otherwise contaminate the resulting range measurement. Experimental results show the potential of the multiple frequency approach; producing high measurement precision while avoiding ambiguity. The results also demonstrate the limitation of this technique, where large errors can be introduced through a combination of a low signal to noise ratio and suboptimal selection of system parameters

Design and construction of a configurable full-field range imaging system for mobile robotic applications

Mobile robotic devices rely critically on extrospection sensors to determine the range to objects in the robot’s operating environment. This provides the robot with the ability both to navigate safely around obstacles and to map its environment and hence facilitate path planning and navigation. There is a requirement for a full-field range imaging system that can determine the range to any obstacle in a camera lens’ field of view accurately and in real-time. This paper details the development of a portable full-field ranging system whose bench-top version has demonstrated sub-millimetre precision. However, this precision required non-real-time acquisition rates and expensive hardware. By iterative replacement of components, a portable, modular and inexpensive version of this full-field ranger has been constructed, capable of real-time operation with some (user-defined) trade-off with precision

Video-rate or high-precision: A flexible range imaging camera

A range imaging camera produces an output similar to a digital photograph, but every pixel in the image contains distance information as well as intensity. This is useful for measuring the shape, size and location of objects in a scene, hence is well suited to certain machine vision applications. Previously we demonstrated a heterodyne range imaging system operating in a relatively high resolution (512-by-512) pixels and high precision (0.4 mm best case) configuration, but with a slow measurement rate (one every 10 s). Although this high precision range imaging is useful for some applications, the low acquisition speed is limiting in many situations. The system’s frame rate and length of acquisition is fully configurable in software, which means the measurement rate can be increased by compromising precision and image resolution. In this paper we demonstrate the flexibility of our range imaging system by showing examples of high precision ranging at slow acquisition speeds and video-rate ranging with reduced ranging precision and image resolution. We also show that the heterodyne approach and the use of more than four samples per beat cycle provides better linearity than the traditional homodyne quadrature detection approach. Finally, we comment on practical issues of frame rate and beat signal frequency selection

Efficient FPGA implementation of homodyne-based time-of-flight range imaging

Time-of-flight range imaging systems illuminate a scene with an amplitude-modulated light source, the light is reflected from objects in the scene, and measurement of the phase of the modulation envelope is performed to determine the object’s distance. As the image sensor is capable of performing this task for every pixel simultaneously, acquisition of the entire scene can be performed at rapid (video) update rates, making the technology ideal for real-time applications. In this paper we present an efficient real-time FPGA algorithm for determining phase, and hence distance, from the raw image sensor output. The algorithm has been implemented on a range imaging system based on the PMD19k-2 image sensor, with range processing performed in real time by a Stratix III FPGA. The scarcest resource in this implementation is RAM, and an analysis is presented to maximise the efficiency of this resource whilst maintaining acceptable processing accuracy. The algorithm can be extended for processing multiple simultaneous modulation frequencies. An efficient method for combining these results to determine unambiguous range, based on the Chinese remainder theorem, is also presented