Full field image ranger hardware

We describe the hardware designed to implement a full field heterodyning imaging system. Comprising three key components - a light source, high speed shutter and a signal generator - the system is expected to be capable of simultaneous range measurements to millimetre precision over the entire field of view. Current modulated laser diodes provide the required illumination, with a bandwidth of 100 MHz and peak output power exceeding 600 mW. The high speed shutter action is performed by gating the cathode of an image intensifier, driven by a 50 Vpp waveform with 3.5 ns rise and fall times. A direct digital synthesiser, with multiple synchronised channels, provides high stability between its outputs, 160 MHz bandwidth and tuning of 0.1 Hz

Range imager performance comparison in homodyne and heterodyne operating modes

Range imaging cameras measure depth simultaneously for every pixel in a given field of view. In most implementations the basic operating principles are the same. A scene is illuminated with an intensity modulated light source and the reflected signal is sampled using a gain-modulated imager. Previously we presented a unique heterodyne range imaging system that employed a bulky and power hungry image intensifier as the high speed gain-modulation mechanism. In this paper we present a new range imager using an internally modulated image sensor that is designed to operate in heterodyne mode, but can also operate in homodyne mode. We discuss homodyne and heterodyne range imaging, and the merits of the various types of hardware used to implement these systems. Following this we describe in detail the hardware and firmware components of our new ranger. We experimentally compare the two operating modes and demonstrate that heterodyne operation is less sensitive to some of the limitations suffered in homodyne mode, resulting in better linearity and ranging precision characteristics. We conclude by showing various qualitative examples that demonstrate the system’s three-dimensional measurement performance

A high resolution full-field range imaging system for robotic devices

There has been considerable effort by many researchers to develop a high resolution full-field range imaging system. Traditionally these systems rely on a homodyne technique that modulates the illumination source and shutter speed at some high frequency. These systems tend to suffer from the need to be calibrated to account for changing ambient light conditions and generally cannot provide better than single centimeter range resolution, and even then over a range of only a few meters. We present a system, tested to proof-of-concept stage that is being developed for use on a range of mobile robots. The system has the potential for real-time, sub millimeter range resolution, with minimal power and space requirements

Development of Extrospective Systems for Mobile Robotic Vehicles.

Extrospection is the process of receiving knowledge of the outside world through the senses. On robotic platforms this is primarily focussed on determining distances to objects of interest and is achieved through the use of ranging sensors. Any hardware implemented on mobile robotic platforms, including sensors, must ideally be small in size and weight, have good power efficiency, be self-contained and interface easily with the existing platform hardware. The development of stable, expandable and interchangeable mobile robot based sensing systems is crucial to the establishment of platforms on which complex robotic research can be conducted and evaluated in real world situations. This thesis details the design and development of two extrospective systems for incorporation in the Victoria University of Wellington's fleet of mobile robotic platforms. The first system is a generic intelligent sensor network. Fundamental to this system has been the development of network architecture and protocols that provide a stable scheme for connecting a large number of sensors to a mobile robotic platform with little or no dependence on the existing hardware configuration of the platform. A prototype sensor network comprising fourteen infrared position sensitive detectors providing a short to medium distance ranging system (0.2 - 3 m) with a 360' field of view has been successfully developed and tested. The second system is a redesign of an existing prototype full-field image ranger system. The redesign has yielded a smaller, mobile version of the prototype system capable of ranging medium to long distances (0 - 15 m) with a 22.2' - 16.5' field-of-view. This ranger system can now be incorporated onto mobile robotic platforms for further research into the capabilities of full-field image ranging as a form of extrospection on a mobile platform

The Waikato range imager

We are developing a high precision simultaneous full-field acquisition range imager. This device measures range with sub millimetre precision in range simultaneously over a full-field view of the scene. Laser diodes are used to illuminate the scene with amplitude modulation with a frequency of 10MHz up to 100 MHz. The received light is interrupted by a high speed shutter operating in a heterodyne configuration thus producing a low-frequency signal which is sampled with a digital camera. By detecting the phase of the signal at each pixel the range to the scene is determined. We show 3D reconstructions of some viewed objects to demonstrate the capabilities of the ranger

Image intensifier characterization

An image intensifier forms an integral part of a full-field image range finder under development at the University of Waikato. Operating as a high speed shutter with repetition rates up to 100 MHz, a method is described to characterise the response, both temporally and spatially, of the intensifier in order to correct for variations in the field of view and to optimise the operating conditions. A short pulse of visible light is emitted by a laser diode, uniformly illuminating the image intensifier, while a CCD camera captures the output from the intensifier. The phase of the laser pulse is continuously varied using a heterodyne configuration, automatically producing a set of samples covering the modulation cycle. The results show some anomalies in the response of our system and some simple solutions are proposed to correct for these

A high-resolution full-field range imaging system

There exist a number of applications where the range to all objects in a field of view needs to be obtained. Specific examples include obstacle avoidance for autonomous mobile robots, process automation in assembly factories, surface profiling for shape analysis, and surveying. Ranging systems can be typically characterized as being either laser scanning systems where a laser point is sequentially scanned over a scene or a full-field acquisition where the range to every point in the image is simultaneously obtained. The former offers advantages in terms of range resolution, while the latter tend to be faster and involve no moving parts. We present a system for determining the range to any object within a camera's field of view, at the speed of a full-field system and the range resolution of some point laser scans. Initial results obtained have a centimeter range resolution for a 10 second acquisition time. Modifications to the existing system are discussed that should provide faster results with submillimeter resolution

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

Shape and deformation measurement using heterodyne range imaging technology

Range imaging is emerging as a promising alternative technology for applications that require non-contact visual inspection of object deformation and shape. Previously, we presented a solid-state full-field heterodyne range imaging device capable of capturing three-dimensional images with sub-millimetre range resolution. Using a heterodyne indirect time-of-flight configuration, this system simultaneously measures distance (and intensity), for each pixel in a cameras field of view. In this paper we briefly describe our range imaging system, and its principle of operation. By performing measurements on several metal objects, we demonstrate the potential capabilities of this technology for surface profiling and deformation measurement. In addition to verifying system performance, the reported examples highlight some important system limitations. With these in mind we subsequently discuss the further developments required to enable the use of this device as a robust and practical tool in non-destructive testing and measurement applications

Heterodyne range imaging as an alternative to photogrammetry

Solid-state full-field range imaging technology, capable of determining the distance to objects in a scene simultaneously for every pixel in an image, has recently achieved sub-millimeter distance measurement precision. With this level of precision, it is becoming practical to use this technology for high precision three-dimensional metrology applications. Compared to photogrammetry, range imaging has the advantages of requiring only one viewing angle, a relatively short measurement time, and simplistic fast data processing. In this paper we fist review the range imaging technology, then describe an experiment comparing both photogrammetric and range imaging measurements of a calibration block with attached retro-reflective targets. The results show that the range imaging approach exhibits errors of approximately 0.5 mm in-plane and almost 5 mm out-of-plane; however, these errors appear to be mostly systematic. We then proceed to examine the physical nature and characteristics of the image ranging technology and discuss the possible causes of these systematic errors. Also discussed is the potential for further system characterization and calibration to compensate for the range determination and other errors, which could possibly lead to three-dimensional measurement precision approaching that of photogrammetry