Abstract:
This thesis documents the development of a time-of-flight (ToF) camera suitable
for autonomous mobile robotics applications. By measuring the round trip time of
emitted light to and from objects in the scene, the system is capable of
simultaneous full-field range imaging. This is achieved by projecting amplitude
modulated continuous wave (AMCW) light onto the scene, and recording the
reflection using an image sensor array with a high-speed shutter amplitude
modulated at the same frequency (of the order of tens of MHz). The effect is to
encode the phase delay of the reflected light as a change in pixel intensity, which
is then interpreted as distance.
A full field range imaging system has been constructed based on the PMD
Technologies PMD19k image sensor, where the high-speed shuttering mechanism
is builtin to the integrated circuit. This produces a system that is considerably
more compact and power efficient than previous iterations that employed an
image intensifier to provide sensor modulation. The new system has comparable
performance to commercially available systems in terms of distance measurement
precision and accuracy, but is much more flexible with regards to its operating
parameters. All of the operating parameters, including the image integration time,
sensor modulation phase offset and modulation frequency can be changed in realtime
either manually or automatically through software.
This highly configurable system serves as an excellent platform for research into
novel range imaging techniques. One promising technique is the utilisation of
measurements using multiple modulation frequencies in order to maximise
precision over an extended operating range. Each measurement gives an
independent estimate of the distance with limited range depending on the
modulation frequency. These are combined to give a measurement with extended
maximum range using a novel algorithm based on the New Chinese Remainder
Theorem. A theoretical model for the measurement precision and accuracy of the
new algorithm is presented and verified with experimental results.
All distance image processing is performed on a per-pixel basis in real-time using
a Field Programmable Gate Array (FPGA). An efficient hardware
implementation of the phase determination algorithm for calculating distance is
investigated. The limiting resource for such an implementation is random access
memory (RAM), and a detailed analysis of the trade-off between this resource and
measurement precision is also presented.
