3D Time-of-flight distance measurement with custom - Universität ...

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OPTICAL TOF RANGE MEASUREMENT 19 Additionally, a large variety of intelligent CW modulation techniques is available. It is worth mentioning pseudo-random modulation and chirping (continuous frequency modulation) [XU]. Pseudo-random modulation, where pseudo noise words (PN modulation) are continuously repeated, offers the advantage of a very high peak in the autocorrelation function. This technique, originally developed for communication technology, is therefore very noise-robust [SC2]. A combination of both CW modulation and pulsed modulation would be ideal, combining their specific advantages of (1) better optical signal-to-background ratio than available from pure CW-modulation, (2) increased eye safety due to a low mean value of optical power and (3) larger freedom in choosing modulation signals and components. Combination in this content means, for example, a “pulsed sine” operation, i.e. a sequence of 1 ms high power sine modulation followed by 9 ms of optical dead time. This dead time can be used for example for post processing tasks. II. COMPONENTS In the following, the necessary components for TOF systems will be described and associated with the corresponding modulation technique. Modulated light sources and electrooptical modulators The emitted light can be modulated in several ways. The use of LEDs or lasers allows direct modulation of the light source by controlling the electrical current. Since the light sent does not have to be coherent or monochromatic, other light sources are possible, in combination with additional large aperture optical modulators such as Kerr cells [HEC], Pockels cells [HEC, SC3], mechanical shutters or liquid crystal shutters. Pulsed operation with fast rise and fall times is only possible with directly controlled laser sources that allow pulses of less than ten femtoseconds [SUT]. The different light sources and modulators can be characterized in terms of light intensity, cut-off frequency, linearity, wavelength, modulation depth (the ratio of signal amplitude to signal offset), eye safety properties, size and price. LEDs are relatively inexpensive. They can be modulated up to some 100 MHz with a 100% modulation depth and a very high linearity. They are available for a wide
20 CHAPTER 2 range of wavelengths from blue (400 nm) to the near infrared (1200 nm) with an optical power of up to several milliwatts. Lasers, even laser diodes, are much more expensive than LEDs and are often larger. However, they offer more optical power and are suited for operation up to some GHz, also at a wide range of wavelengths [SAL]. Practically, lasers and laser diodes are the only light sources suitable for pulsed TOF operation. All other light sources introduced here are actually used for CW modulated light, due to the reduced bandwidth requirements. Kerr cells are based on the quadratic electro-optic effect, where the polarization of a polarized beam is rotated depending on the applied voltage. Together with a polarizer and a modulated control voltage of the cell, a polarized incoming beam can be modulated in intensity. Kerr cells can be used as modulators up to 10 GHz but voltages as high as 30 kV must be switched at this speed [HEC]. Pockels cells, making use of the linear electro-optic effect (pockels effect), work very similarly and also require polarized light for operation. Their cut-off frequency of more than 25 GHz is even higher than that of Kerr cells. The driving voltage requirements are a factor of 10 lower than for an equivalent Kerr cell. However, the Pockels cell still requires some kilovolts to switch from transparent to opaque mode. Therefore, the practical cut-off frequency, which is limited by the cell’s capacitance, reaches only several hundred MHz [HEC, SAL, XU]. Liquid crystal shutters are limited to some kHz of modulation frequency and are therefore not suited for high-resolution measurement applications in the 10 meter range [SAL]. An interesting alternative, however, might be the realization of a mechanical shutter. Miniaturized air turbines with a rotation speed of 427’000 rpm have been reported [SLO]. If one manages to rotate a light grating disc with such an air turbine, shutter frequencies as high as 1 MHz might become possible. For example, a disc of 10cm diameter and a slot width of 1 mm at the edge of the disc would contain about 150 slots. Such a disc would offer a 1x1 mm 2 shutter aperture while allowing 1 MHz shutter frequency (=150*427000(1/min)/60(s/min)). Detectors and Demodulation The easiest and most straightforward way of realizing a TOF-receiver is to use any fast and sensitive electrooptical sensor as a detector. The time of flight can then be determined as follows: a linear analog ramp (or a fast digital counter) is started synchronously with the transmission of a laser pulse. Once the laser pulse reaches the detector the rise of the ramp is stopped. The resulting amplitude is then
Page 1 and 2: 3D Time-of-flight distance measurem
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Page 10 and 11: Abstract Since we are living in a t
Page 12: centimeter accuracy. With the singl
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Page 18 and 19: INTRODUCTION 1 1. Introduction One
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70 CHAPTER 3 Flicker noise is cause
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72 CHAPTER 3 A s = sf ⋅ q C ⎡ V
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74 CHAPTER 3 amount of the charge p
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76 CHAPTER 3 substrate (Equation 3.
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78 CHAPTER 3 charge carriers the CT
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80 CHAPTER 3 without causing short
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82 CHAPTER 3 Over an address decode
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84 CHAPTER 3 Orbit’s 2.0µm CMOS/
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86 CHAPTER 4 Figure 4.1 P lens Lamb
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88 CHAPTER 4 Required optical power
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90 CHAPTER 4 4.2 Noise limitation o
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92 CHAPTER 4 number of pseudo-backg
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94 CHAPTER 4 Cmod = 1 QE(λ) = 0.65
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96 CHAPTER 4 Range accuracy (std) i
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DEMODULATION PIXELS IN CMOS/CCD 99
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IMAGING TOF RANGE CAMERAS 151 6. Im
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IMAGING TOF RANGE CAMERAS 153 contr
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IMAGING TOF RANGE CAMERAS 155 6.1.2
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IMAGING TOF RANGE CAMERAS 157 gate
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IMAGING TOF RANGE CAMERAS 159 6.2 2
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IMAGING TOF RANGE CAMERAS 161 n+ di
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IMAGING TOF RANGE CAMERAS 163 pixel
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IMAGING TOF RANGE CAMERAS 165 Power
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IMAGING TOF RANGE CAMERAS 167 Delay
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IMAGING TOF RANGE CAMERAS 169 6.3 3
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IMAGING TOF RANGE CAMERAS 171 Gate
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IMAGING TOF RANGE CAMERAS 173 Bound
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IMAGING TOF RANGE CAMERAS 175 Figur
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IMAGING TOF RANGE CAMERAS 177 Final
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IMAGING TOF RANGE CAMERAS 179 #5 #6
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SUMMARY AND PERSPECTIVE 181 7. Summ
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SUMMARY AND PERSPECTIVE 183 into th
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SUMMARY AND PERSPECTIVE 185 or phas
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188 CHAPTER 8 8.2 Typical parameter
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190 CHAPTER 8 Spectral luminous int
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192 CHAPTER 8 MCD03S: Conditions SC
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194 CHAPTER 8 MCD06S: Measurement c
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196 [BRK] Brockhaus, “Naturwissen
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198 [KAI] I. Kaisto, et al., “Opt
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200 [SP1] T. Spirig et al., “The
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ACKNOWLEDGMENTS 203 Acknowledgments
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206 Publication list 1. R. Lange, P
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