David K. Wehe

Mobile robot positioning: Sensors and techniques

Abstract : Exact knowledge of the position of a vehicle is a fundamental problem in mobile robot applications. In the search for a solution, researchers and engineers have developed a variety of systems, sensors, and techniques for mobile robot positioning. This paper provides a review of relevant mobile robot positioning technologies. The paper defines seven categories for positioning systems: (1) Odometry, (2) Inertial Navigation, (3) Magnetic Compasses, (4) Active Beacons, (5) Global Positioning Systems, (6) Landmark Navigation, and (7) Model Matching. The characteristics of each category are discussed and examples of existing technologies are given for each category. The field of mobile robot navigation is active and vibrant, with more great systems and ideas being developed continuously. For this reason the examples presented in this paper serve only to represent their respective categories; they do not represent a judgment by the authors. Many ingenious approaches can be found in the literature, although, for reasons of brevity, not all could be cited in this paper. The appendix contains a tabular comparison of the positioning systems discussed in this review that includes system and description, features, accuracy (position), accuracy (orientation), effective range, and source of information. (47 refs.)

ICP registration using invariant features

Investigates the use of Euclidean invariant features in a generalization of iterative closest point (ICP) registration of range images. Pointwise correspondences are chosen as the closest point with respect to a weighted linear combination of positional and feature distances. It is shown that, under ideal noise-free conditions, correspondences formed using this distance function are correct more often than correspondences formed using the positional distance alone. In addition, monotonic convergence to at least a local minimum is shown to hold for this method. When noise is present, a method that automatically sets the optimal relative contribution of features and positions is described. This method trades off the error in feature values due to noise against the error in positions due to misalignment. Experimental results suggest that using invariant features decreases the probability of being trapped in a local minimum and may be an effective solution for difficult range image registration problems where the scene is very small compared to the model.

1-D POSITION SENSITIVE SINGLE CARRIER SEMICONDUCTOR DETECTORS

Abstract A single polarity charge sensing method has been studied using coplanar electrodes on 5 mm cubes of CdZnTe γ-ray detectors. This method can ameliorate the hole trapping problem of room-temperature semiconductor detectors. Our experimental results confirm that the energy resolution is dramatically improved compared with that obtained using the conventional readout method, but is still about an order of magnitude worse than the theoretical limit. A method to obtain the γ-ray interaction depth between the cathode and the anode is presented here. This technique could be used to correct for the electron trapping as a function of distance from the coplanar electrodes. Experimental results showed that a position resolution of about 0.9 mm FWHM at 122 keV can be obtained. These results will be of interest in the design of higher performance room-temperature semiconductor γ-ray detectors.

Multiview registration of 3D scenes by minimizing error between coordinate frames

This paper addresses the problem of large-scale multiview registration of range images captured from unknown viewing directions. To reduce the computational burden, we separate the local problem of pairwise registration on neighboring views from the global problem of distribution of accumulated errors. We define the global problem as an optimization over the graph of neighboring views, and we show how the graph can be decomposed into a set of cycles such that the optimal transformation parameters for each cycle can be solved in closed form. We then describe an iterative procedure that can be used to integrate the solutions for the set of cycles across the graph of views. This method for error distribution does not require point correspondences between views, and can be used to integrate any method of pairwise registration or robot odometry.

POSITION-SENSITIVE SINGLE CARRIER CDZNTE DETECTORS

Abstract Single polarity charge sensing on room temperature semiconductor gamma-ray detectors can be achieved by using the coplanar electrode read-out technique. This mehod can eliminate the hole-trapping problem of the wide band gap semiconductors which are currently available. Our previous results on 5 mm cube CZT detectors confirmed [6] that the energy resolution can be dramatically improved compared with that obtained using the conventional read-out method. This paper explores the application of this technique to CdZnTe detectors of larger volume, namely 1 cm 3 . In our previous work, we suggested a method to obtain γ-ray interaction depth and further progress is reported here. This technique can be used to correct for the electron trapping as a function of distance from the anode. The intrinsic position resolution has been analyzed and energy resolutions of less than 2% FWHM at 662 keV were obtained on both detectors tested. Finally, the factors which inhibit attaining the statistical energy resolution limit of CdZnTe detectors have been explored. These results will be of interest in the design of higher performance, portable and imaging-related, room-temperature semiconductor γ-ray detectors.

Evaluation of a Compton scattering camera using 3-D position sensitive CdZnTe detectors

Abstract A CZT Compton Camera (CCC) is being built using two three-dimensional (3-D) position-sensitive CZT detectors. Expected system performance was analyzed by analytical and Monte Carlo approaches. Based on the measurement of detector energy and position resolution, the expected angular resolution is ∼3° and ∼2° for a ±30° FOV for 511 keV and 1 MeV γ -rays, respectively. The intrinsic efficiency for a point source 10 cm from the first detector surface ranges from 1.5×10 −4 to 8.8×10 −6 for 500 keV–3 MeV.

Temperature dependence of CsI(Tl) gamma-ray excited scintillation characteristics

The gamma-ray excited, temperature dependent scintillation characteristics of CsI(Tl) are reported over the temperature range of −100 to + 50°C. The modified Bollinger-Thomas and shaped square wave methods were used to measure the rise and decay times. Emission spectra were measured using a monochromator and corrected for monochromator and photocathode spectral efficiencies. The shaped square wave method was also used to determine the scintillation yield as was a current mode method. The thermoluminescence emissions of CsI(Tl) were measured using the same current mode method. At room temperature, CsI(Tl) was found to have two primary decay components with decay time constants of τ1 = 679±10 ns (63.7%) and τ2 = 3.34±0.14 μs (36.1%), and to have emission bands at about 400 and 560 nm. The τ1 luminescent state was observed to be populated by an exponential process with a resulting rise time constant of 19.6±1.9 ns at room temperature. An ultra-fast decay component with a < 0.5 ns decay time was found to emit about 0.2% (about 100 photons/MeV) of the total scintillation light. Except for the ultra-fast decay time, the rise and decay time constants were observed to increase exponentially with inverse temperature. At −80°C τ1 and τ2 were determined to be 2.22±0.33 μs and 18.0±2.59 μs, respectively, while the 400 nm emission band was not observed below −50°C. At +50°C the decay constants were found to be 628 ns (70.5%) and 2.63 μs (29.3%) and both emission bands were present. The scintillation yield of CsI(Tl) was observed to be only slightly temperature dependent between −30 and +50°C, peaking at about −30°C (about 6% above the room temperature yield) and monotonically decreasing above and below this temperature. Four different commercially available CsI(Tl) crystals were used. Minimal variations in the measured scintillation characteristics were observed among these four crystals. Thermoluminescence emissions were observed to have peak yields at −90, −65, −40, +20, and possibly −55°C. The relative magnitudes and number of thermoluminescence peaks were found to vary from crystal to crystal.

Single charge carrier type sensing with a parallel strip pseudo-Frisch-grid CdZnTe semiconductor radiation detector

Wide band gap compound semiconducting materials offer great promise for use as room temperature operated radiation detectors. The most common semiconductor radiation detector design incorporates the use of a semiconducting block of material with metal contacts applied at opposite ends of the block. A voltage is applied across the block, which produces an internal electric field capable of drifting free charge carriers to the detector contacts. Gamma ray interactions occurring in the device excite electron-hole pairs which are separated by the applied electric field across the device bulk. Electrons are drifted towards the anode, and holes are drifted towards the cathode. The induced charge produced by the moving free charge carriers can be measured by an external circuit. Shockley 1 and Ramo 2 derived the dependence of the induced current and induced charge produced by point charges moving in an electric field, which was later shown to apply to semiconductor detectors as well. 3‐5 The Shockley‐Ramo theorem states that the induced charge that appears at the terminals of a planar device from moving point charges is proportional to the distance displaced by the moving point charges, regardless of the presence of space charge. Hence, the change in induced charge Q* can be represented by

C-SPRINT: a prototype Compton camera system for low energy gamma ray imaging

An electronically-collimated imaging system is being built using pixellated, low-noise, position-sensitive silicon as the first detector, and a sodium iodide scintillation detector ring as the second detector. The system consists of a single 3/spl times/3/spl times/0.1 cm/sup 3/ silicon pad detector module with 1 keV FWHM (noise-limited) energy resolution centered at the front face of a 50 cm diameter, 10 cm long NaI detector annulus. Custom acquisition and timing electronics have been manufactured to minimize system dead time. Monte Carlo modeling is used to predict system sensitivity and position resolution. Simulations using the existing setup show angular uncertainties of 4.1/spl deg/ and 2.1/spl deg/ FWHM for /sup 99m/Tc and /sup 131/I point sources, respectively (7.2 mm and 3.7 mm at 10 cm). Sensitivity can be improved by more than a factor of a hundred over the existing setup by stacking five 1 mm thick 9/spl times/9 cm/sup 2/ silicon arrays and redesigning the second detector geometry to accept a wider range of scattering angles. Lower bound calculations show that our electronically-collimated camera system challenges current mechanically-collimated systems for both /sup 99m/Tc and /sup 131/I despite the deleterious effects of Doppler broadening. Preliminary measurements show a timing resolution of 41 ns FWHM between the silicon detector and a single SPRINT module.

COPLANAR GRID PATTERNS AND THEIR EFFECT ON ENERGY RESOLUTION OF CDZNTE DETECTORS

Abstract This paper describes diagnostic techniques using depth and radial position-sensing methods that have been applied to identify and remove the non-symmetric effect of coplanar grid electrodes. Our experimental results show that the non-symmetric effect can degrade significantly the energy resolution of single polarity charge sensing CdZnTe detectors and can be minimized by balancing the weighting potentials of coplanar anodes. The coplanar electrode design has been modified based on the knowledge gained from this study and improvements in detector performance have been achieved.