tracking systems and opens up new applications for indoor wireless positioning including robot tracking, patient monitoring, and tracking during computer.
Real-time UWB Indoor Positioning System with Millimeter 3-D Dynamic Accuracy Michael Kuhn*l, Cemin Zhang2, Mohamed Mahfouz l, and Aly E. Fathy2 IMech., Aero., and Biomed. Eng. Department, University of Tennessee, Knoxville, TN 2Electrical Eng. and Comp. Sci. Department, University of Tennessee, Knoxville, TN
Abstract A real-time UWB positioning system has been developed which has dynamic 3-D accuracy of better than 6 mm and static 3-D accuracy of 3-4 mm. This accuracy is an order of magnitude higher than currently available commercial UWB tracking systems and opens up new applications for indoor wireless positioning including robot tracking, patient monitoring, and tracking during computer assisted surgeries. 3-D experimental results are included for both dynamic and static real-time experiments using an optical tracking system with 3-D accuracy of 0.3 mm as a gold standard for reference. Introduction Real-time location systems (RTLS) have seen unparalleled growth in terms of their everyday use, whether it be for tracking military personnel, asset tracking and automated inventory control in industrial environments, or patient monitoring and tracking in a hospital. The required 3-D real-time accuracy is important in all these applications and varies depending on the application. For example, when traveling in a car, the dynamic 3-D accuracy provided by a Global Positioning System, which can vary from 1-2 m up to >20 m [1], is sufficient for car navigation. Conversely, higher accuracy, on the order of 10 cm, is required for indoor asset location in hospital and industrial environments. UWB location systems have certain advantages when used in high accuracy, indoor environments. Clark et al. tested five indoor positioning systems (signal strength, Wi-Fi, ultrasound, RF, and UWB) in multiple locations inside an operating room at a hospital and compared their performance in 3-D indoor localization [2]. The UWB positioning system, the Ubisense real-time location system [3], was the only system in the experiment to consistently achieve 3-D positioning accuracy of less than a meter. Our system achieves 3-D real-time accuracy in the 1-10 mm range, which is over an order of magnitude better than current commercial UWB location systems [3-4]. This paper is organized as follows: the system architecture is discussed including a novel sub-sampling mixer, a robust leadingedge detection algorithm implemented on an FPGA, and low phase noise crystals and LOs. Next, the experimental setup used in measuring the 3-D real-time dynamic and static accuracy of the system is outlined. This is followed by a presentation of the experimental results and a conclusion. System Architecture The transmitter and receiver architectures are outlined in Figs. 1 and 2. As shown in Fig. 1, a 300 ps Gaussian pulse, generated via a ±.5 ppm stable, 10 MHz Vectron crystal, is modulated by an 8 GHz carrier signal, which is generated from
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an Agilent source with low phase noise. The signal is then amplified and transmitted via an omni-directional monopole antenna. At the receiver, shown in Fig. 2, a single element Vivaldi antenna receives the signal, two stages of amplification then lead to a mixer where the signal is downconverted by a separate Agilent signal generator with low phase noise where LO I and L02 are offset in frequency by 130 MHz. The signal then goes through a low-pass filter and is sent through a sub-sampling mixer. More information on the system architecture is discussed in [5-6]. It passes through a 105 MSPS, 10 bit ADC, an FPGA where the leading-edge detection algorithm is run, and finally is sent to a computer. Four base stations are currently used to localize the 3-D position of the tag (a minimum of four are required when using TDOA for 3-D localization) where L02 and PRF2 are synchronized between all four base stations. Experimental Setup The experimental setup for the 3-D real-time experiment is shown in Fig. 3a. A CRS A465 robot moves both an UWB tag and a passive optical probe through 20 static points. This allows both the dynamic accuracy and the static accuracy of the system to be tested over a .6x.2x.15m3 view volume. Figure 3b shows the robot with the UWB tag and passive optical probe attached to its terminal joint. Four base stations are positioned around the robot in a configuration close to a tetrahedron, which provides the best geometric setup for localizing 3-D points in a view volume (reduces overall system geometric dilution of precision). Experimental Results The experimental results are shown in Fig. 4 where Fig. 4a shows the 3-D trajectory taken by the robot overlaying the optical and UWB data (which are synchronized at the CPU) and Fig. 4b shows root-mean-square error (RMSE) at a single static point where 200 samples of UWB and optical data are taken per point. Finally, Table 1 provides a summary of the experiment with final dynamic accuracy of 5.86 mm and static accuracy of 3.86 mm taken over the complete view volume of .6x.2x.15m3 of the robot. Conclusion A novel UWB positioning system has been presented. The system architecture has been described where a sub-sampling mixer, a novel leading-edge detection algorithm, low phase noise crystals and Las, and the use of an La offset all combine to create a UWB positioning system with mm-range static and dynamic 3-D real-time accuracy. This is a milestone in UWB and wireless positioning systems and opens up many new and exciting applications for the future. References [1] F. van Diggelen, C. Abraham, "Indoor GPS technology," in CTIA WirelessAgenda, Dallas, USA, 2001. [2] D. Clarke, A. Park, "Active-RFID system accuracy and its implications for clinical applications", IEEE Symp. on Computer-Based Med. Sys., Salt Lake City, USA, 2006.
[3] Hardware Datasheet, Ubisense, Cambridge, UK, 2007, http://www.ubisense.net/media/pdf/Ubisense%20System%200verview%20V I.I.pdf
[4] Sapphire DART (RTLS) Product Data Sheet, Multispectral Solutions Inc., Germantown, MD, 2008, http://www.multispectral.com/pdf/Sapphire DART.pdf. [5] M. Mahfouz, C. Zhang, B. Merkl, M. Kuhn, A. Fathy, "Investigation of high accuracy indoor 3-D positioning using UWB technology," IEEE Trans Microwave Theory & Tech, 56(6),2008, pp. 1316-1330. [6] C. Zhang, M. Kuhn, A. Fathy, M. Mahfouz, "Real-time noncoherent UWB positioning radar with millimeter range accuracy in a 3-D indoor environment," submitted to IEEE International Microwave Symposium, 2009. Monopole UWBAntenna
Figure 1 - Transmitter architecture showing 300 ps Gaussian pulse modulated by an 8 GHz carrier and transmitted via an omni-direction UWB monopole antenna. Vivaldi Receiving Antenna
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Figure 2 - Receiver architecture where the UWB signal is received via a single element Vivaldi antenna, sent through two stages of amplification, downconverted with a frequency offset of 130 MHz, low-pass-filtered, time extended via a sub-sampling mixer, sent through a 105 MSPS ADC, the leadingedge detection algorithm is run on an FPGA, and finally time-difference-ofarrival (TDOA) is performed on a computer to localize the 3-D tag position.
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(a) (b) Figure 3 - (a) Setup for the 3-D real-time experiment where a CRS A465 robot moves both a UWB tag and a passive optical probe (Polaris Spectra®, Northern Digital, Inc.) through 20 static points inside a .6x.2x.15m3 volume. Data is recorded in real-time via an in-house software interface on the cpu. (b) Picture outlining robot, UWB tag, and optical probe. -2900
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(a) (b) Figure 4 - (a) 3-D trace of optical and UWB data taken along the path traversed by the robot. Three lines were traversed and the robot stopped at 20 static points along these lines. (b) Root-mean-square error (RMSE) at each static point was obtained using 200 samples per point.
Table 1 - Summary of 3-D real-time static and dynamic experiments. Type Accuracy (mm) Samples View Volume Static
3.86
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5.88
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