William R. Michalson

Design of a digital gauss meter for precision magnetic field measurements

The design of a precision gauss meter for measuring magnetic fields is presented. The instrument utilizes a Hall element probe as a transducer and features an analog front end capable of amplifying dc and ac signals. A 16-bit analog-to-digital converter interfaces the analog section to a microcontroller-based digital section which corrects nonlinearities in the Hall element probe as well as gain errors in the amplifiers. The specifications called for 0.25% dc accuracy and an ac bandwidth of 10 kHz. SPICE models were used to determine the frequency response, noise characteristics, and temperature effects of the Hall element probe and the analog section. Finally, the analog section and the Hall element probe were calibrated separately and configuration information was created for storage in EEPROM memory.

Distributed Digital Radios and WLAN Interoperability

In addition to the speaker and the microphone, the radio handset contains the baseband electronics for a digital radio. This includes an Ethernet jack, a full network protocol stack, an OFDM baseband modem and a user interface to place calls: one-to-one (private line) or one-to-many (talk groups). The handset plugs into an ordinary land mobile radio that is used only for up conversion to RF and power amplification. No modification to the radio is needed. Using existing radios means the existing infrastructure and existing channel frequencies are also used. This is a significant cost savings to achieve interoperability, and it immediately puts the technology into the marketplace. A distributed digital radio (DDR) is a 6-way radio: voice/RF, voice/Ethernet and Ethernet/RF. In computer network terminology, a DDR is a hybrid router - hybrid is used because the PHY layer at each of the ports is different: audio, RF or Ethernet. A public safety radio network with DDRs becomes a wireless local area network (WLAN). The WLAN is not WiFi; it is a client mesh with an Ethernet backplane. The DDR provides 2G communications and cost effective IP-based interoperability to all first responders - even to the local and rural public safety agencies with conventional analog radio networks. This paper discusses WLAN interoperability architectures, and it traces the development of the DDR.

A Novel Multi-Probe Resistivity Approach to Inspect Green- State Metal Powder Compacts

This paper describes a new instrumentation approach to the nondestructive testing of green-state powdered metallurgy components. These samples are likely to generate surface-breaking and subsurface defects prior to sintering. Exploiting the principles of electric resistivity or potential drop measurements in solids, a system is configured which is capable of recording surface voltage distributions due to impressed current inputs. At the heart of this novel testing procedure is a multiple-pin sensor which allows for flexible measurement conditions in order to cover wide surface areas. Practical tests with production samples compare well with both analytical and numerical modeling techniques in predicting surface voltage distributions. Furthermore, initial studies of surface-breaking flaws exhibit excellent defect detectability.

Theoretical Data on Support of a Unified Indoor Geolocation Channel Model

In this paper we present theoretical data in support of the unified indoor geolocation channel model namely (1) path loss and (2) multipath distribution models. First, the path loss model is currently accepted to be a function of the transmitter and receiver geometry and frequency of operation. Second, the most widely used and accepted indoor channel multipath distribution models are Nakagami with m degrees of freedom, Rayleigh, Rician, and lognormal. The purpose of this paper is two fold: (1) to provide a better interpretation of the sets of theoretical data for the indoor channel model and (2) to be able to explain the lack of fit of the well-known multipath distribution models from the previous measurement data sets reported in the literature; thus, providing support for the unified indoor channel model theory. The unified path loss model consists of an approach for linking together the path loss models of the three geolocation systems (macrocellular, microcellular, and indoor) with the distance between the transmitter and receiver, R, and the frequency of operation, f. The path loss caused by increase of the transmitter receiver distance is much more severe than the path loss caused by the path loss caused by increase of the frequency of operation. The bottom line here is that we need to design future receivers or propose a signal structure that will account for 40 to 80 dB of signal degradation indoors. The unified multipath distribution consists of a linear transformation of the well-known multipath distribution models such as Nakagami with m degrees of freedom, Rayleigh, Rician, and lognormal. While it is rather straight forward to prove the unified geolocation multipath distribution model when only the contributing individual distributions are Rayleigh, Rician, and lognormal, if we assume that we have a fourth distribution such as Nakagami with m degrees then the process is not straight forward any more. We will investigate this and report the results in the future. The main purpose of the unified multipath distribution model is to enable the calculations of reflections’ gain. Assuming that the channel is composed of individual distributions such as Rayleigh, Rician, and Lognormal we have perform reflection gain calculations. From the theoretical data it appears that reflections with gain 3dB or higher than the LOS gain are on the order of 1 out of 6.2 days. On the other hand, reflections with gain 5dB greater than the LOS gain are of the order of 1 out of ~6072 years. And this is the most important conclusion of this work that for simulation or implementation purposes we should never consider reflections with gains greater than equal to 5 dB greater than the LOS gain.

A new apparatus for non-destructive evaluation of green-state powder metal compacts using the electrical-resistivity method

This paper presents a new apparatus developed for non-destructive evaluation (NDE) of green-state powder metal compacts. A green-state compact is an intermediate step in the powder metallurgy (PM) manufacturing process, which is produced when a metal powder-lubricant mixture is compacted in a press. This compact is subsequently sintered in a furnace to produce the finished product. Non-destructive material testing is most cost effective in the green state because early flaw detection permits early intervention in the manufacturing cycle and thus avoids scrapping large numbers of parts. Unfortunately, traditional NDE methods have largely been unsuccessful when applied to green-state PM compacts. A new instrumentation approach has been developed, whereby direct currents are injected into the green-state compact and an array of spring-loaded needle contacts records the voltage distributions on the surface. The voltage distribution is processed to identify potentially dangerous surface and sub-surface flaws. This paper presents the custom-designed hardware and software developed for current injection, voltage acquisition, pre-amplification and flaw detection. In addition, the testing algorithm and measurement results are discussed. The success of flaw detection using the apparatus is established by using controlled samples, which are PM compacts with dielectric inclusions inserted.

Effects of Push-To-Talk (PTT) delays on CSMA based capacity limited Land Mobile Radio (LMR) networks

Land Mobile Radio (LMR) networks have been traditionally used for transmitting voice signals. Systems like Facsimile, Radio Teletype and Distributed Digital Radios (DDR) use a conventional analog FM radio to transmit data [1], [2]. Digital Radios like Packet Radio and P-25, use complex modulation schemes like Continuous Frequency Shift keying (CFSK), Continuous Phase Modulation (CPM) to transmit voice/data information [3],[4]. The Medium Access Layer (MAC) implemented on such data networks ensures fair access to the half duplex FM channel. For voice transmission the users can themselves act as a default contention resolution mechanism. For data, some variations of channel access mechanisms like Time Division Multiple Access (TDMA) / Frequency Division Multiple Access (FDMA) / Carrier Sense Multiple Access (CSMA) are used [5]. The delays due to the Radio Hardware can significantly influence the throughput of such networks. This paper describes the effects of Push-To-Talk (PTT) delays associated with Land mobile Radios (LMR) for a CSMA based MAC layer. The results show that the PTT delays along with the number of nodes in a network can affect the throughput of a CSMA based capacity limited LMR network.

Accurate synchronization using a full duplex DSSS channel

In the future, geolocation systems will incorporate the characteristics of communications networks with positioning technologies to create systems capable of performing location aware computing. Such systems have wide applications in troop movements, field hospitals and homeland defense, as well as dual-use areas such as firefighter safety and wireless healthcare systems. Of particular interest are systems which may be deployed in an ad hoc manner. By their very nature, these systems cannot make use of pre-existing infrastructure everything necessary to create a functional system must be self-contained, allowing a complete system to be deployed anywhere, at any time. In many of these applications, there are sub-meter accuracy requirements for indoor positioning. For example, it Is important to differentiate which side of a wall a firefighter is on. To achieve this level of accuracy, work is ongoing which exploits the properties of Orthogonal Frequency Division Multiplexing (OFDM) signals and signal coding to mitigate errors associated with multipath interference. However, even if multipath is eliminated, the ability of a system to provide accurate positioning is also critically related to the ability of the transmitters and receivers in the system to establish a reference time. Current simulations indicate that local synchronization must be within few hundred picoseconds. This paper describes a synchronization mechanism, which employs a duplex Direct Sequence Spread Spectrum (DSSS) signal on a single carrier frequency. Recent results indicate that a Maximum Likelihood Estimator (MLE)-based receiver for DSSS signals yields a significant improvement in receiver dynamic range. By using this increased headroom, and knowledge of the signal being transmitted by a node, it becomes possible to extract other DSSS signals on the same carrier frequency. Once a duplex channel is established, local synchronization can be established to an arbitrary accuracy using coding techniques.

Development of a low-cost, self-contained, combined vision and inertial navigation system

In the field of autonomous vehicle research, maintaining accurate estimates of a vehicles velocities, position, and orientation is crucial to successful navigation. Normally, this information can be obtained through the use of a GPS system. However, GPS signals are not always available in some of the more exotic or dangerous locations that a vehicle might be dispatched to. This paper proposes a hybrid navigation system for autonomous vehicles that uses inertial sensors and a stereo camera system to accurately determine the velocities, position, and orientation of a vehicle. Such a system would be ideal for vehicles that have to operate in underground, indoor, extraterrestrial, or other GPS-denied environments.

Error Mechanisms in Indoor Positioning Systems without Support from GNSS

There exist various applications for indoor positioning, amongst which indoor positioning and tracking in urban environments has gained significant attention. Some user communities, like fire fighters, ideally require indoor accuracy of less than one metre, with accuracies of less than six metres acceptable by some other user communities. Achieving this level of accuracy requires a detailed profiling of error sources so that they can be better understood so that, in turn, indoor positioning accuracy in the presence of these errors can be further improved. Some well known error sources like multipath, NLOS (non line of sight), oscillator drift, dilution of precision and others have been studied and can be found in the literature. A less well known error source that can substantially affect indoor positioning accuracy are the effects of the dielectric properties of building materials on propagation delay. Various RF and non-RF based prototypes that claim to be suitable for indoor positioning can be found in the literature. Most of the existing literature discusses algorithms and summarizes the positioning results that were achieved during field tests using a prototype system or, more commonly, simulations. Little of this existing literature provides a breakdown of the total navigation system errors observed with the objective of analyzing the contribution of each error source independently. The paper will first provide a brief overview of the precision personnel locator system developed at the Worcester Polytechnic Institute. The field tests and observed indoor positioning results using this RF prototype will then be summarized and used to provide a baseline to establish a system error budget. The total observed error will be broken down and a detailed analysis of each of the error sources will be presented based on actual measured data in a variety of indoor environments. This leads to a better understanding of how each error source affects indoor positioning accuracy. Each of the error sources can then be independently optimized to minimize the observed errors. Specifically, the interplay between the dielectric properties and multipath profiles will be highlighted. This paper will conclude by presenting an error budget which can be used as a practical lower bound when designing precise indoor positioning systems.

Error mechanisms in an RF-based indoor positioning system

An RF-based indoor precise positioning system being developed for fire fighters is discussed in this paper. The paper will discuss system level overview of the RF prototype and will discuss the NLOS field tests and indoor positioning results using this RF prototype. The position estimation algorithm used is based on Time Difference of Arrival (TDOA) for a multicarrier signal. An error budget for such an RF-based indoor positioning system is presented in this paper with more insight to the sources of errors. Multipath and NLOS conditions indoors are well known error sources but there also exists another not so well known but major sources of error which are due to dielectric properties of the building materials. Basic simulations are presented to better understand the effect of the dielectric properties of building materials on indoor position estimates.