About

Professor Todd A. DeLong is an Assistant Professor in the Department of Electrical and Computer Engineering at the University of Virginia's School of Engineering and Applied Science. He began serving in this role in the Fall of 2015. Prior to his current position, he served at Virginia Commonwealth University and the University of Georgia. His research background includes work as a researcher for the Center for Safety-Critical Systems within UVA's School of Engineering and Applied Science, where he focused on the design and analysis of safety-critical systems used in the transportation (rail) industry. He is a member of the American Society for Engineering Education (ASEE) and a Senior member of the Institute of Electrical and Electronics Engineers (IEEE).

Research topics

• Computer Science
• Algorithm
• Mathematics
• Operating system

Selected publications

• An Analysis of the ATCS Generator Polynomial

  2022 Annual Reliability and Maintainability Symposium (RAMS) · 2021

  1st authorCorresponding
  • Computer Science
  • Computer Science
  • Algorithm

  SUMMARY & CONCL USIONSThe Advanced Train Control Systems (ATCS) specification provides a generator polynomial for calculating a vital Cyclic Redundancy Checksum (CRC) for computer-based communication and control. Unfortunately, very little if any published information exists concerning its design and performance. This paper presents an analysis of a code that uses the generator polynomial, providing what the authors believe is the first evidence of the minimum Hamming distance, thus supporting the use of the generator polynomial to provide vital serial communication in safety-critical railway systems.

  PublisherDOI

• Assessment of Comprehension Retention in a Modern Electrical and Computer Engineering Curriculum

  2018-05-10 · 1 citations

  articleOpen accessSenior author

  His teaching responsibilities have typically been in the area of digital systems, embedded computing, and computer design

  PublisherOA PDFDOI

• ection Technique for ehavioral-Level Models

  2014-12-04

  article1st authorCorresponding

  recognized the importance of incorporating fault tolerance into microelectronic designs. However, they often performed this task late in the process when the design was near completion. As computer systems become more complex, designers must consider fault tolerance throughout the design process to allow early estimation of reliabil-ity and fault coverage. Designers usually perform dependability parameter estimation (DPE) at a high level of abstraction, using stochastic Petri net or queuing models. However, as specifications become more demanding, designers must go to increasingly lower levels of modeled

  Publisher

• A Quantitative Safety Assessment Methodology for Safety-Critical Programmable Electronic Systems Using Fault Injection

  SAE International journal of passenger cars. Electronic and electrical systems · 2009-04-20

  article

  <div class="htmlview paragraph">Given the increased use of programmable embedded electronic systems (PEES) in automotive applications and their vital importance, it is not only important for engineers to design PEES in such a way to meet or exceed safety requirements but also quantify how “safe” these systems are. At the University of Virginia's Center for Safety-Critical Systems, we have developed a safety quantification methodology for embedded real time safety-related systems. The goal of the safety quantification methodology is to provide a generic but rigorous and systematic way of characterizing the dependability behavior of embedded systems that is applicable to a broad range of applications from automotive to nuclear. This paper presents a quantitative safety assessment methodology for safety-critical embedded systems using fault injection (FI). This methodology has been developed, refined and applied to a number of commercial safety-grade systems in the railway, nuclear and avionics industries. Additionally, we present several novel techniques that we developed to overcome long-standing challenges associated with fault injection based safety assessment.</div>

  PublisherDOI

• Dependability Metrics to Assess Safety-Critical Systems

  IEEE Transactions on Reliability · 2005-09-01 · 51 citations

  article1st authorCorresponding

  Metrics are commonly used in engineering as measures of the performance of a system for a given attribute. For instance, in the assessment of fault tolerant systems, metrics such as the reliability, R(t) and the Mean Time To Failure (MTTF) are well-accepted as a means to quantify the fault tolerant attributes of a system with an associated failure rate, /spl lambda/. Unfortunately, there does not seem to be a consensus on comparable metrics to use in the assessment of safety-critical systems. The objective of this paper is to develop two metrics that can be used in the assessment of safety-critical systems, the steady-state safety, S/sub ss/, and the Mean Time To Unsafe Failure (MTTUF). S/sub ss/ represents the evaluation of the safety as a function of time, in the limiting case as time approaches infinity. The MTTUF represents the average or mean time that a system will operate safely before a failure that produces an unsafe system state. A 3-state Markov model is used to model a safety-critical system with the transition rates computed as a function of the system coverage C/sub sys/, and the hazard rate /spl lambda/(t). Also, /spl lambda/(t) is defined by the Weibull distribution, primarily because it allows one to easily represent the scenarios where the failure rate is increasing, decreasing, and constant. The results of the paper demonstrate that conservative estimates for lower bounds for both S/sub ss/ & the MTTUF result when C/sub sys/ is assumed to be a constant regardless of the behavior of /spl lambda/(t). The derived results are then used to evaluate three example systems.

  PublisherDOI

• New Design Approaches for Embedded Safety Critical Systems: Algorithm Based Safety Assurance

  Infotech@Aerospace · 2005-06-15

  article

  Instead of relying on functional testing, fault injection and process inspection methods to validate and verify critical systems, as is done today, we propose a far-reaching and novel approach to this problem. The approach we propose is based on processing or computation that is carried out in entirely in a coded information space. The main benefit of this approach is to intelligently decouple or constrain the aspects of the COTS constituent components (e.g. processors, operating systems) from the application algorithm integrity. Remember, in control systems, SCADA and other embedded systems the application algorithms by design and subsequent refinement define the safety policies of the overall system. By decouple, we explicitly mean detecting the adverse effects the COTS components may have on the functional integrity and safety of the application algorithm. In this paper we present approaches under develop at the University of Virginia that are based on deriving either correctness conditions, or safety from the application algorithm. These conditions are then used to check the safety and the integrity of the computation in real-time.

  PublisherDOI

• VHDL-based distributed fault simulation using SAVANT

  2002-11-27 · 3 citations

  article

  There is a need for simulator-independent, VHDL-based fault simulation. Existing techniques for VHDL-based fault simulation are reviewed. Robust, a simulator-independent fault simulator tool, is described. Slow simulation speed is identified as one limitation of the current Robust tool and distributed simulation on a network of workstations is identified as a feasible way to improve its performance. Previous network-of-workstations fault simulation experiments are reviewed. Current efforts to enhance the Robust tool using SAVANT ate described. A system using Robust (with SAVANT extensions) for fault simulation on a network of workstations is proposed, using the TyVIS VHDL simulation kernel and the Legion distributed processing system.

  PublisherDOI

• An algorithm based fault tolerance technique for safety-critical applications

  2002-11-22 · 5 citations

  article

  The design of safety critical systems is a difficult and time consuming task. The traditional design of safety critical systems involves the use of proprietary hardware and software. The full custom design approach is becoming unacceptable because performing a complete redesign for each different class of control applications is unacceptable. Additionally, traditional safety critical systems are designed based on locating all faults in the system which can affect safety via diagnostics. The increasing complexity of the control algorithms which must be processed in a safety critical fashion makes the fault space of the system increase in size to the point where the design of diagnostics becomes quite difficult and time consuming. One promising methodology which can assist in reducing the design effort associated with safety critical systems while using commercial off-the-shelf (COTS) components is algorithm-based fault tolerance (ABFT). Unfortunately, existing ABFT techniques are currently not suitable for safety critical applications. A new safety critical ABFT (SC-ABFT) technique is presented. The checking scheme for the SC-ABFT method is derived based on verifying the correctness of a given control application as it is being evaluated. Also, the probability of detecting a safety-critical error can made as close to 1.0 as desired by varying certain parameters.

  PublisherDOI

• Determining the expected time to unsafe failure

  2002-11-11 · 5 citations

  article

  The number of applications requiring highly reliable and/or safety-critical computing is increasing. One emerging safety metric is the Mean Time To Unsafe Failure (MTTUF). This paper summarizes a novel technique for determining the MTTUF for a given architecture. The first step in determining the MTTUF for a system is to estimate system Mean Time To Failure (MTTF) and system fault coverage. Once these two parameters are known then the system MTTUF can be calculated. The presented technique allows MTTF and system coverage to be estimated from dependability models that incorporate time varying failure and/or repair rates. Existing techniques for the estimation of MTTUF require constant rate dependability models. For the sake of simplicity, this paper uses Markov models to calculate MTTUF. The presented approach greatly simplifies the calculation of system MTTUF. Finally a comparison is made between reliability expected time metrics (MTTF and MTBF) and safety expected time metrics (MTTUF and MTBUF).

  PublisherDOI

• Safety-critical systems built with COTS

  Computer · 1996-11-01 · 40 citations

  articleOpen access

  In the rail transportation industry competitive pressure has led to the increased use of COTS (commercial off-the-shelf equipment in safety critical systems), making it imperative that we extend proven safety techniques to COTS based systems as well. To this end, we have developed the Vital Framework (V-Frame), which is used to develop a safety critical platform from COTS hardware and software. The key technologies in this framework are formal methods, information redundancy, a proprietary data format, and a concurrent checking scheme. Combining these technologies results in a real time, checkable correctness criterion that is a signature of the application's algorithm structure and is independent of both the hardware and the operating system. V-Frame's most significant attribute is that the fail safe properties of applications do not require the firmware to be correct: the application will operate in a fail safe (or vital) manner even if there are design faults in the operating system and/or the hardware fails. This does not mean that the application does not have to be correctly specified and designed. Formal methods are appropriate in the design of safety critical COTS systems because a generic processing environment is analogous to a formal system: it is designed to apply well defined transformation rules to inputs.

  PublisherOA PDFDOI

Frequent coauthors

• B.W. Johnson

  8 shared

• Harry Powell

  University of Virginia

  4 shared

• D.T. Smith

  Virginia Military Institute

  4 shared

• Ronald Williams

  University of Virginia

  4 shared

• J.A. Profeta

  3 shared

• Carl Elks

  Virginia Commonwealth University

  2 shared

• James P. Hanna

  1 shared

• Joseph Reutzel

  Hitachi (United Kingdom)

  1 shared

Awards & honors

• Member of ASEE
• Senior member of the IEEE