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Now showing 1 - 10 of 872
  • Publication
    Analysis of a Simplified Hopping Robot
    (1988-05-01) Koditschek, Daniel E; Buehler, Martin
    We offer some preliminary analytical results concerning simplified models of Raibert’s hopper. We represent the task of achieving a recurring hopping height for an actuated “ball” robot as a stability problem in the setting of a nonlinear discrete dynamical system. We model the properties of Raibert’s control scheme in a simplified fashion, and provide conditions under which the procedure results in closed loop dynamics possessed of a globally attracting fixed point - the formal rendering of what we intuitively mean by a “correct” strategy. The motivation for this work is the hope that it will facilitate the development of general design principles for “dynamically dexterous” robots. For more information: Kod*Lab
  • Publication
    Transit Technology Today
    (1984-11-01) Vuchic, Vukan R
    This keynote address was presented at the Symposium on Recent Developments in Urban Transit Technology in Taipei, Taiwan on November 27, 1984.
  • Publication
    Holistically Evaluating Agent Based Social System Models
    (2012-01-01) Bharathy, Gnana K.; Silverman, Barry G.
    The philosophical perspectives on model evaluation can be broadly classified into reductionist/logical positivist and relativist/holistic. In this paper, we outline some of our past efforts in, and challenges faced during, evaluating models of social systems with cognitively detailed agents. Owing to richness in the model, we argue that the holistic approach and consequent continuous improvement are essential to evaluating complex social system models such as these. A social system built primarily of cognitively detailed agents can provide multiple levels of correspondence, both at observable and abstract aggregated levels. Such a system can also pose several challenges, including large feature spaces, issues in information elicitation with database, experts and news feeds, counterfactuals, fragmented theoretical base, and limited funding for validation. We subscribe to the view that no model can faithfully represent reality, but detailed, descriptive models are useful in learning about the system and bringing about a qualitative jump in understanding of the system it attempts to model – provided they are properly validated. Our own approach to model evaluation is to consider the entire life cycle and assess the validity under two broad dimensions of (1) internally focused validity/quality achieved through structural, methodological, and ontological evaluations; and (2) external validity consisting of micro validity, macro validity, and qualitative, causal and narrative validity. In this paper, we also elaborate on selected validation techniques that we have employed in the past. We recommend a triangulation of multiple validation techniques, including methodological soundness, qualitative validation techniques, such as face validation by experts and narrative validation, and formal validation tests, including correspondence testing.
  • Publication
    Transportation for Liveable Cities: Problems, Obstacles, and Successful Solutions
    (2010-01-01) Vuchic, Vukan R
    This chapter focuses on urban transportation. It is written by the moderator of the session on Land Transportation. He asked the five speakers to report on positive achievements and challenges in their cities or countries. The chapter starts with the moderator's review of developments and trends in urban transportation and a summary of the five reports by participants presented in the session. It is followed by a brief review of the conditions in urban transportation and its impact on cities. Following a review of problems most cities face and mistakes in transportation planning many cities continue to make, this chapter places emphasis on progressive policies and successful solutions which feasibility has been demonstrated by a number of leading cities.
  • Publication
    Propagation of Schedule Disturbances in Line-Haul Passenger Transportation
    (1969) Vuchic, Vukan R
    Schedule disturbances in public transport operations have a tendency to intensify along the line and propagate to successive vehicles due to the uneven accumulation of passengers. This phenomenon, affecting efficiency and reliability of service, occurs frequently with surface services due to street congestion, as well as with rapid transit when It approaches capacity volumes. In recent years, considerable attention has been given to this problem. Newell and Potts [1] (*), using a deterministic model, derived an expression for the behavior of delays both along the line and of subsequent vehicles at individual stations due to passenger accumulation. They gave a theoretical explanation of the phenomenon of pairing of buses, which later Potts and Tamlin tried to verify through observations of bus operations [2]. While they did observe the tendency for pairing of vehicles, their experiment indicated that numerous other factors in street operation (signals, traffic, etc.) make it difficult to distinguish individual causes of delays. Rapid transit is more convenient for these observations since passenger boarding is the dominant variable factor in operation. Tiercin [3] described a new method of schedule control tested by RATP in Paris for one of the principal « Metro » lines, and London Transport, in planning for « Victoria Line», used computer simulation of rapid transit operation at minimum intervals to derive operational measures to increase stability of service. This work was reported by Welding and Day (4) and in an unpublished Research Report [5]. Recently, Lehmann [6] and Sudmeyer [7] gave an Interesting theoretical analysis of propagation of delays along the line; their discussion was followed by a paper by this author [8] which is incorporated and somewhat expanded here. In this paper a theoretical analysis of the behavior of disturbances is extended to include the changes of disturbances with time (for subsequent vehicles at any given station). Practical implications are discussed and measures to minimize this phenomenon in public transport operations are suggested. A diagram for easy evaluation of stability of any service is also given here.
  • Publication
    Analysis of A Simplified Hopping Robot
    (1991-12-01) Koditschek, Daniel E; Buehler, Martin
    This article offers some analytical results concerning simplified models of Raibert's hopper. We represent the task of achieving a recurring hopping height for an actuated "ball" robot as a stability problem in a nonlinear discrete dynamical control system. We model the properties of Raibert's control scheme in a simplified fashion and argue that his strategy leads to closed-loop dynamics governed by a well-known class of functions, the unimodal maps. The rich mathematical literature on this subject greatly advances our ability to determine the presence of an essentially globally attracting fixed point-the formal rendering of what we intuitively mean by a "correct" strategy. The motivation for this work is the hope that it will facilitate the development of general design principles for "dynamically dexterous" robots.
  • Publication
    Transit Federation -- A Solution for Service Integration
    (1972) Homburger, Wolfgang S; Vuchic, Vukan R
    Inadequate organization of public transport services in urban areas, particularly in the large ones, is one of the major reasons for the unsatisfactory level of service and economic problems of the operating companies. Despite the current trend toward mergers of transit operators into large public agencies, the services in most cities remain fragmented in various degrees; integration is often not in sight due to organizational problems which appear insurmountable. Losses to the users, the operators, and the city from this situation are often very significant. This article briefly analyzes the reasons for this situation, explores its consequences and their importance. A number of solutions for the problem are possible, but none of them is simple and easy to achieve. The federation of transit organizations introduced recently in Hamburg, Germ any, has proved to be so successful that it has received wide attention in international professional circles. This solution therefore deserves a careful study by transit operators as well as government officials of metropolitan areas in the United States and other countries.
  • Publication
    Evaluating UMTA’s Work
    (1976) Vuchic, Vukan R
    The period of economic expansion and affluence since World War II created a strong tendency to base planning on unquestioned extrapolation of past trends. This method is acceptable in areas where the developmental trends are in desirable directions. But when this is not the case, this method does not represent rational planning, since its effect is to continue and encourage undesirable developments. Urban transportation has suffered greatly from this type of planning; many of our national and metropolitan area transportation studies placed a heavy reliance on .continuation of growth in auto ownership, VMT's, decentralization, etc. They failed to set clear goals for public transportation and standards for its service.
  • Publication
    Skip-Stop Operation: High Speed With Good Area Coverage
    (1976) Vuchic, Vukan R
    Increase of transit speeds is one of the most effective ways of increasing the attractiveness of transit for urban travel. While surface transit in particular suffers from low speed, the desirability of higher speeds is not limited to it. Rapid transit has adequate speed for short to medium-distance trips in urban areas. However, for longer. trips, particularly when there is a competing freeway facility, the requirement for speed is rather high. Since many station spacings are adopted on the basis of area coverage, high operating speed of the trains often cannot be achieved. Thus, typical lines of urban rapid transit with average interstation spacings of approximately 800 metres have only limited length on which their speeds are satisfactory; for distances longer than, typically, 8-10 km, they often become too slow. This is becoming an increasing problem with· the spatial spread of cities. This article describes the main alternative solutions to this problem and then focuses on the skip-stop operation, presenting a methodology for its analysis and evaluation of its applicability. The article refers to rail services, but the basic aspects of the problem are common for any technology. For example, there are light rail and bus services for which skip-stop service could be considered utilizing the methodology developed here.
  • Publication
    Light Rail and BRT: Competitive or Complementary?
    (2005-01-01) Vuchic, Vukan R
    Since the 1970s great progress has been made to develop transit systems which provide service considerably better than buses can offer in mixed traffic, but which require significantly lower investment than metro systems with exclusive ways. This “semirapid transit” category of modes, using mostly partially separated ways, has been introduced extensively in medium-sized cities, as well as supplementing metros in suburban areas of large cities.