1,478 research outputs found
Holographic goodness is not that bad: Reply to Olivers, Chater and Watson (2004)
No full textThe holographic approach (HA) to goodness (P. A. van der Helm & E. L. J. Leeuwenberg, 1991, 1996, 1999) is an ideal-observer theory at, in D. Marr's (1982) terms, the computational and algorithmic levels of description. It provides an explanation of the detectability of visual regularities such as mirror symmetry, repetition, and Glass patterns. C. L. N. Olivers, N. Chater, and D. G. Watson (2004) gave a picture of HA as if it were a flawed theory. However, they gave a flawed picture containing factual errors and misconceptions. Most of their alleged counter-evidence actually supports HA's unified account of perfect regularities, perturbed regularities, and nested regularities. Recent evidence indicates that HA may even lead to deeper implementational insight into the processing of spatial frequencies
An Improved mechanical model for the ankle admittance
No full textQuantification of neuromechanical parameters describing joint admittance is used to improve understanding about human motor control, which is relevant for diagnostics and treatment monitoring of patients suffering from movement disorders. Previous studies used a simple mass-spring-damper (MBK) mechanical model to describe the ankle admittance estimated from small mechanical displacements around a constant mean joint angle i.e. working point. Van Eesbeek used the MBK to describe the admittance for several working points over a wide range of motion (RoM) of the passive ankle using a piece-wise linear approach (LPV). However, the MBK model appeared not sufficient to describe the behavior of the ankle in all working points. This discrepancy was the reason for this study, whose goal is to derive a better model structure for the passive ankle over a large RoM. Results show that a mechanical model made up of 2 springs, 1 damper and 1 mass can approximate the ankle admittance more accurately than a MBK model in each working point. The second spring was meant to approximate the mechanical properties of the Achilles tendon. The result endorses the importance of the tendon mechanical properties when estimating the passive ankle admittance. In addition, it may question the validity of older studies that used MBK mechanical models to estimate joint mechanical properties.BMEBiomechanical EngineeringMechanical, Maritime and Materials Engineerin
Driver Steering Support Interfaces Near the Vehicle’s Handling Limits
No full textThe goal of this thesis is to propose steering support systems that can reduce the driver’s control effort, mental load and promote safety. The driver dictates the vehicle’s motion and the support should centralize him/her in the control loop; thus our design philosophy is to increase driver’s responsibility and support him/her in the sense of information rather than automation. Incarnating such an abstract theme into a concrete problem which can be methodologically solved in terms of engineering science, necessitates a milestone-oriented work approach. Thus, the path to realize this development is to systematically sub-divide the concept into distinct milestones allowing to embody this high-level idea into objectively assessed steering interfaces. This milestone-oriented approach can be divided into seven steps: i) Study the state-of-the-art driver support systems and identify the potential space for improvement. ii) Develop the means (driving simulators, vehicular instrumentation and data analysis methods) to aid the driver steering support interface research. iii) Study the driver steering interface without any support. iv) Utilize the gathered knowledge to develop steering support interfaces, assess them in simulation level, v) and adapt the simulation support controllers into real vehicles and test them. vi) Evaluate the influence of the support interface with the real vehicle results. vii) Based upon the assessment, make a road-map for the commercial implementation of the support interface; if it is fruitful promote its further development with ultimate goal the adoption into production vehicles. he aforementioned milestone-oriented approach has been followed for the development of the driver steering support interfaces presented in this thesis. The current summary substantiates the milestones into the distinct goal addressed in Chapters 2 – 7. The goal to develop the hardware and performance evaluation-control methods in order to engineer realistic haptic cues on the steering wheel of our driving simulator is addressed in Chapter 2. A relatively low-cost solution for hardware is deployed, consisting of a velocity-controlled three-phase brushless servomotor, whose high bandwidth control allows for a realistic representation of forces. To test the system, different inertia-spring-damper systems were simulated and evaluated in time and frequency domain. We concluded, that the designed system allowed reproduction of a large range of steering wheel dynamics and forces, comparable to those found in actual cars. Our target to systematically adjust the steering systems properties of the driving simulator so that it matches the steering feedback and vehicle response of a certain vehicle is addressed in Chapter 3. To do so, we employed the steering sensitivity and steering torque gradient, which are two important metrics describing on-centre vehicle dynamics response and steering feedback. We acquired the steering metrics of real cars during double-lane change tests and indicated the key parameters of the vehicle that determine these steering metrics. We instrumented and tested five modern passenger cars, and used a vehicle dynamics model to extract the metrics for multiple vehicular parameterizations (steering ratio, power assist level, etc.) and test speeds. Sensitivity analysis showed that steering sensitivity was mainly influenced by the components that determine the steering ratio whereas the steering torque gradient was also affected by power assist steering settings. By completing this work, we had the foundation to easily assess the realism of our simulated vehicles’ response as well as to easily adapt the vehicular settings to achieve a realistic steering feedback in our driving simulator. Lane departure appears relevant in 179,000 crashes per year and is related to the greatest number of fatal crashes; up to 7,500 fatal crashes per year in the United States. Infiniti predicts that if lane departure prevention (LDP) were fitted to all vehicles, some 12% of all road fatalities could be prevented annually. The problem is that although numerous studies have shown the potential of lane keeping and LDP systems, there are few studies related to their effects during emergency manoeuvres. Thus, Chapter 4 aims to investigate a road-departure prevention (RDP) system during an emergency manoeuvre. We present a driving-simulator experiment which evaluated various steering interfaces of a road-departure prevention (RDP) system in an emergency situation. The interfaces were: 1) haptic-feedback (HF) where the RDP provided advisory steering torque; 2) drive-by-wire (DBW) where the RDP automatically corrected the front-wheel angle; and 3) DBW & HF, which combined both setups. The RDP system intervenes by applying haptic (guidance) feedback torque and/or correcting the angle of the front wheels (drive-by-wire) when road departure is likely to occur. Thirty test drivers tried to avoid an obstacle (a pylon-confined area) while keeping the vehicle on the road. The results showed that HF without DBW had a significant impact on the measured steering torque, but no significant effect on steering-wheel angle or vehicle path. DBW prevented road departure and reduced mental workload, but lead to inadvertent human-initiated counter-steering. It was concluded that a low level of automation, in the form of HF, does not prevent road departures in an emergency situation. A high level of automation, on the other hand, is highly effective in preventing road departures. Chapter 5 has been divided into three parts (A, B, C), all related to real vehicle testing. Our goal to construct a versatile low-cost instrumentation suitable to be fitted on race cars and develop the methods for processing from raw measurements to user-friendly data suitable for driver behaviour studies is addressed in part A. Through a case study on driving behaviour, during the execution of high speed skid-pad manoeuvres, we could easily notice the markedly different driving behaviours between an expert and a novice driver. The experienced driver could learn quickly how to perform repeatable trajectories, unlike the novice driver. The consistently high performance of the expert driver was realized by relatively small correcting inputs (steering wheel angle, throttle). The experienced driver was able to quickly learn how to generate the correct inputs to the vehicle, to yield repeatable vehicle behaviour and consistently perform well. Our aim to investigate driver control actions during high speed cornering with a rear wheel drive vehicle is depicted in Chapter 5, part B. Six drivers were instructed to perform the fastest manoeuvres possible around a marked circle, while trying to retain control of the vehicle and constant turning radius. The data reveal that stabilization of the vehicle is achieved with a combination of steering and throttle regulation. The results show that the drivers used steering control to compensate for disturbances in yaw rate and sideslip angle. Vehicle accustomed drivers had the most consistent performance resulting in reduced variance of task metrics and control inputs. Our target to design controllers that can stabilize the vehicle as an expert driver would is approached in part C of Chapter 5. There, we present data of driver control commands and vehicle response during the execution of cornering manoeuvres at high sideslip angles (drifting) by an expert driver using a RWD vehicle. The data reveal that stabilization of the vehicle with respect to such cornering equilibria requires a combination of steering and throttle regulation. A four wheel vehicle model with nonlinear tire characteristics is introduced and the steady-state drifting conditions are solved numerically to derive the corresponding control inputs. A sliding mode control is proposed to stabilize the vehicle model with respect to steady-state drifting, using steering angle and drive torque inputs. The performance of the controller is validated in a high fidelity simulation environment; the controller can stabilize the vehicle similarly to an expert driver. We also conceptually describe how the proposed controller can motivate a driver steering support drifting interface in the by-wire sense. Our goal to objectively evaluate vehicular steering systems through detailed driver models is substantiated in Chapter 6. It presents a driver model that consists of a preview controller part that responds to visual feedback and a neuromuscular component that reacts to force-feedback. The developed model is sensitive to steering wheel systems with different dynamics, and can predict both goal-directed steering wheel movements, as well as neuromuscular feedback. To provide evidence, we simulated different parameterizations of a steering system and tested them in conjunction with the developed driver model. We concluded that the developed model could predict the expected response for different steering setups. Our milestone goal to propose haptic steering wheel support when driving near the vehicle’s handling limit (Haptic Support Near the Limits: HSNL) is addressed in Chapter 7. The rationale behind the HSNL, derives from the vehicle’s property to reduce the steering “stiffness” (the steering feedback torque as a function of the steering wheel angle) before the vehicle reaches its handling limits and starts to understeer. The HSNL exaggerates the reduction of the steering “stiffness” and makes it profound to the driver, so he/she avoids excessive steering angle inputs which will result in increased tire slip and consequently lateral force loss. Chapter 7 is divided into two parts (A, B). Part A of Chapter 7 studies the influence of the HSNL in (a) driver-in-the-loop simulation and in (b) real track testing with a vehicle (Opel Astra G/B) equipped with a variable steering feedback torque system. In the simulator study (a) 25 drivers attempted to achieve maximum velocity, on a dry skid-pad while trying to retain control of the simulated vehicle parameterized as the Astra. In (b) 17 drivers attempted to achieve maximum velocity, around a wet skid-pad while trying to retain control of the Astra. Driving aids (ABS and traction control) were disabled during testing. Both the driving simulator and the real vehicle tests led to the conclusion that HSNL assisted the test subjects to drive closer to the designated path while achieving effectively the same speed. In the presence of HSNL, the drivers operated the tires in smaller slip angles and hence avoided saturation the front wheels’ lateral forces and excessive understeer. Finally, the support reduced their mental and physical demand. Part B of Chapter 7, studies the influence of HSNL during high speed cornering in a test-track. 17 test subjects drove around a narrow-twisting tarmac circuit, the aforementioned Opel Astra equipped with a variable steering feedback torque system. The drivers were instructed to achieve maximum velocity through corners, while receiving haptic steering feedback cues related to the vehicle’s cornering potentials. Driving aids (ABS and traction control) were disabled during testing. The test-track tests led to the conclusion that HSNL reduced drivers’ mental and physical demand. One of the primal goals of automotive manufacturers is to reduce the driver’s mental and control effort (c.f. Chapter 7); the work that will be presented in this thesis revealed that steering support near the vehicle’s handling limits can reduce the drivers’ mental and physical demand and can potentially promote safety. We can therefore conclude that certain of the developed support interfaces can be implemented into production vehicles.BioMechanical EngineeringMechanical, Maritime and Materials Engineerin
Realization and evaluation of a remotely controlled mobile robot with shared control and underactuated hand to improve the unlatching of doors
No full textRecent disasters, the nuclear meltdown of the Fukushima reactors and the Gulf of Mexico oil spill, have shown us the importance of robotics and how we lack knowledge in that field, at least outside a constructed environment. A bit more frequent and smaller disasters are burning houses. Firefighters risk their own lives by going into burning houses in search of people that might still be inside. To minimize their risks a mobile reconnaissance robot could be sent in first to inspect the house by locating the origin of the fire, possible threats and identify the location of people for the firefighters to rescue. By receiving this information as soon as possible the severity of the threat can be assessed and proper precautions can be taken in time. Robots are usually operated by a human in dangerous environments because of the sophisticated cognitive capabilities of the human. The human can resolve unexpected situations that autonomous robots can still not do. The task should be performed as fast as possible as time is a very critical factor. One of the most time-consuming activities is the opening of doors. Literature suggests a real need for some sort of compliance in the robots to successfully open doors. The reason for that is that compliance decreases the need for accurate positioning of the robot with regard to the door and door handle. One way to accomplish that is to add a compliant hand (e.g. an underactuated hand). Another way to improve a door opening task is use Shared Control to guide the human operator to approach the door and guide him with unlatching the door open. Shared Control helps the operator to be more accurate in his movements. Therefore in this study an experiment was perform to analyze the influence of Shared Control and an Underactuated hand when a human operator remotely controls a non-holonomic, non-redundant mobile robot during the unlatching of a door.BMDBioMechanical EngineeringMechanical, Maritime and Materials Engineerin
Quantitative heritability of anti-citrullinated protein antibody-positive and anti-citrullinated protein antibody-negative rheumatoid arthritis
No full textThe majority of genetic risk factors for rheumatoid arthritis (RA) are associated with anticitrullinated protein antibody (ACPA)-positive RA, while far fewer genetic risk factors have been identified for ACPA-negative RA. This study was undertaken to quantify the contribution of genetic risk factors in general, and of the predisposing HLA-DRB1 shared epitope (SE) alkies in particular, to the ACPA-positive and ACPA-negative subsets of RA, by computing their heritability and assessing the contribution of the HLA SE alleles. Methods. One hundred forty-eight RA twin pairs, in which at least 1 twin of each pair had RA, were tested for ACPAs and typed for HLA-DRB1 genotypes. Heritability was assessed in a logistic regression model including a bivariate, normally distributed random effect, representing the contribution of unobserved genetic factors to RA susceptibility, with the correlation of the random effects fixed according to twin zygosity. The contribution of the HLA SE alleles to genetic variance was assessed using a similar model, except that estimates were based on genotype-specific population prevalences. Results. The heritability of RA among the twin pairs was 66% (95% confidence interval [95% CI] 44-75%). For ACPA-positive RA, the heritability was 68% (95% CI 55-79%), and for ACPA-negative RA it was 66% (95% CI 21-82%). Presence of the HLA SE alleles explained 18% (95% CI 16-19%) of the genetic variance of ACPA-positive RA but only 2.4% (95% CI 1.6-10%) of the genetic variance of ACPA-negative RA. Conclusion. The heritability of ACPA-positive RA is comparable with that of ACPA-negative RA. These data indicate that genetic predisposition plays an important role in the pathogenesis of ACPA-negative RA, for which most individual genetic risk factors remain to be identified. © 2009, American College of Rheumatology
[medal] Penning voor het huwelijk van Ferdinand, zoon van Maria-Theresia, en Marie Beatrice van Modena.. /
No full textOud plaatsnummer O.1771/1De penning bestaat ook in groot formaatRecto: Gekoppelde bustes naar rechts van Ferdinand en Maria Beatrice ; Ferdinand heeft de haren samen gestrikt in de hals ; rondom bovenaan, FERDINAND • A • A • M • BEATRIX EST. SPONSI.Verso: Links komt Mercurius gestapt met staf en helm, en houdt in de linkerhand twee samengebonden wapenschilden van het echtpaar ; rechts zit de gelauwerde watergod met het roer rechtop in zijn linkerhand tussen het riet , zijn rechterhand rust op de kruik waaruit water stroomt met het opschrift, PADUT (de Po) rondom bovenaan, NVMINA FAVENT ; onderaan op de afsnede, NVPTIIS CELEBRATIS / MEDIOLA•D•XV•OCT / I77I. (het huwelijk werd voltrokken te Milaan)Schau- und Denkmünzen welche unter der glorwürdigen Regierung der Kaiserinn Königinn Maria Theresia geprägt worden sind. Medailles frappées sous le regne glorieux de l'impératrice Marie Thérese. Wien: Krauss, 1782, nr. CCXXXIX, p. 335.Bijzondere collectie
Natural convection in high heat flux tanks at the Hanford Waste Site / [by] Mark van der Helm and Mujid S. Kazimi
No full text"February 1996."Series statement handwritten on title-pagePage 118 blankAlso issued as an M.S. thesis written by the first author, and supervised by the second author, MIT Dept. of Nuclear EngineeringIncludes bibliographical references (pages 115-117)A study was carried out on the potential for natural convection and the effect of natural convection in a High Heat Flux Tank, Tank 241-C-106, at the Hanford Reservation. To determine the existence of natural convection, multiple computations based on analytical models were made knowing the tank geometry and contents' thermal characteristics. Each computation of the existence of natural convection was based on the determination of the onset of natural convection generalizing the tank as a 1-D porous medium. Computations were done for a range of permeabilities considering the porous medium alone, with a superposed fluid layer, and with a salt gradient. Considering only the porous medium, the higher permeability value, 3.2 *10-10 ft2, allowed convection, though the lower permeability, 2.6*10-14 ft2, did not. The presence of the superposed layer induced convection throughout the porous medium for the full range of permeabilities.Considering the effect of the salt gradient and superposed layer together, the effect of the superposed layer is expected to induce convection despite the stabilizing salt gradient. Therefore, natural convection is expected to exist in Tank 241-C-106. Secondly, because temperature measurements indicated lower temperatures at a location near the center of the tank, a thermal model was used to compute the local effects of a convective annulus around a thermocouple tree at that location. A conduction model of the tank and surroundings was used to bound the local model. The local model allowing convection in the annulus set the size of the annulus based on the known temperature measurements of the thermocouple tree and the boundary conditions set by the conduction model. Previous published calculations on Tank 241-C-106, allowing for only conduction within the tank, reported a steam region at the bottom of the tank with an approximately 24 foot radius.In the present analysis, using the computer code, TEMPEST, it is found that the cooling effect of the annulus creates a region with a 12 foot radius surrounding the thermocouple tree in which the temperature is suppressed below the saturation temperature due to the effects of the convective annulus. The annulus gap width for matching temperatures and the boundary conditions is on the order of 1 inch
Diagnostic performance of the ACR/EULAR 2010 criteria for rheumatoid arthritis and two diagnostic algorithms in an early arthritis clinic (REACH)
No full textIntroduction: An ACR/EULAR task force released new criteria to classify rheumatoid arthritis at an early stage. This study evaluates the diagnostic performance of these criteria and algorithms by van der Helm and Visser in REACH. Methods: Patients with symptoms ≤12 months from REACH were used. Algorithms were tested on discrimination, calibration and diagnostic accuracy of proposed cut-points. Two patient sets were defi ned to test robustness; undifferentiated arthritis (UA) (n=231) and all patients including those without synovitis (n=513). The outcomes evaluated were methotrexate use and persistent disease at 12 months. Results: In UA patients all algorithms had good areas under the curve 0.79, 95% CI 0.73 to 0.83 for the ACR/EULAR criteria, 0.80, 95% CI 0.74 to 0.87 for van der Helm and 0.83, 95% CI 0.77 to 0.88 for Visser. All calibrated well. Sensitivity and specifi city were 0.74 and 0.66 for the ACR/EULAR criteria, 0.1 and 1.0 for van der Helm and 0.59 and 0.93 for Visser. Similar results were found in all patients indicating robustness. Conclusion: The ACR/EULAR 2010 criteria showed good diagnostic properties in an early arthritis cohort refl ecting daily practice, as did the van der Helm and Visser algorithms. All were robust. To promote uniformity and comparability the ACR/EULAR 2010 criteria should be used in future diagnostic studies. Copyright Article author (or their employer) 2011
The burden of early axial spondyloarthritis
No full textIn this thesis we aimed to investigate the burden of axial spondyloarthritis (axSpA) in an early phase. We have emphasized three main themes: (1) the value of a positive family history of SpA; (2) the impact of axSpA on health outcomes in an early phase of axSpA; and (3) illness perceptions and coping strategies of patients with axSpA.We have shown that when broad HLA-B27 testing is not useful, the presence of a positive family history of axSpA or uveitis could be used to predict HLA-B27 positivity in patients suspected of axSpA. We have demonstrated that health-related quality of life work productivity, and daily activities are seriously impacted even in the earliest phase of the axSpA and remain substantial over time. We have further shown that illness perceptions are important in the relationship between back pain and health outcomes in patients with axSpA. Our results suggest that illness perceptions and coping strategies do not change spontaneously and are rather independent of a decrease in disease status in patients with early axSpA.The knowledge and insights of this thesis could help us in making a timely diagnosis, providing treatment sooner, and exploring additional treatment options, which in turn could hopefully reduce the burden of axSpA. Dutch Arthritis Foundation (ReumaNederland), Pfizer B.V., UCB Pharma B.V., and Chipsoft B.V.LUMC / Geneeskund
Mechanical and histological characterization of thrombi retrieved during thrombectomy for acute ischaemic stroke
No full textBackground: The efficacy of a thrombectomy procedure for acute ischaemic stroke (AIS) is largely dependent on mechanical behaviour of thrombi and interactions with the thrombectomy device. Studies have examined clot analogues and evidence suggests that thrombus mechanical properties largely depend on composition, yet evidence based on thrombi retrieved from AIS patients is still lacking. Therefore, this study aims to characterize the mechanical properties of thrombi retrieved from AIS patients and to determine the relation to thrombus composition. Additionally, results were compared to literature in order to assess clot analogue representativeness.Methods: Directly following a thrombectomy procedure, unconfined compression tests were performed on thrombi retrieved from acute ischaemic stroke patients. For all tested samples, the material properties were characterized and related to the histologically determined composition. Identified histological components were 1) Fibrin & platelets, 2) Red blood cell and 3) Leukocytes. A subgroup analysis was performed to compare values with literature, where samples were stratified into four groups based on fibrin & platelet content (F&P -low, -moderate low, -moderate high and -high). Results: A total of 18 patients yielded 39 samples which were successfully tested and histologically analysed. Sample stiffness was found to be positively correlated to fibrin & platelet content (Rs=0.69, p<0.001). A good histological distribution was present within the data, as the fibrin & platelet content ranged from 7% to 99%. Subgroup analysis showed little difference in mechanical behaviour between the F&P moderate-low and F&P moderate-high subgroups, with the F&P low and F&P high groups respectively exhibiting a decreased and increased stiffness. Comparing to current literature, the results demonstrated that analogues most accurately resemble thrombi with a low fibrin & platelet content. Furthermore, all samples displayed viscoelastic and non-linear stress-strain behaviour.Conclusion: It was found that composition is a strong influencing factor of thrombus mechanical properties. Both at high and low fibrin & platelet contents, the relation between composition and stiffness was strongest, while it was least pronounced at moderate fibrin & platelet contents (approximately 25%-75%).Biomedical Engineerin
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