The role of friction in the measurement of slipperiness,Part 2: Survey of friction measurement devices

This paper seeks to address questions related to friction measurement such as how friction is related to human-centred assessment and actual slipping, and how repeatable friction measurements are. Commonly used devices for slipperiness measurement are surveyed and their characteristics compared with suggested test conditions from biomechanical observations summarized in Part 1. The issues of device validity, repeatability, reproducibility and usability are examined from the published literature. Friction assessment using the mechanical measurement devices described appears generally valid and reliable. However, the validity of most devices could be improved by bringing them within the range of human slipping conditions observed in biomechanical studies. Future studies should clearly describe the performance limitations of any device and its results and should consider whether the device conditions reflect these actual human slipping conditions. There is also a need for validation studies of more devices by walking experiments.     

The role of surface roughness in the measurement of slipperiness

Surface roughness has been shown to have substantial effects on the slip resistance between shoe heels and floor surfaces under various types of walking environments. This paper summarizes comprehensive views of the current understanding on the roles of surface roughness on the shoe and floor surfaces in the measurement of slipperiness and discusses promising directions for future research. Various techniques and instruments for surface roughness measurements and related roughness parameters are reviewed in depth. It is suggested that a stylus-type profilometer and a laser scanning confocal microscope are the preferred instruments for surface roughness measurements in the field and laboratory, respectively. The need for developing enhanced methods for reliably characterizing the slip resistance properties is highlighted. This could be based on the principal understanding of the nature of shoe and floor interface and surface analysis techniques for characterizing both surfaces of shoe and floor. Therefore, surface roughness on both shoe and floor surfaces should be measured and combined to arrive at the final assessment of slipperiness. While controversies around the friction measurement for slipperiness assessment still remain, surface roughness measurement may provide an objective alternative to overcoming the limitations of friction measurements.     

The role of friction in the measurement of slipperiness, Part 1: friction mechanisms and definition of test conditions.

Friction has been widely used as a measure of slipperiness. However, controversies around friction measurements remain. The purposes of this paper are to summarize understanding about friction measurement related to slipperiness assessment of shoe and floor interface and to define test conditions based on biomechanical observations. In addition, friction mechanisms at shoe and floor interface on dry, liquid and solid contaminated, and on icy surfaces are discussed. It is concluded that static friction measurement, by the traditional use of a drag-type device, is only suitable for dry and clean surfaces, and dynamic and transition friction methods are needed to properly estimate the potential risk on contaminated surfaces. Furthermore, at least some of the conditions at the shoe/floor interface during actual slip accidents should be replicated as test conditions for friction measurements, such as sliding speed, contact pressure and normal force build-up rate.     

Human-centred approaches in slipperiness measurement

A number of human—centred methodologies subjective, objective, and combined are used for slipperiness measurement. They comprise a variety of approaches from biomechanically-oriented experiments to psychophysical tests and subjective evaluations. The objective of this paper is to review some of the research done in the field, including such topics as awareness and perception of slipperiness, postural and balance control, rating scales for balance, adaptation to slippery conditions, measurement of unexpected movements, kinematics of slipping, and protective movements during falling. The role of human factors in slips and falls will be discussed. Strengths and weaknesses of human-centred approaches in relation to mechanical slip test methodologies are considered. Current friction-based criteria and thresholds for walking without slipping are reviewed for a number of work tasks. These include activities such as walking on a level or an inclined surface, running, stopping and jumping, as well as stair ascent and descent, manual exertion (pushing and pulling, load carrying, lifting) and particular concerns of the elderly and mobility disabled persons. Some future directions for slipperiness measurement and research in the field of slips and falls are outlined. Human-centred approaches for slipperiness measurement do have many applications. First, they are utilized to develop research hypotheses and models to predict workplace risks caused by slipping. Second, they are important alternatives to apparatus-based friction measurements and are used to validate such methodologies. Third, they are used as practical tools for evaluating and monitoring slip resistance properties of footwear, anti-skid devices and floor surfaces.     

The role of friction in the measurement of slipperiness, Part 2: survey of friction measurement devices.

This paper seeks to address questions related to friction measurement such as how friction is related to human-centred assessment and actual slipping, and how repeatable friction measurements are. Commonly used devices for slipperiness measurement are surveyed and their characteristics compared with suggested test conditions from biomechanical observations summarized in Part 1. The issues of device validity, repeatability, reproducibility and usability are examined from the published literature. Friction assessment using the mechanical measurement devices described appears generally valid and reliable. However, the validity of most devices could be improved by bringing them within the range of human slipping conditions observed in biomechanical studies. Future studies should clearly describe the performance limitations of any device and its results and should consider whether the device conditions reflect these actual human slipping conditions. There is also a need for validation studies of more devices by walking experiments.     

Assessing slipperiness in fast-food restaurants in the USA using friction variation, friction level and perception rating

Although friction variation is speculated to be a significant contributor to slip and fall incidents, it has not been related to a measurement of slipperiness in the literature. This field study investigated the relationship among multiple friction variations, friction levels and the perception ratings of slipperiness in six major working areas of 10 fast-food restaurants in the USA. The mean perception rating score for each working area was correlated with various friction reduction variables across all the restaurants in comparison with its correlation with the mean friction coefficient of each working area. The results indicated that the absolute and relative reductions in friction over the whole working area, among 12 friction reduction variables evaluated, could have a slightly better correlation with the perception rating score (r=0.34 and 0.37, respectively) than the mean friction coefficient of each working area (0.33). However, in friction measurements, more effort and time are needed to quantify friction variations than to obtain the mean friction coefficient. The results of the multiple regression model on the perception rating indicated that adding friction reduction variables into the regression model, in addition to the mean friction coefficient, did not make a significant impact on the outcomes. The results further indicated a statistically significant correlation between the mean friction coefficient and the maximum relative friction reduction over the whole area in each working area across all the restaurants evaluated (r=0.80). Despite a slightly lower correlation with perception rating than the friction variation, the mean friction coefficient of an area is still a reasonably good indicator of slipperiness.     

The role of surface roughness in the measurement of slipperiness.

Surface roughness has been shown to have substantial effects on the slip resistance between shoe heels and floor surfaces under various types of walking environments. This paper summarizes comprehensive views of the current understanding on the roles of surface roughness on the shoe and floor surfaces in the measurement of slipperiness and discusses promising directions for future research. Various techniques and instruments for surface roughness measurements and related roughness parameters are reviewed in depth. It is suggested that a stylus-type profilometer and a laser scanning confocal microscope are the preferred instruments for surface roughness measurements in the field and laboratory, respectively. The need for developing enhanced methods for reliably characterizing the slip resistance properties is highlighted. This could be based on the principal understanding of the nature of shoe and floor interface and surface analysis techniques for characterizing both surfaces of shoe and floor. Therefore, surface roughness on both shoe and floor surfaces should be measured and combined to arrive at the final assessment of slipperiness. While controversies around the friction measurement for slipperiness assessment still remain, surface roughness measurement may provide an objective alternative to overcoming the limitations of friction measurements.     

Friction induced hunting limit cycles: A comparison between the LuGre and switch friction model

In this paper, friction induced limit cycles are predicted for a simple motion system consisting of a motor-driven inertia subjected to friction and a PID-controlled regulator task. The two friction models used, i.e., (i) the dynamic LuGre friction model and (ii) the static switch friction model, are compared with respect to the so-called hunting phenomenon. Analysis tools originating from the field of nonlinear dynamics will be used to investigate the friction induced limit cycles. For a varying controller gain, stable and unstable periodic solutions are computed numerically which, together with the stability analysis of the closed-loop equilibrium points, result in a bifurcation diagram. Bifurcation analysis for both friction models indicates the disappearance of the hunting behavior for controller gains larger than the gain corresponding to the cyclic fold bifurcation point.     

Analysis of undercompensation and overcompensation of friction in 1DOF mechanical systems

This paper investigates the effects of undercompensation and overcompensation of friction in PD controlled 1DOF mechanical systems. The friction force that is acting on the mechanical system and the friction compensation term in the feedback loop are described by a class of discontinuous friction models consisting of static, Coulomb and viscous friction, and including the Stribeck effect. Lyapunov's stability theorem and LaSalle's invariance principle are applied to prove that undercompensation of friction leads to steady-state errors and the properties of the ω-limit set of trajectories of a two-dimensional autonomous differential inclusion are used to show that overcompensation of friction may induce limit cycling. Furthermore, the analysis also indicates that the limit cycling effect can be eliminated by tuning the PD controller gains.     

Logical and information aspects in surface science: friction,capillarity, and superhydrophobicity

Abstract

Logical and information aspects of friction and wetting (including the adhesion, capillarity, and superhydrophobicity) are discussed. Friction involves paradoxes, such as the Painlevé paradoxes of non-existence or non-uniqueness of solutions in mechanical systems of rigid bodies with dry friction. These paradoxes can be treated by introducing ternary logic with the three basic states: rest-motion-undefined. When elastic deformation is introduced, the paradoxical solutions correspond to frictional instabilities leading to rest-motion-unstable as three states of a system. The dynamic evolution of a frictional interface towards a limit cycle can be viewed as a process of erasing the information about the interface due to the instabilities. Furthermore, while friction force is universal, it is not treated as a fundamental force and can be considered as an epiphenomenon of various synergetic mechanisms. This further relates friction to other surface effects, including the capillarity, with its binary logic of wetting states and a possibility of droplet computation for lab-on-a-chip microfluidic reactors. We discuss the logical foundation of biomimetic superhydrophobic surface design and how it is different from the conventional design. Both friction and wetting can be used for novel unconventional logical and computational devices.     

Transducer mechanisms for optical biosensors. Part 1: The chemistry of transduction

The first stage of optical biosensor transduction involves a chemical interaction between the analyte and an indicator phase to produce an optically detectable signal. This stage is critical because it determines stability, selectivity and sensitivity as well as establishing the wavelengths required for the optical measurement. Several types of analyte/indicator interaction are possible. Direct indicators such as those used in optical pH sensors are in equilibrium with the analyte in the sample. They may be used for continuous measurements and can be coupled to the sensing of other analytes such as acidic and basic gases like carbon dioxide and ammonia. Integrating reagents react irreversibly with analyte and require that the rate of product formation be measured. Sensors based on catalysis by an immobilised enzyme involve a steady-state measurement of optically detectable substrate or product. High selectivity can be achieved using antibodies as reagents and basing sensing on competitive binding; however, response time is a serious problem. A variety of methods have been employed to immobilize the indicator phase including adsorption on solid substrates, covalent bonding to a substrate and confinement by membranes permeable to analyte but not to indicator.     

The role of quantitative information about slip and grip force in prosthetic grasp stability

Prosthetic hands introduce an artificial sensorimotor interface between the prosthesis wearer and the environment that is prone to perturbations. We analyze theoretically and evaluate psychophysically the performance in stable grip control in conditions of physical grasps perturbation, such as object slip. Simulation results suggest that user-centered stable grasp control depends on two primal user parameters: reaction time to slip and grip force intensity. Experiments with human users indicate that a user’s response time can be controlled by relaying information about the speed of the slipping object, while minimal grip force intensity can be adjusted with information about grip force at the onset of the slip. Based on our theoretical and experimental findings, we propose a stable grasp control method for prosthetic hands.     

The effect of shoe sole tread groove depth on the friction coefficient with different tread groove widths, floors and contaminants

Slipping and falling are common phenomena in both workplaces and our daily activities. The risks associated with slipping and falling are related to the materials of footwear/floor, contamination condition, and geometric design of the sole. Shoe soles of various tread design are very common. Tread pattern of the shoe affects friction especially under liquid-contaminated conditions. Verification of the effects of tread groove depth is significant in assisting designers in designing proper footwear for workers exposed to slippery floor conditions. In this study, we measured the friction coefficients using the Neolite footwear pads on the terrazzo, steel, and vinyl floors under three liquid-contaminated conditions. A Brungraber Mark II slipmeter was used. The footwear pads had tread grooves with a width of either 3 or 9 mm. The depth of the tread grooves ranged from 1 to 5 mm. The results showed that tread groove depth affected the friction coefficients significantly. Higher friction values were recorded for footwear pads with deeper tread grooves on wet and water–detergent-contaminated floors. The averaged coefficient of friction (COF) gain per tread groove depth increase in millimeter under these two surface conditions ranged from 0.018 to 0.108, depending on the tread groove width, floor, and contaminant.     

Optimal loading conditions in the design and identification of structures. Part 1: discrete formulation

The paper is concerned with a class of structural optimization problems for which loading distribution and orientation are unspecified. The optimal loading conditions correspond to the extremal structural response, which can be used in assessment of structural safety or in generating the maximum structure stiffness or compliance. In identification problems the optimal load distribution is selected in order to minimize the distance norm between model prediction and experimental data. The sensitivity derivatives and optimality conditions are derived in the paper using discretized formulations. The generalized coaxiality conditions of loading and displacement or adjoint displacement vectors generate eigenvalue problems specifying stationary solutions. The paper is illustrated by examples of optimal loading distribution in structure design and identification.     

Delivered Deliverables: The State of the Semantic Web, Part 1

Tangible products have emerged from the semantic Web initiative. The author examines the progress that has been made in regard to specifications, tools, and applications coming that fall under the W3C's initiative.     

The potential for social contextual and group biases in team decision-making: biases,conditions and psychological mechanisms

Abstract

Users' mental models of systems are frequently asserted to be important in their interactions with those systems. Although there are few explicit definitions of mental models in the literature, mental models are assumed to be important and researchers have seemed to accept and understand the concept intuitively. In our research, we have extracted three themes from the definitions of mental models including: (1) the relations among features or components of a system; (2) the capability of the mental model to aid in the control of the system; and (3) the capability of the mental model to enhance the user's understanding of the system. The relationships among the components of users' mental models, including procedural and declarative knowledge, perceptions of the use of system features, and the relations among the similarity of use of system features, were investigated in a field study. Trained and untrained novice users of a new business phone system rated the similarity of use of nine standard features on the phone. These ratings were used to derive cognitive representations of the relations among the system features. Users' ratings of features on univariate scales about the use of the features and scores obtained on a test of procedural knowledge were related to these representations. The two novice groups' mental models were similar except for the perception of one feature; it was more accurately depicted in the trained users' representation than those who did not attend a training programme. Two types of experts' similarity of use ratings were evaluated to determine an ‘appropriate’ representation of the use of the system features; this was then compared to the novices'. This comparison suggested that there were deficiencies in the mental models of novices. Designing training programmes and instructional aids for systems using the discrepancies between novices' and experts' mental models are discussed.     

Developing and sustaining information assurance. The role of community colleges. Part 1

In 2001, articles in several technology journals underscored the shortage of qualified security professionals who understood information assurance (IA) concepts. At the time, only a handful of universities offered academic programs in IA, and those were at the masters and doctoral levels. Although a few colleges had classes that covered IA topics, no undergraduate-level programs existed. Continual training and education are necessary to manage the ever-evolving technologies of computer systems and network administration, which place increasingly heavy demands on public and private entities. New positions open frequently for qualified applicants in IA, sometimes forcing existing employees to step into the job of maintaining secure and available computer infrastructures to support their organizations. To help address the ongoing need for security training; several US community colleges have stepped up to develop academic programs over the past several years. This article presents the case for IA training at that level, setting the stage for further examination of the particular challenges that it entails.