Occupational exoskeletons are designed to reduce workplace injury risk by decreasing work demands. Due to their relatively recent development, there has been limited research into potential unintended and undesirable consequences of wearing them. The goal of this thesis was to investigate the effects of exoskeleton use on reactive balance in response to simulated slips and trips. Five representative exoskeletons were investigated including leg-, back, and shoulder-support exoskeletons. This thesis consists of two studies: a smaller study investigating one exoskeleton and a larger one investigating multiple exoskeletons. Participants stood on a specialized treadmill, then abruptly and unexpectedly changing treadmill belt speed to simulate trip-like forward losses of balance or slip-like backward losses of balance. The results of the first study showed that a passive leg-support exoskeleton adversely reactive balance for both slips and trips. The results of the second study showed that back-support exoskeletons had a greater adverse effect on reactive balance compared to shoulder-support exoskeletons for both slips and trips. These exoskeletons affected reactive balance due to their interaction with stepping kinematics and movement constraints. This thesis provides important information that can be used to warn users of potential increased fall risks and inform exoskeleton manufacturers who may be able to modify designs to reduce any additional fall risk. / Master of Science / Occupational exoskeletons reduce muscle workload for workers during manual tasks. However, because of their additional weight and how they restrict movement, they can increase the risk of falling after a slip or a trip. The goal of this thesis was to see how exoskeletons affect balance after simulated slips and trips. Five exoskeletons were studied; These exoskeletons supported the legs, back, and shoulders. This thesis includes two studies: a smaller study with one exoskeleton and a larger one with multiple exoskeletons. In order to simulate a slip and trip, participants stood on a treadmill and then the treads would unexpectedly accelerate to a speed to make them lose their balance. The results of the first study showed that an exoskeleton that supported the legs negatively affected balance for both slips and trips. The results of the second study showed that exoskeletons that supported the back negatively affected balance more than those that supported the shoulders for both slips and trips. These exoskeletons affected balance due to them interacting with the legs and affected stepping. This thesis provides important information that can be used to warn workers of potential increased fall risks and inform exoskeleton manufacturers who may be able to help reduce any fall risk.

This thesis studies the present methods used to predict the ultimate tripping strength of stiffened panels under compressive axial stress. The current methods involve the use of a bifurcation, or eigenvalue, approach to predicting failure stress. The effects of initial transverse eccentricity of the stiffener are ignored using such a method. Six panels were modeled and tested with ABAQUS, a finite element software package, and the results were compared to output from ULSAP, a closed-form ultimate strength analysis program. The ultimate strengths predicted by ABAQUS changed with the influence of initial deflection of the stiffener flange, while the results from ULSAP did not change. This thesis attempts to use beam-column analysis on the imperfect stiffener flange to predict the tripping strength. It was determined that the procedure presented in this thesis does not accurately model the true failure mode of stiffeners in tripping. The resulting ultimate strengths are extremely conservative and neglect the importance of the stiffener web's role in tripping. Future work is recommended to expand on these findings and to incorporate the influence of the stiffener web into a beam-column solution. / Master of Science

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Fall related injuries are a major public health concern due to their high associated medical costs and negative impact on quality of life. Obese and older adults are reported to fall more frequently than their normal-weight and young counterparts. To help identify potential mechanisms of these falls the purpose of the research within this dissertation was to investigate the effects of obesity and age on the likelihood of tripping and subsequent balance recovery. Four experimental studies were conducted. The purpose of the first study was to investigate the effects of obesity, age and gender on the likelihood of tripping during level walking. Likelihood of tripping was assessed with median minimum foot clearance (MFC) and MFC interquartile range (IQR). Obesity did not increase the likelihood of tripping suggesting the increased rate of falls among obese adults is not likely due to a greater likelihood of tripping over an unseen obstacle. Additional results suggested females and individuals of shorter stature have an increased likelihood of tripping compared to their male and taller counterparts. The purpose of the second study was two-fold. First, the effects of load carriage and ramp walking on the likelihood of tripping were investigated, followed by investigating the effects of age and obesity on the likelihood of tripping during load carriage and ramp walking. Again, likelihood of tripping was assessed with median MFC and MFC IQR. Load carriage increased the likelihood of tripping during both level and ramp walking and obesity and age increased the likelihood of tripping during selected combinations of load carriage and/or ramp walking. These results suggest that the increased rate of falls during load carriage and the increased rate of falls among obese and older adult workers reported elsewhere may be due in part to an increased likelihood of tripping. The third study proposed a new method for investigating the likelihood of tripping as a function of obstacle height. The proposed method aimed to clear up ambiguous results often encountered when using MFC central tendency and variability to quantify likelihood of tripping. The method used trip probability curves and a statistical bootstrapping technique to compare trip probability at specific obstacle heights between groups of interest. An additional benefit of this method was that it was able to identify effects of factors not identifiable by the commonly used ANOVA analysis using MFC central tendency and variability. The purpose of the fourth study was to investigate the effects of obesity, age and gender on balance recovery following a lab induced trip perturbation. Measures of balance recovery included fall rate, stepping strategy and characteristics, and trunk kinematics. Obese, older, and female adults fell more frequently after tripping and this higher fall rate may help explain the higher fall rates among obese, older and female adults reported elsewhere. Failed recoveries were associated with higher peak trunk angles and angular velocities in addition to the use of a lowering strategy. Obese, older, and female adults had higher peak trunk angles and angular velocities and older adults and females used lowering strategies more often. These alterations in trunk kinematics and stepping strategy may have contributed to the higher fall rate among these individuals. / Ph. D.

This thesis derives the proper load shedding scheme for a distribution system with wind power generating units to maintain the power supply reliability for the islanding operation of micro grid systems after fault disturbance. The comparison of operation performance and control scheme between the fixed speed and the variable speed wind power generators are made. The seasonal wind power energy by the wind turbine is calculated by applying the exponential rate (Power Law) and Weibull possibility distribution model with the actual minutely wind speed data in Hengchun and Penghu area in 2005. The mean values of seasonal wind power output and standard deviation are determined for the design of load shedding scheme for the islanding operation of the distribution feeder. Moreover, a practical distribution feeder BX31 in Fengshan District in Taiwan Power Company (TPC) is selected for the computer simulation of micro grid systems. The systems voltage and power variation of the distribution feeder are investigated when the wind turbine is connected to the feeder by considering the weekday and weekend load models. The voltage sag on the distribution feeder and transient stability of wind turbine are simulated for the fault contingency with three-phase short-circuit fault occurred on the feeder outlet. Consequently, synchronous condensers are connected and the blade angle of wind turbines is then adjusted to improve the transient response of output characteristic of wind turbine. It is found that the power supply reliability of micro grid with wind power generations will be deteriorated following the tripping of wind turbines due to the oscillation of terminal voltage introduced by long fault clearing time. To solve the problem, the proper design of the tripping for the micro grid systems with wind turbines is derived according to the transient stability analysis. To restore the systems stability of the islanding systems, the proper amount of load shedding is determined and the switching of control modes of the wind turbine is executed, according to the power mismatch between wind turbine generation and load demand of the distribution feeder.

The distance required for flow entering a laboratory channel to become fully-developed and uniform can be substantial. Given the need to establish fully-developed uniform flow, if the length of a laboratory channel is not substantial then it likely that the flume cannot be used to conduct open-channel flow research. In laboratory studies where the channel bed is hydraulically rough, the noted problem can be lessened by minimizing the length over which the flow becomes fully-developed and uniform (Lunif). For this study it is hypothesized that if bed material with a roughness height (ks, ∆) is placed at the channel entrance and ks, ∆ is greater has the roughness height of bed material placed throughout the channel (ks, bed) then Lunif can be reduced. The length over which the larger bed material is referred to as the tripping zone length (∆). A second hypothesis for this study is that if ∆ is longer, then Lunif will be shorter. The primary objective of this study is to test the above mentioned hypothesis and to develop a relationship for predicting Lunif as a function of Δ. For this study, physical tests were performed in a rectangular Plexiglas flume with a variable slope. The flume was 6.1 m long, 45.7 cm wide, and 45.7 cm deep. The channel has smooth walls and the bed was lined with gravel (median particle size, d50 = 8.5 mm or 22 mm). Similarly tripping zone was lined with gravel of larger size (median particle size, d50 = 13 mm or 58 mm).Twelve tests were conducted for the study. For each test, longitudinal point velocity measurements (u) were made along the channel center, at five elevations (z), and at twelve longitudinal stations (x). An Acoustic Doppler Velocimeter was used to measure u. Lunif was determined by considering four indications of flow uniformity. Results indicate that having a tripping zone decreases Lunif and the magnitude of the decrease in Lunif was dependent on ∆. A function is presented for predicting Lunif /H = f (Rep, Fr, and Δ/H) where Rep is the Reynold's particle number, Fr is the Froude number and H is the flow depth.

yes / Trips over obstacles are one of the main causes of falling in older adults, with vision playing an important role in successful obstacle negotiation. We determined whether a horizontal-vertical illusion, superimposed onto low-height obstacles to create a perceived increase in obstacle height, increased foot clearances during obstacle negotiation thus reducing the likelihood of tripping. Eleven adults (mean ± 1 SD: age 27.3 ± 5.1 years) negotiated obstacles of varying heights (3, 5, 7 cm) with four different appearance conditions; two were obstacles with a horizontal-vertical illusion (vertical stripes of different thickness) superimposed on the front, one was a plain obstacle and the fourth a plain obstacle with a horizontal black line painted on the top edge. Foot clearance parameters were compared across conditions. Both illusions led to a significant increase in foot clearance when crossing the obstacle, compared to the plain condition, irrespective of obstacle height. Superimposing a horizontal-vertical illusion onto low-height obstacles can increase foot clearance, and its use on the floor section of a double-glazing door frame for example may reduce the incidence of tripping in the home.

Due to the consideration of economic dispatch or system operation, the regional severe unbalance between power generation and load demand will introduce large amount of power flow over the inter-area tie lines. The tripping of transmission lines due to power system fault contingency will result in the increase of power flow over the inter-area tie lines, which may violate the constraint of transient stability. The whole power system could be collapsed due to further tripping of more generators unless proper remedy actions are executed in tie. This thesis proposes an effective approach to determine the generators to be tripped for power system protection. All of the generators in the power system are divided into critical group and non-critical group based on the methods of Energy-gap and Impedance-gap. The power flow capability limit of transmission lines on inter-areas have been derived by transient stability analysis. Once fault contingency occurs on the power system, the power flow of transmission lines on inter-area is compared to the above transmission limit to determine the generators in the critical group to be tripped. By this way, the stable operation of power system can be maintained after the contingency. To demonstrate the effectiveness of the proposed methodology, the Taipower system is selected for computer simulation to verify the generator tripping by system transient stability analysis. Different scenarios of system fault contingencies on primary EHV substations with various power flow conditions over the inter-area tie lines have been investigated to confirm the power system performance by applying the proposed generator tripping.

The World Health Organization claims that there are more than 285 million blind and visually impaired people in the world. In the US, 25 million Americans suffer from total or partial vision loss. As a result of their impairment, they struggle with mobility problems, especially the risk of falling. According to the National Council On Aging, falls are among the primary causes for fatal injury and they are the most common cause of non-fatal trauma-related hospital admissions among older adults. Visibility, an organization that helps visually impaired people, reports that people with visual impairments are twice as likely to fall as their sighted counterparts. The Centers for Disease Control and Prevention reported that 2.5 million American adults were treated for fall-related injuries in 2013, leading to over 800,000 hospitalizations and over 27,000 deaths. The total cost of fall injuries in the United States in 2013 was $31 billion, and the financial total is expected to rise to $67.7 billion by 2020. Reducing the amount of these unexpected hospital visits saves money and expands the quality of life for the affected population. Technology has completely revolutionized how nowadays activities are conducted and how var- ious tasks are accomplished, and mobile devices are at the center of this paradigm shift. According to the Pew Research Center, 64% of American adults own a smartphone currently, and this number is trending upward. Mobile computing devices have evolved to include a plethora of data sensors that can be manipulated to create solutions for humanity, including fall prevention. Fall prevention is an area of research that focuses on strengthening safety in order to prevent falls from occurring. Many fall prevention systems use sensing devices to measure the likelihood of a fall. Sensor data are usually processed using computer vision, data mining, and machine learning techniques. This work pertains to the implementation of a smartphone-based fall prevention system for the elderly and visually impaired. The system consists of two modules: fall prevention and fall detection. Fall prevention is in charge of identifying tripping hazards in the user’s surroundings. Fall detection is in charge of detecting when falls happen and alerting a person of interest. The proposed system is challenged by multiple problems: it has to run in near real time, it has to run efficiently in a smartphone hardware, it has to process structured and unstructured environments, and many more related to image analysis (occlusion, motion blur, computational complexity, etc). The fall prevention module is divided into three parts, floor detection, object-on-floor detection, and distance estimation. The evaluation process of the best approach for floor detection achieved an accuracy of 92%, a precision of 88%, and a recall of 92%. The evaluation process of the best approach for object-on-floor detection achieved an accuracy of 90%, a precision of 56%, and a recall of 78%. The evaluation process of the best approach for distance estimation achieved a MSE error of 0.45 meters. The fall detection module is approached from two perspectives, using inertial measuring units (IMU) embedded in today’s smartphones, and using a 2D camera. The evaluation process of the solution using IMUs achieved an accuracy of 83%, a precision of 89%, and a recall of 58.2%. The evaluation process of the solution that uses a 2D camera achieved an accuracy of 85.37% and a recall of 70.97%.

In recent years, the world has seen a significant growth in energy requirements. To meet this requirement and also driven by environmental issues with conventional power plants, engineers and consumers have started a growing trend in the deployment of distributed renewable power plants such as photovoltaic (PV) power plants and wind turbines. The introduction of distributed generation pose some serious issues for power system protection and control engineers. One of the major challenges are power system protection. Conventional distribution power systems take on a radial topology, with current flowing from the substation to the loads, yielded unidirectional power flow. With the addition of distributed generation, power flow and fault current are becoming bi-directional. This causes loss of coordination between conventional overcurrent protection devices. Adding power sources downstream of protection devices might also cause the upstream protection device to be blinded from faults. Conventional overcurrent protection is mainly based on the fault levels at specific points along the network. By adding renewable sources, the fault levels increase and become dynamic, based on weather conditions. In this dissertation, power system faults are modelled with sequence components and simulated with Digsilent PowerFactory power system software. The modeling of several faults under varying power system parameters are combined with different photovoltaic penetration levels to establish a framework under which protection challenges can be better defined and understood. Understanding the effects of distributed generation on three phase power systems are simplified by modeling power systems with sequence networks. The models for asymmetrical faults shows the limited affect which distributed generation has on power system protection. The ability of inverter based distributed generators to provide active control of phase current, irrespective of unbalanced voltage occurring in the network limits their influence during asymmetrical faults. Based on this unique ability of inverter based distributed generators (of which PV energy sources are the main type), solutions are proposed to mitigate or prevent the occurrence of loss of protection under increasing penetration levels of distributed generation. The solutions include using zero and negative sequence overcurrent protection, and adapting the undervoltage disconnection time of distributed generators based on the unique network parameters where it is used. Repeating the simulations after integrating the proposed solutions show improved results and better protection coordination under high penetration levels of PV based distributed generation. / Dissertation (MEng)--University of Pretoria, 2017. / Electrical, Electronic and Computer Engineering / MEng / Unrestricted