Resumen de Development, analysis and validation of a passive system compensator of accelerations applicable to transport delicate loads
Pedro José Fernández Concellón
This thesis deals with the field of passive compensation of horizontal accelerations and introduces a new approach to transport delicate loads or patients in vehicles. Passive compensation of accelerations has been studied largely and some of its applications are found in vehicles, devices and buildings. However, here it is introduced and analysed a 3D multibody system used to transport loads capable of compensating accelerations passively at a specific rate (no necessarily at 100%, it is defined according to the requirements of the load transported in it).
Firstly, a literature review has been carried out during which different systems applied to railway transport, road transport, buildings and other devices, such as medical equipment, have been found. Although these systems share the aim of compensating accelerations passively with the system analysed in this thesis, they differ from it in the way of achieving this compensation and/or in the capabilities for compensating accelerations. This is why after reviewing the state of the art in this field, the design of this new approach has continued. In this review some problems that may arise during the road transportation of delicate loads and people have been considered for the design of the system.
Having a general design of the system, without limiting its use for a specific purpose, the system has been parameterized in order to pose a mathematical model of its equations of motion. In this stage, the dimensions of its components, their masses, inertias and the damping coefficient of its dampers have been considered.
The solids that form the 3D multibody system are considered to behave as rigid solids, can move in the tri-dimensional space, have their mass concentrated in their geometrical centre, their inertia tensors have values only in their principal axes and the dampers have been modelled characterizing only their viscous behaviour, without considering the phenomena of friction neither between them nor in the links between the solids.
The equations of motion of the system have formed a nonlinear system of second order differential algebraic equations which has been solved through the Newton-Raphson numerical integration method. On it, several modifications have been carried out in order to make that the method solved the system as efficiently as possible. It has been acted on the size of the step, on the type of selection of the step (fixed or variable) and on the formulae used to approximate the derivatives.
The results obtained with the proposed integration method have been compared against the results obtained after simulating the motion of the system in a software of dynamic analysis for multibody systems, MSc Adams. After being validated computationally the general mathematical model, it has been particularized the analysis of the 3D multibody system for its application for transporting patients in ambulances. For this application, the configuration of the system that has been analysed behaves kinematically like the articulated quadrilateral mechanism, because it is a significative example that eases the analysis of the motion of the system. The dimensions of the system and the characteristics of its dampers needed for this particular application have been obtained after applying two types of analysis. They have not considered the phenomena of friction between the parts of the system neither the transmission of vibrations among them nor among them and the vehicle. This requires a modelling of the system more complex and realistic and a specific characterization of the vehicle in which the system is going to be installed. The first analysis, through applying the Design of Experiments theory approach, has analysed the influence that two dimensional parameters and the masses of the system (because they are the parameters that can be modified most easily during experimentation) have on the motion of the base of the system (the two dimensional parameters) and on its natural mode of vibration (the two dimensional parameters and the masses).
In the second analysis, based on the results obtained with the first analysis and having into account the dimensional limitations imposed by the specific application to which it is intended to be applied, the dimensions that the system should dispose in order to achieve a specific rate of compensation of accelerations during its motion have been determined and so its damping ratio in order to obtain a vibrational response appropriate for transporting patients in a recumbent supine posture. In this point, the results obtained in previous researches have been considered in what respect to discomfort and motion sickness for people who maintain a seated posture and are exposed to different types of oscillations (pure lateral, pure roll y and fully roll-compensated lateral oscillation).
The experimental part of this research is divided into two parts that are detailed below: The first part is focused on the motion of the system. For it, two different prototypes of the 3D multibody system were built with which a experimental part has been carried out at the University of Zaragoza that has allowed to validate the theory on which the design of the system is based and on which some problems and limitations have been found during their assembly and design.
The second part has been focused on the effect that different types of oscillations have on the discomfort of people. As a study of the discomfort of people who maintain a recumbent supine posture and considering roll-compensation of lateral oscillations at different rates had not been carried out previously, a research stay was carried out at the Institute of Sound and Vibration Research at the University of Southampton for it. Continuing with the studies carried out previously for people who maintain a seated posture, additionally to consider the types of oscillations previously analysed (pure lateral, pure roll and fully roll-compensated lateral oscillation) the experiment include a type of oscillation in which the lateral acceleration was roll-compensated at a rate of 25%, all for people who maintain a recumbent supine posture, like if they were lying on a stretcher. The results of this experiment have served to determine the range of frequencies and the types of oscillations under which people are more sensitive to lateral accelerations when they maintain a recumbent supine posture. Improvements in the perception of discomfort have been found only in a limited range of frequencies over the range of frequencies studied in the experiment. This research leaves open several future lines of research. The general model that has been parameterized can incorporate more parameters that can be of interest (materials or friction, among others) and also it can consider a behaviour for the solids that form part of the system different to the rigid solid behaviour. With appropriate means, the validation of the model can be done experimentally and not only computationally. The experimentation with people may consider in addition to discomfort, the problematic of motion sickness and for both, oscillations with more rates of roll-compensation of lateral oscillation. Additionally to the application of transporting patients, this system may be of interest for the manufacturers of delicate loads (like glass or ceramic products), but a specific analysis of the design of the system for its particular application is needed.
