This thesis is divided into six chapters.
The first two chapters concern to the state-of-art that supports all the research required for the data processing presented in this study. Chapter 1 shows a brief historical per- spective, the objectives and motivation of the present thesis. Chapter 2 presents the re- quired theoretical background for the field of research, including basic operation of the accelerometer, its advantages and operating criteria, the composition of the acquired sig- nals and the existing machine learning techniques to be used.
Chapter 3 is related with the materials and methods used for the acquisition system adopted in this work. Other ACC databases used for testing the developed framework are referred in this section as well and represented in the following figure 1.3.
1. INTRODUCTION 1.4. Thesis Overview
Chapter 4 and 5 correspond to the results obtained in this work. In particular, chapter 4 presents a set of features similar to those used in [35] (time, statistical and spectral do- mains) and a new set of implemented features inspired on audio recognition methods, including features from the time-frequency domain.
The choice of features is a fundamental step to apply machine learning methodologies to sensor data and it influences the outcome of any approach. The proposed methodology for feature selection and the influence of several parameters such as window size and HMM application are also described.
Chapter 5 carries out all results achieved from all studies referred before in chapter 4 with clustering performance evaluation methods.
Chapter 6 the main conclusions of the present work are drawn.
Appendix B presents additional results and appendix C consists on two papers: one re- cently submitted to the 8th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2015), held in Lisbon in January 2015, and other published in Elsevier - Procedia Computer Science.
Figure 1.3: Overall structure of the framework developed in the present work for Hu- man Activity Recognition (HAR) systems. The first stage was related to the acquisition of ACC data from three databases: Foundation Champalimaud Human Activity (FCHA) database, PAMAP database and clinical database (vide chapter 3). All databases were used in all stages of the framework: feature extraction, clustering and HMM applica- tion. Feature extraction was computed with features from all four domains (statistical, time, frequency and time-frequency) whose output was used as input of the unsuper- vised learning methods (K-means, Affinity Propagation, DBSCAN and Ward). The Hid- den Markov Model was applied to the clustering results and this process is evaluated according to two performance evaluation methods: Adjust Rand Index (ARI) and HA (vide chapter 5).
2
Theoretical Background
2.1
Accelerometry and Accelerometer
Recently, the use of systems based on accelerometers to quantify and characterize human movement has increased significantly. These systems are becoming indispensable in the field of diagnosis, treatment and research by providing qualitative and quantitative data [27].
Overall, there are several types of accelerometers such as fluid, magnetic, strain gauge, piezoresistive, capacitive and piezoelectric [27], [58]. These latter three classes are more commonly applied in the classification and study of the human movement. The general and basic mechanism of the Micro-Electro-Mechanical-Systems (MEMs) will be described in the following section 2.1.1 [27].
2.1.1 The Accelerometer System Function
The underlying basis of operation for the measurement of acceleration is represented as a mass-spring system [19],[18], [27]. In MEMs sensors, the accelerometric system works accordingly to the Hooke’s and Newton’s second law, where: F = kx (eq. 1) and F = ma (eq. 2), respectively. F denotes the force (N), m is the mass of the system (kg), k is the spring constant, x is the displacement (m) and a is the acceleration (ms-2). When a com- pressive or extension force is detected due to a given movement, the spring-mass system will react to produce a proportional force to the initially imposed force [18], [27], [35]. If the system’s mass is known, as well as the spring constant, it is possible to determine the acceleration of a given movement, accordingly to its displacement by: (eq. 1)=(eq. 2).
2. THEORETICALBACKGROUND 2.1. Accelerometry and Accelerometer
Figure 2.1: Representation of the operating principle of MEMs accelerometer where k is the spring constant, x is the displacement of the system with mass m and a is the acceleration of the mass-spring system, calculated from the equations 1 and 2.
2.1.2 The Accelerometer Signal
The accelerometer signal is composed of a static component, known as the gravitational acceleration (in g), which is always present (Fig. 2.2) and qualifies the accelerometer as an inclinometer. In these conditions, an horizontal sensing axis detects acceleration around 0 g and a vertical sensing axis (aligned with the center of the Earth) can detect an accel- eration value around 1 g [27]. Therefore, the gravitational acceleration offers information concerning space orientation of the accelerometer and thus, the subject’s posture.
In addition to the gravitational component, accelerometer-based systems show inertial components, in a local coordinate system in the 3D accelerometer case. Each component or plane responds to the frequency and intensity of the movement of the subject [19], [27]. There are uniaxial, biaxial and triaxial devices, for 1D, 2D or 3D accelerometers respec- tively, and its selection depends on many factors such as the aim of the investigation and its budget.
Due to the characteristics of the accelerometer signal, calibration is an important pro- cedure and it is performed by placing the accelerometer in a known static orientation position relative to the given body under study [23]. The reference point is taken as 0 g in a free-fall situation. The acquired data from the accelerometer depends on several conditions and criteria, which will be further discussed in section 2.1.3.
2.1.3 The Accelerometer Functioning Criteria for Human Activity Analysis The body acceleration components change continuously during the movement of the subject, regardless of the accelerometer location. The output data will certainly present background noise, as the result of electronic factors, motion artefacts and others [48]. An accelerometer can detect rotational and translational accelerations during movement. The selection of the accelerometer placement becomes very important in this context in order to reduce these motion effects [14], [28], [40]. The placement of the sensor must be
2. THEORETICALBACKGROUND 2.1. Accelerometry and Accelerometer
studied in order to ensure the patients’ comfort and to assure that the study is performed as planned [35]. In some situations, where the goal is to study ‘whole body’ movements, the most used placement is close to the center of mass of the human body, for instance the sternum [28], waist [19] or lower back, in the lumbar region [41].
The task performed by the subject will produce variations in the detected accelerations. If the subject is at rest, acceleration is determined by its position according to the grav- itational direction and it is possible to determine the posture of the subject. It is also important to consider the orientation of the accelerometer relatively to the body location for better understanding and analysis of the output data.
Figure 2.2: Representation of a 1D accelerometer on the wrist and respective sensing axis (in blue) accordingly to the subject’s movement. The orange arrows represent the gravitational acceleration. In (b) the output of the sensing axis is aligned with the global horizontal and the gravitational component is null. In (a) and (c) the sensing axis is no longer perpendicular to the gravitational vector (unlike in b) and will show not just the body acceleration but a part of the gravitational component (from 0 to 1 g).
2.1.4 Advantages of Accelerometer System
The MEMs technology presents several advantages over more traditional and subjective analysis, such as:
• Relative low cost and simple to operate compared with traditional optical gait anal- ysis such as optical markers [13], [18], [19], [27]. It can be applied in real life situa- tions such as to backup rehabilitation;
• Portability [54], [58]. It can be used in different environments and infrastructures, not just in a laboratory [10], [13], [23], [27];
• Small size of the accelerometry apparatus allowing the possibility of more test movements without restrictions [13], [19], [27], [58];
• Allows direct measurement of acceleration in three dimensions, reducing error due to displacement and velocity [27], [53];
2. THEORETICALBACKGROUND 2.2. Accelerometry Signal Processing