Achieving stability of an IP has become a common engineering challenge for researchers and the problem has been discussed theoretically by several authors [1�C5] and experimentally demonstrated by others [4�C7].There are Site URL List 1|]# many examples of the IP model, both man made and found in the natural world. In control theory the challenge of control made the IP system a classic tool in control laboratories [8�C12]. The balancing of an IP by moving a cart along a horizontal track is a classic problem in the area of control. Usually one of the state variables of the system which is also the controlled variable, the sway angle, is evaluated from expensive measurement system (e.g., encoder) placed at the pivot joint.
The main limitation of this approach is that placing the encoder at the pivot is not always possible.
Arguably the most prevalent example of an IP is a human being. A standing human looks like an IP with the center of mass well above the ground [13�C15]. The mechanism to keep balance during the standing posture has intrigued scientists from several fields for a long time  and has much significance in clinics. Several authors focused on the estimation Dacomitinib of the sway angle since this measurement is related to oscillation of the center of mass of a subject and to the neural control of posture during perturbed and unperturbed stance. However, recent studies [16,17] focused on the contribution of hip and knee to balance control.
Given the implications that non-rigidity at the knee and hip may have Brefeldin_A for the postural control during unperturbed stance, a comprehensive analysis of the ankle, knee and hip movements is required.
The analysis revealed that the one-segment IP model is an oversimplification of reality. A multilink model which takes into account thigh, shank and arm-trunk-head segments should be adopted to have a more accurate description of standing balance. From this perspective, the IP model plays the role of the basic element of a multilink chain [18,19]. Kinematics analysis of a multi-link chain should be analyzed using the framework of multiple IPs .In the last years, sensing hardware developments have made available on the market miniaturized inertial (accelerometers and/or gyroscopes) and magnetic sensors.
These sensors have found applications in robotics and biomechanics because of their low cost, small size and weight, low power consumption, ease of use and portability. The problem of accurate tracking of orientation by means of these sensors has thus become important in several domains since these wearable sensors can be considered the most valuable opportunity to monitor kinematics and dynamics of human subjects outside specialized laboratories.