_Article_  
Hardware-In-the-Loop Equipment for the  
Development of an Automatic Perturbator for Clinical  
Evaluation of Human Balance Control  
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are just one of the possible challenges to balance maintenance. Unexpected impact forces to the body, as may be provided by collisions with other people and moving objects, are a frequent cause of falls in many sports activities, working conditions, and in everyday life. Balance perturbations due to body collisions elicit different postural reflexes and body reactions than surface displacements, but have been poorly investigated, possibly also due to the lack of adequate testing devices which account for the individual physical parameters of the subject body. In particular, the same perturbation can stress a human body in different ways depending on its height, weight, and inertia around affected joints. In order to achieve a standardized scalability of the perturbation, it is particularly relevant to develop techniques and systems capable of generating customized and accurate stimuli to subjects with different anthropometric characteristics. For this reason, our work has focused on the development of a mechanical perturbator device with these characteristics, which can operate autonomously or possibly in collaboration with other systems, for example based on a mobile support base, to provide even more complex patterns of perturbation, if required by the specific clinical investigation. Besides the perturbation system, posturography requires appropriate systems of sensors to detect and record the body response, consistent with the planned analysis. Typically, postural responses can be described using kinetics and/or kinematic means, as well as by the analysis of muscular activity, as detected by electromyography [12]. However, among the several measurements available, the ones related to the displacement of the Center of Pressure, defined as the instantaneous position of the mean of the base of support reactive forces to the subject's weight, are commonly used to analyze postural response both in static and in dynamic condition [5; 7; 9; 13; 14; 15; 16]. In addition, analyses based on 3-D motion capture are also performed to study the movement of the human body during posture recovery by means of optoelectronic systems [17; 18] and inertial measurement units (IMUs) [19; 20; 21; 22]. All these data may be finally correlated with the perturbation to render an objective assessment of the subject's postural control. Models of postural control systems are also currently implemented to support the study of postural reactions [2; 23; 24; 25; 26]: these are generally described by the combination of an active (neuro-muscular) and a passive (inertial and viscoelastic) response, the former being particularly affected by aging or neuro-muscular diseases. Human-machine physical interaction is one of the most relevant topics in modern robotics, being strategic in many applications, e.g., safety contacts in collaborative robots [27; 28; 29], tele-manipulation tasks [30], techniques for robot-aided rehabilitation [31; 32] or surgery [33]. Due to the different complexity of these applications, several control techniques for contact force regulation have been developed, with different performance and specifications. In all these applications, it is fundamental to monitor contacts between the machine and the human being. This can be addressed by both passive strategies (e.g., by endowing a mechanical structure with compliant elements) and active control strategies. In the latter, it is necessary to implement monitoring devices and actuators to control in real time the entity of the interaction force. Some examples of active control architectures developed for contact force regulation are hybrid force/position [34] and impedance or admittance controllers [35; 36]. In the first architectures, the system is shifted from a position reference to a force reference when a contact force is detected over a specific threshold.