## 1 Introduction  
Postural control can be enlisted as one of the most critical functionalities of the human body. It is fundamental to perform everyday life activities, but also for managing random perturbations coming from the interaction with the environment. The central nervous system implements different postural strategies thanks to the information provided by the somatosensory, vestibular, and visual systems. The apparatus continuously commands the contraction and relaxation of the body's muscles to correct the posture and reduce the risk of falling. In order to get objective results about postural control performance, it is possible to perform balance analyses through dedicated instrumentation and protocols. Posturographic trials are performed in static (unperturbed) and dynamic (perturbed) conditions, depending on the scope of the analyses. In general, unpredictable mechanical perturbations are applied to the body of the patient to generate a loss of balance that is objectively evaluated and correlated to the features of the stimuli. Some devices directly impart a force by pushing/pulling the subject to/from a chosen point of application [1; 2; 3; 4; 5; 6]. This approach can ease the configuration of the stimuli, however, it presents several complications regarding the control of the perturbation's magnitude and duration. Other devices exert the perturbation through sliding/tilting of the base of support (BoS) [7; 8; 9] or by means of obstacles placed on the path that disturb the patient during walking [10; 11], which translate into a disturbance to the body's center of mass (CoM). This kind of stimulation, however, does not include the possibility to replicate a rather simple and typical balance perturbation as the one defined by a collision between the patient and an external body. Moreover,it does not allow to freely configure the characteristics of the stimuli, e.g., in terms of point of application and direction. The postural response to the stimuli is measured by center of pressure (CoP) related measurements [12; 13; 14; 15], motion analysis [16; 17; 18; 19], and/or electromyography [20; 21; 22]. A posturographic trial involves the interaction between a perturbation system (i.e., the perturbator) and the human being, i.e., the patient. Moreover, the perturbation system could be directly maneuvered by another human being, i.e., the operator. Therefore, it is particularly relevant to study the complex human-machine interfaces resulting from this clinical task. Mechatronic systems dealing with the human body are typically the object of research activities in a wide set of applications, as collaborative robots (cobots) environment [23; 24] or robotics for rehabilitation [25; 26] and surgery [27]. In many applications, it is critical to keep the interaction forces below specific thresholds or to control the actual evolution of forces rising due to human-machine collisions over time. For these reasons, active control solutions based on the real-time monitoring of physical quantities from the system (i.e., the plant) are often implemented. Given a certain application, control schemes with different complexity are designed in order to achieve reasonable performance in these tasks, which shall require the ability to track desired force profiles during sudden contacts, thus assuring the safety required. In previous works from the authors [28; 29; 30], a pneumo-tronic perturbator device for dynamic posturography has been developed, aimed at providing controlled stimuli with defined amplitude and duration to the body of a patient. Based on a linear pneumatic actuator, this device has only partially allowed fulfilling the application's specifications, especially regarding the duration of the stimuli. Since it is advisable to end each stimulation before the actual balance control intervention, to reduce any overlap between active neuromuscular response and the force imparted by the device, it is particularly relevant to limit the duration approximately below 75 ms [31].