## 1 Introduction  
In the case of deep boring processes, the use of high aspect ratio tools may be unavoidable. As a result, due to the tool low stiffness, unstable vibrations (i.e., chatter) [1] may occur compromising surface quality and tool life. Chatter onset depends on (i) system dynamics (i.e., tool Frequency Response Function); (ii) cutting parameters (i.e., radial depth of cut, spindle speed and feed per revolution); (iii) tool insert geometry; (iv) tool and workpiece material. As presented in [2], two different strategies may be used to ensure a stable machining: exploiting the lobbing effects, limiting stable and unstable cutting conditions (by means or out-of-process or in-process strategy) or changing the process/system dynamic behavior. In the out-of-process methods, chatter prediction models [3] are used to detect cutting parameters that gives a stable machining. However, these approaches require several inputs, such as the dynamic behavior of the tooling system and accurate cutting force model. On the contrary, in the in-of-process methods, cutting parameters (usually spindle speed [4]) are corrected during machining, as soon as chatter is detected. However, exploiting the lobbing effect is not effective in cases of high compliance tools, where stability field is limited and stable cutting parameters are associated to low productivity levels. Hence, increasing the limits by changing the system or process behavior is preferred. The process stability can be modified by dedicated cooling strategy (e.g., cryogenic [5]), which affects the mechanism of chip formation,and hence, could stop or release vibrations. However, these techniques depend on the material to be machined. Therefore, it is generally preferable to improve the tooling system dynamics by means of passive or active techniques. Passive techniques employ devices that, once mounted on a structure, modify its dynamic response. In particular, if correctly designed (i.e., tuned), they can reduce structure peak amplitude and, in case of machining process, increase chatter stability limits [6]. Even if these devices, compared to active ones, are cheaper and do not require external power, their effect on the system response is limited to a short frequency range. This implies that dynamic characterization of the component, on which passive devices will be installed, is required to properly design the device. Moreover, the same device is not effective for different configurations [7]. Active approaches require a more complex architecture able to monitor, diagnose and act on the process in order to suppress chatter vibration. Amplitude vibration is continuously acquired by sensors and the signal is processed in real time to detect chatter onset. Control logic, properly designed and implemented, receives the signal and drives the actuators in order to contrast chatter. Despite the complexity, these approaches are more suitable for industrial application, since they are able to mitigate chatter in different configurations (e.g., different tool set-up, different cutting parameters). The design of an active boring bar involves the design of monitoring system (sensors), actuation system and control logic. Up to now, research works has mainly focused on the development and implementation of different control logics, such as optimal control [8], direct position feedback [9] and direct velocity feedback (DVF) [10; 11]. Among all, DVF is the most common and mature control logic, since it was demonstrated to be a robust approach [12] and its effectiveness was proved in several applications for chatter mitigation [13; 14]. Beside control logic, active strategies effectiveness is strongly related to the inertia of the applied sensors and actuators. However, no work was dedicated to the design of the structure of such devices. Indeed, few design solutions were presented in literature focusing only on building prototypes to demonstrate control logic effectiveness.