## 1 Introduction  
Most underwater exploration and inspection tasks are achieved by remotely operated vehicles (ROVs), whose maneuverability and reliability are at the heart of a mission's success [1, 2]. Maintenance missions include the inspection of ship hulls, pontoons, offshore wind farms, oil platforms, risers, and seabed pipelines as well as all artificial sensing structures integrated in the oceans for monitoring the seas or conducting physics experiments, such as the neutrino telescope in the Mediterranean Sea [3]. ROVs are linked to a surface control vessel by a tether that can transmit data and even supply power if necessary [4, 5]. However, the tether is likely to apply undesirable forces on the ROV through its attachment point [6, 7], which can affect the ROV's mobility and its planned trajectory while increasing power consumption [8, 9, 10, 11]. Tether constraints are even more important for small and less powerful ROVs, which are also widely used in shallow waters. A slack, passive tether increases the risk of entanglement and seabed drag, resulting in premature wear [12, 13, 14]. To take advantage of the cables linked to underwater robots, a control system for varying cable length is required. One of the main challenges in underwater robotics is to provide robots with more navigation autonomy and maneuverability, whereas tether deployment remains under the control of a skilled human operator. In the literature, there exist three main solutions for the management of subsea tethers, namely involving tether customization/instrumentation, use of surface winches, or use of an underwater tether management system (TMS), which can be implemented by an auxiliary robot. Tethers are often customized by buoys and ballasts to change their buoyancy, shape, or behavior [7; 15; 16]. Such systems are passive, and their positive impact on cable management is limited when they are not used in conjunction with active length control. Cables can also be fitted with external or internal sensors to monitor their behavior and shape, e.g., taking measurements at several specific nodes along the cable through inertial or tension sensors [7; 17] or continuously along the entire cable through embedded fiber optic solutions [18; 19; 20]. The first solution generates an irregular shape, whereas the second can be very expensive. Surface winches are used for cable winding and unwinding and are often operated either manually or simply based on cable tension [21; 22]. They are commonly placed on the surface vessel, but they can also be embedded on the ROV itself, which involves technological challenges [15]. tether management systems (TMSs) are widely used for deep water systems [1; 23; 24; 25]. Located between the surface and the ROV, they manage the portion of the cable connected to the ROV. They are usually combined with a depressor weight to limit the transmission of heave motion from the surface vessel to the ROV via the tether. An auxiliary ROV can also play the role of a TMS [2]. It can be remotely controlled by an operator or automatically manage the shape of passive weighing cables [26]. However, this solution introduces the potential risk of collision between the robots. This paper presents a mechatronic solution for automating the deployment of a tether that links a remotely operated vehicle to the surface while limiting unwanted disturbances to the ROV. This solution is designed to be installed easily on an existing neutrally buoyant and lightweight cable. It is a low-cost solution that consists of combining two buoys and a ballast located between the buoys, and a flex sensor is mounted at the ballast. The buoy-ballast system is also built to be neutrally buoyant and behave as a compliant system in preventing the ROV from being pulled by the cable and enabling active cable management using the flex sensor. Unlike most existing solutions designed for weighing cables, the system presented in this article is original and suitable for neutrally buoyant cables, which do not carry energy.