## 4 Rigid Robotic Exoskeletons for Hand Rehabilitation
### Interactive Experience Features
[61] proposed a device in which an admittance control with a real-time varying admittance model is implemented to effectively support the patient's desired motion. Reversibility is another desirable property of exoskeletons. Firstly, a mechanically reversible device is efficient from the energy consumption point of view. Reversibility enhances device safety since the device can be always forced by the user in case of issues and allows the user's interaction to be detected. Reversibility can be obtained by choosing high-efficiency actuators and properly designing the transmission systems [45]. When the employed actuators do not allow this property, for instance, linear actuators, which usually have lower efficiency levels, the reversibility can be obtained by the device sensing and control system [32]. Rigid hand or finger exoskeletons are usually connected to the hand by flexible and adaptable fabric components. The actuators are usually positioned on the back of the hand [34; 44; 52] or on the forearm [25]. Usually, a rigid support for the actuators is present, connected to the hand palm or to the forearm with flexible and adaptable Velcro straps or similar systems. The parts of the exoskeleton activating the fingers are connected to the phalanges through rigid or flexible rings. In some cases, the structure is connected directly to the fingertip through a thimble [34]. * _Control._ Often, in rigid hand exoskeletons, the control system consists of a microcontroller that processes information, such as sensor data, and generates commands for the actuation unit. Additionally, an auxiliary data processing unit, for example, a tablet or a computer, accessible to both the user and specialist is used to monitor the user's activities and progress [32]. In the state-of-the-art technology, different control methods are employed in the rigid hand exoskeleton to provide various forms of rehabilitation, like active, passive, resistive, and active-assisted. The most common types of control are those based on force, position, or torque control. Other control signals can be used, such as bio-signals, voice signals [28], and simple trigger signals. Rigid hand exoskeletons mostly rely on position and force control. Meanwhile, others incorporate bio-signal-based controls. The trigger control exploits buttons to activate specific exercises for the user's rehabilitation. Usually, these exercises consist of pre-defined actions such as opening/closing the hand and grasping an object [46; 47]. Force-based control is an interactive modality that aims to create an active rehabilitation tailored to the user. In this case, rigid hand exoskeletons are actuated on the basis of the force exerted by the user. By using this approach, the users are actively engaged, allowing them to improve their hand skills throughout the rehabilitation process: the exercises employed enhance the manipulation of objects and help to increase the hand grip force. Consequently, this type of exoskeleton control is also used in active-assistive rehabilitation, based on the specific user's needs. For instance, [PERSON] et al. [61] implemented an admittance control system for a hand exoskeleton to be applied in Virtual Reality (VR)-based rehabilitation tasks, which varies the control parameters to properly render the force sensation and to adapt to the user's motion intentions. Position-based control is usually implemented by using pre-defined inputs to obtain desired exoskeleton motions. This method is preferred for the passive rehabilitation modality, where the exercises are conducted by giving an input command to the controller, which is then processed and consequently sent to the actuators, which are controlled in terms of their positions. Current research in the field of rigid hand exoskeleton indicates that PID (Proportional-Integral-Derivative) and PD controllers are predominantly employed [23; 24; 25; 27; 30; 34; 36; 41; 53]. Among the different control models, position-based control and force-based control are the most commonly used approaches [31; 32; 35; 43; 45].