6.B) .ASOCIACIONES EMPRESARIALES DE CENTROS ESPECIALES DE EMPLEO

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exoskeleton state of the art is also focused on the interaction between man and machine: in fact, the necessity of command signals that reflect the human intention is of fundamental importance. In this particular context, the surface electromyography (sEMG) has achieved promising results in the prosthetic and exoskeleton fields. The benefits, in this last case, are related not only to the controllability of the device but can also be extended to rehabilitation efficiency, as shown in (Hu et al. 2013). In fact, a rehabilitation workout is more effective when it involves a voluntary effort instead of simple passive movements. There are a lot of sEMG-driven exoskeletons, designed specifically with the only aim of performing slow opening and closing cycles for the hand rehabilitation (Ho et al. 2011) (Loconsole et al. 2013) (Mulas, Folgheraiter, and Gini 2005). In recent years, the exoskeleton systems go further, exploiting the characteristics of electromyography in terms of decoding the intention of movement in order to achieve an active rehabilitation, or “functional” one. Not only it permits slow hand movements, but it also allows object grasping, and more in general the interaction with the external world. Therefore, it descends that the current hand exoskeletons are eligible both for rehabilitation and assistance, i.e. permitting the user to perform activities of daily living (ADLs).

3.3 Design requirements of a rehabilitation exoskeleton device

From the previously section, after an accurate analysis of the state of the art in hand rehabilitation and empowering devices, it is possible to highlight several common requirements:

1. Limiting the range of motion to the physiological one with mechanical stops for safety;

2. Coincidence of the exoskeleton’s center of rotation with the rotational axis of the humans joints;

3. Small size; 4. Low weight;

5. Simple structure and easy attachment;

6. Actuation of the main degrees of freedom for each finger;

7. Free palm, in order to allow the interaction of the human body with the environment.

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As pointed out in the previous list, it is possible to define and explore some guidelines for the design of a new generation of rehabilitation and assistive hand exoskeleton. According with (Pons 2008) and (Iqbal and Baizid 2015) it is possible to summarize the design requirements in the following bullets:

• _{Range of motion;} • _Compactness;

• _{Energy consumption;}

• _{Comfort-ability and wear-ability;} • _{Force levels;}

• _{Intention movements control;} • _Safety.

Let’s go deeper in the details of each point:

Range of Motion (RoM): this first constraint concerns the shape and the dimension of

the workspace. For exoskeleton devices where the mechanisms are connected with human limbs, the workspace must follow as much as possible with the human range

of motion (RoM). In particular, the exoskeleton’s RoM must be at least a physiological

subset of the human one, but in any case, the RoM of the device cannot constrain the limb with accidental and unnatural movements.

Compactness: a rehabilitation or empowering exoskeleton should be compact, in order

to perform both grasps and manipulability tasks. The compactness considers structural weight and inertia of the links, and the size of the device. The device’s size is most important also from the safety point of view and for the comfort- and wear-ability.

Energy consumption: the energy consumption is a big issue for every portable device,

where the energy issue is strictly connected with the number and the size of the actuators. In rehabilitation, this issue can be partially solved by reducing the number of actuators, introducing self-balance and passive mechanisms for an optimization of the energy consumption.

Comfort-ability: exoskeletons or in general rehabilitation devices need to be

comfortable in order to be worn and used for many hours without generating pain or suffering on the users. The comfort-ability and wear-ability are also connect with the weight and dimensions of the device.

3.3 Design requirements of a rehabilitation exoskeleton device

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Force levels: in order to obtain an efficient rehabilitation, the exoskeleton must

generate a sufficient force level to allow the limb movements, without generating undesirable reaction as muscle cramps caused by fast movements. In addition, the force level must be high enough to help the user in empowering tasks.

Intention movement control: another important aspect regards the control strategy of

rehabilitation devices is the easy to use; in fact, the efficiency of the rehabilitation is conditioned by the control. In particular, the use of bio-signals, as input signals, is better than other types as it enhances the recovery of the neural network paths (Fig. 33).

Fig. 33. Path form the intention (brain) to active assistance (ReHand2) passing to intention transmission (motor neurons) and the acquisition of muscle activities (sEMG sensors).

Safety: all the previous requirements are intrinsically conditioned by the safety

requirement. For instance, in order to avoid injuries is important to reduce the weight and the inertia and provide a compliant RoM. In general, any medical device has to strictly satisfy restrictions related to risks (Bien and Stefanov 2004). In particular, ISO 14971:2012 collects the standards related to the risks management in order to determine the safety of medical devices.

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Finally, after an accurate reasoning of the design requirements, we chose to develop a hybrid exoskeleton able to perform functional rehabilitation and empowering tasks, for stroke patients (rehabilitation feature) and for elderly and SLA patients (empowering feature), aiming to recover the normal hand functionality or to help suffering people during all the activities of daily living (ADLs). In particular, we chose to exploit the principle of underactuation, already used in the prosthetic field with excellent results. The underactuation permits to coordinate multiple outputs (in our case the fingers) with only one actuator, thus with a simple control strategy, as will be described in next chapters.

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