Due to the growing demand for assistance in rehabilitation therapies for hand movements, a robotic system is proposed to mobilize the hand fingers in flexion and extension exercises. The robotic system is composed by four, type slider-crank, mechanisms that have the ability to fit the user fingers length from the index to the little finger, through the adjustment of only one link for each mechanism. The trajectory developed by each mechanism corresponds to the natural flexoextension path of each finger.
The amplitude of the rotations for metacarpophalangeal joint (MCP) and proximal interphalangeal joint (PIP) varies from 0 to 90∘ and the distal interphalangeal joint (DIP) varies from 0 to 60∘; the joint rotations are coordinated naturally. The four RRRT
mechanisms orientation allows a 15∘ abduction movement for index, ring, and little fingers. The kinematic analysis of this mechanism was developed in order to assure that the displacement speed and smooth acceleration into the desired range of motion
and the simulation results are presented. The reconfiguration of mechanisms covers about 95% of hand sizes of a group of Mexican adult population. Maximum trajectory tracking error is less than 3% in full range of movement and it can be compensated by the additional rotation of finger joints without injury to the user.
The number of people with disabilities is increasing; thus, the demand of rehabilitation services is increasing too, due to the population growth and ageing, emerging chronic diseases, and the medical advances that preserve and extend life expectancy .The World Health Organization reported “an estimated 10% of the world’s population, some 650 million people, experience some form of impairment or disability”; about 80% of people with disabilities live in developing countries. The majority are poor and experience difficulties in accessing basic health services, including rehabilitation services , an alternative to address this problem is the use of robotic systems in rehabilitation therapies. Robotic systems have already proven to enhance hand therapies through incorporating intensive and interactive exercises [2,3]. Levanon confirms that “advanced technology can enrich treatment and can help patients who cannot come to the clinic regularly for treatment” . “Disorders of the upper extremities specifically limit the independence of affected subjects”  and impairment of hand affects significantly
the execution of activities of daily living (ADL). There are injuries like fractures, sprains, and dislocations that cause temporary disability and they require mobilization exercises
as part of rehabilitation therapy . Fasoli et al. concludes that “robotic therapy may complement other treatment approaches by reducing motor impairment in persons with
moderate to severe chronic impairments” . On the other hand, Carey et al. concluded “that individuals with chronic stroke receiving intensive tracking training showed improved tracking accuracy and grasp and release function, and these improvements were accompanied by brain reorganization” . Thus, Kitago et al. stablish that there is a great need to develop new approaches to rehabilitation of the upper limb after stroke. Robotic therapy is a promising form of neurorehabilitation that can be delivered in higher doses
than conventional therapy . Additionally, rehabilitation robots also can be a platform for quantitative monitoring on the recovery process in a rehabilitation program due to the standardized experimental setup and the high repeatability of motion tasks.
Different robotic devices for upper limb rehabilitation have been developed over the past two decades to provide hand motor therapy . There are different design philosophies
applied to robotic therapies, determining the degrees of freedom considered and technologies used. The objective is to develop a training platform that helps patients regain hand range of motion and the ability to grasp objects, ultimately allowing the impaired hand to partake in activities of daily living .
In the specific case of the fingers of the hand, exoskeletons, wearable orthosis and gloves, haptic interfaces, and end effector-based devices have been developed and evaluated in order to facilitate the rehabilitation process [3, 5]. Exoskeletons are devices with a mechanical structure that mirrors the skeletal structure of the limb; that is, each segment of the limb associated with a joint movement is attached to the corresponding segment of the device. This design allows independent, concurrent, and precise control of movements
in a few limb joints. It is, however, more complex than an end-effector-based device . An example of this approach is the HEXORR, Hand EXOskeleton Rehabilitation Robot .
This device has been designed to provide full range of motion (ROM) for all of the hand’s digits. The thumb actuator allows for variable thumb plane of motion to incorporate different degrees of extension-flexion and abduction-adduction. The finger four-bar linkage is driven by a direct current, brushless motor. The mechanisms of HEXORR only have one rotation axis for all the metacarpophalangeal joints for index to little fingers, but the rotation axes of the finger joints are not collinear. This device does not consider the distal interphalangeal joins of the fingers.
Glove devices are wearable, such as the robotic glove, which utilizes soft actuators consisting ofmolded elastomeric chambers with fiber reinforcements that induce specific
bending, twisting, and extending trajectories under fluid pressurization. These soft actuators were mechanically programmed to match and support the range of motion of
individual fingers .These devices require a pneumatic or hydraulic facility, which is more complex than electric supply, especially for domestic use. The variation in hand size can be a complication for the use of these devices.
The haptic devices form another group of systems interacting with the user through the sense of touch and the mobilization of the limb. Haptic devices can be classified as
either active or passive, depending on their type of actuator.
An example of this approach is the “haptic knob” which is a two-degree-of-freedom robotic interface to train movements and force control of wrist and hand. The “haptic knob” uses an actuated parallelogram structure that presents two movable surfaces that are squeezed by the subject . This device is oriented to perform many ADL such as grasping and manipulating objects.
The advantage of the end-effector-based systems is their simpler structure and thus less complicated control algorithms.
However, it is difficult to isolate specific movements of a particular joint. The Rutgers Hand Master II is a force feedback glove powered by pneumatic pistons positioned in the palm of the hand and provides force feedback to the thumb, index, middle, and ring fingertips . The fingertips develop a linear trajectory, whose amplitude depends on the
length of the pneumatic pistons. Amadeo is a commercially available device that provides endpoint control of each of the hand digits along linear fixed trajectories electric motor . In this case, the fingertips develop a linear trajectory too.
The design of a reconfigurable robotic system proposed, Ro-Share, has advantages with respect to the devices mentioned.
First, it is designed so that each fingertip develops a natural flexoextension trajectory considering the joint coordination of each finger kinematic chain. Each of the
fingers is free to move without forcing the rotation axis alignment of its joints. Only one actuator is necessary for each mechanism that mobilizes one finger. Each mechanism can
be adjusted to the finger length by the length adjustment of its crank link. The variation of hand length can be up to 16% for a male and female adult from 18 to 90 years old specific
population .Hence, a robotic system to guide the fingertip of fingers, index, middle, ring, and little finger, in flexion and extension exercises is proposed, which must be able to fit finger sizes through only one link length adjustment.
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