UNIVERSITA’ DI PISA
Scuola di Ingegneria
Corso di Laurea Magistrale in Ingegneria Biomedica
Tesi di Laurea
Design and Development of a Hand Exosuit for
Rehabilitation and Assistance in Spinal Cord Injuries
Relatori
Prof. Marco Controzzi Prof. Antonio Frisoli Dott. Daniele Leonardis
Candidato
Tommaso BagneschiThe dedication of this thesis is split seven ways: to my mom, to my dad, to Beatrice, to Vera, to Omar, to Minou, and to you, if you have stuck with Tommy until the very end.
SUMMARY
This thesis presents a novel hand exosuit for rehabilitation and assistance in Spinal Cord Injuries (SCI). The developed prototype was designed with three distinctive features: a) the actuation of the exoskeleton is based on four-tendon model, which simulate the four groups of tendons inside fingers; b) all its components are compact on the hand dorsum without taking up space on the patient's forearm, this allows the portability of the device and makes it suitable for personal patient assistance; c) its finger motion units are composed of a series of flexible open rings and a thimble on the fingertips all crossed by cables for the actuation, this makes the units modular and adjustable according to the length of the patient's fingers. The design and development were carried out at the PERCRO laboratories of the Scuola Superiore Sant'Anna. A characterization of the prototype with healthy participants has been performed in the laboratory on the basis of grasping force and grasping pressure. Then, a preliminary experimentation was carried out at the Careggi Hospital in Florence with some patients suffering from SCI with motor disabilities in the hands.
CONTENTS
SUMMARY 1
-CONTENTS 2
-CHAPTER 1: INTRODUCTION 4
-1.1 Spinal Cord Injury and Rehabilitation 4 -1.1.1 The hand and the Spinal Cord Injury 4
-1.1.1 Rehabilitation and robotics 5
-1.2 Design requirements for hand exoskeletons 6
-1.2.1 Rigid link-based exoskeletons 9
-1.2.2 Hand soft exoskeletons 11
-1.2.3 Haptic exoskeletons 14
-1.3 Objectives of the thesis 15
-CHAPTER 2: APPROACH 17
-2.1 Cables 19
-2.2 Fingers unit 21
-2.3 Routing 26
-2.4 Control 27
-2.4.1 Servo motor characterization 29
-2.5 Sensing objects 30
-CHAPTER 3: FIRST PROTOTYPE 32
-3.2 Tests on healthy subjects 36
-3.3 Results and discussion 38
-CHAPTER 4: SECOND PROTOTYPE 42
-4.1 Actuation 42
-4.1.1 Components 43
-4.1.2 Assembly 44
-4.2 Palm and Dorsum Support 46
-4.2.1 Routing 48
-4.3 Other improvements 49
-4.3.1 Finger unit and cable adjusting 49
-4.3.2 Flexible wristband 50
-4.4 Control 52
-4.4.1 Servo motor characterization 53
-4.5 Tests with healthy subjects 55
-4.5.1 Results and discussion 59
-4.6 Preliminary test with subjects with Spinal Cord Injuries 64
-CHAPTER 5: DISCUSSION AND CONCLUSIONS 69
-LIST OF TABLES 76
-LIST OF FIGURES 76
-REFERENCES 84
-CHAPTER 1: INTRODUCTION
1.1 Spinal Cord Injury and Rehabilitation
1.1.1 The hand and the Spinal Cord Injury
The hand is very complex both from a mechanical point of view and in terms of neural connections. It is composed by 15 bones for the fingers and the wrist bones units, 20 Degrees of Freedom (DoF) and more than 25 muscles (Mcmath, Mitchell, and Taylor 1955)(Buchholz and Armstrong 1991). Even though the hand comprises only 1% of our body weight, about 30% of our central nervous systems (CNS) capacity is related to its control (Hofer et al. 2019). It also has a big relevance for sense of touch. This sense and the mobility of the joints contribute to hand dexterity and haptic capabilities (Robles-de-la-torre 2006). The hands are involved in four out of six Activities of Daily Living (ADLs) (Lawton and Brody 1969), so they are one of the most important organs for self-maintenance.
The pathologies related to mobility and hand use are various. One of these is the Spinal Cord Injury (SCI). This type of injury affects the proper communication between the central and peripheral nervous systems. Depending on the level of the lesion, the patient may lose the full part of the use and strength of the lower and upper limbs and consequently also of the hands. Causes include motor vehicle accidents (36–48%), violence (5–29%), falls (17– 21%), and recreational activities (7–16%) (Spinal cord injury facts and figures at a glance 2014). Most individuals regain motor function in the first six months after injury, but only a small proportion regain functional strength below the injury level (Stauffer 1984), (McDonald and Sadowsky 2002). The goal of research in this area is to improve the style
and quality of life for these people. People subject to SCI have different life priorities according to the type of their injury (Simpson et al. 2012). When considering patients who have suffered from high level spinal cord injury and suffer from tetraplegia, their priority is the functional recovery of the upper limbs and hands, so as to gain more independence to execute ADLs (Anderson 2004), (Wagner et al. 2007).
1.1.1 Rehabilitation and robotics
Physical rehabilitation is indispensable for the treatment of patients with physical or neurological disabilities (Kwakkel et al. 2004). Such a therapy mostly focuses on increasing the effective range of motion of the impaired joints, and repeating ADLs. Traditional rehabilitation therapy approaches need the active presence of an expert operator, who guides the patient during each movement, providing the right support and loads. A more innovative approach is the use of robots for assisted therapy. The advantages are various including the possibility of objective and large-scale measurements, less physical effort from therapists, fewer therapists needed, increased patient independence, patients more involved in the exercise (e.g. exercises with a purpose), customized therapy on patient. The use of exoskeletons is an excellent non-invasive solution for rehabilitation. Studies have demonstrated the effectiveness of the use of robotic devices for the upper limbs in stroke rehabilitation, particularly on motor learning and motor control and strength (Pollock et al. 2014), (Kwakkel, Kollen, and Krebs 2008a). Kwakkel et al. has demonstrated that a robot-assisted therapy has induced a significant improvement in the upper limb motor functions (Kwakkel, Kollen, and Krebs 2008b). An exoskeletal robot can be used in a functional rehabilitation combined with virtual reality in order to improve motor performance of the hand (Sgherri et al. 2017). Technologies such as Brain Computer Interface (BCI) could restore more effective motor control for people after stroke or other traumatic brain
disorders, helping to guide activity-dependent brain plasticity with the use of EEG brain signals to indicate the patient's current state of brain activity and to allow the user to lower abnormal activity later. Alternatively, by using brain signals to supplement the control of compromised muscles, BCIs could increase the effectiveness of a rehabilitation protocol and thus improve muscle control for the patient (Daly and Wolpaw 2008).
The use of a robotic device allows to perform active and highly repetitive movements. The improvement in motor recovery compared to traditional therapy has been shown (Barreca et al. 2003), (M. Barsotti et al. 2015), (Michele Barsotti et al. 2017). Therefore, hand exoskeletons can be also used in bimanual rehabilitation sessions, based on coordination of the two upper limbs (Leonardis et al. 2015). The training of patients with two-handed tasks improves the efficiency of grasping movements on the damaged side (S. M. Waller et al. n.d.) with changes accompanied by a reorganization of brain mapping on the affected hemisphere. Evidence indicates that simultaneous movement of both limbs helps the neuro-muscular system regain some stability and improve the use of the damaged limb (S. M. C. Waller and Whitall 2008), even in grasping tasks (S. M. Waller et al. n.d.).
1.2 Design requirements for hand exoskeletons
The clinical exoskeleton adapts to the human anatomy in order to maintain kinematic compatibility during motion promoting physiological movements. Due to the complexity of the human musculoskeletal system and intra/intersubjects variability, powered exoskeletons are prone to human–robot misalignments. These induce undesired interaction forces that may compromise safe operation (Zanotto et al. 2015), (Tong, Ho, Pang, Hu, Tam, Fung, Wei, Chen, et al. 2010). Hand exoskeletons can be divided into three macro-categories:
rehabilitation exoskeletons, assistive exoskeletons and haptic exoskeletons (Sarac, Solazzi, and Frisoli 2019).
(a) (b) (c)
Figure 1.1 Generic hand exoskeletons for different applications: (a) rehabilitation (Tong, Ho, Pang, Hu,
Tam, Fung, Wei, Chen, et al. 2010), (b) assistive (Gasser et al. 2017a) and (c) haptic (Choi et al. 2016) Rehabilitation exoskeletons are designed to treat disabilities in a clinical environment (Figure 1.1A). They are suitable for repetitive tasks, such as opening and closing the hand, mimicking ADLs (Tong, Ho, Pang, Hu, Tam, Fung, Wei, and Chen 2010). Earlier rehabilitation exoskeletons repeatedly move the patient's fingers while patients sit passively.
Some models of rehabilitation exoskeleton can interact with a virtual environment to involve patients in serious games. Serious games are games designed for a primary purpose other than pure entertainment (Djaouti, Alvarez, and Jessel n.d.). In the rehabilitation area, the patients are asked to play task-specific videogames helped by the rehabilitation robot. It has been showed that games contribute to increase motivation in rehabilitation sessions, which is the major problem in therapy sessions, caused by the repetitive nature of exercises (Krichevets et al. 1995), (Burke et al. 2009). If patients interact with virtual reality, they do not need the palm of their hand to be free. This means that the palm area can be occupied by kinematic structures to move the fingers. Instead, if patients interact with real objects, this space must remain free.
These exoskeletons allow patients to apply high output forces, to monitor fingers position and to evaluate the performance, so they are preferred to interact with real objects. They are also adaptable to the type of task to be performed even if patients with severe disabilities do not benefit from them due to the loss of isolated individual finger movement (Welmer, Holmqvist, and Sommerfeld 2008), (Sarac, Solazzi, and Frisoli 2019).
Assistive exoskeletons (Figure 1.1B) are designed to assist ADLs patients in their daily life, not necessarily in a clinical environment (Gasser et al. 2017b). They have a strong psychological impact on the patient who regains a part of autonomy for simple but essential tasks such as holding a glass of water (Anderson 2004). Unlike earlier rehabilitative exoskeletons, patients actively and voluntarily move their hands and the exoskeleton assists the movement by directing it and increasing the force generated by the hand. So, the exoskeleton increases the grasp force on a real object. For this reason, they often need a palm support to interact with it. They are portable and easily wearable and adjustable to perform different tasks. They generate less force than rehabilitation exoskeletons, but enough to grasp and hold real objects, while rehabilitation exoskeletons focus on the opening and closing movement. (Sarac, Solazzi, and Frisoli 2019). Clinical studies revealed the positive impact of realism and active participation of the patient on motor learning (Hubbard et al. 2009), (Bayona et al. 2005), and motivated the designers to fuse rehabilitation and assistive exoskeletons to let users interact with real objects during physical therapy.
The design of the exoskeletons varies according to different choices. A first item to decide is the number of how many fingers and degrees of freedom have to be controlled independently. Exoskeletons for studying finger movement can move even a single finger (e.g. thumb finger exoskeletons (Aubin, Sallum, Walsh, and Correia 2013), (Maeder-York et al. 2014) or 2-fingered exoskeletons (Cempini et al. 2013)). Burton ed al. have designed
a link-based rehabilitation exoskeleton moving all five fingers independently (Burton et al. 2011); Hasegawa et al. have developed an assistive exoskeleton controlled by bioelectric potentials that supports wrist and hand activity by moving all five fingers (Hasegawa et al. 2008). Most studies focus on developing 3-finger exoskeletons. Exoskeletons that move three fingers, typically thumb, index and middle finger, are easier to draw, develop and control, and most ADLs involve only these three fingers (Sarac, Solazzi, and Frisoli 2019).
Another feature to decide is how many DoFs to assign to fingers. Fiorilla et al. have developed an exoskeleton with a 1 DoF mechanism that only repeatedly flex/extends the metacarpophalangeal joint (Fiorilla et al. 2009), but common ADLs need fingers with more than one DoF. For this reason, in the literature we find exoskeletons that individually control multiple joints of the fingers with 2 DoF (Aubin, Sallum, Walsh, Stirling, et al. 2013), 3 DoF (Jones et al. 2010), 4 DoF (Polotto et al. 2012) mobility. As said before, human fingers have 4 DoFs. An exoskeleton that gives the hand all 20 DoF increases mobility but also the complexity of the design both in terms of control and space occupied by each component. For this reason, decisions are often made to simplify the model. A first simplification is to leave finger abduction/adduction passive (Agarwal et al. 2013) or negligible at all; another is to mechanically couple the joints between the proximal and intermediate phalanx and the intermediate and distal phalanx to the metacarpophalangeal joint (Yamaura et al. 2009).
1.2.1 Rigid link-based exoskeletons
Most of the developed rehabilitation exoskeletons are rigid link-based devices. These types of exoskeletons use mechanical links to form the finger components and interacts with user’s finger to move finger joints. They can have independent control thanks to individual actuators for each assisted finger joint (Figure 1.2A), (Agarwal et al. 2013). These types of
actuators are mainly remote to reduce space. By mechanically coupling the joints as mentioned before, the device can be simplified by interacting with the user's finger in multiple points moves all joints together, so that one finger can be controlled with only one actuator (Figure 1.2B), (Troncossi et al. 2012). Another type of rigid exoskeletons interacts only with the user's fingertip from a single point and control only the fingertip regardless how the joints move (Figure 1.2C), (Sun, Miao, and Li 2009).
(a) (b) (c)
Figure 1.2 Examples of rigid link-based exoskeletons: (a)independent control device (Agarwal et al. 2013); (b) coupled linkage device (Troncossi et al. 2012); (c) fingertip linkage device (Sun, Miao, and Li 2009).
Rigid link-based exoskeletons are able to apply considerable force to the fingers. They have high repeatability and precise control, but to be effective links and joints must be correctly aligned to the patient's hand. If there is a human-robot misalignment, as explained above, and therefore the joints are not aligned and are not compatible with the kinematics of the hand on which they are mounted, the robot generates interaction forces with the wrong hand and moves without moving the fingers correctly. Therefore, correct alignment often requires the constant presence of a technician during dressing or a specially developed exoskeleton for a single patient. Novel exoskeletons have an intrinsic adaptation of the rigid kinematics parallel to the fingers of the hand, but to work well they must be attached even
better to the fingers. In general a rigid link-based exoskeleton structure is cumbersome and difficult to adapt to different hands, moreover these exoskeletons are expensive and not very affordable.(Sarac, Solazzi, and Frisoli 2019).
1.2.2 Hand soft exoskeletons
(a) (b)
Figure 1.3 Examples of soft exoskeleton actuation: (a) pneumatic actuation (Yap et al. 2015); (b) cabled actuation (Decker and Kim 2017).
Soft exoskeletons have been developed to overcome the limits of rigid robots. They consist of flexible gloves for assistance and rehabilitation that assist the movement of the fingers, actuated pneumatically or by cables. The materials they are made of are different: they vary from fabric gloves (Decker and Kim 2017) to polymer-based gloves (Kang et al. 2019) to open gloves (Takahashi, Furuya, and Koike 2020), so they offer a particular wearing comfort that is not provided by other rigid exoskeletons. Having no well-defined joints and links, they do not suffer from human-robot misalignment, however they are capable of generating lower forces than a link-based robot. Even though their wearability can be improved using Velcro connections in the palm or half gloves (Decker and Kim 2017), patients still have to reach an initial pose to wear the glove. In addition, the movement of the fingers and phalanges cannot be fully guided as in the case of rigid exoskeleton kinematics.
The use of air chambers placed on the finger dorsum allows you to flex and extend the finger according to the air flow (Connelly et al. 2010), (Koo, Kang, and Cho 2013). The work of Yap et al. allows to have pneumatic actuators of variable stiffness in different points to follow the profile of the fingers (Yap et al. 2015). The cable actuation interacts directly with the fingertips and does not directly take into account the movement of all joints. By pulling the tip from above the back of the finger, it is extended. Pulling the tip from the other side results in flexion. The cables are sheathed and held in place as close to the finger surface as possible to avoid undesired deformation of the glove. Such an actuation is called bio-inspired, since it is inspired by the inner tendons of the fingers, which promote flexion and extension. (Xiloyannis et al. 2018) (Dwivedi et al. 2019) (Kang et al. 2016).
The soft exoskeletons actuated by cables can be distinguished by several characteristics: the position of the cables, the number of cables, the number of actuators and the position of the actuator unit. The position of the extension cables is generally on the dorsal part of the finger, while the extension cables are placed on the volar part. On the volar part the extension cables can be placed centrally (Decker and Kim 2017) or laterally (In et al. 2015) to the finger. The number of cables also varies depending on how many fingers need to be actuated and with which tension pattern. The simplest case is to have only one cable per finger for flexion and one elastic element on the dorsal part for extension (Gerez and Dwivedi 2020). The most common case is two cables per finger, one for extension and one for flexion (Decker and Kim 2017). There can be up to three (two for flexion and one for extension) (In et al. 2015) or four cables per finger (two for flexion and two for extension) in order to perfectly mimic the geometry of the four major muscle-tendon units (Lee, Landers, and Park 2014). The number of actuators may vary depending on the strategies and simplifications considered. Given the same considerations explained above, the more mobility is given to
the fingers individually and the more actuators are needed, the more space will be occupied and the more complex will be the model. For this reason, the actuation unit is often dislocated from the hand and depending on the use made of the exoskeleton, it can be placed on the user. This is the case of Delph et al. who have placed the actuator unit in a backpack behind the patient, so their device can move all five fingers (Delph et al. 2013). Popov et al. placed it on the arm and In et al. on the forearm (Popov, Gaponov, and Ryu 2017), (In et al. 2015).
(a) one cable per finger (b) two cables per finger
(c) three cables per finger (d) four cables per finger
Figure 1.4 Examples of soft exoskeletons actuated by different number of cables: (a) one cable per finger
(Gerez and Dwivedi 2020); (b) two cables per finger (Decker and Kim 2017); (c) three cables per finger (In et al. 2015); (d) four cables per finger (Lee, Landers, and Park 2014).
1.2.3 Haptic exoskeletons
Haptic gloves are a different kind of exoskeletons with a different purpose. These are designed for healthy subjects to interact with a virtual environment (Choi et al. 2016). These exoskeletons must track user’s movements and reflect them into virtual environments or for teleoperated applications. Haptic devices introduce sense of touch relying on kinaesthetic feedback provided to the user, tactile stimulation, vibrotactile or electro-tactile stimulation. Cutaneous feedback also plays an important role in improving the perception of virtual environments. Therefore, devices that provide different feedback have been developed. Systems that simulate the contact of the fingertips reproduce the surface orientation rendering force feedback at the fingerpad (Gabardi et al. 2016). Another approach is to simulate the skin deformation after the contact with the virtual object, e.g. devices that render both tangential skin stretch and normal deformation on the skin of the user’s fingerpads (Fan et al. 2014). An hybrid approach is to display change in contact surface by means of the displacement of a tactor, and once in contact exploit three dimensional skin stretch deformation of the fingerpad to display forces (Leonardis et al. 2017). Although these haptic gloves are used in different conditions, not pathological, therefore with healthy and not spastic hands, they offer interesting ideas for the design of assistive or rehabilitative exoskeletons if a goal is to simulate the grasp of an object in a virtual environment.
1.3 Objectives of the thesis
This thesis work presents the development of an innovative soft exoskeleton, inspired by effective design solutions proposed in the state of the art, and with further innovative development of the critical points of ergonomics and the implementation of active flex-extension of the fingers of the hand with a single actuation unit.
In particular the objectives of this thesis have been:
1. To examine the current models of soft exoskeletons by hand and evaluate their problems.
2. To adopt design guidelines based on adaptability, comfort and compactness.
3. To take advantage of modern additive manufacturing techniques such as fused deposition modelling to increase the customization of the exoskeleton according to the patient's hand size;
4. To evaluate the prototype in a laboratory setting and to further improve the prototype based on preliminary experimental evaluation;
5. To evaluate the prototype on SCI patients in clinal environment and to further evaluate the critical points.
The structure of the thesis consists of a first part in which the decisions that have been taken on the basis of the current state of the art are explained, how the tendon model and the modular structure of the finger units were chosen.
The next part illustrates the design and development of a first prototype: a leather semi-glove with modular finger units that implements active finger flexion. Then, this prototype was evaluated in a laboratory environment on healthy subjects.
In the next chapter the problems of the first prototype were evaluated and a new prototype was developed to improve its characteristics. Ergonomics and comfort were improved, and an actuation unit was developed that implements both flexion and extension with a single actuator. It was also evaluated in a laboratory environment and then tested on SCI patients to assess its critical points with injured subjects.
Finally, all the thesis work was discussed. After commenting on the results obtained, possible future developments for this exoskeleton were identified.
CHAPTER 2: APPROACH
Most of the cable actuated soft exoskeletons presented above show two relevant problems: the lack of compactness and the low adjustability according to the size and shape of the hands. Many exoskeletons are designed to actuate multiple fingers independently or move joints individually. This implies a need for a larger number of actuators and consequently a need for space to place actuators and pulleys. For this reason, the actuator unit is often dislocated from the hand. Instead, as far as adjustability is concerned, gloves with enclosed fingers have the constraint of hand size, so not all possible patients can use the same glove. As a result, more models of larger sizes would be needed and the costs for a possible clinical facility that would want to use these devices would increase. The purpose of this work was to design a soft exoskeleton according to specific requirements to try to solve the problems of conventional soft exoskeletons, i.e. to design a compact and adjustable device, compliant as much as possible to different hand sizes and shapes.
At first, the tendon model, actuated by cables, was chosen as actuation model. To ensure the compactness of the device, a simplification was made: it was decided to actuate only the first three fingers with a multi-finger dependent approach. This means moving three fingers simultaneously and guiding all joints with a single actuation, developing a device capable of assisting the closing and opening of the hand. This approach is consistent with many of the ADLs. The choice of a single actuator simplifies the model by minimizing the space needed to position the actuator unit and allows it to be positioned on the back of the hand. The aim is also to free the wrist and forearm from cables, because in cable actuated exoskeletons these areas are always occupied by cables, thus limiting wrist movements so as not to pull
the cables unintentionally. For this reason, it was decided to place the actuation on the dorsum of the hand, thus obtaining maximum compactness.
To improve adaptability, it was decided not to design a closed, one-piece glove, but to design an open half glove with modular finger units. It was decided to design a series of rings and a thimble to assemble the finger unit. In this way it would be possible to vary the number and size of the rings according to the size of the fingers. Running the cables through the rings and securing them at the tip of the thimble would have been a great way to adjust the length of the finger unit without the need for a whole glove of constant size.
Figure 2.1 Ideal 3D model of the compact and adjustable soft exoskeleton for hand rehabilitation and assistance
2.1 Cables
Figure 2.2 Anatomy of index finger tendons (Takahashi, Furuya, and Koike 2020).
Each finger of the hand has four DoFs: three for flex-extension and one for adduction-abduction. Each movement of the bones is associated with muscled and tendons. Two group of tendons can be distinguished, one for flexion and one for extension (Figure 2.2 shows an example of anatomy of the index finger). In particular, the muscles responsible for the flexion are the “flexor digitorum profundus” (i.e. deep muscle) and the “flexor digitorum superficialis” (i.e. shallow muscle), which line along the fingers. The flexor digitorum profundus is a long muscle connected near the distal interphalangeal (DIP) joint that passes through the inside of the flexor digitorum superficialis connected around the proximal interphalangeal (PIP) joint, and both muscles line toward the metacarpophalangeal (MCP) joint (Takahashi, Furuya, and Koike 2020). Although the muscles responsible for the finger extension do not have such a complex structure as the flexors, their structure is similar but placed on the dorsal side of the finger. The design principle of the soft exoskeletons cables has its origin in this biomechanical structure. This principle has been followed for the design of a new exoskeleton but with the necessary simplifications.
It was decided to have four cables per finger to increase the stability of the finger units. Fabric exoskeletons with only two cables per finger, one for extension and one for bending (Decker and Kim 2017) can suffer glove deformation due to cable pulling (Figure 2.3A). As a result, when the glove is deformed, the finger is flexed less than expected. In order to avoid this problem, it was decided to follow the model proposed by In et al. (In et al. 2015). Their device has two cables in parallel for bending, fixed laterally to the finger and at the tip with a thimble, thus avoiding deformation of the glove. It was decided to use Nylon braided cases, given their good mechanical strength and to use two cables also for the extension to distribute the load on two cables and increase the stability. In addition, to reduce friction, the cables were routed in Teflon sheaths.
(a) (b)
Figure 2.3 A comparison of the interfaces for tendon attachment. The green line on the dorsal side represents the extensor tendon, and the red line on the volar side represents the flexor tendon. (a) A case in
which the tendon is directly attached to the glove. (b) A case in which the tendon is attached through a thimble-like strap and is fixed near the finger by straps. (In et al. 2015)
2.2 Fingers unit
Open rings have been designed to support the cable actuation model. As anticipated by In et al., the cables to be placed and held in the right position need their own support (in their case they had a leather-like glove, the cables in the appropriate sheaths were sewn to the glove). Instead Kang et al. have created an innovative semi-open glove made entirely of a soft polymer (Kang et al. 2019). Even there the cables run through specific guides to flex and extend the fingers. In common these two models have that must be known the length of the finger and place the cable support in a precise position of the phalanges. To overcome this limit, it was decided to design rings not connected to any fabric or polymer support. It was decided to increase the number of rings in order to have a modular unit, able to adapt to different finger length.
Figure 2.4 Ideal finger unit diagram. Note the different number of ring elements and the cables running through them. In red the cables for bending. In blue in extension cables.
(a) (b)
Figure 2.5 Designed part for finger units: (a) flexible open ring; (b) thimble.
Two parts have been designed with Creo Parametric 3D Modelling Software to fix the cables at the fingertip and slide them parallel, two above the finger for extension and two below for flexion. One is the open ring (Figure 2.5A), the other is the thimble (Figure 2.5B). The cables slide inside the upper and lower holes of the two parts. In each hole is placed a piece of Teflon sheaths to reduce friction. The ring is the fundamental part which makes modular the finger unit. It is designed as a combination of two ellipses to better fit around the finger and to prevent the ring from rotating around the finger longitudinal axis. It is open so that when the flexion cables are relaxed the ring doesn’t apply any force to the finger, when they are pulled the ring edges tighten around the finger and by friction the ring doesn’t move from its position. The design is inspired by the Kang model of clench rings (Kang et al. 2019). Fixing the parts firmly on the hand is a critical point of the exoskeletons to properly transmit strength to the limbs but it is problematic because usually the bands constantly tighten the limbs. While with this type of ring when relaxed the fingers are less stressed. The ring has a rectangular hole in which an elastic ribbon, used in the first prototype, slides (presented below). Instead, the thimble has the function of fix the cables at the fingertip and
maintain the position during the movements. It is also semi-open at the lower part to fit more comfortably the finger and the close tightly during the actuation. It has the same holes for cables and a buckle for the elastic ribbon. Two version have been designed: one close on the tip (Figure 2.6A), one open (Figure 2.6B). The open-tip version was experimentally observed to be more comfortable because the closed-tip version applies higher forces to the fingertip during operation. The closed version after prolonged wearing creates discomfort and slight pain due to pressure on the fingertips. The open version also helps to ventilate the finger and to reduce sweating.
(a) (b)
Figure 2.6 Two versions of the thimble: (a) close tip; (b) open tip.
Both parts are designed to be printed with the Fused Deposition Modelling (FDM) printer. Unnecessary undercuts and details of less than 0.5 mm have therefore been avoided. The position of the pieces on the printing table was decided on the basis of the internal resistance that the pieces would have, i.e. more resistant on the printing table and less resistant between layers. It is necessary to print different pieces in number and size according to the fingers of each patient. With FDM this can be done quickly and cost effectively as there is no need for
large-scale production at the moment. The parts are printed in Thermoplastic Polyurethane (TPU 95A) to give flexibility and comfort on the finger by Makerbot X2 printer.
(a) (b)
Figure 2.7 Diagrams of forces and constraints applied to the parts: (a) forces applied to the ring; (b) forces applied to the thimble.
Since the pieces would have been subjected to considerable forces during the handling and following tests, a finite element method (FEM) analysis has been performed using Ansys software. The finger unit has been designed as a series of rings crossed by cables that pass through the Teflon sheaths and these are attached to the thimble. The cables are the medium through which a force is applied to the components. For the FEM analysis the worst case was considered, i.e. the pulling of the lower parts of rings and thimble being the overhanging and less constrained parts. For both parts, the surface in contact with the finger was considered a fixed constraint. For the thimble the force was considered to be applied longitudinally to the lower holes. Two hypotheses have been made for the rings. The worst case considered was a force applied at 45° compare to the axis of the ring holes, associated with the closing of the fingers. In addition, the force parallel to the axis of the holes was considered to be zero, due to the sheaths that should reduce friction. The resulting force is perpendicular to the hole axis. Figure 2.7 shows the diagram of the forces and constraints
applied to the rings and to the thimble. It was considered a static torque of 1.3 Nm (average torque among the common servomotors that could have been used), converted into force with the cable pulley radius and divided by the number of bending cables. The results, shown in the figure, showed that despite the elastic deformations we remain in a safety region well below the yield stress. The results are shown in Figure 2.8 and Figure 2.9.
(a) (b)
Figure 2.8 Ansys results for the thimble: (a) Equivalent Stress; (b) Equivalent Elastic Strain.
(a) (b)
2.3 Routing
Figure 2.10 Ideal routing of the device
As mentioned above, the cables must provide for the bending and extension of the fingers with a parallel sliding of pairs of cables. Conventional hand exoskeletons have extended routing from the dorsum and palm of the hand to the forearm, often stiffening the wrist and the actuation is dislocated by the patient. In this case, wanting a compact exoskeleton, it was thought a routing that would wrap the hand and bring the cables on the dorsum where the actuation unit will be placed. So, after a linear segment the cables, responsible for the bending, bends along soft angles to go from the palm to the back. An important feature is that this routing during actuation brings the parts of the palm and back of the hand closer and tightens them together. In this way, it would allow the exoskeleton a greater grip on the hand. In addition, it is important to attach the cables to the palm instead of the wrist as conventional tendon-based exoskeletons do. Hooking the tendons to the wrist during actuatation would tend to lift a rope between the fingers and wrist reducing the grip on the object. Instead, the attachment to the palm reduces this rope and therefore promotes grip.
2.4 Control
The actuation part has been placed on the hand dorsum and consists of a smart servo motor with a single pulley for the winding of bidirectional cables (one direction of rotation to roll flexion cables and unroll extension cables and one direction to roll extension cables and unroll flexion cables). The smart servo motor has integrated a position sensor and a temperature sensor. It has been connected to a Teensy 3.2 microcontroller; this is connected to the computer. The Teensy microcontroller is programmed to execute low-level commands. It sends a command package in binary code to the motor. Each command has its own packet of different length and content. The implemented commands to be sent to the motor were: the control of the motor speed according to a voltage value, the position reading and the temperature reading. It was decided to read the temperature to observe its variations inside the motor and thus avoid excessive overheating. Instead the high command and user control is on MATLAB Simulink to communicate with the microcontroller. This model writes to the Teensy the voltage to be applied to the motor, reads the position and forms a closed loop to perform a position control. In this way the user can make the motor run various tasks by giving it a path to follow (Figure 2.11).
In particular, on the Simulink model the operator can choose the wave form to guide to motor position (e.g. Figure 2.12 shows a sine wave and a square wave), this wave form is compared with the position reference for its position. Then, a voltage signal is processed to be sent to the motor and readings commands for the sensors integrated in the motor are timed and all packed in an output buffer for the microcontroller. During the reading phase, a buffer is unpacked from which various data are extracted: voltage and current applied to the motor, temperature read by the sensor integrated into the motor, position read from the encoder. Pressure and force data are also read from two sensors, presented below.
The motor speed can be modulated using a control package because the servo motor driver accepts a voltage reference and then applies it to the motor. Ignoring the motor dynamics, a voltage reference corresponds to a current corresponding to a torque. This feature has been designed to guide the movement of the exoskeleton.
Figure 2.12 MATLAB Simulink model: top left the block for the operator to choose the waveform; top right the block for processing, timing and writing commands on the microcontroller; below the reading and unpacking of the reading buffer. Note the negative feedback of the read position of the encoder for the control in position.
2.4.1 Servo motor characterization
Since all future tests and use would be performed under almost static conditions, it was necessary to characterize the servo in order to modulate the force according to the voltage reference.
Figure 2.13 Lewansoul LX-16A motor on its support for characterization with Futek LSB200 sensor
The first considered actuator was the LewanSoul LX-16A Servo, it consists of a gear reduced DC motor with embedded drive electronics and position sensor and it has been characterized with a test to extract the force that it generates varying the applied voltage. A specific support for this test has been designed and printed by FDM (Figure 2.13). On the support it has been placed a force sensor (Futek LSB200) and on the motor a metal rod in contact with the sensor. The distance between the axis of rotation of the motor and the contact point was 20 mm. Through the model on MATLAB Simulink a series of ten voltage
ramps has been applied to the servo motor (Figure 2.14A) and the force has been read by the force sensor. It was also a series of ramps, but out of phase compared to voltage reference. The force ramps have been averaged to extract the force-voltage relation. Figure 2.14B shows how the motor generates a force proportional to the voltage applied to it, but only after exceeding a voltage threshold. This threshold is present due to static friction inside the motor. This friction can be quantified as the intercept of the fitting line of the linear section with the force-axis, i.e. about -12.52 N.
Figure 2.14 Results of the characterization test: (a) in blue measured force as function of time, averaged over ten repetitions, in orange the voltage reference; (b) measured force as function of voltage, the extracted
linear fitting and its equation.
2.5 Sensing objects
After this, it was thought two ways to measure to action of the exoskeleton and then compare them. One way was to estimate the grasping pressure for the exoskeleton-assisted
hand with a pressure sensorized plastic bottle (Figure 2.15). The sensor (Bosch BMP280 pressure sensor) was placed under the bottlecap to measure the increasing pressure inside the bottle when pressing it. The bottle was filled with water to increase its stiffness. The second way was to measure the grasping force using a rigid handle including a force sensor (Figure 2.16). Two rigid hollow part have been designed and printed to enclose the sensor (Futek LSB200 force sensor). These two sensorized objects have been integrated in the Simulink model for the exoskeleton control and associated to another Teensy microcontroller, programmed to read the pressure and the force from the sensors.
(a) (b)
Figure 2.15 Pressure sensor: (a) sensorized bottle with pressure sensor; (b) Bosch BMP280 pressure sensor inside the bottlecap.
(a) (b)
Figure 2.16 Force sensor: (a) sensorized cylinder with force sensor; (b) Futek LSB200 force sensor in the cavity inside the cylinder.
CHAPTER 3: FIRST PROTOTYPE
A first prototype of the exoskeleton has been made to test and verify the correct work of the rings and thimble, the correct slide of the cables through to sheaths and the particular routing presented above. So, it was thought to test only flexion and initially only three fingers to further simplify the configuration and afterwards verify the extension. Nylon cables were used to implement flexion (two parallel cable sliding through the little holes in the bottom part of the rings) and laterally folded around the palm and actuated by a small radius pulley connected to the servomotor shaft. The cables of index and middle fingers folded the hand from the same side, the cables of the thumb from the other side. Instead, the extension was passive by means of elastic elements placed on the finger dorsum and fixed to the thimbles with adjustable buckles.
Figure 3.1 Scheme of the tendon routing and actuator arrangement for the first prototype. Elastic ribbon in blue; Nylon cables in yellow, orange and purple.
3.1 The glove
The main structure of the soft exoskeleton consists of a leather glove. Leather was quick and easy solution for a prototype. Easy to cut and sew, a leather glove offers a good support for the hand and the mechanical part over it. It was made laterally open to make it easier to wear for an unhealthy patient and foldable with Velcro straps.
Figure 3.2 Preliminary prototype of the compact soft exoskeleton. Actuator and tendon transmission are arranged around palm and hand dorsum, with no remote mechanical parts.
Other three parts have been designed to connect fingers units with cables and elastic element. A support for the servomotor with sheath connections(Figure 3.3), bolted to the glove and to the motor, printed in Polylactic Acid (PLA) to have more rigidity and stability during the actuation by Ultimaker 3 printer. To print this, it was necessary to use a support material printed, parallel to the PLA, with a second extruder, due to the presence of undercuts. The support material was the Polyvinyl Alcohol (PVA), that is soluble in water
at ambient temperature. Adjustable buckles (Figure 3.4A)to adjust the length of the elastic ribbons, bolted to the glove, printed in TPU, a base for the palm in which the cables run (Figure 3.4B), filled with Teflon sheaths, bolted to the glove and printed in TPU.
Figure 3.3 Dorsum support with holes for connection to the motor and the glove and sheath connection.
(a) (b)
The cables, brought into position on the back of the hand, wrap around a double groove pulley (Figure 3.5A), one groove for two cables of the thumb, one groove for four cables of index and middle fingers, even it printed in PLA for rigidity and stability. The grooves have the precise dimension to hold four cables for three rotations in each groove. The design of this pulley was done after evaluating the critical points of the commercial aluminum pulley associated with the servomotor. The aluminum pulley did not have well defined grooves and the cables, during winding, came out of their position until they got stuck between the aluminum and the motor. In addition, the cables coming from two different directions were subject to knotting, making actuation complicated. Instead, once the new pulley is assembled, cables are aligned with the sheath guides on the support, separated in the two grooves and spaced from the motor. The pulley has special holes and guides, they carry the cables outward through the pulley flange. The cabled are held in position by two screws and flat washers. This positioning allows to adjust the cable length according to the patient’s finger. The pulley is fixed on the driving horn by pressure and by a screw. The horn has splines that are too small to be reproduced with a common FDM printer. So, as a first approach it was decided to make the hole of the pulley slightly smaller and fixed by pressure. Later it was observed experimentally to be an optimal and fast solution, because the pulley remained in position despite the considerable torsional forces involved. The pulley shaft has a minimal length to reduce the bending momentum during rotation and a minimal radius to reduce the torsional momentum.
(a) (b)
Figure 3.5 Pulley and Support: (a) double groove pulley with holes and guides for cables and ideal internal splines; (b) Cable routing on support and pulley, blue arrows for index and middle finger cables, red arrow
for thumb cables.
Figure 3.6 Assembly of the pulley, the support and the Lewansoul LX-16A servomotor (the yellow block).
3.2 Tests on healthy subjects
A preliminary study was carried out with a healthy subject using the sensorized objects, presented above, to test this prototype. Dimensions of the participants’ hand have been measured: length from the base of the wrist to the tip of the middle finger, middle finger
length, palm width. He was asked to wear the exoskeleton and to extend his fingers, then the length of cables was adjusted. To test this first prototype instead of a closed loop position control, an open loop control was performed by driving the motor with a voltage reference. It has been decided to send a voltage step function to the exoskeleton making it close until Time 1, making it open until Time 2 and resting until Time 3, repeating for ten cycles. In order to estimate three characteristic times, the model was launched with a voltage of 2.5 V and the exoskeleton-assisted hand started to close around the sensorized bottle counting the time to reach the maximum pressure read by the pressure sensor. Then the voltage was inverted to reach the initial position and count Time 2. The voltage was set to zero for a short time to distinguish one cycle from the next and Time 3 was counted (Table 3.1, Subject 1). The voltage step function was generated with these three times (Figure 3.7), so this voltage reference could be applied to the motor to obtain a closing and opening sequence (Figure 3.8).
Figure 3.7 Voltage step function sequence as reference
The first test started with the sensorized bottle (Figure 2.15) to measure grasping pressure during the manipulation of the exoskeleton. The subject asked to open their hand and to relax it passively and leave the exoskeleton open and close it by the applied voltage reference. From the motor characterization the stall output torque at 2.5V corresponds to 19 N pulling force applied to the tendon transmission. So, the measured pressure was averaged over ten repetition. The experiment was repeated with the second sensorized object (Figure 2.16) to measure a grasping force for comparison. For a correct force reading, the object has been positioned so that the index and middle fingers make force perpendicularly to the sensor with the thumb in opposition, so that shear forces not readable by the sensor have been reduced to a minimum.
Other five healthy subjects were enrolled to repeat these tests. Their characteristic hand sizes, too, were measured and characteristic times were recounted (Table 3.1). After that, they performed ten repetitions of grips to measure the grasping pressure from the sensorized bottle and ten repetitions to measure the grasping force from the sensorized cylinder.
3.3 Results and discussion
In Figure 3.9A, the measured pressure averaged over ten repetitions is shown. Results of the force measurement are shown in Figure 3.9B. Figure 3.9A shows a short time during which no pressure is applied, while the extended hand must come into contact with the surface of the bottle, and an interval of time during which with a positive voltage a gradual increase in pressure up to a peak is observed. Immediately afterwards, with the inversion of voltage, the hand relaxes, and the pressure drops. The same is observable for the measured force in Figure 3.9B.
Figure 3.9 Measured grasping pressure (a) and grasping force (b) with passive user averaged over ten trials. Orange line represents the voltage reference applied to the servomotor.
Figure 3.10highlights the compliance introduced in the system by the soft parts, by the fixing interface of the exoskeleton to the hand, and by soft tissues of the hand itself. Compliance of the system can be modeled with a quadratic model, obtained performing a quadratic regression to fit the measured data.
Figure 3.10 Measured grasping pressure as a function of position in degrees (solid line) and quadratic model fit to the measured data (dotted line).
For each subject measured pressure and force were averaged over ten repetition. For comparison the maximum averaged pressure and force and their standard deviation were taken (Table 3.2). Obtained this data, the maximum force value from each subject was used to extract a normalized value that represents the efficiency of the system. The force generated by the exoskeleton has been related to the value of force corresponding to 2.5 V. This value of force is extracted from the obtained from the characterization of the motor described above where it has been put in relation the force generated from the motor with the value of voltage applied to it and to 2.5 V corresponds approximately to 19 N. These normalized efficiency values are reported in the last column of Table 3.2. On average, the efficiency of the six subjects is about 60%. This has been attributed to the internal friction of the cable actuation system, which despite being reduced by Teflon sheaths and Nylon cables is still present. Another interesting consideration was evaluated with this data. The force generated by the device on each subject was related to their hand length. Although the number of subjects is reduced, in Figure 3.11 it is possible to observe a certain linear trend that correlates the force generated by the glove with the length of the hand.
Table 3.1 Personal data of each healthy subject: finger size, characteristic times for voltage step function (1, 2, 3).
Size [mm] Time 1 [s] Time 2 [s] Time 3 [s]
Subject 1 80; 183; 85 5 7.5 8.5 Subject 2 78; 184; 88 3.5 5.2 6.2 Subject 3 79; 173; 85 4.1 5.7 6.7 Subject 4 88; 200; 86 7.3 10.7 11.7 Subject 5 80; 180; 86 5 7.5 8.5 Subject 6 86; 196; 93 7.3 10.7 11.7
Table 3.2 Maximum pressure and force and their standard deviation for each subject.
Max Pressure [mBar] Std for Pressure Max Force [N] Std for Force 𝑭𝒆𝒙𝒐/𝑭𝒎𝒐𝒕𝒐𝒓 Subject 1 118.38 8.95 11.55 1.49 0.60 Subject 2 106.05 5.03 11.09 0.78 0.58 Subject 3 60.30 4.86 7.93 0.31 0.41 Subject 4 79.93 13.75 12.79 0.92 0.67 Subject 5 67.15 5.00 11.76 1.18 0.61 Subject 6 111.95 6.36 13.67 0.86 0,71
CHAPTER 4: SECOND PROTOTYPE
The first prototype has shown a good functionality of finger flexion and sufficient grasping pressure and force, but it has shown even some problems. During the manipulation the leather glove and the routing support tended to pull away from the palm. The thumb routing resulted unnatural and uncomfortable. Instead the elastic band were too weak to bring the finger back into position, in addition the bolted buckle on the glove dorsum makes the glove unsuitable to different hand sizes. A prolonged wearing, subjected to high forces, caused localized pains in areas where there are screws and bolts. Ultimately, the Lewansoul LX-16A servo has caused problems during the control because it has an encoder based a potentiometer. It has position reading limits (it guarantees reading within a range of 270 degrees), so it does not allow multi-turn reading, which in necessary in this case where the windings are from two to three turns to accomplish a full open-closing of the fingers. So, with a second prototype the goals were to improve the ergonomics avoiding pains and to maintain the glove tight to the hand and to remove the elastic ribbons and implement the finger active extension with cables.
4.1 Actuation
The second prototype differs from the first in the number of cables and the way they are handled. In the first prototype the pulley was directly connected to the crankshaft, in the second it is not. Another pulley connected in the same way, but wider, would have taken up too much space for the right finger extension and the bending moment on it would have been too high to ensure stability during the drive. In addition, the new pulley was designed to handle twice as many cables as the previous one: twelve cables, six for bending and six for
extension (four per finger as the ideal model presented above Figure 2.10) with six grooves, one for two cables. It was positioned on the left side of the motor and it was necessary to develop a gear to transmit the rotation. The gear would have transmitted the rotation from the motor to the pulley allowing to wind in one direction the cables for the flexion and unwind those for the extension and in the opposite direction wind the cables for the extension and unwind those for the flexion.
4.1.1 Components
Figure 4.3A shows how all the components of the gear have been designed and assembled. The optimal distance of the shaft parallel to the motor was decided and the gear ratio was decided 1:1, because the reduction of the servomotor was adequate for this use, consequently the diameter of the wheels. So, two gear wheels have been designed and worked with a CNC milling machine in Polyoxymethylene (POM, Delrin). The first wheel would be screwed to the flange fixed to the motor shaft, the second connected to the parallel shaft by aluminum tab. The shaft has been worked at the lathe in aluminum, providing central cavity and six holes for cables and housings for tab and grains. A box has been designed to support the rotating shaft and printed in carbon fiber reinforced Onyx (Nylon mixed with chopped carbon fiber) by Markforged Mark TWO printer to increase mechanical resistance. Its holes were made big enough to house shaft and two ball bearings. The six-grove pulley has been designed to be a coating for the aluminum shaft, because it would have too difficult shape to be developed as a single piece and also because avoid section change avoids internal tension peaks in the shaft sections. The shaft diameter has been dimensioned to support high forces. Since the device would work at low speeds, it was decided to consider the case quasi-static in case the motor applies maximum torque. The worst case was taken in which there is a force applied only on the central point of the shaft and as resulting forces only those of
the bearings (Figure 4.1A). Therefore, both the shaft diameter and the bearings were oversized. The model was then verified on time on the Ansys software which carries out an FEM analysis. The results of deformation and stress were negligible compared to the breaking point of the aluminum (Figure 4.1B). So, the pulley was printed by FDM in Onyx and then connected to the shaft by two grains. A flange was added on the outside of the box to secure cables with screws for later adjustment and a case to cover the excess cable length and prevent them from getting caught on the gear. On the side of the wheels, a spacer and a lock have been added to avoid axial translation of the gear wheel. These four pieces have also been printed in Onyx.
(a) (b)
Figure 4.1 (a) diagram of the forces applied on the aluminum shaft; (b) resulting total deformation by Ansys.
4.1.2 Assembly
To assemble the actuator unit the pulley was placed inside the Onix box and passed through the aluminum shaft aligning the six holes of the pulley to it and connected by two grains. The outer flange was positioned to adjust the cables and its cover by fixing it to the shaft with one grain. Then the box was screwed to the servomotor and the two gear wheels were assembled, one on the motor with screws and one on the shaft with the aluminum key. The one on the shaft was spaced from the Onix box with the small internal spacer and held
in position with the external block fixed with another setscrew. the assembly result is shown in Figure 4.2 and Figure 4.3.
Figure 4.2 Pulley shaft section. From the left: aluminum shaft, gear wheel lock, gear wheel, gear wheel spacer, carbon fiber reinforced gear box, six-grove pulley, cable adjustment flange, cable cover (tab,
bearings and grains not present).
(a) (b)
4.2 Palm and Dorsum Support
The leather glove has been replaced by elastic wrist glove covering the entire palm and hand dorsum. In this way the glove in more adaptable to different hand size. A side zipper was added anticipating the difficulty of wearing a glove by patients with motor disabilities. In addition, new supports for the motor and the routing has been developed. A generic hand 3D model was used to model two supports, one on the palm for the routing of finger flexion like for the first prototype, one on the hand dorsum for the connection with the actuator and the routing of finger flexion and extension (Figure 4.4).
(a) (b)
Figure 4.4 Two glove-hand support modelled on a generic 3D hand model: (a) routing palm support; (b) motor and routing dorsum support.
These two supports were modelled to have compliance with concavity and curved surfaces of the hand and to replace screws and bolts with sewing over the elastic glove. In
this way, pains were avoided, and the glove resulted more ergonomic. They have small raised holes to allow cable sheaths to pass through. The palm support provides the passage of three sheath for the flexion cables, one for the thumb, one for the index finger and one for the middle finger. The dorsum support provides, in addition to the passage of flexion cable, the passage of three sheath for the extension cables and the connection with the motor. Figure 4.5A and Figure 4.6Ashow how cables are brought to the pulley level. Blue cables represent flexion cables, they run over the palm to the dorsum a distance with gentle curves. Red arrows represent extension cables, they run a shorter distance, but they curve at almost ninety degrees. Two small parts were added to guide the sheaths for thumb cables, both for flexion and extension (Figure 4.5B and Figure 4.6B). The cables were brought lined up to the left of the motor and then rolled around the six-groove pulley. All these four parts were designed to be printed in Acrylonitrile-Butadiene-Styrene (ABS) by Stratasys printer. This is an FDM printer with two extruders: one for the main material that the piece is made of (in this case ABS), the other for printing a soluble support material.
(a) (b)
Figure 4.5 Cable routing on the palm of the hand: (a) CAD drawing, flexion cables in blue; (b) routing photo on worn glove and adjusted cables length.
(a) (b)
Figure 4.6 Cable routing on the dorsum of the hand: (a) CAD drawing, extension cables in red, flexion cables in blue; (b) routing photo on worn glove and adjusted cables length.
4.2.1 Routing
First of all, sheaths have been placed on palm and dorsum support, minimizing their length and softening the curves, and their ends have been fixed by interlocking with smaller diameter holed to the support. Once pieces of actuation box have been assembled according to Figure 4.2, cables have been loaded on pulley. First, they have been passed through the holes of the Onyx pulley inside the aluminum shaft, then pulled out and fixed on the cable adjustment flange. Before running cables through the sheaths, only extension cables (Figure 4.6A, red arrows) have been preloaded with four windings around the pulley, because the initial finger configuration would have been extended, so maximum available length for flexion cables and minimum for extension cables. So, the cables have been passed through the sheaths and through the finger unit (from rings to thimbles, to the aluminum plate to be fixed by screws as explained above). At this point, the motor could be mounted on the
support and the gear box with the pulley to it. Once worn the exoskeleton, the cable length could be adjusted according to the finger size.
4.3 Other improvements
4.3.1 Finger unit and cable adjusting
From first prototype it was observed the right functionality of the finger unit, both rings and thimbles. So, they have been preserved, but with some changes. The number of cables was increased from two to four (two, as before, to implement the flexion, tested in the first prototype, and other two, placed over the ring instead of the elastic ribbon, to implement the extension of the finger). In addition, the fixing of cables at the thimble has been improved. In the first prototype cables were fixed to the thimble by passing through two holes and then simply they were knotted (Figure 4.7A). A plate in aluminum have been placed in the thimble instead of the ribbon with four threaded holes. Flexion cables have been fixed by screws to the first two holes, extension cables to the seconds, in this way the fixing was reversible and more easily adjustable (Figure 4.7B).
(a) (b)
Figure 4.7 Fixing of cables: (a) first prototype; (b) second prototype
4.3.2 Flexible wristband
In first prototype the glove was made of a semi-rigid leather and it was very tight thanks also to Velcro straps. This allowed to unload the reaction forces generated by the pulling of the cables. Pulling the cables towards the palm of the hand generated a reaction force on the glove directed towards the fingertips. The glove being very tight and semi-rigid unloaded the force on the hand and the glove stays in place. The second elastic glove did not allow this. That’s why three more pieces have been added, one flexible wristband (Figure 4.8A) and two regulator bundle (Figure 4.8A). These pieces have been clipped onto the motor support and unloaded the reaction force on the wrist.
Figure 4.8 TPU 95A wrist band (A) and regulator bundle (B)
Figure 4.9 Final appearance of the prototype after having assembled each component, worn and adjusted the flexible wrist band the length of the cables to have extended fingers.
4.4 Control
The Lewansoul LX-16A servo motor had a position reading limit. It guaranteed a correct reading in a range of 270 degrees. This created problems with the repeatability and accuracy of the device control, since it took two to three motor axis rotations to switch between the extended and flexed finger configuration. For this reason, the Lynxmotion Smart Servo LSS ST1 motor has been used to improve the control and make it more accurate and repeatable. The new motor guarantees an accurate reading in a range of 360 degrees and has in its internal firmware the ability to store a "virtual angular position" is a feature which allows for rotation beyond 360 degrees, permitting multiple rotations of the motor shaft. So, the teensy coding has been changed. The microcontroller still executes low-level commands, but the command packets are generated differently. Differently from the first servo which communicated by short binary packet, this servo communicates by ASCII strings converted in binary code. The low-level commands executed by Teensy are the same: apply a voltage to the ends of the engine, reading the position and internal temperature. The Simulink model has also been modified to communicate correctly with the microcontroller. Since the servo motor communicates in ASCII code, two conversions are made on the Simulink model to manage the data. The read data are first converted into ASCII strings, knowing in advance the maximum possible length that each data can have, then converted into integers. Vice versa, the data that must be sent are first converted in ASCII strings and then converted in binary code.
The motor control has been made accurate and repeatable by closing the loop for position control. For the previous tests with the first prototype were measured the three characteristic times, explained above, to have all pressure and force peaks in succession and synchronized.
Despite this, it was observed a slight phase shifting of the curves and the difficulty to control the device passing from one subject to another, having to modify each time the Simulink model according to these times. For this reason, the position control was the best solution to guarantee the accuracy and precision of the device. Instead of deciding which voltage waveform to send to the motor, the user decides a position waveform to guide the motor. Figure 4.10 shows a simplified version of the model to highlight the difference between open loop and closed loop for in position control using the discrete PID controller.
Figure 4.10 User area to switch from open loop control with voltage examples to position control with a wave form and discrete PID controller
4.4.1 Servo motor characterization
Similar to how it was done for the first motor, it was necessary to do a characterization of the engine to extract the characteristic function that correlates the force generated by the motor according to a voltage applied to it. A new support has been designed for the LSS ST1 motor and printed by FDM (Figure 4.11). On the support it has been placed the force sensor (Futek LSB200) and on the motor a metal rod in contact with the sensor. The distance
