Mechanical Design of a Finger Exoskeleton for Haptic Applications VCM-2016 November 25-26 2016, Can Tho University, Vietnam VCM-2016 Mechanical Design of a Finger Exoskeleton for Haptic Applications Tin Huu Le Quang Nguyen1, Viet-Hong Tran1 1Department of Mechatronic Engineering, Faculty of Mechanical Engineering, Ho Chi Minh city University of Technology, VNU-HCM, Ho Chi Minh city, Vietnam Email: 21203870@hcmut.edu.vn, tvhong@hcmut.edu.vn

Speaker: TIN HUU LE QUANG NGUYEN Introduction Materials and Methods Results and Discussion Conclusion Speaker: TIN HUU LE QUANG NGUYEN

Introduction Figure 1: Hand exoskeleton as a haptic device Haptic devices have been used in many applications, such as tele-manipulation, virtual reality, assistive devices and medical applications Haptic glove is a kind of haptic device. It is an exoskeleton, which is mounted on human hand. It allows user to have force feedback in simulated or tele-operated environment Key functions: Measure the kinematic configuration (position, velocity, acceleration) of the user’s fingers Display contact forces to the user Existing problems: Bulky, heavy Limited range of motions and maximum contact forces Figure 1: Hand exoskeleton as a haptic device Source: Ma Z and Ben-Tzvi P., (2015) “Design and Optimization of a Five-Finger Haptic Glove Mechanism”, ASME. J. Mechanisms Robotics, 7(4), p. 041008 INTRODUCTIOn

Introduction Cable-driven system characteristics: Linkage-based Soft actuator Cable-driven Cable-driven system characteristics: n DoF system is completely actuated with at least n+1 cables Cables can only transmit forces with positive tension Highest rigidity Linear force transmission Bulky High stresses Complicated structure High compliance Low inherent stiffness Safe interaction High power to weight ratio Long setup time Shear force on attachment point Limited DoF High rigidity Linear force transmission Lightweight, compact Actuated remotely Lower efficiency Cables’ slack-prevention problem Current drive system for hand exoskeleton can be divided into 3 main categories For haptic glove, cable-driven is preferred because of its low inertia and it allows actuators to be placed far away, results in low stress on user’s hand. INTRODUCTIOn

Introduction Objectives: Mechanical design of haptic finger exoskeleton Cable routing scheme Choosing cables in order to satisfy static equilibrium INTRODUCTIOn

Materials and Methods (b) (a) Figure 2: Human finger model; In previous research, we have established the kinematic model along with necessary forward, inverse kinematic equations and Jacobian. This model consists of 4 mechanism constants: d, d1 and position of A0; 6 parameters: p1 p2 p3 l1 l2 l3; 6 variables: theta1,2,3 and alpha1,2,3 The kinematic length of our linkage can be calculated in 2 ways: Optimize for a particular user Or Finding lower and upper limits which help it accommodate several users (a) (b) Figure 2: Human finger model; Exoskeleton model MATERIALs and methods

Materials and Methods - Optimization for a user Objective function: Constraints: => Nonlinear multi-objective optimization => Monte Carlo algorithm The objectives of this linkage geometry optimization method is to maximize force transmission ratio and force contact. It is represented through finding the minimum of 2 objective functions: - Minimize total friction loss during operation - Minimize the correspondent Jabcobian elements so that we can get the highest contact force with the same amount of joint torque Along with it are 3 constraints, which is prerequisite to a collision-free performance and make it easy for joint design. MATERIALs and methods

Materials and Methods - Optimization for a user Finger’s RoM is used to ensure no hinder to natural movements Collision detection must be made after to make sure there’re no collision during operation MATERIALs and methods

Materials and Methods - Linkage geometry limits for several users Requirements: Does not hinder natural movements Collision-free performance MATERIALs and methods

Materials and Methods – Linkage design For generality, today, we focus on a design that can change the mechanism links’ length In order to do that, we add translational elements to the existing rotational elements. However, realizing a 2 DoF joint whose translation can be locked is not good in our case because it will make the mechanism’s width unnecessary thick. Hence, another way is to separate DoFs, it may restrict the maximum changing range of the geometry but it helps the mechanism become more compact. Figure 3: Mechanical design concepts MATERIALs and methods

Materials and Methods – Joints’ RoM Inverse kinematics: Finding α, knowing θ Where So, for each set of parameter l1 l2 l3, we can find the mechanism joints’ RoM using inverse kinematics equations along with the range of typical human index finger’s RoM MATERIALs and methods

Material and Methods – Cable routing – Choosing cables The routing allows fully and linearly control of the joints Figure 4: Cable routing structure Using the minimal number of cables, in this case, 4, we want to fully control the joints. There are many routing structure can be synthesized, however, we want to minimize the number of joints a cable has to pass through, and also cables can be routed compactly from distal link to the base. Hence, we chose the structure shown in Figure 4. These equations shows that we can fully, and also, linearly control all the joints. But, as we can see, because of insufficient number of equations, we cannot have a definite solution. In addition, all cable tension must have positive values in order to transmit forces. This yields a problem to the controller. This problem can be solved beautifully when we apply the torque resolver, or adding some rectify elements to these equations. MATERIALs and methods

Materials and Methods – Cable routing – Choosing cables Where is a arbitrary positive parameter which: Does not have influence on the equation Can act as pretension to ensure the positive cable tensions. Define operator To put it in a simple term, let’s take a look at the first equation. A viable positive solution to this equation can be written as In order to generalize the solution, let’s define some new operators. So we got a general positive solution to a 2-variable equation. The beautiful thing is, this operator can be computed easily by the controller, or even with some electrical circuits. MATERIALs and methods

Materials and Methods – Cable routing – Choosing cables Apply that to the rest of our equations, we got a general positive solution to all of the cable tensions Hence, the maximum tension for each cable can be calculated MATERIALs and methods

Results and Discussion – Geometry optimization Input parameters: d = 5mm; yA0 = 10mm; xA0 = 0mm d1 = 10mm Links’ length step: 0.1mm Joints’ angle step: 10 Finding range: l∈[𝑝;3𝑝] Table 1: Author’s index finger dimension and workspace Finger joint Angular motion range (deg) Finger link length (mm) MCP [-90;30] 45 PIP [-120;0] 28 DIP [-80;30] 13 In the next section, Result and Discussion, firstly, we use the mentioned procedure to find linkage geometry for a customized product with following input parameters results and discussion

Results and Discussion – Geometry optimization Table 2: Optimization result Z Joint Joints’ RoM (deg) Links’ length (mm) 37.9591 1 [3.64;159.23] 110 2 [-153.57;-123.17] 83 3 [-53.87;125.32] 14 Here’s the result: we can see that the human finger’s workspace is a subset of exoskeleton’s workspace. Therefore, it ensure no hinder to natural movements during operation. Figure 5: Workspace checking results and discussion

Results and Discussion – Geometry limits Input parameters: 18 datasets d = 5mm; yA0 = 10mm; xA0 = 0mm d1 = 10mm Links’ length step: 0.1mm Joints’ angle step: 10 Finding range: l∈[ 𝑝 ;3 𝑝 ] Result: 𝑙 1 ∈ 50;85 (mm) 𝑙 2 ∈ 35;55 (mm) 𝑙 3 ∈[15;45] (mm) Similarly when we calculate linkage geometry limits for general product. With the input parameters: We found the results: results and discussion

Results and Discussion – Linkage design Based on previous results, we can have the mechanical design of the device. The exoskeleton is mounted on a finger-cut fabric glove, which is worn by user Translational joints can be moved within predetermined limits and then, locked in place by bolts. Moments at Rotational joints are transmitted by 2-sided cut faces. Total mass ~= 4.6% Maximum Force-> can be neglected Maximum width: 13mm Total mass: 46.33gr Figure 6: Mechanical design results and discussion

Results and Discussion – Joints’ RoM α 1 ∈[ −15 0 ; 150 0 ] α 2 ∈[ −140 0 ; −40 0 ] α 1 ∈[ −85 0 ; 125 0 ] Singularity check: Since Which mean A0, A1, A2 are on a straight line => Mechanism does not encounter singularity during its operation After calculating the joints’ RoM, we want to check if the mechanism will encounter any singularity during its operation. We can see here that mechanism’s jacobian equal 0 when 3 points A0 A1 A2 are on a straight line. Since the maximum value of our alpha2 is only -40, we can be sure that our mechanism can avoid singularity configuration. results and discussion

Results and Discussion – Design checking In order to check our design, we interest in finding its critical configuration. As the torque calculation equations depict, Joint 1’s torque has maximum value when the moment arm from A0 to P3 is the longest, hence we have the stance where alpha2 at maximum and A2P3 is parallel to A0A1. Similarly, Joint 2 and 3’s torque become maximum when A1, A2 and P3 on a straight line. (b) Figure 7: Critical Stances (a) Joint 1’s critical; (b) Joint 2&3’s critical results and discussion

Results and Discussion – Design checking – Joint 1 Here, we check the static structure of Joint 1. We can see that the dangerous zones are these because the linkage is thin there (2mm thick and 1mm width) The minimal safety factor of the design is about 2.55 Figure 8: Equivalent Stress & Safety Factor of Joint 1 results and discussion

Results and Discussion – Design checking – Joint 2&3 In Joint 2&3 checking, the dangerous zones are also on the links with slot for similar reasons. Our safety factor has a minimum at about 2.06 Figure 9: Equivalent Stress & Safety Factor of Joint 2&3 results and discussion

Results and Discussion – Choosing cables Miniature steel cable from Techni-cable: Diameter: 1mm Structure: 7x7 (flexible) Strength: 580N ⇒ Using established equations, the maximum positive tensions of the cables are calculated. So we choose a flexible-type steel cable type with the strength of 580N as our drive cable. results and discussion

Conclusion Contributions: Future works: Finding mechanism’s geometry for customized product and general product. Mechanical design and cable routing scheme of the device Choosing drive cables for the system Future works: Control system Prototyping In conclusion, in this research, we have Conclusion

Appendix – D-H notations ai-1 di θi 1 α1 1’ d1 -90o 2 l1 α2 3 l2 α3 3’ l3 E d

Intermediate Phalanges Appendix – Datasets No. Proximal Phalanges Intermediate Phalanges Distal Phalanges 1 45 28 13 10 46 25 2 40 20 11 60 30 3 26 12 50 24 4 52 21 5 14 55 6 15 7 16 8 49 31 27 17 9 35 18

Appendix – The shortest satisfied linkage geometry No. l1 l2 l3 1 64.52632 39.07368 18.48424 10 62.52632 26.48424 2 56.52632 35.07368 22.48424 11 78.52632 53.07368 32.48424 3 37.07368 20.48424 12 66.52632 43.07368 24.48424 4 68.52632 13 5 14 72.52632 28.48424 6 15 60.52632 41.07368 7 16 8 17 9 49.07368 18

Appendix – Force sensor