1. Mechanical Design of a Finger Exoskeleton for Haptic Applications
VCM-2016
November ,
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

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

3. 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
INTRODUCTIOn

4. 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

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

6. 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

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Results and Discussion – Geometry optimization
Table 2: Optimization result Z
Joint
Joints’ RoM (deg)
Links’ length (mm) 1
[3.64;159.23]
110 2
[ ; ] 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

17. 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

18. 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

19. Results and Discussion – Joints’ RoM
α 1 ∈[ −15 0 ; ]
α 2 ∈[ −140 0 ; −40 0 ]
α 1 ∈[ −85 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

20. 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

21. 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

22. 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

23. 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

24. 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

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

26. 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

27. Appendix – The shortest satisfied linkage geometry
No. l1 l2 l3 1 10 2 11 3 12 4 13 5 14 6 15 7 16 8 17 9 18

28. Appendix – Force sensor