Biomechanical considerations in the design of an artificial hand

This paper presents the design of a hand prosthesis and the implementation of an anthropomorphic finger with three degrees of freedom. The design integrates anatomical considerations, which will be emulated with alternative materials that meet the biological characteristics of the hand. The built model is inspired by the real anatomy of a human finger, in order to provide all the different handholds.

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BIOMECHANICAL CONSIDERATIONS IN THE

DESIGN OF AN ARTIFICIAL HAND

Oscar F Avilés., Daniel H Sánchez, Oswaldo Rivera,

Mauricio Mauledoux, Oscar I Caldas

Universidad Militar Nueva Granada, Programa de Ingeniería Mecatrónica, Grupo de Investigación en Mecatrónica DAVINCI, Cr 11 No 101-80 Bogotá D.C, Colombia

ABSTRACT

This paper presents the design of a hand prosthesis and the implementation of an anthropomorphic finger with three degrees of freedom The design integrates anatomical considerations, which will be emulated with alternative materials that meet the biological characteristics of the hand The built model is inspired by the real anatomy of a human finger, in order to provide all the different handholds The calculations of the direct and inverse kinematics are made in order to know the workspace and points of the final effector of the anthropomorphic finger

Keywords: Prosthesis, Anthropomorphic Finger, Anatomy, Tendons, Cartilage,

Kinematics, Grip Taxonomy

Cite this Article: Oscar F Avilés., Daniel H Sánchez, Oswaldo Rivera, Mauricio

Mauledoux, Oscar I Caldas, Biomechanical Considerations in the Design of an

Artificial Hand International Journal of Mechanical Engineering and Technology

10(12), 2019, pp 330-342

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=12

1 INTRODUCTION

The success of mankind evolution is mostly due to its ability to explore and study the surrounding world, but also to the workability of the human being With the hand objects grabbing, holding and manipulating can be performed with great skill, which is important for performing tasks of daily living with tools

Robotic hands are clearly human-inspired, and their models have led to innovative and expensive fingers prototypes to be used as functional parts of anthropomorphic robots Several work can be found in literature, such as in Stanford / JPL Hand [1], UTAH / MIT Hand, TUAT/Karlsruhe Humanoid Hand, DLR Hand, NASA Robonaut or UMNG Hand Other related works are the fingers of m Cecarelli et al [8], who proposed the motion kinematics of human fingers, using sequences of video recording and photo as a methodology

to determine the function of the movement Garcia et al [9] used a system which emulates the performance of the finger muscular system, represented by a transmission system based on tendons Fabrizio Lotti et al [10] developed a finger that has two degrees of freedom and low weight, using rigid links connected with flexible elements and actuators for linear displacement In Table 1, a review of some related works on robotic hands is presented [14]

Table 1: Technical characteristics of various artificial hands

Size (norm.)

# of fingers DOF

Actuators (type) Control

# of Sensors

Weight (g)

Force (N)

Speed (s)

(Muscl) E ≈17’000 ≈ 400 > 300 0.25 Ottobock

SUVA (Otto Bock, 2002) ≈1 3 1 1 (E) I 2 600 < 100 < 1

Southampton (Light, 2000) > 1 5 6 6 (E) E - 400 38 2.5 1

Utah/MIT (Jacobsen, 1983) > 2 4 16 32 (P) E 16 - 31.8 -

Stanford/JPL (Salisbury, 1983) ≈ 1.2 3 9 12 (E) E - 1100 1 -45 -

Robonaut (Lovchik, 1999) ≈ 1.5 5 12 +

2 14 (E) E

43 +

UB

(Bonivento,

Barret

(Townsend,

UMNG (Ocampo, 2004) ≈ 1 4 12 +

Currently, advanced robotics is moving from the classical concept of precise and rigid structures, often heavy and complex, to more flexible and lightweight structures, with the prospect of an increase in performance, high mechanical simplicity and therefore a considerable cost reduction The robots, especially humanoid and industrial manipulators, are designed to assist in various jobs in industry and at home Therefore, these robots need to be able to perform tasks that humans normally do, and they must be provided with an end effector (tool) that can manipulate objects with the same skill of a human being, as shown in Fig 1

The design of an anthropomorphic hand is highly complex and depends on the tasks desired to be performed The main objective is the realization of common tasks in daily life, including the different types of grip as shown in Fig 1

Figure 1 Schematic of a human hand

2 ANATOMY OF THE HAND

The different components of the prehensile system are inspired in the human hand The objective is to mimic the hand skills and interaction with different environments [8]

The human hand is connected to the wrist and has twenty-two DOF moved by closely forty muscles [9, 10] From fig 1, the thumb is fixed more proximal than the other fingers and thanks to its carpal and metacarpal bones [9, 14] it can close and rotate, so the orientation plane can be changed allowing the thumb to oppose the other fingers, allowing the extension/abduction movement (in and out from the hand)

2.1 Geometric Model

The knowledge of the geometric model of the human hand is one of the purposes in this work,

in order to properly design a finger and an artificial hand Besides of the possible simulations, the geometric model allows to evaluate the proposed theoretical hypotheses and facilitates the understanding of position and orientation of the fingers with respect to a reference point

2.2 Skeletal Description of the Hand

The five fingers of the human hand can be referenced like this: thumb ; index ; middle ; ring ; and pinky The last four fingers are formed by four bones referenced as follows: Metacarpal phalange (Pi0), proximal phalanx , middle phalanx and distal phalanx The thumb is the shortest of all (it has two phalanges), and it is composed of five bony pieces: scaphoid bone , trapezius , first metacarpal , first phalanx and second phalanx Table 3 shows the dimensions of the aforementioned bones

2.3 Two-Finger Interphalangeal Biomechanics

All the joints are formed by reciprocal interconnections in each of the phalanges The adjustment in these joints is maintained and stabilized by the lateral ligaments Depending on the bony surfaces in contact and the arrangement of the ligaments in the joints, fingers can have 1, 2 or 3 degrees of freedom Excluding the thumb fingers have two types of joints: metacarpo-phalangeal (Oi1) and proximal and distal inter-phalangeal (Oi2), (Oi3), the thumb has three: the trapezo-metacarpal (O01), metacarpophalangeal (O02), interphalangeal (O03), (see Fig 2)

The human hand has 25 active DOF divided into 5 DOF per finger, those DOF differ between the thumb and the other fingers For a correct and agile manipulation, one must

analyze how an object is grasped and manipulated and what forces must be applied to it The development of a robotic hand requires knowledge of the geometric relations of the manipulator-object system, therefore, there are two main kinematic models in a multi-finger manipulation system: hand kinematics and contact kinematics In Fig 2, a model of the hand can be seen as a kinematic chain according to the mobility of each of the joints

Figure 2: Biomechanical equivalence in an artificial hand

The muscles and joints of the hand allow great deal of prehensile configurations which can be divided in two main groups, the prehensile and non-prehensile In the first group, the object can be grabbed partially or fully within the hand, meanwhile the second one does not involve grabbing actions, instead the objects are lifted with the entire hand or just the fingers (see Fig 3)

Figure 3: Taxonomy of human prehensile models

For the design of an anthropomorphic finger, the following measurements are considered according to the approximation of the lengths stipulated in Table 2 and Table 3

Table 2 Average anthropomorphic data for index finger

Phalanx

Phalanx measurements [mm]

Distance between phalanx [mm]

19,67±1,03

5,58 ± 0,92

24,67 ± 1,37

7,57 ± 0,45

43,57 ± 0,98

15,57 ± 0,84

71,57 ± 5,60

Table 3 Human hand articulations and boundaries

Element Joint DOF Flex-Ext

angle

Abd-Add angle

2.4 Kinematics

For the formulation of the kinematic model of the human hand the Denhavit & Hartenberg -

DH formalism is used With this method modeling is done through local vectors, where the positions and orientations of various points of interest are obtained and used for the graphic construction of the model, through primitive elements

From the definition of the joints of the skeletal structure, define a reference system for each point Oi Fig 4 shows the structure of a human finger and its corresponding coordinate systems, as well as the joint variables that are considered to obtain the model

2.5 Direct Kinematics

With the acquired data from the finger, a prototype for the hand is designed (Fig 6) keeping

in mind the following requirements in the designing process:

 Absence of all DOF in the wrist (it is designed to be part of a robotic arm) The only movable pieces will be the fingers

 The metacarpal phalanx articulation only has one DOF

 The distal inter phalanx and proximal inter phalanx articulations are revolution joints (1DOF)

By using the Denavit-hartenberg parameters (table in Fig 4) where are the angles for each phalanx, p is the length of the palm, and

are the distances of each phalanx

Joint  l1 d1 i

Figure 4: Geometric representation of a finger 3 DOF

(1)

Where is the angle between phalanges, ( is a homogenous transformation matrix, is the length of each phalanx and is the position of the fingertip Having the matrices of transformation of each joint, the next step is to multiply them all to obtain the general matrix that indicates the final position in which the anthropomorphic finger is The last generated matrix is different from the others because it is constant while the previous matrices depend on the joints Taking into account that y the transformation matrix is: [ ] (2) Where:

These equations give the value of the position of the fingers of the hand as a function of the joint coordinates With the data obtained from the solution of the problem of direct kinematics, the points that the fingertip can reach by performing the movements of flexion and extension were found Table 3 indicates the range of motion of each bone and the reach of the finger can be observed as shown in Fig 5

Figure 5: Graph of reachable points by the human finger

3 ARTIFICIAL ANTHROPOMORPHIC FINGER

Next, the built model of a biologically inspired finger is presented The motion transmission system is made through high resistance nylon wires, Figure 6

Figure 6: Artificial finger prototype

Once having the range of flexion and extension of a human finger and the anthropomorphic finger, a comparison is made to calculate the margin of error obtained in order to determine the trajectory described by the artificial finger

As can be seen in Fig 7, the anthropomorphic finger does not reach the limit of flexion However, its range is not limited since it can reach different positions just like the human finger does The percentage of error obtained is 9%

Figure 7: Artificial finger workspace graphic

3.1 Tools and Software

For the design of an artificial hand based on the previously described finger, the computer-aided design (CAD) SolidWorks® software was used, which is then linked to Matlab® to perform dynamic and control simulations

Matlab was used to load the design of the prosthesis made in SolidWorks, simulate the kinematic behavior of the hand and to create a user-machine interface

3.2 Virtual Simulation Environment and Simulation

To design the model of the hand, an assembly-type file was created using the SolidWorks software where all the pieces were joined and the final design of the hand was generated, as seen in Fig 8; since the aim of the design is to show the movement of the possible prosthesis Matlab was chosen as work software due to the possibility of creating a user-machine interface to interact with the designed prosthesis The simulation design was made using Simulink, in which the data of angles entered by the user are applied to the joints of the corresponding finger

The assembly made in SolidWorks was imported using the SimMechanics toolbox This toolbox allows to see the behavior of the hand when executing the simulation, as well as the location of the center of mass of each one of the parts of the hand, along with its orientation

Figure 8 Assembly design of the SolidWorks hand and simulation environment

For the user-machine interface a Guide was made in Matlab, which is presented in Fig 8 This interface allows the user a gradual variation of the joint angles of the Proximal, Middle and Distal Phalanges for the little fingers, annular, heart and index For the thumb we worked with the joints of Metacarpo, Proximal Falange and Distal Falange

The diagram of Fig 9 was made in Simulink and shows how each part of the prosthesis is connected in the palm Also showing the 5 fingers

Figure 9 Simulation diagram of the simulated prosthesis

For each finger, Figure 10, a simulation is performed in which a connection is made between each phalanx by existing joints These joints enter the angle that the user determines

Figure 10 Simulation finger diagram in Simscape

Finally, Figure 11 shows the different grip configurations achieved with the virtual hand

Figure 11 Movements obtained in the virtual environment

3.3 Experimental Validation

To validate the three degrees of freedom of the anthropomorphic finger, a montage is made as per the architecture presented in Fig 12, where the movements of the virtual finger are synchronized with the proposed finger In the system, the desired angle value is sent through serial communication to the embedded system, which sends the command to the motors and the movement is made in the artificial finger and at the same time it sends the same angle to the virtual finger

Figure 12 Architecture experimental validation system

Fig 13, shows movements performed by the prototype designed utilizing index finger and thumb