Design of a hybrid mechanism for shoulder joint motion.

Grecu, Valentin ; Dumitru, Nicola ; Grecu, Luminita 等

1. INTRODUCTION

In the non technical area of applications, exoskeleton are becoming important in assisting elderly or physically weak people function without help, in rehabilitating people injured in accidents or in war, and in temporarily supporting people suffering from muscle atrophy. In the cases of muscle atrophy especially, such a device can be used together with physical therapy in order to accelerate the recovery process.

Among the human joint, the shoulder joint is an important one, as many human motions require its use. This joint is one of the most complex and therefore the design of exoskeleton device for the shoulder joint is quite difficult to be done.

2. KINEMATIC DESIGN AND ANALYSIS

2.1 Centres of Rotation

Human arm shoulder has 7 degree of freedom. Many of the mechanisms are already made but they do not include the motion of the center of rotation of the humerus with respect to the scapula. In the existing mechanisms there are preferred the motion of the hand not of the center of rotation, so their wearing is uncomfortable. The design of the exoskeleton mechanism presented in this paper, aims to reproduce the human movement of the humerus with respect to the scapula (Papadopoulos & Patsianis, 2007).

In this exoskeleton mechanism two parts are important first Geneva mechanism and second the four-bar mechanism. This combination permits us to refer to the mechanism as to a hybrid mechanism.

In most existing mechanisms, the glenohumeral joint is modeled as a 3-DOF ball and socket joint and therefore, it doesn't include translation of the glenohumeral joint and thus of centers of rotation.

As we know in a human body at a shoulder joint we have two center of rotation of the humerus head with respect to the scapula. The distance between these centers of rotation should be 3 to 4 mm. They are represented in Fig. 1.

The side hand motion up to 30[degrees] will move with respect to CR1 (center of rotation 1) then till 60[degrees] it will shift to CR2 (center of rotation 2). At last the side hand motion will go up to 180[degrees] (McCormick, 1970).

[FIGURE 1 OMITTED]

2.2 Geneva Mechanism

Geneva mechanism is a type of an intermittent motion mechanism. When holding of position become necessary then we use the Geneva mechanism (Vidya & Kumaresan, 2000). Geneva mechanism is used in this hybrid mechanism due to its intermittent motion nature. In Geneva mechanism two parts are mainly important: star wheel and crank wheel. Star (Geneva) wheel is a wheel with number of slots. Crank wheel is a simple wheel on which a pin becomes project. In Geneva mechanism crank wheel and star wheel are placed at calculated distance. Crank wheel get the rotary motion and its pin enters in the Geneva wheel tangentially.

3. PRESENT WORK

In modeling and simulation of this mechanism there are necessary CAD tools. In the present work for modeling the parts of the hybrid mechanism and for assembling them Solidworks 2005 tools are used. The computational steps and simulation is done with programming tools Visual Basic 6.0.

4. MODELING OF THE MECHANISM PART

For modeling the upper limb of the human body we used three important parts: a Geneva Mechanism, Connecting Links, Hand Cover (Kiguchi & Fuduka, 2004). A Modeling of Hybrid Mechanism includes two parts which are modeled: crank wheel and Geneva wheel. In Fig.2 there are four inputs for the crank wheel: crank radius, crank depth, pin distance and pin depth.

[FIGURE 2 OMITTED]

In Fig.3, there can be noted the inputs for the Geneva wheel: crank wheel radius, crank wheel depth, number of slots, angle between the slots and pin radius.

[FIGURE 3 OMITTED]

5. ASSEMBLY AND SIMULATION OF MECHANISM

After the computational steps made in Visual Basic, the output dates are stored in files which are them used by Solidworks environment. The entire model consisted in Geneva wheel, crank wheel, hand cover and connecting links will be assembled. Simulation is only possible in assembly mode.

Mechanism parts should not be interacted to each other otherwise simulation will not be started. For simulation option chooses clockwise direction for the crank wheel to shift the center of rotation of limb in upper side for lower motion chooses vice versa. Take a short time step in simulation to see the small changes at the time of motion of mechanism. Choose only crank wheel rotation for simulation because in actual condition crank wheel is rotated by motor.

The centerline of the slot must be tangent to the circle with radius r, described by the center of the pin at the position in which the pin enters or leaves the slot. This condition dictates that the center distance of the two wheels must be r [square root of 2].

The expression [phi] = [phi] ([theta]) that relates the star wheel displacement to the displacement of the driving wheel.

r sin [theta] = x sin [phi] (1)

r cos [theta] + x cos [phi] = r [square root of 2] (2)

where r representing the radius of the pin circle and x the distance between the driving pin and the center of star wheel. The desired expression [phi] = [phi] ([theta]) is given by

[phi] = [tan.sup.-1] (sin [theta]/[square root of 2] - cos [theta]) (3)

The corresponding velocity of the star wheel, [??], is given by,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

and the corresponding acceleration by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

with - [45.sup.0] [less than or equal to] [phi] [less than or equal to] [45.sup.0] and [37.sup.0] [less than or equal to] [theta] [less than or equal to] [143.sup.0].

The total weight of the proposed mechanism was estimated about 1.6 kg and the maximum stress about 230 N/[mm.sup.2] with the maximum displacement at 2.6 mm (Grecu & Dumitru, 2009). This mechanism main deal with motion of center of rotation and step-by-step arm motion (Fig.4).

[FIGURE 4 OMITTED]

The nature of the required torque for the abduction and adduction of the human arm is seen in Fig.5, noting that the maximum torque is about 2 Nm.

[FIGURE 5 OMITTED]

6. CONCLUSION

The developed mechanism is well suited to help people who are suffering from muscle atrophy especially. Such a device can be used together with physical therapy in order to accelerate the recovery process. In the present time many existing equipment does not have a property to shift the center of rotation of limb. They directly give the side arm motion. This kind of mechanism is very useful for side arm motion without pain because it gives a step-by-step motion. In this hybrid mechanism, Geneva mechanism is the main part to give step-by-step motion. The modeling of Geneva mechanism is programmed by programming tools of Visual Basic 6.0 interfacing with the modeling software Solidworks 2005. The simulation of this mechanism is also done by Solidworks software in assembly mode. By this interfacing we can save time and reduce the complexity of Geneva mechanism.

7. REFERENCES

Grecu, V. & Dumitru, N. (2009). Analysis of Human Arm Joints and Extension of the Study to Robot Manipulator, Proceedings of the International MultiConference of Engineers and Computer Scientists, Hong Kong, pp 13481351

Kiguchi, K. & Fuduka, T. (2004). A 3 DOF Exoskeleton for Upper Limb Motion Assist: Consideration of the Effect of Bi-articular Muscles, Proceeding of International Conference on Robotics and Automation, pp 2424-2429.

McCormick, E.J. (1970). Human Factor Engineering, 3rd ed. New York, Ed. McGraw-Hill

Papadopoulos, E. & Patsianis, G. (2007). Design of Mechanism for the Shoulder Joint, Nation Technical University, Athens, Greece

Vidya, V. & Kumaresan, P. (2000). Design of a Micromachined Geneva Wheel as a Mechanism to Obtain Intermittent Motion from a Constantly Rotating Source', University of California, Berkeley