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Design of Pneumatic Exoskeleton using Artificial Muscles
1Vishal Parshuram Patil, 2Saurabh Krishna Mhatre, 3Nitin Yashwant Shingade, 4Sachin Bharatsing Pawar,
5N.A.Meshram
1,2,3,4,5Department of Mechanical Engineering Konkan Gyanpeeth College of Engineering Karjat, India
Abstract— This work presents the design of exoskeleton made up of mechanical links and control by mechatronics system using Artificial Pneumatic Muscles (PAM).
Exoskeleton could be regarded as wearable robots: A wearable robot is a mechatronic system that is designed around the shape and function of the human body, with segments and joints corresponding to those of the person it is externally coupled with. Robotic exoskeleton systems are one of the highly active areas in recent robotic research.
These systems have been developed significantly to be used for the human power augmentation, robotic rehabilitation, human power assist and haptic interaction in virtual reality. The upper extremity presented here will help to increase the strength of human which will help to lift the load more easily than human can with biological muscles.
In this, Air Muscle is used as the analogy of the biological motor for lifting the load. It has advantages like the passive damping, good power-weight ratio and usage in rough environments. This paper gives the characteristics, design, working of Exoskeleton along with Air Muscle in terms of contraction with variation of pressure at different loads and also in terms of volume of air trapped in it with variation in pressure at different load.
Keywords— Pneumatic Exoskeleton, Design, Analysis, Pneumatic Artificial Muscles (PAM), Working, Control.
I. INTRODUCTION
The main function of a powered exoskeleton is to assist the wearer by boosting their strength and endurance.
They are commonly designed for military use, to help soldiers carry heavy loads both in and out of combat. In civilian areas, similar exoskeletons could be used to help firefighters and other rescue workers survive dangerous environments. The medical field is another prime area for exoskeleton technology, where it can be used for enhanced precision during surgery or as an assist to allow nurses to move heavy patients.
While the exoskeleton that is presented here - with multi-degree of freedom in the body and an extended exoskeleton covering more limbs gives extra strength to the arm muscles to lift heavy objects with the help of pneumatic power. For example in a factory, on a construction site exoskeleton enables to interact with machine components which are too heavy to transport
from one place to other place with the help of muscular power only. This also avoids the accidents at construction site and protect the workers against injury by transferring the major part of the load to the exoskeleton. The upper body consist of Pneumatic Muscles as a power source, links and external metallic structure to support arm and back. The new system should have high power and force to volume or weight and good positional and force control. Different types of actuators are in practice such as electrical actuators, hydraulic actuators, pneumatic actuators, PAM etc. But only PAM can give softness of natural muscle, which makes it unique to use in exoskeleton.
II. WHY PAM?
Air Muscle has combined characteristics softness and compliance with powerful and accurate motion. Variety of techniques and materials are used in construction of Air Muscle but fundamental principles of fabrication are same. When weight of Air Muscles with weight of pneumatic cylinder is compared, Air Muscle is found to be quite advantageous. Weight of Air Muscle is only 10
% of pneumatic cylinder‟s weight with the same diameter. Initial force during contraction is 10 times bigger. It can exert force 400 times its weight. Typical DC motors or Pneumatic actuators (piston cylinders) can exert about 16 times their weight. Pneumatic artificial muscle works without stick-slip effect and therefore, small and slow motions are possible. The construction of these muscles gives good dynamic performance and position control.
III. THEORETICAL BACKGROUND
The technology of Exoskeletons is divided into two parts, lower extremity exoskeletons and upper extremity exoskeletons. The reason behind separating the two parts is that people can envision great applications for either part.
In the early 1960s, the Defense Department of the United States expressed their interest in the development of a powered suit of armor that would make soldiers lift and carry heavier weights. There are several attempts
has been done on designing exoskeleton using different systems and actuators like electrical, hydraulic, pneumatic. As we are concerned with pneumatic exoskeleton, the various exoskeleton which works on pneumatics are as follows:
A. Pneumatic exoskeleton with torso
Important scientific research started in the 1970‟s where the group around Vukobratovic played a pioneer role.
They had a clear goal in mind to help patients with defects in their locomotor system to regain walking capabilities. At this time, lack of computer processor power, heavy actuators (both pneumatic and electrical), and heavy power supplies limited the realization of interesting theoretical results. But they continued their work that led to interesting results as can be seen in Fig.1.
Fig. 1. Patient with pneumatic exoskeleton with torso B. Powered Lower Limb Orthosis
The powered lower limb orthosis developed at the University of Michigan aims at rehabilitation of patients with neurological injuries. This helps to restore lost locomotion capabilities of patients more effectively, concerning the quality of progress, and will reduce the costs of the whole process. The orthosis is powered at the knee and ankle joint with artificial pneumatic muscles. Experiments showed that the artificial muscles were able to produce a substantial torque contribution to the movement. This relieved the patient from 30–50% of the body weight.
Fig. 2. Powered lower limb orthosis
C. RoboKnee
RoboKnee is an exoskeleton developed by the company Yobotics, a spin-off from the Massachusetts Institute of Technology Leg Laboratoy founded in 2000. The device is supporting the knee motion with a series elastic actuator attached to the thigh and shank as shown in fig.2. The control system calculates the actuator force based on the knee torque necessary for maintaining a statically stable pose and according to this the actuation system will perform the desired task. The next steps of development involve inclusion of hip and ankle actuation and better detection of the operator‟s movement intent.
Fig. 3. RoboKnee
Moreover, Yamamato created an exoskeleton for assisting nurses with patient handling where lower limbs include pneumatic actuators for flexing and extending the hips and knees. The pneumatic power is provided by air pumps mounted directly onto each actuator.
Operators input is determined by force sensing resistors (FSR) attached to the operator‟s skin. These measuring resistors along with other information such as the joint angles are used to determine the input torques required for various joints.
IV. EXOSKELETON HARDWARE
This chapter describes the mechanical construction and the pneumatic actuation of the exoskeleton. In the following section (A) general requirements of an exoskeleton to design arm, shoulder joint, back support frame are specified. Those requirements lead to a design which is described in section (B) The actuation of this exoskeleton is described in section (C).
A. Requirement
Exoskeleton requires metallic links for arm and back frame to transfer load from arm to back. The size, material, properties required depends on which type and how much load we are going to lift. Here we have considered around 200N of load to be lifted by the model. It also consist of PAM which is the main actuating component of the system works on air pressure. It should be kept in mind that the power of the actuator determines its size, weight, and power consumption. For those parameters no definite limits can be given, since they are a matter of comfort and
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acceptance. The shoulder joint which here required depends on how many degree of freedom we are introducing in our system.
B. General Design
Our design has different components:
1) Arm
It is made up of material aluminium to reduce weight and increase toughness, having rectangular cross section of 20mm×5mm.The different linkages are connected by rivets to eliminate the problems encounter in welding.
So it is easy to assemble and dissemble. This is the main component to which we are going to applied load to be lifted with the help of PAM powered by air.
Fig. 4. CAD Model of exoskeleton arm 2) Shoulder Joint
It is made up of alloy steel having three degrees of freedom of angular motion in x-x, y-y and z-z direction.
3) Frame
It also made up of alloy steel of hollow square bar of 20mm×20mm cross section. The frame is very useful because it is the member who is going to take load which is acting on hand.
Fig. 5. CAD Model of back frame C. Numerical Analysis
While designing the Exoskeleton arm and to calculate various forces acting on it, we are considering some assumptions for the sake of simplicity. Such as, we have considered side link of the arm i.e. ABC as shown in Fig. is of equal length to the lower length where load is attached at its extreme end. Hence considering the horizontal length equals to 340mm and vertical length of link AD is of 150mm. Two muscles are attached to each side link across BD. If Ɵ is the angle made by force P which is resultant force of two muscles
P = 2×F (1) Where F = Force exerted by each muscle
Considering 200 N of load to be lifted by entire arm Therefore load acted on each link is 100N i.e. half of total load
Fig.6. Force calculation of arm links From ΔABD,
Ɵ = tan-1(AD/AB)
= tan-1(150/60) = 68.20
Taking moment about point A,
ƩMA = 0 = P × SinƟ × 60 – 100 × 340 P × Sin (68.2) × 60 = 34000 P = 610.31 N
From Eq. (1)
F = (610.31/2) = 305.15 N
Hence 305.15 N is the force acting in each air muscle.
If L is the length of air muscle then, L2 = AD2 + AB2
= 1502 + 602 L = 161.55 mm D. FEA Analysis
We have done design and stress analysis both on
„Autodesk Inventor Professional 2016‟ as follows:
1) Displacement
The maximum displacement is 20.41mm which is at the extreme end of lower link where load is attached.
Fig.7. Displacement analysis 2) Safety Factor
Safety factor is minimum at end point of side link which is 0.14 ul that means the section is critical for safety purpose.
Fig.8. Safety factor analysis 3) 1st Principal Stress
The 1st principal stress is maximum at the same critical section as that of safety factor and that value is 772.1 MPa.
Fig.9. 1st Principal stress analysis 4) Von Mises Stress
For this stress also, the critical section is same with value 1432 MPa.
Fig.10. Von mises stress analysis E. Actuation
Regarding the actuation, one has to decide between several fundamental concepts: rotatory actuators (like electric motors with harmonic drives), or linear actuators (electric motors coupled to ball screws, pneumatic or hydraulic pistons, pneumatic muscles). Here we decide to go with Pneumatic Artificial Muscles (PAM) which has many advantages as we discussed already in previous session.
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Air Muscle is an actuator that works very similarly to a human muscle. It contracts by thickening. High pressure air pumped through the rubber tube it inflates like balloon causing the Muscle to shorten by as much as 40%. It has very high force and weight ratio. Air Muscles are being used for number of years as actuators.
According to Baldwin physician Joseph L. Mckibben introduced as orthotic actuator in the late 1950‟s: due to resemblance of artificial muscle with human muscle it became ideal choice for orthotic actuator. In late 1980‟s Bridgestone reintroduced as the Rubber actuator and since then these types of actuators are used to power robots, mainly of anthropomorphic design, prostheses and orthotics.
The basic Air Muscle design consists of a rubber tube enclosed at one end, and sheathed by a nylon jacket.
One end of the inner tubing is sealed, and the other end is connected to an air pressure source. The unique action of the Air Muscle comes from the presence of the nylon sheath. When the inner tube is pressurized, it seeks to expand. However, the weave along the nylon sheath is such that it does not allow the inner tube to expand in thickness. Fig. shows the construction details of Air Muscle. Air muscle is constructed with a long party balloon rubber tube in polyester braided sheath. One end of balloon tube is connected with air inlet and other end is firmly plugged to avoid any air leakage.
Fig. 11. Construction details of Air Muscle The maximum diameter of the nylon sheath increases with decreasing length, and this is an inherent characteristic of the nylon woven sheath. Due to the unique woven nature of the sheath, the expansion of the Air Muscle leads to the thickening of the inner tube until a certain limit. After this stage, any further increase in pressure causes the Air Muscle to contract, thus decreasing the length of the nylon sheath, and subsequently increasing the maximum diameter of the weave of the nylon sheath, and giving more room for the inner tube to expand. This in result shortens the length
of Air Muscle. The net result of this action is that, with the application of a pressure source, an Air Muscle decreases in length, but increases in thickness. If a load is attached to the sealed end of the Air Muscle, this decrease in length will lead to a net displacement of the load, provided the size of load is well within the operating region of the Air Muscle.
For a given absolute internal air pressure (p), work input done on Air Muscle while inflating is given by
dWin = p×dV
and the output work produced in shortening of Air Muscle is
dWout = F×dx
where, x is contracted length of Air Muscle.
And F is the force exerted by the Air Muscle.
According to conservation principle, dWin = dWout
Therefore, F×dx = p×dV and hence F = p×(dV/dx)
Fig. 12. Operation of air muscle
With the experiment, it has been found that at constant load with increasing pressure in the Air Muscle volume trapped in Air Muscle increases and at the same time contracted length of Air Muscle decreases. This can be understood with the fig.13 as shown below.
Fig. 13. Operation of air muscle at constant load At constant load i.e., hanged weight (W), variations in pressure, volume and its contracted lengths are expressed as
p1 < p2 < p3 < p4 v1 < v2 < v3 < v4 x1 > x2 > x3 > x4
Similarly, at constant pressure with increasing load, contraction in length decreases and at the same time volume trapped in Air Muscle increases. This can be realized with the fig.14 as shown.
Fig. 14. Behaviour of air muscle at constant pressure At constant pressure p,
F1 = 3W; F2 = 2W; F3 = W;
F1 > F2 > F3, v1 < v2 < v3, x1 > x2 > x3.
V. DISCUSSION
Need of Pneumatic exoskeleton has studied successfully.
The structure of exoskeletal frame is designed with dimensions and as per requirements. Analysis is done for all possible stresses and displacement for applied load and given constraints and we get safe values for design. The behavior of Air muscle under varying
parameters and different theoretical as well as experimental aspects are investigated. Contraction of the Air muscle under varying loads as well as the amount of volume trapped in Air muscle have been thoroughly studied. Subsequent upon the increase in air pressure to muscle, it is observed that less pressure increase is needed initially to let air muscle contract. The designed exoskeleton model is capable to work for expected performance.
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