design of lower limb exoskeleton
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Design of Exoskeleton for Paraplegic PatientsAlok Bharadwaj | Aditya S. N | Anirudh V KaushikR V College of EngineeringBangalore
OverviewKinematicsDynamicsDesignAnalysisControllingDesign Features Comfort level,Manufacturability, Compactness and structural stability.
Kinematics of Mechanism
Kinematics of MechanismHip (Passive joint)
Kinematics of MechanismHip and Knee jointUse of mechanical stoppers to limit the angular movement of limbs.
Kinematics of MechanismFoot joint (Ball and Socket)
Dynamics of MechanismAssumptions
Mass of all links = 4kgMass of all motors = 4kgMass of battery pack = 2 kg Weight of human considered = 1000N
Dynamics of MechanismCase 1 - Standing upFigure AFigure B
Dynamics of MechanismStanding up
Torque required by motor at M2 = (1000 N) X (450 mm) = 450000 Nmm
However, as the motion begins, the position of upper body keeps changing. Thus, due to change of Center of gravity, required torque is much lower progressively.
Also, the crutches take up most of the body weight as the person can easily bend upper body to stand up.
This value is rounded off to 50000 Nmm shared by each leg. Thus, torque at each motor M2 is 25 Nm.
Dynamics of MechanismCase 2 - Climbing stairs
Torque required by knee motor= (1100 N) X (335 mm)=368500 Nmm
After considering the load taken up by using crutches and bending forward in order to climb stairs, this value is considerably lesser. (Assumed to be 50 Nm for each knee motor in each leg)
Thus, the maximum torque at knee considering previous case too becomes 50 Nm.
Dynamics of MechanismCase 3 Raising leg (maximum extent)
Torque required by motor at hip= (110 N) X (450 mm)=49500 Nmm
Thus, the maximum torque required at the hip joint is around 50 Nm at each motor of the hip joint.
Design Complete Assembly Ez Walk
DesignChoosing the Battery:For given maximum load @ 20rpm, the torque required is 50Nm.Required power = 100W per motor at 24V DCTotal max power input = 400W for motorsOverall power output = 500W Battery assumed to last for two hoursCapacity needed 40000mAhWeight of LiPo batteries ~ 3.5Kg
AnalysisMaterial Used: Al 6061Ultimate strength: 310MpaYield strength:276MpaShear strength:207MpaFOS considered:3Failure mechanism: Von Mises
AnalysisCritical Link: Thigh link
Controlling the mechanism
Initiating walkingAngle measurement using strain gauge measures the angle of upper body with respect to ground.Length of step proportional to step length.
Use of Ultrasonic Sensors2 sensors are used in each leg one at hip strap and one at footHip sensor senses variation in the terrainFoot sensor senses the vertical distance to the ground
Different cases requiring controlWalking
Different cases requiring controlClimbing Stairs UP
Different cases requiring controlClimbing Stairs DOWN
Safety FeatureCrutches are provided to give proper balance when the person bends forwardWhile climbing down, if the person leans forward (variation in strain gauge), the mechanism is turned off preventing his fall.
Advantages of DesignComfortUse of straps for even distribution of loadWeight evenly distributed on both sidesWeight of mechanism not being transferred to the body.
Advantages of DesignManufacturabilityLinks used and Aluminum 6061 are readily available in the market.Assembly is simple. Most assembly is just through press fit. Fasteners are required only at a few places.No requirement of a CNC for manufacturing of any componentBall socket joints can be readily bought off the shelf
Advantages of DesignWeightAluminum has a very low density making the mechanism very lightEvenly distributed loads at all locations. No point loads.Weight evenly distributed b/w both legs resulting in no lateral CG offset. This makes the mechanism easy to useLow weight motors
Advantages of DesignSpecial featuresAdjustable linksPortability ease of assembly. Links can be press fitted into each other. Very less use of fasteners.