POWERED ANKLE-FOOT PROSTHESISDarshana Unnikrishnan Reg. No: 110925010 M.Tech Control Systems CONTENTS • • • • • • • Introduction Biomimetic design goals Shock tolerance Force bandwidth Energy economy Conclusion References • Because of their passive nature. . especially at moderate to fast walking speeds.INTRODUCTION • Compared to intact persons.walking amputees require 10-60% more metabolic energy. • In contrast. the human ankle performs positive net work and has greater peak power over stance period. such prosthesis cannot generate more mechanical energy than is stored during each walking step. • Commercially available prosthesis comprise spring structures that store and release elastic energy while walking. a powered prosthesis must be position-and impedancecontrollable. • A transtibial amputee overcomes these energy deficiencies by using hamstring muscles to aggressively extend the hip throughout early stance.Cont. • Also. . • Robotic ankle controllers follow preplanned kinematic trajectories during walking.. • But human ankle is believed to operate in impedance control mode during stance and position control mode during swing. • Ankle-foot mechanisms for humanoid robots are often too heavy or not sufficiently powerful to meet the human-like specifications required for a powered prosthesis. Biomimetic design goals • Size and mass: – prosthesis height is 18cm from the ground.2 rad/s – Torque = 1. • Torque and speed: – Peak velocity = 3.5 Hz. – Bandwidth = 3.5% of the total mass of the body.2 Nm/kg – Power = 3 + 1 W/kg • Torque bandwidth: – defined as that frequency range over which 70% of the total signal power was captured.6 + 0. . – Prosthesis mass is 2.6 + 0. Cont… • Net positive work: – Average positive work done at the human ankle per unit body mass for self-selected speed is 0. 175 cm.05 J/kg. respectively” . and foot length are 78 kg. and 27 cm. walking at a self-selected speed of 1. • Controlled dorsiflexion stiffness: – The average human ankle stiffness per unit body mass at a self-selected walking speed is 8 + 1 Nm/rad/kg.25 m/s.21 + 0. “ankle-foot prosthesis for a nominal male subject. height. whose body mass. we combine a linear model of an ankle-foot prosthesis with a series elastic actuator( SEA). Me = ImR2 Fe = ¼ TmR be = ¼ bmR . • To understand how much series compliance is needed.Shock Tolerance • Series motor compliance has been used in humanoid leg design to effectively lower shock loads and protect the motor transmission from damage. .Cont… • Me is the effective mass • Im is the rotary internal inertia • Fe is the linear motor force • be is the damping • bm is motor friction from bearings and brushes • Assume that the foot is a rigid body of negligible mass. as foot mass is relatively small compared with the effective motor inertia. the negative work done on the leading leg is increasing from 6.00 m/s. we assume that the prosthesis worn on the leading leg has to absorb all 27 J of energy during anklecontrolled plantar flexion. • To determine the series stiffness that adequately protects the transmission.Cont… • the trailing leg performs positive external work on the body’s center of mass. • the leading leg performs predominantly negative external work.5 J at 0.8 J at the maximum walking speed of 2.75m/s to 26. . • In the study. Torque (Nm/kg) Ankle angle (rad) Fig 1: Ankle torque is plotted versus ankle angle . – a ballscrew transmission (Nook ECN-10030-LG. .4 g/cm2 • be = 8.250 Ns/m based on experimental measurements.Cont… • Components used: – a 200-W dc brushless motor (Maxon EC-Powermax 30) • Im = ¼ 30. 10 mm33 mm) • specifically sized for a nominal male foot size of 27cm • maximum transmission load rating of 5 kN • A series stiffness value of 600 kN/m results in a peak transmission force approximately equal to the 5-kNload limit of the ball-screw transmission. • In series compliance.FORCE BANDWIDTH • The actuator system should not saturate within the desired operating range of torque and speed. because of motor saturation. . the openloop force bandwidth is reduced when a spring is placed in series with the motor and transmission. • 7-Hz open-loop torque bandwidth. • Find the series compliance necessary to produce at least that bandwidth by equating θ equal to zero. Fig 2: Series Compliance . • In summary. the open loop bandwidth becomes 20Hz. • Thus parallel motor elasticity is introduced to the motor architecture. . • With parallel elasticity. the maximum level of series stiffness that adequately protects the transmission from damage during heel strike fails to satisfy bandwidth requirements. Fig 3: A powered prosthesis with both series and parallel elasticity . ENERGY ECONOMY • A powered prosthesis must operate for at least one full day on a single battery charge. • To calculate the values of stiffness for the series and parallel springs that minimize the prosthesis COT. . consider the standard dc motor model. • Energy economy is the electrical energy required to transport unit body weight (amputee + prosthesis) in unit distance. 05. .000 steps of powered walking.22-kg) would enable 5. • Li-Polymer battery (0.• The prosthesis COT is calculated as 0. Fig 4: Powered ankle-foot prosthesis with series and parallel elasticity. . . • To minimize prosthesis COT and motor or transmission size. • Thus. orthotic. • In future investigations. and exoskeletal applications. we can apply the ankle-foot design to robotic.CONCLUSION • The minimum level of series compliance that adequately protects the transmission from damage during foot collision fails to satisfy bandwidth requirements. parallel motor elasticity is used to lower the forces borne by the SEA. we select a parallel stiffness that supplies the necessary ankle stiffness during early stance period dorsiflexion. enhancing system force bandwidth. 2) E. pp. ‘‘Energy/speed relation of below-knee amputees walking on motor-driven treadmill. vol.’’ Eur. S. 173–185. 3) G. 272–278. Gonzalez. Rehabil. pp. vol. J. Colborne. ‘‘Analysis of mechanical and metabolic factors in the gait of congenital below knee amputees: A comparison of the SACH and Seattle Feet. Naumann. Phys. Berbrayer. pp. 1973. H.. Reyes. and D. 31. R. L. . 3. J. Longmuir. 55. vol. ‘‘Energy expenditure in B/K amputees: Correlation with stump length. 1992. E. P. G.’’ Arch. Physiol. Phys. and R. no. Corcoran. 3.. Med. no.’’ Am. 1974.REFERENCE 1) N. 111– 119. Appl. P. J. Med. Molen. 5. no. 71.. Rehabil.