Admittance shaping controller for exoskeleton assistance of the lower extremities
The control method for lower-limb assistive exoskeletons assists human movement by producing a desired dynamic response on the human leg. Wearing the exoskeleton replaces the leg's natural admittance with the equivalent admittance of the coupled system formed by the leg and the exoskeleton. The control goal is to make the leg obey an admittance model defined by target values of natural frequency, resonant peak magnitude and zero-frequency response. The control achieves these objectives objective via positive feedback of the leg's angular position and angular acceleration. The method achieves simultaneous performance and robust stability through a constrained optimization that maximizes the system's gain margins while ensuring the desired location of its dominant poles.
1. An exoskeleton system for assisted movement of legs of a user comprising:
a harness worn around a waist of the user;
a pair of arm members coupled to the harness and to the legs;
a pair of motor devices, wherein one of the pair of motor devices is coupled to a corresponding arm member of the pair of arm members moving the pair of arm members for assisted movement of the legs; and
a controller coupled to the motor controlling movement of the assisted legs, the controller shaping an admittance of the system facilitating movement of the assisted legs by generating a target DC gain, a target natural frequency and a target resonant peak, wherein the controller comprises:
an angle feedback compensator; and
an angular acceleration feedback compensator.
2. The exoskeleton system of claim 1 , wherein the angle feedback compensator generates a target DC gain.
3. The exoskeleton system of claim 1 , wherein the angle feedback compensator generates a target DC gain on the leg's admittance to compensate for the stiffness and gravitational torque on the legs.
4. The exoskeleton system of claim 1 , wherein the angular acceleration feedback compensator generates a target natural frequency and target resonant peak.
5. The exoskeleton system of claim 4 , wherein the angular acceleration feedback compensator generates target values of natural frequency and resonant peak magnitude of the leg's admittance.
6. The exoskeleton system of claim 1 , wherein the dynamics of the leg are modeled as the transfer function of a linear time-invariant (LTI) system, the controller replacing the natural admittance of the leg by the equivalent admittance of the coupled system formed by the leg and the exoskeleton.
7. The exoskeleton system of claim 1 , wherein the desired dynamic response of the assisted leg is given by an integral admittance model defined by X d h (s)=I/I d h (s 2 +2ζ d h ω d nh s+ω d nh 2 ), where I d h , ω d nh , and ζ d h are desired values of the inertia moment, natural frequency and damping ratio of the leg.
8. The exoskeleton system of claim 4 , wherein the angular acceleration feedback compensator matches the dominant poles of the coupled system with those of the target admittance, through a pole placement technique.
9. The exoskeleton system of claim 3 , wherein the angular acceleration feedback compensator prevents dominant poles from crossing to a right-hand side of a complex plane (RHP) or imaginary poles.
10. A device for controlling an exoskeleton system comprising:
a controller shaping an admittance of the system facilitating movement of assisted legs coupled to the system, wherein the controller models dynamics of one of the legs as a transfer function of a linear time-invariant (LTI) system, the controller replacing admittance of the one of the legs by an approximate equivalent admittance of a coupled leg and system by generating a target DC gain, a target natural frequency and a target resonant peak.
11. The device of claim 10 , wherein the controller approximately matches a dynamic response of the assisted legs to an integral admittance model defined as X d h (s)=I/I d h (s 2 +2ζ d h ω d nh s+ω d nh 2 ), where I d h , ω d nh , and ζ d h are predefined values of the inertia moment, natural frequency and damping ratio of the one of the legs.
12. The device of claim 10 , wherein the controller comprises: an angle feedback compensator; and an angular acceleration feedback compensator.
13. The device of claim 12 , wherein the angle feedback compensator generates the target DC gain.
14. The device of claim 12 , wherein the angle feedback compensator generates the target DC gain compensating for stiffness and gravitational torque on the legs.
15. The device of claim 12 , wherein the angular acceleration feedback compensator generates the target natural frequency and target resonant peak.
16. The device of claim 12 , wherein the angular acceleration feedback compensator increases a natural frequency of the legs and a magnitude peak of the legs admittance.
17. The device of claim 14 , wherein the angular acceleration feedback compensator matches dominant poles with the target admittance through a pole placement technique.
18. A method for an exoskeleton assistive control comprising:
calculating ratios between unassisted leg movement and a desired value through natural frequencies, resonant peaks and DC gains of the exoskeleton;
calculating angular position feedback gain k DC of the exoskeleton system;
calculating target admittance parameters ω d nh and ζ d h ;
obtaining a dominant pole of a target admittance as p h d =−σ h d +jw dh d ;
obtaining parameters {σ f , ω d,f } of a feedback compensator of the exoskeleton system; and
obtaining a loop gain K L and an inertia compensation gain I c of the coupled exoskeleton system and legs of a user.
19. The method of claim 18 , wherein obtaining the parameters {σ f , ω d,f } of the feedback compensator of the exoskeleton system comprises performing constrained optimization.