201 7 . 5 . 15 Control of lower extremity exoskeleton based on admittance method - PowerPoint Presentation

Slide1

2017.5.15

Control of lower extremity exoskeleton based on admittance method

Authors:Wenhao

Chen;

Shengli

Song

Catalogue 1、Human-exoskeleton Coupling Dynamics Mode 2、Admittance Control 4、Simulation

5、Conclusion3

、Stability Analysis and Parameter Optimization

1、Human-exoskeleton Coupling Dynamics Mode

Where

,

,

are

the moment of inertia, joint damping coefficient and joint stiffness coefficient of the human calf

respectively;

is

the knee joint angle trajectory

and

is

the net muscle torque acting on the joint.

Where is the coupling torque representing the interaction between exoskeleton and the human calf. is the damping coefficient and is the stiffness coefficient

is the damping coefficient and is the coefficient

Exoskeleton only provides power to human calf

exoskeleton

Human calf

2.1、Admittance

Transfer function

s

are given by

2.2、Admittance ControlThe control consists of a positive feedback control

From

the fig,

we can

get

2、Admittance Control

When the parameter a, b, c are greater than 0, the exoskeleton’s admittance characteristics can be offset by the control function , which makes the exoskeleton be equivalent to a small mass. Thereby reducing the coupling with the human calf, can play a lower tracking error and coupling torque.

3.1 Stability Analysis

For the parameters

,

,

,

and

are greater than 0, a , b and c can be obtained :

In

order to make the system have better stability, the

designed

system is the minimum phase system, that is, the open-loop transfer function has no pole and no zero in the right half of the complex plane.

In addition, the system gain margin

and phase margin

should also be met:   

3.2 Parameter OptimizationWhen the human calf swing at the angular velocity , the module of the closed-loop transfer function 𝐺(𝑗𝜔) should be the smallest. As the human calf can swing at different angular velocities, so set the objective function: 

Optimization results are a=0.89, b=0.91, c=0.95

.

4、Simulation The figure show that the phase of the calf swing in the case 1(wearing exoskeleton without power) is significantly lagging behind that in the case 2(the calf is free to swing without exoskeleton), and the amplitude is also smaller, indicating that wearing exoskeleton without power, exoskeleton is a burden for human body. And the calf's swing in the case 3(adopts the admittance control) is very close to that in the case 2.

4、Simulation The figure shows that the tracking error adopted the optimized parameters is smaller than that adopted unoptimized parameters. And the peak of tracking error adopted unoptimized parameters is about 0.5°

5、Conclusion The admittance control can shield the mechanical admittance of the exoskeleton itself, thus reducing the tracking error and the coupling torque of the exoskeleton. And the optimized admittance control can achieve more accurate motion tracking.

Thank You！