Inverse Kinematics

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# Inverse Kinematics - PowerPoint PPT Presentation

Inverse Kinematics. Set goal configuration of end effector calculate interior joint angles. Analytic approach – when linkage is simple enough, directly calculate joint angles in configuration that satifies goal. Numeric approach – complex linkages

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Presentation Transcript

Inverse Kinematics

Set goal configuration of end effector

calculate interior joint angles

Analytic approach – when linkage is simple enough, directly calculate joint angles in configuration that satifies goal

At each time slice, determine joint movements that take you in direction of goal position (and orientation)

Forward Kinematics - review

Pose – linkage is a specific configuration

Pose Vector – vector of joint angles for linkage

Degrees of Freedom (DoF) – of joint or of whole figure

Types of joints: revolute, prismatic

• Tree structure – arcs & nodes
• Recursive traversal – concatenate arc matrices
• Push current matrix leaving node downward
• Pop current matrix traversing back up to node

Inverse Kinematics

End Effector

L1

q3

q2

L3

L2

q1

Goal

Inverse Kinematics

Underconstrained – if fewer constraints than DoFs

Many solutions

Overconstrained – too many constraints

No solution

Reachable workspace – volume the end effector can reach

Dextrous workspace – volume end effector can reach in any orientation

Inverse Kinematics - Analytic

Given arm configuration (L1, L2, …)

Given desired goal position (and orientation) of end effector: [x,y] or [x,y,z, y1,y2, y3]

Analytically compute goal configuration (q1,q2)

Interpolate pose vector from initial to goal

Analytic Inverse Kinematics

Multiple solutions

Goal

(X,Y)

Analytic Inverse Kinematics

L2

180- q2

L1

(X,Y)

q1

qT

Analytic Inverse Kinematics

L2

180- q2

L1

(X,Y)

q1

Y

qT

X

Analytic Inverse Kinematics

L2

(X,Y)

L1

180- q2

q1

qT

Y

X

Iterative Inverse Kinematics

When linkage is too complex for analytic methods

At each time step, determine changes to joint angles that take the end effector toward goal position and orientation

Need to recompute at each time step

Inverse Jacobian Method

a2

d2=EF-J2

End Effector

a2 x d2

q2

- Compute instantaneous effect of each joint

- Linear approximation to curvilinear motion

- Find linear combination to take end effector towards goal position

Inverse Jacobian Method

Instantaneous linear change in end effector for ith joint

= (EF - Ji) x ai

Inverse Jacobian Method

What is the change in orientation of end effector induced by joint i that has axis of rotation ai

and position Ji?

Angular velocity

Inverse Jacobian Method

Solution only valid for an

instantaneous step

Angular affect is really

curved, not straight line

Once a step is taken, need

to recompute solution

Set up equations

yi: state variable

xi : system parameter

fi : relate system parameters to state variable

Inverse Jacobian Method

- Mathematics

Matrix Form

Inverse Jacobian Method

- Mathematics

Inverse Jacobian Method

- Mathematics

Use chain rule to differentiate equations to relate changes in system parameters to changes in state variables

Inverse Jacobian Method

- Mathematics

Matrix Form

Change in position (and orientation) of end effector

Change in joint angles

Linear approximation that relates change in joint angle to change in end effector position (and orientation)

Inverse Jacobian Method

= (S - J1) x a1

= w1

Inverse Jacobian Method

The Matrices

N DoFs

V – desired linear and angular velocities

3x1, 6x1

J – Jacobian

Matrix of partials

3xN, 6xN

N x 1

q – change to joint angles (unknowns)

LU decomposition

Solving using the Pseudo Inverse

A solution of this form

When put into this formula

Like this

After some manipulation, you can show that it…

…doesn’t affect the desired configuration

But it can be used to bias

The solution vector

Desired angles and corresponding gains are input

‘z’ is H differentiated

Form of the Control Term

Bias to desired angles

(not the same as hard joint limits)

Where the deviation is large, you bump up the solution vector in such a way that you don’t disturb the desired effect

Include this in equation

Isolate vector of unknown

Rearrange to isolate the inverse

Some Algebraic Manipulation

LU decomp.

Solving the Equations

Control Term

Use to bias to desired mid-angle

Does not enforce joint angles

Does not address “human-like” or “natural” motion

Only kinematic control – no forces involved

Other ways to numerically IK

Jacobian transpose

Alternate Jacobian – use goal position

Damped Least Squares

CCD

Jacobian Transpose

Use projection of effect vector onto desired movement

Alternate Jacobian

G

Use the goal postion instead of the end-effector!!??

!?

substitution

Solve

Damped Least Squares

G

RA (q1,q2 ,q3) , RB ( q4), RC(q5,q6 ,q7)

3 DoF

G

Decompose into simpler subproblems

1 DoF

Set hand position and rotation based on relative position of Goal to shoulder

3 DoF

Fix wrist position – use as Goal

L

L1

L2

s

Set q4 based on distance between shoulder and wrist

w

e

Assume axis of elbow is perpendicular to plane defined by s, e, w use law of cosines

s

q4

w

e

Elbow lies on circle defined by w, s & q4

Determine elbow position based on heuristics

For example:

project forearm straight from hand orientation

if arm intersects torso or a shoulder angle exceeds joint limit (or exceeds comfort zone) –

Clamp to inside of limits

s

q4

w

e

From e and s, determine RA

From e and w and hand orientation, determine RB

Cyclic-Coordinate Descent

Traverse linkage from distal joint inwards

Optimally set one joint at a time

Update end effector with each joint change

At each joint, minimize difference between end effector and goal

Easy if only trying to match position; heuristic if orientation too

Use weighted average of position and orientation.

Cyclic-Coordinate Descent

Rotational joint:

.

Cyclic-Coordinate Descent

Rotational joint:

.

Cyclic-Coordinate Descent

Rotational joint:

.

Cyclic-Coordinate Descent

Rotational joint:

.

Cyclic-Coordinate Descent

Translational joint:

..

IK w/ constraints

Chris Welman, “Inverse Kinematics and Geometric Constraints for Articulated Figure Manipulation,” M.S. Thesis, Simon Fraser University, 2001.

Basic idea:

Constraints are geometric, e.g., point-to-point, point-to-plan, specific orientation, etc.

Assume starting out in satisfied configuration

Forces are applied to system

Detect, and cancel out, force components that would violate constraints.

IK w/ constraints

Point-on-a-plane constraint

Fa

Fc

Ft

Given: geometric constraints & applied forces

Determine: what constraints will be violated & what (minimal) forces are needed to counteract the components of the applied forces responsible for the violations.

Constraints

To maintain constraints, need:

Notation:

.

Constraints

IK Jacobian

.

Generalized force

Constraint Jacobian

example constraint

Geometric constraint on point that is function of pose

Usually sparse

.

Computing the constraint force

Applied force

Yet to be determined constraint force

To counteract ga’s affect on constraints:

g should lie in the nullspace of JcK

.

Computing the constraint force

The system is usually underconstrained

Restrict gc to move the system in a direction it may not go

Solve linear system to find Lagrange multiplier vector.

.

Solving for Lagrange Multipliers

Shortest distance from A to B passing through a point P

A

Constrain P to lie on g

B

g

Points on ellipse are set of points for which sum of distances to foci is equal to some constant

Solving for Lagrange Multipliers

Use truncated SVD with backsubstitution on

diagonal

Nullspace basis.

Range basis

Feedback term

Spring that penalizes deviation from constraints.

Implementation

Handles on skeletons

Point handle

orientation handle

Center-of-mass handle

Each handle must know how to its value from q

Each handle must know how to compute the Jacobian.

Constraints on handles

Constraining a point handle to a location

Constraining a point handle to a plane

Constraining a point handle to a line

Constraining an orientation handle to an orientation

.

Dataflow approach

Constraint function block

Knows how to compute

Its function in term of x

Knows its Jacobian wrt x

..

Example network

Jc

C

c1

c2

h4

h2

h1

h3

..

q