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 satisfies goal. Numeric approach – complex linkages

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Inverse Kinematics

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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 satisfies goal Numeric approach – complex linkages At each time slice, determine joint movements that take you in direction of goal position (and orientation)

Forward Kinematics - review Articulated linkage – hierarchy of joint-link pairs 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 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

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

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

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)

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 Adding a Control Term 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

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

Jacobian Transpose Use projection of effect vector onto desired movement

Hueristic Human-Like Linkage (HAL) 7 DoF linkage 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 Hueristic Human-Like Linkage (HAL) 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

Hueristic Human-Like Linkage (HAL) 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