haptic rendering in complex environments€¦ · haptic rendering in complex environments workshop...

www.DLR.de • Slide 1 > VROOS: Haptics & Physcis > Mikel Sagardia > 2012.10.09

Haptic Rendering in Complex EnvironmentsWorkshop zu interaktiven VR-Technologien für On-Orbit Servicing

Mikel SagardiaDLR, Institute of Robotics and Mechatronics


Haptic Rendering Physical RenderingCollision Detection and

Force ComputationMovement Simulation

1 ms !!

LOGIC

I am the Demiurge that sees EVERYTHING…


Overview

- Haptic Rendering: Collision Detection and Force Computation- Scenario and Structure Overview- Voxelmap-Pointshell (VPS) Algorithm at DLR

- Data Structures- Level-of-Detail Accurate Collision Detection

- Multi-Body Collision Environments- Net Operation- Data Traffic

- Physical and Haptic Force Post-Processing- Physical Meaning of the VPS Forces- Friction & Gravity- Normalization- Calibration- “God-Object” Methods for Haptic Display- Movement Simulation


Data Structure Generation(Offline ~1 sec.)

Polygonalmodels

Voxmap Pointshell

Collision Detection and Force Computation(Online <1 msec.)

• Pointshell is transformed into voxmap coordinates

• Likely colliding points are checked for collision

• Normal vectors of colliding points are scaled with local and global penetration in the voxmap yielding single collision forces

Voxmap Pointshell (VPS) Algorithm: Principle


Level-of-Detail Accurate Collision Detection

Classical Ideal Classical Ideal

Point-Sphere Hierarchy Interpolation with Distance Fields

Pointshells strongly influence the quality of the force direction and the

computation time

Voxmaps strongly influence the quality of the

force magnitude


Point-Sphere Hierarchies (1/2)

1) Localize likely colliding regions: faster computation2) Use high resolutions in colliding areas: better force quality


Point-Sphere Hierarchy (2/2)


Local Penetration Interpolation (1/2)

Discrete layered voxmaps generate discontinuities in the force values

A continuous penetration would avoid these discontinuities


Local Penetration Interpolation (2/2)

180º 360º


Overview






Multi-Body Environments: Requirements

- Multi-body collision detection is in worst case a quadratic problem:

- N objects N(N-1)/2 checks- 11 objects 55 checks

- We need a graph/net of objects (vertices) and collision detection threads (edges)…

- able to compute collisions between arbitrary geometries

- able to deliver the collision force of each object every 1 ms

- easily scalable- modifiable online


LOGIC

I can see EVERYTHING

…

Multi-Body Environments: Online Workflow (1/4)


LOGIC

PAUSE!Handle grasped module: pause

collision detection

between them



LOGIC

PAUSE!Module in

Satellite: pause collision detection

between them



LOGIC

RESUME!Handle left

Module: resume collision detection

between them



1. Each object (Oi) gets independently its position xi2. Each collision detection algorithm (Aij) gets the position of its respective

objects (xi, xj) and computes the collision force (Fij) between both objects3. Each object (Oi) gets collision forces (Fij) from all its respective algorithms

(Aij), sums up all forces upon itself and delivers its total collision force (Fi)

Multi-Body Environments: Net Operation


Overview






Physical Force Post-Processing Pipeline

VPS Friction & Gravity

Other Physical Forces

Contact Forces

Ideally forces should be a normalized signal

[0 1], scalable on demand

Movement Simulation

Force magnitude strongly affects the

solving of movement equations

God-Object Method for

Haptic Display

Objects should remain on surface and we should display hard

contacts

Fx

Fh

Fp

Normalization

Calibration

Forces should have a real physical meaning, not a

virtual one!


Friction Computation (1/2): Contact Point

- Linear (vB) and angular (wB) velocities of pointshell can be derived from its transformation matrix BHW

- Artificial contact point* Q is defined as the point in the pointshell with maximum penetration

- Velocity field of rigid bodies enables computing velocity in contact point vQ

* We could have several contact points, but Q is likely a good and fast approximation


Friction Computation (2/2): Contact Friction Force

- Coulomb dry friction model: friction force magnitude is at most a fraction of the normal contact force

- Contact point Q and its velocity*(vQ) define the direction of the friction force (FR)

- Friction coefficient μ should be function of contact area and contact point velocity (vQ)

* The breakthrough of this method is probably filtering appropriately the contact point velocity





Contact Forces



Movement Simulation




Haptic Display


contacts

Fx

Fh

Fp

Normalization

Calibration


virtual one!


Force Magnitude Correction Factor (1/2)

Main pitfall of the VPS algorithm: unlike in the reality, there must be an overlapping volume to compute a collision force (penalty based)

and this volume affects the magnitude of the resulting force


Nn

Force Magnitude Correction Factor (2/2)

N number of pointshell pointsn number of colliding pointsp high stiffness percentage: 0.01α exponent: 2

Goal Force Scaling





Contact Forces



Movement Simulation




Haptic Display


contacts

Fx

Fh

Fp

Normalization

Calibration


virtual one!


Voxmap Pointshell (VPS) Algorithm:Physical Meaning

- Penalty based: there must be penetration to detect force

- Differentials replaced by voxels and points (higher resolutions)

- Volumetric forces: generalization of the Achimedes’ principle

- VPS forces require volume scaling?


Voxmap Pointshell (VPS) Algorithm: Archimedes


Calibration

- There is a deviation (ΔF) virtual and real forces

- Objective: find a correlation thatmaps online virtual forces to besimilar to real ones

- Method- Definition of parametrized

contact scenarios that abstractmost common contactscenarios: resolution, geometry(curvature), contact areas, etc.

- Experiments- Computation of equivalent

inner-trajectories (EA)- Identification of dependencies

between parameters- Finding the correlation function





Contact Forces



Movement Simulation




Haptic Display


contacts

Fx

Fh

Fp

Normalization

Calibration


virtual one!


VPS God-ObjectUser Userxh

p, FVPS

xs

F

Key: compute God-Object position xs

xh

“God-Object” MethodDisplaying Objects on the Surface and Generating Hard Contacts

- Dynamic object A coupled with haptic device penetrates static object B (A: xh)

- God-Object A remains on the surface (A: xs)

- Difference between xs and xh is used to compute force





Contact Forces



Movement Simulation




Haptic Display


contacts

Fx

Fh

Fp

Normalization

Calibration


virtual one!


Physical Movement Simulation

- Object pose along time (movement) SHW(t)=[SRW(t)|xG(t)] can be obtained by solving the Newton-Euler equations*, subjected to all necessary constraints (gi)

- The input of these equations are the external forces (F) and torques (T), mainly computed by VPS the output movement will be strongly affected by the magnitude and quality of these variables (F,T)

* Most physics engines, however, integrate a state vector composed by

the position vector, rotation matrix, linear moment and angular moment

*


Conclusions and Future Work (1/2)


- Improvements in force direction and magnitude quality by applying hierarachies in pointshell and distance field interpolation in voxelmaps

- Multi-Body Collision Detection and Force Computation- Analysis of time delays due to data traffic- Grouping of threads to avoid overflow and enable complex

scenarios with mechanisms


Conclusions and Future Work (2/2)

- Physical and Haptic Force Post-Processing- Physical Meaning of the VPS Forces- Friction Computation

- Implement, test and evaluate different friction models- Calibration of Forces

- Find the correlation that maps real and virtual forces for more realistic haptic and physical display

- Haptic Display of Forces by Means of the “God-Object” Method- Full implementation for the 6 DoF case

- Movement Simulation- Stabilization of the Physics Engine


Thank you!Workshop on Virtual Reality for On-Orbit Servicing (VROOS)

Haptic Rendering in Complex Environments

… or trying to figure out the relation between bunnies, dragons, and satellites

Mikel SagardiaDLR-RMC2012.10.09

haptic rendering in complex environments€¦ · haptic rendering in complex environments workshop...

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