Friction & Workspaces

Transcription

1 Friction & Workspaces CPSC / Sonny Chan University of Calgary

2 Today s Agenda Rendering surfaces with friction Exploring large virtual environments using devices with limited workspace [From C.-H. Ho et al., Presence 8(5), 1999.]

3 Coulomb Friction What is it, and how do we render it?

4 Coulomb Friction Friction force proportional to normal force F f = µf N Static (sticking) friction: F s apple µ s F N Kinetic (sliding) friction: F k = µ k F N

5 Rendering Friction Basic case: avatar µ = µ s = µ k F N = F c surface F c

6 Rendering Friction F N = F c cos Do we move the avatar? F f = F c sin??? surface Move avatar if sin >µcos tan >µ

7 Rendering Friction How far do we move the avatar? how far? surface = tan 1 µ

8 = tan 1 µ Friction Cone

9 avatar Friction Cone in 3D device position

10 Static & Kinetic Friction = tan 1 µ k Case where static and kinetic friction are distinct: = tan 1 µ µ s 6= µ k Two different coefficients of friction gives two friction cones: - static friction cone - dynamic friction cone

11 Static & Kinetic Friction Do we move the avatar?

12 Static & Kinetic Friction Do we move the avatar? how far?

13 Static & Kinetic Friction Do we move the avatar?

14 Static & Kinetic Friction Do we move the avatar?

15 Static & Kinetic Friction Do we move the avatar?

16 Static & Kinetic Friction Do we move the avatar?

17 Coulomb Friction: Summary Friction force is proportional to normal (contact) force Construct friction cone(s) from coefficients of friction With a proxy algorithm, we can render forces of static and kinetic friction - In general, μs > μk When do we switch between static and kinetic friction cones? - This is the crux of Assignment #2, Part II

18 Friction Demo

19 Workspace Management

20 Your Haptic Device How much workspace does the Novint Falcon haptic device have? < 10 cm

21 How can we deal with this limitation?

22 Strategy #1: Absolute Position Scale r A = s r D/W + r W PROS CONS natural, direct mapping loss of spatial resolution - serious adverse effects on haptic navigate large virtual environments performance and stability fixed workspace - or small ones

23 How does your computer mouse interface handle this? < 25.6

24 v a v d trajectory of the end-effector trajectory of the device trajectory of the avatar trajectory of the avatar [From F. Conti & O. Khatib, Proc. IEEE World Haptics Conference, 2005.]

25 Strategy #2: Ballistic Control

26 Strategy #2: Ballistic Control r A = s(v D ) r D/W PROS CONS adaptive spatial resolution workspace gets shifted or offset span large workspaces when moving fast essentially requires indexing to correct offset - can use a button or switch on device precise control when slow

27 How about mimicking these devices?

28 Strategy #3: Rate Control v A = s r D/W PROS CONS infinite workspace! cannot perform fast motions in different directions no indexing needed - scratching a rough surface viable for Assignment #3 - tapping a hard object force feedback is awkward

29 workspace drift f a Workspace Drift Control François Conti & Oussama Khatib, 2005 trajectory of the avatar r a r d workspace drift

30 Main Ideas Take advantage of our imprecise proprioception Move the centre of the virtual workspace to coincide with the centre of the device workspace Use the device operator s own motion to apply workspace drift

31 virtual object Initial condition avatar virtual workspace of the avatar physical workspace boundaries of the device. [From F. Conti & O. Khatib, Proc. IEEE World Haptics Conference, 2005.]

32 Motion toward object [From F. Conti & O. Khatib, Proc. IEEE World Haptics Conference, 2005.]

33 local interaction avatar-object Interacting with object v wa velocity and direction of the workspace drift. Want to move workspace centre to location of interaction [From F. Conti & O. Khatib, Proc. IEEE World Haptics Conference, 2005.]

34 v wa Drifting the workspace -v wd direction of hand drift. [From F. Conti & O. Khatib, Proc. IEEE World Haptics Conference, 2005.]

35 Workspace Drift Control v W = s kv D k r D/W Drift only occurs when the operator s hand is in motion Velocity of workspace drift is proportional workspace drift motion of the avatar to the speed of hand motion avatar motion of the hand Can be combined with ballistic control or rate control if needed Figure 7 - Moving an avatar along a straight line during a workspace drift requires compensating for the shift by slightly moving the end-effector in the opposite direction of the workspace drift.

36 Workspace Management: Summary Many feasible strategies exist for managing a device s limited workspace Absolute position scale is easy and direct, but loss of resolution is bad Ballistic control works well for mice, but requires indexing or clutching Rate control makes haptic interaction difficult, but can be effective when used carefully in specific workspace regions Workspace drift control tries to combine the best qualities of these strategies