Advanced Programming for 3D Applications CE00383-3

1. Advanced Programming for 3DApplicationsCE00383-3 Introduction to Human Motion Lecture 2 Bob Hobbs Staffordshire university

2. General Outline • Human Skeleton • Muscle Groups • How Robots simulate humans • Kinematics • Gait • Locomotion

3. Human Dynamics • Users described as participants • basic interaction involves control of camera (viewpoint) • exploratory navigation / locomotion • Walk through systems • More advanced environment allow interaction • Touch , selection, manipulation • referred to as direct manipulation

4. Simulation of Body • Body model is the description of the interface • eyes are viual interface, ears are audio interface • geometric description drawn from egocentric point of view • description of hand and fingers forms basis of grasping simulation for picking up objects (Boulic 1996)

5. Simulation of Body- Building the body • The more points represneting the body the more realistic the movement • Up to 90 points for motion-capture in animation • Standard for human skeleton (H-Anim 1999) • More typically head, Torso, Both hands • Inferred movement from limited points • Inverse kinematics problem - infinite possibilities of movement in virtual environment, consistent restraint • Elbow position in 4- Tracker system (Badler, 1993)

6. H-Anim Humanoid L Hip L Knee L Ankle L Midtarsal Sacroiliac R Hip R Knee R Ankle R Midtarsal L Shoulder L Elbow L Wrist vl5 R Shoulder R Elbow R Wrist Skullbase

7. Simulation Of body - tracking the participant • Choice of system depends on 5 factors • accuracy, resolution, range, lag, update rate • Many different tracking technologies • Meyer 1992 • frequency and time • ultrasonic time-of-flight measurement • Pulsed Infra-red • GPS • Optical Gyroscopes • Phase difference

8. Simulation Of body - tracking the participant • Spatial Scan • Outside-in • Inside-out • Inertial sensing • mechanical gyroscope • Accelerometer • Mechanical Linkages • Direct - Field Sensing

9. Interaction with virtual Body • Limitations mean reliance on metaphors for • object manipulation (grasping and moving) • locomotion (movement) • Limitations in haptics mean that restraint on the virtual environment exists

10. Muscles • http://www.youtube.com/watch?v=T-ozRNVhGVg&feature=PlayList&p=37A3DC6AF2D7C881&index=5 • http://www.youtube.com/watch?v=pbTah5NVOtU&NR=1

11. The Musculotendinous Unit • Tendon- spring-like elastic component in series with contractile component (proteins) • Parallel elastic component (epimysium, perimysium, endomysium, sarcolemma) F x PEC: parallel elastic component CC: contractile component SEC: series elastic component

12. II. Mechanics of Muscle Contraction • Neural stimulation – impulse • Mechanical response of a motor unit - twitch T: twitch or contraction time, time for tension to reach maximum F0: constant of a given motor unit Averaged T values Tricep brachii 44.5 ms Soleus 74.0 ms Biceps brachii 52.0 ms Medial Gastrocnemius 79.0 ms Tibialis anterior 58.0 ms

13. Summation and tetanic contraction (ms)

14. Generation of muscle tetanus 100Hz 10 Hz Note: muscle is controlled by frequency modulation from neural input very important in functional electrical stimulation

15. Wave summation & tetanization Critical frequency

16. Motor unit recruitment • All-or-nothing event • 2 ways to increase tension: • - Stimulation rate • Recruitment of more motor unit Size principle Smallest m.u. recruited first Largest m.u. last

17. Robots • Springs • Screws • Metal parts • Servos • Rubber Models • Springs • Joints • Segments • Muscles simple, fast, easy to understand

18. Robotic Basics • Have moveable segments • Connected with joints • Robots spin wheels and pivot jointed segments with some sort of actuator • Some robots use electric motors and solenoids as actuators some use a hydraulic system and some use a pneumatic system (a system driven by compressed gases). • Robots may use all these actuator types. • Robots usually have some sort of sensor

19. Actuators • Electrical current drives actuators controlling individual joints • Directly to motors or solenoids • To valves controlling flow of fluids to hydraulic or pneumatic systems

20. Robot arm • Simplest sort of robot • Typical arm has 7 segments, 6 joints • 6DOF • Human arm 7DOF • Usually driven by Step Motors • Main use is in manufacturing

21. Robot Arm • Fitted with end effector • Usually interchangeable • Artificial Hand , paint gun, welding rod • Pressure sensor needed to prevent crushing • Programmed by incremental steps which are then replicated ad infinitum

22. Step Motor • electromagnetic, rotary actuator, that mechanically converts digital pulse inputs to incremental shaft rotation. • The rotation not only has a direct relation to the number of input pulses, but its speed is related to the frequency of the pulses.

23. Step Motor Each pulse corresponds to an angular rotation

24. Step Motors • Between steps holds position w/o brake or clutch • Can be programmed to move a precise number of steps and then hold position • Possible to be bi-directional • Rapid acceleration, deceleration and reversal • cf DC Servo motors

25. Choosing the right motor • Basic Types: • Variable Reluctance, • Permanent Magnet, • Hybrid • Parameters to be considered • Distance to be traversed. • Maximum time allowed for a traverse. • Desired detent (static) accuracy. • Desired dynamic accuracy (overshoot).

26. More parameters • Settling time • Required step resolutiong • System friction • System inertia. • Speed/Torque characteristics of the motor: When selecting a motor/drive, the capacity of the motor must exceed the overall requirements of the load. • Torque-to-inertia Ratio • Torque Margin: Selecting a motor drive that provides at least 50% margin above the minimum required torque is ideal.

27. Frameworks, Chains (or Skeletons) • A lot of mechanical objects in the real world consist of solid sections connected by joints • Obviously robot arm but also • Creatures such as humans and animals. • Car Suspension • Ropes, string and Chains

28. Frameworks, Chains (or Skeletons) • Sections and joints of robot arm are known as a 'chain‘ • In creatures could be referred to as a skeleton • Moveable sections correspond to bones • Attachments between bones are joints.

29. Frameworks, Chains (or Skeletons) • Motions of chains can be specified in terms of translations and rotations. • Forward Kinematics - From the amounts of rotation and bending of each joint in an arm, for example, the position of the hand can be calculated. • Inverse Kinematics - If the hand is moved, the rotation and bending of the arm is calculated, in accordance with the length and joint properties of each section of the arm.

30. Joint Translation-Rotation • We can use a transform (T) to transform each point relative to the body to a position in world coordinates. • If we want to model both linear and angular (rotational) motion then we need to use a 4x4 matrix to represent the transform

31. ? End Effector Base What is Inverse Kinematics? • Forward Kinematics

32. End Effector Base What is Inverse Kinematics? • Inverse Kinematics

33. ? End Effector Base What does looks like?

34. Infinite number of solutions ! Solution to • Our example Number of equation : 2 Unknown variables : 3

35. Our example • System DOF = 3 • End Effector DOF = 2 Redundancy • System DOF > End Effector DOF

36. Redundancy • A redundant system has infinite number of solutions • Human skeleton has 70 DOF • Ultra-super redundant • How to solve highly redundant system?

37. Iterative solution • Start at end effector • Move each joint so that end gets closer to target • The angle of rotation for each joint is found by taking the dot product of the vectors from the joint to the current point and from the joint to the desired end point. Then taking the arcsin of this dot product. • To find the sign of this angle (ie which direction to turn), take the cross product of these vectors and checking the sign of the Z element of the vector.

38. Goal Potential Function • “Distance” from the end effector to the goal • Function of joint angles : G(q)

39. Goal distance End Effector Base Our Example

40. Ground reaction force (N) time (ms) m1 Energetic losses may increase performance! Nonlinear spring-damper element k m2 Dynamics of the long jump Seyfarth et al. (1999) J. Biomech.

41. Joint Structures • This allows two nodes to be attached to each other in a flexible way so that forces in the plane of the joint will be transmitted through it, but forces perpendicular to the joint will cause it to bend. This will provide IK like capabilities

42. Name • Symbol • DOF • Revolute joints • R • 1 • Prismatic joints • P • 1 • Helical joints • - • 1 • Cylindrical joints • RP • 2 • Spherical joints • 3R • 3 • Planar joints • RRP • 3 Types of Joint

43. Joint Structures • In character animation, only 2 types of joint need to be considered. These are the "revolute" and "prismatic" joints. All other types can be based on these two. • 1 degree of freedom: • rotational joint - wheel. • hinge - similar to rotational joint above but with limits to motion (end stops) • 2 degrees of freedom • ball & socket joint

44. Dynamics • Forward Dynamics - The movements are calculated from the forces, such as, force = mass * acceleration. • Inverse Dynamics - Constraints are applied which specifies how objects interact, for example, they may be linked by a hinge joint or a ball joint, and from this the forces can be calculated

45. Forward Dynamics • If no forces act on a particle, the particle retains its linear momentum. • The rate of change of the linear momentum of a particle is equal to the sum of all forces acting on it. • When two particles exert forces upon each other, these forces are equal in magnitude and opposite in direction.

46. Forward Dynamics • These laws can also be applied to rigid bodies by assuming that the forces are acting on the centre of mass of the object. • Assuming that the mass is constant then the second law becomes: • force = mass * acceleration • Euler extended these laws to include rotation. So there are equivalent laws for rotation such as: • torque = inertia * angular acceleration.

47. What is a robot? • Joseph Engelberger, a pioneer in industrial robotics, once remarked "I can't define a robot, but I know one when I see one." • Many different machines called robots • Everybody has a different idea of what constitutes a robot • Name from robota– forced labour

48. What relevance to us? • VR models use robotic principles • Avatars behave like robots • Simulations of robots used to test real robots • May be used to control remote robotics

49. Virtual Actors: Autonomous or Guided Guided Actors are Slaved to the Motions of a Human Participant Using Body Tracking – Optical, mechanical, . . . – A.K.A. Avatar • Autonomous Actors Are Controlled by Behavior Modeling Programs, and Can - Augment or replace human participants - Serve as surrogate instructors - Act as guides in complex synthetic worlds • Hybrid Control Desirable - VRLOCO uses interaction to invoke and control locomotion behaviors

50. The Weiss 6-Level Motor Organization Hierarchy Organism Level 6. Motor Behavior 5. Motor Organ System 4. Motor Organ 3. Muscle Group 2. Muscle 1. Motor Unit Neuron Level 3. Muscle Group - Coordinated action of several muscles - Motion at one joint 2. Muscle - Muscle contraction 1. Motor Unit - Neuron + muscle fibers - Twitching, shivering