Mobile Robot Locomotion PDF

2. MOBILE ROBOT LOCOMOTION 1 Autorska prava © Ivan Petrović. Sva prava pridržana.  What is locomotion? Locomotion is a process that causes motion Opposite of manipulation  Robot arm is fixed and uses force to affect...

2. MOBILE ROBOT LOCOMOTION 1 Autorska prava © Ivan Petrović. Sva prava pridržana.  What is locomotion? Locomotion is a process that causes motion Opposite of manipulation  Robot arm is fixed and uses force to affect and move the objects  The space is fixed, while the MR moves by applying forces Locomotion mechanisms enable the MR to move through space  Actuators which generate forces and mechanisms that form wanted kinematic and dynamic MR properties 2 Autorska prava © Ivan Petrović. Sva prava pridržana.  Locomotion concepts Research robots can walk, jump, run, skate, swim, fly, sliding, roll Inspired by the Nature Difficult to imitate No wheels (bipeda walking can be approximated with a rolling polygon) Most common locomotion mechanisms: wheels and legs 3 Autorska prava © Ivan Petrović. Sva prava pridržana. Locomotion concepts in the Nature 4 Autorska prava © Ivan Petrović. Sva prava pridržana.  Criteria for choosing MR locomotion Maneuvering capabilities => ability to change direction and velocity of motion Controlability => practical and not too complex mechanism Traction force => minimize skid Climbing capability => depends on terrain Stability => should not tip over Efficiency => rational energy consumption Maintenance => easy maintenance, high reliability Effect on the environment => as small as possible Navigation capabilites => accuracy of the odometry system 5 Autorska prava © Ivan Petrović. Sva prava pridržana. Efficiency of the locomotion concept? 6 Autorska prava © Ivan Petrović. Sva prava pridržana.  Wheels or legs? Why do legs consume more energy? Legs Periodic motion Actuators in joints work against each other Wheels Continuous motion Natural pendulum and spring oscilations can generate automatically periodic motion – passive walkers 7 Autorska prava © Ivan Petrović. Sva prava pridržana.  2.1. Walking mobile robot Pros:  Can pass all the terrains that a human can (e.g. forest, ruins,...) Cons:  High number of degrees of freedom  Complex to maintain stability 8 Autorska prava © Ivan Petrović. Sva prava pridržana.  Stability of walking robots Robot with 2 legs; a-b area of Static stability: static stability  Center of mass is always inside the polygon whose vertices are points of contact between the legs and the ground Area of stability of a robot with 6 legs Dynamic stability:  Robot center of mass can be outside => stability is maintained with a control algorithm 9 Autorska prava © Ivan Petrović. Sva prava pridržana.  Number of legs Robots with 1, 2, 4, 6, 8, even 12 legs. Robot must have 4 legs for static stability => while one leg moves 3 form the polygon of stability Robot with more legs can peform more complex motion Robots with 1 or 2 legs can only be dynamically stable => challenge of being stable while static – prevent them from falling Robot with 2 legs is similar to the human – human- robot interaction research 10 Autorska prava © Ivan Petrović. Sva prava pridržana.  Four-legged robots Dog Aibo (Sony, Japan) Human-robot interaction (pet) Emulation of learning and changes in behaviour 11 Autorska prava © Ivan Petrović. Sva prava pridržana.  Insect-like MRs 6 and more legs  insect level antenae as sensors Measure small distances Working in swarmes 12 Autorska prava © Ivan Petrović. Sva prava pridržana.  Leg designs Minimum 2 degrees of freedom to lift and move the leg (combination of prismatic and rotation joints) Usually have 3 degrees of freedom to execute a more natural walk 13 Autorska prava © Ivan Petrović. Sva prava pridržana.  Gait control - 1 Gait of a four-legged MR: Picture shows:  Solid line – area of static stability  Dashed line – MR starting position  dot – MR center of mass  arrow – direction of motion 14 Autorska prava © Ivan Petrović. Sva prava pridržana.  Gait control - 2 Dynamic gait: complex mechanical design instead of complex contorl algorithm Good for uneven terrains Example: jumping single- legged robot (Raibert hopper) 15 Autorska prava © Ivan Petrović. Sva prava pridržana.  2.2. MRs with wheels Rolling wheel: Simple mechanical desing, stable focus – stability, maneuverability and controllability issues – medial and lateral skid, non-constant wheel radius, uneven terrain 16 Autorska prava © Ivan Petrović. Sva prava pridržana.  Characteristics of MRs with wheels Most fitting design for most applications Stability guaranteed with three wheels (possible with two) Center of mass must be inside of a triangle whose vertices are at the wheels and ground contact points Increased stability with 4 and more wheels Not good at uneven terrains Wheels can be controled by rolling and steering Complex design, causes errors in odometry 17 Autorska prava © Ivan Petrović. Sva prava pridržana.  Four basic wheel types a) b) a) Standard wheel:  2DOF - rotation about the wheel shaft and about the contact point b) Castor wheel:  2DOF - rotation about the wheel shaft and the castor axis 18 Autorska prava © Ivan Petrović. Sva prava pridržana.  Four basic wheel types c) d) c) Mecanum wheel:  3DOF- rotation about the wheel shaft, contact point and rollers (small wheels distributed on main wheel surface d) Spherical wheel:  As in older computer mouse designs  Technically complex 19 Autorska prava © Ivan Petrović. Sva prava pridržana.  Wheel configurations 2 wheels 3 wheels Synchrounous drive Omnidirectional drive 20 Autorska prava © Ivan Petrović. Sva prava pridržana.  Wheel configurations 4 wheels 6 wheels 21 Autorska prava © Ivan Petrović. Sva prava pridržana.  Drive configurations Nonholonomic configurations (will be defined in lecture #3): Differential drive Synchronous drive Tricycle drive Ackerman (car) drive Holonomic cofigurations (omnidirectional drives) 22 Autorska prava © Ivan Petrović. Sva prava pridržana.  Differential drive Most common configuration, especially with VL smaller robots Difference in the left and right wheel velocity VR determine the rotation rate Typical configuration (birds-eye view) Pioneer 2DX Examples: Khepera 23 Autorska prava © Ivan Petrović. Sva prava pridržana.  Differential drive Pros:  Simple design  Low price  Enables in-place rotation => important in close spaces Cons:  Difficult to maintain straight motion, since it is very sensitive to small errors in left and right wheel velocities which can appear due to:  Unequal wheel radius,  Uneven terrain, Cause problems  Wheel baseline changes during turns, for robot motion  Wheel skid during acceleration and breaking estimation 24 Autorska prava © Ivan Petrović. Sva prava pridržana. Syncronous drive All wheels move with same velocity and have the same steering angle (two motors – for turning and rolling) Pros:  Independent motors used for straight and rotating motion simplify control  Straight motion ensured mechanically Cons:  Orientation drifts with time due to wheel skid 25 Autorska prava © Ivan Petrović. Sva prava pridržana.  Ackerman drive VFL Simple design VFR Adapted to outdoors Complicated motion planning Straigh motion ensured VBL mechanically VBR Kinematic challenge of parallel parking: - Wheels have limited rotation - Impossible in-place rotation Navlab & ALVINN 26 Autorska prava © Ivan Petrović. Sva prava pridržana. Omnidirectional drive Each wheel has a motor, i.e. velocities can be controlle indipendently => most common omnidirectional design Pros:  Enables complex motion, in all directions all the time (holonomic robot) Cons:  Mechanically does not ensure straight motion  Complex design, high price 27 Autorska prava © Ivan Petrović. Sva prava pridržana.  Omnidirectional drive Wheel chair Discover ‘97 (among to 10 innovations) Sage – museum guide 28 Autorska prava © Ivan Petrović. Sva prava pridržana.  Complex designs MR with multiple trailers: Application example: (airports) 29 Autorska prava © Ivan Petrović. Sva prava pridržana. MR with caterpillar tracks Pros:  Simple and robust drive mechanism Cons:  Skid and bad odometry – uncertainty in the center of rotation  Requires high power Remotec Andros V vehicle Autorska prava © Ivan Petrović. Sva prava pridržana. 30  Wheel and leg combination EPFL Shrimp 6 wheels Passive locomotion concept for uneven terrains Can negotiate obstacles twice as large as the wheel radius 31 Autorska prava © Ivan Petrović. Sva prava pridržana.