Locomotion Flashcards
Locomotion
The act of moving from place to place.
It is also the study of the interaction forces between the robot and the environment. Interaction forces are related to mechanisms, actuators and interfaces (tyres, shields…etc).
Cost of transport
COT = |mech. energy| / (weigth * dist. traveled)
How can we generate motions?
- with manual motion design
- with a control based approach (ZMP and inverse pendulum)
- from learning algorithms (parameterized by a nervous system CPG or search algorithms GA, DL and RL)
- from human data (learning by watching/imitation)
Mechanical locomotion
Locomotion generated by a pure mechanical motion, such as rolling. In practice, ground robots almost always roll using wheels or tracks.
Holonomic vs nonholonomic robots
- holonomic: many approaches in AI robotics assume that the robot is holonomic. It means that the device can be treated as a massless point capable of instantaeously turning in any direction.
- Nonholonomic: in reality, robots are nonholonomic. They have mass and there is always some skitter (sbandamento) where the robot crabs (cade) to one side when it turns.
Advantages of holonomic systems
Since we consider the robot as a massless point:
- it allows the designer to ignore the complexities involved in the mechanical control of the robot (dynamic kinematics)
- it greatly simplifies path planning and localization
Types of wheel
- standard wheel: 2 DoFs
- castor wheel: 3 DoFs
- omnidirectional/swedish wheel: ~ 3 DoFs, there are contact points in which the wheel is not optimally positioned (this causes some sliding effects)
- spherical wheel: 3 DoFs
draw some skecthes…
Which issues wheels can manage?
- stability: is guaranted by 3 wheels and improved with 4+ wheels (dynamic stability)
- maneuverability: combined effect of wheel spinning and wheel steering
- controllabilty: defines the positions achiveable by manipulating the velocity control inputs
Omnidirectional wheels holonomicity
They have rollers that allow perpendicular movements so the robot can moves sideways. Unfortunately these wheels are not suitable for outdoors environements.
Synchro-drive robot
- Drive motor: defines the speed of the vehicle
- Steering motor: steers spontaneously the 3 wheels
- Synchro drive platform: 2 DoFs (any x, y position but no control on the orientation)
draw the robot
Steering mechanisms for nonholonomic systems
- Ackermann steering: used by cars. The wheels in front turn to guide the vehicle. The Ackermann mechanism adjusts the wheels so that the wheel on the inside of the turn is rotated more than the other wheel in order to compensate for slight difference between the diameters of the circular paths
- skid steering: aka differential drive steering is how a tank or a bulldozer is steered. The tracks on each side can be controlled independently. Skid steering can approximate holonomicity but it depends on many variables.
Circular path of a differential drive robot
2 constraints: pure rolling, no lateral sliding
ΔΘ = VlΔt / (R-w/2) = VrΔt / (R+w/2)
- find R
- substitute R into one of the two member
Circular path of a car-like tricycle robot
Rd = L / sin(s)
where L is the distance between the axis of the driven wheels and the steering wheel.
ΔΘ = vΔt / Rd = vΔt*sin(s) / L
What is biomimetic?
Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems.
Types of biomimetic locomotion
- crawling: the agent overcomes (sfrutta) friction through longitudinal vibration or movement (e.g. caterpillar)
- sliding: the agent overcomes friction through transverse vibrations or movements (e.g. snake)
- running: the agent overcomes kinetic energy with an oscillatory movement of a multi-link pendulum that leads to a predominately horizontal motion
