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
Energy required by the different types of locomotion systems (rank by the most energy consuming to the least)
- crawling/sliding
- running
- tires on soft ground
- walking
- railways wheels
Advantages and “but(s)” of legged locomotion
- enables the agent to find isolated footholds
- provides active suspension for the body
- relatively energy efficient
BUT:
- many DoFs must be controlled in a coordinate way
- the robot must see detailed elements of the terrain
Static balance for legged motion with the support polygon
Walking can be modeled as a rolling of a polygon with side of length equals to the length of the step.
If the COM of the agent is inside the support polygon then the agent is statically balanced.
Static stability
If the robot is static balanced both when is stationary and when is moving then it is static stable.
A robot with good static stability is able to maintain its position even when it is subjected to external forces.
Static stability when walking is also called static walking.
Dynamic balance
Is the ability of a robot to maintain its balance while it is running or moving in a way such that static stability is not applicable.
Most animals relies on dynamic balance.
Dynamic balancing involves force balancing and moment balancing.
How the dynamic balance is reached in legged motion?
Considering the Zero Moment Point, we have the dynamic balance if the legs are placed such that each leg is in the ZMP.
The ZMP is the angle where the horizontal forces of momentum and friction are balanced and thus the robot should not fall. It is used to computed where to place a leg.
How do you actually make a robot move in practice?
- In practice, mechanical locomotion is the most used because wheels and tracks are easy to build and keep the platform stable compared to the multijointed legs.
- Biomimetic locomotion (crawling, sliding or walking) does not guarantee not be energy efficient. In theory legged locomotion is the most versatile but in practice is still a long way off.
