Basilio Bona DAUIN – Politecnico di Torino

Name: Basilio Bona DAUIN – Politecnico di Torino
Uploaded: 2017-07-13T15:56:12+00:00
Duration: PTM22S14
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Description: Basilio Bona DAUIN – Politecnico di Torino

Basilio Bona DAUIN – Politecnico di Torino
28/04/2017 ROBOTICS 01PEEQW Basilio Bona DAUIN – Politecnico di Torino di 23

Mobile & Service Robotics Kinematics
28/04/2017 Mobile & Service Robotics Kinematics di 23

Fundamental problems in mobile robotics
28/04/2017 Fundamental problems in mobile robotics Locomotion: how the robot moves in the environment Perception: how the robot perceives the environment Mapping: how to build the map of the environment Localization: where is the robot wrt the map Representation: how the robot organizes the knowledge about the environment Path planning/action planning: what the robot shall do to go from here to there; what are the actions to be performed to complete a specified task Supervision and control: how are the command to actuators generated to perform simple or complex tasks. How to generate tasks Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW di 23

Kinematics robot kinematics depends on the adopted locomotion stuctures Almost all mobile robots are underactuated, i.e., they have less actuators (and the relative control signals) that is less than the number of degrees of freedom of the structure Various wheels arrangements exist Each one has a different kinematic model We will study the following structures Differential drive robots Bicycle-like robots And a brief introduction to quadcopters Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Examples of underactuated structures
A car has two actuators, thrust and steer, and moves in a 2D space (3dof) A fixed-wing aircraft has four actuators, forward thrust, ailerons, elevator and rudder, and moves in a 3D space (6dof) A helicopter has four actuators: thrust vector magnitude and direction from the main rotor, and yaw moment from the tail rotor, and moves in a 3D space (6dof) A quadcopter has four actuators, the four rotor thrusts , and moves in a 3D space (6dof) A ship has three actuators: two propellers on parallel axes and one rudder, and moves in a 2D space (3dof) Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Instantaneous Curvature Centre (ICC)
The wheel axes meet in a common point called ICC If an ICC exists, each wheel rotates without slippage ICC Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

ICC ? wheels slip = skid steering
If an ICC does not exist, the wheel motion occurs with slippage ? wheels slip = skid steering Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Kinematics of a wheeled robot (aka rover)
the steering angle is the angle between the velocity vector and the steered wheel direction is equivalent to the angle between the normal to the velocity vector and the steered wheel rotation axis the robot pose is the position and the orientation (with respect to some give inertial axis) Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Kinematics Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Kinematics – Fixed wheel
The kinematic model represents a rover frame with a generic active non-steering wheel, located at a given position, with a local orientation The kinematic equations describe the relations and constraints between the wheel angular velocity and the angular velocity of the frame given constants Linear velocity of the wheel at the contact point with the plane Angular velocity of the wheel Angular velocity of the robot Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Kinematic constraints
To avoid slippage the tangential velocity at the wheel contact point, due to the rover rotation around its center, must be equal to the advance velocity of the wheel Hypothesis In the wheel reference frame we have Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Differential Drive (DD) Rover
Left wheel, active, non steering Passive support (castor, omniwheel) Right wheel, active, non steering Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Differential Drive Rover
Left wheel Right wheel Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Differential Drive Kinematics
ICC Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

ICC Consider the generic time instant Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

after Turn left Turn right before Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Considering the sampled time equations and assuming constant values during the time interval Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Approximation when sampling period is small (A) Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

EULER APPROXIMATION Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

RUNGE-KUTTA APPROXIMATION Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

EXACT INTEGRATION (B) (A) and (B) are the same Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Path Planning Basilio Bona - DAUIN - PoliTo
ROBOTICS 01PEEQW

2D Path Planning free space obstacles final pose final pose
initial pose initial pose Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

F I 3D Path Planning Basilio Bona - DAUIN - PoliTo
ROBOTICS 01PEEQW

arcs of zero radius represent turn-in-place maneuvers
Path Planning A robot path from pose A to pose B can always be decomposed into a series of arcs of different radius arcs of zero radius represent turn-in-place maneuvers arcs of infinite radius represents straight line maneuvers Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Path Planning Driving a vehicle to a goal (no orientation specified)

Path Planning Driving a vehicle to a goal (orientation specified)

Path Planning Basilio Bona - DAUIN - PoliTo
ROBOTICS 01PEEQW

Unicycle Unicycle is the simplest example of a wheeled robot. It consists of a single actuated steering wheel  is the steering velocity states/coordinates control commands (underactuated) Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Unicycle motion Basilio Bona - DAUIN - PoliTo
ROBOTICS 01PEEQW

Unicycle-like robot (polar coordinates)

Bicycle-like robot NON HOLONOMIC CONSTRAINT steering angle orientation

Bicycle equations 1 1) 2) 1’) Basilio Bona - DAUIN - PoliTo
ROBOTICS 01PEEQW

Bicycle equations 2 Basilio Bona - DAUIN - PoliTo
ROBOTICS 01PEEQW

ROBOTICS 01PEEQW

Bicycle vs unicycle motion

Quadcopters Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Quadcopter model Basilio Bona - DAUIN - PoliTo
ROBOTICS 01PEEQW

Quadcopter equations Basilio Bona - DAUIN - PoliTo
ROBOTICS 01PEEQW

Odometry Odometry is the estimation of the successive robot poses based on the wheel motion. Wheel angles are measured and used for pose computation Odometric errors increase with the distance covered, and are due to many causes, e.g. Imperfect knowledge of the wheels geometry Unknown contact points: in the ideal DD robot there are two geometric contact points between wheels and ground. In real robots the wheels are several centimeters wide and the actual contact points are undefined Slippage of the wheel wrt the terrain Errors can be compensated sensing the environment around the robot and comparing it with known data (maps, etc.) Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Odometry Errors Estimated path based only on odometry Desired path Map

Visual odometry Visual odometry is the estimation of the robot poses based only on the acquisition of successive images of the environment Stereo visual odometry uses two cameras to estimate the ego-motion of a camera by tracking features in a video sequence. If the camera system is calibrated then the full 3D Euclidean geometry can be reconstructed Using a single camera, the reconstruction is up to an unknown scale factor A RGB-D (Kinect or similar) camera can be used Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Other important problems that must be addressed
Mapping: how to build environment maps (geometrical vs semantic maps) Localization: where am I in the map? Simultaneous Localization and Mapping (SLAM): build a map and at the same time localize the robot in the map while moving Path planning: the definition of an “optimal” geometric path in the environment from A to B, and avoid obstacles Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Path Planning – A map is given
Path planning algorithm: it computes which route to take, from the initial to the final pose, based on the current internal representation of the terrain possibly considering a “cost function” (minimum time, minimum energy, maximum comfort, etc.) Complete path generation: the path is created as a sequence of successive moves from the initial pose to the final one Motion planning: is the execution of the above defined theoretical route; it translates the plan from the internal representation to the physical movement of the wheels Next move selection: at each step, the algorithm must decide which way to move next. An efficient dynamic implementation is required Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Introduction to AI Robotics (MIT Press)
Path Planning Where am I going? Mission planning What’s the best way there? Path planning Where have I been? Map making Where am I? Localization Navigation Carto- grapher Mission Planner deliberative Behaviors How am I going to get there? reactive Introduction to AI Robotics (MIT Press) Robin Murphy 2000 Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Path Planning B A Mobile obstacles Fixed obstacles Velocity
Acceleration constraints Velocity Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Which commands shall I give to the rover?
Path Planning Which commands shall I give to the rover? Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Path Planning B Final pose Path B Path A A Initial pose

Motion Planning B Move 3:rotation Final pose Move 2: go straight
Initial pose Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Non-holonomic constraints
Non-holonomic constraints limit the possible incremental movements in the configuration space of the robot Robots with differential drive move on a circular trajectory and cannot move sideways Omni-wheel robots can move sideways Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Holonomic vs. Non-Holonomic
Non-holonomic constraints reduce the control space with respect to the current configuration (e.g., moving sideways is impossible) Holonomic constraints reduce the configuration space Basilio Bona - DAUIN - PoliTo ROBOTICS 01PEEQW

Academic year 2010/11 by Prof. Alessandro De Luca

Basilio Bona DAUIN – Politecnico di Torino

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