robotics Flashcards
Mechanism
System of rigid links (bodies) connected through joints
Robot
mechanism + actuated joints + end effector
Joint
Set of 2 surfaces that can slide, keeping contact to one another
Revolute joint
The relative position of two links is defined by a joint angle (theta)
Prismatic joint (P)
The relative position of 2 links is defined by a joint displacements (d)
Kinematics chain
Sequence of rigid elements linked through active joints in order to perform a task efficiently
Robot characteristics
Robot movements
Movement capability
Movement precision
Dynamical characteristics
- Robot movements
Degree of freedom
Robot positioning
Robot orienting
- Movement Capability
Working volume
Maneuvering degree
Accessibility
- Movement precision
Precision
Repetitiveness
- Dynamical characteristics
Payload
Velocity
Stability
Degrees of freedom (DoF)
Correspond to the number independent movements a robot can make.
In many cases, the number of actuators coincides with the number of degrees of freedom, except:
- Underactuated robobts: number actuators < DoF
- Redundant robots: number actuators > DoF
- Coupled Actuation: One actuator might control more than one degree of freedom
Robot positioning
The end effector in the 3D space, required three DoF, either obtained from rotations or displacements (x, y and z)
Coordinate systems
- Cartesian coordinates
- Cylindrical coordinates
- Spherical coordinates
Robot orienting
Orienting the end effector in the 3D space, required three additional DoF to produce the three rotations (roll, tilt, pan)
Joint types examples (extra note)
A joint adds a degree of freedom to the manipulator structure, if it offers a new movement to the end effector that can not be produced by any other joint or a combination of them
Working space
Workspace of a robot
All the possible positions and orientation (total volume) that the robot’s end-effector can reach (examples, a gripper, tool or a sensor)
Reachable shape is more important than the volume
Maneuvering degree
Capacity to reach a given position and orientation (pose) from different paths (different configurations)
Usually implies the presence of redundant joints (degrees of manipulability or degrees of redundancy)
Forced access (without redundancy) / Multiple access (with redundant DoF)
Accessibiliy
Capacity to change the orientation at a given position. Strongly depends on the joint limits.
Precision
Capacity to place the end effector into a given position and orientation (pose) within the robot working volume, from a random initial position. Error (e) increases with the distance to the robot axis.
What does precision depend on?
- Mechanical play (backlash)
- Sensors offset
- Sensors resolution
- Misalignments in the position / size rigid elements (eg. end-effector)
Error (e) increases …
with the distance to the robot axis
Repetitively
Capacity to place the end effector into a given position and orientation (pose) within the robot working volume, from a given initial position.
Repetitively depends on:
• Mechanical play (backlash)
• Target position
• Speed and direction when reaching the target
Payload
The load (in Kg) the robot is able to support in a continuous and precise way to the most distance point.
The values usually used are the maximum load and nominal at acceleration = 0. The load of the End-Effector is not included.
Velocity
Maximum speed (mm/sec.) to which the robot can move the End-Effector.
If a joint is slow, all the movements in which it takes part will be slowed down. For short movements it can be more interesting the measure of acceleration.
Classical architectures
- Cartesian
- Cylindrical
- Spherical (polar)
- Angular / Rotational
Cartesian (advantages)
+ linear movement in 3 dimensions
+ simple kinematical model
+ rigid structure
+ easy to display
+ possibility of using pneumatic actuators, which are cheap, in pick&place operations
+ ctt resolution
Cartesian (drawbacks)
- requires a large working volume
- the working volume is smaller than the robot volume (crane structure)
- requires free area between the robot and the object to manipulate
- guides protection
Cylindrical (advantages)
+ simple kinematical model
+ easy to display
+ good accessibility to cavities and open machines
+ larger forces when using hydraulic actuators
Cylindrical (drawbacks)
- restricted working volume
- requires guided protection (linear)
- the back side can surpass the working volume
Spherical (advantages and drawbacks)
A
+ large reach from a central support
+ it can bend to reach objects on the floor
+ motors 1 and 2 close to the base
D
- complex kinematics model
- difficult to visualize
Angular / Rotational (advantages)
+ maximum flexibility
+ large working volume with respect to the robot size
+ joints easy to protect (angular)
+ can reach the upper and lower side of an object
Angular / Rotational (drawbacks)
- complex kinematical model
- difficult to display
- linear movements are difficult
- no rigid structure when stretched
SCARA (advantages & drawbacks)
+ high speed and precision
- only vertical access
Joint space (configuration space)
is the space in which the joint angles (theta i; and the joint displacements (di) are defined
Cartesian space (operational space)
is the space in which the end-effector position (x,y,z) and the end-effector orientation (afla, beta, y) are defined
note about joint angles and displacements
angles: only defined for revolute joints
displacements: only defined from prismatic joints
Joint space vector (q)
represents the vector describing the position and orientation of the robot’s end effector in Cartesian space (operational space)
Generalized joint coordinates (q)
refers tot he joint position variables, whether angular or linear displacements
Cartesian space vector (x)
represents the vector describing the position and orientation of the robot’s end effector in Cartesian space (operational space)
