Welcome to robotics
Welcome
to
robotics world
Learning contents –
- Introduction and Robot kinematics
- Robot drives and control
- Robot sensors
- Robot cell design and application
- Methods of Robot Programming
Introduction:
Definition of industrial robots:
The Robotic Industries Association (RIA) defines Robots as follows:"A robot is a reprogrammable, multifunctional manipulator designed to move material, parts, tools or specialized devices through variable programmed motions for the performance of a variety of tasks.“The modern definition of a robot can be an electro-mechanical device that follows a set of instructions to carry out certain jobs, but literally, robot means a ‘slave’.
But what exactly is a robot?
A robot has these essential characteristics:
Sensing:
First of all your robot would have to be able to sense its surroundings. It would do this in ways that are not similar to the way that you sense your surroundings. Giving your robot sensors: light sensors (eyes), touch and pressure sensors (hands), chemical sensors (nose), hearing and sonar sensors (ears), and taste sensors (tongue) will give your robot awareness of its environment.
Movement:
A robot needs to be able to move around its environment. Whether rolling on wheels, walking on legs, or propelling by thrusters a robot needs to be able to move. To count as a robot either the whole robot moves, like the Sojourner, or just parts of the robot, like the Canada Arm.
Energy:
A robot needs to be able to power itself. A robot might be solar-powered, electrically powered, battery powered. The way your robot gets its energy will depend on what your robot needs to do.
Intelligence:
A robot needs some kind of "smarts." This is where programming enters the picture. A programmer is a person who gives the robot its 'smarts.' The robot will have to have some way to receive the program so that it knows what it is to do
Need and scope of industrial robots:
Typical applications of robots
- welding
- painting
- assembly
- pick and place for printed circuit boards
- packaging and labeling
- product inspection, and testing
- They can assist in material handling.
Tasks are:
- Dangerous
- Space exploration
- chemical spill cleanup
- disarming bombs
- disaster cleanup
- Boring and/or repetitive
- Welding car frames
- part pick and place
- manufacturing parts.
- High-precision or high-speed
- Electronics testing
- Surgery
- precision machining.
Automation –Machinery designed to carry out a specific task
- Bottling machine
- Dishwasher
- Paint sprayer
Notations of link Each joint is connected to two links, an input link, and an output link. Joint provides controlled relative movement between the input link and output link.
A robotic link is the rigid component of the robot manipulator. Most of the robots are mounted upon a stationary base, such as the floor.
From this base, a joint-link numbering scheme may be recognized as shown in Figure.
The robotic base and its connection to the first joint are termed as link-0. The first joint in the sequence is joint-1. Link-0 is the input link for joint-1, while the output link from joint-1 is link-1—which leads to joint-2. Thus link 1 is, simultaneously, the output link for joint-1 and the input link for joint-2. This joint-link-numbering scheme is further followed for all joints and links in the robotic systems.
Joints used in robotics
a) Linear joint (type L joint)
The relative movement between the input link and the output link is a translational sliding motion, with the axes of the two links being parallel.
b) Orthogonal joint (type O joint)
This is also a translational sliding motion, but the input and output links are perpendicular to each other during the move
c) Rotational joint (type R joint)
This type provides rotational relative motion, with the axis of rotation perpendicular to the axes of the input and output links.
d) Twisting joint (type T joint)
This joint also involves rotary motion, but the axis of rotation is parallel to the axes of the two links.
e) Revolving joint (type V-joint, V from the “v” in revolving)
In this type, the axis of the input link is parallel to the axis of rotation of the joint. However, the axis of the output link is perpendicular to the axis of rotation.
Common Robot Configurations
a. Polar configuration
It consists of a sliding arm L-joint, actuated relative to the body, which rotates around both a vertical axis (T-joint) and a horizontal axis (R-joint).
b. Cylindrical configuration
It consists of a vertical column. An arm assembly is moved up or down relative to the vertical column. The arm can be moved in and out relative to the axis of the column. A common configuration is to use a T-joint to rotate the column about its axis. An L-joint is used to move the arm assembly vertically along the column, while an O-joint is used to achieve radial movement of the arm
c. Cartesian co-ordinate robot
It is also known as a rectilinear robot and an x-y-z robot. It consists of three sliding joints, two of which are orthogonal O-joints
d. Jointed-arm robot
It is similar to the configuration of a human arm. It consists of a vertical column that swivels about the base using a T-joint. The shoulder joint (R-joint) is located at the top of the column. The output link is an elbow joint (another R joint).
e. SCARA
Its full form is ‘Selective Compliance Assembly Robot Arm'. It is similar in construction to the jointer-arm robot, except the shoulder and elbow rotational axes are vertical. It means that the arm is very rigid in the vertical direction, but compliant in the horizontal direction.
End effector
An end effector is usually attached to the robot's wrist, and it allows the robot to accomplish a specific task.
This means that end effectors are generally custom-engineered and fabricated for each different operation.
There are two general categories of end effectors viz. grippers and tools.
Grippers grasp and manipulate the objects during the work cycle.
Typically objects that are grasped are the work parts that need to be loaded or unloaded from one station to another.
Grippers may be custom-designed to suit the physical specifications of work parts.
End Effectors: grippers
Sensors in Robotics:
There are generally two categories of sensors used in robotics. these are sensors for internal purposes and for external purposes.
Internal sensors are used to monitor and control the various joints of the robot. They form a feedback control loop with the robot controller. Examples of internal sensors include potentiometers and optical encoders, while tachometers of various types are deployed to control the speed of the robot arm.
External sensors are external to the robot itself and are used when we wish to control the operations of the robot. External sensors are simple devices, such as limit switches that determine whether a part has been positioned properly, or whether a part is ready to be picked up from an unloading bay.
Sensors in Robotics
Types of robots
Pick and place
Moves items between points
Continuous path control
Moves along a programmable path
Sensory
Employs sensors for feedback
A SCARA robot (Selective Compliant Articulated Robot Arm): A pick-and-place robot with angular
x-y-z positioning (Adept Technology)
A six-axis industrial robot ($60K)(Fanuc Robotics), but an additional $200K is often spent for tooling and programming.
Moves items from one point to another
Does not need to follow a specific path between points
Uses include loading and unloading machines, placing components on circuit boards, and moving parts off conveyor belts.
A Cartesian robot for picking and placing circuits on circuit boards
Continuous path control
Moves along a specific path
Uses include welding, cutting, machining parts
Robotic seam welding
Sensory
Uses sensors for feedback.
Closed-loop robots use sensors in conjunction with actuators to gain higher accuracy – servo motors.
Uses include mobile robotics, telepresence, search, and rescue, and pick and place with machine vision.
Measures of performance
Working volume
The space within which the robot operates.
A larger volume costs more but can increase the capabilities of a robot
Speed and acceleration
Faster speed often reduces resolution or increases the cost
Varies depending on position, and load.
Speed can be limited by the task the robot performs (welding, cutting)
Resolution
Often a speed tradeoff
The smallest step the robot can take
Performance (cont.)
Accuracy
The difference between the actual position of the robot and the programmed position
Repeatability
Will the robot always return to the same point under the same control prerequisites?
Enriched cost
Varies depending on position, load
Robot kinematics
The analytical study of the geometry of the motion of a mechanism:
- concerning a fixed reference coordinate system,
- without regard to the forces or moments that cause the motion.
- To control and program a robot we must have knowledge of both its spatial arrangement and a means of reference to the environment.
Robot kinematics studies the relationship between the dimensions and connectivity of kinematic chains and the position, velocity, and acceleration of each of the links in the robotic system, to plan and control movement and to compute actuator forces and torques.
OPEN CHAIN MANIPULATOR KINEMATICS
The mechanics of a manipulator can be depicted as a kinematic chain of rigid bodies (links) merged by revolute or prismatic joints.
One end of the chain is constrained to a base, while an end effector is mounted to the other end of the chain.
The consequential motion is obtained by the manuscript of the elementary motions of each link concerning the previous one
CLOSED KINEMATIC CHAIN
Much more difficult.
Even analysis has to take into account statistics, constraints from other links, etc.
Synthesis of closed kinematic mechanisms is challenging.
Kinematic equations
- A Fundamental tool in robot kinematics is the kinematics equations of the kinematic chains that form the robot.
- These non-linear equations are used to map the joint parameters to the configuration of the robot system.
- Kinematics equations are also used in biomechanics of the skeleton and computer animation of articulated characters
- Forward kinematics uses the kinematic equations of a robot to compute the position of the end-effector from specified values for the joint parameters.
- The reverse process that computes the joint parameters that achieve a specified position of the end-effector is known as inverse kinematics.
- The dimensions of the robot and its kinematics equations define the volume of space reachable by the robot, known as its workspace.
Forward kinematics
Forward kinematics specifies the joint parameters and computes the configuration of the chain.
For serial manipulators, this is achieved by direct substitution of the joint parameters into the forward kinematics equations for the serial chain.
For parallel manipulators substitution of the joint parameters into the kinematics equations requires a solution of the set of polynomial constraints to determine the set of possible end-effector locations.
Inverse kinematics
Inverse kinematics specifies the end-effector location and computes the associated joint angles.
For serial manipulators, this requires the solution of a set of polynomials obtained from the kinematics equations and yields multiple configurations for the chain.
For parallel manipulators, the specification of the end-effector location simplifies the kinematics equations, which yields formulas for the joint parameters.
Serial manipulators are the most common industrial robots. They are designed as a series of links connected by motor-actuated joints that extend from a base to an end-effector. Often they have an anthropomorphic arm structure described as having a "shoulder", an "elbow", and a "wrist".
A parallel manipulator is a mechanical system that uses several computer-controlled serial chains to support a single platform or end-effector
Robot trajectories:
Trajectory planning: IMPORTANT aspect in robotics, VERY IMPORTANT for the dimensioning, control, and use of electric motors in automatic machines (e.g. packaging). The origin of the interest in the control area was the substitution of mechanical cams with electric cams in the design of automatic machines.
Manipulators with multiple degrees of freedom for accomplishing various complex manipulations in the workspace.
Path: only geometric description Trajectory: timing included
Trajectory planning:
Trajectories can be defined in joint space or in Cartesian space. They are then directly provided as the input for the controller. Trajectories are important because they enable the system to ensure:
- feasibility: the motion can be verified to respect the dynamic constraints of lower-level controllers.
- safety and comfort: the trajectories can limit velocity, acceleration, and jerk, which are directly related to the safety and comfort of humans.
- optimization: optimization can integrate both geometry and time.
- flexibility: trajectories allow the definition of a lot of tools to adapt and transform them.
Joint space vs operational space description
Robot dynamics is concerned with the relationship between the forces acting on a robot mechanism and the accelerations they produce. Typically, the robot mechanism is modeled as a rigid-body system, in which case robot dynamics is the application of rigid-body dynamics to robots. The two main problems in robot dynamics are:
- Forward dynamics: given the forces, work out the accelerations.
- Inverse dynamics: given the accelerations, work out the forces.
Forward dynamics is also known as "direct dynamics," or sometimes simply as "dynamics." It is mainly used for simulation. Inverse dynamics has various uses, including online control of robot motions and forces, trajectory design and optimization, design of robot mechanisms, and as a component in some forward-dynamics algorithms.
Other problems in robot dynamics include:
Calculating the coefficients of the equation of motion.
Inertia parameter identification --- estimating the inertia parameters of a robot mechanism from measurements of its dynamic demeanor.
Configuration of robot controller:
Elements included in the robot controller include:
- Joint servo controller.
- Joint power amplifier.
- Mathematical processor.
- Program memory.
- Input device.
- General robot controller element.
Working of Robot?
Motion commands are executed by the controller from two possible sources:
a) Programme input b) Programme memory
- For each motion command the executive processor informs the mathematical processor of the coordinate calculation transformation that must be made
- When the transformation computation is completed, the executive processor downloads the results to the joint controllers as position commands.
- Each joint controller then drives its corresponding joint actuator using the power amplifier.
Safety while using robots:
Human Errors:
Human error occurs in day-to-day activity and this is no different about a robotic work cell. Whether it is programming, preventative maintenance, or teaching pendant control, operators have the potential to place themselves in hazardous positions due to over-familiarity or lack of knowledge of the robot’s motion path.
Control Errors:
Errors in the controls software and hardware can lead to hazards within a robotic work cell. If the control system faults, the system response may lead to a dangerous working environment if it is closely coupled with human interaction.
Unauthorized Access:
Access by an unauthorized operator into a safeguarded robotic work cell. If an operator is unfamiliar with the safety hardware associated with the robotic work cell, they can find themselves in a dangerous and potentially fatal area.
Mechanical Failures:
During the design and programming stages, mechanical part failure is not always taken into account. When an unexpected failure occurs, this can lead to a potentially hazardous situation for the operator.
Environmental Sources:
Outside factors and communication interference can create an undesirable effect on a robotic work cell. Unsuppressed power surges or power loss can lead to injury if they are not planned for during the initial stages of the project.
Power Systems:
Power sources that communicate to the robotic cell can be disrupted and lead to undesired actions. This can produce a release of energy, creating a hazardous environment for an operator.
Improper Installation:
Any time an industrial robot is installed it is vital to the success of the project and the safety of the operators that the system is installed correctly before it is fully operational. If the robotic work cell is incorrectly set up, future hazards may occur due to variance from the original design.
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