Unit 1: Introduction to Robotics and Robot Anatomy - Subjective Questions

The evolution of robotics can be explained through the following stages:

• Ancient mechanical devices: Early inventors developed mechanical figures and automated devices using gears, pulleys, and water power.
• Industrial automation: During the Industrial Revolution, machines were introduced to perform repetitive manufacturing operations.
• First programmable machines: In the twentieth century, numerical control and programmable machines made automated production more flexible.
• Development of industrial robots: The first industrial robots were introduced for tasks such as material handling, welding, and painting.
• Microprocessor era: Microprocessors improved robot control, sensing, accuracy, and programmability.
• Modern robotics: Present-day robots use sensors, artificial intelligence, machine vision, networking, and autonomous decision-making.

Thus, robotics has progressed from simple mechanical automation to intelligent systems capable of interacting with people and adapting to changing environments.

A robot is a reprogrammable, multifunctional manipulator designed to move materials, parts, tools, or specialized devices through programmed motions to perform a variety of tasks.

Important characteristics of a robot include:

• Reprogrammability: Its sequence of operations can be changed through software.
• Multifunctionality: The same robot can perform different tasks by changing its program or end-effector.
• Sensing capability: Sensors provide information about position, force, vision, temperature, or the surrounding environment.
• Programmable motion: The robot can move through a defined sequence of positions and orientations.
• Physical manipulation: It can handle objects or tools using a mechanical structure.
• Adaptability: Advanced robots can respond to variations in objects and operating conditions.

A conventional automated machine usually performs a fixed sequence of operations, whereas a robot is more flexible and can be reprogrammed for different applications.

Robots can be classified using several criteria:

• According to application:

  • Industrial robots for welding, assembly, painting, and material handling.
  • Service robots for healthcare, cleaning, education, hospitality, and domestic work.
  • Military, agricultural, space, and underwater robots for specialized operations.

• According to control method:

  • Limited-sequence robots that follow simple programmed motions.
  • Point-to-point robots that move between specified positions.
  • Continuous-path robots that control the path between points.
  • Intelligent robots that use sensors and decision-making algorithms.

• According to mobility:

  • Stationary or fixed-base robots.
  • Wheeled, tracked, legged, aerial, and underwater mobile robots.

• According to mechanical configuration:

  • Cartesian, cylindrical, spherical or polar, articulated, SCARA, and parallel robots.

This classification helps in selecting a robot suitable for a particular task, workspace, accuracy requirement, and environmental condition.

A complete robotic system consists of the following major components:

• Manipulator: The mechanical structure consisting of links and joints. It provides the required physical motion.
• Actuators or drives: Motors, hydraulic cylinders, or pneumatic devices that produce motion at the joints.
• Controller: The computer-based unit that stores programs, processes sensor signals, and generates commands for the actuators.
• Sensors: Devices that measure position, velocity, force, torque, temperature, proximity, or visual information.
• End-effector: A gripper, welding gun, spray nozzle, drill, or other tool attached to the robot wrist.
• Power supply: Provides electrical, hydraulic, or pneumatic energy to the system.
• Programming device: A teach pendant or computer interface used to create, modify, and test robot programs.
• Communication and safety system: Enables interaction with other equipment and protects operators through emergency stops, guards, and safety sensors.

All components work together to sense the environment, compute the required action, and execute the desired task.

Degrees of freedom (DOF) represent the number of independent movements required to completely specify the position and orientation of a robot or object.

• A translation along the , , or axis contributes one degree of freedom.
• A rotation about the , , or axis also contributes one degree of freedom.
• A rigid body in three-dimensional space can have a maximum of six DOF: three translational and three rotational.

Examples include:

• A linear slide has one translational DOF.
• A planar two-link arm generally has two rotational DOF.
• A six-axis articulated robot can independently control three-dimensional position and three-dimensional orientation.

DOF are important because they determine the robot's ability to reach locations, orient tools, avoid obstacles, and perform complex tasks. A robot with insufficient DOF may reach a point but may not be able to orient its end-effector correctly.

The motion of a rigid body in three-dimensional space can be divided into translation and rotation.

• Translation along the three coordinate axes gives:
  
• Rotation about the three coordinate axes gives:
  

Therefore, the total number of degrees of freedom is:

The six DOF are:

• Translation along the axis
• Translation along the axis
• Translation along the axis
• Rotation about the axis
• Rotation about the axis
• Rotation about the axis

Thus, a robot with six independent axes can theoretically control both the position and orientation of its end-effector arbitrarily within its reachable workspace, subject to mechanical and joint limits.

Coordinate systems provide reference frames for describing the position and orientation of robot components and objects.

• World coordinate system: A fixed reference frame attached to the workcell or floor. It is useful for describing the overall layout of the robot and surrounding equipment.
• Base coordinate system: A reference frame attached to the robot base. Robot motion and position are commonly specified relative to this frame.
• Joint coordinate system: Describes motion directly in terms of individual joint variables, such as joint angles or linear displacements.
• Tool coordinate system: A frame attached to the end-effector or tool. It is useful for programming tool movement and specifying the tool center point.
• User or workpiece coordinate system: A frame attached to a workpiece, fixture, or task surface. It simplifies programming when the workpiece is moved.

Transformations between these coordinate systems allow the controller to convert positions and orientations so that the robot can execute the required task accurately.

The robot workspace is the total volume or region that can be reached by the robot's end-effector.

• Reachable workspace: The set of all points that the end-effector can reach with at least one orientation. It depends on link lengths, joint limits, and the mechanical configuration.
• Dexterous workspace: The set of points that the end-effector can reach with all or a specified range of orientations. It is generally smaller than the reachable workspace.

For example, an articulated robot may be able to place its gripper at a point but may not be able to rotate the gripper into the orientation required for a welding operation. That point belongs to the reachable workspace but may not belong to the dexterous workspace.

Workspace analysis is important for workcell design, robot selection, collision avoidance, and determining whether all task positions can be accessed.

The anatomy of a typical industrial robot includes:

• Base: Supports the robot and may be fixed to the floor, wall, ceiling, or a mobile platform.
• Links: Rigid structural members that connect the joints and determine the robot's reach.
• Joints: Movable connections that provide relative motion between links. They may be rotary or prismatic.
• Arm: The main group of links and joints that positions the wrist within the workspace.
• Wrist: A set of joints near the end of the arm that controls the orientation of the tool.
• End-effector: The device that performs the task, such as a gripper, welding torch, or paint gun.
• Actuators and transmissions: Produce and transmit power to the joints.
• Controller: Executes the program, coordinates motion, and processes feedback from sensors.

The arm mainly controls position, while the wrist generally controls orientation. Together they allow the robot to place and operate its tool accurately.

Robot joints provide relative movement between connected links and determine the robot's degrees of freedom.

A rotary joint changes the orientation of one link relative to another. A prismatic joint changes the distance between links by sliding one link along an axis.

The combination of these joints determines the robot's mechanical configuration, workspace, speed, load capacity, and positioning ability.

The major manipulator configurations differ as follows:

• Cartesian manipulator: Uses three mutually perpendicular prismatic joints. Its workspace is generally rectangular or box-shaped. It provides high accuracy and is used in gantry systems, machining, and pick-and-place operations.
• Cylindrical manipulator: Usually combines a rotary base joint with prismatic joints. Its workspace is approximately cylindrical. It is suitable for machine loading and material handling.
• Spherical or polar manipulator: Uses rotary joints combined with a prismatic joint. Its workspace is approximately spherical or part-spherical. It has been used for handling and welding applications.
• Articulated manipulator: Uses several rotary joints resembling a human arm. It has a large and flexible workspace and is widely used for welding, painting, assembly, and handling.

The choice depends on required reach, flexibility, accuracy, payload, obstruction constraints, and task complexity.

SCARA manipulators and parallel manipulators are specialized robot configurations.

SCARA manipulator:

• SCARA stands for Selective Compliance Assembly Robot Arm.
• It typically has rotary joints for horizontal-plane motion and a prismatic joint for vertical motion.
• It is compliant in the horizontal direction but rigid in the vertical direction.
• It is fast and accurate for assembly, insertion, packaging, and electronic component handling.

Parallel manipulator:

• It consists of multiple kinematic chains connected between a fixed base and a moving platform.
• Several actuators share the load and control the platform simultaneously.
• It offers high stiffness, low moving mass, and rapid motion.
• It is used in high-speed pick-and-place systems, flight simulators, precision positioning, and motion platforms.

SCARA robots are especially effective for planar assembly, whereas parallel robots are preferred for high-speed and high-precision motion.

The differences between fixed-base manipulators and mobile robots are as follows:

A fixed manipulator generally provides high repeatability and payload capacity. A mobile robot provides flexibility and access to multiple locations but requires additional systems for navigation, localization, and safe movement.

Major types of mobile robots include:

• Wheeled robots: Use wheels for locomotion. They are efficient, fast, and mechanically simple on smooth surfaces, but their performance is limited on rough or discontinuous terrain.
• Tracked robots: Use continuous tracks. They provide good traction and stability on uneven ground, but they consume more energy and may damage delicate surfaces.
• Legged robots: Use legs to step over obstacles and operate on irregular terrain. They are highly adaptable but require complex control and consume considerable energy.
• Aerial robots: Use propellers or wings to fly. They can access difficult or hazardous locations and are useful for surveillance and inspection, but they have limited flight time and payload.
• Underwater robots: Operate below the water surface for inspection, research, and maintenance. They can work in inaccessible areas but face communication, pressure, and navigation challenges.

The appropriate type depends on terrain, payload, speed, endurance, energy supply, and environmental conditions.

Industrial and service robots differ in their intended use and operating conditions.

• Purpose: Industrial robots mainly automate manufacturing and production tasks. Service robots assist people or perform useful tasks outside traditional industrial production.
• Operating environment: Industrial robots usually work in controlled, structured environments. Service robots often operate in homes, hospitals, offices, public spaces, or outdoor environments.
• Interaction: Industrial robots commonly work behind safety guarding or in controlled collaborative cells. Service robots frequently interact directly with people and must respond safely to unpredictable situations.
• Programming: Industrial robots often execute precise, repetitive programs. Service robots require greater perception, navigation, and decision-making ability.
• Examples: Industrial robots include welding arms, painting robots, and assembly robots. Service robots include vacuum-cleaning robots, hospital delivery robots, agricultural robots, and rehabilitation robots.

Both types use mechanical structures, actuators, controllers, and sensors, but their requirements for flexibility and human interaction are different.

The operation of a robotic system follows a sense, decide, and act sequence.

• Sensors: Measure the robot's internal state and external environment. Encoders measure joint position, while force sensors, proximity sensors, cameras, and inertial sensors provide additional information.
• Controller: Receives sensor data, compares the actual state with the desired state, calculates control commands, and coordinates the robot's movements. It also manages programs, communication, and safety functions.
• Actuators: Convert control commands and supplied energy into mechanical motion. Common actuators include electric motors, hydraulic cylinders, and pneumatic cylinders.

A feedback loop can be represented as:

This loop enables accurate motion, correction of errors, and adaptation to changes in the operating environment.

Point-to-point control and continuous-path control differ in the way robot motion is specified.

• Point-to-point control: The robot is required to reach a sequence of selected positions. The exact path between these positions is usually not important. It is suitable for spot welding, pick-and-place, and machine loading.
• Continuous-path control: The complete path, including intermediate positions, velocity, and sometimes acceleration, is controlled. It is required when the tool must follow a precise trajectory.
• Applications of continuous-path control: Arc welding, spray painting, cutting, polishing, and sealing.
• Accuracy requirements: Point-to-point control emphasizes accurate final positions, while continuous-path control emphasizes accurate movement throughout the trajectory.

Continuous-path control is more computationally demanding because the controller must coordinate multiple axes while maintaining the required path.

The workspace of a manipulator is determined by several mechanical and operational factors:

• Link lengths: Longer links generally increase the maximum reach.
• Joint types: Rotary and prismatic joints produce different workspace shapes.
• Joint limits: Mechanical stops and software limits restrict the possible positions and orientations.
• Robot configuration: Cartesian, cylindrical, spherical, articulated, and parallel structures generate different workspace geometries.
• Interference between links: Parts of the robot may obstruct certain positions.
• End-effector dimensions: The tool shape and size affect usable access to objects.
• Obstacles and fixtures: Workcell equipment can reduce the practical workspace.
• Orientation requirements: A point may be reachable, but the required tool orientation may not be possible.
• Payload and speed: Heavy loads or high speeds may reduce the practically usable region.

Workspace evaluation ensures that the robot can access every required task location safely and effectively.

Robot anatomy describes the physical arrangement and functional relationship of its mechanical parts.

• Kinematic chain: A sequence of links connected by joints. It transmits motion from the base to the tool.
• Base: The supporting structure from which the robot's main motion begins.
• Arm: The links and joints that provide the reach and position the wrist.
• Wrist: The joint assembly near the end of the arm that controls tool orientation.
• End-effector: The device attached to the wrist that directly performs the operation.

The base provides support, the arm determines the position of the wrist, and the wrist determines the orientation of the end-effector. The end-effector may be a gripper for handling parts or a process tool such as a torch, drill, or spray nozzle.

This arrangement separates gross positioning from fine tool orientation and makes the robot suitable for a wide range of tasks.

Robots provide several important benefits in industry:

Advantages:

• High repeatability and consistent product quality.
• Ability to work continuously for long periods.
• Improved productivity and shorter production cycles.
• Safe performance of hazardous, hot, toxic, or physically demanding tasks.
• Reduced material waste when motion is precisely controlled.
• Easy adaptation to product changes through reprogramming.

Limitations:

• High initial cost for the robot, tools, fixtures, programming, and safety systems.
• Need for skilled personnel for installation, maintenance, and programming.
• Limited ability to handle highly unpredictable situations without advanced sensing and intelligence.
• Ongoing maintenance and energy requirements.
• Possible need to redesign the workcell or manufacturing process.

Robots are most effective when tasks are repetitive, measurable, hazardous, or require consistent precision.