Industrial robots are designed for use in industrial automation. They are programmable machines used specifically to automate production-related tasks.
Typically, several robots are installed on an assembly line, each responsible for a specific objective. For example, some select and position workpieces, some interact with different equipment (such as a lathe or milling machine), and others perform assembly work. They can also be reprogrammed to run different types of applications.
A robot is a system built from several subsystems. These subsystems interact with each other and with the workspace (like an assembly line) to perform specific tasks.
Although industrial and service robots can perform different tasks, they share common components and structure. In this article, we will learn about the components of an industrial robot or robotic arm.
Components
There are five main components of a robotic arm.
1. Manipulator arm
2. End effector
3. Actuators and transmission
4. Controller
5. Sensors
The robot/robotic arm is typically mounted on a fixed or mobile base.
The manipulator arm
The manipulator arm is a programmable mechanical arm that functions similarly to a human arm. Its purpose is to move as needed, providing reach in the workspace. Functions as part of a complex robot or performs independent tasks. Essentially, it is a moving chain of successively coupled segments or links.
The segments are called cross slides because they can move over each other in a workspace. One end of the manipulator remains fixed to the ground or base, and the other remains free to hold the end actuator.
The manipulator arm is designed with a specific coordinate system, with several designs available. The simplest is a two- or three-axis arm. The axis or degree of freedom refers to the independent movement of the segment or link. The point where two segments or links are coupled is called a joint. The segments are connected to each other by some lower pair connector.
Here are the types of bottom pair connectors or joints:
1. Revolution joint (R): has one degree of freedom (DOF), allowing relative rotational movement of the output link with the axis of rotation perpendicular to the input and output link axis.
2. Prismatic joint (P): has one DOF, allowing a translational sliding movement between the input and output link. The axes of both links are parallel.
3. Helical joint (H): has a DOF, allowing rotational movement of the output link while translating around a screw axis perpendicular to the axis of the input link.
4. Cylindrical joint (C): has two DOFs — the first is the rotation of the output link, with an axis of rotation perpendicular to the axis of the input link. The second degree of freedom is the translational sliding motion of the output link along the axis perpendicular to the axis of the input link.
5. Universal joint (S): has two DOFs — the first is the rotation of the output link around the axis perpendicular to the axis of the input link. The second is the rotation of the input link around the axis perpendicular to the axis of the output link.
6. Ball joint (T): has three DOFs – the first is the rotational movement of the output link around its own axis. The other two include the translational movement of the output link around axes perpendicular to the axis of the input link.
Industrial robots are primarily designed in a body-wrist configuration. In a 6 DOF robotic arm, three links constitute the body that places the end effector in the desired location on the work area, and three links form the wrist of the manipulator, placing the end effector in the desired orientation.
Swivel and prismatic joints connect most of the manipulator arm links.
The end effector
The end effector is the gripper or tool arm mounted on the wrist of the manipulator arm. Depending on the robot task or robotic application, a specific end effector is attached to the wrist. For example, there are different end effectors for tasks related to material handling and processing.
There may also be different end effectors for the same task. For example, grasping can be done in different ways, such as mechanical fixation, magnetic grasping or suction gripping. Each method uses a different type of end effector.
Actuators and transmission
Actuators or drives are needed to move the links over the joints. The movement takes place during the transport of a load desired by the robot. The payload can be an arm tool or a workpiece.
There are three types of actuators or drives used to build industrial robots.
1. Pneumatic actuators: These actuators use compressed air to move the link around the joint. The movement can be translational or rotational. Pneumatic drives are typically used for linear or translational motion. They are simple to build, economical, fast and reliable. However, the drives are only suitable for small and light payloads as there can sometimes be delay in movement and reduced repeatability.
2. Hydraulic Drives: These units use oil to move the link through the joint. Oil is pumped from a tank to the hydraulic actuator through a control valve. Both linear and rotational movement can be driven through these actuators. Hydraulic drives can move heavy loads and are easy to maintain. They are expensive and not always as accurate as other types of units.
3. Electric Drives: These are the electric motors used to move the link over the joint. There are many different types of electric motors, including brushless DC motors, stepper motors, DC servo motors, and reversible AC servo motors. The one used is based on the desired movement of the link and the required control and repeatability. Electric motors are highly precise, reliable and can support many payloads. Engines come with different price tags depending on features and application.
Typically, transmission elements are required between the link and the inverter. The actuator or inverter output may exhibit kinematics different from the desired motion. For example, a DC motor produces rotational motion that must be converted into translational motion of the link. Actuator output may be unsuitable for direct link application.
For example, an engine may have a high RPM output, but its RPM must be reduced or converted to equivalent torque. Another case where transmission is required between the drive and the link is when the actuator is too large to fit along the link. In these cases, with proper transmission, the drive is placed in a suitable location around the link, and any conflict between the movement of interconnected links is resolved.
The controller
A controller is a computing unit that controls the movement of links in a programmable manner. The controller can be microcontrollers, specialized controllers, or computers.
The controller receives feedback from the sensors, controlling the outputs of the actuators so that the robot moves sequentially to perform its task. The sensors and actuators interface with the controller through hardware interfaces. Controllers may also have a user interface for reprogramming or human input.
Sensors
The sensors interact with the robot's workspace and evaluate the movement and orientation of the manipulator arm and end effector. The controller then considers further actions.
Two types of sensors are used in building industrial robots, tactile and non-tactile. Tactile sensors make physical contact for detection, generating analog or digital signals proportional to the desired physical quantity.
Tactile sensors include force, torque, pressure, touch, and position sensors. Non-tactile sensors do not make physical contact for detection, but use a magnetic field, radio waves, or ultrasonic sound waves. Non-tactile sensors include proximity, imaging, ranging imaging, and optical sensors.
