Introduction to Robotics

Robotics has been on people’s minds ever since we started building things. With the current advancements in Machine Learning, Artificial Intelligence, and Manufacturing Processes, the applications of Robotics and the demand for Robotics engineers is tremendous. In this article, I try to summarise what robotics actually is and give a basic understanding of it.

Robotics comes from the work ‘Rabota’, meaning worker. Robotics is the branch of technology that deals with the design, construction, and use of robots to perform tasks done traditionally by human beings.

What is a Robot?

Put simply, a robot is any autonomously operated machine that tries to reduce human effort; even if it doesn’t resemble a human in any way is a robot.

In general, robots are controlled by a computer that runs a program. This is what separates a robot from a simple manipulator(which is usually controlled by a human). The motions of a robot are controlled by a controller and supervised by a computer program.

An ideal robot is one that can perform many different tasks and is flexible in what it can do by just changing the program and not having to redesign it.

Classification of Robots

According to the Japanese Industrial Robot Association(JIRA), there are 6 classes of Robots:

Class I: Manual Handling Device: A device with multiple degrees of freedom actuated by an Operator.

Class II: Fixed Sequence Robot: A device that performs a task according to a predetermined, unchanging method. This is hard to modify.

Class III: Variable Sequence Robot: Same as in class II, but easier to modify.

Class IV: Playback Robot: This is a class of robots in which an operator performs the task, leading the robot and it records these motions. Then later the robot repeats the same motions according to the recorded information.

Class V: Numerical Control Robot: Operator provides the robot with movements using a program rather than teaching it manually.

Class VI: Intelligent Robot: A robot that can understand its environment and successfully complete a task despite the changing environment.

The Robotics Institute of America(RIA) considers only classes 3–6, above as robots. The Association Française de Robotique(AFR) has 4 classes of Robots:

Type A: Handling Devices with manual control.

Type B: Automatic Handling Devices with predetermined cycles

Type C: Programmable, servo-controlled robots.

Type D: Same as Type C, but with the added functionality to acquire information from the environment.

Robot Components

A Robot, in a broad sense, has the following components:

Manipulator/rover: The main body of the robot. It consists of links, joints, and other structural elements of the robot.

End Effector: This part is connected to the last joint(hand) of a manipulator that handles objects, makes connections to other machines, or performs tasks. These are designed specifically for a purpose. Either controlled by the robot’s controller or by a separate end effector controlling device(like PLC).

Actuators: These are the muscles of the manipulators. The controller sends signals to actuators, which, in turn, move the links and joints.

Sensors: Used to get information about the state of the robot or to communicate with the outside environment.

Controller: It receives data from the computer(the brain of the robot), controls the motion of the actuators according to that data, and coordinates the motions with the sensory feedback information.

Processor: This is the brain of the robot. It calculates the motions of the robot’s joints, determines how much and how fast each joint should move to achieve the desired speed and motion. The processor is generally a computer but specifically dedicated to this purpose.

Software: Three types of software are used in robots. One is the operating system to operate the processor. Second, the robotic software calculates the necessary motion of each joint based on the kinematic equations of the robot. Third, is the collection of application-oriented routines and programs for specific tasks the robot needs to perform like assembly, machine loading, vision routines, etc.

Robot Degrees of Freedom

To locate a rigid body in space, we need to specify its location, and therefore require 3 pieces of information. After the location is specified, there are an infinite number of ways to orient that object. We need 3 additional pieces of information to specify the orientation of the body.

There need to be 6 degrees of freedom(3 coordinates — x, y, z, and 3 axes of rotation — alpha, beta, gamma) available to fully place the object in space and orient it as desired.

Therefore, any robot needs at least 6 degrees of freedom to place and orient it in its workspace. There are many commercial robots with 7 degrees of freedom, and thus many infinitely many ways to position and orient a part at the desired location. This is done for obstacle avoidance.

The end effector of the robot is never considered as an additional degree of freedom. All robots have this functionality.

Partial Degrees of Freedom are cases where a joint may have the ability to move but its motion is restricted. In this case, the convention is to assign a 1/2 degree of freedom to that joint. Another way a joint can have a 1/2 degree of freedom is if it's allowed to move only at particular values. For example, if a joint is made to be only at 0, 30, 60, 90 degrees.

Robot Joints

There are different types of joints like linear, rotary, sliding, or spherical.

Most robots are two types of joints:

• Prismatic: Linear, hydraulic or pneumatic cylinders or electric linear actuators. They are used in gantry, cylindrical or spherical robot variations.
• Revolute: Rotary, although hydraulic and pneumatic rotary joints are common, most are electrically driven(by servo motors or stepper motors).

Robot Coordinates

Robot configurations generally follow the coordinate frames with which they are defined. Some common ones are(Prismatic joints are denoted by P, revolute joints are denoted by R):

• Cartesian: 3P
• Cylindrical: PRP
• Spherical: P2R
• Articulated: 3R
• SCARA(Selective Compliance Assembly Robot Arm): Two/Three revolute joints that are parallel(allow the robot to move horizontally), plus an additional prismatic joint that moves vertically.

A robot with 3 prismatic and 3 revolute joints is denoted by 3P3R.

Robot Reference Frames

Robots are moved relative to different coordinate frames. In each frame, the motions will be different. Three common reference frames that are used are:

• World Reference Frame: This is the Universal Reference Frame, defined by the x, y, z-axis. Used to define motions of the robot relative to other objects, define other parts and machines with which the robot communicates, and define motion trajectories.
• Joint Reference Frame: Used to specify the movements of individual joints of the robot. In this case, each joint is accessed and moved individually.
• Tool Reference Frame: This specifies movements of the robot’s hand relative to a frame attached to the hand, and all motions are relative to this local n, o, a frame. This frame is a moving frame that changes continuously as the robot moves(It moves with the robot). Extremely useful when the robot is to approach and depart from other objects or to assemble parts.

Robot Programming Modes

The robot can be programmed in many ways. Some of the common ones are:

Physical Set-up: The operator sets up switches and hard stops that control the motions of the robot. Usually used with other devices like Programmable Logic Controllers(PLC).

Lead Through/Teach Mode: Robot’s joints are moved with a teach pendant. When the desired location is achieved, the location is entered into the controller. During playback, the robot moves the joints to the same locations. This is usually point-to-point, i.e., the motion between the points is not specified.

Continuous Walk-Through Mode: All joints are moved simultaneously, while the motion is continuously recorded. During playback, the exact motion is repeated.

Software Mode: The program is written offline or online and is executed by the controller to control the motions. This is the most sophisticated and versatile mode of programming but requires a working knowledge of the programming syntax of the robot.

Robot Characteristics

• Payload: The weight the robot can carry and still remain within its other specifications. The maximum load capacity of the robot may be much more than its specified payload.
• Reach: Maximum distance a robot can reach within its workspace.
• Precision(validity): How accurately a specified point can be reached. Depends on the resolution of actuators, as well as the robot’s feedback.
• Repeatability(variability): How accurately the same position can be reached if the motion is repeated several times. The radius of the circle formed by repeated motions is called repeatability. This is more important than precision, as precision produces the same error every time, which can be corrected; but repeatability produces random errors which cannot be accounted for.

Robot Workspace is the collection of points a robot can reach depending on the robot’s configuration and the size of its links and wrist joints.

References

• Introduction to Robotics, Analysis, Control, Applications by Saeed B. Niku.