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Subject of discussion: Robotic Arm Design and How It Works
What is a Robotic Arm?
Robotic Arm is a mechanism made up of linkages, interconnected through suitable joints, to achieve the required degrees of freedom and spatial movement for the intended work. The robotic manipulator can often be programmed for specific tasks. Due to its functional similarity to a human hand, it is also referred to as anthropomorphic.
Consider a manufacturing industry where humans use tools but carry out end to end tasks of making a product themselves. Now, a robotic arm can perform many operations ranging from metal cutting, metal joining, pick & place and component assembly to product labeling, spray painting, etc., by itself without human intervention. The use of electrical motors and electronic devices such as micro-controllers to drive the mechanical linkages and joints makes a manipulator self-sufficient and important part of Robotic Arm Design.
A robotic arm’s ability to reproduce the results with minimal error increases its efficiency and speed of operation, hence ideal robotic arm design reducing the product cycle time and cost. Without human intervention, the risk of injury is also reduced greatly, making it easier to handle hazardous materials.
Types of Robotic Arm
Cartesian robot / Gantry robot: The spatial movement and location is defined in the cartesian coordinate system, and its arm consists of three prismatic joints. Uses of this robot range from pick and place work, handling of machine tools, performing arc welding, sealant application, and carrying out assembly operations.
Cylindrical robot: The axes of this robot are set up in the cylindrical coordinate system. It is used for handling die cast machines, handling machine tools, carrying out assembly operations, and spot welding.
Spherical robot / Polar robot: Its axes form the polar coordinate system, and it is used in gas, arc, and spot welding operations, fettling mechanism die casting, and handling machine tools.
SCARA robot: It implies Selective Compliance Assembly Robot Arm, which is particularly useful for small robotic assembly operations. As the name suggests, it provides compliance in one plane with two parallel rotary joints and it is rigid in the third direction. It is used to handle machine tools, sealant application, assembly operations, and pick and place work.
Articulated robot: The arm of this robot has at least three rotary joints. Robotic Arm Design uses range from gas & arc welding, spray painting, fettling machines, die casting, and assembly operations.
Parallel robot: A robot with concurrent prismatic or rotary joints. Famous examples are Stewart Platform and Delta robot. This type of robot is used in cockpit flight simulators and optical fiber alignment.
Anthropomorphic robot: A robot design that resembles a human hand with independent fingers.
What is meant by Robotic Arm Design ?
Mechanical Design of Robotic Arm
Inspired by a human hand, the mechanical aspect of a robotic arm design constitutes of several linkages which can be thought of as to form a kinematic chain. The links are connected by joints, which provide the necessary rotational and translational capabilities to the mechanism. The part of a robotic arm design which interacts with the environment is usually the last link and it is called the end effector, or end of arm tooling (EOAT). This is where the hand would be in a human arm.
Degrees of Freedom
In Robotic Arm Design, the degree of freedom (DoF) of a robot is determined using the total number of freedom of the rigid body minus the number of constraint on it’s motion. These constraints of motion often come from the joints. For example, revolute and prismatic joints each offer a single degree of freedom between the two bodies they connect. A universal joint offers two relative DoFs, and a spherical joint offers three relative DoFs.
In Robotic Arm Design of serial and parallel manipulator systems, the end effector is positioned with five degrees of freedom, consisting of three translational DoF and two for orientation. A direct relation can thus be obtained between actuator position and manipulator configuration.
Grubler’s formula is repeatedly used to determine the DoF of a robotic arm, which consider that the constraints on condition that by the joints are self-governing.
Degree of freedom is descriptive of a robotic arm. For example, in the instance of a serial robot, the number characteristically denotes to the number of single-axis rotational joint in the arm, where a greater number specifies improved flexibility in aligning a tool so it’s an important parameter for robotic arm design.
The robot workspace (also known as reachable space) is defined by the collection of all points which can be reached by the end effector. There are many variables on which the workspace depends: the link lengths, rotational and translational limits, overall configuration of the mechanism, etc. The workspace of a serial robotic arm design is described in the figure below. It is a typical workspace of a 4 DoF robot arm. The degree of freedom (DoF) offered by wrist rotation is not included as the robot’s workspace does not depend on its orientation.
The work volume created in this manner defines the workable space for the robot, which can be altered by changing the link lengths and allowable degrees of freedom for the mechanism.
The mechanical design may be limited to 6 DoF as it permits all necessary movements. This can help to keep a check on the cost and complexity of the robot.
Typical workspace representation for various types of robotic arms is given below:
Electronics Design of Robotic Arm
Servo Motor Control
Depending on the input power source, servos are either AD or DC (battery operated) motors. In general, servo motors provide high torque to inertia ratio, which is achieved through an inbuilt gear system. The feedback control loop enables very high precision. The small and compact DC servo motors are highly popular with toys, educational robotic applications, and RC planes. Most servo motors have a rotational limit of about 90 to 180 degrees.
However, some motors can provide higher angular movements. The ability to offer an extremely high level of precision for spatial orientation makes the servo motors an ideal choice for use in robot arms and legs, rack and pinion steering, and sensor scanner. It is easy to implement the velocity and angle control loops as these servos are entirely self-contained.
Servo Wiring: Typically, servo motors have three wires: Ground is identified through Black or Brown. Power is identified through Red. Signal wire is identified through Yellow, Orange, or White (3-5V).
Servo Voltage (Red and Black/Brown wires): Servo motor operating voltage typically varies from 4.8V to 6V. Some micro-sized servo motors operate at a lesser voltage, and some Hitec servo motors are also available, which operate at higher rated voltage.
Signal Wire (Yellow/Orange/White wire): While power is provided to the servo motor through the black and red wires, the commands to operate the servo are provided through the signal wire. Generally, a logic square wave of a specific wavelength (~50Hz) is sent to the servo, which orients it to a particular angle as the wavelength maps directly to the servo angle. For example, in case of Arduino Mega, it receipts i/ps from the PC to generate the square wave, which then controls the angular orientation of the servomotor.
Microcontroller (Basic Concept of Arduino)
Servo motors make use of microcontrollers to control their precision and angular location. Arduinos (a single-board microcontroller) are one such example can be programmed as per the application. It is intended for an Atmel AVR processor, with on-board I/O structures power with USB connections.
Robotic Arm Control
The robotic arms can have manual control or an autonomous capability. In manual mode, a robot is taught to do its task by a trained operator (programmer) who uses a portable control device (a teach pendant) to carry out the objective. This is a relatively slow procedure.
A typical robotic arm has a multi-level control setup, including a microcontroller, driver, and a computer based user interface. Concepts of inverse kinematics are used to provide flexibility in programming and control methods. This implementation is also possible through manual mode. A typical microcontroller has an associated development/programming board.
The basic concept of Forward Kinematics is to determine the orientation and position of the end effector when the joint angles and link lengths of the robot arm are known. The reverse happens in Inverse Kinematics when the desired position of the end effector is known, and aim is to find out the joint angles to achieve the objective.
For example, consider the representation of a planar 2 DoF robotic arm, as shown above. To locate the end effector at a known position in the cartesian space, the coordinates of the end effector will become the input variables x and y with respect to the base, which is taken as origin.
Robots are used for a variety of applications. The end effector must be chosen to fulfill the said objective. It can either be a hand like gripper intended for pick and place operations to a specified location as shown below, or a welding interface to hold the electrodes. The manipulator can have an interface of a spray paint gun for painting purposes or a platform for simulators, making it a complex mechanism and the most crucial part of the robotic arm. The end effector can be pneumatic, electric, or hydraulic based. A servo motor usually controls the end effectors.
Advantages and Disadvantages of Robotic Arm
Advantages of Robotic Arm
- Increased productivity.
- This are capable of effective utilization of resources and raw materials.
- This provides Flexibility at work.
- Reduces cycle time of product manufacturing.
- Lesser product rejection counts on account of defects.
- Extremely high repeatability and precision, thereby minimizing errors and enhancing performance.
- Safer to handle hazardous material as the risk of life is reduced.
Disadvantages of Robotic Arm
- Responsible for labor unemployment.
- High facility and equipment setup costs.
- Flexibility & functionality limited by design as compared to the human hand, which can multi-task.
- Programming for high accuracy tasks is a challenge.
- Extensive requirement of sensor installation for feedback to perform precision work.
- Upcoming challenges related to artificial intelligence and machine vision.
- Post breakdown maintenance and production line delays.
Notable applications of Robotic Arm
Robotic arms affect our lives on a large scale as they play an essential role in industries ranging from food packaging to automobile manufacturing to space applications. Some noteworthy examples are listed below:
In space, the International Space Station (ISS) is installed with the Canadarm and its successor Canadarm2, which are both multi DoF robotic arms. Canadarm1, officially recognized as the Shuttle Remote Manipulator System (SRMS), was employed to deploy, maneuver and carry payload on space shuttle orbiters. It was also fitted with the Orbiter Boom Sensor System (OBSS) to assess the damage to thermal protection system.
Canadarm-2 plays a vital role in the assembly and maintenance of ISS and supports the docking of spacecraft and spacewalks by astronauts.
The Curiosity rover, which landed on the planet Mars, used a robotic arm to pick and place instruments and collect samples from the terrain. Another Mars lander called InSight boasts a robotic arm called Instrument Deployment Arm (IDA), which is around 1.8m long with shoulder elbow and wrist joints to carry out functions like deployment of heat flow probe deep into the terrain. It also has a five-finger grapple and provision for mounting cameras.
NASA’s mission to study asteroids and take samples using the spacecraft OSIRIS-Rex, makes use of TAGSAM robotic arm for collecting the samples.
For human safety and to assist the armed forces, unique robotic arms are made. The robotic arm design can synchronize its movement with the operator, who is at a distance.
The FDA approved da Vinci Surgical System consists of three to four interactive robotic arms that provide surgical assistance with a minimally invasive approach.
To know about Pick and Place Robotic Arm, click here.