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This article offers industry insights about automation systems with a primary focus on arms and actuators. Read further to learn more about:
An automation system is an integration of sensors, controls, and actuators designed to perform a function with minimal or no human intervention. The field concerned in this subject is called Mechatronics which is an interdisciplinary branch of engineering that combines mechanical, electrical, and electronic systems.
Most automation systems are derived from manual processes such as drilling, cutting, welding, and so on. These systems use robotic arms to manipulate the movement of the tool that performs the original function. Other applications, particularly in the field of process control, use automation to monitor and control process parameters. This is done by manipulating the operation of equipment such as heaters, motors, pumps, and compressors or by opening or isolating process lines using control valves. Automation systems are available in different configurations even for one specific function. The most common applications of automation systems are:
The main objective of an automation system is to reduce human intervention. A human operator is prone to errors and fatigue which can lead to a variety of problems. Adapting an automation system will produce substantial benefits on profit, production rate, safety, and quality. Enumerated below are the advantages and disadvantages of using automation systems.
More Consistent Production: Robotic systems, when designed correctly, can reduce the production time required by doing complicated movements efficiently. A higher production rate means a larger production volume and better profit. To achieve faster actions, there must be sufficient force supplied by the driver and the flow of movement of the links and joints must be smooth without any unnecessary transitions. Moreover, the processing speed of a computer is faster and more efficient than humans. Humans can indeed process more complex information than computers, but when human error, breaks, and sick days are taken into account, computers perform better.
Increased Repeatability: A production line is efficient because of the repeated sequence of movements that are somehow "programmed" to the operator performing the work. This requires minimal decision making where the actions are governed mostly by a predetermined, specific set of instructions. These repeated movements can be broken down into simple translations and rotations that can be programmed into a robot. Since the actuators are designed with a near-constant range of motion, repeatability is increased.
Precision and Accuracy: As mentioned earlier, the actuators are designed to perform movements with a constant range. The characteristics of an actuator‘s movement will not change unless there is a feedback signal or change in the control variables. The automation system can be calibrated to deliver the same output with minimal or no deviation.
An automation system consists of a device capable of receiving input (sensor, human-machine interface, etc.), a computing system (processor), and the manipulators that perform the actual work (actuator). The most important part of the three is the computing or control system. It can be divided into two types: open loop and closed loop (feedback) control. Open-loop control is a system where the controller sends signals to the actuator to obtain the desired response. The signals sent are only governed by the program initially fed to the controller. In a closed-loop system, a feedback signal is involved. The feedback signal is generated by the sensor which measures directly or indirectly the response made by the actuator. This feedback signal is processed by the controller to compare the actual output with the desired output. The controller then makes the necessary adjustments to the signals being sent to the actuator. The loop is repeated until the correct response is attained.
The input component can be a human-machine interface or a sensor. The human-machine interface is where the human operator communicates with the controller. This is where variables or commands are entered to modify the expected output. A sensor is used to measure the output by utilizing different physical or electromagnetic characteristics such as pressure, temperature, magnetism, radiation, and so on. The measured physical property is converted into an electronic signal that can be read and used by the controller.
The actuator is the part that produces the actions. The actuator is composed of a driver and an assembly of joints and links. The driver provides the required force or torque used to move the links connected by joints. Drivers can be considered as electric, hydraulic, or pneumatic. Electric actuators are motors or solenoids that convert electrical energy into a mechanical output. Hydraulic and pneumatic systems operate using fluid pressure compressed on pistons, cylinders, vanes, or lobes. These systems, in their most basic concept, can be considered electric as well since the fluid is controlled by the opening and closing of solenoid valves.
The links can move relative to each other depending on the degree of freedom allowed by the joint. Degrees of freedom is defined as the allowed motion of the links on a three-dimensional axis. There are six degrees of freedom, three for translation (up and down, left and right, forward and backward) and three for rotation (pitch, yaw, and roll). Most joints allow only one or two degrees of freedom for simplicity since creating a highly movable arm is difficult, expensive, and impractical.
The arm is where the end-of-arm-tools are mounted. An arm is an assembly of links and joints that have a fixed range of motion. A link is typically a rigid component capable of transferring force. Links are connected by joints which are categorized as revolute or prismatic joints. Revolute joints allow rotational movement while prismatic joints allow translational movement. Combinations of these links and joints yield varying degrees of freedom or range of motion. Arm configurations can be classified as follows.
Cartesian Robot: A Cartesian robot is composed of three prismatic joints. The name Cartesian is derived from the three-dimensional Cartesian coordinate system which consists of X, Y, and Z axes. This is the simplest system since it is easy to calculate the movements needed to manipulate the end effector from one place to another. This is suitable for applications that only require movement at right angles without the need for end effector rotation. An example of a Cartesian robot is a gantry machine.
Polar Robotic Arm: This type is also known as spherical robots. Its range of movement can be visualized as a sphere with the radius having the length of the link connecting the second revolute joint and the end effector. This link is allowed to be extended using a prismatic joint. Thus, this robotic arm is composed of two revolute joints and one prismatic joint.
Cylindrical Robotic Arm: This type of robotic arm consists of one revolute joint and two prismatic joints. The revolute joint is located at the base of the arm. This joint allows rotation of the links about its axis. This forms a cylindrical range of motion. The prismatic joints can extend which can be visualized as changing the height and radius of the cylinder.
Selective Compliant Articulated Robot Arm (SCARA): A SCARA is a robot that consists of an arm that is compliant or flexible horizontally in the X-Y plane but rigid vertically or in the Z-axis. This describes its "Selective Compliant" characteristic. Its "Articulated Robot Arm" is similar to a human arm composed of two links attached by a joint at their ends. This allows the robotic arm to extend or fold.
Articulated or Anthropomorphic Robot: This robot adds two more degrees of freedom to the end effector, in contrast with SCARA robots. Articulated robots have arms that are connected by a revolute joint at one end, similar to SCARA. However, they do not have a vertical linear guide. Rather, one arm is mounted into a swivel joint with a fixed base which allows more flexible movement.
End-of-arm-tools (EOATs), also known as end effectors, is the tool or operator designed to interact with the product or process. Most EOATs are grippers that lift and drop objects used for transferring or reorienting objects. Grippers have different methods of handling objects and are classified as impactive (mechanical jaws), ingressive (needles), astrictive (vacuum and magnetism), and contigutive (adhesion). Tools can also be fitted for more specific applications such as milling and welding.
Mechanical Grippers: These are used for basic pick-and-place robotic systems. Grippers have one to three sets of mechanical jaws that are driven typically by servo motors or pneumatic actuators. These jaws are composed of one line which is connected to the wrist by a revolute or prismatic joint. To control the gripping force when using servo motors, feedback is generated by strain gauges or the motor current. For grippers using pneumatic actuators, the gripping force can be increased without damaging the item due to the inherent compressibility of air. The jaws can be constructed as forks, fingers, parallel plates, or surfaces following the shape of the payload. A better grip is achieved by lining the surfaces with resilient, high friction materials.
Vacuum or Suction Cups: These are used for picking objects with smooth surfaces such as films, glass, and plates. A common way of producing a vacuum is through the use of a venturi supplied with compressed air. To create a larger suction force, an array of suction cups is used. Vacuum EOATs are cleaner than mechanical grippers and can allow some positional deviation. This type of EOAT is not suitable for rough, porous, or irregular surfaces. Moreover, the object can slip out of the suction cup when accelerated too quickly.
Magnetic Grippers: These types of EOATs use electromagnets for lifting ferromagnetic objects. Permanent magnets are also used since it does not continuously consume power. However, it needs a mechanical device for removing the collected object. Electromagnets are preferred due to their simple operation since the object can be lifted or dropped simply by supplying or cutting power to the electromagnet. However, aside from the limitation of its use on ferromagnetic materials, it also causes the parts to be magnetic. Also, it cannot be accelerated too quickly since the attached object can slip.
Inflatable Collars and Cylinders: An inflatable collar can be visualized as a looped elastomer tube supported by a rigid structure on its outer periphery. It grips the object by expanding the tube while releasing, which is done by deflating. These are commonly used in the two-dimensional gripping of tubular or cylindrical products.
Needle Grippers: These types perform gripping action by penetrating the object or bulk with needles or hackles. These EOATs are usually static without any moving links of joints. Needle grippers are used in handling porous or fibrous objects such as textiles, carbon and glass fibers where small penetrations are not an issue.
Adhesive Grippers: As the name suggests, these types of grippers grasp the product through surface adhesion. A special type of adhesive is coated onto the surface of a pad or plate which contacts the product to be lifted. The main advantage of adhesive grippers is their ability to operate without any air or power supply. However, they are limited to handling light objects and they tend to lose gripping effectiveness over time.
Tools (Permanent and Changeable): Tools can be fitted at the outermost link of the wrist instead of a gripper. The tool can be permanently attached or changeable. Common tools for end effectors are screwdrivers, wrenches, drills, rotating cutters, lasers, waterjet nozzles, paint spray nozzles, welding electrodes, and solders. Other specialized end effectors include inspection systems with mounted sensors. An example of this is a camera or other type of optical device which is used for non-contact testing and 3D measurements. The resulting measurements are exact in the order of tenths of a millimeter due to the intrinsic repeatability, precision, and accuracy of robotic systems.
Existing tools installed to the robotic arm can also be changed over time due to modifications brought about by new product requirements, system improvements, or part obsolescence. In deciding whether the new tool is applicable, several factors must be verified:
Anthropomorphic and Adaptive Grippers: In comparison with mechanical grippers, anthropomorphic grippers have more complicated links and joints. Mechanical grippers typically have one link connected to the wrist by a revolute or prismatic joint. Anthropomorphic grippers, on the other hand, have two or more links chained together by revolute joints. They can be configured to provide two- or three-dimensional gripping by having two or three sets of fingers. To be adaptive, each finger is actuated independently with mounted sensors for checking proximity and grip strength. Anthropomorphic and adaptive grippers are useful in applications where the objects are frequently varying such as in sorting and multiple product line packaging systems.
Actuators are the components that provide force or torque to create movement. These are attached to the links and joints through tendons, gears, chains, cams, or shafts to form the main actuation system. They are classified as electric, hydraulic, and pneumatic.
Electric Actuators: Electric actuators are the most widely used actuators for industrial robots. The most common type of electric actuator is a servo motor energized by a DC power supply. The rotational movement of the motor can be converted into linear action by various mechanical transmission systems such as belts, cables, and chains. Electric actuators that create direct linear motion also exist in the form of linear motors and solenoids. The main types of electric actuators are summarized below:
Servo motors: This type of electric actuator operates through a closed-loop or feedback system which processes an output signal to control its position, velocity, and acceleration. The motors used in the servomechanism can be a brushed DC motor, brushless DC motor, AC motors, and even linear motors. The servomechanism has a sensor, transducer, or potentiometer called an encoder that measures the position and speed of the motor and converts it into an electronic signal. The signal, either digital or analog, is fed to an amplifier and controller which then alters the voltage or frequency of the electric power supplied to the motor.
Stepper Motors: Unlike the servomotor, stepper motors do not need a feedback loop. They operate through the continuous energization and de-energization of stator poles that pull the poles of the rotor. The stator has poles energized separately to pull the rotor poles and create a stepping or indexing rotation. The rotor is made of laminated ferromagnetic material with a different number of poles than the stator. The difference in the number of poles of the rotor and the stator allows only one set or pair of poles to be attracted at a time. A controller and amplifier power the poles according to the programmed speed of the motor. Stepper motors are simpler than servo motors but are less powerful. If the load is exceeded, the motor can slip. Since there is no built-in feedback loop, there is no way to correct the deviation.
Pneumatic Actuators: Pneumatic actuators operate using compressed air typically at pressures around 6 to 10 bars. The flow of compressed air is controlled by solenoid valves. The most common types of pneumatic actuators are cylinders or rams. A pneumatic cylinder has a piston that extends or retracts upon the application of pressure inside the cylinder. One side of the piston is connected to a rod which couples to the robot arm. Other methods of coupling are also possible such as cables and magnets. The amount of force generated depends on the pressure and the effective area of the piston.
Pneumatic cylinders can be single-acting or double-acting. A single-acting cylinder has only one inlet port in which the compressed air pushes the piston in one direction only. The return stroke is achieved by an external force such as spring force or gravity. On the other hand, a double-acting cylinder has two ports on both ends of the cylinder that acts as both inlet and exhaust ports. Compressed air is supplied on one end and is released on the other. This allows the piston to move and exert force in both directions. A less common type of pneumatic cylinder is a telescoping cylinder which is composed of nested shells that extend when compressed air is introduced. Telescoping cylinders can be single or double-acting.
For creating rotary motion using compressed air, pneumatic motors are used. Common pneumatic motors are rotary vanes and turbines. Rotary vanes operate through the positive displacement of air as it passes the rotor. Turbines create rotation using the kinetic energy of the passing air. Aside from pneumatic cylinders and motors, other types of pneumatic actuators exist such as tubes, bellows, and diaphragms. Though different in construction, they function the same way as cylinders and motors.
Hydraulic Actuators: Hydraulic actuators operate the same way as pneumatic actuators. The only differences are the magnitude of force created, the robustness of construction, its fluid circuit, and the ability to be servo-controlled. Hydraulic actuators can exert very large forces suited for carrying heavy payloads. This can be attributed to the incompressibility of hydraulic fluids or oil. Pressures can go as high as 130 bars. Because of the high pressures involved, hydraulic actuators are constructed with very thick and rigid metals. The rams and pistons are surface treated and sealed to prevent any fluid leakage.
Pneumatic circuits are typically open where the air is not recirculated within the system. In hydraulic circuits, the fluid is returned to the pumping unit where oil is filtered and cooled before recirculation. When compressed at very high pressures, the fluid tends to heat up which can accelerate its degradation.
Another desirable characteristic of hydraulic actuators is their ability to be servo-controlled. Pneumatic cylinders are only capable of fully extending or retracting. Hydraulic cylinders, on the other hand, are capable of being servo-controlled in which their extension length and speed can be precisely controlled.