UNIT-2 ROBOT DRIVE SYSTEMS AND END EFFECTORS

Transcription

1 UNIT-2 ROBOT DRIVE SYSTEMS AND END EFFECTORS

2 CONTENTS 2.1 Pneumatic Drives 2.2 Hydraulic Drives 2.3 Mechanical Drives 2.4 Electrical Drives 2.5 D.C. Servo Motors 2.6 Stepper Motor 2.7 A.C. Servo Motors 2.8 End Effectors 2.9 Grippers Pivoting movement: Linear or Translational Movement: Linkage actuation: Screw actuation: Gear and rack actuation: Rope and pulley actuation: Cam actuation: 2.10 Mechanical Grippers 2.11 Pneumatic and Hydraulic Grippers 2.12 Magnetic Grippers Types of magnetic grippers Electromagnets Permanent magnets 2.13 Vacuum Grippers 2.14 Two Fingered and Three Fingered Grippers Jaw Gripper 2.15 Internal Grippers and External Grippers External gripper Internal gripper 2.16 Selection and Design Considerations

3 TECHNICAL TERMS Sensor: Instruments used as input devices for robots, which enable it to determine aspects regarding the robot's environment, as well as the robot's own positioning. Actuators: A power mechanism used to effect motion of the robot; a device that converts electrical, hydraulic, or pneumatic energy into robot motion. Vacuum Cup Hand: An end-effector for a robot arm which is used to grasp light to moderate weight objects, using suction, for manipulation. Resolvers: A resolver is a type of rotary electrical transformer used for measuring degrees of rotation. It is considered an analog device Optical Encoder: A detection sensor, which measures linear or rotary motion by detecting the movement of markings past a fixed beam of light. This can be used to count revolutions, identify parts, etc. Stepper motor: A stepper motor (or step motor) is a brushless DC electric motor that divides a full rotation into a number of equal steps. Gears: A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque Power screws: Power Screws are used for providing linear motion in a smooth uniform manner. They are linear actuators that transform rotary motion into linear motion. Gripper: An end effector that is designed for seizing and holding (ISO 8373), and "grips" or grabs an object. It is attached to the last link of the arm. Hooks Scoops: a component of machinery to carry things Ladles: a serving device, typically for soup or a foundry ladle used to carry and pour molten Metal Wrist: An interconnected set of links and powered joints between the arm and end effector, which supports, positions and orientates the end effector. 2.1 PNEUMATIC DRIVES Pneumatic drive systems make use of air-driven actuators. Since air is also a fluid, many of the same principles that apply to hydraulic systems are applicable to pneumatic systems. Pneumatic and hydraulic motors and cylinders are very similar. Since most industrial plants have a compressed readily available energy source. This makes the installation of robots that use pneumatic actuator drives easier and less costly than that of hydraulic robots. For lightweight pick-and-place applications that require both speed and accuracy, a pneumatic robot is potentially a good choice. Pneumatic actuator drives work at high speeds and are most useful for

4 small-to-medium loads. They are economical to operate and maintain and can be used in explosive atmospheres. However, since air is compressible, precise placement and positioning require additional components to achieve the smooth control possible with a hydraulic system. These components are discussed in later chapters. It is also difficult to keep the air as clean and dry as the control system requires. Robots that use pneumatic actuator drives are noisy and vibrate as the air cylinders and motors stop. 2.2 HYDRAULIC DRIVES Many earlier robots were driven by hydraulic actuator drives. A hydraulic drive system uses fluid and consists of a pump connected to a reservoir tank, control valves, and a hydraulic actuator. Hydraulic drive systems provide both linear and rotary motion using a much simpler arrangement than conventional electric-drive systems, Figure The storage tank supplies a large amount of instant power, which is not available from electric-drive systems. Hydraulic actuator drives have several advantages. They provide precise motion control over a wide range of speeds. They can handle heavy loads on the end of the manipulator arm, can be used around highly explosive materials, and are not easily damaged when quickly stopped while carrying a heavy load. However, they are expensive to purchase and maintain and are not energy efficient. Hydraulic actuator drivers are also noisier than electric-drive actuators and are not recommended for clean-room environments due to the possibility of hydraulic fluid leaks. 2.3 ELECTRICAL DRIVES Three types of motors are commonly used for electric actuator drives: ac servo motors, dc servo motors, and stepper motors. Both ac and dc servo motors have built-in methods for controlling exact position. Many newer robots use servo motors rather than hydraulic or pneumatic ones. Small and medium-size robots commonly use dc servo motors. Because of their high torque capabilities, ac servo motors are found in heavy-duty robots, Figure A stepper motor is an incrementally controlled dc motor. Stepper motors are rarely used in commercial

5 industrial robots, but are commonly found in educational robots, Figure Conventional, electric-drive motors are quiet, simple, and can be used in clean-air environments. Robots that use electric actuator drives require less floor space, and their energy source is readily available. However, the conventionally geared drive causes problems of backlash, friction, compliance, and wear. These problems cause inaccuracy, poor dynamic response, need for regular maintenance, poor torque control capability, and limited maximum speed on longer moves. Loads that are heavy enough to stall (stop) the motor can cause damage. Conventional electric-drive motors also have poor output power compared to their weight. This means that a larger, heavier motormust be mounted on the robot arm when a large amount of torque is needed. The rotary motion of most electric actuator drives must be geared down (reduced) to provide the speed or torque required by the manipulator. However, manufacturers are beginning to offer robots that use direct-drive motors, which eliminate some of these problems. These high-torque motors drive the arm directly, without the need for reducer gears. The prototype of a direct drive arm was developed by scientists at Carnegie-Mellon University in The basic construction of a direct-drive motor is shown in Figure Coupling the motor with the arm segment to be manipulated eliminates backlash, reduces friction, and increases the mechanical stiffness of the drive mechanism. Compare the design of a robot arm using a direct-drive motor in Figure 2-25 to one with a conventional electric-drive (Figure 2-22). Using direct-drive motors in robots results in a more streamlined design. Maintenance requirements are also reduced. Robots that use directdrive motors operate at higher speeds, with greater flexibility, and greater accuracy than those that use conventional electric-drive motors. Applications currently being performed by robots with direct-drive motors are mechanical assembly, electronic assembly, and material handling. These robots will increasingly meet the demands of advanced, high-speed, precision applications.

6 2.4 D.C. SERVO MOTORS A servomotor is a motor which forms part of a servomechanism. The servomotor is paired with some type of encoder to provide position/speed feedback. A stepper motor is one type of servomotor. A stepper motor is actually built to move angular positions based upon each possible step around the entire rotation, and may include microsteps with a resolution such as 256 microsteps per step of the stepper motor. A servomechanism may or may not use a servomotor. For example, a household furnace controlled by a thermostat is a servomechanism, because of the feedback and resulting error signal, yet there is no motor being controlled directly by the servomechanism. 2.5 STEPPER MOTOR Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator. Figure 1 illustrates one complete rotation of a stepper motor. At position 1, we can see that the rotor is beginning at the upper electromagnet, which is currently active (has voltage applied to it). To move the rotor clockwise (CW), the upper electromagnet is deactivated and the right electromagnet is activated, causing the rotor to move 90 degrees CW, aligning itself with the active magnet. This process is repeated in the same manner at the south and west electromagnets until we once again reach the starting position. DC brush motors rotate continuously when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple "toothed" electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, such as a microcontroller. To make the motor shaft turn, first, one electromagnet is given power, which makes the gear's teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. Each of those slight rotations is called a "step", with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle.

7 2.6 A.C. SERVO MOTORS These servo motors are basically two-phase, reversible, induction motors modified for servo operation. Ac servo motors are used in applications requiring rapid and accurate response characteristics. To achieve these characteristics, these ac servo motors have small diameter, highresistance rotors. The ac servo motor's small diameter provides low inertia for fast starts, stops, and reversals. High resistance provides nearly linear speed-torque characteristics for accurate servo motor control. An induction motor designed for servo use is wound with two phases physically at right angles or in space quadrature. A fixed or reference winding is excited by a fixed voltage source, while the control winding is excited by an adjustable or variable control voltage, usually from a servo amplifier. The servo motor windings are often designed with the same voltage/turns ratio, so that power inputs at maximum fixed phase excitation, and at maximum control phase signal, are in balance. The inherent damping of servo motors decreases as ratings increase and the servo motors are designed to have a reasonable efficiency at the sacrifice of speed-torque linearity. Induction type servo motors are available in fractional and integral horsepower sizes. 2.7 END EFFECTORS The end effector is the robot s hand, or the end-of-arm tooling on the robot. It is a device attached to the wrist of the manipulator for the purpose of grasping, lifting, transporting, maneuvering, or performing operations on a workpiece. The end effector is one of the most important components of a robot system. The robot s performance is a direct result of how well the end effector meets the task requirements. The area within reach of the robot s end effector is called its work envelope.

8 2.8 GRIPPERS A robot gripper serves as the physical interface that a robot performs an application. The grippers are mainly used in a robot for grasping / holding an object. The grasped objects will be moved to the preferred place with the help of a robot. The grippers are capable of carrying work parts, bottles, tools, and so on. There are four robot gripper types: vacuum grippers, pneumatic grippers, hydraulic grippers, and servo-electric grippers. 2.9 MECHANICAL GRIPPERS Robot mechanical grippers and its actuating mechanisms can be classified into several methods. (i) The first method is based on the type of finger movement. During this arrangement, the opening and closing of the fingers can be actuated by either pivoting, or linear or translational movement Pivoting movement: In this motion, the rotation of fingers is concerned with the fixed pivot points of the gripper for providing open and close actions. It uses the linkage mechanism for achieving this movement LINEAR OR TRANSLATIONAL MOVEMENT: During linear motion, the guide rails are used to move the fingers parallel to each other for accomplishing closing and opening. In translational movement, the fingers are maintained in a parallel orientation to each other. As like pivoting movement, it also uses the linkage mechanism for actuation. (ii) The second method of classifying the mechanical grippers is based on the type of kinematic device used for the actuation of finger motions. It can be accomplished by anyone of these types: linkage, screw, gear and rack, rope and pulley, or cam actuation LINKAGE ACTUATION: The design of linkage actuation helps in finding out the conversion of gripper s input force into the gripping force, the time taken to actuate the gripper, and the maximum capability to open the finger. It has plenty of designs for opening and closing the finger, and some of its types are shown below.

9 2.9.4 SCREW ACTUATION: The screw-type actuated gripper consists of a screw connected with a threaded block. To rotate the screw, a motor is used along with a speed reduction device. If the screw is turned in one direction, the threaded block is moved in one direction. Similarly, the threaded block moves in the opposite direction if the screw is turned on the other direction. As the threaded block is attached with gripper, it makes the fingers to open and close Gear and rack actuation: For this actuation, the gear and rack are connected with a piston, which provides lineartype movement. The two partial pinion gears are driven when the rack is moved. As it is linked with gripper, the opening and closing of fingers are accomplished Rope and pulley actuation: In this actuation, a tension device is required to go up against the rope movement in the pulley. Suppose, if the pulley is activated in one direction for opening the gripper, the tension device will provide slack in the rope. Similarly, the gripper is closed by activating the pulley on the other direction Cam actuation: As like linkage actuated gripper, it also has a wide range of designs for opening and closing the gripper fingers. One of its types is described shortly here. A cam actuated gripper with spring loaded follower can be used to provide open and close actions of fingers. The spring is incorporated for forcing the gripper to close if the cam is moved in one direction, while the movement of cam on the other direction causes the gripper to open. This type can be useful for holding various sizes of work parts PNEUMATIC GRIPPER This is the most popular type of gripper due to its compact size, light weight, and force. They can easily be incorporated into tight work cells, which can come in handy in manufacturing. Pneumatic grippers can be either opened or closed, earning them the nickname "bang bang" actuators because of the "bang" sound created by the metal-on-metal punctuation when operated HYDRAULIC GRIPPERS These grippers for robots provide the most strength and are often used for applications that require a significant amount of force. They generate their strength from pumps that can provide 2000 psi and more. Although they are strong they can bring more mess with oil used in the pumps and may damage the end effector with their great force MAGNETIC GRIPPERS Magnetic grippers are most commonly used in a robot as an end effector for grasping the ferrous materials. It is another type of handling the work parts other than the mechanical grippers and vacuum grippers Types of magnetic grippers: The magnetic grippers can be classified into two common types, namely: Magnetic grippers with Electromagnets Permanent magnets

10 Electromagnets: Electromagnetic grippers include a controller unit and a DC power for handling the materials. This type of grippers is easy to control, and very effective in releasing the part at the end of the operation than the permanent magnets. If the work part gripped is to be released, the polarity level is minimized by the controller unit before the electromagnet is turned off. This process will certainly help in removing the magnetism on the work parts. As a result, a best way of releasing the materials is possible in this gripper Permanent magnets: The permanent magnets do not require any sort of external power as like the electromagnets for handling the materials. After this gripper grasps a work part, an additional device called as stripper push off pin will be required to separate the work part from the magnet. This device is incorporated at the sides of the gripper. The advantage of this permanent magnet gripper is that it can be used in hazardous applications like explosion-proof apparatus because of no electrical circuit. Moreover, there is no possibility of spark production as well. Advantages This gripper only requires one surface to grasp the materials. The grasping of materials is done very quickly. It does not require separate designs for handling different size of materials. It is capable of grasping materials with holes, which is unfeasible in the vacuum grippers. Drawbacks The gripped work part has the chance of slipping out when it is moving quickly. Sometimes oil in the surface can reduce the strength of the gripper. The machining chips may stick to the gripper during unloading VACUUM GRIPPERS This type of gripper has been the standard EOAT in manufacturing, allowing for the most flexibility out of all the robot gripper types. Vacuum grippers can be funneled through a rubber or polyurethane suction cup. They may also be funneled through a closed-cell foam rubber layer, thus generating a vacuum suction of compressed air.

11 2.14 Two Fingered and Three Fingered Grippers The most popular style of gripper, all 2 Jaw grippers (angular, parallel and toggle) provide 2 mounting locations for the fingers that come in contact with the part to be grasped. The jaws move in a synchronous motion opening and closing toward the central axis of the gripper body Jaw Gripper A more specialized style of gripper, all 3 Jaw grippers (parallel and toggle) provide 3 mounting locations for the fingers that come in contact with the part to be grasped. The jaws move in a synchronous motion opening and closing toward the central axis of the gripper body. 3 Jaws provide more contact with the part to be grasped and more accurate centering than 2 jaw models INTERNAL GRIPPERS AND EXTERNAL GRIPPERS Grippers are used in two different holding options, External and Internal. The option used is determined by the geometry of the part to be grasped, the process to be performed, orientation of the parts to be grasped and the physical space available ETERNAL GRIPPER External gripping is the most common way to hold parts. The closing force of the gripper is used to hold the part INTERNAL GRIPPER Internal gripping is used when the part geometry will allow and when the process to be performed need access to the outside surface of the part grasped. The opening force of the gripper is used to hold the part SELECTION AND DESIGN CONSIDERATIONS The industrial robots use grippers as an end effector for picking up the raw and finished work parts. A robot can perform good grasping of objects only when it obtains a proper gripper selection and design. Therefore, Joseph F. Engelberger, who is referred as Father of Robotics has described several factors that are required to be considered in gripper selection and design. The gripper must have the ability to reach the surface of a work part. The change in work part size must be accounted for providing accurate positioning. During machining operations, there will be a change in the work part size. As a result, the gripper must be designed to hold a work part even when the size is varied. The gripper must not create any sort of distort and scratch in the fragile work parts. The gripper must hold the larger area of a work part if it has various dimensions, which will certainly increase stability and control in positioning. The gripper can be designed with resilient pads to provide more grasping contacts in the work part. The replaceable fingers can also be employed for holding different work part sizes by its interchangeability facility.

12 QUESTION BANK PART-A 1. Define End effector:- End effector is a device that is attached to the end of the wrist arm yo perform specific task. 2. Give some examples of Robot End effector:- Gripper Tools Welding equipments End of arm Tooling (EOAT) 3. What is meant by Gripper? Gripper is the End effector which can hold or grasp the object. 4. What are the types of Grippers? Magnetic Gripper Mechanical Gripper Hooking Gripper Vacuum Gripper 5.What is a stripping device? A device used to remove work piece from the magnetic gripper 6 What are the types of mechanical Grippers? Linkage actuation gripper Gear and rack actuation gripper Cam actuated gripper Screw actuated gripper 7. Give some examples of tool as Robot End effector Spot Welding Tools Arc welding Torch Spray painting nozzle Water jet cutting tool 8. Define compliance of a robot? It refers to the displacement of the wrist end in response to a force or taught exerted against it. 9.Name some feedback devices used in robotics? Potentiometer Resolver Encoder 10.What are the types of encoders? A. Linear encoder B. Rotary encoder (i) Absolute encoder (ii) Incremental encoder 11. List the various actuating mechanisms used in mechanical grippers. Linkage actuation gripper Gear and rack actuation gripper Cam actuated gripper Screw actuated gripper 12. What is the principle of vacuum cup? The principle used in vacuum pump and Venturi

13 13. List the advantages of magnetic grippers? Pickup times are very fast Variations in part size can be tolerated. The grippers do not have to be designed for one particular work part. They have the ability to handle metal parts with holes. They require only one surface for gripping 14. List the disadvantages of magnetic grippers? Residual magnetism Side slippage More than one sheet will be lifted by the magnet from a stack 15. Name the types of magnetic grippers. 1) Electromagnetic grippers 2) Permanent magnet grippers 16.Define adhesive grippers? Grippers in which an adhesive substance performs the grasping action for handling fabrics and other lightweight material are called Adhesive grippers. 17. What are the limitations of adhesive grippers? Adhesive substances losses his tackiness on repeated usage. Reliability is diminished with successive operations 18. How can we overcome the limitation of adhesive grippers? The adhesive material is loaded in the form of a continuous ribbon into a feeding mechanism that is attached to the robot wrist. 19. List the advantages/features of suction cup grippers Requires only one surface of the part for grasping. Applies uniform pressure distribution on the surface of the part. Relatively a lightweight gripper Applicable to a variety of different material 20. What are the means by which air is removed from the vacuum cup? 1. Vacuum pump 2. Venturi PART-B (16 Mark) 1. Explain the robot drive system and control system with neat sketch Refer INDUSTRIAL ROBOTICS by Mitkell P.Groover Pg. No Explain mechanical grippers in detail with figures Refer INDUSTRIAL ROBOTICS by Mitkell P.Groover Pg. No Explain the following with neat sketch. 1) Vacuum grippers 2) Magnetic grippers 3) Adhesive Grippers Refer INDUSTRIAL ROBOTICS by Mitkell P.Groover Pg. No 127 4) (i) Discuss on the applications of tools as robots end effectors. (ii) List and explain the factors to be considered in the selection and design of grippers. 5) With suitable sketches, explain the operation of motor which provides the output in the form of discrete angular motion increments.

14 6) Explain working principle of Steeper Motor 7) Explain the difference between stepper motor and servo motors. 8) End effectors design is very important in robot explain the statement, state various types of grippers used in robotics system and explain any one of them. 9) Explain different types of robot drive system state advantages and disadvantages of them 10) Distinguish between series wound and shunt wound motor?