Robot Drive system and End Effector types
- 1. UNIT II ROBOT DRIVE SYSTEMS AND END EFFECTORS
- 2. Syllabus Pneumatic Drives – Hydraulic Drives – Mechanical Drives – Electrical Drives – D.C. Servo Motors, Stepper Motor, A.C. Servo Motors – Salient Features, Applications and Comparison of Drives End Effectors – Grippers –Mechanical Grippers, Pneumatic and Hydraulic Grippers, Magnetic Grippers, Vacuum Grippers; Two Fingered and Three Fingered Grippers; Internal Grippers and External Grippers; Selection and Design Considerations
- 3. Drives • The main motive power of a robots is in the drives • At present direct drives to the arms of the robots are preferred • Power is transmitted or applied to various joints of the robot through gears, belts, cable chains and many other means
- 4. Types of Drives There are four basic types of drives as follows • Pneumatic drives • Hydraulic drives • Electrical drives • Mechanical drives
- 5. Factors consideration 1) The drive systems are chosen based on the following factors • Power consumption • Positional accuracy • Repeatability • Speed of operation • Stability • Reliability • Cost 2) The drive methods are selected also on the basis of using • Open loop controls • Closed loop controls
- 6. Hydraulics drives Pump: Transforms mechanical energy in to hydraulic energy. Thus a pump produces flow Types of pumps: • Hydrodynamic (or) Non-positive displacement pumps • Hydrostatic (or) positive displacement pumps
- 7. Hydrodynamic (or) Non-positive displacement pumps: Types of hydrodynamic pumps: • Centrifugal pump (Impeller) • Axial pump (Propeller) There is a sufficient clearance between the rotating and the stationary elements The flow rate of the pump depends on the speed at which the propeller or impeller is driven and the restriction of the outlet or resistance of the external system It creates pressure due to the rotary motion of the propeller or impeller and flow only takes place at the expense of the pressure
- 8. Hydrostatic (or) positive displacement pumps Types of hydrostatic pumps: • Gear pumps (internal and external) • Vane pumps (balanced and unbalanced) • Piston pumps (radial piston pumps, axial piston pumps- bent axis and swash plate) It delivers a fixed quantity of fluid per revolution of the pump shaft Its output flow is constant at the rated speed of the pump
- 9. Hydraulic Drive System • A hydraulic drive system is a quasi- hydrostatic drive or transmission system that uses pressurized hydraulic fluid to powerhydraulicmachinery • A hydraulic drive system consists of three parts: Generator (e.g. a hydraulic pump), driven by an electricmotor Valves, Filters, Piping etc. (to guide and control the system); The actuator (e.g. a hydraulic motor or hydraulic cylinder)todrivethemachinery.
- 10. Advantages of Hydraulic actuator • Hydraulic actuators are rugged and suited for high-force applications. They can produce forces 25 times greater than pneumatic cylinders of equal size. They also operate in pressures of up to 4,000 psi. • Hydraulic motors have high horsepower-to-weight ratio by 1 to 2 hp/lb greater than a pneumatic motor. • A hydraulic actuator can hold force and torque constant without the pump supplying more fluid or pressure due to the incompressibility of fluids • Hydraulic actuators can have their pumps and motors located a considerable distance away with minimal loss of power.
- 11. Disadvantages of Hydraulic actuator • Hydraulics will leak fluid. Like pneumatic actuators, loss of fluid leads to less efficiency. However, hydraulic fluid leaks lead to cleanliness problems and potential damage to surrounding components and areas. • Hydraulic actuators require many companion parts, including a fluid reservoir, motors, pumps, release valves, and heat exchangers, along with noise- reduction equipment. This makes for linear motions systems that are large and difficult to accommodate.
- 12. Pneumatic drives system • Pneumatic systems use air as the medium which is abundantly available and can be exhausted into the atmosphere after completion of the assigned task. • The pneumatic drive systems are especially used for the small type robots, which have less than five degrees of freedom.
- 13. Components of Pneumatic Drive Systems
- 14. Components of Pneumatic Drive Systems Air filters: These are used to filter out the contaminants from the air. Compressor: Compressed air is generated by using air compressors. Air compressors are either diesel or electrically operated. Based on the requirement of compressed air, suitable capacity compressors may be used. Air cooler: During compression operation, air temperature increases. Therefore coolers are used to reduce the temperature of the compressed air. Dryer: The water vapor or moisture in the air is separated from the air by using a dryer. Control Valves: Control valves are used to regulate, control and monitor for control of direction flow, pressure etc. Air Actuator: Air cylinders and motors are used to obtain the required movements of mechanical elements of pneumatic system. Electric Motor: Transforms electrical energy into mechanical energy. It is used to drive the compressor. Receiver tank: The compressed air coming from the compressor is stored in the air receiver.
- 15. Pneumatic drives Compressor: compress air from atmospheric pressure to a higher level of pressure at the expense of reduction of volume Types of compressors: • Positive displacement compressor (reciprocating piston) • Vane type rotary compressor • Screw type rotary compressor
- 16. Actuators Actuators can be categorized by the energy source they require to generate motion. For example: • Hydraulic actuators use liquid to generate motion. • Pneumatic actuators use compressed air to generate motion. • Electric actuators use an external power source, such as a battery, to generate motion.
- 17. Actuators (Pneumatic/Hydraulic) Actuators are devices that convert energy extracted out of a fluid to mechanical work Types of actuators: 1) Linear actuators (or) pneumatic/hydraulic cylinders – Single acting – Double acting 2) Rotary actuators (or) pneumatic/hydraulic motors – Gear motor – Vane motor – Piston motor
- 18. Example
- 19. Example
- 20. Advantages of Pneumatic actuator • The benefits of pneumatic actuators come from their simplicity. • Pneumatic actuators generate precise linear motion by providing accuracy, for example, within 0.1 inches and repeatability within .001 inches. • Pneumatic actuators typical applications involve areas of extreme temperatures. • In terms of safety and inspection, by using air, pneumatic actuators avoid using hazardous materials. They meet explosion protection and machine safety requirements because they create no magnetic interference due to their lack of motors.
- 21. Disadvantages of Pneumatic actuator • Pressure losses and air’s compressibility make pneumatics less efficient than other linear-motion methods. Compressor and air delivery limitations mean that operations at lower pressures will have lower forces and slower speeds. A compressor must run continually operating pressure even if nothing is moving. • To be truly efficient, pneumatic actuators must be sized for a specific job. Hence, they cannot be used for other applications. Accurate control and efficiency requires proportional regulators and valves, but this raises the costs and complexity. • Even though the air is easily available, it can be contaminated by oil or lubrication, leading to downtime and maintenance.
- 22. Mechanical drives (Drive Mechanisms) • It is necessary to get the motion in linear or rotary fashion • When motors are used, the rotary motion is converted in to linear motion through the following Rack and pinion gearing, Ball bearing screws (ball rolls between the nut and screw) Gear trains – Spur, helical & worm gears Cam and follower
- 23. Electric Actuators • Electric Actuators are devices powered by motor that converts electrical energy to mechanical torque • Types Of Electric Actuators: There are three types of pneumatic actuator: they are – DC Motors- is an electric motor that runs on direct current (DC) electricity. – AC Motors - is an electric motor driven by an alternating current. – Stepper Motors- (or step motor) is a brushless DC electric motor that divides a full rotation intoa number of equal steps.
- 24. Advantages • Electrical actuators offer the highest precision-control positioning. • Their setups are scalable for any purpose or force requirement, and are quiet, smooth, and repeatable. • Electric actuators can be networked and reprogrammed quickly. They offer immediate feedback for diagnostics and maintenance. • They provide complete control of motion profiles and can include encoders to control velocity, position, torque, and applied force. • In terms of noise, they are quieter than pneumatic and hydraulic actuators. • Because there are no fluids leaks, environmental hazards are eliminated.
- 25. Disadvantages • The initial unit cost of an electrical actuator is higher than that of pneumatic and hydraulic actuators. • Electrical actuators are not suited for all environments, unlike pneumatic actuators, which are safe in hazardous and flammable areas • A continuously running motor will overheat, increasing wear and tear on the reduction gear. • The motor can also be large and create installation problems.
- 26. Electrical drives • Electrical drives use D.C. motors for robot articulation • They provide clean drives in comparison to hydraulic and pneumatic drives • Electrically driven robots exhibits good repeatability • There are two types of motors used in the field of automation and robotics: Permanent magnet motors and motors with wound field coils • The torque developed on the motor shaft depends on the magnetic field flux in the stator field and the current in the motor armature
- 27. Types of Electrical drives • D.C. motors • A.C. motors • Stepper motor
- 28. D.C. Motor • DC motor can supply power to carry desired loads • D.C motors have high torque to volume ratios • For high precision D.C. motors are suitable with closed loop servo controls • D.C. motors have a stator and a rotor • Permanent magnets are used to generate the stator magnetic field by supplying directly the electrical current in to the armature winding of the rotor through brushes and commutators
- 32. Working Principle of DC Motor When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn towards the right, causing rotation. When the coil turns through 90°, the brushes lose contact with the commutator and the current stops flowing through the coil. However the coil keeps turning because of its own momentum.
- 33. • The efficiency of the DC Motor increases by: • Increasing the number of turns in the coil • Increasing the strength of the current • Increasing the area of cross-section of the coil • Increasing the strength of the radial magnetic field
- 34. • The efficiency of the DC Motor increases by: • Increasing the number of turns in the coil • Increasing the strength of the current • Increasing the area of cross-section of the coil • Increasing the strength of the radial magnetic field
- 36. Cont... • In the shunt wound motor, a stator field winding is connected in parallel with the armature winding • In series wound motor, the stator winding (electromagnet) is connected in series with the armature winding • In the compound wound motor, two stator windings are connected-one in series and the other in parallel with the armature winding • In brushless D.C. motors, electronic commutation in matching with the rotor and the stator magnetic fields is made replacing the conventional brush-communication system
- 37. Stepper Motor Advantages:- Low cost for control achieved Ruggedness Simplicity of construction Can operate in an open loop control system Low maintenance Less likely to stall or slip Will work in any environment
- 38. Disadvantages:- Require a dedicated control circuit Use more current than D.C. motors High torque output achieved at low speeds
- 39. The top electromagnet (1) is turned on, attracting the nearest teeth of a gear-shaped iron rotor. With the teeth aligned to electromagnet 1, they will be slightly offset from electromagnet 2 The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the nearest teeth slightly to the right. This results in a rotation of 3.6° in this example. Practical Stepper motor operation
- 40. The bottom electromagnet (3) is energized; another 3.6° rotation occurs. The left electromagnet (4) is enabled, rotating again by 3.6°. When the top electromagnet (1) is again enabled, the teeth in the sprocket will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation in this example.
- 42. End-Effector (or) Gripper • Robot end effector (or) end of arm tooling is the bridge between the robot arm and the environment around it • A robot end effector which is attached to the wrist of the robot arm is a device that enables the general purpose robot to grip materials, parts and tools to perform a specific task • An end effector of a robot can be designed to have several fingers, joints and degrees of freedom
- 43. Types of End Effectors (or) Grippers • Mechanical gripper • Pneumatic gripper • Hydraulic gripper • Magnetic gripper • Vacuum gripper • Hooking and lifting gripper • Scooping or ladling gripper • Adhesive or electrostatic gripper
- 44. The grippers may be classified in to: • Part handling grippers: used to grasp and hold objects that are required to be transported from one point to another or placed for some assembly operations. The part handling applications include machine loading and unloading, picking parts from a conveyor and moving parts, etc • Tools handling grippers: to hold welding gun or spray painting gun to perform a specific task. It may also hold a deburring tool • Special grippers: specialized devices like Remote Centre Compliance (RCC) to insert an external mating component in to an internal member, viz. inserting a plug in to a hole
- 45. Mechanical Grippers • It uses mechanical fingers actuated by a mechanism to grip an object • The fingers of the grippers makes contact with the object • The fingers are either attached to the mechanism or an internal part of the mechanism • The use of replaceable fingers allows for wear and interchangeability • Different sets of fingers for use with the same gripper mechanism can be designed to accommodate different parts models
- 47. Friction between the fingers and workpart In this approach the fingers must apply a force that is sufficient for friction to retain the part against gravity, acceleration and any other forces
- 48. Friction between the fingers and workpart
- 49. Types of gripper mechanism • Classification based on finger movement – Pivoting Movement • Linear actuation • Gear and rack actuation • Cam actuation • Screw actuation • Rope and pulley actuation • miscellaneous – Linear or translational movement
- 50. Pivoting Gripper Mechanisms – Linear Actuations
- 51. Pivoting Gripper Mechanisms – Gear and Rack method
- 52. Pivoting Gripper Mechanisms – Cam Actuated Method
- 53. Pivoting Gripper Mechanisms – screw Actuated Method
- 54. Pivoting Gripper Mechanisms – rope and pulley Actuated Method
- 55. Mechanical gripper with interchangeable fingers
- 58. Single Acting Cylinder Control • Direct and Speed Control
- 59. Double Acting Cylinder Direct control Single control
- 61. Characteristics of various control systems
- 62. Magnetic Grippers • Magnetic Grippers are used extensively on ferrous materials • Magnetic Grippers can use either electromagnets or permanent magnets
- 63. Electromagnet Type • Electromagnetic grippers are easier to control • It require a source of DC power and an appropriate controller • When the part is to be released, the control unit reverses the polarity at a reduced power level before switching off the electromagnet • This produces acts to cancel the residual magnetism in the workpiece ensuring a positive release of the part
- 64. Permanent Magnet Type • Permanent magnet do not require an external power • They can be used in hazardous and explosive environments, because there is no danger of sparks which might cause ignition in such environments • When the part is to be released at the end of the handling cycle some means of separating the part from the magnet must be provided
- 65. Figure
- 66. Advantages of Magnetic Grippers In general magnetic grippers offer the following advantages in robotic handling operations: • Variations in part size can be tolerated • Pickup times are very fast • They have ability to handle metal parts with holes • Only one surface is required for gripping
- 67. Disadvantages of Magnetic Grippers • The residual magnetism remaining in the workpiece may cause problems • Problem of picking up one sheet at a time from a stack • The magnetic attraction tends to penetrate beyond the top sheet in the stack, resulting in the possibility that more than a single sheet will be lifted by the magnet
- 68. Vacuum Grippers • Vacuum grippers are used for picking up metal plates, pans of glass, or large lightweight boxes • The vacuum cups are made of elastic materials • For handling softer materials, cups made of harder materials are used • A compressed air supply and a venturi are used to create a gentle vacuum that lifts the part • Instead of a venturi, a vacuum pump powered by an electrical motor may also be used • The lift capacity of the suction cup depends on the effective area of the cup and the negative air pressure(pressure difference between the inside and the outside of the vacuum cup) between the cup and the object
- 69. figure
- 70. Adhesive Grippers • An adhesive substance can be used for grasping action in gripper design • The requirement on the items to be handled are that they must be gripped on one side only • The reliability of this gripping device is diminished with each successive operation cycle as the adhesive substance loses its tackiness on repeated use • To overcome this limitation, the adhesive material can be loaded in the form of a continuous ribbon in to a feeding mechanism attached to the robot wrist
- 71. Hooks • Hooks can be used as end effector to handle containers and to load and unload parts hanging from overhead conveyors • The item to be handled by a hook must have some sort of handle to enable the hook to hold it
- 72. Ladles and Scoops • Ladles and scoops can be used to handle certain materials in liquid or powder form • One of the limitations is that the amount of material being scooped by the robot is sometimes difficult to control
- 73. Inflatable devices • An inflatable bladder is expanded to grasp the object • Inflatable bladder is fabricated out of some elastic material like rubber, which makes it appropriate for gripping fragile objects • The gripper applies a uniform grasping pressure against the surface of the object
- 74. Figure
- 75. Internal and External gripper • Robot end effectors can be classified on the basis of the mode of gripping as external and internal gripping • The internal gripper grips the internal surface of the objects with open fingers • The external gripper grips the exterior surface of objects with closed fingers
- 76. Internal Gripper
- 77. External Gripper
- 78. Two Fingered -Gripper Pivoting or swinging gripper mechanisms: • This is the most popular mechanical gripper for industrial robots • It can be designed for limited shapes of an object, especially cylindrical workpiece
- 79. Gripper using Slider Crank Mechanism & Swing Block Mechanism • If actuator that produce linear movement are used, like pneumatic piston cylinders, the device contains a pair of slider crank mechanisms or swing block mechanism
- 80. Gripper with a Rotary Actuator • Actuator is placed at the cross point of the two fingers • Each finger is connected to the rotor and the housing of the actuator respectively • The actuator movement directly produces grasping and releasing actions
- 81. Cam actuated Gripper • The cam actuated gripper includes a variety of possible designs • A cam and follower arrangement, often using a spring loaded follower, can provide the opening and closing action of the gripper • The advantage of this arrangement is that the spring action would accommodate different sized objects
- 82. Screw Type Gripper • The screw is turned by motor usually accompanied by a speed reduction mechanism • Due to the rotation of the screw, the threaded block moves, causing the opening and closing of the fingers depending on the direction of rotation of the screw
- 83. Two Fingered -Gripper Translation gripper mechanisms: • Translation mechanisms are widely used in grippers of industrial robots • The finger motion corresponds to the piston movement without any connecting mechanisms between them • The drawback is that sometimes it is difficult to design the desired size of the gripper, because here the actuator size decides the gripper size
- 84. Figure
- 85. Gripper using Rotary actuators
- 86. Three Fingered Gripper • The increase of the number of fingers and degrees of freedom will greatly aid the versatility of grippers • It is capable of grasping the object in three spots, enabling both a tighter grip and the holding of spherical objects of different size keeping the centre of the object at a specified position • Three point chuck mechanisms are typically used for this purpose • Each finger motion is performed using ball-screw mechanism • Electrical motor output is transmitted to the screws attached to each finger through bevel gear trains which rotate the screws • When each screw is rotated clockwise or anticlockwise, the translational motion of each finger will be produced, which results in the grasping-releasing action
- 87. Figure
- 88. Selection and Design Considerations There are two ways of constraining the part in the gripper • In the first way, the gripper fingers may enclose the part to some extent, thereby constraining the motion of the part • This is accomplished by designing the contacting surface of the fingers to be in the approximate shape of the part geometry • The second way of holding the part is by friction between the fingers and the object • In this approach the finger must apply a force that is sufficient for friction to retain the part against gravity, acceleration and any other force that might arise during the holding portion of the working cycle
- 89. Cont... The gripping force that must be applied is Fg = (m * g * sinƟ)/ (µ * n) m= mass, Kg g= acceleration due to gravity, m/s^2 µ = coefficient of friction Ɵ = angle subtended with the horizontal n = number of pairs of contact surfaces