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Unit 3 Computer Numerical Control

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Unit 3 Computer Numerical Control

  1. 1. Industrial Control and Automation Unit-3 Computer Numerical Control
  2. 2. Contents • Introduction to CNC Systems • Types –Analogue & Digital CNC System • Absolute and Incremental • Open Loop System and Closed Loop System • CNC Drives • Classification and Operation • Feedback Devices-Adaptive Control • CNC Part Programming. 19-09-2016 suryaprakash.s.eee@kct.ac.in 2
  3. 3. Introduction to CNC Systems • CNC is defined as an NC system whose microcontroller is based on a dedicated microcomputer rather than on a hard- wired controller. • The numerical data required to produce a part is provided to a machine in the form of program, called part program or CNC (computer numerical control) • The program is translated into the appropriate electrical signals for input to motors that run the machine. 19-09-2016 suryaprakash.s.eee@kct.ac.in 3
  4. 4. Milestones in the development of CNC Machine 1775 •John Wilkinson- cannon boring machine (lathe) 1881 •Eli Whitney- milling machine 1947 •Mr. John Parsons began experimenting for using 3-axis curvature data to control the machine tool motion for the production for aircraft components. 1949 •parsons- first NC machine 1951 •MIT was involved in the project 1955 •after refinements NC became available in industry 2016 •Today, modern machinery are CNC milling machines and lathes. 19-09-2016 suryaprakash.s.eee@kct.ac.in 4
  5. 5. Features of CNC Machine • Storage of more than one part program • Various form of program input • Program editing in machine tool • Fixed cycles and program subroutines • Interpolation • Positioning feature for setup • Cutter length and tool compensation • Acceleration and deceleration calculation • Communication interface 19-09-2016 suryaprakash.s.eee@kct.ac.in 5
  6. 6. • Diagnostic tools • Control start-up diagnostics • malfunction and failure analysis • Extended diagnostics for individual components • Tool life monitoring • Preventive maintenance notices • Programming diagnostics 19-09-2016 suryaprakash.s.eee@kct.ac.in 6
  7. 7. Machine control unit for CNC 19-09-2016 suryaprakash.s.eee@kct.ac.in 7
  8. 8. Subsystems of MCU for CNC • Central Processing Unit (CPU) Control section Arithmetic-logic unit Immediate Access Memory • Memory • Input/ Output Interface • Control for Machine Tool Axes and Spindle Speed • Sequence Control for other machine tool function • Personal Computer and the MCU 19-09-2016 suryaprakash.s.eee@kct.ac.in 8
  9. 9. Analysis on Positioning Systems • The NC positioning system converts the coordinate axis values in the NC part program into relative positions of the tool and work part during processing. • The worktable moves linearly by means of a rotating lead screw, which is driven by a stepping motor or servomotor. • The velocity of the worktable, which corresponds to the feed rate in a machining operation, is determined by the rotational speed of the lead screw, 19-09-2016 suryaprakash.s.eee@kct.ac.in 9
  10. 10. • There are two types of positioning systems used in NC systems: (a) open loop and (b) closed loop, 19-09-2016 suryaprakash.s.eee@kct.ac.in 10
  11. 11. Open Loop Positioning System • Operates without verifying the actual position achieved in the move is same as the desired position • Cost is less than the Closed loop system and are appropriate when the force resisting the actual motion is minimal 19-09-2016 suryaprakash.s.eee@kct.ac.in 11
  12. 12. • It uses a stepping motor to rotate the lead screw a stepping motor is driven by a series of electrical pulses, which are generated by the MCU in an NC system. • Each pulse causes the motor to rotate a fraction of one revolution called the step angle. • The possible step angles must be consistent with the following relationship: 19-09-2016 suryaprakash.s.eee@kct.ac.in 12
  13. 13. • The angle through which the motor shaft rotates is given by • The motor shaft is generally connected to the lead screw through a gear box, which reduces the angular rotation of the lead screw. 19-09-2016 suryaprakash.s.eee@kct.ac.in 13
  14. 14. • The linear movement of the worktable is given by the number of full and partial rotations of the lead screw multiplied by its pitch: • Control pulses are transmitted from the pulse generator at a certain frequency, which drives the worktable at a corresponding velocity or feed rate in the direction of the lead screw axis. • The rotational speed of the lead screw depends on the frequency of the pulse train . • The table travel speed in the direction of lead screw axis is determined by the rotational speed. 19-09-2016 suryaprakash.s.eee@kct.ac.in 14
  15. 15. Closed Loop System • It uses feedback measurements to confirm that the final position of the worktable is the location specified in the program. • Mostly used for continuous path operations such as milling or turning, in which there are significant forces resisting the forward motion of the cutting tool 19-09-2016 suryaprakash.s.eee@kct.ac.in 15
  16. 16. • It uses servomotors and feedback measurements to ensure that the worktable is moved to the desired position • Commonly used feed back sensor is Optical Encoder • optical encoder consists of a light source and a photo detector on either side of a disk • The disk contains slots uniformly spaced around the outside of its face. These slots allow the light source to shine through and energize the photo detector. 19-09-2016 suryaprakash.s.eee@kct.ac.in 16
  17. 17. • The flashes are converted into an equal number of electrical pulses. • By counting the pulses and computing the frequency of the pulse train, worktable position and velocity can be determined 19-09-2016 suryaprakash.s.eee@kct.ac.in 17
  18. 18. • In the basic optical encoder, the angle between slots in the disk must satisfy the following requirement: • The pulse count can he used to determine the linear x- axis position of the worktable by factoring in the lead screw pitch and the gear reduction between the encoder shaft and the lead screw. • The velocity of the worktable, which is normally the feed rate in a machining operation, is obtained from the frequency of the pulse train. 19-09-2016 suryaprakash.s.eee@kct.ac.in 18
  19. 19. • The pulse train generated by the encoder is compared with the coordinate position and feed rate specified in the part program and the difference is used by the MCV to drive a servomotor, which in turn drives the worktable. • A digital-to-analog converter converts the digital signals used by the MCU into a continuous analog current that powers the drive motor. • Metal cutting machine tools that perform continuous path cutting operations, such as milling and turning, fall into this category 19-09-2016 suryaprakash.s.eee@kct.ac.in 19
  20. 20. CNC Software • Operating System Software- to interpret the NC part programs and generate the corresponding control signals to drive the machine tool axes. 1. Editor -which permits the machine operator to input and edit NC part programs and perform other file management functions 2. Control Program- which decodes the part program instructions, performs interpolation and acceleration /deceleration calculations, and accomplishes other related functions to produce the coordinate control signals for each axis. 3. Executive Program-which manages the execution of the CNC software as well as the I/O operations of the MCU. 19-09-2016 suryaprakash.s.eee@kct.ac.in 20
  21. 21. • Machine Interface Software-to operate the communication link between the CPU and the machine tool to accomplish the CNC auxiliary functions • Application Software- applications in the user's plant. 19-09-2016 suryaprakash.s.eee@kct.ac.in 21
  22. 22. Driving System • It must response accurately according to the programmed instruction • Mostly electric motor for small tools and hydraulics for large tools • Motor is coupled directly to the gear box to the machine lead screw to slide or spindle • Four types of electrical driving system used are  DC Servo Motor  AC Servo Motor  Stepping Motor  Linear Motor 19-09-2016 suryaprakash.s.eee@kct.ac.in 22
  23. 23. DC Servo Motor 19-09-2016 suryaprakash.s.eee@kct.ac.in 23
  24. 24. AC Servo Motor 19-09-2016 suryaprakash.s.eee@kct.ac.in 24
  25. 25. Stepping Motor 19-09-2016 suryaprakash.s.eee@kct.ac.in 25
  26. 26. Linear Motor 19-09-2016 suryaprakash.s.eee@kct.ac.in 26
  27. 27. Feed back Devices • In order to operate accurately machine should be updated with good positional and velocity feedback devices. • Positional Feedback Devices 1. Linear Transducer 2. Rotary Encoder • Velocity Feed back Devices 19-09-2016 suryaprakash.s.eee@kct.ac.in 27
  28. 28. Encoders 19-09-2016 suryaprakash.s.eee@kct.ac.in 28
  29. 29. Tachometer 19-09-2016 suryaprakash.s.eee@kct.ac.in 29
  30. 30. CNC Part Programming • planning and documenting the sequence of processing steps to be performed on an NC machine. • Traditional input medium dating back to the first NC machines in 1950s is l-inch wide punched tape • More recently the use of magnetic tape and floppy disks were used due to high data intensity. • The methods are: 1. manual part programming, 2. computer-assisted part programming, 3. part programming using CAD/CAM. and 4. manual data input. 19-09-2016 suryaprakash.s.eee@kct.ac.in 30
  31. 31. NC Coding System • Low Level Languages- Binary Number - Binary Coded Decimal • High Level Languages-APT( Automated Programming tools) 19-09-2016 suryaprakash.s.eee@kct.ac.in 31
  32. 32. EIA and ISO Codes 19-09-2016 suryaprakash.s.eee@kct.ac.in 32
  33. 33. 19-09-2016 suryaprakash.s.eee@kct.ac.in 33
  34. 34. Types of Axes • 3 Linear Axis • 3 Rotational Axis 19-09-2016 suryaprakash.s.eee@kct.ac.in 34
  35. 35. Dimension system • Incremental System • Sequence of Points • Error will be accumulated 19-09-2016 suryaprakash.s.eee@kct.ac.in 35
  36. 36. • Absolute System • All the reference are made to origin 19-09-2016 suryaprakash.s.eee@kct.ac.in 36
  37. 37. Definition of Programming “all machining data is compiled and where the data are translated into language which can be understood by the control system of machine tool” Machine Sequence- classification of process, tool start point, cutting depth , tool path etc., Cutting Condition spindle speed, feed rate, coolant, etc Selecting of cutting tools 19-09-2016 suryaprakash.s.eee@kct.ac.in 37
  38. 38. Programming Structure 19-09-2016 suryaprakash.s.eee@kct.ac.in 38
  39. 39. 19-09-2016 suryaprakash.s.eee@kct.ac.in 39
  40. 40. Commonly Used Functions 19-09-2016 suryaprakash.s.eee@kct.ac.in 40
  41. 41. 19-09-2016 suryaprakash.s.eee@kct.ac.in 41
  42. 42. 19-09-2016 suryaprakash.s.eee@kct.ac.in 42
  43. 43. 19-09-2016 suryaprakash.s.eee@kct.ac.in 43
  44. 44. 19-09-2016 suryaprakash.s.eee@kct.ac.in 44
  45. 45. 19-09-2016 suryaprakash.s.eee@kct.ac.in 45
  46. 46. 19-09-2016 suryaprakash.s.eee@kct.ac.in 46
  47. 47. Steps for Part Programming 19-09-2016 suryaprakash.s.eee@kct.ac.in 47
  48. 48. 19-09-2016 suryaprakash.s.eee@kct.ac.in 48
  49. 49. For feed back and suggestion mail me to suryaprakash.s.eee@kct.ac.in 21-06-2016 suryaprakash.s.eee@kct.ac.in 49