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									An overview of computerised Numerical Control and
         Programmable Logic Control
                                                     RAVI RAJ

Abstract— Over the past few years, people have been working in
machine automation areas so as to meet the increasing demands
to manufacture complicated components of high accuracy in
large quantities. So industries have gradually moved from
conventional relay logic control to programmable logic control
and then to computerised numeric control. Here, we shall deal
how a programmable logic controller works in automation and
the control of a computerised numeric controller on PLC to
provide automation with accuracy.

Keywords— CNC, PLC, Part programming, Ladder diagram

                         1. INTRODUCTION
   The present day computer can be considered as a direct
consequence of the progress in the field of numerical control
of machine tools. A real breakthrough was achieved around
1965 when numerical control machines were fitted with
minicomputers which introduced the name Computer                          2.1 Input Devices
Numerical Control. Computer Numerical control (CNC) is a          a. Floppy Disk Drive
specialized and versatile form of soft automation and its
application cover many kinds, although it was initially           Floppy disk is a small magnetic storage device for CNC data
developed to control the motion and operation of machine          input. It has been the most common storage media up to the
tools.                                                            1970s.
   Computer Numerical control may be considered to be a           b. USB Flash Drive
means of operating a machine through the use of discrete
numerical values fed into machine. Controlling a machine tool     A USB flash drive is a removable and rewritable portable hard
by means of prepared program, which consists of blocks, or        drive with compact size and bigger storage size than a floppy
series of commands/numbers, is known as numerical control.        disk.
In simple words, a CNC system receives numerical data,            c. Serial communication
interpret the data and then control the action accordingly. The
first step in the process of implementing automation in any       The data transfer between a computer and a CNC
industry is to manufacture parts or components through            machine tool is often accomplished through a serial
automation using machines and machine tools with little           communication port.
human intervention.                                               d. Ethernet communication
                     2. CNC SYSTEM
                                                                      2.2 Machine Control Unit
    A CNC system consists of major elements:
    a. Input Device                                                 The machine control unit is the heart of the CNC system.
    b. Machine Control Unit                                       There are two sub-units in the machine control unit: the Data
    c. Machine Tool                                               Processing Unit (DPU) and the Control Loop Unit (CLU).
    d. Driving System                                                 a. Data Processing Unit
    e. Feedback Devices                                             On receiving a part programme, the DPU firstly interprets
    f. Display Unit                                               and encodes the part programme into internal machine codes.
                                                                  The interpolator of the DPU then calculate the intermediate
                                                                  positions of the motion in terms of BLU (basic length unit)
                                                                  which is the smallest unit length that can be handled by the
                                                                  controller.
                                                                    b. Control Loop Unit
   The data from the DPU are converted into electrical signals     as the position of the machine slide, the spindle RPM, the feed
in the CLU to control the driving system to perform the            rate, the part programmes, etc. In an advanced CNC machine,
required motions. Other functions such as machine spindle          the Display Unit can show the graphics simulation of the tool
ON/OFF, coolant ON/OFF, tool clamp ON/OFF are also                 path so that part programmes can be verified before the
controlled by this unit according to the internal machine codes.   actually machining. Much other important information about
                                                                   the CNC system can also displayed for maintenance and
    2.3 Machine Tool                                               installation work such as machine parameters, logic diagram
                                                                   of the programmer controller, error messages and diagnostic
  This can be any type of machine tool or equipment. In            data.
order to obtain high accuracy and repeatability, the design and
make of the machine slide and the driving lead screw of a             CNC machines are widely used in the metal cutting
CNC machine is of vital importance. The slides are usually         industry and are best used to produce the following types of
machined to high accuracy and coated with anti-friction            product:
material in order to reduce the stick and slip phenomenon.            • Parts with complicated contours
                                                                      • Parts requiring close tolerance and/or good repeatability
    2.4 Driving System                                                • Parts requiring expensive jigs and fixtures if produced on
                                                                   conventional machines
   The driving system is an important component of a CNC              • Parts that may have several engineering changes, such as
machine as the accuracy and repeatability depend very much         during the development stage of a prototype
on the characteristics and performance of the driving system.         • In cases where human errors could be extremely costly
The requirement is that the driving system has to response            • Parts that are needed in a hurry
accurately according to the programmed instructions. This             • Small batch lots or short production runs.
system usually uses electric motors although hydraulic motors
are sometimes used for large machine tools. The motor is
coupled either directly or through a gear box to the machine
lead screw to moves the machine slide or the spindle. Three
types of electrical motors are commonly used.

  a.   DC Servo Motor
  b.   AC Servo Motor
  c.   Stepping Motor

    2.5 Feedback Devices

   In order to have a CNC machine operating accurately, the
positional values and speed of the axes need to be constantly
updated. Two types of feedback devices are normally used
positional feedback device and velocity feedback device.
   a. Positional Feed Back Devices
   There are two types of positional feedback devices: linear                        Fig. Classification of CNC Machine
transducer for direct positional measurement and rotary
encoder for angular or indirect linear measurement.                                   3.   CNC Part Programming
   b. Velocity Feedback Device
   The actual speed of the motor can be measured in terms of         A computer program to specify:
voltage generated from a tachometer mounted at the end of the          which tool should be loaded on machine spindle,
motor shaft. DC tachometer is essentially a small generator            what are cutting condition (speed, feed, coolant),
that produces an output voltage proportional to the speed. The         The start and end of the motion segment,
voltage generated is compared with the command voltage                 How to move tool with respect to machine.
corresponding to the desired speed. The difference of the
voltages can is then used to actuate the motor to eliminate the        3.1 Dimension system in CNC part programming
error.
                                                                        1. Incremental System
    2.6 Display Unit                                                  This type of control always uses a reference to the
                                                                   preceding point in a sequence of points. The disadvantage of
   The Display Unit serves as an interactive device between        this system is that if any error occurs, it will be accumulated.
the machine and the operator. When the machine is                       2. Absolute System
running, the Display Unit displays the present status such
  All references are made to the origin of the coordinate                                     4.   Programmable Logic controller
system. All commands of motion are defined by the absolute
coordinate referred to the origin.                                          When the first electronic machine controls were designed,
                                                                            they used relays to control the machine logic. There were few
        3.2 Programme Structure                                             limitations: Relays fail, the delay when relay turns on/off, and
                                                                            there is entire wall of relays to design/wire.
      A CNC programme consists of blocks, words and                         A single PLC can be programmed to replace thousands of
      addresses.                                                            relays.
           Block: A command given to the control unit is
               called a block.                                              4.1 PLC Programming
           Word: A block is composed of one or more
                                                                                    Ladder Diagram - most common
               words. A word is composed of an identification
               letter and a series of numerals.                                     Structure Text Programming (ST)
           Address: The identification letter at the
               beginning of each word is called address.                            Functional Block Programming (FB)
          Example: N20 G01 X20.5 F200 S1000 M03
          N is the address, N20 is the word.                                        Instruction List (IL)
It is also known as G-M code because of the use of G and M
words used in blocks.
    A “G” letter specifies certain machine preparations such as
                                                                            4.2 CNC-PLC Interface
inch or metric modes, or absolute versus incremental modes.
    A “M” letter specifies miscellaneous machine functions.                 The diagram below shows a CNC-PLC interface of a CNC
Some of the Keys words are given below:                                     system. The part program determines the operational sequence
                                                                            of the machine. It executes the part program as follows:
                                                                            CNC sets a predetermined flag in PLC for M-code M24. PLC
                                                                              resets “read in enable” in CNC to stop further execution of
      KEY LETTERS                   G CODES                 M CODES           part Program. PLC sends an output (signal 3) to clamp
                                                                              solenoid from the ladder logic. Pressure switch signal
 O            Program        G00          Rapid       M00     Programme
                                                                              comes as input to PLC. Once clamping operation is
              number                   transverse                stop         completed. PLC sets the “read in enable” in CNC and
                                                                              execution of part program is resumed.
 N           Sequence        G01         Linear       M01      Optional
              number                  interpolation           programme          N40 ..........
                                                                 stop
                                                                                N50 M08 sets a PLC flag. PLC resets “read in enable” in
 G          Preparatory      G02,        Circular     M02     Programme         CNC. PLC sends output to coolant solenoid. Once
             function        G03      interpolation              end            coolant flow starts flow switch sends input to PLC. PLC
                                       CW, CCW                                  sets “read in enable” (signal 10) in CNC and part program
                                                                                execution is resumed till M02.
X,Y,Z      X-axis, Y-axis,   G17,     XY, XZ,YZ       M03,     Spindle on
               Z-axis        G18,       Plane         M04         CW
            designation      G19

R,I,J        Circular        G20,     Inch, Metric    M08     Coolant on
            designation      G21         units

  F          Feed rate       G41,         Cutter      M09     Coolant off
                             G42      compensation
                                        left, right

  S        Spindle speed     G90        Absolute      M10      Clamps on
                                       Positioning

 M         Miscellaneous     G91      Incremental     M30     Programme
             function                 positioning             stop. Reset
                                                                to start
Advantages of CNC machines over Conventional machines:
    Increased flexibility
    Improved quality
    Higher productivity
    Economical batch production
    Reduced scrap generation
    High precision and efficiency


            5.   COMPUTER ADDED MANUFACTURING
In manual preparation of a CNC part programme, the
programmer is required to define the machine or the tool
movement in numerical terms. If the geometry is complicated
3D surfaces cannot be programmed manually.
With the development of the CAD/CAM system, interactive
graphic system is integrated with the CNC part programming.
The part programmer can create the geometrical model in
CAM package or directly extract the geometrical model from
CAD/CAM data base.
Flow Chart of a CAM system is below.




                         6. CONCLUSIONS
   This paper gives an overview of PLC and CNC controller
in machine automation. It mainly deals with CNC elements,
PLC-CNC interface, part programming and use of CAD/CAM
in machine automation. CNC machines are considered to be a
very important aspect in engineering design process. Its
application are very wide and it is versatile with advantages
over other conventional machining. CNC machines tool
features high efficiency, high speed and high accuracy.

                     AKNOWLEDGEMENT

I would like to acknowledge and
At BHEL, Haridwar for their continuous support and
motivation during the whole training period.

								
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