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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