Numerical control for machines .tools were introduced in 1950’s by Prof.John T Parsons. The first NC machine was built at the Massachusetts institute of Technology in 1953 by joint efforts of US Air Force, the MIT and parson’s cooperation. NC is control by numbers .NC is control recorded information called part program, which is set of coded instructions given as numbers for automatic control of am machine in a pre-determined sequence. Numerical control can be defined as a technique of controlling a machine tool by the direct insertion of numerical data at some point of the system .
The functions that are controlled on the machine tool are displacement of the slide members, spindle speeds ,tool selection etc.At first ,the numerical control was used to produce geocentrically complex parts ,but later used for added efficiency in medium batch production of turned and milled parts presently, Numerical control is employed in all sectors of production . Rapid development in the field of electronics such as integrated circuit, large scale integrated circuits and development of minicomputer lead to the development of minicomputers based CNC systems. Further development and the electronic “chip” revolution have ushered in the current generation “compact and powerful” Microprocessor based CNC systems. Development of computer numerically controlled (CNC) machines is an outstanding contribution to the manufacturing industries. It has made possible the automation of the machining process with flexibility to handle small to medium batch of quantities in part production. Initially, the CNC technology was applied on basic metal cutting machine like lathes, milling machines, etc. Later, to increase the flexibility of the machines in handling a variety of components and to finish them in a single setup on the same machine, CNC machines capable of performing multiple operations were developed. To start with, this concept was applied to develop a CNC machining centre for machining prismatic components combining operations like milling, drilling, boring and taping. Further, the concept of multi-operations was also extended for machining cylindrical components, which led to the development of turning centers.
Computer Numerical Control (CNC) is a specialized and versatile form of Soft Automation and its applications cover many kinds, although it was initially developed to control the motion and operation of machine tools. Computer Numerical Control may be considered to be a means of operating a machine through the use of discrete numerical values fed into the machine, where the required ‘input’ technical information is stored on a kind of input media such as floppy disk, hard disk, CD ROM, DVD, USB flash drive, or RAM card etc. The machine follows a predetermined sequence of machining operations at the predetermined speeds necessary to produce a workpiece of the right shape and size and thus according to completely predictable results. A different product can be produced through reprogramming and a low-quantity production run of different products is justified. ADVANTAGE OF CNC MACHINES
Higher flexibility
Increased productivity
Consistent quality
Reduced scrap rate
Reliable operation
Reduced non productive time
Reduced manpower
Shorter cycle time
High accuracy
Reduced lead time
Just in time (JIT) manufacture
Automatic material handling
Lesser floor space
Increased operation safety
Machining of advanced material
The definition of CNC given by Electronic Industry Association (EIA) is as follows: “A system in which actions are controlled by the direct insertion of numerical data at some point. The system must automatically interpret at least some portion of this data.” In a simple word, a CNC system receives numerical data, interpret the data and then control the action accordingly.
CNC SYSTEMS
INTRODUCTION
Numerical control (NC) is a method employed for controlling the motions of a machine tool slide and its auxiliary functions with input in the form of numerical data. A computer numerical control (CNC) is a microprocessor-based system to store and process the data for the control of slide motions and auxiliary functions of the machine tools. The CNC system is the heart and brain of a CNC machine which enables the operation of various machine members such as slides, spindles, etc. as per the sequence programmed into it, depending on the machining operations. The main advantage of a CNC system lies in the fact that the skills of the operator hitherto required in the operation of a conventional machine is removed and the part production is made automatic. The CNC systems are constructed with a NC unit integrated with a programmable logic controller (PLC) and some times with an additional external PLC (non-integrated). The NC controls the spindle movement and the speeds and feeds in machining. It calculates the traversing path of the axes as defined by the inputs. The PLC controls the peripheral actuating elements of the machine such as solenoids, relay coils, etc. Working together, the NC and PLC enable the machine tool to operate automatically. Positioning and part accuracy depend on the CNC system’s computer control algorithms, the system resolution and the basic mechanical machine accuracy. Control algorithm may cause errors while computing, which will reflect during contouring, but they are very negligible. Though this does not cause point to point positioning error, but when mechanical machine inaccuracy is present, it will result in poorer part accuracy. Computer Numerical Control (CNC) is a specialized and versatile form of Soft Automation and its applications cover many kinds, although it was initially developed to control the motion and operation of machine tools. Computer Numerical Control may be considered to be a means of operating a machine through the use of discrete numerical values fed into the machine, where the required ‘input’ technical information is stored on a kind of input media such as floppy disk, hard disk, CD ROM, DVD, USB flash
drive, or RAM card etc. The machine follows a predetermined sequence of machining operations at the predetermined speeds necessary to produce a work piece of the right shape and size and thus according to completely predictable results. A different product can be produced through reprogramming and a low-quantity production run of different products is justified. Control Systems
Open Loop Systems :
Open loop systems have no access to the real time data about the performance of the system and therefore no immediate corrective action can be taken in case of system disturbance. This system is normally applied only to the case where the output is almost constant and predictable. Therefore, an open loop system is unlikely to be used to control machine tools since the cutting force and loading of a machine tool is never a constant. The only exception is the wirecut machine for which some machine tool builders still prefer to use an open loop system because there is virtually no cutting force in wirecut machining.
Block Diagram of an Open Loop System
Close Loop Systems:
In a close loop system, feedback devices closely monitor the output and any disturbance will be corrected in the first instance. Therefore high system accuracy is achievable. This system is more powerful than the open loop system and can be applied to the case where the output is subjected to frequent change. Nowadays, almost all CNC machines use this control system. Block Diagram of a Close Loop System CONFIGURATION OF THE CNC SYSTEM
Fig.1 shows a schematic diagram of the working principle of a NC axis of a CNC machine and the interface of a CNC control.
CNC system
Fig.1 Schematic diagram of a CNC machine tool
A CNC system consists of the following 6 major elements:
a.Input Device
b.Machine Control Unit
c.Machine Tool
d.Driving System
e.Feedback Devices
f.Display Unit
Input Devices
a. Floppy Disk Drive
Floppy disk is a small magnetic storage device for CNC data input. It has been the most common storage media up to the 1970s, in terms of data transfer speed, reliability, storage size, data handling and the ability to read and write. Furthermore, the data within a floppy could be easily edited at any point as long as you have the proper program to read it. However,this method has proven to be quite problematic in the long run as floppies have a tendency to degrade alarmingly fast and are sensitive to large magnetic fields and as well as the dust and scratches that usually existed on the shop floor.
b. USB Flash Drive
A USB flash drive is a removable and rewritable portable hard drive with compact size and bigger storage size than a floppy disk. Data stored inside the flash drive are impervious to dust and scratches that enable flash drives to transfer data from place to place. In recent years, all computers support USB flash drives to read and write data that make it become more and more popular in CNC machine control unit.
c. Serial communication
The data transfer between a computer and a CNC machine tool is often accomplished through a serial communication port. International
standards for serial communications are established so that information can be exchanged in an orderly way. The most common interface between computers and CNC machine tools is referred to the EIA Standard RS-232. Most of the personal computers and CNC machine tools have built in RS232 port and a standard RS-232 cable is used to connect a CNC machine to a computer which enables the data transfer in reliable way. Part programs can be downloaded into the memory of a machine tool or uploaded to the computer for temporary storage by running a communication program on the computer and setting up the machine control to interact with the communication software. Direct Numerical Control is referred to a system connecting a set of numerically controlled machines to a common memory for part program or machine program storage with provision for on-demand distribution of data to the machines.(ISO 2806:1980) The NC part program is downloaded a block or a section at a time into the controller. Once the downloaded section is executed, the section will be discarded to leave room for other sections. This method is commonly used for machine tools that do not have enough memory or storage buffer for large NC part programs. Distributed Numerical Control is a hierarchical system for distributing data between a production management computer and NC systems. (ISO 2806:1994) The host computer is linked with a number of CNC machines or computers connecting to the CNC machines for downloading part programs. The communication program in the host computer can utilize two-way data transfer features for production data communication including: production schedule, parts produced and machine utilization etc.
Serial communication in a Distributed Numerical Control system
Machine Control Unit (MCU)
The machine control unit is the heart of the CNC system. There are two sub-units in the machine control unit: the Data Processing Unit (DPU) and the Control Loop Unit (CLU). a. Data Processing Unit
On receiving a part programme, the DPU firstly interprets 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 lengthunit) which is the smallest unit length that can be handled by the controller. The calculated data are passed to CLU for further action. b. Control Loop Unit
The data from the DPU are converted into electrical signals in the CLU to control the driving system to perform the required motions. Other functions such as machine spindle ON/OFF, coolant ON/OFF, tool clamp ON/OFF are also controlled by this unit according to the internal machine codes. Machine Tool
This can be any type of machine tool or equipment. In order to obtain high accuracy and repeatability, the design and make of the machine slide and the driving lead screw of a CNC machine is of vital importance. The slides are usually machined to high accuracy and coated with anti-friction material such as PTFE and Turcite in order to reduce the stick and slip phenomenon. Large diameter recirculating ball screws are employed to eliminate the backlash and lost motion. Other design features such as rigid and heavy machine structure; short machine table overhang, quick change tooling system, etc also contribute to the high accuracy and high repeatability of CNC machines.
Ball Screw in a CNC machineBall screw structure
Driving System
The driving system is an important component of a CNC machine as the accuracy and repeatability depend very much on the characteristics and performance of the driving system. The requirement is that the driving system has to response accurately according to the programmed instructions. This 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
This is the most common type of feed motors used in CNC machines. The principle of operation is based on the rotation of an armature winding in a permanently energized magnetic field. The armature winding is connected to a commutator, which is a cylinder of insulated copper segments mounted on the shaft. DC current is passed to the commutator through carbon brushes, which are connected to the machine terminals. The change of the motor speed is by varying the armature voltage and the control of motor torque is achieved by controlling the motor’s armature current. In order to achieve the necessary dynamic behaviour it is operated in a closed loop system equipped with sensors to obtain the velocity and position feedback signals.
DC Servo Motor
b. AC Servo Motor
In an AC servomotor, the rotor is a permanent magnet while the stator is equipped with 3-phase windings. The speed of the rotor is equal to the rotational frequency of the magnetic field of the stator, which is regulated by the frequency converter. AC motors are gradually replacing DC servomotors. The main reason is that there is no commutator or brushes in AC servomotor so that maintenance is virtually not required. Furthermore, AC servos have a smaller power-to-weight ratio and faster response. AC Servo Motor
c. Stepping Motor
A stepping motor is a device that converts the electrical pulses into discrete mechanical rotational motions of the motor shaft. This is the simplest device that can be applied to CNC machines since it can convert digital data into actual mechanical displacement. It is not necessary to have any analog-to-digital converter nor feedback device for the control system. They are ideally suited to open loop systems. However, stepping motors are not commonly used in machine tools due to the following drawbacks: slow speed, low torque, low resolution and easy to slip in case of overload. Examples of stepping motor application are the magnetic
head of floppy-disc drive and hard disc drive of computer, daisy-wheel type printer, X-Y tape control, and CNC EDM Wire-cut machine.
Stepping Motor
Feedback Device
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 transducer for direct positional measurement and rotary encoder for angular or indirect linear measurement. Linear Transducers –
A linear transducer is a device mounted on the machine table to measure the actual displacement of the slide in such a way that backlash of screws; motors, etc would not cause any error in the feedback data. This device is considered to be of the highest accuracy and also more expensive in comparison with other measuring devices mounted on screws or motors.
Linear Transducer Rotary Encoders –
A rotary encoder is a device mounted at the end of the motor shaft or screw to measure the angular displacement. This device cannot measure linear displacement directly so that error may occur due to the backlash of screw and motor etc. Generally, this error can be compensated for by the machine builder in the machine calibration process.
Incremental and Absolute Rotary Encoder b. Velocity Feedback Device
The actual speed of the motor can be measured in terms of voltage generated from a tachometer mounted at the end of the motor shaft.DC tachometer is essentially a small generator that produces an output voltage proportional
to the speed. The voltage generated is compared with the command voltage corresponding to the desired speed. The difference of the voltages can is then used to actuate the motor to eliminate the error.
Tachogenerator
Display Unit
The Display Unit serves as an interactive device between the machine and the operator. When the machine is running, the Display Unit displays the present status such as the position of the machine slide, the spindle RPM, the feed rate, the part programmes, etc. In an advanced CNC machine, the Display Unit can show the graphics simulation of the tool path so that part programmes can be verified before the actually machining. Much other important information about the CNC system can also displayed for maintenance and installation work such as machine parameters, logic diagram of the programmer controller, error massages and diagnostic data.
Servo Drive
A servo drive consists of a servo amplifier and a servo motor. The main task of a servo amplifier (also called amplifier, servo controller, or just controller) is the control of the motor current. In addition, ESR servo amplifiers offer a broad spectrum of functionality While most of the electrical drives are operated at constant speed, a servo drive has a rather “hectic” life. Often it has to accelerate to the rated speed within a few milliseconds only to decelerate a short time later just as quick. And of course the target position is to be reached exactly with an error of a few hundredths of a milli -meter. Compared to other controlled drives servo drives have the advantage of high dynamics and accuracy, full stall torque, and compact motors with high power density. Servo drives are used where high dynamics (i. e. fast acceleration and deceleration) and good accuracy at reaching target positions are important. The good control behaviour allows the optimal adaptation to the application (e. g. positioning without overshoot). But also the smooth run (due to sinusoidal commutation) and the possibility of exact synchronisation of two or more drives open a wide
field. Because of their wide speed range servo drives can be used in a huge number of applications. Servo drives run in large, highly automated installations with several dozens of axes as well as in machines with only a few axes which perhaps operate independently. Servo motor
Servo motors are electric motors that are designed specially for high dynamics. Servo motors by ESR distinguish themselves by a compact design with high power density and a high degree of protection (up to IP 65). They come as AC servo motors (brush less) or DC servo motors (with brushes for the commutation). The high power density is achieved by permanent magnets made of neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo), or ferrite material. The servo motor is equipped with a position sensor which provides the controller with position and speed information. As a standard, the AC servo motors are equipped with resolvers. In combination with the digital servo amplifiers sincos encoders (absolute encoder, single-turn or multi-turn) and high-resolution incremental encoders may be used as well, in case higher accuracy or dynamics is required. The DC servo motors can be equipped with tacho generators and/or incremental encoders. For dimensioning the motor the following data are important: the mass of the parts to be moved, the cycle time of the application, and the friction torque. With these data the rated and peak torque (maximum acceleration or deceleration) and the rated speed can be calculated. If required, gears are used to match the moment of inertia of the motor to the moment of inertia of the application. Servo amplifier
The servo amplifier (also called amplifier, servo controller, or just controller) controls the current of the motor phases in order to supply the servo motor with exactly the current required for the desired torque and the desired speed. The essential parts of a servo amplifier are the power section and the control loops. The power section consists of a mains rectifier, a DC-bus, and a power circuit which supplies the individual motor phases with current. The control loops (analogue or digital) drive the power circuit and by constantly comparing setpoint with actual values ensure that the motor keeps exactly to the desired motions even under varying load.
Operator Control Panel
Fig.4 shows a typical Hinumerik 3100 CNC system’s operator control panel. The operator control panel provides the user interface to facilitate a two-way communication between the user, CNC system and the machine tool. This consists of two parts:
Video Display Unit (VDU)
Keyboard
Video Display Unit (VDU)
The VDU displays the status of the various parameters of the CNC system and the machine tool. It displays all current information such as: Complete information of the block currently being executed
Actual position value, set or actual difference, current feed rate, spindle speed Active G functions
Main program number, subroutine number
Display of all entered data, user programs, user data, machine data, etc. Alarm messages in plain text
Soft key designations
In addition to a CRT, a few LEDs are generally provided to indicate important operating modes and status. Video display units may be of two types: 1. Monochrome or black and white displays
2. Color displays
Operator’s and machine panel
Keyboard
A keyboard is provided for the following purposes:
Editing of part programs, tool data, and machine parameters. Selection of different pages for viewing.
Selection of operating modes, e.g. manual data input.
Selection of feed rate override and spindles speed override. Execution of part programs.
Execution of other toll functions.
Machine Control Panel (MCP)
It is the direct interface between operator and the NC system, enabling the operation of the machine through the CNC system. Fig.5 shows the MCP of Hinumerik 3100 system. During program execution, the CNC controls the axis motion, spindle function or tool function on a machine tool, depending upon the part program stored in the memory. Prior to the starting of the machine process, machine should first be prepared with some specific tasks like, Establishing a correct reference point
Loading the system memory with the required part program
Loading and checking of tool offsets, zero offsets, etc.
For these tasks, the system must be operated in specific operating mode so that these preparatory functions can be established.
Control elements of the machine control panel
Modes of operation
Generally, the CNC system can be operated in the following modes:
Manual mode
Manual data input (MDI) mode
Automatic mode
Reference mode
Input mode
Output mode, etc.
Manual mode:
In this mode, movement of a machine slide can carried out manually by pressing the particular jog button (+ or -). The slide (axis) is selected through an axis selector switch or through individual switches (e.g., X+, X-, Y+, Y-, Z+, Z-, etc.). The feed rate of the slide movement is prefixed. CNC system allows the axis to be jogged at high feed rate also. The axis movement can also be achieved manually using a hand wheel interface instead of jog buttons. In this mode slides can be moved in two ways: Continuous
Incremental
Continuous mode: In This mode, the slide will move as long as the jog button is pressed. Incremental mode: Hence the slide will move through a fixed distance, which is selectable. Normally, system allows jogging of axes in 1, 10, 100, 1000, 10000, increments. Axis movement is at a prefixed feed rate. It is initiated by pressing the proper jog+ or jog- key and will be limited to the no of increments selected even if the jog button is continuously pressed. For subsequent movement the jog button has to be released and once again pressed. Manual Data Input (MDI) Mode
In this mode the following operation can be performed:
Building a new part program
Editing or deleting of part program stored in the system memory Entering or editing or deleting of:
—— Tool offsets (TO)
—— Zero offsets (ZO)
—— Test data, etc.
Teach-in
Some system allows direct manual input of a program block and execution of the same. The blocks thus executed can be checked for correctness of dimensions and consequently transferred into the program memory as part program. Playback
In setting up modes like jog or incremental, the axis can be traversed either through the direction keys or via the hand wheel, and the end position can be transferred into the system memory as command values. But the required feed rates, switching functions and other auxiliary functions have to be added to the part program in program editing mode. Thus, teach-in and playback operating method allows a program to created during the first component prove out.
Automatic Mode (Auto and Single Block)
In this mode the system allows the execution of a part program continuously. The part program is executed block by block. While one block is being executed, the next block is read by the system, analyzed and kept ready for
execution. Execution of the program can be one block after another automatically or the system will execute a block, stop the execution of the next block till it is initiated to do so (by pressing the start button). Selection of part program execution continuously (Auto) or one block at a time (Single Block) is done through the machine control panel. Many systems allow blocks (single or multiple) to be retraced in the opposite direction. Block retrace is allowed only when a cycle stop state is established. Part program execution can resume and its execution begins with the retraced block. This is useful for tool inspection or in case of tool breakage. Program start can be effected at any block in the program, through the BLOCK SEARCH facility.
Reference Mode
Under this mode the machine can be referenced to its home position so that all the compensations (e.g., pitch error compensation) can be properly applied. Part programs are generally prepared in absolute mode with respect to machine zero. Many CNC systems make it compulsory to reference the slides of the machine to their home positions before a program is executed while others make it optional. Input Mode and Output Mode (I/O Mode)
In this mode, the part programs, machine setup data, tool offsets, etc. can be loaded/unloaded into/from the memory of the system from external devices like programming units, magnetic cassettes or floppy discs, etc. During data input, some systems check for simple errors (like parity, tape format, block length, unknown characters, program already present in the memory, etc.). Transfer of data is done through a RS232C or RS422C port. Other Peripherals
These include sensor interface, provision for communication equipment, programming units, printer, tape reader/puncher interface, etc
INTERFACING
Interconnecting the individual elements of both the machine and the CNC system using cables and connectors is called interfacing. Extreme care should be taken during interfacing. Proper grounding in electrical
installation is most essential. This reduces the effects of interference and guards against electronic shock to personnel. It is also essential to properly protect the electronic equipment. Cable wires of sufficiently large cross-sectional area must be used. Even though proper grounding reduces the effect of electrical interference, signal cable requires additional protection. This is generally achieved by using shielded cables. All the cable shields must be grounded at control only, leaving other end free. Other noise reduction techniques include using suppression devices, proper cable separation, ferrous metal wire ways, etc. Electrical enclosures should be designed to provide proper ambient conditions for the controller. MONITORING
In addition to the care taken by the machine tool builder during design and interfacing, basic control also includes constantly active monitoring functions. This is in order to identify faults in the NC, the interface control and the machine at an large stage to prevent damages occurring to the work piece, tool or machine. If a fault occurs, first the machining sequence is interrupted, the drives are stopped, the cause of the fault is stored and then displayed as an alarm. At the same time, the PLC is informed that an NC alarm exits. In Hinumerik CNC system, for example, the following can be monitored: Read-in
Format
Measuring circuit cables
Position encoders and drives
Contour
Spindle speed
Enable signals
Voltage
Temperature
Microprocessors
Data transfer between operator control panel and logic unit
Transfer between NC and PLC
Change of status of buffer battery
System program memory
User program memory
Serial interfaces
DIAGNOSTICS
The control will generally be provided with test assistance for service purposes in order to display some status on the CRT such as: Interface signals between NC and PLC as well as between PLC and machine Flags of the PLC
Timers of the PLC
Counters of the PLC
Input/output of the PLC
For the output signals, it is also possible to set and generate signal combinations for test purposes in order to observe how the machine react to a changed signal. This simplifies trouble shooting considerably. MACHINE DATA
Generally, a CNC system is designed as a general-purpose control unit, which has to be matched with the particular machine to which the system is interfaced. The CNC is interfaced to the machine by means of data, which is machine specific. The NC and PLC machine data can be entered and changed by means of external equipment or manually by the keyboard. These data are fixed and entered during commissioning of the machine and generally left unaltered during machine operations. Machine data entered is usually relevant to the axis travel limits, feed rates, rapid traverse speeds and spindle speeds, position control multiplication factor, Kv factor, acceleration, drift compensation, adjustment of reference point, backlash compensation, pitch error compensation, etc. Also the optional features of the control system are made available to the machine tool builder by enabling some of the bits of machine data. Applications of CNC Machines CNC machines are widely used in the metal cutting industry and are best used to produce the following types of product: • Parts with complicated contours
• Parts requiring close tolerance and/or good repeatability • Parts requiring expensive jigs and fixtures if produced on
conventional machines • Parts that may have several engineering changes, such as during the development stage of a prototype
• In cases where human errors could be extremely costly • Parts that are needed in a hurry
• Small batch lots or short production runs
Some common types of CNC machines and instruments used in industry are as following: • Drilling Machine
• Lathe / Turning Centre
• Milling / Machining Centre
• Turret Press and Punching Machine
• Wirecut Electro Discharge Machine (EDM)
• Grinding Machine
• Laser Cutting Machine
• Water Jet Cutting Machine
• Electro Discharge Machine
• Coordinate Measuring Machine
• Industrial Robot
PLC PROGRAMMING
Programmable Logic Controller (PLC)
A PLC matches the NC to the machine. PLCs were basically introduced as replacement for hard wired relay control panels. They were developed to be reprogrammed without hardware changes when requirements were altered and thus are reusable. PLCs are now available with increased functions, more memory and large input/output capabilities. Fig.7 gives the generalized PLC block diagram. In the CPU, all the decisions are made relative to controlling a machine or a process. The CPU receives input data, performs logical decisions based upon stored programs and drives the outputs. Connections to a computer for hierarchical control are done via the CPU. The I/O structure of the PLCs is one of their major strengths. The inputs can be push buttons, limit switches, relay contacts, analog sensor, selector switches, proximity switches, float switches, etc. The outputs can be motor starters, solenoid valves, position valves, relay coils, indicator lights, LED displays, etc. The field devices are typically selected, supplied and
installed by the machine tool builder or the end user. The voltage level of the field devices thus normally determines the type of I/O. So, power to actuate these devices must also be supplied external to the PLC. The PLC power supply is designated and rated only to operate the internal portions of the I/O structures, and not the field devices. A wide variety of voltages, current capacities and types of I/O modules are available.The principle of operation of a PLC is determined essentially by the PLC program memory, processor, inputs and outputs. The program that determines PLC operation is stored in the internal PLC program memory. The PLC operates cyclically, i.e. when a complete program has been scanned, it starts again at the beginning of the program. At the beginning of each cycle, the processor examines the signal status at all inputs as well as the external timers and counters and are stored in a process image input (PII). During subsequent program scanning, the processor the accesses this process image. To execute the program, the processor fetches one statement after another from the programming memory and executes it. The results are constantly stored in the process image output (PIO) during the cycle. At the end of a scanning cycle, i.e. program completion, the processor transfers the contents of the process image output to the output modules and to the external timers and counters. The processor then begins a new program scan.
Fig.6 System with peripheral devices
Fig.7 Generalized PLC block diagram
What does ‘PLC’ mean?
A PLC (Programmable Logic Controllers) is an industrial computer used to monitor inputs, and depending upon their state make decisions based on its program or logic, to control (turn on/off) its outputs to automate a machine or a process. PROGRAMMABLE LOGIC CONTROLLER
“A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions such as logic sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or
processes”. Traditional PLC Applications
*In automated system, PLC controller is usually the central part of a process control system. *To run more complex processes it is possible to connect more PLC controllers to a central computer. Disadvantages of PLC control
– Too much work required in connecting wires.
– Difficulty with changes or replacements.
– Difficulty in finding errors; requiring skillful work force. – When a problem occurs, hold-up time is indefinite, usually long. Advantages of PLC control
* Rugged and designed to withstand vibrations, temperature, humidity, and noise. * Have interfacing for inputs and outputs already inside the controller. * Easily programmed and have an easily understood programming language. Major Types of Industrial Control Systems
Industrial control system or ICS comprise of different types of control systems that are currently in operation in various industries. These control systems include PLC, SCADA and DCS and various others: PLC
They are based on the Boolean logic operations whereas some models use timers and some have continuous control. These devices are computer based and are used to control various process and equipments within a facility. PLCs control the components in the DCS and SCADA systems but they are primary components in smaller control configurations. DCS
Distributed Control Systems consists of decentralized elements and all the processes are controlled by these elements. Human interaction is minimized so the labor costs and injuries can be reduced.
Embedded Control
In this control system, small components are attached to the industrial computer system with the help of a network and control is exercised. SCADA
Supervisory Control And Data Acquisition refers to a centralized system and this system is composed of various subsystems like Remote Telemetry Units,
Human Machine Interface, Programmable Logic Controller or PLC and Communications. Hardware Components of a PLC System
Processor unit (CPU), Memory, Input/Output, Power supply unit, Programming device, and other devices.
Central Processing Unit (CPU)
CPU – Microprocessor based, may allow arithmetic operations, logic operators, block memory moves, computer interface, local area network, functions, etc. CPU makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early. System Busses
The internal paths along which the digital signals flow within the PLC are called busses.
The system has four busses:
– The CPU uses the data bus for sending data between the different elements, – The address bus to send the addresses of locations for accessing stored data, – The control bus for signals relating to internal control actions, – The system bus is used for communications between the I/O ports and the I/O unit. Memory
System (ROM) to give permanent storage for the operating system and the fixed data used by the CPU. RAM for data. This is where information is stored on the status of input and output devices and the values of timers and counters and other internal devices. EPROM for ROM’s that can be programmed and then the program made permanent. I/O Sections
Inputs monitor field devices, such as switches and sensors.
Outputs control other devices, such as motors, pumps, solenoid valves, and lights. Power Supply
Most PLC controllers work either at 24 VDC or 220 VAC. Some PLC controllers have electrical supply as a separate module, while small and medium series already contain the supply module. Programming Device
The programming device is used to enter the required program into the memory
of the processor. The program is developed in the programming device and then transferred to the memory unit of the PLC. PLC OPERATIONS:
Input Relays
These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors. Internal Utility Relays
These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task.
Counters
These do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed. Some manufacturers also include highspeed counters that are hardware based. Timers
These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type.
Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s. Output Relays
These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen. Data Storage
Typically there are registers assigned to simply store data. Usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC.
The Simatic S5 PLC is an automation system based on PLC. It was manufactured and sold by Siemens. Such automation systems control process equipment and machinery used in manufacturing. STEP 5 programming language is used for
writing user programs for SIMATIC S5 programmable controllers. The program can be written and entered into the programmable controller as in: Statement list (STL), Fig.12 (a)
Control system flowchart (CSF), Fig.12 (b)
Ladder diagram (LAD), Fig.12 (c)
(a)
Fig.12 Programmable controller
The statement list describes the automation task by means of mnemonic function designations. The control system flowchart is a graphic representation of the automation task. The ladder diagram uses relay ladder logic symbols to represent the automation task. The statement is the smallest STEP 5 program component. It consists of the following: Operation, i.e. what is to be done?
E.g.A = AND operation (series connection)
O= OR operation (parallel connection)
S= SET operation (actuation)
Operand, i.e. what is to be done with?
E.g.I 4.5, i.e. with the signal of input 4.5
The operand consists of:
Operand identifier (I = input, Q = output, F = flag, etc.)
Parameter, i.e. the number of operand identifiers addressed by the statement. For inputs, outputs and flags (internal relay equivalents), the parameter consists of the byte and bit addresses, and for timers and counter, byte address only. The statement may include absolute operands, e.g. I 5.1, or symbolic operand, e.g. I LS1. Programming is considerably simplified in the later case as the actual plant designation is directly used to describe the device connected to the input or output. Typically, a statement takes up one
word (two bytes) in the program memory. STRUCTURED PROGRAMMING
The user program can be made more manageable and straightforward if it is broken down into relative sections. Various software block types are available for constructing the user program. Program blocks (PB) contain the user program broken down into technologically or functionally related sections (e.g. program block for transportation, monitoring, etc.). Further blocks, such as program blocks or function blocks can be called from a PB. Organization blocks (OB) contain block calls determining the sequence in which the PBs are to be processed. It is therefore possible to call PBs conditionally (depending on certain conditions). In addition, special OBs can be programmed by the user to react to interruptions during cyclic programming processing. Such an interrupt can be triggered by a monitoring function if one or several monitored events occur. Function block (FB) is block with programs for recurrent and usually complex function. In addition to the basic operations, the user has a extended operation at his disposal for developing function blocks. The program in a function block is usually not written with absolute operands (e.g. I 1.5) but with symbolic operands. This enables a function block to be used several times over with different absolute operands. For even more complex functions, standard function blocks are available from a program library. Such FBs are available, e.g. for individual controls, sequence controls, messages, arithmetic operations, two step control loops, operator communications, listing, etc. These standard FBs for complex functions can be linked it the user program just like user written FBs simply by means of a call along with the relevant parameters. The Sequence block (SB) contain the step enabling conditions, monitoring times and conditions for the current step in sequence cascade. Sequence blocks are employed, for example, to organise the sequence cascade in communication with a standard FB. The data blocks (DB) contain all fixed or variable data of the user program. CYCLIC PROGRAM PROCESSING
The blocks of the user program are executed in the sequence in which they specified in the organisation block. INTERRUPT DRIVEN PROGRAM PROCESSING
When certain input signal changes occur, cyclic processing is interrupted at the next block boundary and an OB assigned to this event is started. The
user can formulate his response program to this interrupt in the OB. The cyclic program execution is the resumed from the point at which it was interrupted. TIME CONTROLLED PROGRAM EXECUTION
Certain Obs are executed at the predetermined time intervals (e.g. every 100ms, 200ms, 500ms, 1s, 2s, and 5s). For this purpose, cyclic program execution is interrupted at the block boundary and resumed again at this point, once the relevant OB has been executed. Fig.13 gives the organisation and execution of a structured user program.
Fig.13 Organisation and execution of a structured user program EXAMPLES OF PLC PROGRAM
Before attempting to write a PLC program, first go through the instruction set of the particular language used for the equipment, and understand the meaning of each instruction. Then study how to use these instructions in the program (through illustration examples given in the manual). Once the familiarization task is over, then start writing the program. Follow the following steps to write a PLC program.
List down each individual element (field device) on the machine as Input/Output. Indicate against each element the respective address as identifier during electrical interfacing of these elements with the PLC. Break down the complete machine auxiliary functions that are controlled by the PLC into individual, self contained functions. Identify each individual function as separate block (PBxx/FBxx) Once the PBs and FBs for each function are identified, take them one by one for writing the program. List down the preconditions required for the particular function separately. Note down the address of the listed elements.
Write down the flow chart for the function.
Translate the flow chart into PLC program using the instructions already familiarized. Complete the program translation of all individual functions in similar lines. Check the individual blocks independently and correct the program to get the required results. Organize all the program blocks in the organization block depending upon the sequence in which they are supposed to
be executed as per the main machine function flow chart. Check the complete program with all the blocks incorporated in the final program.
Example 1: Spindle ON
PreconditionsFeedback elementsAddressFault indicationAddressRemark
Tool clampPressure switchI 2.4LampQ 2.1
Job clampProximity switchI 3.2LampQ 1.7
Door closeLimit switchI 5.7LampQ 4.0
Lubrication ONPLC output bitQ 1.0LampQ 7.7
Drive readyInput signal from I 4.6LampQ 0.4
Drive unit
PB 12 written is the individual function module for spindle ON for all the preconditions checked and found satisfactory. This function is required to be executed only when the spindle rotation is requested by the NC in the form of a block in the part program. Whenever NC decodes the part program block, it in turn informs the PLC through a fixed buffer location that spindle rotation is requested. Say Flag bit F 100.0 is identified for this information communication. With this data, spindle ON function module can be recalled in the organisation block OB1 as follows. OB 1
……
A F 100.0
JC PB12
……
BE
Now, spindle ON function module PB12 will be executed only when F 100.0 is set. Otherwise the function execution will be bypassed.
FLOW CHART
