PLC Based Industrial Automation System

Transcription

1 PLC Based Industrial Automation System 1 Gavali Amit Bhimrao, 2 Patil Mahadev S. Department of Electronics and Telecommunication Rajarambapu Institute of Technology Rajaramnagar, Islampur, Sangli, India 1 amitgavali9011@gmail.com, 2 mahadev.patil@ritindia.edu Abstract - This paper describes how the present automation system comes in to existence through its various stages. In the past, automation is done through relays and contactor logics. Since the human intervention is more, the scope of errors was also more. But with the advent of microprocessors & microcontrollers several new tools as PLCs (Programmable Logic controllers) come in to use. These have reduced human intervention. This in turn has increased accuracy, precision and efficiency. Keywords Automation, PLC, PLC based Control Panel II. Programming logic controllers (PLC) I (a) RELAY SYSTEM: The relay, which operates much like a solenoid, is an electromagnetic device that consists of a coil with a soft iron core and electric contacts mounted on, but insulated from, the moving armature or plunger. The relay allows one circuit to control another. I. INTRODUCTION Automation, a Greek word means self dedicated. Automation is the key to modernization and has been conceptually understood as a way to increase efficiently and to improve productivity. The process of having machine follow a predetermined sequence of operation with little or no human labor, using special equipment and devices that perform and control manufacturing processes is known as automation. The goals of automation are Integration of various aspects of manufacturing operations to reduce labor cost. The fundamental constituents of any automated process are (1) a power source, (2) a feedback control mechanism, and (3) a programmable command as shown in figure 1. Figure 1: Fundamental constituents automated process Aautomation tools: I. Relays and contactor logic Figure 2: Relay mechanism Relay Operation: When current is supplied to the coil, the resulting magnetic field pulls the armature and the contacts toward the coil. The relay is in energized position when the armature is fully seated. Contacts that were open are now closed. These contacts are referred to as the normally open () contacts. Contacts that were closed are now open. These contacts are called the normally closed (NC) contacts. Therefore, the terms normally open and normally closed refer to the condition of the contacts when the relay coil is deenergized. A relay may have one or both types of contacts and may also have multiple sets of contacts for controlling several circuits at once. When the coil circuit is opened, current cannot flow in the coil, the magnetic field collapses, and the relay de-energizes. A spring attached to the armature returns the contacts to their normal positions. 33

2 Relays comes in different configurations are SPST (Single Pole Single Throw) is the simplest only two contacts, SPDT (Single Pole Double Throw) has three contacts. The contacts are usually labled common, normally open & normally closed. The normally closed contact will be connected to the common contact when no power is applied to the coil. The normally open contact will be open (i.e. not connected) when no power is applied to the coil. When the coil is energized the common is connected to the normally open contact and the normally closed contact is left floating. The double pole version are the same as the single pole version except there are two switches that open and close together. Select a relay with contact that can handle the voltage and current requirements of the load. Keep in mind that some loads (such as motor) draw more current when first turned on than they do not at steady state. Relay Application: The relay can be used to open and close high-voltage or high-current circuits with relatively little voltage and current in the coil circuit. The controlling circuit is electrically isolated from the controlled circuit. In other words, current from the controlling circuit does not flow in the controlled circuit. The link between the two circuits is strictly magnetic. The relay is also useful for remote be located at one point and other circuit components located some distance away. Because the controlling circuit is usually operated by a low-current power source, relatively small gage wires can be used to connect the remote switch device. Relay Ratings: The electrical contacts are rated for the voltage and current that they can safely switch. The relay is also rated for the voltage and current required to properly energize the coil. If a time delay is desired, thus value must also be specified. The value of these ratings will depend on how the relay is used in the system. Disadvantages of Relay System: The maintenance cost of the relay system is quite high, as the frequent maintenance is required in it. A driver circuit is required along with larger relays. Very high human intervention is required in this system. They do not allow manual adjustment of the relay state when the control power is off. These are only use for ON/OFF control. Intermediate controlling is not possible by the relays. They increased electrical noise when conducting. contact system is the current carrying part of the contactor. This includes Power Contacts, Auxiliary Contacts, and Contact Springs. The electromagnet system provides the driving force to close the contacts. The enclosure system is a frame housing the contact and the electromagnet. Enclosures are made of insulating materials like Bakelite, Nylon 6, and thermosetting plastics to protect and insulate the contacts and to provide some measure of protection against personnel touching the contacts. Open-frame contactors may have a further enclosure to protect against dust, oil, explosion hazards and weather. The major disadvantage of relays and controller logic is that it does not have any redundant system it means that if system fails then, it does not have any controlling system to continue its controlling and monitoring process. So if system fails then we have to rectify the control system so that controlling & monitoring could be restarted. II PROGRAMMING 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. A Programmable Logic Controller or Programmable Controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly. PLCs are used in many industries and machines. Unlike general- purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory. A PLC is an example of a hard real time system since output results must be produced in response to input conditions within a limited time. I (b) CONTACTOR LOGIC: A contactor is an electrically controlled switch (relay) used for switching a power circuit. A contactor is activated by a control input which is a lower voltage / current than that which the contactor is switching. Contactors come in many forms with varying capacities and features. Unlike a circuit breaker a contractor is not intended to interrupt a short circuit current. Figure 3: Block diagram of PLC Basic Features of Contactor Logic are as follows: A PLC has 4 main units: 1) Programmable Memory- The contactor is composed of three different systems. The instruction for logical control sequence is stored here. 2) 34

3 Data Memory- The status of the switches interlocks, past values of data and other working data is stored here. 3) O/P Devices- These are hardware or software drives for industrial process actuators, such as solenoid, switches, motor and valves. 4) I/P Devices- These are hardware or software driver for industrial process sensors such as switch status sensors, proximity detectors interlock setting. Working principal of PLC: Inputs Modules: Inputs come from sensors that physically translate into electrical signals. The simplest form of inputs are digital. In smaller PLCs the inputs are normally built in and are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8, 16, 32, 64, 96 inputs of the same type on each card. Scanning Principle of PLC: The PLC program is executed as part of a repetitive process referred to as a scan. A PLC scan starts with the CPU reading the status of inputs. The application program is executed using the status of the inputs. Once the program is completed, the CPU performs internal diagnostics and communication tasks. The scan cycle ends by updating the outputs, then starts over. The cycle time depends on the size of the program, the number of I/Os, and the amount of communication required. Figure 6: Scan cycle of PLC Figure 4: Example of input card Outputs Modules: Output modules rarely supply any power, but instead act as switches. External power supplies are connected to the output card and the card will switch the power on or off for each output. A common choice when purchasing output cards is relay transistors or triacs. Relays are the most flexible output devices. They are capable of switching both AC and DC outputs. But, they are slower, cost more, and they will wear out after millions of cycles. The most important consideration when selecting relays, or relay outputs on a PLC, is the rated current and voltage A PLC resolves the logic of a ladder diagram (program) rung by rung, from the top to the bottom. Usually, all the outputs are updated based on the status of the internal registers. Then the input states are checked and the corresponding input registers are updated. This process is run in a endless cycle. The time it takes to finish one cycle is called the scan time. In some controllers the idle state is eliminated. In this case, the scan time varies depends on the program length.scan cycle depends upon number of I/Os. More I/Os more scan time. Application decides the scan time of a PLC. Scan time in normal applications is 1 ms to 2 ms & in bigger applications 100 ms to 200 ms Advantages of using PLC s are as follows: Cost effective for controlling complex systems. Flexible and can be reapplied to control other systems quickly and easily. Computational abilities allow more sophisticated control. Trouble shooting aids make programming easier and reduce downtime. Reliable components make these likely to operate for years before failure. II. SWITCHED MODE POWER SUPPLY Figure 5: Example of output card A switched-mode power supply is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a source, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics. An SMPS is usually employed to efficiently provide a regulated output voltage, typically at a level different from the 35

4 input voltage. (Figure7. Shows DELTA SMPS supply 230 VDC, 5A) IV. RELAY BOARD III. Figure 7: Switched Mode Power Supply MINIATURE CIRCUIT BREAKER A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance.(figure.8 shows SEIMENS MCB 2P C4A, 1P C2A) Figure 9: Relay board A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. Relays were used extensively in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contractor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays". (Figure. 9 shows CONNECT WELL 8 channel, 5A, coil-24vdc) V. SOFTWARE FOR PLC Figure 8: Miniature Circuit Breaker All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must detect a fault condition, in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit, some mechanically-stored energy ( such as springs) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated. CoDeSys V2.3: CoDeSys is a complete development environment for Nexgenie messung PLC. CoDeSys stands for Controlled Development System. CoDeSys puts a simple approach to the powerful IEC language at the disposal of the PLC programmer. Use of the editors and debugging functions is based upon the proven development program environments of advanced programming languages. POU (Program Organization Unit) Functions, function blocks, and programs are POUs which can be supplemented by actions. Each POU consists of a declaration part and a body. The body is written in one of the IEC programming languages which include IL, ST, SFC, FBD, LD or CFC. CoDeSys supports all IEC standard POUs. POU can call up other POUs. However, recursions are not allowed. Function A function is a POU, which yields exactly one data element (which can consist of several elements, such as fields or structures) when it is processed, and whose call in textual language can occur as an operator in expressions. 36

5 VI. Figure 10: Example of Ladder diagram ELECTRICAL CONTROL PANEL In an automated system, a PLC controller is usually the central part of a process control system. With the execution of a program stored in program memory, PLC continuously monitors status of the system through signals from input devices. Based on the logic implemented in the program, PLC determines which actions need to be executed with output instruments. Most disadvantages of a classic control panel are too much work required in connecting wire, Difficulty with changes or replacements, Difficulty in finding errors; requiring skillful work force, When a problem occurs, hold-up time is indefinite, usually long. Figure 11: Example of conventional control panel (Contactor logic control panel) Systematic approach in designing a process control system: First, we need to select an instrument or a system that we wish to control. Function of a process control system is constantly watched by input devices (sensors) that give signals to a PLC controller. In response to this, PLC controller sends a signal to external output devices (operative instruments) that actually control how system functions in an assigned manner. Input devices are various switches, sensors and such. Output devices are solenoids, motors, relays as well as instruments for sound and light signalization. Third, we wrote ladder diagram for a program by sequence of operations that was determined in the first step. Finally, program uploaded into the PLC controller memory. When finished with programming, checkup is done for any existing errors in a program code (using functions for diagnostics) and, if possible, an entire operation is simulated. Before this system is started, we need to check once again whether all input and output instruments are connected to correct inputs or outputs. By bringing supply in, system starts working. Advantages of control panel that is based on a PLC controller can be presented in few basic points: Compared to conventional process control system, number of wires needed for connections is reduced. Consumption is greatly reduced because a PLC consumes less than a bunch of relays. Diagnostic functions of a PLC controller allow for fast and easy error detection. Change in operating sequence or application of a PLC controller to a different operating process can easily be accomplished by replacing a program through a console or using a PC software (not requiring changes in wiring, unless addition of some input or output device is required). It is cheaper compared to a conventional system, especially in cases where a large number of I/O instruments are needed and when operational functions are complex Reliability of a PLC is greater than that of an electro-mechanical relay or a timer (shown in figure 12) Material which is required for making PLC based control panel for PLC based fully automatic hydraulic press machine shown in figure 12. Specifications of PLC used are: Nexgenie NG16DL, Input power: 24VDC, 2.4W. Expansions: 4 maximum, Program software: IEC compatible programming software CoDeSys, Programming languages: Ladder language, Expansions modules (NE08IX): Nexgenie PLC expansion 8 i/p, 24VDC, Seimens MCB 2P C4A, MCB 1P C2A, MCB 2P C16A, Seimens 9A Contactor, Delta SMPS supply 230AC, output 24VDC 5A, Connect Well 8 ch. coil- 24VDC, connecting strips, 16 pin, 10 pin 16A malefemale socket. Figure 12: PLC based electrical control panel Secondly, we need to specified all input and output instruments that will be connected to a PLC controller. We are designed PLC operated fully automatic hydraulic press machine. For that sequence of operation is when 37

6 start Push Button is pressed the motor starts running. There are two modes of operation Auto and Manual mode Start Auto Mode: For Auto Mode you need press cycle start push button, initially both the valves will be in off state, when cycle start push button is pressed the solenoid valve 1 is made ON as soon as the valve turns ON the hydraulic ram moves forward to do its job now when the ram comes in contact with down sensor the valve 1 turns off and automatically Solenoid valve 2 turns ON hence making the Ram to move backwards to its home position, valve 2 turns off when the Ram comes in contact with Up sensor. (Flow chart shown in figure 14) SETUP MODE Motor start/ running push button Motor running Is temp >= temp SET Motor stop Manual (Setup) Mode: For the machine to run in Manual (setup) mode you need to press separate push buttons for two different operations. Firstly press the main ram forward push button the valve 1 turns ON, ram moves forward when the ram gets comes in contact with down sensor the valve 1 turns OFF, now release the main ram forward push button & press main ram return push button now the valve 2 turns ON hence making the ram move backwards to its home position, valve 2 turns off when the Ram comes in contact with Up sensor. Motor acts as universal element the modes of operation should only operate when the motor is running. The master stop push button is used to stop the motor at any instance of operation. The emergency main ram return is used to ON the Solenoid Valve 2 (i.e. main ram to move backwards) at any Instance of operation. Note: emergency main ram return works only for Auto Mode. (Flow chart shown in figure 15) Is solenoid valve 1 Is solenoid valve 2 RAM forward Is down sensor detected metal? RAM backward Is M.S Motor stop Stop RAM forward Is M.S Motor stop Is up sensor detected metal? Figure 13: PLC operated fully automatic hydraulic press machine Figure 13 shows example of PLC based industrial automation system (i.e. PLC based fully automatic hydraulic press machine) suitable for different types of heavy duty works in different commercial and industrial applications. This press machine can be used for cutting, punching, compression, modeling etc and can be used on the assembly lines of automotive industries. It also can be used in plastic industries for pressing application. Stop RAM backward Stop Figure 15: Flow chart for Setup (Manual) Mode 38

7 1 Solenoid valve 1 active C B Start Select AUTO MODE STATUS 1. Cycle start 2. Emg. main ram return 3. Master stop 2 Return to home position A Stop motor 3 VII. CONCLUSION A PLCs is specifically made to fit in industrial environment, where it is exposed to hostile condition such as heat,dust, humidity, unreliable power,mechanical shocks and vibration. PLCs have both hardware and software features that make them attractive as controllers of wide range of industrial equipments. A PLC can work in the harsh and extreme conditions of the industry where other systems such as a Micro-Controller cannot. Programming is a lot easier as compared to other embedded system which makes the job easy for the technicians. PLC s allow end user to configure and control the application which is more useful in industries. RAM forward Stop REFERENCES Is down sensor detected metal? B [1] Xiaoli Wang, Yu Wa, Control System Design for 100KN Double Column Universal hydraulic machine, Journal and applied Information Technology, vol. 48(1), 10 th Feb 2013 Stop RAM forward Solenoid valve 2 active 1 1.E.M.R.R. 2.M.S 3.N.B.P 3 2 [2] LV Yuan-jun, The control of paperboard pressing machine, Fifth IEEE International Conference on Intelligent Computation Technology and Automation, 2012 [3] M. Sumaila, A. Okonigbon, Design and Manufacture of a 30-ton hydraulic press, AU J.T. vol.14,no.3, pp , Jan 2011 C RAM backward B Return to home position C [4] Liping Guo, Design projects in a programmable logic controller (PLC) course in electrical engineering technology The technology interface journal vol.10 No.1, Fall 2009 Is up sensor detected metal? A Stop motor [5] Sanjiv Kumar, Devendra Kumar, Industrial Automation System, XXXII National System Conference, December 17-19,2008 Stop RAM backward [6] George K. Adam, Design and Control of a Mechatronic Hydraulic Press System, IEEE International Conference on Mechtronics, 2004 A Figure 14: Flow chart for Auto Mode 39