DMT 3533 Programmable Logic Controller PLC PLC System
DMT 353/3 – Programmable Logic Controller (PLC) PLC System Mohd Khairul Md Kamil School of Microelectronic Engineering
PLC vs. PC PLC PC Environment designed for harsh conditions with electrical noise, magnetic fields, vibration, extreme temperatures or humidity not designed for harsh environments. Industrial PCs are available but cost more Ease of Use friendlier to technicians since they are in ladder logic and have easy connections. Operating systems like Windows are common. Connecting I/O to the PC is not always as easy Flexibility in rack form are easy to exchange and add parts. They are designed for modularity and expansion Typical PCs are limited by the number of cards they can accommodate and are not easily expandable. Speed execute a single program in sequential order. by design, are meant to handle The have better ability to handle events in simultaneous tasks. They have difficulty real time handling real time events Reliability never crashes over long periods of time. ("Never" may not be the right word but its close enough to be true. ) A PC locking up and crashing is frequent Programming languages Languages are typically fixed to ladder logic, function block or structured text A PC is very flexible and powerful in what to use for programming Data management Memory is limited in its ability to store a lot of data. This is where the PC excels because of it's hard drive. Any long term data storage, history and trending is best done on a PC.
PLC Hardware • PLC hardware falls into two physical configurations: 1) Fixed- has all of its components (input section, CPU, memory, power supply, output section) built into one self-contained unit, and are not removable. 2) Modular- comes as separate pieces of power supplies, CPU, input and output cards or I/O modules. * flexibility
Fixed I/O PLCs
Modular I/O PLCs
PLC System
PLC System • A PLC system consists of a Central Processing Unit (CPU) containing an application program and Input and Output Interface modules, which is directly connected to the field I/O devices. • The program controls the PLC so that when an input signal from an input device turns ON, the appropriate response is made. The response normally involves turning ON an output signal to some sort of output devices. • The Central Processing Unit (CPU) is a microprocessor that co-ordinates the activities of the PLC system. It executes the program, processes I/O signals & communicates with external devices.
PLC System • There are various types of memory unit. It is the area that holds the operating system and user memory. • The operating system is actually a system software that co-ordinates the PLC Ladder program. • Timer and Counter values are stored in the user memory. • Depending on user’s need, various types of memory are available for choice: Ø ROM Ø RAM Ø EPROM Ø EEPROM
PLC System (a) Read —Only Memory (ROM) ROM is a non-volatile memory that can be programmed only once. It is therefore unsuitable. It is least popular as compared with others memory type. (b) Random Access Memory (RAM) RAM is commonly used memory type for storing the user program and data. The data in the volatile RAM would normally be lost if the power source is removed. However, backing up the RAM with a battery solves this problem.
PLC System (c) Erasable Programmable Read Only Memory (EPROM) EPROM holds data permanently just like ROM. It dose not require battery backup. However, exposing it to ultraviolet light can erase its content. A prom writer is required to reprogram the memory. (d) Electrically Erasable Programmable Read Only Memory (EEPROM) EEPROM combines the access flexibility of RAM and the non-volatility of EPROM in one. Its contents can be erased and reprogrammed electrically, however, to a limit number of times.
Input Devices • Intelligence of an automated system is greatly depending on the ability of a PLC to read in the signal from various types of automatic sensing and manual input field devices. • Push buttons, keypad and toggle switches, which form the basic man-machine interface, are manual input device. • On the other hand, for detection of work piece, monitoring of moving mechanism, checking on pressure and or liquid level and many others, the PLC will have to tap the signal from the specific automatic sensing devices like proximity switch, limit switch, photoelectric sensor, level sensor, etc. . • Types of input signal to the PLC would be of ON/OFF logic or analogue. These input signals are interfaced to PLC through various types of PLC input module.
Output Devices • An automatic system is incomplete and the PLC system is virtually paralyzed without means of interface to the field output devices. • Most commonly controlled devices are motors, solenoids, relays indicators, buzzers and etc. • Through activation of motors and solenoids the PLC can control from a simple pick and place system to a much complex servo positioning system. • These type of output devices are the mechanism of an automated system and so its direct effect on the system performance.
Output Devices • However, other output devices such as the pilot lamp, buzzers and alarms are merely meant for notifying purpose. Like input signal interfacing, signal from output devices are interfaced to the PLC through the wide range of PLC output module.
Programming Devices • The programming device is used to input the desired instructions. These instructions determine what the PLC will do for a specific input • The PLC can be programmed by; 1) Personal Computer – (PC programming device) 2) Programming Console – (Hand held programming device)
PC Programming Device • Manufacturers software required. • Large amounts of logic can be displayed. • Circuit elements can be highlighted in color to indicate status. • More than one program can be stored on the computers hard drive • The computer can be used to document the program. • PC software provides cut-and-paste features for program developing and editing. • Allows easy monitoring of data tables. • Easy to make copies of the program on floppy disk, CD-ROM, or hard drive
Manufacturer Programming software Mitsubishi MELSOFT Siemens SIMATIC Rockwell Automation RSLogix OMRON Telemecanique CX-Programmer Pro. Worx 32
Hand Held Programming Device • Has a connecting cable so that it can be plugged into a PLC programming port. • Are compact, inexpensive, and easy to use. • Contains keys for instruction entering and editing, and navigation keys for moving around the program
PLC Operation • A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps CHECK INPUT STATUS EXECUTE PROGRAM UPDATE OUTPUT STATUS
PLC Operation • Step 1 - CHECK INPUT STATUS -First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third. . . It records this data into its memory to be used during the next step. • Step 2 - EXECUTE PROGRAM -Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.
PLC Operation • Step 3 - UPDATE OUTPUT STATUS -Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true. • After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above.
Q&A
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