Plc notes
- 1. Programmable Logic Controller INTRODUCTION PLC stands for Programmable Logic Controllers. A PLC is basically a microprocessor based devicethat is used for controlling any machine( electrical, mechanical, electronic).It is also used in assembly lines controlling in the industries. It is similar to a computer. It is typically based on RISC architecture. It is programmed in specific languages based on the real time purpose. It is connected to sensors, actuators, relays, contactors, etc. it is characterized by the number and type of I/O ports they provide and by their I/O scan rate The National Electrical Manufacturers Association (NEMA) defines a PLC as 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 I/O modules various types of machines or processes. A PLC is able to receive (input) and transmit (output) various types of electrical and electronic signals and can control and monitor practically any kind of mechanical and/or electrical system. Therefore, it has enormous flexibility in interfacing with computers, machines, and many other peripheral systems or devices. Control is the process in a system in which one or several input variables influence other variables. Need for PLC Hardwired panels were very time consuming to time, debug and change The following requirements for computer controllers to replace hardwired panels: Solid state not mechanical Easy to modify input and output devices Easily programmed and maintained by plant electricians Be able to function in an industrial environment
- 2. Comparison Hardwired control systems PLC systems The functions are determined by physical wiring The functons are determined by a program stored in the memory Changing the function means changing the wiring The control fucntions can be changed by simply changing the program Can be contact making type(relays, contactors) or electronic type (logic circuits) Consists of a control device, to which all the sensors and actuators are connected PLC architecture The basic architecture of the PLC can be described as below:
- 3. The basic components of a PLCare: 1. A Central Processing Unit(CPU) 2. Input module 3. Output module 4. Programming device 5. Powersupply Central ProcessingUnit: It is the heart of the PLCsystem. The CPU is a microprocessor based control system that replaces central relays, counters, timers and sequencers. One bit processors are adequate for dealing with logic operations. The operations of a CPU are as follows: The CPU acceptsdata from various sensing devices, executes the user program from memory and sends appropriate output commands to control devices. A DC powersource is required to produce a low-levelvoltageused by processor and I/O modules. This power supply can be housed in the CPU or may be a separately mounted unit, depending on the PLC system manufacturer. Input and Output Modules: First lets see what are the inputs and outputs. Inputs: Inputs are basically the physicalentities that control the on and off of the machine. These devices are controlledby the either the machine operator manually or automatically sent after starting the machine. Some off the devices can be listed down as: Push buttons Limiter switches Pressure switches Flow switches Proxy sensor
- 4. Emergency switch Outputs: Outputs are the devices which receivethe signals given by the PLC and perform the actions accordingly. The output devices can be listed down as: Relays Contactors Solenoid valves Lamp indicators. Input-output module: The I/O modules connectthe input deviceswith the controller.They convertthe electricalsignals used in the devices into electronic signals that can be used by the control system and translate the real worldvalues to IOtable values. Blockdiagram of input module: Blockdiagram of output module:
- 5. Circuit diagram of isolator circuit The number of I/Odevices used within a controlsystem is called its “point count”. Programmingdevice(keyboardandmonitor): Keyboard and monitor is used forprogramming the PLC. The data is entered in the processor with the help of the keyboard in the formof ladder diagram. This ladder diagram can be seen on the monitor screen. The programmer can communicate withthe processor with the help of the programming devices. The programming unit communicates with the processor of the PLC viaa serial or parallel data communication link. PLC powersupply: The PLC is power by the AC mains supply. Howeversome of the PLC components utilize power of about only 5V dc. PLC power supply converts the AC powersupply into DC and supports thesse components. The overall blockdiagram of the PLC:
- 6. The input signals are given to the Data Acquisition System (DAS) and then further sent to the input module. Further the signals are sent to the processor. The processor is connectedwith the programming devices. The signal from the processor is then sent to the output module and then from the output module further to the output devices. PLC operating sequence Self test: testing of its own hardware and software for faults. Input scan: if there are no problems, PLC will copy all the inputs and copy their values into memory. Logic solve/scan: using inputs, the ladder logic program is solved once and outputs are updated. Output scan: while solving logic, the output values are updated only in memory when ladder scan is done, the outputs will be updated using temporary values in memory.
- 7. Number systems and codes The integral operation of programmable controllers is largely based on numbers. Numbers emulated by electronic circuitry are used to encode and store information that in turn se allows these devices to process instructions and data required to perform each and every operation. PLC also relies on number systems to perform even the most basic operations and store various information. The number systems commonly used in PLC are given below: In PLC, decimal number system is usually used ofr timer and counter presets, math operations and general numerical quantities. Binary number system, bytes and words are the common units for storing digital information in memory. They are used to represent letters, numbers, punctuation marks, machine instructions and virtually any information type. In PLC in many cases, base 8 is used for addressing memory and input/output terminal locations. Using octal for numbering is convenient in these cases since memory is comprised of groupings of 8 binary digits(bytes) and I/O modules are normally in one or more groups of eight points per module. Hex numbers easily express coded digital data which is otherwise expressed in binary. Such codes are typically used when devices communicate. While the binary data may be easily
- 8. interpreted by the receiving device, hex coded characters are really used for convenience of human operators. Automation: Automation is basically the delegation of human control functions to technical equipment aimed towards achieving higher productivity, superior quality of end product, efficient usage of energy and raw materials, improved safety in working conditons etc. Advantages of PLC in Automation: To reduce human efforts To get maximum efficiency from machine and control them with human logic To reduce complex circuitry of entire system To eliminate the high costs associated with inflexible, relay controlled systems. Areas of application of PLC in automation: Manufacturing/machining Food/beverage Textile industry Travel industry Aerospace Printing industry Programming of PLC: The PLC can be programmed in different ways. There are various methods for entering and interconnecting various logic elements. Some of these can be explained as follows: 1. Entry of ladder logic diagram 2. Low level computer languages 3. High level computer languages 4. Functional blocks 5. Sequential function chart ProgrammingDevicesare: Programming Console, Hand Programmer, PC
- 9. Ladder Logic Diagram: Among these, ladder logic diagram is most frequently used as whenever a personnel wants to change the PLC, he does not have to learn an entirely new programming language. Only the knowledge of the circuitdiagram is enough. This method includes the direct entry of the logic diagram into the PLC memory. This method requires the use of a keyboard and a display screen with graphics capability to display the symbols of the components and their inter relationships in the ladder logic diagram. Programming is accomplished by inserting appropriate components in the rungs of the ladder diagram. Ladder logic diagram uses graphic symbols similar to relay schematic circuitdiagrams. It consists of twovertical lines representing the powerrails Circuits are connected as horizontal lines between these twovertical lines. Such horizontal lines are called as Rungs. Eachrung contains atleast one input and one output. A particular input or output can appear in more than one rung of ladder. Output is connectedat right side and input is connectedand input is connected at left side (foreach rung) Output (whichis on right side) cannot be directly connected withleft side The ladder logic diagram consists of twotypes of components: o Contacts o Coils Contacts are used to represent loads such as motors, relays, solenoids, timers, counters, etc. The program is entered rung by rung in the logic diagram. Ladder logic diagrams are based on the ON-OFFoperation. They are entirely based on the logic level 1 and logic level 0. This can be a disadvantage in using the ladder logic programming. The various devices that are based on the binary logic can be stated as follows: DEVICE ONE/ZERO INPUT Limit switch Contact/no contact Photodetector Contact/no contact Pushbutton switch On/off Timer On/off Control relay Contact/no contact Circuit breaker Contact/no contact OUTPUT Motor On/off Alarm buzzer On/off Control relay Contact/no contact Lights On/off
- 10. Valves Closed/open Clutch Engaged/not engaged Solenoid Energized/not energized The ladder diagrams are based on three logic controls. 1. AND 2. OR 3. NOT Logic AND operation: Logic OR operation: Logic NOToperation: Ladder logic symbols:
- 11. Implementing the switching operations using ladder symbol: A simple example of a ladder logic diagram: Consider a start delta motor starter: In this circuitthe motor is supposed to run on star configuration for some time after starting and then automatically the connections are converted into delta configuration. The power circuitfor start delta starter can be given as follows:
- 12. This diagram can be converted into PLC ladder logic as follows: Memory System:
- 13. All PLCs contain both RAM and ROM in varying amounts depending upon the design of the PLC. The use of PLC’s memory is determined again by the design of the unit. However, all PLC memories can be subdivided into at least five major areas. A typical memory utilization map for a PLC is depicted in the following figure. a. Executive Memory: The operating system or executive memory for the PLC is always in ROM since, once programmed and developed by the manufacturer, it rarely needs changing. It is the one that actually does the scanning in a PLC. The operating system is a special machine language program that runs the PLC. It instructs the microprocessor to read each user instruction, helps the microprocessor to interpret user programmed symbols and instructions, keeps the track of all the I/O status, and is responsible for maintaining / monitoring the current status of the health of the system and all its components. b. System memory In order for the operating system to function, a section of the memory is allotted for system administration. As the executive program performs its duties, it often requires a place to store intermediate results and information. A section of RAM is installed for this purpose. Normally this area is allotted for use of the operating system only and is not available to the user for programming. It might be thought of as a scratch pad for the operating system to doodle on as necessary. Some PLCs use this area for storing the information which passes between programmer and operating system, e.g. the operating system generates certain error codes store in the specific address in this area during the execution of user program which can be read by user program; or the user may also give additional information to the operating system before execution of user program by writing some codes in the specific address in this area, etc. c. I/O Status Memory - I/O Image Table
- 14. Another portion of RAM is allocated for the storage of current I/O status. Every single input/output module has been assigned to it a particular location within the input/output image table. The location within the input and output image tables are identified by addresses, each location has its own unique address. During the execution of user program, the microprocessor scans the user program and interpret the user commands, the status of input modules used are read from the input image table (not directly from the input module itself). Various output device status generated during the execution of user program are stored in the output image table (not directly to output modules). (Find out about input scan and output scan.) d. Data Memory Whenever timers, counters, mathematics and process parameters are required, an area of memory must be set aside for data storage. The data storage portion of memory is allocated for the storage of such items as timers or counter preset/accumulated values, mathematics instruction data and results, and other miscellaneous data and information which will be used by any data manipulation functions in the user program. Some manufacturers subdivide the data memory area into two sub-memories, one for fixed data and other for variable data. The fixed data portion can only be programmed via the programming device. The CPU is not permitted to place data values in this area. The variable portion of the data memory is available to the CPU for data storage. e. User Program Memory The final area of memory in a PLC is allocated to the storage of the user program. It is this memory area that the executive program instructs the microprocessor to examine or 'scan' to find the user instructions. The user program area may be subdivided if the CPU allocates a portion of this memory area for the storage of ASCII messages, subroutine programs, or other special programming functions or routines. In the majority PLCs, the internal data storage and user program areas are located in RAM. Several systems do offer an option that places both the user program and the fixed data storage areas in EPROM type memory. The user can develop program in RAM and run the system to ensure correct operation. Once the user is satisfied that the programming is correct, a set of EPROMs is then duplicated from the RAM. Then the user can shut down the CPU and replaces the RAM with the newly programmed EPROM. Any future change would require that the EPROMs be reprogrammed. Discrete I/O A “discrete” data point is one with only two states on and off. Process switches, pushbutton switches, limit switches, and proximity switches are all examples of discrete sensing devices. In order for a PLC to be aware of a discrete sensor’s state, it must receive a signal from the sensor through a discrete input channel. Inside the discrete input module is (typically) a light-emitting diode (LED) which will be energized when the corresponding sensing device turns on. Light from this LED shines on a photo-sensitive device such as a phototransistor inside the module, which in turn activates a bit (a single element of digital data) inside the PLC’s memory. This opto-coupled arrangement makes each input channel of a PLC rather rugged, capable of isolating the sensitive computer circuitry of the PLC from transient voltage “spikes” and other electrical phenomena capable of causing damage.
- 15. Indicator lamps, solenoid valves, and motor contactors (starters) are all examples of discrete control devices. In a manner similar to discrete inputs, a PLC connects to any number of different discrete final control devices through adiscrete output channel. Discrete output modules typically use the same form of opto-isolation to allow the PLC’s computer circuitry to send electrical power to loads: the internal PLC circuitry driving an LED which then activates some form of photosensitive switching device. Alternatively, small electromechanical relays may be used to interface the PLC’s output bits to real-world electrical control devices. Analog I/O In the early days of programmable logic controllers, processor speed and memory were too limited to support anything but discrete (on/off) control functions. Consequently, the only I/O capability found on early PLCs were discrete in nature2. Modern PLC technology, though, is powerful enough to support the measurement, processing, and output of analog (continuously variable) signals. All PLCs are digital devices at heart. Thus, in order to interface with an analog sensor or control device, some “translation” is necessary between the analog and digital worlds. Inside every analog input module is an ADC, orAnalog-to-Digital Converter, circuit designed to convert an analog electrical signal into a multi-bit binary word. Conversely, every analog output module contains a DAC, or Digital-to-Analog Converter, circuit to convert the PLC’s digital command words into analog electrical quantities. Analog I/O is commonly available for modular PLCs for many different analog signal types, including: • Voltage (0 to 10 volt, 0 to 5 volt) • Current (0 to 20 mA, 4 to 20 mA) • Thermocouple (millivoltage) • RTD (millivoltage) • Strain gauge (millivoltage) Example 1: Lighting controlsystem: A lighting control system is to be developed. The system will be controlled by four switches, SWITCH1, SWITCH2, SWITCH3, and SWITCH4. These switches will control the lighting in a room based on the following criteria: 1. Any of three of the switches SWITCH1, SWITCH2, and SWITCH3, if turned ON can turn the lighting on, but all three switches must be OFF before the lighting will turn OFF.
- 16. 2. The fourth switch SWITCH4 is a Master Control Switch. If this switch is in the ON position, the lights will be OFF and none of the other three switches have any control. The first item we may accomplish is the drawing of the controller wiring diagram. All we need do is connect all switches to inputs and the lighting to an output and note the numbers of the inputs and output associated with these connections. The remainder of the task becomes developing the ladder diagram. The wiring diagram is shown in Figure 1. Notice that all four switches are shown as normally open selector switches and the output is connected to a relay coil CR1. We are using the relay CR1 to operate the lights because generally the current required to operate a bank of room lights is higher than the maximum current a PLC output can carry. For this wiring configuration, the following definition list is apparent: INPUT IN1 = SWITCH1 INPUT IN2 = SWITCH2 INPUT IN3 = SWITCH3
- 17. INPUT IN4 = SWITCH4 (Master Control Switch) OUTPUT OUT1 = Lights control relay coil CR1 This program requires that when SWITCH4 is ON, the lights must be OFF. In order to do this, it would appear that we need a N/C SWITCH4, not a N/O as we have in our wiring diagram. However, keep in mind that once an input signal is brought into a PLC, we may use as many contacts of the input as we need in our program, and the contacts may be either N/O or N/C. Therefore, we may use a N/O switch for SWITCH4 and then in the program, we will logically invert it by using N/C IN4 contacts. The ladder diagram to implement this example problem is shown in Figure 2. The ladder was printed using graphics characters (extended ASCII characters). Notice the normally closed contact for IN4. A normally closed contact represents an inversion of the assigned element, in this case IN4, which is defined as SWITCH 4. Remember, SWITCH 4 has to be in the OFF position before any of the other switches can take control. In the OFF position, SWITCH 4 is open. This means that IN4 will be OFF (de-energized). So,in order for an element assigned to IN4 to be closed with the switch in the OFF position, it must be shown as a normally closed contact. When SWITCH 4 is turned ON, the input, IN4, will become active (energized). If IN4 is ON, a normally closed IN4 contact will open. With this contact open in the ladder diagram, none of the other switches will be able to control the output. REMEMBER: A normally closed switch will open when energized and will close when de-energized.
- 18. Example 2: Devices: Device Function X0 X0=ON when the detected input signal from the bottle bottom is sheltered X1 X1=ON when the detected input signal from the bottle neck is sheltered Y0 Pneumatic pushing pole Program description If the bottle on the conveyor belt is upstanding, the input signal from the moitoring photocell at both bottle bottom and bottle neck will be detected. In this case, X0=ON and X1=ON. The normally open (NO) contact X0 will be activated as well as the normally closed (NC) contact X1. Y0 remains OFF and pneumatic pushing pole will not perform any action. If the bottle from the conveyor belt is down, only the input signal from monitoring photocell at the bottle bottom will be detected. In this case, X0=ON, X1=OFF. The state of the output Y0 will be ON because the NO contact X activates and the NC contact X1 remains OFF. The pneumatic pushing pole will push the fallen bottle out of the conveyor belt
- 19. Example 3: Devices: Device Function X0 X0 turns ON when the botton switch is turned to the right X1 X1 turns ON when the top switch is turned to the right Y1 Stair light Program Description If the states of the bottom switch and the top switch are the same, both ON or OFF, the light will be ON. If different, one is ON and the other is OFF, the light will be OFF. When the light is OFF, users can turn on the light by changing the state of either top switch or the bottom switch of the stairs. Likewise, when the light is ON, users can turn off the light by changing the state of one of the two switches.
- 20. PLC features and benefits