Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering Date:

Experiment No – 1 (B) Aim: Objectives:        Theory:

To study hardware and software requirements of PLC & SCADA. Basics of Process Automation Block Diagram & Working of PLC Introduction to Programming Scan Cycle of PLC Rules for programming Various Instruction for Programming Understanding Laboratory Setup

Basics of Process Automation Automation is the use of control systems and information technology to reduce human efforts by reducing its intervention while controlling. Process automation is specific to manufacturing or any such process wherein we are creating a product of required specifications with the help of automation. To control such a process first we need to sense the parameters to be controlled which is done through a sensor. A SENSOR is a device used to convert a physical parameter such as room temperature, pressure, flow etc. into a signal that can be measured. It is not necessary that these signals are in the form of electrical signals. So to understand or manipulate the output of sensor, the signals of sensors should be in electrical form. This is achieved by TRANSDUCERS. The manipulation for control is done by a controller which can be a PLC or DCS. The controllers are placed at a distance from the field. For signals to reach there without any drop TRANSMITTERS are used. After processing the input signal, the processor generates output depending on the logic which it has to be performed. The output signal is then forwarded to an output device. If the output device is a motor, then the signal from processor will give command to the Motor Control Centre (MCC). If output device is a VFD or electrically operated control valve then signal from processor can directly go to it. If output device is a valve that too pneumatically operated then a device called converter is used after controller to convert its electrical signal to pneumatic signal which can then be given to the control valve. A CONVERTER thus is a device which converts a standard signal to the signal which can be understood by the device to be interfaced to the converter. A CONTROL VALVE is an element which controls the flow of the substance to control process parameters such as flow, temperature, level, etc. Thus there is a concept of Process Input/ Output and Controller Input/ Output. A sensor or transmitter acts as process output but acts as input to controller whereas a control

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering valve acts as process input and output to controller. This basic understanding of the process and controller I/O is a base to start study of PLC hardware and its programming. Block Diagram of PLC Figure 1 (Draw on blank side )shows the block diagram of PLC. As it can be seen in Figure 1 block diagram of PLC consists of CPU, input module, output module and power supply as its main units. We will see further each unit’s function and operation and main features in detail. 1.

Central Processing Unit: This is the brain of PLC which performs all the operation i.e. executes the logic or controls the process. It has two parts:

a)

Microprocessor: The computers centre that carries out mathematic and logic operations. For performing these operations the processor needs to be powerful which is decided by its clock speed and bit size. Memory: The area of the CPU in which data and information is stored and retrieved. Hold the system software and user program. The part of the memory which holds the ladder logic or user program is called user memory. The part of the memory that holds predefined system variables is called data memory. The organization of user and data memory can be understood deeply through a memory map. Figure 2(Draw on blank side ) shows memory map. Generally user memory occupies larger space of total memory as compared to data memory. This can be seen from memory map shown in Figure 2. Also we can observe various sections of data memory from fig2.

b)

Addressing: Addressing in PLC, follows octal addressing. There are two types of addressing: I)

Internal Addressing: Internal addressing is the addressing made for variables which are internally created in the memory for simulation purpose. Also the tagging of timers, counters etc. i.e. variables of data memory come into this type of addressing. With the change in the manufacturer of PLC, change in the addressing takes place. The addressing of this type will be explained while programming, in coming sections.

II)

External addressing: The field sensors i.e. input devices and output devices i.e. valves, motors etc. are communicating with the CPU with the help of input modules and output modules respectively. In order that CPU understands the information is coming from which devices external addressing is necessary. This addressing is done during installation of PLC. It is a part of configuring PLC during installation and commissioning of any PLC. The addressing is explained below with the help of Figure 3.

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering In a process plant where large process is to be controlled for obtained a product on continuous basis, there are large number of input and outputs. Thus there will be large number of input and output modules which are arranged as shown in Fig-3. There are several panels in which there are several racks and terminal boxes placed. These panels are considered to be groups. Inside racks there are several slots in which input and output modules are placed. The slots contain backplane through which the input and output modules can communicate to CPU and modules and CPU can get power supply from the power supply unit in the rack. The standard which is followed while addressing is first group is addressed, then rack, slot and lastly the channel number of the module or card attached. This concept of channel will be explained further in section of input and output module. Always in group 0 i.e. 1st panel in the 1st rack i.e. rack 0, slot no 1 or slot no.2 is always the CPU no other group or rack or slot will contain the CPU. In some cases CPU can be placed in Group 0, Rack0, and Slot no.0 (CPU addressing) only when power supply unit is too big and cannot be accommodated in a rack. The first slot in the rack always accommodates power supply to provide supply to CPU and cards, whereas last slot of each rack is always accommodating a communication card, for CPU to communicate with the other racks. A Rack is available in standard slot size of 2, 4, 8or 16.

(Figure3) 2.

Power Supply Unit: The electrical supply that converts alternating current (AC) line voltages to various operational DC values. In the process, the power supply filters and regulates the DC voltages to ensure proper computer operation. The Figure 4 shows block diagram of the power supply unit. (Draw Figure 4 on blank side and label)

3.

Programmer / Monitor: The programmer/ monitor (PM) is a device used to communicate with the circuits of PLC, to place the program / logic prepared by the user into the PLC processor to execute it. This is where the SCADA resides. This is generally a personal computer (PC) for industrial

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering

4.

purpose. Thus the name programmer / monitor. To place the program in the PLC’s processor is called downloading and taking the last configuration and program available in the PLC to PC is called uploading. Input Module: There are four tasks which an input module needs to accomplish for its proper working which are as follows: a) It senses the presences or absence of an input signal at each of its input terminals. This tells which of the switch, sensor or other signal is on or off in the process being controlled. b) It converts the input signal for high, or on, to a DC level usable by the module’s electronic circuit. For a low, or off, input signal, no signal is converted, indicating off. c) The input module carries out electronic isolation by electronically isolating the input module output from its input. d) Its electronic circuit must produce an output, via output logic, to be sensed by the PLC CPU. This can be understood from block diagram shown below in Figure 5 of an input card / module.

(Figure5) The input module consists of various terminals or channels which decides the number of input devices that can be connected to a particular module or card. As per the standard IEC61131 which a standard for PLC, the input card are available with 2, 4, 8, 16 or 32 channel’s i.e. these number of devices can be connected to the card at a time. Moreover there are several types of input and output cards available; their classification is shown below in Figure 6. (Draw Figure 6 on blank side) The classification of input module cards can be done on the basis of following points: a) Based on the power supply provided to the card, they can be classified as AC input card and DC input card. These can be further classified on the basis of various voltage levels as shown in Figure 6. b) Based on the types of devices connected to the card the devices are classified as analog and digital card. If the output of the sensor is of type on or off i.e. digital it will be connected to a digital input card. If the output of the sensor is continuously varying output with time then such a sensor will be connected to analog input card, as the sensors output is of analog form. Apart from these basic classifications other special types of input cards which are available in the market are as follows:

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering   

RTD input card Thermocouple input card Pulse input card etc. The simple representation of a digital input card is shown below in Figure 7.

(Figure7) Figure 8, 9 and 10 show the circuit diagram i.e. internal circuit of AC/ DC input cards. Apart from these there is another concept in case of input module i.e. the way the devices are connected. Based on this concept input modules are classified as source type or sink type cards. The connection of the same is shown in Figure 11 and 12. This concept mainly deals with the flow of current between devices and the PLC. If the current flows from device to the card then such a type of card are called sink type card shown in Figure11. If the current flows from card to device then such a type of card is called source type of card shown in Figure 12.

(Figure8)

(Draw Figure 9 on blank side)

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering

(Figure 10)

(Figure 11)

(Figure 12) 5.

Output Module: The output module does exactly opposite of what the input module does i.e. it passes the signal coming from the CPU to the output devices connected to the output module. This can be understood from block diagram shown below in Figure 13 of an output card / module.

(Figure13)

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering The output module consists of various terminals or channels which decides the number of output devices that can be connected to a particular module or card. As per the standard IEC61131 which a standard for PLC, the output card are available with 2, 4, 8, 16 or 32 channel’s i.e. these number of devices can be connected to the card at a time. Moreover there are several types of output cards available; their classification is shown below in Figure 14. The classification of output module cards can be done on the basis of following points: Based on the power supply provided to the card, they can be classified as AC input card and DC input card. These can be further classified on the basis of various voltage levels as shown in Figure 14 (Draw Figure 9 on blank side) . a) Based on the type of device used for switching action of the output devices connected to the cards, they are classified as shown in Figure 14 and explained below. (i) Relay Type output card: Figure 15 shows the circuit diagram for the same. Output from a PLC is used to operate a relay which is able to switch currents of the order of a few amperes in an external circuit. It also isolates PLC from external circuit apart from switching currents. This circuit can be used for both types of AC and DC supply output cards. They can withstand high surge currents & voltage transients.

(Figure15) (ii) Transistor Type output card: Figure 16 shows the circuit diagram for the same. This type of card uses a transistor to switch current through external circuit. This gives fast switching action. It is strictly for DC switching & is destroyed by Overcurrent & high reverse voltage. For protection either a fuse or built in electronic protection is used. This circuit can thus be used only for DC supply output cards.

(Figure16)

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering (iii) Triac Type output card: Figure 17 (Draw on the blank side) shows the circuit diagram for same. Opto-isolators for isolation, this circuit can be used to control external loads that can be used to control external loads that are connected to the AC power supply. It is strictly for AC operation & is very easily destroyed by Overcurrent. Fuses are virtually always included to protect such outputs. As explained above it can be used for only those output cards which are working on AC supply. (Figure17) The simple representation of a digital output card is shown below in Figure 18.

(Figure18) Apart from these there is another concept in case of output module i.e. the way the devices are connected. Based on this concept output modules are classified as source type or sink type cards. The connection of the same is shown in Figure 19 and 20. This concept mainly deals with the flow of current between devices and the PLC. If the current flows from device to the card then such a type of card are called sink type card shown in Figure19. If the current flows from card to device then such a type of card is called source type of card shown in Figure 20.

(Figure19)

(Figure20) Basics of Programming

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering

1.

2.

3.

4.

5.



In order to program the Programmable Logic Controller there are several languages available. Each of the software available in the market can support one of these languages or all depending on the manufacturer. The languages available are as follows: Ladder Logic: Ladder Logic has been evolved out of relay control panels, since PLCs adopted legacy concepts, which were applicable to such panels. Also to facilitate maintenance and modification of the physically wired control logic, the control panel was systematically organized so that each control formed a rung much like a rung on a ladder. The development of PLCs retained the ladder logic concept where control circuits are defined like rungs on a ladder where each rung begins with one or more inputs and each rung usually ends with only one output. This is thus the first programming language which was developed with the advent of PLC in market. This language will be studied in detail over here in coming practical’s. Functional Block Diagram (FBD): This type of programming evolved after the ladder logic programming came into market. This is the easiest language in which we can program the PLC. This is an emerging language in market nowadays. The concept used in this type of programming is that a number of small instructions are combined to form a block which performs a major function. E.g.: AND function Block, Timer Function Block etc. We just need to conFigure these blocks in order that these blocks perform their functions. This type of programming we will just be observing, we won’t be learning this language in detail. Sequential Flow Charts (SFC): This language is generally used in batch process control where a sequence of operation is followed to obtain a product on a certain lot from a big amount of raw material. This language is like flow chart preparation defining the sequence of operation. This type of programming we will just be observing, we won’t be learning this language in detail. Instruction Set Language (IS): This type of language is similar to programming done in microcontroller assembly language and C or C++ language. The programming of controller with this programming requires an expert or a lot of practice. Thus this language is not popular in industries. Thus we won’t be studying this language in coming sections. Structural Text Language (ST): This type of language is similar to programming done in microcontroller assembly language and C or C++ language. The programming of controller with this programming requires an expert or a lot of practice. Thus this language is not popular in industries. Thus we won’t be studying this language in coming sections. Introduction to Ladder Logic Programming Each manufacturer of PLC systems has own style of writing the instructions. Different PLCs has different instruction sets but even some common basic instructions are shared by all the PLCs. All manufacturers give different software packages for programming PLCs. Ladder is most commonly used programming language. Prior to PLCs, relay logic was used in industry. Ladders were developed to mimic or imitate relay logic. Basics of Relay Logic Nowadays, people use relays for isolation but not for control logic. A relay is simple magnetic device which acts as a control switch shown in Figure 21. When the switch is on,

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering current will flow through the coil on iron piece. This iron core acts as electromagnet and due to the magnetic field upper contact gets attracted towards lower one and circuit gets completed, allowing current to flow from load. This describes the operation or working of a relay in brief.



(Figure 21) Ladder Programming Ladder diagram is popular language of programming the PLCs. Ladder diagram shows the sequence of the logic execution which is presented diagrammatically in Figure 22. In ladder diagram, there are two vertical lines generally called as Phase (positive) and neutral. Rungs which show current flow in horizontal direction are the sequence in which the logic executes. The Analogous to relay, ladder has two main symbols which are contacts and output coil. Generally each rung has inputs (contacts) on left hand side and outputs (coil) on the right hand side. These contacts and coils are called as bits of the relays. Each input and output is individual bit in I/O files. An instruction in ladder instructs PLCs how to respond to the bits in I/O files which are stored in the memory. Input contacts are the condition area; the conditions must be fulfilled to change the status of the output coils.

(Figure 22) Most commonly used relay instructions used in PLC programming are as shown in the table 1 below. Instruction Symbol Description Examine On An input condition that is open when (Normally de-energized Open) Examine Off (Normally Close)

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An input condition that is close when de-energized

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering

Output Coil

An output instruction that is true when the input conditions become true

Negated Output Coil

An output instruction that is true all the time except when all input conditions are true

Latched Output Coil

To hold an output ON

Unlatched Output Coil

To unlatch the latched ON output

Working of PLC The ladder program once made has to be executed. Execution can be done only when program is loaded in the controller i.e. PLC. The process of loading the program in PLC is called downloading. The process of loading the program from PLC to PC is called uploading. Once the program is downloaded and the connections of input devices with input cards and output devices with output cards is done, then PLC is ready to run i.e. execute the logic and thus control the process. Earlier in the section we saw memory mapping where we saw two tables which were assigned space one was input image status table and other output image status table. Their existence or importance we will be studying here. A PLC is continuously running through its program and updating it as a result of the input signals. Each such loop is termed a scan cycle. PLCs could be operated by each input being examined as it occurred in the program, its effect on the program determined, and the output correspondingly changed. This mode of operation is termed continuous updating. Because there is time spent interrogating each input in turn with continuous updating, the time taken to examine several hundred input/output points can become comparatively long. To allow more rapid execution of a program, a specific area of RAM is used as a buffer store between the control logic and the input/output unit which is known as input image status table and output image status table. Each input/output has an address in this memory. At the start of each program cycle the CPU scans all the inputs and copies their status into the input/output addresses in RAM. As the program is executed, the stored input data is read, as required, from RAM and the logic operations are carried out. The resulting output signals are stored in the reserved input/output section of RAM. At the end of each program cycle all the outputs are transferred from RAM to the appropriate output channels. The outputs then retain their status until the next updating. This method of operation is termed mass I/O copying. The sequence can be summarized as follows and can be observed from Figure 23 and 24: (To be drawn of blank side.)

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Chandubhai S. Patel Institute of Technology, Changa M. & V. Patel Department of Electrical Engineering

1. Scan all the inputs and copy into RAM. 2. Fetch, decode, and execute all program instructions in sequence, copying output Instructions to RAM. 3. Update all outputs. 4. Repeat the sequence. (Draw both Figures Figure23 and Figure 24 on blank side) The time taken to complete a cycle of scanning inputs and updating outputs according to the program instructions, that is, the cycle time, though relatively quick, is not instantaneous and means that the inputs are not watched all the time, but instead that samples of their states are taken periodically. A typical cycle time is on the order of 10 to 50 ms this means that the inputs and outputs are updated every 10 to 50 ms and thus there can be a delay of this order in the system reaction. It also means that if a very brief input cycle appears at the wrong moment in the cycle, it could be missed. In general, any input must be present for longer than the cycle time. Special modules are available for use in such circumstances. Consider a PLC with a cycle time of 40 ms what is the maximum frequency of digital impulses that can be detected? The maximum frequency will be if one pulse occurs every 40 ms, that is, a frequency of 1/0.04 ¼ 25 Hz. The cycle or scanning time for a PLC, i.e. its response speed, is determined by: 1. 2. 3. 4.

The CPU used. The size of the program to be scanned. The number of inputs/outputs to be read. The system functions that are in use; the greater the number, the slower the scanning time.

Response Time: The time taken between an input occurring and an output changing depends on such factors as the electrical response time of the input circuit, the mechanical response of the output device, and the scan time of the program. A ladder program is read from left to right and from top to bottom. Thus if an output device, such as an internal relay, is set in one scan cycle and the output has to be fed back to earlier in the program, it will require a second scan of the program before it can be activated.

Marks/Grade

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Sign of faculty in charge

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