PROGRAMMABLE LOGIC CONTROLLER BY: Salsabila Ahmad

CHAPTER 4 BASIC FUNCTION OF PLC AND APPLICATION BY: Salsabila Ahmad

BASIC FUNCTION OF PLC AND ITS APPLICATION Introduction to Pendant Programming Input and Output Internal Relay ‘AND’ and ‘OR’ circuit Latching Relay Program Editing Program Observations Q&A

PROGRAM SCAN

Program Scan During each operating cycle, the processor reads all inputs, takes these values, and energizes or de-energizes the outputs according to the user program. This process is known as a scan. I/O scan – records status data of input devices. Energizes output devices that have their associated status bits set to ON (1) Program scan – instructions are executed sequentially Because the inputs can change at any time, the PLC must carry on this process continuously.

Scan Process Scan time indicates how fast the controller can react to changes in inputs. Scan times vary with computer model and program content, and length. If a controller has to react to an input signal that changes states twice during the scan time, it is possible that the PLC will never be able to detect this change. Scan time may be a concern in high speed operations

Scan Process The scan is a a continuous and sequential process Read inputs The scan is a a continuous and sequential process Adjusts outputs Run program

Data Flow Overview Input image Output table file image table file modules data Examine data Check/compare/examine specific conditions Output data modules Take some action Return results Input image table file Output image table file Program

Scan Process Input Module Output Module Output device Input file Program When the input is closed, the input module senses a voltage and an ON condition (1) is entered into the input table bit I:3/6 The processor turns light output O:4/7 ON during the next I/O scan I:3/6 O:4/7 During the program scan the processor sets instructions I:3/6 and O:4/7 to ON (1)

SCAN PATTERN There are two basic scan patterns that different PLC manufacturers use to accomplish the scan function: Horizontal scan E.g. Allen-Bradley Vertical scan E.g. AEG Modicon

Horizontal Scanning Order The processor examines input and output instructions from the first command top left in the program horizontally rung by rung End of ladder In addition to the program itself, the scan time is also dependent on the clock frequency of the processor!

Vertical Scanning Order The processor examines input and output instructions from the first command, vertically, column by column page by page. Pages are executed in sequence. End of ladder Misunderstanding the way the PLC scans can cause programming bugs!

PLC PROGRAMMING LANGUAGE Methods use for user to communicates information with PLC 3 most common language structures Ladder diagram language Boolean language Function chart

LADDER DIAGRAM LANGUAGE The most commonly used for PLC. Ladder diagram language

BOOLEAN LANGUAGE Addressing format for input and output varies for different PLC models Boolean language

COMPARING PROGRAMMING LANGUAGE PB1 CR1 CR2 SOL Relay Schematic LS1 PB1 CR1 CR2 LS1 SOL Equivalent ladder diagram language Equivalent Boolean language

FUNCTIONAL CHART Advantages: Block of logic can be programmed as a module Simpler programming Faster scan time Enhanced maintainability Ease of future enhancements Functional chart

SEQUENTIAL FUNCTION CHART (SFC) PROGRAMMING A functional chart program is a pictorial representation or a special type of flowchart of a sequential control process. It allows the description of the process to become the actual program. Functional chart programming uses function blocks made up of steps and transition units.

Sequential Function Chart (SFC) Programming Check for stop Wait for start Check for part Lock the clamp Press the part Unlock clamp Move conveyor One cycle Continuous Check mode

PROGRAM INSTRUCTIONS

Relay-Type Instructions The ladder diagram language is basically a symbolic set of instructions used to create the controller program. These ladder instructions symbols are arranged to obtain the desired control logic.

EXAMINE IF CLOSED (XIC) INSTRUCTION

Similar to the normally open relay contact Similar to the normally open relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) CLOSED (XIC) Symbol Typically represents any input. Can be a switch or pushbutton, a contact from a connected output, or a contact from an internal output. Has a bit-level address which is examined for an ON condition. The status bit will be either 1 (ON) or 0 (OFF). The status bit is examined for an ON condition.

I:012 04 If the status bit is 0 (OFF), then the instruction is FALSE. If the status bit is 1 (ON), then the instruction is TRUE.

EXAMINE IF OPEN (XIC) INSTRUCTION

Similar to the normally closed relay contact Similar to the normally closed relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) OPEN (XIO). Symbol Typically represents any input. Can be a switch or pushbutton, a contact from a connected output, or a contact from an internal output. Has a bit-level address which is examined for an OFF condition. The status bit will be either 1 (ON) or 0 (OFF). The status bit is examined for an OFF condition.

I:012 I:012 04 If the status bit is 1 (ON), then the instruction is FALSE. If the status bit is 0 (OFF), then the instruction is TRUE. I:012 04

OUTPUT ENERGIZE (OTE) INSTRUCTION

Similar to the relay coil Similar to the relay coil. The processor makes this instruction true (equivalent to energizing a coil) when there is path of true XIC and XIO instructions in the rung. Symbol Typically represents any output that is controlled by some combination of input logic. Output can be a connected device or an internal output (internal relay). If any left-to-right path of input conditions is TRUE, the output is energized (turned ON).

OUTPUT ENERGIZE instruction - FALSE OUTPUT ENERGIZE instruction - TRUE 01 11 15

EXAMPLE

Status Bit Examples Input module A Bit status Button not actuated Button actuated OFF ON False True Button not actuated True False ON OFF Output A A Output

Ladder Rung A B C D Output instruction Input conditions Output instruction A ladder rung consists of a set of input conditions, represented by contact instructions, and an output instruction at the end of the rung, represented by the coil symbol.

For an output to be activated or energized, at least one left-to-right path of contacts must be closed. A complete path is referred to as having logic continuity. When logic exists the rung condition is said to be TRUE. Each contact or coil symbol is referenced with an address number that identifies what is being evaluated and what is being controlled. The same contact instruction can be used throughout the program whenever that condition needs to be evaluated.

Rung Continuity 1 The Examine If Closed instruction is TRUE Bit in memory Bit in memory 1 The Examine If Closed instruction is TRUE making the rung TRUE The Examine If Closed instruction is FALSE making the rung False LS_1 SOL_5

INSTRUCTION ADDRESSING

Allen-Bradley SLC-500 Controller Addressing Energized output 6 O:0:4 Address terminal O0:4/6 Output image table file 0 O:0:4/6 Bit address Input image table file 1 I:3/12 Bit address I1:3 12 User-programmed rung Address input terminal I1:3/12 Closed input

Structure of A 16-Bit Word OFF ON 1 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Word (16 bits) I/O Connection Diagram L1 L2 PB1 LS1 I:4/5 I:4/6 O:2/3 O:3/6 SOL 1 PL 1 R

BRANCH INSTRUCTIONS

Parallel Input Branch Instructions Branch instructions are used to create parallel paths of input condition instructions. If at least one of these parallel branches forms a true logic path, the logic is enabled.

Parallel Output Branching D E On most PLC models, branches can be established at both the input and output portion of the rung. With output branching, you can program parallel outputs on a rung to allow a true logic path to control multiple outputs.

Nested Input and Output Branches A nested branch starts or ends within another branch. Input and output branches can be nested to avoid redundant instructions and to speed up the processor scan time.

Nested Contact Program B C D E Y Nested contact On some PLC models, the programming of a nested branch circuit cannot be done directly. Contact instruction C repeated A B C D E Y Reprogrammed to obtain the required logic.

PLC Matrix Limitation Diagram Max series contacts Max parallel lines No. outputs per rung and location of the output in the rung There may be limitations to the number of series contacts instructions, number of parallel lines, and the number of outputs and their location on the rung.

Programming of Vertical Contacts B C D E Y Original program Y = (AD) + (BCD) + (BE) + (ACE) A D B C E Y Reprogrammed to obtain the required logic

Programming for Different Scan Patterns Original program Y = (ABC) + (ADE) + (FE) + (FDBC) A D B E F Y A B D E Y F C Reprogrammed to obtain the required logic

INTERNAL RELAY

Internal Control Relay The internal output operates just as any other output that is controlled by programmed logic; however, the output is used strictly for internal purposes. The internal output does not directly control an output device. The advantage of using internal outputs is that there are many situations where an output instruction is required in a program, but no physical connection to a field device is needed. Their use in this type of instance can minimize output card requirements.

Extending the Number of Series Contacts Using an Internal Control Relay Internal relay coil Rung 1 Rung 2 Internal relay contact Discrete output (requires one physical connection on the output module)

AND and OR circuit

LATCH

LATCHING RELAY

LATCHING RELAY It is necessary to have a relay "latch" ON so if the device that activated the relay is switched OFF the relay remains ON Useful for making a momentary pushbutton switch perform like a maintained switch Might as well we use maintained switch?

Consider, for example, the pushbuttons that switch a machine on and off Refer to basic STOP/START circuit Here we use the contact of the motor to hold the circuit This can be done we include a relay in the circuit that is wired as a latch as shown in the ladder diagram

When power is applied to the rails CRI is initially de-energized N/O CRI contact is also open Assuming S1 has not been pressed there is no path for current to flow through the rung it will be OFF.

Next, when S1(START) is press it provides a path for current flow through S1, S2, and coil of CR1 It energizes CRI As soon as CR1 energizes the N/O CRI contact in parallel with S1 closes (since the CRI contact is operated by the CRI coil)

When the relay contact closes switch S1 is no longer needed to maintain a path for current flow through the rung because it is provided by the N/O CR1 contact and N/C pushbutton S2 At this point, we can release S1 and the relay CRI will remain energized

The N/O CR1 contact "seals" or "latches" the circuit ON This type of contact configuration has several commonly used names including a sealing contact, seal-in contact, or latching contact

The circuit is de-energized by pressing the STOP switch S2 This interrupts the flow of current through the rung de-energizes the CRI coil opens the CRI contact in parallel with S1.

When S2 is released, there will still be no current flow through the rung because both S1 and the CR1 N/O contact are open

WHY DON’T WE USE MAINTAINED SWITCH? The latch circuit has one other feature that cannot be obtained by using a maintained switch. Should power fail while the machine is on the latch rung will, of course, de-energize However, when power is restored, the machine will not automatically restart It must be manually restarted by pressing S1 This is a safety feature that is required on all heavy machines

PROGRAM ENTERING AND EDITING

Entering the Ladder Diagram A personal computer is most often used to enter the ladder diagram. The computer is adapted to the particular PLC model using the relevant programmable controller software.

SETTING

Specify File name Type of device/ processor used Network type

CX Programmer Main Screen Different screens, toolbars and windows dialog boxes are used to navigate through the Windows environment

INSTRUCTION TOOL BAR

Tool Bar for Creating Ladder Diagram Vertical line Function e.g -Counter -Timer -End NO contact NC contact NO branch NC branch Horizontal line Output coil Output NOT Tool Bar for Creating Ladder Diagram

Branch contain in the same rung Output must located at far right Error Input located at right After complete program must have END instruction

To connect PC with PLC (online) To transfer program from PC to PLC To read program from PLC to PC

Run Mode Work online Transfer to PLC Program PLC monitoring Monitor Transfer from PLC Debug

PROGRAM OBSERVATION

Monitoring a Ladder Logic Program Operation of the logic is apparent from the highlighting of rungs of the various instructions on screen, which identifies the logic state in real time and has logic continuity. Highlighted rungs indicate the instruction is true

MODES OF OPERATION A processor has basically two modes of operation: program mode run mode And some of its variation

PROGRAM MODE May be used to enter a new program edit or update an existing program upload files download files document programs change software configurations When the PLC is switched into the program mode, all outputs from the PLC are forced off regardless of their rung logic status, and the ladder I/O scan sequence is stopped.

RUN MODE and its variations is used to execute the user program. Input devices are monitored and output devices are energized accordingly. Test Mode is used to operate, or monitor, the user program without energizing any outputs. Remote Mode allows the PLC to be remotely changed between program and run mode by a personnel computer connected to the PLC processor.