Navigation

  • Page 1

    Module 4 Programmable Logic Control Systems Version 2 EE IIT, Kharagpur 1

  • Page 2

    Lesson 19 The Software Environment and Programming of PLCs Version 2 EE IIT, Kharagpur 2

  • Page 3

    Instructional Objectives After learning the lesson students should be able to A. Describe the structure of a PLC Program B. Describe the execution of a PLC Program C. Describe the typical elements of an RLL Diagram D. Design RLL Diagrams for simple industrial logic control problems Structure ...

  • Page 4

    Such facilities of structured programming result in programs, which are easier to read, write, debug and maintain. Program Execution There are different ways and means of executing a user program. Normally a cyclic execution program is preferred and this cyclic operators are given due prioriti...

  • Page 5

    Interrupt Driven and Clock Driven Execution Modes A cyclically executing program can however be interrupted by a suitably defined signal resulting in an interrupt driven mode of program execution (when fast reaction time is required). If the interrupting signal occurs at fixed intervals we can a...

  • Page 6

    Rung Logic ] Rung ••(Virtual) Power Rails Fig. 19.1(a) The structure of Relay Ladder Logic Programs for PLCs Relay coil Fig. 19.1(b) The internal structure of a simple Rung RLL Programming Paradigms: Merits and Demerits For the programs of small PLC systems, RL...

  • Page 7

    Example: Forward Reverse Control IN001IN002IN001 IN003OP001 OP001 OP001 OP002 OP002 OP002 Fig. 19.2 RLL Diagram for the Forward Reverse Control Problem This example explains the control process of moving a motor either in the forward direction or in the reverse direction. The dir...

  • Page 8

    coil. Once the coil contacter gives the logic ‘1’, the following consequences takes place simultaneously A. The auxillary contact OP001 in the second rung becomes opened,which stops the voltage for reverse motion of the motor. At this stage, the second rung is not turned on even the reverse r...

  • Page 9

    status of the process image independently to the output modules. This method enables faster program execution because access to the process image is executed much faster than access to the I/O – modules. In RLL Programs, inputs are represented as contacts. Two types of contacts are used, namely...

  • Page 10

    carried out with the signal “plant in operation”. At signal status “1” of this signal the indicating light is either permanently on or is flashing. Timer These are special operands of a PLC, which represent a time delay relay in a relay logic system. The time functions are a fixed comp...

  • Page 11

    Input Output DelayFig. 19.5 A Typical Input output waveform for an On-Delay Timer I01O01O02 O01O01PR= DelayValueFig. 19.6 The realization of an On-delay timer from a general timer. Realization of on-delay timer: The realization of on-delay timer using the basic timer sh...

  • Page 12

    Input Output Delay Delay TimerStarts But resets DelFig. 19.7 A Typical Input output waveform for an Off-Delay Timer Realization of off-delay timer: The realization of on-delay timer using the basic timer shown in the previous fig is explained here. The realization is as shown in t...

  • Page 13

    same time and remains on for a fixed time then becomes ‘off’. The output pulse width is independent of input pulse width. Input Output Fig. 19.9 A Typical Input output waveform for a Fixed Width Timer I01 O01O02I01I01 I01 O03O02 PR= Pulse width Fig. 19.10 The realization of a ...

  • Page 14

    Reset T A BCInput ACC A + B + C = TFig. 19.11 A Typical Input output waveform for a Retentive Timer Non-retentive timer: The input and output signals of the non-retentive timer are as shown in the Fig. 19.11. This is implemented internally in a register. When the input is ‘on’, t...

  • Page 15

    Stores Current count Output coil Goes high When CR = PR Set terminalCount valvePreset Count Enable/ResetLogic Count Logic CR incremented by 1 every time Count logic goes high Counter Fig. 19.13 Structure of a Typical Counter User defined Data If the memory capacity of the flag are...

  • Page 16

    Numeric or Arithmetic functions Program control functions and Other statements Binary function combines primarily binary signal status with logic operations. The logic functions (binary logic operation) are the AND and the OR functions, according to the series and the parallel circuit arrange...

  • Page 17

    Communication tasks to supervisory computers for say trend updates can be implemented with clock-driven tasks. Point to Ponder: 2 A. What is the basic difference between Input and Auxiliary Contacts? Ans: Input contacts correspond to physical devices that can be asserted either by external ag...

  • Page 18

    Off Delay timer OP001 OP002 OP003 Off Delay Fixed Pulse Width I01 003 Fixed Pulse width Version 2 EE IIT, Kharagpur 18

x