6.1 Explain the term program when referring to a PLC - NSC Electrical Technology Power Systems - Question 6 - 2017 - Paper 1

PLC stands for Programmable Logic Controller.

Three basic devices used in programming a PLC include:

1. User interface (on the PLC unit - screen & buttons).
2. Computer or laptop with interface cable.
3. Handheld programming device.

The circuit diagram can be represented as follows:

          Supply
            |\n         +--+
         |  |  (Switch A)
         +--+
            |
            +---- OR ----+ 
            |             |
         +--+           +--+
         |  |  (Switch B) 
         +--+           |  |  (Lamp)
            |          +--+
            +----------|
                       +-----+

This demonstrates how the lamp will light up if either switch A or B is closed.

The ladder logic diagram for an OR-gate can be illustrated as:

|----[ ]---------|
|    Switch A    |
|                |
|----[ ]---------|
|    Switch B    |
|                |
|----( )---------|
|    Output       |

This representation shows that if either Switch A or Switch B is activated, the output will be on.

Three programming languages commonly used in PLC programming include:

1. Ladder Logic (LL).
2. Instruction List (IL).
3. Structured Text (ST).

To simplify the Boolean expression:

1. Start with the original expression: [ A\overline{B}C + ABC + A\overline{B} + ABC\overline{A} ]
2. Combine like terms: [ A\overline{B}C + ABC + A\overline{B} + ABC(1) ]
3. Apply the Consensus Theorem: [ BC + A\overline{B} ]
4. Result: [ B(C + ar{A}) ]
5. Final simplification: [ B ]

The maintenance of a PLC system should be less than that of a relay system for several reasons. PLCs typically use fewer components than relay systems, which means there are fewer parts that can fail. Additionally, PLCs often have built-in diagnostic features that can alert users to faults or issues, making troubleshooting faster and easier.

The Boolean equation of the control system can be expressed as: [ F = A imes B + A imes C + B imes C ] This equation signifies that the output F will be true when at least two of the three sensors (A, B, C) detect a position.

Using a Karnaugh map to simplify the Boolean expression derived:

1. Place 1s in the K-map corresponding to the outputs from combinations of sensors:
   • Rows represent combinations of A, B, and C producing outputs.
2. Group the 1s in pairs or larger groups to reduce terms.
3. The simplified expression after grouping will yield: [ F = A imes B + A imes C + B imes C ]

The gate network for the simplified expression can be illustrated as:

       +---+       +---+
       | A |       | B |
       +---+       +---+
          |           |
          |           |
       +---+       +---+
       |   |-------|   |  
       | OR|       | AND|  
       |   |-------|   |  
       +---+       +---+  
           |         |     
           +---------+
           |   F     |
           +---------+

This diagram illustrates how the signals from sensors are combined using OR and AND gates to produce the output.