Файл: Programmable logic controllers. Methods and Applications (Hackworth J., Prentice Hall).pdf

Chapter 1 - Ladder Diagram Fundamentals

There are a few important details that have been added along with the switches and lamp. Note that the added wires have been assigned the wire numbers 3 and 4 and the added components have been assigned the reference designators PB1, PB2, and L1. Also note that the switches are on the left and the lamp is on the right. This is a standard convention when designing and drawing machine circuits. The controlling devices (in this case the switches) are always positioned on the left side of the rung, and the controlled devices (in this case the lamp) are always positioned on the right side of the rung. This wiring scheme is also done for safety reasons. Assume for example that we put the lamp on the left side and the switches on the right. Should there develop a short to ground in the wire from the lamp to the switches, the lamp would light without either of the switches being pressed. For a lamp to inadvertently light is not a serious problem, but assume that instead of a lamp, we had the coil of a relay that started the machine. This would mean that a short circuit would start the machine without any warning. By properly wiring the controlled device (called the load) on the right side, a short in the circuit will cause the fuse to blow when the rung is activated, thus de-energizing the machine controls and shutting down the machine.

For those more familiar with logic diagramming, the OR gate representation of the OR circuit in Figure 1-23 and Equation 1-2 is shown in Figure 1-24. Again, when drawing machine controls diagrams, we do not use this schematic representation.

SWITCH1

LAMP2

SWITCH2

Figure 1-24 - OR Circuit

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Chapter 1 - Ladder Diagram Fundamentals

We can now add this circuit to our ladder diagram as another rung as shown in

Figure 1-25. Note that since the switches SWITCH1 and SWITCH2 are the same ones used in the top rung, they will have the same names and the same reference designators when drawn in rung 2. This means that each of these two switches have two N/O contacts on the switch assembly. Some designers prefer to place dashed lines between the two PB1 switches and another between the two PB2 switches to clarify that they are operated by the same switch actuator (in this case the actuator is a pushbutton)

When we have two or more components in parallel in a rung, each parallel path is called a branch. In our diagram in Figure 1-25, rung two has two branches, one with PB1 and the other with PB2. It is possible to have branches on the load side of the rung also. For example, we could place another lamp in parallel with LAMP2 thereby creating a branch on the load side.

It is important to note that in our ladder diagram, it is possible to exchange rungs 1 and 2 without changing the way the lamps operate. This is one advantage of using ladder diagramming. The rungs can be arranged in any order without changing the way the machine operates. It allows the designer to compartmentalize and organize the control circuitry so that it is easier to understand and troubleshoot. However, keep in mind that, later in this text, when we begin PLC ladder programming, the rearranging of rungs is not

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Chapter 1 - Ladder Diagram Fundamentals

SWITCH1 and SWITCH2 and SWITCH3

SWITCH1 and SWITCH2 and SWITCH4

SWITCH1 and SWITCH3 and SWITCH4

SWITCH2 and SWITCH3 and SWITCH4

SWITCH1 and SWITCH2 and SWITCH3 and SWITCH4

Figure 1-27 - OR-AND Lamp Circuit

The Boolean expression for the OR-AND circuit is shown in Equation 1-4

These two rungs will now be added to our ladder diagram and are shown in Figure 1-28. Look closely at the circuit and follow the possible power paths to energize LAMP3 and LAMP4. You should see two possible paths for LAMP3:

SWITCH1 AND SWITCH2

SWITCH3 AND SWITCH4

Either of these paths will allow LAMP3 to energize. For LAMP4, you should see four possible paths:

SWITCH1 AND SWITCH3

SWITCH1 AND SWITCH4

SWITCH2 AND SWITCH3

SWITCH2 AND SWITCH4.

Any one of these four paths will energize LAMP4.

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Chapter 1 - Ladder Diagram Fundamentals

Now that we have completed a fundamental study of ladder diagram, we should begin investigating some standard ladder logic circuits that are commonly used on electric machinery. Keep in mind that these circuits are also used in programming programmable logic controllers.

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Chapter 1 - Ladder Diagram Fundamentals

Ground Test

Earlier, we drew a ladder diagram of some switch circuits which included the control transformer. We connected the right side of the transformer to ground (the frame of the machine). For safety reasons, it is necessary to occasionally test this ground to be sure that is it still connected because loss of the ground circuit will not affect the performance of the machine and will therefore go unnoticed. This test is done using a ground test circuit, and is shown in Figure 1-29.

Figure 1-29 - Ground Test Circuit

Notice that this rung is unusual in that it does not connect to the right rail. In this case, the right side fo the lamp L1 has a wire with a lug that is fastened to the frame of the machine under a screw. When the pushbutton S1 is pressed, the lamp L1 lights if there is a path for current to flow through the frame of the machine back to the X2 side of the control transformer. If the lamp fails to light, it is likely that the transformer is no longer grounded. The machine should not be operated until an electrician checks and repairs the problem. In some cases, the lamp L1 is located inside the pushbutton switch S1 (this is called an illuminated switch).

The Latch (with Sealing/Latching Contacts)

Occasionally, it is necessary to have a relay “latch” on so that if the device that activated the relay is switched off, the relay remains on. This is particularly useful for making a momentary pushbutton switch perform as if it were a maintained switch. Consider, for example, the pushbuttons that switch a machine on and off. This can be done with momentary pushbuttons if we include a relay in the circuit that is wired as a latch

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