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

Chapter 1 - Ladder Diagram Fundamentals

is pressed, we have a current path through the pressed switch to the coil of TDR1. The time delay relay TDR1 begins to count time. As long as we hold either switch depressed,

TDR1 will time out in ½ second. When this happens, the N/C TDR1 contact in the first rung will open, and the rung will be disabled from energizing, which, in turn, prevents the machine from running. At this point, the only way the first rung can be enabled is to first reset the time delay relay by releasing both S1 and S2.

If S1 and S2 are both pressed within ½ second of each other, the TDR1 N/C contact in the first rung will have not yet opened and CR1 will be energized. When this happens, the N/O CR1 contact in the first rung seals across the TDR1 contact so that when the time delay relay TDR1 times out, the first rung will not be disabled. As long as we hold both palm switches on, CR1 will remain on and TDR1 will remain timed out.

If we momentarily release either of the palm switches, CR1 de-energizes. When this happens, we loose the sealing contact across the N/C TDR1 contact in the first rung. If we re-press the palm switch, CR1 will not re-energize because TDR1 is still timed out and is holding its N/C contact open in the first rung. The only way to get CR1 re-energized is to reset TDR1 by releasing both S1 and S2 and then pressing both again.

S2

Figure 1-32 - 2-Handed Operation with Anti-Tie Down and

Anti-Repeat

Single Cycle

When actuated, the machine must perform only one cycle and then stop, even if the operator is still depressing the RUN switches. This prevents surprises and possible injury for the operator if the machine should inadvertently go through a second cycle. Therefore,

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Chapter 1 - Ladder Diagram Fundamentals

because the contacts CR1 and CR3 are sealed by LS1A. Also, in the third rung, it has no immediate affect because LS1B is open which disables the entire rung. Eventually, the machine finishes it’s cycle and the cam switch rides into the cam detent. This causes LS1A to open and LS1B to close. In the third rung, since the N/C CR1 contact is closed (CR1 is off because S1 is released), closing LS1B switches on CR3. In the second rung, when LS1A opens CR2 de-energizes (because the N/O CR1 contact is open). This stops the machine and prevents it from beginning another cycle. The circuit is now back in it’s original state and ready for another cycle.

2. If the operator does not release the START switch before the machine finishes it’s cycle, CR1 remains energized. Eventually, the machine finishes it’s cycle and the cam switch rides into the cam detent. This causes LS1A to open and LS1B to close. In the third rung, the closing of LS1B has no effect because N/C CR1 is open. In rung 2, the opening of LS1A causes CR2 to de-energize, stopping the machine. Then, when the operator releases S1, CR1 turns off, and CR3 turns on. The circuit is now back in it’s original state and ready for another cycle.

There are some speed limitations to this circuit. First, if the machine cycles so quickly that the cam switch “flies” over the detent in the cam, the machine will cycle endlessly. One possible fix for this problem is to increase the width of the detent in the cam. However, if this fails to solve the problem, a non-mechanical switch mechanism must be used. Normally, the mechanical switch is replaced by an optical interrupter switch and the cam is replaced with a slotted disk. This will be covered in a later chapter. Secondly, if the machine has high inertia, it is possible that it may “coast” through the stop position. In this case, some type of electrically actuated braking system must be added that will quickly stop the machine when the brakes are applied. For our circuit, the brakes could be actuated by a N/C contact on CR2.

Combined Circuit

Figure 1-35 shows a single cycle circuit with the START switch replaced by the two rungs that perform the 2-handed, anti-tie down, and anti-repeat functions. In this circuit, when both palm switches are pressed within 0.5 second of each other, the machine will cycle once and stop, even if both palm switches remain pressed. Afterward, both palm switches must be released and pressed again in order to make the machine cycle again.

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Figure 1-35 - 2-Handed, Anti-Tie Down, Anti-Repeat, Single-

Cycle Circuit

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1-5. Machine Control Terminology

There are some words that are used in machine control systems that have special meanings. For safety purposes, the use of these words is explicit and can have no other meaning. They are generally used when naming control circuits, labeling switch positions on control panels, and describing modes of operation of the machine. A list of some of the more important of these terms appears below.

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2 HAND OPERATION

A control design method in which a machine will not RUN or CYCLE unless two separate buttons are simultaneously pressed. This is used on machines where it is dangerous to hand-feed the machine while it is cycling. The two buttons are positioned apart so that they both cannot be pressed by one arm (e.g., a hand and elbow). Both buttons must be released and pressed again to have the machine start another cycle.

1-6. Summary

Although this chapter gives the reader a basic understanding of conventional machine controls, it is not intended to be a comprehensive coverage of the subject. Expertise in the area of machine controls can best be achieved by actually practicing the trade under the guidance of experienced machine controls designers. However, an understanding of basic machine controls is the foundation needed to learn the programming language of Programmable Logic Controllers. As we will see in subsequent chapters, the programming language for PLCs is a graphic language that looks very much like machine control electrical diagrams.

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Chapter 1 - Ladder Diagram Fundamentals

Chapter 1 Review Questions

1.What is the purpose of the control transformer in machine control systems?

2.Whys are fuses necessary in controls circuits even though the power mains may already have circuit breakers?

3.What is the purpose of the shrouded pushbutton actuator?

4.Draw the electrical symbol for a two-position selector switch with one contact. The switch is named “ICE” and the selector positions are “CUBES” on the left and “CRUSHED” on the right. The contact is to be closed when the switch is in the “CUBES” position.

5.Draw an electrical diagram rung showing a N/O contact CR5 in series with a N/C contact CR11, operating a lamp L3.

6.A delay-on (TON) relay has a preset of 5.0 seconds. If the coil terminals are energized for 8 seconds, how long will its contacts be actuated.

7.If a delay-on (TON) relay with a preset of 5.0 seconds is energized for 3 seconds, explain how it reacts.

8.If a delay-off (TOF) relay with a preset of 5.0 seconds is energized for 1 second, explain how the relay reacts.

9.Draw a ladder diagram rung similar to Figure 1-30 that will cause a lamp L5 to illuminate when relay contacts CR1 is ON, CR2 is OFF, and CR3 is OFF.

10.Draw a ladder diagram rung similar to Figure 1-30 that will cause a lamp L7 to be OFF when relay CR2 is ON or when CR3 is OFF. L7 should be ON at all other times. (Hint: Make a table showing all the possible states of CR2 and CR3 and mark the combinations that cause L7 to be OFF. All those not marked must be the ones when L7 is ON.)

11.Draw a ladder diagram rung similar to Figure 1-30 that will cause relay CR10 to energize when either CR4 and CR5 are ON, or when CR4 is OFF and CR6 is ON. Then add a second rung that will cause lamp L3 to illuminate 4 seconds after CR10 energizes.

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