This start switch shall be easily identified. The second operation step may be something like, “Depress start push button PS”. Analyze what will happen to each control element in the system when the start signal is given, and record their state in their corresponding blocks. Study both the electrical circuit and the hydraulic circuit together to determine what state may change to other control elements when the state of one element has changed. In this example, when the start push button is momentarily depressed, relay R is energized, which in turn makes two R contacts change their state to close. In turn, this causes the relay S in the second rung to be energized. Contact S in the second rung becomes closed. As a result of the solenoid being energized in the third rung, the directional valve is shifted to the left position to extend the cylinder. While the cylinder is extending, the limit switch LS1 is released to return to its normally open state. The state of each element after momentarily pressing the start push button PS is tabulated in the second row of Figure 2.36.
Figure 2.35: Element states at rest
Figure 2.36: Complete state chart for the system
f.Determine which input device causes the next operation step.
Identify what conditions cause the state of change for an output. The span between the start of an operation to the next operation is referred to as transition. During a transition, some input devices may change their state, yet they do not cause the change in any output. The state changes in the input devices during a transition can be ignored. Those input state changes that cause the output state change are referred to as significant switching states. We need to carefully examine the physical system and ladder diagram to identify the significant switching states for the next operation.
In this sample project, the next significant switching state is the cylinder making contact with limit switch LS2. When LS2 is actuated, the power holding in the second rung of the circuit is broken to de-energize the relay S. This causes the solenoid to be de-energized, contacts S-1 and S-2 to change back to open, and the cylinder to retract. The state of each element at this step is shown in the third row of Figure 2.36.
g.Repeat step f above to successively identify the significant switching states for the next operations.
Use the same procedure in step f to find the significant switching states for the next operation. Repeat the same procedure until the operation cycle is completed. In the sample circuit, the next significant step will be the cylinder touches the limit switch LS1 to make it closed. This provides a continuity to energize the relay S, which in turn energizes the solenoid to extend the cylinder. The state of each element in this step is given in the fourth row of Figure 2.36.
The system operation will be repeated continuously between steps c and d. No further analysis is needed until momentarily pressing the stop button PT. This is the last step of the system operation. Press PT to break the power holding in the first rung for system control relay R. The two contacts R-1 and R-2 return to their normally open state. The continuity in rung 2 is also broken to de-energize the solenoid relay S to cause the cylinder to retract. The operation cycle ends when the cylinder is fully retracted. The last row of Figure 2.36 shows the state of each element when the PT is momentarily pressed.
h.Give a complete description of the control system.
After the analysis cycle is completed, we will be able to fully describe the control system and explain its operation in a concise way. “Continuous reciprocation of a hydraulic cylinder between two limit switches” is a good example of giving a concise description of this control system.
Review Questions
1. What are the three types of control elements used in electrical control circuits?
2. What is a ladder diagram?
3. Use drawings to illustrate the wiring conventions for conductors connected and conductors not connected.
4. How do you define the continuity along a control rung in relay diagrams?
5. Give some general guidelines for labeling ladder diagrams.
6. What are the two ways control elements can be arranged along a rung in the control circuits?
7. Use a drawing to illustrate the series connection.
8. Use a drawing to illustrate the parallel connection.
9. Explain how series and parallel connection are related to the AND and the OR function.
10. Describe the purpose of input devices.
11. What is the common characteristic of discrete input devices?
12. Explain the basic construction of a switch.
13. Draw the symbol for a single pole-single throw (SPST) switch.
14. Draw the symbol for a single pole-double throw (SPDT) switch.
15. Draw the symbol for a double pole-double throw (DPDT) switch.
16. Describe how to determine the number of poles and throws, respectively.
17. Explain what normally open (NO) and normally closed (NC) mean.
18. Draw the schematic symbols for the following switches:
a. limit switch (NO)
b. limit switch (NC)
c. pressure switch (NO)
d. pressure switch (NC)
e. push button (NO)
f. push button (NC)
g. selector switch (4 positions)
h. temperature switch (NO)
i. temperature switch (NC)
j. liquid level switch (NO)
k. liquid level switch (NC)
l. proximity switch (NO)
m. proximity switch (NC)
19. What is a logic element?
20. Use a drawing to help describing the operation principle of an electromechanical relay.
21. Draw the schematic symbol for an electromechanical relay and their contacts (NO and NC).
22. Describe the purpose of using a timer.
23. What are the two main types of timers?
24. Explain the operation principle of an on-delay timer.
25. Draw the schematic symbols for on-delay timers and their contacts.
26. Describe the operation principle of an off-delay timer.
27. Draw the schematic symbols for off-delay timers and their contacts.
28.
