Figure 1-10.-AND symbol and circuit.

symbol is shown, which can be compared to the

starting and running magnetic contractors, an auto

transformer, a thermal overload relay, and a mercury

electrical circuit in figure 1-10, view B.

timer to control the duration of the starting cycle.

NOTE: Both switches, A AND B, must be closed

to energize the lamp.

Logic Controllers

In figure 1-11, view A, art OR symbol is shown,

Some of the controlled equipment that you will

which can be compared to the electrical circuit in fig-

encounter use logic systems for circuit control. For

ure 1-11, view B, where either switch A OR B needs to

additional information in this area the Navy Electricity

be closed to energize the lamp.

and Electronics Training Series (NEETS), module 13,

Using the characteristics of the AND and OR logic

is an excellent basic reference.

symbols, we will now discuss how they can be used in

The basic concept of logic circuits is shown in

a logic controller.

figures 1-10 and 1-11. In figure 1-10, view A, an AND

One common application of logic control that is

being incorporated on newer ships is the elevator

system. Since this system is large and consists of many

symbols, we will show only a small portion of this

system.

Let us assume that the elevator platform is on the

third deck and that you require it on the main deck--Refer

to figure 1-12. Three conditions must be met before the

elevator can be safely moved. These conditions are

detected by electronic sensors usually associated with

the driven component. One of the conditions is that the

platform must be on EITHER the second or third deck

(on a certain deck as opposed to somewhere in between).

If this condition is sensed, the OR symbol will have an

input, and since only one input is needed, the OR symbol

also will have an output.

Figure 1-10.-AND symbol and circuit.

The other two conditions to be met are that the

locking devices must be engaged and the access doors

must be shut. If the sensors are energized for these two

conditions, the AND symbol will have the three inputs

necessary to produce an output. This output will then set

up a starting circuit, allowing the motor to be started at

your final command.

The advantages of these electronic switches over

mechanical switches are low power consumption, no

moving parts, less maintenance, quicker response, and

Figure 1-11.-OR symbol and circuit.

Figure 1-12.-Basic logic circuit.

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