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Figure 2.2: Horizontal ladder diagram
Figure 2.3: Wiring conventions
Control components are placed between two sides of the ladder with two symbols of wiring conditions — conductors connected and conductors not connected — as shown in Figure 2.3. Two wires with a dot in their intersection represent an electrical junction, whereas two wires crossing without a dot indicate no electrical junction (not electrically connected).
2.2.1Continuity
Continuity is defined as having power flowing from one rail to the other rail along a rung (Figure 2.4). All connecting elements in the rung must be closed to provide continuity. No continuity means the continuity in the rung is broken. It means that at least one of the connecting elements becomes open (Figure 2.5). No power can flow from one rail to the other rail in a rung that does not have continuity.
Figure 2.6 shows the ladder diagram of a control circuit for a one-shot cylinder reciprocation of a pneumatic circuit. The system consists of one relay coil (CR), one push button (PB), one limit switch (LS), two contacts (CR-1 and CR-2), and one solenoid (SOL).
Figure 2.4: Continuity
Figure 2.5: No continuity
Figure 2.6: Ladder diagram for a pneumatic circuit
The circuit shown in Figure 2.6 works as follows:
a.At rest, no continuity is provided to both rungs of the circuit. Both relay coil (CR) and solenoid (SOL) are de-energized. The cylinder in the pneumatic circuit is fully retracted.
b.When the push button (PB) is momentarily depressed, the continuity is provided in the first rung to energize the relay coil (CR) (Figure 2.7). The two contacts, CR-1 and CR-2, controlled by the relay coil CR change their state to ON. The contact CR-1 and limit switch LS now form the continuity for the first rung even when the push button PB is released. This power holding circuit continuously holds the power to energize the relay coil CR. The solenoid SOL in the second rung is continuously energized, which in turn extends the cylinder.
c.When the cylinder contacts the limit switch LS, the continuity in the first rung is broken to de-energize the relay coil CR. Both CR-1 and CR-2 change their state to OFF (Figure 2.8). The solenoid SOL is de-energized to shift the valve to its normal position, which in turn retracts the cylinder. The cycle stops when the cylinder is fully retracted.
Figure 2.7: Circuit states after PB is depressed
Figure 2.8: Circuit states after limit switch LS is actuated
2.2.2Series and Parallel Connection
The control elements can be arranged in two ways along a rung: series and parallel. Figure 2.9 is a series circuit in which elements are connected in the series pattern. A parallel circuit has more than one branch along a rung (Figure 2.10). The continuity can be formed in any one of the branches. In fact, the series connection is the AND function; the parallel connection is the OR function. It is possible to use a combination of parallel and series connections in the circuit (Figure 2.11).
Figure 2.9: Series connection
Figure 2.10: Parallel connection
Figure 2.11: Parallel-series connection
2.2.3Labeling
Adding labels to control elements and rungs in control circuits facilitates the reading of control circuits. Some general guidelines in labeling ladder diagrams (Figure 2.12) are:
a.Use the letter R and a number to represent each rung and its location in the circuit. The labels, R1 and R2, for example, represent the first rung and second rung in the control circuit.
b.Assign an abbreviated label to each control element. The label should reflect what the element type is. The label CR, for example, indicates a relay coil, and LS a limit switch. A number may be added to indicate that more than one element of the same type is used and its location of use. The labels, LS-1 and LS-2, indicate there are two limit switches and where they are located.
c.The labels of contacts must be the same as their controlling relay coils. This means that a relay coil and those contacts controlled by it must have the same label. When more than one contact that is controlled by a relay is used, add a number to the contact. For example, CR-1 and CR-2 represent two contacts controlled by relay CR.
Figure 2.12: Labeling control elements
2.3 Input Devices
Input devices in an electrical control circuit provide command signals (start, stop, etc.) and feedback signals (position, pressure, flow level, etc.) to be used as the basis for control decision making. Input devices generally include such discrete sensors as limit switches, push button switches, pressure switches, proximity switches, temperature switches, flow level switches, and photodetectors. One common characteristic of these input devices is their ability to generate an ON/OFF or Connected/Not Connected signal when devices are actuated or non-actuated in their normal states. For example, a normally open (NO) limit switch generates an ON (or Connected) signal when it is actuated by an actuator (Figure 2.13). These connected and not connected conditions determine electrical continuity status in the circuits.
The basic construction of a switch is made up of contacts and a conductive element, called a pole, which is used to provide continuity between the contacts, as shown in Figure 2.14. Switches can be categorized according to configuration, type, and method of actuation.
2.3.1Switch Configuration
The construction of switches varies according to the number of poles, the number of throws, and the pole-contact schemes used. The number of poles indicates the number of external conductors controlled by the switch. One set of contacts and one pole is required for each conductor. Switches may have one, two, or any number of poles, with one to three being the most common. Figure 2.15 illustrates single pole (SP), double pole (DP), and triple pole (TP) configurations.
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