Файл: Programmable logic controllers. Methods and Applications (Hackworth J., Prentice Hall).pdf
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Chapter 1 - Ladder Diagram Fundamentals
It is important to remember that many of the schematic symbols used in electrical diagrams are different than the symbols for the same types of components in electronic diagrams. Figure 1-14 shows the three most common relay symbols used in electrical machine diagrams. These three symbols are a normally open contact, normally closed contact and coil. Notice that the normally open contact on the left could easily be misconstrued by an electronic designer to be a capacitor. That is why it is important when working with electrical machines to mentally “shift gears” to think in terms of electrical symbols and not electronic symbols.
CR1 |
CR101 |
CR1 |
Figure 1-14 - Relay Symbols
Notice that the normally closed and normally open contacts of Figure 1-14 each have lines extending from both sides of the symbol. These are the connection lines which, on a real relay, would be the connection points for wires. The reader is invited to refer back to Figure 1-13 and identify the relationship between the normally open and normally closed contacts on the physical relay and their corresponding symbols in Figure 1-14.
The coil symbol shown in Figure 1-14 represents the coil of the relay we have been discussing. The coil, like the contacts, has two connection lines extending from either side. These represent the physical wire connections to the coil on the actual relay. Notice that the coil and contacts in the figure each have a reference designator label above the symbol. This label identifies the contact or coil within the ladder diagram. Coil CR1 is the coil of relay CR1. When coil CR1 is energized, all the normally open CR1 contacts will be closed and all the normally closed CR1 contacts will be open. Likewise, if coil CR1 is deenergized, all the normally open CR1 contacts will be open and all the normally closed CR1 contacts will be closed. Most coils and contacts we will use will be labeled as CR (CR is the abbreviation for “control relay”). A contact labeled CR indicates that it is associated with a relay coil. Each relay will have a specific number associated with it. The range of numbers used will depend upon the number of relays in the system.
Figure 1-15 shows the same relay symbols as in Figure 1-14, however, they have not been drawn graphically. Instead they are drawn using standard ASCII printer characters (hyphens, vertical bars, forward slashes, and parentheses). This is a common method used when the ladder diagram is generated by a computer on an older printer, or when it is desired to rapidly print the ladder diagram (ASCII characters print very quickly). This printing method is usually limited to ladder diagrams of PLC programs as we will see later. Machine electrical diagrams are rarely drawn using this method.
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Chapter 1 - Ladder Diagram Fundamentals
CR1 |
CR101 |
CR1 |
----| |---- |
----|/|---- |
----( )---- |
Figure 1-15 - ASCII Relay Symbols
Relays can range in size from extremely small reed relays in 14 pin DIP integrated circuit-style packages capable of switching a few tenths of an ampere at less than 100 volts to large contactors the size of a room capable of switching thousands of amperes at thousands of volts. However, for electrical machine diagrams, the schematic symbol for a relay is the same regardless of the relay’s size.
Time Delay Relays
It is possible to construct a relay with a built-in time delay device that causes the relay to either switch on after a time delay, or to switch off after a time delay. These types of relays are called time delay relays, or TDR’s. The schematic symbols for a TDR coil and contacts are the same as for a conventional relay, except that the coil symbol has the letters “TDR” or “TR” written inside, or next to the coil symbol. The relay itself looks similar to any other relay except that it has a control knob on it that allows the user to set the amount of time delay. There are two basic types of time delay relay. They are the delay-on timer, sometimes called a TON (pronounced Tee-On), and the delay off timer, sometimes called a TOF (pronounced Tee-Off). It is important to understand the difference between these relays in order to specify and apply them correctly.
Delay-On Timer (TON) Relay
When an on-timer is installed in a circuit, the user adjusts the control on the relay for the desired time delay. This time setting is called the preset. Figure 1-16 shows a timing diagram of a delay-on time delay relay. Notice on the top waveform that we are simply turning on power to the relay’s coil and some undetermined time later, turning it off (the amount of time that the coil is energized makes no difference to the operation of the relay). When the coil is energized, the internal timer in the relay begins running (this can be either a motor driven mechanical timer or an electronic timer). When the time value contained in the timer reaches the preset value, the relay energizes. When this happens, all normally open (N/O) contacts on the relay close and all normally closed (N/C) contacts on the relay open. Notice also that when power is removed from the relay coil, the contacts immediately return to their de-energized state, the timer is reset, and the relay is ready to begin timing again the next time power is applied. If power is applied to the coil and then switched off before the preset time is reached, the relay contacts never activate.
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Chapter 1 - Ladder Diagram Fundamentals
TDR |
||||||
On |
||||||
Coil |
||||||
Delay |
||||||
Power |
||||||
TDR
N/O
Contact
TDR
N/C
Contact
Figure 1-16 - Delay-On Timer Relay
Delay-on relays are useful for delaying turn-on events. For example, when the motor is started on a machine, a TON time delay relay can be used to disable all the other controls for a few seconds until the motor has had time to achieve running speed.
Delay-Off Timer (TOF) Relay
Figure 1-17 shows a timing diagram for a delay off timer. In this case, at the instant power is applied to the relay coil, the contacts activate - that is, the N/O contacts close, and the N/C contacts open. The time delay occurs when the relay is switched off. After power is removed from the relay coil, the contacts stay activated until the relay times-out. If the relay coil is re-energized before the relay times-out, the timer will reset, and the relay will remain energized until power is removed, at which time it will again begin the delay-off cycle.
TDR |
Off |
|||
Coil |
||||
Delay |
||||
Power |
||||
TDR
N/O
Contact
TDR
N/C
Contact
Figure 1-17 - Delay-Off Timer Relay
Delay-off time delay relays are excellent for applications requiring time to be “stretched”. As an example, it can be used to operate a fan that continues to cool the machine even after the machine has been stopped.
1-4. Fundamentals of Ladder Diagrams
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Chapter 1 - Ladder Diagram Fundamentals
Basic Diagram Framework
All electrical machine diagrams are drawn using a standard format. This format is called the ladder diagram. Beginning with the control transformer, we add a protective fuse on the left side. As mentioned earlier, in many cases the fuse is part of the transformer itself. From the transformer/fuse combination, horizontal lines are drawn to both sides and then drawn vertically down the page as shown in Figure 1-18. These vertical lines are called power rails or simply rails or uprights. The voltage difference between the two rails is equal to the transformer secondary voltage, so any component connected between the two rails will be powered.
H1 |
H3 |
H2 |
H4 |
|
F1 |
T1 |
|||
2 |
X1 |
X2 |
1 |
|
Figure 1-18 - Basic Control Circuit
Notice that the right side of the control transformer secondary is grounded to the frame of the machine (earth ground). The reason for this is that, without this ground, should the transformer short internally from primary to secondary, it could apply potentially lethal line voltages to the controls. With the ground, an internal transformer short will cause a fuse to blow or circuit breaker to trip farther “upstream” on the line voltage side of the transformer which will shutdown power to the controls.
Wiring
The wires are numbered. In our diagram, the left rail is wire number 2 and the right rail is wire number 1. When the system is constructed, the actual wires used to connect the components will have a label on each end (called a wire marker), as shown in
Figure 1-19, indicating the same wire number. This makes it easier to build, troubleshoot, and modify the circuitry. In addition, by using wire markers, all the wires will be identified, making it unnecessary to use more than one color wire to wire the system, which reduces the cost to construct the machine. Generally, control circuits are wired with all black, red,
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Chapter 1 - Ladder Diagram Fundamentals
or white wire (do not use green - it is reserved for safety ground wiring). Notice that in
Figure 1-18 the wire connecting T1 to F1 is not numbered. This is because in our design we will be using a transformer with the fuse block included. Therefore, this will be a permanent metal strap on the transformer and will not be a wire.
The wire generally used within the controls circuitry is AWG14 or AWG16 stranded copper, type MTW or THHN. MTW is an abbreviation for “machine tool wire” and THHN indicates thermoplastic heat-resistant nylon-coated. MTW has a single PVC insulation jacket and is used in applications where the wire will not be exposed to gas or oil.
It is less expensive, more flexible, and easier to route, bundle, and pull through conduits. THHN is used in areas where the wire may be exposed to gas or oil (such as hydraulically operated machines). It has a transparent, oil-
resistant nylon coating on the outside of the insulation. The drawback to THHN is that it is more expensive, is more difficult to route around corners, and because of its larger diameter, reduces the maximum number of conductors that can be pulled into tight places
(such as inside conduits). Since most control components use low currents, AWG14 or AWG16 wire is much larger than is needed. However, it is generally accepted for panel and controls wiring because the larger wire is tough, more flexible, easier to install, and can better withstand the constant vibration created by heavy machinery.
Reference Designators
For all electrical diagrams, every component is given a reference designator. This is a label assigned to the component so that it can be easily located. The reference designator for each component appears on the schematic diagram, the mechanical layout diagram, the parts list, and sometimes is even stamped on the actual component itself. The reference designator consists of an alphabetical prefix followed by a number. The prefix identifies what kind of part it is (control relay, transformer, limit switch, etc.), and the number indicates which particular part it is. Some of the most commonly used reference designator prefixes are as follows:
T |
transformer |
CR |
control relay |
R |
resistor |
C |
capacitor |
LS |
limit switch |
PB |
pushbutton |
S |
switch |
SS |
selector switch |
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Chapter 1 - Ladder Diagram Fundamentals
TDR or TR |
time delay relay |
M |
motor, or motor relay |
L |
indicator lamp or line phase |
F |
fuse |
CB |
circuit breaker |
OL |
overload switch or overload contact |
The number of the reference designator is assigned by the designer beginning with the number 1. For example, control relays are numbered CR1, CR2, etc, fuses are F1, F2, etc. and so on. It is generally a courtesy of the designer to state on the electrical drawing the “Last Used Reference Designators”. This is done so that anyone who is assigned the job of later modifying the machine will know where to “pick up” in the numbering scheme for any added components. For example, if the drawing stated “Last Used Reference Designators: CR15, T2, F3", then in a modification which adds a control relay, the added relay would be assigned the next sequential reference designator, CR16. This eliminates the possibility of skipping a number or having duplicate numbers. Also, if components are deleted as part of a modification, it is a courtesy to add a line of text to the drawing stating “Unused Reference Designators:” This prevents someone who is reading the drawing from wasting time searching for a component that no longer exists.
Some automation equipment and machine tool manufacturers use a reversed component numbering scheme that starts with the number and ends with the alphabetical designator. For example, instead of CR15, T2, and F3, the reference designators 15CR, 2T, and 3F are used.
The components in our diagram example shown in Figure 1-18 are numbered with reference designators. The transformer is T1 and the fuse is F1. Other components will be assigned reference designators as they are added to the diagram.
Boolean Logic and Relay Logic
Since the relays in a machine perform some type of control operation, it can be said that they perform a logical function. As with all logical functions, these control circuits must consist of the fundamental AND, OR, and INVERT logical operations. Relay coils, N/C
contacts, and N/O contacts can be wired to perform |
LAMP1 |
|
these same fundamental logical functions. By properly SWITCH1 |
||
SWITCH2 |
||
wiring relay contacts and coils together, we can create |
||
any logical function desired. |
AND
Generally when introducing a class to logical
operations, an instructor uses the analogy of a series Figure 1-20 - AND Lamp Circuit
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Chapter 1 - Ladder Diagram Fundamentals
connection of two switches, a lamp and a battery to illustrate the AND function. Relay logic allows this function to be represented this way. Figure 1-20 shows the actual wiring connection for two switches, a lamp and a battery in an AND configuration. The lamp,
LAMP1, will illuminate only when SWITCH1 AND SWITCH2 are ON. The Boolean expression for this is
Lamp1 = (Switch1) • (Switch2) |
(1-1) |
If we were to build this function using digital logic chips, the logic diagram for Equation 1-1 and Figure 1-20 would appears as shown in Figure 1-21. However, keep in mind that we will not be doing this for machine controls.
SWITCH1
LAMP1
SWITCH2
Figure 1-21 - AND Circuit
To represent the circuit of Figure 1-21 in ladder logic form in an electrical machine diagram, we would utilize the power from the rails and simply add the two switches (we have assumed these are to be pushbutton switches) and lamp in series between the rails as shown in Figure 1-22. This added circuit forms what is called a rung. The reason for the name “rung” is that as we add more circuitry to the diagram, it will begin to resemble a ladder with two uprights and many rungs.
H1 |
H3 |
H2 |
H4 |
|
F1 |
T1 |
|||
2 |
X1 |
X2 |
1 |
|
SWITCH1 |
SWITCH2 |
LAMP1 |
||
3 |
4 |
|||
PB1 |
PB2 |
L1 |
||
Figure 1-22 - Ladder Diagram |
||||
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