Use PLC Automation Manufacturing System 2.1.1 ladder Logic

2.1.1 ladder Logic
2.1.1 trapezoid Logic

 

Ladder logic is the main programming method used for PLCs. as mentioned before, ladder logic has been developed to mimic relay logic. the describe to use the relay logic diagrams was a strategic one. by selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly partitioned CED.

Modern Control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in figure 2.1. when a voltage is applied to the input coil, the resulting current creates a magnetic field. the magnetic field pulls a metal switch (or Reed) towards it and the contacts touch, closing the switch. the contact that closes when the coil is energized is called normally open. the normally closed contacts touch when the input coil is not energized. relays are normally drawn in schematic form using a circle to represent the input coil. the output contacts are shown with two parallel lines. normally open contacts are shown as two lines, and will be open (non-conducting cting) when the input is not energized. normally Closed Contacts are shown with two lines with a diagonal line through them. when the input coil is not energized the normally closed contacts will be closed (conducting ).

Trapezoid logic is the main programming method used by PLC. As mentioned above, the development of trapezoid logic is imitating the logic of relays. The logic diagram of the used relay is one of the strategies. By selecting the trapezoid sequence as the main programming method, the funds required for training engineers and technicians are greatly reduced.

Modern Control systems still include relays, but they are rarely used in logical control. A relay is a simple device that uses a magnetic field to control the switch, as shown in figure 2.1. When the voltage is applied to the input coil, the generated current generates a magnetic field. Magnetic Field pulls a metal switch (or Reed), electric shock closes, turn off the switch. The contact is closed when the coil is charged. Closed contact closed when the input coil is not powered. In the circuit diagram, the relay usually uses a circle to represent the input coil. The output contact is expressed in two parallel lines. When the input coil is not powered on, the regular open contact is displayed as two vertical lines and will open (not conductive ). Normally Closed Contacts are displayed as diagonal lines through two vertical bars. When the input coil fails, the normally closed contact will be closed (conductive ).

 

 

Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. an example of a relay in a simple control application is shown in Figure 2.2. in this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input. the second relay is normally open and will not allow current to flow until a voltage is applied to the input B. if current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for Output C. this circuit wowould normally be drawn in the ladder logic form. this can be read logically as C will be on if
Is Off and B is on.

Relays are used to hold one power off the other (usually high current) while keeping them isolated. A simple example of a relay control program is shown in Figure 2.2. In this system, the first relay on the left uses a normally closed point, allowing the current to flow when the voltage is added to the input point. The second relay is normally open. When the voltage is added to input B, the current flow is not allowed. If the current flows through the first two relays, the current will flow through the coil of the third relay and turn off the switch that outputs C. This circuit is usually drawn as a trapezoid logic form. This can be interpreted as C on if A is off and B on.

 

The example in Figure 2.2 does not show the entire control system, but only the logic. when we consider a PLC there are inputs, outputs, and the logic. figure 2.3 shows a more complete representation of the PLC. here there are two inputs from push buttons. we can imagine the inputs as activating 24 v dc relay coils in the PLC. this in turn drives an output relay that switches 115 v ac, that will turn on a light. note, in actual PLCs inputs are never relays, but outputs are often relays. the ladder logic in the PLC is actually a computer program that the user can enter and change. notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. do not think that the ladder logic in the PLC needs to match the inputs or outputs. many beginners will get caught trying to make the ladder logic match the input types.

The example in Figure 2.2 does not show the entire control system, but only the logic. When we consider that a PLC should have input, output, and logic. Figure 2.3 shows a more comprehensive PLC representation. There are two buttons in the input format. We can imagine that the input is a 24 v dc Drive PLC relay coil. This in turn drives another output relay and switches on and off the 115 v ac, which will turn on a lamp. Please note that the input in the actual PLC system never uses relays, but the output is often relays. The PLC ladder diagram logic is actually a computer program that users can input and change. Note that the push buttons of the two inputs are always open, but the logic of the PLC ladder diagram has a regular open contact and a regular closed contact. Do not consider that the logic in the PLC ladder diagram must match the actual input or output. Many beginners will try to make the ladder map logic match the actual input type.

 

Returns relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. the circuit shown in Figure 2.4 is an example of this, it is called a seal in circuit. in this circuit the current can flow through either branch of the circuit, through the contacts labeled A or B. the input B will only be on when the output B
Is on. if B is off, and a is energized, then B will turn on. if B turns on then the input B will turn on, and keep output B on even if input a goes off. after B is turned on the output B will not turn off.

Many relays also have multiple outputs (switches), which enables an output relay to be simultaneously input. The circuit shown in Figure 2.4 is an example, which is called the keep circuit. The current in this circuit flows through any branch and is labeled as a contact or B contact. When the output end is on, input end B can only be on. If B is disabled and A is powered on, B is turned on. If B is enabled, input B is enabled, and output B is on even if input a is disabled. After on, output B will not be closed.