PLC Hardware Components Introduction to PLC DMT 3533

PLC Hardware Components Introduction to PLC DMT 353/3

Seal-In Circuits A seal-in circuit is a method of maintaining current flow after a momentary switch has been pressed and released. Hardwired Seal-In Circuit Start Stop Motor starter coil M M Seal-in contact The seal-in auxiliary contact of the starter is connected in parallel with the Start button to keep the starter coil energized when the Start button is released.

Programmed Seal-In Circuit Ladder logic program Inputs L 1 Stop Start PB Start M Stop PB Motor starter coil (M) Output L 2 M Motor starter coil (M) Both the Start and Stop buttons are examined for a closed condition because both buttons must be closed to cause the motor starter to operate.

Latching Relay Latching relays are used where it is necessary for contacts to stay open and/or closed, even though the coil is energized momentarily. L 1 L 2 ON L OFF U PL Contact shown with relay in the unlatched position Electromagnetic latching relay circuit When the ON button is momentarily actuated, the latch coil is energized to set the relay to its latched position. The relay does not have to be continuously energized to hold the contact closed.

Programmed Latching Relay Instruction Command OTL OTU Name Symbol OTL sets the bit to “ 1” when the rung becomes true, and retains its state when the rung loses continuity or a power cycle occurs OUT resets the bit to “ 0” when the rung becomes true and retains it Output Latch Output Unlatch XXX Description Latch and unlatch coils have the same address

Latching Relay Program ON ON Latch rung OFF Unlatch rung Output rung 1 0 When the unlatch rung becomes true (OFF button actuated), When the ON button is momentarily actuated, the latch rung the status bit (10) is reset back to 0 and so the output is switched becomes true and the latch status bit (10) is set to 1, and so the off. output is switched on. This status bit will remain on (1) when logic continuity of the latch rung is lost.

PLC Water Level Program Pump running Low level High level The program for this process is designed to control the level of water in a storage tank by turning a discharge pump on and off. Modes of Operation High sensor switch Low sensor switch Pump motor Automatic Mode: Off Position: the pump will Manual Mode: the pump Status Indicating Lights: - if the level of water in the stop if it is running and will start if the water in the - water pump running light tank reaches a high point, the not start if it is stopped. tank is at any level except (green) water pump will start so that low. - low water level status light water can be removed from (red) the tank, thus lowering the - high water level status level light (yellow) - when the water level reaches a low point, the pump will stop

PLC Water Level Program OFF /Man Positions ( water at a low level)

PLC Water Level Program Manual Mode ( water between high and low levels)

PLC Water Level Program Automatic Mode ( water at high level)

Sequential Control Process Sequential controls are required for processes that demand certain operations to be performed in specific order. Filling operation Capping operation Bottle movement In this filling and capping operation, the tasks are: (1) fill bottle (2) press on cap These tasks must be performed in the proper order.

Combination Control Process Combination control processes require that operations be performed without regard to the order in which they are performed. Labeling process Label solenoid 1 Label solenoid 2 In this labeling operation, the tasks are: (1) place label 1 on bottle (2) place label 2 on bottle The order in which the tasks are preformed does not really matter.

Automatic Control Automatic control involves maintaining a desired setpoint. Automatic furnace temperature control Furnace PLC Touch screen interface In this operation the set-point is selected by means of the touch screen interface. If there is a deviation from that set point, an error is determined by comparing the output against the set point and using this error to make a correction. This requires some form of feedback from the output back to the control for the input.

Hardwired Sequential Process The sequential task is as follows: 1. Start button is pressed. 2. Table motor is started. 3. Package moves to the limit switch and stops. STOP RUN Auxiliary Features: Ø An emergency stop button that will stop the table, for any reason, before the package reaches the limit switch position Ø A red pilot light to indicate the table has stopped Ø A green pilot light to indicate the table is running Package Motor Limit switch (LS 1) Emergency STOP START

Hardwired Sequential Process Start PB 1 Emergency Stop PB 2 CR 1 1 LS 1 CR 1 -1 2 CR 1 -2 3 STOP RUN R G CR 1 -3 4 M CR 1 -4 OL Summary of the control task: • Start button is actuated. • CR 1 -1 closes to seal in CR 1 • CR 1 -2 opens, switching the red stop pilot light off • CR 1 -3 closes, switching the green run pilot light on • CR 1 -4 closes to energize the motor starter and motor • The package moves to the limit switch to actuate it and de -energize coil CR 1 • CR 1 -1 opens to open the sealin contact • CR 1 -2 closes, switching the red pilot light on • CR 1 -3 opens, switching the green pilot light off • CR 1 -4 opens to de-energize the starter coil, stop the motor, and end the sequence

Programmed Sequential Process

Writing a Program from a Narrative Description : A simple drilling operation requires the drill press to turn on only if there is a part present and the operator has one hand on each of the start switches. This precaution will ensure that the operator's hands are not in the way of the drill. Drill motor PB 1 PB 2 Switches Part sensor

Writing a Program from a Narrative Description : A simple drilling operation requires the drill press to turn on only if there is a part present and the operator has one hand on each of the start switches. This precaution will ensure that the operator's hands are not in the way of the drill. PB 1 and PB 2 and the part sensor must be activated to make the drill motor operate.

Motorized Door Program Description : A motorized overhead garage door is to be operated automatically to preset open and closed positions.

Motorized Door Program

Continuous Filling Operation Program Description : A continuous filling operation requires boxes moving on a conveyor to be automatically positioned and filled. Hooper Solenoid Motor Level switch Photo switch START STOP PL Run PL Standby PL Full

Continuous Filling Operation Program

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