Create a ladder diagram for controlling a batch mixing process. Implement a PLC program for the mixing tank or Mixing Process using PLC Ladder Logic.
PLC Program for Mixing Tank
Fig : Mixing tank
A tank is used to mix two liquids.
The required control circuit operates as follows:
A. When the START button is pressed, solenoids A and B energize. This permits the two liquids to begin ﬁlling the tank.
B. When the tank is ﬁlled, the ﬂoat switch trips. This de-energizes solenoids A and B and starts the motor used to mix the liquids together.
C. The motor is permitted to run for 1 minute. After 1 minute has elapsed, the motor turns off and solenoid C energizes to drain the tank.
D. When the tank is empty, the ﬂoat switch de- energizes solenoid C.
E. A STOP button can be used to stop the process at any point.
F. If the motor becomes overloaded, the action of the entire circuit will stop.
G. Once the circuit has been energized, it will continue to operate until it is manually stopped.
PLC Logic Solution
A relay schematic that will perform the logic of this circuit is shown in Below Figure. The logic of this circuit is as follows:
Fig : Relay Schematic
A. When the START button is pushed, relay coil CR is energized. This causes all CR contacts to close. Contact CR-1 is a holding contact used to maintain the circuit to coil CR when the START button is released.
B. When contact CR-2 closes, a circuit is completed to solenoid coils A and B. This permits the two liquids that are to be mixed together to begin ﬁlling the tank.
C. As the tank ﬁlls, the ﬂoat rises until the ﬂoat switch is tripped. This causes the normally closed ﬂoat switch contact to open and the normally open con- tact to close.
D. When the normally closed ﬂoat switch opens, solenoid coils A and B de-energize and stop the ﬂow of the two liquids into the tank.
E. When the normally open contact closes, a circuit is completed to the coil of a motor starter and the coil of an on-delay timer. The motor is used to mix the two liquids together.
F. At the end of the one minute time period, all of the TR contacts change position. The normally closed TR-2 contact connected in series with the motor starter coil opens and stops the operation of the motor.
Normally open TR-3 contact closes and energizes solenoid coil C which permits liquid to begin draining from the tank. The normally closed TR-1 contact is used to assure that valves A and B cannot be re-energized until solenoid C de-energizes.
G. As liquid drains from the tank, the ﬂoat drops. When the ﬂoat drops far enough, the ﬂoat switch trips and its contacts return to their normal positions. When the normally open ﬂoat switch contact reopens and de-energizes coil TR, all TR contacts return to their normal positions.
H. When the normally open TR-3 contact reopens, solenoid C de-energizes and closes the drain valve. Contact TR-2 recloses, but the motor cannot restart because of the normally open ﬂoat switch contact.
When contact TR-1 re-closes, a circuit is completed to solenoids A and B. This permits the tank to begin reﬁlling, and the process starts over again.
I. If the STOP button or overload contact opens, coil CR de-energizes and all CR contacts open. This de- energizes the entire circuit.
Note : The PLC Program also will be very much similar to the above relay schematic.
Assignment For You
Analyse the below animation and share the circuit operation through comments.
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