Topic: design PLC program to realize liquid level control. There are two water pumps in the pool. The water pump 1 works alternately with the water pump 2 to keep the liquid level in the pool constant. Solution: the liquid level control can be controlled by water pump, and the alternate operation of water pump can be controlled by timer. The specific operation is as follows: 1. Connect the water pump 1 and the water pump 2 to the PLC output ports 1 and 2 respectively, and connect the liquid level sensor in the pool to the PLC input port. 2. Set two timers T 1 and T2 to control the working time of water pump 1 and water pump 2 respectively. After the T 1 timer starts, the water pump 1 starts, and when the time reaches the set value, the water pump 1 stops; After the T2 timer starts counting, the water pump 2 starts, and when the time reaches the set value, the water pump 2 stops. 3. Set a comparator to compare the relationship between the liquid level value detected by the liquid level sensor and the set value. If the liquid level is lower than the set value, start the water pump to work; If the liquid level is higher than the set value, stop the pump. 4. The structure of 4.PLC program is as follows: Step 1: Initialize the output state of water pump 1 and water pump 2, and the input state of liquid level sensor. Step 2: Start the cycle operation, including the following steps: Step 3: Start the timer T 1 to start timing, and detect the state of the liquid level sensor at the same time. Step 4: If the liquid level is lower than the set value, that is, the output signal of the liquid level sensor is low level, the output signal of the water pump 1 is set to high level to keep the water pump 1 working; If the liquid level is higher than the set value, the output signal of the water pump 1 is set to low level, and the water pump 1 stops running. Step 5: When the timer T 1 reaches the set time, the output signal of the water pump 1 is set to a low level to stop the operation of the water pump 1. Step 6: Start the timer T2 to start counting and detect the state of the liquid level sensor at the same time. Step 7: If the liquid level is lower than the set value, that is, the output signal of the liquid level sensor is low level, the output signal of the water pump 2 is set to high level to keep the water pump 2 working; If the liquid level is higher than the set value, the output signal of the water pump 2 is set to a low level to stop the operation of the water pump 2. Step 8: When the timer T2 reaches the set time, the output signal of the water pump 2 is set to a low level to stop the operation of the water pump 2. Step 9: Return to Step 3 and continue the cycle operation. 5.PLC program test: put the pool in the laboratory, connect the PLC, inject a certain amount of water, and start the PLC running program. When the water level is lower than the set value, the pump will start automatically, and when the water level is higher than the set value, the pump will stop automatically. By observing the state of water level float and liquid level sensor, the correctness of PLC program is determined.