Write a ladder logic program to control a heated mixing tank in a chemical process.

DESCRIPTION OF THE EQUIPMENT:

The equipment consists of a 500 gallon tank equipped with a single mixer and three heating elements. The mixer runs at 6 rpm. and has a sensor that detects rotation of the shaft. The rotation sensor becomes momentarily FALSE once each rotation. There is also a sensor that detects the level of liquid in the tank and another that detects the temperature of the fluid.

DESCRIPTION OF THE PROCESS:

No part of the process can operate until the sensor indicates that the fluid in the tank is at the correct level.

When the level is correct, the mixer starts. There is a watch-dog timer which indicates that the mixer is running.

The heaters come on only if the liquid level is correct, if the mixer is running, and the fluid temperature is below the set point. The heaters come on in sequence at 5 second intervals. If the fluid level falls below the correct level or if the mixer stops, the heaters immediately shut off.

When the sensor shows that the temperature is correct, the heaters turn off, but the mixer continues to run. The process runs for 10 seconds after the fluid reaches the correct temperature and then the entire system shuts off.

For this problem, you may assume that the tank will never overfill and the temperature will never go over the set point.

DESCRIPTION OF THE PROGRAM:

Use input X2 to simulate the fluid level sensor. When X2 is TRUE, the fluid level is correct. Turn on a Green light to indicate correct level. The mixer should start when the level is correct.

Use X3 to simulate the sensor on the mixer shaft. The watch dog must be correctly sensing the mixer rotation in order for the heaters to operate. Turn on a Yellow light to show that the mixer is running correctly (i.e. the watch dog is correctly sensing the rotation)

Use X1 to simulate the temperature sensor. When the sensor is TRUE, the temperature is below the set point. When the sensor becomes FALSE, the temperature is correct.

With the fluid level correct, the watch dog correctly sensing the mixer, and the temperature is not at the set point, the heater sequence should start. The first heater starts immediately; the second starts 5 seconds later; the third starts after another 5 seconds. Turn on a Red light for

heater #1; turn on a Yellow light for heater #2; turn on a Green light for heater #3. If the heaters are not on, the lights should be off.

When X1 is FALSE, the temperature is correct. Turn off the heaters when the temperature is correct (the lights should go off).

When the heaters turn off, keep the mixer running. Let the process run for 10 seconds after the heaters turn off; then turn everything off.

If the fluid level becomes incorrect while the process is running, the heaters and the mixer must turn off.

If the mixer stops, the heaters must turn off.

If you use any SET commands, be sure to RESET them appropriately. Remember that a bit remains SET until you explicitly RESET it.