Automatic mixer

The tank in the diagram is filled with a fluid, agitated for a length of time, and then emptied. A state description is similar to a flowchart in computer programming. This sequential process is the kind of process that can easily be handled by a programmable controller.

A ladder diagram is a diagram with a vertical line (the power line) on each side. All the components are placed between these two lines, connecting the two power lines with what look like rungs of a ladder - thus the name, ladder diagram. The letter symbols in the diagram are defined in succeeding paragraphs.

Relay ladder diagrams are universally understood in industry, whether in the process industry, in manufacturing, on the assembly line, or inside electric appliances and products. Any new product increases its chances of success if it capitalizes on widely held concepts. Thus, the PC's ladder diagram language was a logical choice.

Some mention should be made at this point about electrical and electronics symbol designations, hi general, there is a difference between electrical and electronics symbols and symbol designations. These two industries grew up somewhat independent of each other, and therefore differences exist. For example, the electronics symbol for a resistor is a zigzag line with a symbol designation of ri. The same symbol in the electrical or industrial world is a rectangle with lines out the ends and with a symbol designation of 1R. These differences can sometimes be confusing. In this chapter, we will use the industrial symbols and symbol designations because the programmable controller developed as an industrial machine.

Now, let us follow the series of events for the full control cycle the automatic mixer process. At the start of the process, the start push button (1PB) is pressed. The start button energizes a control relay (1CR) located in the start/stop switch box.

It is located in the first line, or rung, of the ladder diagram. They are shown in the normally open position (abbreviated NO). The same symbol with a slash drawn through it represents the normally closed (NC) relay contact.

When the relay (1CR) is energized (or pulled in or picked up), these relay contacts change state; in this case, they close. When the 1CR contact under the 1PB switch closes, it allows current to continue through the coil of the 1CR relay, even though the start push button (1PB) is released. This circuit holds the 1CR relay in as long as the power line power is applied, the stop button (2PB) is not pushed, and the timing relay (1TR) has not timed out.

Another 1CR contact is located in the second rung of the ladder diagram. When this 1CR closes, current can flow through solenoid A. Solenoid A is an electromechanical device that is electrically activated to mechanically open a valve, which allows fluid to flow into the tank. Fluid flows because the float switch (1FS) in rung 2 is closed.

When the tank has filled, the float switch (1FS) changes to the filled position. This change de-energizes solenoid A, starts the timer relay, and operates the mixer solenoid (MS).

After the timer has timed out, relay 1TR switches off the mixer and energizer solenoid B, which empties the tank. When the tank is empty, float switch (1FS) shuts off solenoid B and places the system in the ready position for next manual start.

Notice that pressing the start switch (1PB) again one the cycle has started will have no adverse effect on the cycle. This protective logic should be designed into all processes, whether a PC or a computer is used.