Programmable controllers

In the 1960s, computers were considered by many in industry as the ultimate way of increasing efficiency, reliability, productivity, and automation of industrial processes. Computers possessed the ability to acquire and analyze data at extremely high speeds, make decisions, and then disseminate information back to the control process. However, there were disadvantages associated with computer control, such as high cost, complexity of programs, hesitancy on the part of industry personnel to rely on a machine, and lack of personnel trained in computer technology. Thus, computer applications through the 1960s were mostly in the area of data collection, on-line monitoring, and open-loop advisory control.

During the mid 1960s, though, a new concept of electronic controllers evolved, the programmable controllers (PCs). This concept developed from a mix of solid-state computer technology and traditional sequential controllers, such as the stepping drum (a mechanical rotating switching device) and the solid-state programmer with plug-in modules. This new device first came about as a result of problems faced by the auto industry, which had to scrap costly assembly line controls each time a new model went into production. The first PCs were installed in 1969 as electronic replacements of electromechanical relay controls. The PC presented the best compromise of existing relay ladder schematic techniques (a topic that will be discussed later in this chapter) and expanding solid-state technology. It increased the efficiency of the auto industry's system by eliminating the costly job of rewiring relay controls used in the assembly line process. The PC reduced the changeover downtime, increased flexibility, and considerably reduced the space requirements formerly used by the relay controls.

In this chapter, we will first discuss the basic concept of sequential control. Next, we will describe the programmable controller. We end the chapter with a description of a specific PC, the Allen-Bradley Bulletin 1772. The PC has gained wide acceptance in industry because of its ability to reconfigure processes economically and to be programmed easily. However, the 1772 is not the only PC on the market. Currently, there are about thirtyeight companies in the United States that manufacture PCs.

Sequential Control

Traditionally, industrial processes and control systems used relays, timers, and counters. These devices constitute a class of control systems used to control processes known as sequential control processes. A sequential process is a process in which one event follows another until the job is completed.

In this section, we first present an example of a sequential process. This process could be controlled either by the traditional electromechanical devices or by a PC.

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. In 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.

Programmable Controller

In 1978, the National Electrical Manufactures Association (NEMA) released a standard for programmable controllers. This standard was the result of four years of work by a committee made up of representatives from PC manufacturers. NEMA Standard ICS3 – 304, defines a programmable controller as "a digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various of machines or processes. A digital computer which is used to perform the functions of a programmable controller is considered to be within this scope. Excluded are drum and similar mechanical type sequencing controllers."

There is a tendency to confuse PCs with computers and programmable process controllers that are used for numeral control and for position control. Numeral and position control are used where a very large number of incremental positions are needed to complete a task. Examples are a lathe and a drilling machine. These tasks are not normally handled well by a PC, although that situation is changing.

What is the difference between a PC and a computer? To start with, all PCs are computers. The PC's block diagram structure is the same as that given in Chapter 11 for the computer. However, not all computers are PCs. The major differences that distinguish a PC from a computer are the PC's ability to operate in harsh environments, its different programming language, and its ease of troubleshooting and maintenance.

Programmable controllers are designed to operate in industrial environments that are dirty, are electrically noisy, have a wide fluctuation in temperatures (0° - 60°C), and have relative humidities of from 0 % to 95 %. Air conditioning, which is generally required for computers, is not required for PCs.

The PC's programming language has been, by popular demand, the ladder diagram with standard relay symbology. The reason for the ladder diagram's popularity is that plant personnel are very familiar with relay logic from their previous experience with sequential controls. There are other PC languages in use, however. One language involves Boolean statements relating logical inputs such as and, or, and invert to a single statement output. This language uses instructions such as AND, OR, LOAD, STORE, and so on. Thus type of language is very similar to computer assembly language.

The last major difference between PCs and computers is that of troubleshooting and maintenance. The PC can be maintained by the plant electrician or technician with minimal training. Most of the maintenance is done by replacing modules rather than components. Many times, the PC has a diagnostic program that assists the technician in locating bad modules. Computers, on the other hand, require highly trained electrionics specialists to maintain and troubleshoot them.