Programmable Logic Controllers

Learning Outcome
Upon successful completion of this module, the student will be able to describe the major components of programmable logic controllers (PLCs), their basic functions, and the types of tasks they perform.
Enabling Objectives
The student will be able to:
(a) Define the basic PLC applications.
(b) Define discrete-state process control.
(c) Define the sequence of events.
(d) Define flow charts in terms of the sequence of events.
(e) Define ladder logic as applied to control the sequence of events.
(f) State how to program the PLC using ladder logic.

INTRODUCTION
The majority of industrial process control installations involve more than simply regulating a dynamic variable, such as the pressure in a vessel or the level in a tank These simple control situations have applications in industry and are referred to as continuous control. There also are a great many processes in industry in which it is not a variable that has to be controlled, but a sequence of events. For example, a certain startup sequence must be followed before a boiler can run continuously. This startup sequence is made up of many discrete states, each discrete state specifying a condition of the process. An example would be the sequence for purging a boiler: dampers fully open, gas valve off, fan running, and so on.
Programmable logic controllers (PLCs) were originally designed to control processes that required a sequence of events to be followed. Today, PLCs have advanced to the stage of being incorporated in SCADA (Supervisory Control and Data Acquisition) systems, continuous control systems, and the like. However, for an introductory look into the operations of a PLC, this module will concentrate on discrete control, using PLCs to control a sequence of events.

COMPONENTS OF A PROGRAMMABLE LOGIC CONTROLLER
For the purposes of this discussion, the major components of a typical PLC will be broken down as follows:

1. Definition of inputs/outputs.
2. Sequence of events.
3. Flowcharts of the event sequence.
4. Ladder logic.
5. Programming a PLC.

Inputs/Outputs
PLCs are capable of inputting information from the process, and based upon theinformation at a particular point in time, generating a control signal. Fig. 1 shows a block diagram of the I/O (inputs/outputs) of a typical PLC.

Figure 1: Inputs/Outputs of Typlical PLC

The following terms are commonly used in the industry:
DIs - Discrete inputs are signals which have only two positions; for example, open/closed or on/off.
Als - Analog inputs are ever-changing signals, such as temperature. The temperature can be any value between its upper and lower range.
DOs - Discrete outputs are the control signals for devices which have only two positions; for example, to turn a pump on or off.
AOs - Analog outputs are ever-changing signals which control the process. An example is the control of a valve to perhaps 30% flow, or some other value between its upper and lower range.
It should be noted that the remainder of this module concentrates on the use of DIs and DOs. Other I/0s are also available on some PLCs, but the I/0s summarized in Fig. 1 make up the majority of PLC points.

Sequence of Events

Fig. 2 shows a schematic diagram of a simple boiler, illustrating the points to be monitored and controlled.

Figure 2: Boiler Schematic

LS - Level switch low(drum level low)
GH - Gas high pressure
GL - Gas low pressure
DS - Damper fully open (proof)
FS - Flame status
IC - Ignition control
The startup sequence for a boiler of this type is as follows:

1. Receive the start signal.
2. Check gas pressure high and low; if high or low, abort.
3. Check drum level; if low, abort.
4. Open dampers fully, if they fail to open, then alarm.
5. Start combustion fan.
6. Wait 30 seconds (the time could vary).
7. Position dampers to minimum fire position.
8. Start ignition
9. Open gas pilot valve.
10. Check for flame. If flame does not establish itself within 6 seconds, alarm and proceed with post purge.
11. Everything is normal; the pilot is established and the boiler can operate under another control system, such as modulating control.

Fig. 3 is a block diagram of a typical PLC, showing the same I/O points.

Figure 3: A Typical PLC

These I/O points are hardwired into the PLC and, under static conditions, do not change. The sequence of events listed has to be entered into the PLC, so that the PLC can control the boiler. Flow charts may be used in order to define the sequence of events into a logical flow.
Flowcharts
Flow charts are used to describe the sequence of events, to eliminate the possibility of ambiguities, and to more precisely document these sequences. Flow charts basically consist of three symbols, as shown in Fig. 4.

Figure 4: Three Symbols

 Although there are many sophisticated types of flowcharts and symbols, the symbols listed in Fig. 4 are adequate to describe the boiler example covered in this module. A suitable flow chart describing the boiler’s sequence of operations is shown in Fig. 5. This flow chart clearly defines the sequence of operations as started in the English language description. This flow chart now has to be described in terms that the PLC can understand, namely ladder logic.

Figure 5: Flow Chart of Boiler Startup

Ladder Logic
Ladder logic, or ladder diagrams, are used extensively when programming PLCs, but these are not the only method of implementing control sequences. Since it is so widely accepted, some of the simpler tasks performed by ladder logic will be described. The symbols that will be used to implement the boiler example are listed in Fig. 6.

Figure 6: Symbols

Rather than discuss the design methods used in developing ladder logic circuits, we will look at a suitable ladder logic diagram which could be used to control the boiler example. It is not intended that students understand the ladder logic completely, but rather that the student be exposed to the concepts of ladder logic. A suitable ladder logic diagram for the boiler control is shown in Fig. 7.

Figure 7: Ladder Logic Diagram

Programming and Using a PLC
Now that the sequence of events has been defined in terms of ladder logic, this has to be transferred into a PLC. Fig. 8 shows the connection of a PLC programmer to a PLC. A PLC programmer is a device which connects to the PLC so that someone can enter the ladder logic directly into the PLC.
Various forms of PLC programmers exist, but the point to note is that some physical device must be used to configure or program the PLC to carry out the proper sequence of events. The sequence can be altered any time a PLC programmer is connected. Normally, however, after the ladder logic has been entered, the PLC programmer is disconnected and the PLC is left to operate the process. Hence the PLC now becomes a dedicated controller. Under normal conditions, the PLC operates the process (in this case, the boiler) from the I/O points and the boiler start switch. PLCs are not limited to controlling a single process but can, if need be, control several processes at a time.

Figure 8: Connection of a PLC Programmer to a PLC