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01 October 2003

PLCs are logical choice

The application of digital techniques to process control requires an understanding of digital system fundamentals.

In binary logic control systems, binary numbers 1 and 0 are represented by voltage level, relay contact status, switch position, and other elements. For example, in transistor-transistor logic gates, a binary 1 equals a voltage signal in the range of 2.4 to 5.0 volts, and a voltage level between 0 and 0.8 volts represents a binary 0.

Electronic circuits can manipulate digital signals to perform a variety of logical functions such as NOT, AND, OR, and others. In hardwired electrical logic systems, relays implement logic functions.

An important logic function is the NOT or inversion function. It produces an output opposite to the input. Thus, in a binary system an input of a 0 has the output 1, and the input 1 has the output 0.

An OR function, with two or more inputs and a single output, operates in accordance with the following definition: The output of an OR function assumes the 1 state if one or more inputs assume the 1 state.

An AND function has two or more inputs and a single output, and it operates in accordance with the following rule: The output of an AND gate assumes the 1 state if and only if all the inputs assume the 1 state.

There are two common sets of symbols used in process control applications to represent logic function: graphic symbols and ladder symbols.

The graphic symbols are generally used on engineering drawings to convey the overall logic plan for a discrete or batch control system, and they are based on ANSI/ISA 55.2 Binary Logic Diagrams for Process Operations.

The ladder logic symbols describe a logic control plan if relays are implementing the control system or if programmable controller ladder logic is running the control strategy.

Figure one compares these two sets of symbols for the most common logic functions encountered in logic control design.

LADDER LOGIC UTILITY

Ladder diagrams are a traditional method used to describe electrical logic controls. These circuits are ladder diagrams, because they look like ladders with rungs. Each rung of a ladder has a number so that we can easily cross-reference between sections on the drawing that describes the control scheme.

We can appreciate the utility of ladder logic diagrams by investigating a simple control example. Figure two shows a typical process level control application.

In this application, let us assume that the flow into the tank is random, and we need to control the level in the tank by opening or closing the on/off electric solenoid valve (LV-1) based on the level sensed in the tank by a level switch (LSH-1).

We will also provide the operator with a three-position hand, off, and automatic (HOA) switch to manually turn the valve on or off, and the option to select automatic control using the level switch to maintain the proper level in the tank.

The ladder logic design for this application is in figure three. If the HOA switch is in the automatic position and the level switch closes, the solenoid energizes.

This is a simple example of a logical AND function in process control. If the HOA switch is in the hand or manual position, the valve will also turn on.

If we designate the logic variables as follows, A for hand position, B for automatic position, C for level switch LSH-1, and Z for the solenoid valve LV-1, then the logic equation for the control system is Z = A + BC.

BOIL DOWN LIQUID CONCENTRATE

A more complex application might be to control tank level between two level switches, a level switch high (LSH) and a level switch low (LSL). In this application, a pump supplying fluid to a tank is turned on and off to maintain the liquid level in the tank between the two level switches.

The process is shown in figure four, and it uses steam at a regulated flow to boil down a liquid to produce a more concentrated solution, which is drained off periodically by opening a manual valve on the bottom of the tank.

Note that a small diamond symbol indicates that the pump interlocks with the level switches on the tank.

SYSTEM WILL CYCLE ON AND OFF

The electrical ladder diagram used to control the feed pump and hence the liquid level in the process tank is in figure five. To explain the logic of the control system, we will assume the tank is empty, the low level switch is closed, and the high level switch is closed.

When a level switch activates, the normally open contacts close and the normally closed contacts open. To start the control system, the operator depresses the start push button (PB1).

This energizes the control relay (CR1), which seals in the start push button with the first set of contacts, denoted as CR1 (1) on the ladder diagram.

At the same time, the second control relay (CR2) is energized in rung 3 through contacts on LSH-1, LSL-1, and CR1.

This turns on the pump starter relay Kl. The first set of contacts on CR2 is used to seal in the low level switch contacts, so when the level in the tank rises above the position of the low level switch on the tank, the pump will stay on until the liquid level reaches the high level switch.

After the high level switch is activated, the pump will be turned off. The system will now cycle on and off between the high and low levels until the operator depresses the stop push button PB2.

This pump control application is a typical example of logic control in the process industries. The logic control system can implement using a hardwired relay based logic system, a programmable logic controller, or a distributed control system. IT

Nicholas Sheble (nsheble@isa.org) writes and edits the Control Fundamentals department. The source for this piece is Thomas A. Hughes' book Programmable Controllers, ISA. Hughes is a senior member of ISA and has over twenty-five years experience in instrumentation and control systems design. He has written another book also published by ISA, Measurement and Control Basics.