Back to Basics

In the cab of an 1800’s steam locomotive, the steam pressure gauges and water supply indicators were the crew’s “Operator Interface.” These OI devices linked the crew to the status of the boiler and enabled them to make decisions.

A Little History

In the 20th century, dials, gauges, status lamps, and chart recorders became available to help plant personnel check status and make decisions in industrial manufacturing operations. Pushbuttons, switches, and analog controls let technicians supply inputs to modify and control events.

Figure 1. Operator Interfaces: Then and Now

Eventually, as computer and Programmable Logic Controller (PLC) technology became available, great volumes of information could be observed and manipulated by operators. Because unpredictable events can occur, humans continue to be an important part of plant floor control and monitoring.

However, the volume of available information has sometimes led to unmanageable “forests” of hardwired display and entry devices on Operator Interface panels. (See Figure 2.)

Figure 2. A Hardwired Panel

The Role of Plant Floor “Operators”

Plant floor personnel interact with hard-wired devices on control panels as well as with electronic operator interface devices. Both need to support the same kinds of functions. Examples include:

To be effective, OI devices must provide efficient and flexible methods to support these types of OI functions.

OI Technology Before the 1980s

In the 1970s and 1980s, the Programmable Logic Controller became the dominant plant-floor electronic tool for control, sequencing, and data storage. Conventional hardwired, panel-mounted operator interface devices were connected as PLC inputs (pushbuttons, thumbwheels, etc.) and outputs (lamps, dials, numeric and text readouts). (See Figure 3.)

Figure 3. Typical Pre-1980 OI Panel

Figure 4 shows that, with a conventional control panel, every single panel device consumes input/output ( I/O) capacity of the PLC . This becomes a three-part burden on the PLC, that of input, output, and processing:

Figure 4. Burdens on the PLC

The end result is that the devices on a conventional hardwired panel use up much of the PLC that could otherwise be used for the control and monitoring of real devices on a machine or in the plant.

There are further consequences to adding several dozen hardwired operator interface devices to a panel. They:

The Electronic Operator Interface

As mentioned earlier, electronic operator interfaces typically feature some type of luminous display, and often include a method for operators to perform inputs and entries.

Electronic OIs can be as small as a paperback book, with focused functionality, and can sell for under $500.

They can also feature displays as large—or larger—than 15” diagonal, perform a multitude of functions, and sell for upwards of $10,000.

Figure 5 shows a typical modern Electronic OI. Notice there is no keyboard or mouse: operators touch the screen icons to input data.

Figure 5. Typical Modern Electronic OI

Electronic OI not only solves some of the input, output, and processing problems of conventional wiring; it enhances the “usability” of the system, and extends the information across networks.

Let's contrast this diagram of electronic OI connectivity to the PLC (see Figure 6) with the conventional control panel described in the previous section.

Figure 6. Single-Wire Connection from Connection OI to PLC

Although the design details of the electronic OI and how it communicates to the PLC will be taken up later, the key advantages are:

Electronic OI devices have become extremely popular because of their flexibility and their power. Ultimately, they help plant personnel do a better job of making decisions and taking action, which keeps plant processes running as intended.

Figure 7. Using an Electronic OI Panel