Displays and control panels increasingly are comprised of modules that are combined to provide custom requirements.

CRTs and flat-panel display technologies are assuming an ever-larger role in industrial equipment and consumer products. On operator panels where a few indicator lights might have sufficed years ago, there is more of an emphasis on providing a clear-cut and straightforward user interface. This trend has fostered a need for operator controls that are easy to understand. Displays play an important part in simplifying such controls.

In more specialized instrumentation, indicator lights and alphanumeric readouts continue to play an important role. Liquid-crystal displays (LCDs) and electroluminescent displays are increasingly providing custom readouts in small, compact instrumentation.


The simplest cathode-ray tube (CRT) displays are monochromatic units used to provide displays containing only alphanumeric characters, no graphics. Such CRTs come in typical sizes ranging from 4-in. diagonals on up to 19-in. diagonals and larger. The average price for such displays, when housed in ruggedized industrial enclosures, is generally a few hundred dollars or less. Typical applications include providing readouts of position on CNC or other industrial machinery.

CRTs that provide more than just readouts of alphanumeric information increasingly are classified as being components of operator interface panels. In such applications, the CRT is frequently called upon to display icons and other graphic information that aids and abets the operator in interacting with the machine or system being controlled.

The most common format for graphic video displays of this sort is the VGA standard that is widely used on personal computers. Graphic resolution is 640 X 480 pixels in standard mode displaying 256 colors. Higher resolutions are possible. One advantage of the VGA format for such applications is that, because it is used in high volumes, costs associated with both the CRT tube and the ancillary driving electronics are reasonably low.

VGA displays are frequently bundled with the needed drive electronics and display-generating software, comprising modules that are, in turn, built into control panels. Moreover, the increased use of laptop personal computers has made VGA-format LCDs increasingly cost competitive for use in a variety of industrial and consumer-oriented control panels.

A number of operator interface panels now consist of a VGA-format display coupled with some sort of touchscreen input. These devices, which can use either CRTs or LCD flat panels when space is at a premium, can be found in applications as diverse as blood analysis machines to curbside baggage check-in machines, and from shopping mall kiosks to building automation systems.

There are two main types of operator interface panels that use the VGA/touchscreen approach. The first merely combines an ordinary VGA display with a separate touchscreen applied over top the video display. Of the two types, this is the least expensive.

The second type incorporates the touchscreen matrix within the display enclosure, so that the touchscreen and display comprise a single, closed unit. Typical construction practice in this sort of module is to include subsets of desktop computer functions such as hard disk drives, a large main memory, or the ability to work with other peripherals. This approach gives these display modules the capability to serve as stripped-down terminals or as full-blown computers.

One problem developers have had with touchscreens used as separate overlays concerns calibrating the touch-sensitive areas on the overlay with the locations of patterns on the screen. This is basically a software task that, although straightforward, can be time consuming.

Other problems have cropped up when touch displays must be used in applications governed by industry-standard safety regulations. Sometimes, for example, systems must undergo certification by Underwriters Laboratories or other organizations. Installing an overlay on a UL-certified video terminal may void the certification. The assembly must then go through a potentially lengthy requalification process. Fully integrated touch-control screens overcome such problems.

Resistive
touch systems employ a membrane stretched over a glass or plastic substrate. Small plastic insulators create a gap between the two layers. Touching a pad on the face of the CRT produces a voltage drop that sensing electronics read as an indication of a touch.

Resistive touch sensors are often preferred on industrial systems because they do not have the drawbacks of either capacitive or infrared methods. The only real problem with resistive screens is that the touch membrane eventually wears out. However, this is not an issue for the majority of actual industrial uses.

Capacitive
touch technology, in contrast, generally uses discrete touch pads fused directly onto the surface of the glass faceplate. Oscillators are also employed. Touching the pads with a finger or other conductive probe changes the capacitance of the oscillator. This modifies either the oscillating frequency or the waveform shape, which can, in turn, be an indication of a touch.

One problem with capacitive systems is that they are insensitive to nonconductive objects. That means that a user cannot wear gloves when operating the screen. In addition, the sensitivity of these touch systems may be affected by moisture and by the varying natural body capacitance of different operators.

Infrared
touchscreens generally employ LEDs and light sensors placed around the periphery of the screen. The LEDs emit a grid of infrared light. When an object interrupts these beams, light sensors register a touch.

A benefit of infrared touchscreens is that the LEDs and sensors can often be sealed in the terminal to meet NEMA-4 requirements so that the entire assembly can be cleaned with a hose. But operational drawbacks may make other technologies a better choice for industrial applications. For example, anything that interrupts an LED is interpreted as a touch. This makes infrared systems prone to false actuations from factors such as shirt sleeves, shavings spinning off a part being machined, or even smoke.

Other less-serious drawbacks include parallax, where the operator points to one target but triggers another. This may occur because the infrared grid is physically separated from the screen image by as much as 1.5 in. It is usually a problem only near the screen edges, however, where the CRT curves most severely. Ruggedness may also be an issue, since the unit malfunctions if the LEDs are misaligned.

Interfacing touch subsystems typically involves connecting to a host computer and generating software to produce application screens. When an operator touches the display to make a selection, a touch report goes to a host computer. This report is generally a numerical value indicating the location of the touch. The host then translates this value into a corresponding command.

Most touch-control units provide signals following ANSI x3.64-1979, the same format used for DEC VT-100 and other widely used terminals. This ANSI standard establishes escape sequences for standard control functions such as setting character attributes, moving the cursor, and clearing the display.

Connections from industrial touch terminals to the outside world are generally through RS-232C, RS-422A, Ethernet, Novell, Token Ring, RS-485 multidrop, or through custom connections.

Touch subsystems having multidrop capabilities allow several touch terminals to connect with a common host through a single serial port. Terminals must be equipped with an RS-485 interface to provide such features. The RS-485 standard uses a balanced voltage scheme identical to RS-422A.