Guide to Industrial Automation -- Components and Hardware: Operator Interfaces

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Machine operators need to interact with machinery in order to activate devices or processes and get status feedback. Historically this was done with push buttons, switches, and pilot lights. As technology has advanced, these have been replaced with dedicated text and graphic displays with membrane push buttons and touch screens. Industrial computers with a built-in monitor using a keyboard and pointing device such as a mouse are another form of machine interface. Computer interfaces with dedicated controllers are also widely used. The operator interface includes hardware (physical) and software (logical) components. Like the earlier description of automated systems, operator interfaces provide a means of:

  • Input, allowing the user to send signals or data to a system or controller;
  • Output, allowing the system to control the effects of the users' manipulation

An operator interface is programmed by means of software on a standard computer terminal. The interface should be designed to produce a user interface that makes it simple and efficient to operate the machinery. The operator should be able to provide minimal input to produce the desired result, and the interface should provide only the desired information back to the operator. This requires careful planning of the screen menu structure and machine representative graphics and icons and consistently organized displays for an effective interface.

Other terms associated with operator interfaces are MMI (man machine interface), HMI, GUI (graphical user interface), and OIT (operator interface terminal).

2.1 Text-Based Interfaces

Operator interfaces may be text based, simply providing instructions or machine status to the operator. They may or may not include buttons for input. The displays are typically backlit LCD but may be vacuum fluorescent bulbs. LED arrays are also common in larger message-only-type displays with a greater viewing distance.

Individual lights are arranged in a pattern, allowing selected points to illuminate in the shape of alphanumeric characters. These may be arranged in multiple rows or columns, depending on the required message length and size of characters. FIG. 3 shows production data displayed in a workcell. Colors are configurable within each field to make the display more readable; it can even be programmed for fields to change color based on numerical limits.

2.2 Graphical Interfaces

FIG. 3 Text display. (Courtesy of Mills Products.)

With improvements in technology, it has become standard for machines to use graphic interfaces with pictorial representations of the machine or production line for diagnostic purposes. These may be either monochrome or color and have membrane-type buttons, touch screens, or both.

Graphic interfaces are manufactured by most PLC or DCS manufacturers and also by third parties who specialize in these products. They may use a proprietary operating system or be based on a computer platform, such as Microsoft Windows. Software for programming these operator interfaces is almost always proprietary to the manufacturer. Drivers are generally available for most popular controls platforms.

Graphical interfaces provide the ability to create a virtually unlimited number of screens and interface objects. Smaller screens can be superimposed over larger ones or minimized like the Windows operating system.

Faceplates may also be used with a graphical interface. A faceplate is an object that contains a standardized arrangement of buttons and indicators that may be populated via software with different device data. Thus if there are many similar devices, such as motors or conveyors, they can all use the same faceplate with their own start and stop buttons and status indicators.

2.3 Touch Screens

A touch screen is an electronic visual display that can detect the presence and location of a touch within the display area. Determining the location of a touch requires two measurement values, one on the X axis and one on the Y. The term generally refers to touching the display of the device with a finger or hand. It enables one to interact directly with what is displayed, rather than indirectly with a pointer controlled by a mouse, trackball, or touch pad.

Measurements coordinates are in analog form and are generally converted using a 10-bit analog to digital converter, providing 1024 positions in the X and Y directions. Touch points are then passed to a computer or HMI microprocessor using serial communication.

Following are some of the different types of touch screen technologies available.


Resistive touch screens are made of several layers of material. A hard outer surface provides insulation between an operator's finger and the inner conductive materials. Behind this layer are two thin electrically conductive layers separated by a small gap or space. The gap is separated by an array of very small transparent insulating dots; when the outer surface is pressed, the inner layers touch and the panel acts as a pair of voltage dividers. This creates electrical currents that indicate where the screen was pressed. This data is then sent to the controller for interpretation based on the address of the button or control that was drawn in that spot.

Resistive touch screens are cost-effective and are often used in restaurants and hospitals in addition to factories because of their resistance to liquids and other contaminants. A disadvantage of this technology is that it is easily damaged by sharp objects, such as tools.

It also provides only about 75 percent optical transparency because of the extra layers and insulators.

Surface Acoustic Wave (SAW)

Touch screens using surface acoustic wave (SAW) technology use a glass surface and so are more resistant to sharp objects than resistive touch screens. SAW devices consist of two interdigital transducer arrays (IDTs) that transmit ultrasonic waves across the surface of the screen. Touching the screen surface absorbs a portion of the wave, registering the location on the surface. This is sent to the touch screen controller for interpretation.

Image clarity using SAW technology is better than that of resistive or capacitive touch screens because there are no extra layers between the image and the glass. Multiple touched points can also be sensed simultaneously. Contaminants on the surface of the screen can interfere with the ultrasonic waves, however. Because of the transmitting and receiving transducers being exposed along the edges of the screen, they are also not completely sealable and can be damaged by large amounts of liquids, dirt, or dust. They also must be touched with a fairly wide object, such as a finger; a hard stylus will not work.


A capacitive touch screen panel consists of an insulator such as glass coated with a transparent conductor. The human body is also an electrical conductor, so touching the surface of the screen results in a distortion of the screen's electrostatic field, measurable as a change in capacitance. The location is then sent to the controller for processing.

Unlike a resistive touch screen, one cannot use a capacitive touch screen with an electrically insulating material, such as a standard glove. A special capacitive stylus or glove with fingertips that generate static electricity are sometimes used, but this can be inconvenient for everyday use.

There are several different capacitive technologies used for touch screens, each with its own cost or technical advantages. Surface capacitance technology provides a fairly durable and inexpensive product but has limited resolution, is prone to false triggering, and needs calibration during the manufacturing process. Projected capacitive touch (PCT) screens are more accurate because of a greater resolution. The top layer is also glass, making it more impervious to sharp objects. Because the conductive layer is etched, the clarity and light transmission are reduced, however.

Mutual capacitive touch (MCT) screens have a capacitor at the intersection of each row and column. This allows the registration of multiple touches to be detected, but these are more expensive than surface capacitive screens. Self-capacitive sensors can also be used with the same X-Y grid. They provide a stronger signal than the mutual capacitance type but cannot resolve more than one finger or touch at a time.


An infrared touch screen uses an array of infrared LED and photodetector pairs around the edges of the screen to detect an interruption in the pattern of beams. These LED beams cross each other in a vertical and horizontal or X-Y pattern allowing the sensors pick up the exact location of the touch. This type of technology can detect nearly any input, including a finger, gloved finger, stylus, or pen. It is generally used in applications that cannot rely on a conductor (such as a bare finger) to activate the touch screen. Infrared touch screens do not require any patterning on the glass, which increases durability and optical clarity of the overall system, unlike capacitive or resistive technologies.

Optical Imaging

Optical imaging uses charge-coupled device (CCD) image sensors similar to a digital camera along with an infrared backlight. An object is then detected as a shadow. This can be used to detect both the location and the size of the touching object. As the cost of CCD components has lowered, this technology has become more popular. It is very versatile and scalable, especially for large-screen applications.

Dispersive Signal Technology

This technology uses sensors to detect the piezoelectricity generated in the glass due to a touch. Since the mechanical vibrations are used to detect the contact, any object can be used to touch the screen. Like SAW and optical imaging technologies, since there are no objects or etching behind the screen, the optical clarity is excellent. Because of the mechanical aspect of the technology, after the initial touch the system cannot detect a motionless finger.

Acoustic Pulse Recognition (APR)

Another interesting technology is acoustic pulse recognition (APR). Four small transducers along the edges of the screen detect the sound of an object hitting the glass. This sound is then compared using a lookup table to prerecorded sounds for every position on the glass.

APR ignores ambient sounds since they do not match the stored digitized sounds. Like dispersive signal technology, after the initial touch the motionless finger cannot be detected, but the table lookup method is much simpler than the complex algorithm used to detect the piezoelectric contact.

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