Guide to Industrial Automation -- Components and Hardware: Controllers

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Automated systems use a wide range of mechanical and electrical products from a great variety of manufacturers.

Catalogs from the manufacturer, whether in paper or electronic form, can be a great resource for technical information not only on the manufacturer's specific product, but on general control and automation techniques.

This section on "Components and Hardware" is divided into dedicated sub-sections:

  1. Controllers
  2. Operator Interfaces
  3. Sensors
  4. Power Control, Distribution, and Discrete Controls
  5. Actuators and Movement
  6. AC and DC Motors
  7. Mechanisms and Machine Elements
  8. Structure and Framing

1 Controllers

Controllers provide the computing, calculation, and I/O management part of an automation system. They may act as a nucleus or be networked together and distributed throughout the system.

1.1 Computers

In addition to being used as a tool to write the programs for control systems, computers are also used as the actual controller for some machines. Computers have the advantage of being relatively low cost because of their wide availability. Since computers already have a monitor and some sort of pointing device such as a mouse, human machine interface (HMI) programs can also be easily implemented using standard computers.

Computer operating systems are not usually optimized for performing real-time control on machines. Most PC systems run a variety of the Microsoft Windows operating system, which by its nature contains many components not required or wanted in a control system. Because of this, a special platform, Microsoft Windows CE, was developed to remove many of the features not required for a control system. Windows CE is less memory intensive and component based and therefore more appropriate for real-time control. Embedded controllers are beginning to use Windows CE as a standard platform.

1.2 Distributed Control Systems (DCSs)

Distributed control systems (DCSs) are often found in process control applications such as chemical plants. They are used extensively in processes that are continuous or batch oriented. DCSs are connected to sensors and actuators and use set point control to control the flow of material through the plant. The most common example is a set point control loop containing a pressure sensor, controller, and control valve. Pressure or flow measurements are transmitted to the controller, usually through the aid of a signal conditioning I/O device. When the measured variable reaches a certain point, the controller instructs a valve or actuation device to open or close until the fluidic flow process reaches the desired set point. Large oil refineries have many thousands of I/O points and employ very large DCSs. Processes are not limited to fluidic flow through pipes, however, and can also include things like paper machines and their associated variable speed drives and motor control centers, cement kilns, mining operations, ore-processing facilities, and many others.

A typical DCS consists of functionally and/or geographically distributed digital controllers capable of executing from 1 to 256 or more regulatory control loops in one control box. The I/O devices can be integral with the controller or located remotely via a field network.

Another name for this is distributed I/O. Today's controllers have extensive computational capabilities and, in addition to PID control, can generally perform logic and sequential control.

A DCS may employ one or several workstations and can be configured at the workstation or by an off-line personal computer.

Local communication is handled by a control network with transmission over twisted pair, coaxial, or fiber-optic cable. A server and/or applications processor may be included in the system for extra computational, data collection, and reporting capability.

1.3 Programmable Logic Controllers (PLCs)

Programmable logic controllers (PLCs), are widely used to control plant floor automation systems. They are essentially digital computers used to control electromechanical processes. PLCs are used in many different industries and machines, such as packaging and semiconductor machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or nonvolatile memory. A PLC is an example of a real-time system since output results must be produced in response to input conditions within a bounded time; otherwise unintended operation will result.

The main difference from other computers is that PLCs are armored for severe conditions (dust, moisture, heat, cold, and so on) and have the facility for extensive I/O arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches or other sensors, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The I/O arrangements may be built into a simple "brick"-style PLC, or the PLC may have digital and analog I/O modules that plug into a PLC rack. Rack mounted communication modules can also be used to interface remote I/O blocks into the processor. An example of a rack-based PLC is shown in FIG. 1.

Major PLC manufacturers also sell the software to program their platforms. These software packages are specific to the platform; they cannot be used on other manufacturers' hardware. Additional software is often necessary to configure network communications and program HMIs; this may be packaged in a common software suite.

Prior to the advancement of computer systems, logic would be drawn manually using the same techniques as those used to design physical relay control systems and then converted into a shorthand that could be entered using a handheld keypad or text-based computer. As technology advanced, logic could be drawn on a computer screen. This was usually still converted into the same shorthand and available for documentation. Logic was often still printed out in a graphical format also.

Because of memory limitations, descriptive comments for coils, contacts, and other instructions were not stored in the PLC memory.

Symbols could be used to create a reference for these devices, but generally they were simply referred to by bit or integer number.

Memory registers were generally reserved for the type of data that was used; bit, word, or floating point values. Timers and counters also reserved areas as well as math registers.

FIG. 1 Allen-Bradley ControlLogix PLC.

In more modern PLCs, memory size is much less of an issue.

More descriptive tags are allowed to be stored and other methods such as structured text and sequential function charts are able to be used. Programmers can even use a combination of these and ladder logic based on what is most appropriate. Further information on PLC software is contained in Section 6.

1.4 Embedded Controllers and Systems

Embedded systems are special-purpose computing systems that are usually designed to perform one or a few dedicated functions, often with real-time computing constraints. It is usually embedded as part of a complete device including hardware and mechanical parts. In contrast, a general-purpose computer, such as a personal computer, can do many different tasks depending on programming. Embedded systems control many of the common devices in use today. Common control components of an embedded system are the microprocessor or CPU, RAM or random-access memory, and flash memory.

In general, embedded system is not an exactly defined term, as many systems have some element of programmability. For example, handheld computers share some elements with embedded systems- such as the operating systems and microprocessors that power them-but are not truly embedded systems, because they allow different applications to be loaded and peripherals to be connected.

Temperature and Process Controllers

One type of special-purpose controller in wide use is the temperature controller. These devices may perform simple on/off heater control in reaction to a sensed temperature or control multiple zones using PID control. Controllers may be designed for a specific type of temperature sensor or configurable by software or dip switches.

Stand-alone temperature controllers are often sized using the DIN (Deutsches Institut für Normung) system, a German standard.

They may thus be classified as 1/16 DIN, 1/8 DIN, or 1/4 DIN sizes.

Timers and counters are also often sized this way. This ensures that controllers will fit a certain-size panel cutout. FIG. 2 shows a 1/4 DIN panel mount temperature controller. Parameters such as the process value (PV) and set value (SV) are displayable in different colors for ease of use. Setting of parameters is done using the membrane keypad buttons on the front of the controller.

Temperature is not the only parameter that may be controlled by a separate controller. Nearly any process variable can be controlled using a stand-alone panel-mounted unit. Process controllers can be used to control the position of a valve based on flow or pressure; they effectively use an input variable to control an output variable as shown in Fig. 2.3, a closed-loop feedback system. Process controllers physically resemble temperature controllers; the major difference is the type of input circuitry.

FIG. 2 1/4 DIN Temperature Controller. (Courtesy of Omron.)

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