This section of the primer deals with the various pieces of hardware used to design and assemble an automated system or machine.
3. COMPONENTS AND HARDWARE
Automated systems use a wide range of 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.
In addition to being used 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 due to their wide availability. Since computers already have a monitor and some sort of pointing device such as a mouse, HMI (Human Machine Interface) 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.
3.1.2 Distributed Control Systems (DCS)
Distributed Control Systems 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 setpoint control to control the flow of material through the plant. The most common example is a setpoint 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 Input/Output (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 setpoint. 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 input/output devices (I/O) can be integral with the controller or located remotely via a field network. Today’s controllers have extensive computational capabilities and, in addition to proportional, integral, and derivative (PID) control, can generally perform logic and sequential control.
DCSs 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.
3.1.3 Programmable Logic Controllers (PLC)
PLCs, or Programmable Logic Controllers, 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 non-volatile 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, etc) and have the facility for extensive I/O arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, 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 input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.
3.1.4 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, 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 which power them — but are not truly embedded systems, because they allow different applications to be loaded and peripherals to be connected.