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When you're looking at programmable automation controllers for your application, here are some considerations to keep in mind.
For a new system, a PAC offers monitoring, control, and data acquisition capabilities plus connections with company computers. Full and
control, motion control, the ability to interface with devices, and should all be PAC capabilities. You may not need them all now, but needs have a way of expanding.
Also, look for PAC features that can reduce expenditures on network hardware, such as additional built-in network interfaces. The PAC shown at right, for example, includes two independent Ethernet network interfaces plus four serial ports configurable for RS-232 or RS-485/422.
For an existing installation, a PAC can consolidate separate systems and link them with company computers, so that control, production, and monitoring data can be exchanged as needed. Verify that new PACs are compatible with all legacy systems, including all networks and protocols.
Location and extent of system
Is your system distributed over a broad area? Do you need to monitor assets in remote locations? Because PACs combine distributed control with remote communication options, they can efficiently handle extensive systems with remote installations.
Choose the PAC size and options that suit the size of your system. For example, a PAC that mounts on the I/O rack is probably more suited to cell control, RTU-type installations, or smaller systems. A more powerful, standalone PAC may be needed for more extensive distributed systems.
For the most efficient PAC-based system, choose one that utilizes , not just a distributed architecture. Distributed intelligence offloads many control functions to remote processors co-located with distributed I/O. Distributed intelligence shortens wiring runs, reduces network traffic, maintains critical control should communications fail, and frees the central controller for supervisory tasks.
Networks and communications
Choose a PAC that has all the networking and communication options you need—and anticipate needing—built in.
Networks may include (either wired or wireless), , or others. Communication options might include , or , , Allen-Bradley DF1, and other standards on the control side. For communication with computer networks, you may also need protocols such as TCP/, for email, for network management, and for file transfer.
Unlike a personal computer (PC), PACs are industrially hardened and built to withstand normal industrial use. Check the PAC's specs for temperature and humidity tolerances. If your application involves extreme temperatures, vibration, dampness, dust, electrical noise, or other exceptional conditions, provide necessary enclosures and protection just as you would for a traditional control system.
Types of sensors, actuators, and devices
Signal requirements for inputs and outputs vary widely. Look for PAC-based systems offering reliable I/O with the digital, analog, and serial signals you need. These may include temperature, rate, RMS, pH/ORP, load cell, and others in addition to voltage and current. The PAC should be able to communicate with all signal types natively, rather than requiring signal conditioners.
Where cabinet space is limited, high-density I/O is a good choice. Software-configurable I/O—for example, an input module configurable as any of several thermocouple types—offers flexibility and reduces the number of spares you need to have on hand.
Advanced control capabilities
PAC-based systems have advanced control capabilities built in. Make sure that your requirements for high-speed digital control, motion control, , and mathematically complex logic, for instance, can be satisfied without add-ons.
Because a PAC is similar to a PC, the integrated software that comes with it includes advanced programming features such as subroutines, string handling, complex conditions, and floating-point math. In addition, a PAC can often be programmed in C or other standard programming languages.
While it may seem easier to continue using programming tools you're familiar with (such as ladder logic for a PLC), you may find that software designed for the PAC is more efficient for your expanding needs. It's a good idea to explore any software suite designed for the PAC. Such an will likely save development time and effort, and it may be easier to learn than you think.
Data acquisition and database connectivity
For data acquisition applications, choose a PAC system with two strengths: first, substantial memory for acquiring and storing data; and second, the ability to share data directly with corporate databases over an Ethernet network.
If control networks and computer networks need to be separated, consider how you'll accomplish this. One way is to using independent Ethernet network interfaces on the PAC itself.
Check out the PAC-based system's options. The integrated software development environment should use a single tagname database, so that once you define variables and I/O in the control software, you can immediately use them in the HMI software.
A PAC should also offer communication with third-party HMIs using OPC. And other options, such as a touchscreen terminal, may be available as well.
Plan for the future. When your needs change, additional distributed I/O can be handled by the same PAC—as can process, discrete, and motion control. All of these types of control should be programmable in the same software as part of the same system, and most changes should require no middleware or add-ons.
More information on PACs