FEATURE HMIS, PLCS & INDUSTRIAL PCS
When specifying machine controllers, the three main options for machine builders are smart relays, PLCs/PACs and IPCs. Each option has its place, and making the right choice will help keep costs and complexity down while providing the required level of performance for your application, writes Don Pham, product manager at IDEC
Building-in control A Right:
Figure 1. Smart relays are typically supplied with an integral display, and often with an external display
mimicking the same information
wide variety of machine controllers are available, and we can reduce the leading options down to just three: smart relays, PLCs/PACs and PCs. But what are the advantages of each? For many small-scale
Figure 2. Newer micro PLCs are approaching the capabilities of older model PACs, putting pressure on PACs to add new features and capabilities
applications, relay-based controllers are the price/performance leader. These smart relays usually have an alphanumeric display which shows operating information and allows for limited interaction. While many can be programmed via this, a better option is to use the PC-based programming software which is usually supplied with the relay. These often feature an Ethernet port
plus a serial or proprietary communication port. The Ethernet port can be used for connection to the PC for initial
programming, and then for connection to other devices (such as an external text display panel, other smart relays or an Ethernet switch) once programming is complete. Of additional benefit,
some newer smart relays have a built-in web server capability, allowing web pages to be programmed and stored in the unit for remote access via any web browser; and some suppliers are writing smartphone apps to enable communications with their smart relays
via Apple and Android phones, increasing speed and ease of remote access. Ladder logic programming makes it easy
to write efficient programs for simple operations. Internal program storage is limited, so operations can’t be overly complex. Smart relay I/O used to be strictly discrete, but analogue I/O is now available on some newer units. With respect to maximum allowable
I/O, IDEC’s FL1F SmartRelay can handle up to 60 I/O consisting of a maximum of 24 discrete inputs, 20 discrete outputs, eight analogue inputs and eight analogue outputs.
PLCS AND PACS Programmable logic controllers (PLCs) and programmable automation controllers (PACs) are the most widely used machine controllers. In general, PACs add more capabilities than PLCs, particularly for control of very complex systems. They can also handle advanced motion control, incorporate vision systems and perform advanced control of analogue loops. These controllers range from micro PLCs with about 32 built-in I/O points to full-featured PACs capable of handling tens of thousands of I/O. For example, the Micro PLC pictured in Figure 2 can handle up to 520 I/O total, and 126 of these I/O points can be analogue. Although this controller is labelled as a Micro PLC, it approaches PAC power in many respects including execution speed and program memory. PLCs and PACs are capable of controlling almost any machine, and many now have built-in PID and motion control. Copious on-board memory allows real-time and high-speed control of many I/O points and control loops, even for sophisticated applications like multi-axis motion control. In addition, networking choices are numerous, with almost all PLCs and PACs equipped with multiple Ethernet ports and support for various Ethernet protocols.
Characteristics
Maximum I/O Footprint
Local expansion capability Remote expansion capability Programming languages
Programming software cost Hardware cost Program memory Ease of use Flexibility Connectivity to other systems
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Comparing the different controllers PLCs/PACs
Smart relay
• About 60 • Smallest • n/a
• n/a
• Ladder, some function blocks
• Free to flow
• Low • Low • Easy • Low • One comm port & protocol
SEPTEMBER 2016 | DESIGN SOLUTIONS
• 10,000 plus • Depends on I/O quantity • Medium to high
• Medium to high
• Multiple - ladder, structured text, function, block, etc.
• Medium to high
• Medium to high • High • Medium • High • Multiple communication ports and protocols
• High
• Same as PLC/PAC, plus all common PC programming languages
• High
• High • High • Most complex • Very high • Virtually unlimited
HIGH PERFORMANCE PCS The most powerful machine controllers are Industrial PCs (IPCs). Being expensive, however, these are a good choice for very demanding applications. Many modern industrial PCs now come
Industrial PCs
• 10,000 plus • Depends on I/O quantity • High
in rack-mount versions, so are similar in appearance and form factor to a PLC or PAC. IPC I/O also looks similar, with local rack mount and remote options, easily expandable to tens of thousands of I/O points. IPCs are the most open of all controllers, so adding virtually any type of I/O is possible – often through the addition of a plug-in card to handle a particular protocol. Furthermore, because IPCs are so open, connectivity options are extensive. Program and data storage memory can be expanded as required, allowing both execution of large real-time control programs and storage of manufacturing data.
IDEC
www.idec.com
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