Smaller PLCs retain popularity

Though the controller landscape is now a place of constant change, PLCs have not yet been replaced by PCs. One glance at products released lately shows PLC manufacturers have adapted to threats from PC-based, "open", and embedded control. New form factors exploit economies of size. Functions previously reserved for large PLCs are routinely found in micro units, for example, floating-point...


Though the controller landscape is now a place of constant change, PLCs have not yet been replaced by PCs. One glance at products released lately shows PLC manufacturers have adapted to threats from PC-based, 'open', and embedded control. New form factors exploit economies of size. Functions previously reserved for large PLCs are routinely found in micro units, for example, floating-point math, high-speed counters, and analog functions. Sometimes the controller just looks like a PLC; inside it is really a PC.

Control Engineering's 2000 study of the perceptions and attitudes of readers involved in specifying, recommending, and/or buying Programmable Logic Controllers (PLCs) revealed that 98% of respondents have such a role. The study also explored:

  • Types of PLCs used;

  • Major applications for PLCs;

  • How PLCs are interconnected with overall control systems; and

  • Preferences for programming.

Control Engineering polled 1,500 of its readers. There were 384 responses for a rate of just over 25%.

PLCs were categorized by number of I/O points supported. Micro has less than 128; medium contains 128 to 512; and large PLCs have over 512 points. More respondents (77%) use Micro PLCs than other sizes. Medium PLCs were used by 70% of readers. Only 40% reported using large PLCs. While these proportions are similar to the 1999 study, micros retained purchasing strength at the expense of large units. Explains GE Fanuc Automation's (Charlottesville, Va.) business leader, Bill Black, 'While PC-based control continues to gain momentum in manufacturing, PLCs remain a viable and integral part of the manufacturing process. For small, stand-alone control of a single process, PLCs are more cost effective, more compact, and easier to use than PC-based solutions. Micro PLCs were designed to address these basic control needs.'

Dave Quebbemann, Omron's (Schaumburg, Ill.) industrial automation marketing manager, adds, 'I expect PLCs to remain [viable] for a long time into the future. Smaller PLCs are very cost-effective. Small to mid-size OEMs will gravitate to PLCs because of low cost. Users want rugged controls without having to worry about compatibility difficulties. Yet, the traditional PLC is changing to add more connectivity while controlling small work cells.'

Think that PLCs are a device for discrete manufacturing alone? Consider this: while eight of 10 reported using them for machine control, two-thirds use PLCs in process control, and one-third cite batch process applications. Obviously, many engineers have both process and machine control applications in their plants, since the result is greater than 100%. Only a few years ago, integrated motion control in a PLC was often considered a tricky application. Now, 40% of respondents are using PLCs in motion applications. PLCs have found a niche in virtually all types of manufacturing.

Says Dale Snyder, Logix business manager, Rockwell Automation (Mayfield Heights, O.), 'Automation technology is headed toward tighter integration of multiple control disciplines, such as high-speed discrete, motion, process, and drive control. Eliminating the need to deal with multiple control editors, communication interfaces, and HMI will significantly reduce engineering time and maintenance costs, as well as improve data sharing and productivity.'

Ethernet leads connectivity

Almost half (46.5%) of respondents use Ethernet for networking PLCs. This number is up six percentage points from 1999. Many network protocols received increased responses over 1999, led by Profibus-up from 4.0 to 9.0% of mentions and DeviceNet-up by three points to 15.4% of mentions. Stephan Borres, director of PLC systems, Schneider Electric (North Andover, Mass.), explains, 'Seamless information flow has become the key to automation success. The use of open architecture, including the Ethernet, PCs, and the web, is now driving the market. The PLC has become a component of the flow, not the organizer. Customers are shifting to smaller but more powerful PLCs with embedded intelligence and Internet technologies.'

Almost all (93%) program PLCs with Ladder Diagram editors, virtually the same as 1999. About one-in-five use function block programming, an increase of three points. International programming language standard IEC 61131-3 gained much publicity in 1999, and some companies state compliance with the standard prominently in marketing communications. Respondents were almost evenly split over whether adherence to the standard is 'somewhat important' (41%) or 'not important at all' (39%). Seven percent said adherence was a must.

In the future, 35% said that they expected to buy PLCs with more remote I/O modules. About three in 10 plan to purchase PLC I/O modules to network with PCs, and two in 10 expect to replace PLCs with PCs running soft logic. One in 12 (8%) look to add web-enabled PLCs.

Paul Ruland, PLC product manager, (Cumming, Ga.), says, 'Small PLCs will benefit from leveraging commercial technology such as Microsoft Windows CE, allowing them to become a generic target for software applications. This trend will change the typical PLC buying decision to one of hardware specifications, peripheral and communication connectivity, similar to buying a PC, versus the traditional decision process, which requires evaluating a controller partially based on its programming software. This should also help quell the debate over which programming language is the best since ladder logic, flowcharts, state logic, function blocks, IEC61131 combinations, along with Visual Basic and C++ can all easily program a Windows CE-based PLC. Direct connectivity to other Microsoft-based systems in the enterprise, including web publishing and data serving, sheds the PLC stigma of a silent black box.'

Wayne King, business manager for automation products at Siemens Energy & Automation (Alpharetta, Ga.) sees the future as, 'intelligent distributed architecture. There will be more Ethernet plus Profibus. In a decentralized architecture, intelligence is pushed out to stations where devices have more intelligence. Software is going beyond just programming, to something more like a project management tool. Programmers not only must control the source, but also must see that everything is synced up.'

PLCs are still alive and kicking, but things have greatly changed over the past few years. They no longer are found in a huge centralized enclosure, filled with banks of large I/O modules. PLCs are found in much smaller packages these days, in smaller enclosures, and located closer to the action. They are becoming increasingly networked and are occasionally even PCs in PLC clothing.

Programmable Logic Controllers

For more information on programmable logic controllers, visit .

Palm-size comes to PLCs

Charlottesville, Va .-VersaMax Micro PLC is available in 14 or 28 I/O points, yet fits in one hand. The small, DIN-rail or panel-mount controller saves panel space and installs quickly. Features include 9K words of memory, floating-point math, real-time clock, PID support, high-speed counter, and motion control. Communications capabilities include RS-232 and RS-485 ports with Ethernet connectivity to be available by 3Q00. Built-in expansion capability supports up to 84 I/O points on the 28-point base unit and 70 points on the 14-point base unit. Programmers use VersaPro Windows-based programming software. GE Fanuc Automation

Active redundancy in PLC

Alpharetta, Ga .-Simatic S7-400H is a fault-tolerant version of the S7-400 PLC family. It offers active redundancy of mission-critical applications. It is configured as two S7-400 PLCs sharing a common backplane operating in a parallel, hot back-up mode. In case of fault, bumpless changeover occurs between the affected and standby unit without lost data. Each unit can accommodate multiple CPUs and various communications processors and function modules. Siemens Energy & Automation

Motion control added to PLC

Mequon, Wis .-Enhancements to ControlLogix platform support Rockwell's unified architecture approach to control across disciplines. Three motion instructions and two axis types have been added to motion control capabilities. These include indexing, gearing, position and time camming, and high-speed registration. The controller supports both sequential and motion control with one CPU and software package. It supports up to 16 motion modules for a total of 32 axes. Also, RSLinx software can now issue a single request to the ControlLogix 5500 to receive the entire list of tag data, rather than accomplishing it one tag request at a time. Finally, a system browse tool has been added to allow users to navigate control system networks graphically. Rockwell Automation

Ultra-slim PLC

Schaumburg, Ill .- Measuring just 33 x 65 x 90 mm, CPM2C offers several communication methods, including RS-232, host link, and no-protocol communications of standard serial devices, such as bar code readers. When used with an Omron NT631/31 HMI, the CPM2C micro PLC can be programmed via a programming console embedded into each HMI. Programming can also be accomplished through a PC using Omron's CPT support software or SSS software, or through a hand-held programming console. Expandable up to 140 I/O points, other specifications include: 14 basic and 105 special instructions (185 variations), 4,096- word program capacity, and 256 timers/counters. Built-in features such as real-time clock, RS-232 port, and synchronized pulse control help reduce system cost by eliminating more costly peripherals. Omron Electronics

Real-time control over Ethernet

Palatine, Ill .-Two new models of the Modicon Momentum M1E processor adapter are available with full IEC program control capability. An integral Ethernet port in the processor allows customers to perform such functions over Ethernet as data acquisition, peer-to-peer communications, and I/O scanning. Five embedded web pages enable use of a standard web browser to read status and diagnostic information from the processor. Processor specifications include 544KB RAM and 1MB of flash memory with Ethernet and I/O bus ports or RS-485 ports. Using standard IP device addressing and a simple 'fill-in-the-blank' menu, M1E users can select the I/O peer communications to service. Programming options include 5 IEC 61131 program editors using Modicon Concept software or the traditional ladder logic using Modicon Concept or ProWorx NxT packages. Schneider Electric

Visual programming for windows CE PLC

Cumming, Ga .WinPLC, a Windows CE PLC that fits into the CPU slot of DL205 Series PLC bases, uses backplane communication to DL205 I/O modules. For users who wish to develop Visual C/C++ or Visual Basic applications on Windows CE, the 100 MHz H2-WPLC2 is now available. To develop an application, users purchase the appropriate Visual Toolkit for Windows CE from Microsoft and WinPLC hardware. A WinPLC Software Development Kit will be available at no charge to manage the upload/download/debug process. This kit includes the backplane API, headers and libraries, and utilities to configure and monitor the WinPLC hardware, to read and write to the I/O modules and serial port for debug purposes, and to interact remotely with Windows CE in the WinPLC. Application debugging and downloading is done through the WinPLC's onboard 10Base-T Ethernet port.

Small PLC switches 10 A outputs

Vernon Hills, Ill .-Alpha fits in a new controller class aimed at meeting basic control needs in electrical contracting, building automation, home automation, and other industrial and commercial markets. The small (71 x 90 x 55 mm, min) controller can switch output circuits of 10 A. The built-in operator interface is used for on-unit programming, as well as message and data display. Other features include real-time clock, analog capability, and I/O ranges of up to 30 points. Function block control requires no relay ladder logic programming knowledge. Mitsubishi Electric Automation

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