PLCs Maximize Machine, Motion Control

Control engineers now have at their disposal a wide variety of logic devices to program processes and machines. Computers and computer technology provide almost unlimited control possibilities no matter what the application. PLCs remain a strong part of the mix—a recent survey finds more than half expect to increase PLC spending in the next 12 months.

By Dick Johnson December 1, 2005

Control engineers now have at their disposal a wide variety of logic devices to program processes and machines. Computers and computer technology provide almost unlimited control possibilities no matter what the application. PLCs remain a strong part of the mix—a recent survey finds more than half expect to increase PLC spending in the next 12 months. More than 40% expect to spend as least much as they did in the past 12 months.

Early machine and motion control were rooted in basic logic, the same basic logic that computers still leverage today. Depending on the machine and its function, mid-20th-century logic could be mechanical, pneumatic, or electrical, or a hybrid of any of the three. Years past, control engineers used basic relay-ladder logic to map out the sequence and build control systems for manufacturing machines and lines of all types. This type of ladder logic used many components (relays, timers, coils, switches, pilot lights, pushbuttons, etc.) and an enclosure, adding protection (and wiring). Even though the process to create this type of control system was straightforward, demands for faster, cheaper, more accurate, and more complex machinery clearly demanded something more sophisticated. Enter the programmable logic controller or PLC.

PLCs are now, and have been for some time, the control device of choice for a wide variety of control applications. According to the most recent Control Engineering /Reed Research online poll (using feedback from more than 300 respondents, who specify, recommend, or buy PLCs), a majority use PLCs for machine-control applications. Process control applications are a close second, followed by motion control, batch control, and diagnostic applications. Most applications are in-plant (54%), with the reminder OEM (25%) or both (17%).

Spending up, size down

The 2005 survey found that 51% of respondents expected to increase their spending on PLCs, while 8% expected to spend less. Remaining respondents (42%) expected spending would remain the same over the next year. These figures set a solid spending trend; in the 2002 survey only 29% of respondents expected an increase in spending, 64% expected spending to remain the same, and 7% expected a decrease. The 2005 survey saw a 2-5% increase in nano PLCs (&15 I/O connections), micro PLCs (12 to 128 I/O), and medium PLCs (128-512 I/O). Large PLCs (>512 I/O) and PC-based control each fell 4%.

The slight “decline” in PC-based control among PLC users may be attributed to a melding of product categories. Use of PC-based silicon in a PLC form factor, (often called a programmable automation controller or PAC), tends to blur the line between PCs and PLCs. Some manufacturers even refer to their PACs as PLCs or “controllers.” That terminology also blurs the line between PLCs and products traditionally known as “loop controllers,” covered in a separate July 2005 article.

Software-based PLCs and embedded control use stayed nearly the same in this 2005 PLC survey.

Mike Miclot, division product manager at Rockwell Automation, sees identical trends from his vantage point as a supplier—smaller PLCs are gaining ground over larger and PC-based configuration. Miclot says three reasons push the move to smaller PLCs. First, newer, smaller controllers have more power and can replace their larger counterparts. Next, the controls community has embraced a distributed architecture that uses networks of smaller PLCs (performance and autonomy are the key factors here). The third reason, Miclot says, is lower overall cost.

Is there a limit to how small these devices can be? If PLCs do get any smaller than the current “nano” configuration, it likely will be because the electronic components that manufacturers put inside continue to be miniaturized. “But even as these devices become smaller and more powerful, there will be a point where they will become impractical within the factory environment—just too complex or difficult to install or work on,” Miclot warns.

Pushing these limits of shrinkage, Divelbiss Corp. (see online link for more information) even touts a silicon-based PLC.

One rung at a time

The control engineering profession has its enduring traditions. There is neither an engineer nor a tech unfamiliar with one of its oldest and longest-lasting PLC programming languages—the venerable ladder diagram. Firmly rooted in the days of relay logic, the ladder diagram remains the overwhelming top pick among the respondents in the 2005 survey (see accompanying diagram). Little has changed in this portion of the 2005 survey from 2004’s, and even before that. The top three programming languages remained ladder diagram, function block, and sequential function chart, in that order. Changes at the bottom of the list saw both C programming and instruction list move up in the standings.

According to Joe Rubino, PLC product manager for Omron Corp., ladder diagrams may hold this top slot for some time to come. “The requirement for ladder diagram programming is directly driven through supporting the equipment on the manufacturing floor. The domestic market of PLC vendors established deep-seated roots with ladder diagram programming by migrating the legacy equipment control panels, which used relays to perform the control logic, to programmable logic controllers. Ladder diagram programming offered the easiest migration path, mimicking the physical wiring of the legacy relay panels. This greatly reduced the learning curve for using the PLC. Existing personnel, who previously supported relay panels, are able to develop and support the PLC’s electronic representations of the physical panel wiring that they were accustom to,” says Rubino.

Does Omron see other programming languages as eventually unseating the ladder diagram? “No,” Rubino retorts, “Instead of function blocks or sequential function charts overtaking ladder diagrams in the future, we see them complementing the way to write PLC control code. The key is in offering the programmer flexible ways of creating control code.”

No failure to communicate

Networking trends varied only slightly from the previous survey. Most respondents reported that most of their PLCs were standalone (32.9%). A slightly smaller number claimed that they networked with personal computers (26.5%). Respondents also reported that the ability to network with other PLCs (22.8%) and distributed control systems (17.6%) were necessary to the functioning of their control network. Overall, respondents reported that the greater majority of their PLCs worked on some sort of a network, the margin being slightly greater than two to one.

When asked which features they would expect to specify in future PLC purchases, respondents chose the same attributes for the top three positions, in slightly different order than the earlier survey (see accompanying chart). Based on the responses, it is clear that the ability to network programmable logic controllers (and do so easily) is on the mind of the control engineer. Ability to use universal programming software for multiple targets/ platforms claimed the number one position. Number two among the features deemed very important for future purchase was the ability to network PLC I/O connections with a PC. Third most-wanted feature was the availability of PLCs with more remote I/O subsystems.

When Mike Shulim, chief technical officer for DST Controls, a San Francisco Bay area systems integrator, was asked which features “got his vote” as the most important advances in PLC development, he cited PLCs with onboard PC processors, nano/micro PLCs, and PLCs with more remote I/O subsystems, in that order. Even though Shulim’s picks do not mirror survey results, it is easy to see that working control engineers like PLCs that are small, fast, and smart. Doing a job for less money (PLCs with remote I/O subsystems save on wiring-related costs) can’t hurt either.

PLC-type products

For other manufacturers, visit ; for system integrators, go to ; for more on these products, see the Web sites below.

Cost-effective online editing

The Allen-Bradley MicroLogix 1100 controller gives end-users and OEMs a cost-effective, low-end microcontroller with the flexibility of online editing, says Rockwell Automation. List priced at $550, the new controller helps users make program modifications without interrupting operation and make program adjustments, including PID loops, while still online. An embedded port for EtherNet/IP protocol communications enables users to easily share data across a production facility, while eliminating time and costs associated with separate network connections. In addition, an embedded, Ethernet Web server gives users remote access to control information, further optimizing production efficiency by sharing real-time data on the plant floor with OEM partners or remote maintenance engineers. Aimed at applications where distributed control across an Ethernet network is required (such as material handling or packaging), it suits applications using supervisory control and data acquisition (SCADA) with the controller as a remote terminal unit (RTU). Rockwell Automation/Allen Bradley

PLC combines technology, motion control

The 317T Integrated Technology PLC is said to save time and money by combining technology and motion control into one Simatic PLC. Using Siemens Step-7 software, only one program is needed for PLC and motion control, simplifying configuration and programming while reducing training and hardware costs. The PLC can control up to 16 axes. Motion control functions available include positioning in absolute, relative, additive, and superimposed operating modes. Other functions include geared synchronous motion, electronic cam disc and cam functionality, moving to a positive stop, and measuring probe functionality (used for print mark correction). Siemens Energy & Automation

Improving communication

AutomationDirect has introduced two new communications modules for the DL05/DL06 micro PLC family. The D0-DCM module adds two serial ports to any DL05 or DL06 PLC, using RS-232 or RS-422/485 (only ModBus supports RS-485) data interfaces up to 115.2 Kbps. Port 1 supports ModBus RTU, DirectNet, and K-Sequence slave protocols; Port 2 can also operate as a DirectNet or ModBus master. The F0-CP128 CoProcessor module offers three serial communication ports that are configurable using the module’s full Basic programming capability. The module features a 100-MHz clock speed, RS-232 or RS-485 data interface options, and rates up to 512 Kbps. Either module can be installed in the option slot of the DL05 or in four option slots available on the DL06 PLC models. AutomationDirect

Program with Bluetooth or Ethernet

Telemecanique’s Twido nano-controller, the first nano PLC said to have embedded Ethernet capability, can now be programmed for either Bluetooth wireless or Ethernet communications, extending its functionality and ease of use in many applications. Its new built-in wireless capabilities simplify diagnostics and troubleshooting. Users can remotely monitor and control these PLCs over Ethernet, using the Twido processor with an embedded Ethernet connection or by using the new Twido Modbus to Ethernet bridge. Bluetooth wireless capability makes set-up and adjustment much easier with wire-free connection to the PLC up to 30 ft via a laptop with TwidoSoft programming software or with a new TwidoAdjust tool for pocket PCs. Schneider Electric

Multiplatform portability

PACSystems RX3i controller has a control engine and universal programming environment to provide application portability across multiple hardware platforms. It is said to offer a high level of automation functionality in a compact, cost-effective package. Features include a high-speed processor and patented technology for faster throughput without information bottlenecks, dual-backplane bus support per module slot, high-speed, PCI-based processor for fast throughput of new advance I/O connections, a serial backplane for easy migration of existing Series 90-30 I/O devices, and advanced analog modules with diagnostics and interrupt capability. It also provides an Intel Celeron 300-MHz CPU with 10 Mbytes memory for advanced programming and performance, ability to store logic, program documentation, and machine documentation files, as well as open communications support (serial, Ethernet protocols, Genius, HART, Profibus, and DeviceNet). GE Fanuc Automation

Improved memory motion integration

FX3U SuperMicro compact PLC is said to offer users faster throughput, larger memory, and improved motion integration. It also expands the range of application opportunities, enhances usability, and increases performance for compact PLC users. Features include six, 100-kHz high-speed inputs, three, 100-kHz high-speed outputs, 64-kbyte standard programming-memory, and simultaneous communications capability via three RS networks. It also has a new Profibus Master Module and the new SSCNET 3 2-axis optical network module for high-speed connection to Mitsubishi servo amplifiers. Mitsubishi Electric

Big capability, compact size

Omron CJ1 Series is said to be designed to deliver significant PLC performance from a product the size of the smallest micro controllers on the market, requiring only 40-60% of the panel space of traditional mid-size PLCs. The device has processor speeds of 20 ns, and accommodates up to 2,560 I/O points. Modules are each roughly the size of a deck of playing cards and lock together with simple connectors, eliminating the need for a bulky PLC rack. Omron’s PC21 bus, designed for its mid-sized CS1 PLC, provides high-speed communication from the CJ1 CPU to individual modules. The PLC also includes communication options generally associated with larger platforms, leveraging Omron’s FINS protocol to transparently tie Ethernet protocols, Controller Link, serial, and device-level networks together for data exchange and programming. Programming is via Omron’s CX-Programmer software. Omron Electronics LLC

Small but mighty

ELC delivers abundant module selection in a small package. One-third the size of competitive offerings, it can retrofit more I/O points into a single space or deliver cost savings by reducing cabinet size, compared to other options. Features include multiple communications ports, remote I/O ability, data collection storage area, high-speed counters, high-speed pulse outputs, ramps, trapezoids, floating point math, interrupts, timer resolution to 1 ms, and PID capability. Users can expand it from 10 to 256 I/O points using the same controller, so they no longer have to count I/O to determine which controller to use. Its no-rack design allows modules to be added by simply snapping them into their mating connectors. Connecting to other networks is simple using its advanced communication. Slave connectivity can be added to DeviceNet, Profibus, or Modbus TCP, and users can share data with other networks. Eaton Corp.

New programmable automation controllers

Opto 22 is introducing two new versions of its PAC systems, now named the Snap PAC family. First is the Snap PAC-S series controller, a high-performance standalone controller suitable for distributed control systems and applications with I/O point counts above 128 points or complex Ethernet network architectures. Snap-PAC-S1 model features a 32-bit multitasking processor with floating point unit, 32 MB RAM, 16 MB flash memory, and 8 MB battery-backed RAM. Second is the Snap PAC-R series controller, an “on-the-I/O-rack” controller for cell control and smaller point count applications where I/O needs are geographically contained. Opto 22

Double duty: motion, PLC

MintDriveII is a programmable motion controller featuring a PLC operating in the background. It integrates a powerful motion controller and servo drive into an intelligent compact package. It is programmed in an easy-to-use basic multitasking language, which coordinates motion, HMI, and PLC tasks. The controller includes on-board digital and analog I/O capabilities that can be used to control many parts of a machine. For PLC operation, true and false conditions, along with logical operators, are used to set up actions including: jog, set user outputs, drive enable or disable, and others. Digital and analog I/O points can be expanded with either its developer’s range of CAN I/O modules operating over the CAN bus, or CANopen devices conforming to DS401 I/O. Other fieldbus options include DeviceNet and Profibus-DP. Baldor Inc.

Compact, many features, affordable PLC

XC200 is said to be a high-end feature, low-end price PLC. It offers a small footprint—60 x 100 mm, and scan times of 0.5ms per 1K of instructions. It also features an IEC-61131 programming environment, large capacity memory, removable memory card, a USB port, multi-tasking processor, an internal RAM disk, integrated I/O with high speed counters and interrupts, an RS-232 connection, onboard fieldbus networking, and a scalable back plane. Advanced software tools can program, configure, debug, commission, and design Java-enabled Web pages. The controller also has an intuitive interface and built-in libraries to make programming easier. Plug-and-play design allows for replacement of hardware and software with just a screwdriver. Fast Ethernet protocol access allows program changes and data access from virtually anywhere. Moeller Electric