Any delineation of types of control systems is somewhat arbitrary, but for the purposes of this article we'll work within the following parameters:
* PLC--Combines power supply, processor, and local I/O in a single chassis or rack. Primarily programmed in ladder logic, its chief function is logic control. A single-vendor solution.
* PC--Standard general-purpose personal computer, either desk or rack-mount, often industrially hardened. Runs Windows NT or some other operating system primarily designed for non-industrial applications.
* Embedded Controller--Any controller not classified as a PLC or a PC, including motion and servo controllers. Runs Windows CE or some other embedded operating system. Designed for a specific purpose rather than general purpose. Can be designed to limit user access. End user does not usually specify brand or model.
Easy Embedded
One of the chief advantages of embedded control is the industrial OEM gets to pick the brand and model. "If we used PLCs, each customer would tend to specify not only a particular brand, but also in many cases a specific model," says Walter Adam, project engineer with Johann A. Krause (http://www.jakrause.com), Auburn Hills, Mich. Krause uses Phoenix Contact (http://www.phoenixcon.com) embedded controllers and is one of the world's largest manufacturers of assembly machinery for the auto industry.
The ubiquity of PLCs makes them a prime choice in customer specifications, but this is not so for other types of controllers. "Customer specifications usually include PLC specs, but they rarely specify the type of motion or servo controller," observes Kenny Teixeira, systems engineer with Winpak (http://www.winpak.com), San Bernadino, Calif. "This is one of the main reasons we use a motion controller with PLC functionality instead of vice-versa."
Winpak makes machines that form, fill, and seal pouches for food and beverage industry applications. The machines are controlled by Bosch Rexroth (http://www.boschrexroth-us.com) motion and logic controllers. Most of the control code for Winpak's machines is motion-related rather than sequential logic, and the company's engineers found traditional PLCs weak in motion control. "The Bosch controller is not as capable as a PLC for logic control, but its strengths in motion control make it a good fit for our application," adds Teixeira.
The stealth nature of embedded controllers, and to some extent PC-based controllers, is a major selling point for industrial machine OEMs. OEM customers not only are less likely to specify brand and model, they are less likely to view and/or modify the control program. On the other hand, because PLCs are widely used in manufacturing plants, operations personnel are comfortable making changes to PLC ladder logic.
This can cause two major problems for machine builders. First, modified programs can cause machinery to malfunction, often resulting in lost production, equipment damage, and even personnel injury. "It's too easy for end users to tamper with and change PLC programs," says Dennis Kelly, controls manager with HMS Products Co. (http://www.hms-group.com), Detroit. "Our transfer systems move tons of material, and any mishap can cause damages and possible injury, so it is critical to have tamper-proof control." HMS's transfer systems for the metal-stamping industry use a Phoenix Contact embedded VME-based controller powered by a Motorola 68030 processor.
The second major problem with PLC ladder logic is exposure of proprietary software code. Most PLCs don't have tools to conceal source code, prevent downloads, and thwart reuse. Most embedded controllers are programmed with high-level languages such as C. Compiled code is downloaded to the controller, and the OEM retains the source code. Most PC-based control software products have a C tool kit that can be used to protect proprietary code.
Truly Scalable
All control systems claim scalability, but some industrial OEMs beg to differ because of their unique machine control needs. Krause's assembly machines perform a wide range of tasks in various auto industry applications. Instead of designing custom machinery for each application, the company designs discrete mechanical units that perform different tasks. These mechanical units are then joined together to perform assembly operations.
Each mechanical unit needs its own controller, so Krause needs true scalable control. Phoenix Contact provided a family of embedded controllers classified in four different levels. Control capability ranges from basic control devices with simple closed-loop logic functions such as motor starters at Level 0, to PC-based plug-in embedded controllers at Level 3. Krause uses controllers from each level depending on the control needs of each mechanical unit. These controllers are then linked via Profibus, Interbus, or Ethernet to create an integrated control system.
PC-based controllers cannot scale down in price, and PLCs have other limitations that preclude use in Krause's applications. "PLCs need a control cabinet, and our controllers must be able to operate without control cabinets, often mounted directly to machine frames," says Adam. "Low-cost micro and nano PLCs can't meet our requirements for speed, memory, and diagnostics."
Other factors contribute to true scalable control. All the different control units should be programmable with a single software package. After programming, the compiled code should be downloadable to any of the vendor's entire family of controllers, subject to limitations on program size and complexity. The programming software package should allow industrial OEMs to use multiple languages including ladder, flow chart, statement list, and C.
Advanced Measurements (http://www.advmeas.com) is a system integration company in Calgary, Alberta. It recently developed an embedded controller for a power tong used to apply the precise amount of torque needed to connect 30-ft. pipe stems in oil-drilling applications. The control strategy was developed with LabView RT, a real-time control development package from National Instruments (http://www.ni.com).
The initial application was done on an embedded PCI bus controller installed in the HMI PC. The National Instruments embedded controller contains a real-time control engine as well as discrete and analog I/O. "PC-based control was not viable because of requirements for speed, determinism, and reliability," says Steve Conquergood, president of Advanced Measurements.
The second application of the same control strategy will employ a National Instruments FieldPoint real-time controller. Advanced Measurements will use the same control flow charts because the only change is in the compiled code downloaded to the controller. The FieldPoint controller is a better fit for this second application because it is DIN rail-mounted, more compact, and less expensive than the PCI bus embedded controller. National also sells a PXI real-time controller that executes the same control flow charts.
Embedded controllers let industrial OEMs control model and brand choices. It also allows them to protect the control program from tampering. For many applications, embedded controls are more scalable. Cost was once a huge advantage for embedded controls as compared to PLCs, but the differential is rapidly narrowing.
Can't Beat The Price?
In the past, the price advantage of embedded controllers over PLCs was substantial. Many high-volume industrial OEMs did not consider PLCs because of the relatively high cost, and instead developed their own embedded controllers. Because these controllers were designed for a specific application, per-unit costs were extremely low. There were usually considerable front-end development costs, but the volume over years of use fully recovered these costs.
Many machine builders developed custom embedded controls prior to the introduction of low-cost PLCs and see no reason to switch. But others are taking a new look at PLCs. Largely in response to machine builder needs, PLC vendors introduced a slew of low-cost nano and micro PLCs during the past few years. Prices in the $100 range for a PLC with an integrated power supply and I/O are understandably piquing the interest of many industrial machine OEMs.
Remstar International (http://www.remstar.com), Westbrook, Maine, sells more than 5,000 automated storage and retrieval systems annually. Each vertical carousel, horizontal carousel, and vertical-lift module contains an embedded controller. With these volumes, it's no surprise that PLC vendors have been trying to persuade Remstar to abandon embedded control for more than 20 years.
"If PLCs could meet out cost requirements, we could concentrate our internal efforts on applications instead of on controller hardware and operating system development and maintenance," says Tim Robey, Remstar vice-president of technology. Remstar has always been interested in the PLC option for a number of reasons, but stuck with its custom embedded controllers because PLCs were simply too expensive.
"PLC vendors can continuously capitalize on industry-wide technological advancements, and they offer unmatched reliability because they have hundreds of thousands or even millions of units in the field," adds Robey. "We have finally found a PLC that is price competitive." Remstar is testing an AutomationDirect (http://www.automationdirect.com) $199 DL06 PLC in a vertical carousel, and if the test is satisfactorily completed, the company plans to build new systems with that PLC. Some of Remstar's systems require Ethernet, so the company plans to use an optional Ethernet module when needed. Robey feels a built-in Ethernet port would probably make the base PLC cost prohibitive.
Even at the low prices, Remstar is satisfied with tech support. "Tech calls to PLC vendors are often referred back to local distributors, and many of them are technically weak," argues Robey. "AutomationDirect has great direct tech support, and an actual person answers the phone." These front-line contacts are supposed to direct callers to the right expert, but Robey finds they actually answer about 60% of his questions.
Remstar has a high-volume and fairly low-end application, but many industrial machine OEMs have more complex needs. When a machine needs an HMI, real-time control, and networking, a PC is often an attractive option. OEMs realize the benefits of using a PC for multiple tasks, but they have been searching for a way to make PCs more suitable for real-time control.
Embedded Inside
Many machine builders like PC-based control because a single platform can serve multiple needs. The PC can run one of the many excellent PC-based HMI software packages. It can connect to other applications via Ethernet or other industry-standard protocols with low-cost plug-in cards. The PC also can serve as a development station for programming and troubleshooting the control system. I/O cards can plug directly into the PC bus, creating a full-fledged controller.
Despite all these benefits, PC-based control presents a problem for many industrial OEMs. "We simply could not get the speed, determinism, and reliability that we needed with straight PC-based control," says Advanced Measurements' Conquergood. One of the main constraints for all PC-based controllers is the Windows operating system.
Windows NT and its variants are huge, monolithic general-purpose operating systems. Windows is designed to do many things well, but it's not optimized for any particular application. There are tremendous varieties of Windows applications on the market, and the operating system must be able to support everything from games to spreadsheets to desktop publishing.
Faro Technologies (http://www.faro.com), Lake Mary, Fla., manufactures a portable coordinate measurement machine used to obtain three-dimensional measurements. Faro uses a Motorola embedded controller (http://www.motorola.com/semiconductors) instead of a PC for a number of reasons. "The PC form factor is relatively large and power requirements are substantial," says Andy Helm, electrical engineering manager. "In addition, we have found that the embedded PC market is fairly unstable. You can design around a certain product and it could be discontinued on a moment's notice."
Some industrial OEMs solve the Windows problem and take full advantage of PC benefits by using a Windows PC with an embedded real-time control card installed directly in the PC bus. The plug-in card runs a completely different operating system, so it continues to execute its real-time control program even if Windows crashes. It is also possible to reboot Windows without interrupting real-time control.
End users can load as many different programs as they want onto Windows without slowing down the processing speed of the plug-in card. "We have used plug-in, real-time, PC-based controllers from National Instruments in a number of different applications," observes Conquergood. "Most of our controllers need deterministic cycle times of 1 msec with reliability much greater than that provided by Windows applications."
One such application makes use of the advantages of a PC. Advanced Measurements replaced a 25-year-old custom controller with an embedded PC controller on a machine corrugating, hole-punching, and cutting to length sheet steel. A real-time plug-in controller from National Instruments was installed in a PC's PCI bus, and the PC also executed the LabView HMI program. Discrete and analog I/O on the plug-in card connected to field devices.
The PC also was used to interface with a drawing database. Operators simply selected the correct drawing, and the PC extracted the relevant information from the database to determine machine operation. One PC was able to act as an HMI, a network interface, a programming station, a report generator, and a real-time controller.
VME-based controllers are close relatives of PCs, and plug-in cards for VME buses offer many of the same benefits as PC-based plug-in controllers. Goodrich (http://www.goodrich.com) in Troy, Ohio, uses a VME backplane with two CPUs for embedded control of a test stand. One CPU is used for HMI and networking, while the second CPU runs a VxWorks real-time operating system from Wind River Systems (http://www.windriver.com) and performs control and data acquisition tasks.
Embedded VME controllers can provide superior performance as compared to similarly priced PLCs. "The PLC is not a good platform for high-speed data acquisition and buffering," says Joe Britton, systems engineer with Goodrich. "We use 1 GHz processors from VMIC (http://www.gefanuc.com), and they have plenty of horsepower to perform the required tasks for this application."
Like the PC bus, the VME bus is a true industry standard with a large number of suppliers. "We have access to a wide variety of manufacturers for CPUs, I/O boards, and power supplies," adds Britton. "We also can write all our applications in C instead of trying to shoehorn our program into ladder logic." PLCs restrict users to one vendor for all replacement and expansion hardware, and this can escalate prices.
Can Embedded Controllers Stave Off PLCs?
Embedded control used to mean custom design, but this is no longer the case. Embedded operating systems such as Windows CE, embedded XP, and embedded Linux let vendors and machine OEMs create custom operating systems from a menu of building blocks.
These custom operating systems contain only the functionality needed for the application, thus avoiding operating system bloat and unreliability. These compact custom operating systems also require much less processing power and memory than conventional PC operating systems.
Another way to deploy embedded control without custom design is via PCI and VME bus plug-in cards. PCI cards in particular are allowing users to take advantage of all of the benefits of a PC without giving up the reliability and determinism of a PLC. These plug-in cards can operate at higher scan rates than PLCs, and these scan rates are increasing rapidly as PC technology advances.
PLCs are fighting back with a new generation of low-cost nano and micro controllers. These controllers are displacing custom embedded controllers in some OEM applications, and they allow OEMs to use an off-the-shelf solution. As a result, industrial machine OEMs can concentrate on improving and applying their machines, instead of spending time and money on controller hardware and operating system development and maintenance.
Table 1: Top Ten Reasons Why an OEM Might Use an Embedded Controller Instead of a PLC
1 Embedded controllers are usually more compact.
2 If customers specify different PLC brands, the OEM must support each brand and model.
3 If customers specify a PLC, they often select a more expensive brand and model than needed.
4 Embedded controllers are faster.
5 Embedded controllers often can be field installed without control cabinets.
6 Embedded controllers are more scalable, especially at the low end.
7 Embedded controller software programs are more secure from end user tampering.
8 Embedded controllers are usually programmed with C, which is more powerful than ladder logic.
9 CMOS level I/O can be provided with less space and at lower cost than with a PLC.
10 If few I/O and many serial ports are needed, than embedded controls are a better option.
Contributed in part by Dick Matthews, Director of Hardware Engineering, Z-World /Rabbit Semiconductor
Table 2: Top Ten Reasons Why OEMs Might Use an Embedded Controller Instead of a PC
1 Embedded controllers are faster.
2 Embedded controllers can be deterministic, PC-based controllers are not.
3 Embedded controller operating systems are more reliable.
4 Embedded controllers are generally less expensive than PCs.
5 Embedded controllers have more flexible mounting options.
6 Embedded controllers are more scalable, especially at the low end.
7 Embedded controllers can be industrially hardened from the ground up
8 Embedded controllers generally require less system resources and power than a PC.
9 Embedded controllers are generally smaller than PCs.
10 Embedded controller outputs are generally higher current than would be available on a PC.
Contributed in part by Dick Matthews, Director of Hardware Engineering, Z-World/Rabbit Semiconductor
Table 3: Top Ten Reasons Why OEMs Might Use a PLC Instead of an Embedded Controller
1 PLC requires less internal development and support.
2 OEM customers may be more willing to accept PLC control.
3 PLCs can be lower cost, especially for low-volume OEMs.
4 OEM personnel may be more familiar with PLCs than with embedded controllers.
5 PLCs have unmatched worldwide support and distribution.
6 One PLC brand can usually support a machine OEM's entire line of products.
7 The PLC program is usually portable across a vendor's line of PLCs.
8 There is only one vendor for the entire control system including CPU, I/O, and programming software.
9 Agency approvals for entire systems are already completed by the vendor.
10 There are a wide variety of available I/O, both local and remote.
Contributed in part by Paul Ruland, PLC and I/O Product Manager; AutomationDirect
