The lines that divide embedded control, PLCs, and PC-based control are blurring. As technology change begets terminology change, even product vendors are not sure how to classify their new industrial controllers. Machine builders share in the confusion, not sure themselves how to categorize the control systems that provide the intelligence for their machines.
Around this time each year, we examine these competing control strategies to look for field evidence about what's working and where. The results haven't yet defined an emerging front runner. So, if we're ever to make some sense of it, we need a starting point. This article proposes a definition of embedded, PLC, and PC-based controls based on control system functionality, controller packaging, and end user perceptions. We'll also examine applications where embedded control is advancing because of new technology, lower costs, and the unique needs of industrial machine OEMs.
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.
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.
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.