Single Board Computers for Control Applications
Haydn Povey, senior product manager at embedded processor designer ARM, points out: “People are moving from PLCs to high-end microcontrollers as their control algorithms become more complex.”
According to David Pursley, applications engineer for Kontron, changes in the engineering community also fuel a trend away from traditional control technology: “In the last two or three years, we’ve had engineering managers say newer engineers coming out of universities are familiar with how to program a PC, but not a PLC.”
But, PC-based control systems have problems, too.
“The standard PC suffers from a number of issues,” Povey says. “First, they’re built to keep costs down, so they’re not very robust. Also, the operating system on a PC is built to be very open, because you never know what people will use it for. An embedded platform tends to be a bit more focused.”
This article presents a third option—an embedded control system using an single-board computer (SBC) as controller. Eric Lai, platform product manager for embedded systems at Advantech America, lists SBC advantages as:
More processing power. Current SBC solutions offer from P4 to Dual or Quad Core processor performance;
Flexibility for expansion. A 4U SBC system can provide up to 20 PCI slots;
Relatively low cost of ownership. SBC is a stable and field-proven solution with standardized platforms, and a choice of standard operating systems. Application software and hardware are widely available.
Wikipedia defines an SBC thus: “Single-board computers are complete computers built on a single circuit board. The design is centered on a single or dual microprocessor with RAM, I/O, and all other features needed to be a functional computer on the one board.” Contrast this with what is inside the PC on your desk. There, the random access memory (RAM) chips, for example, are soldered to auxiliary boards called “sticks” that mount in connectors that hold them at right angles to the main or “mother” board. This arrangement is often referred to as a “mezzanine” card. Other necessary components may be mounted on “daughter” boards that mount parallel to the mother board on standoffs.
|Single board computers provide all the circuitry necessary for a complete computing solution (processors, memory, data communications) on a single board.|
Some SBC embedded systems also use mezzanine cards and daughter boards. The distinction is that these auxiliary cards perform functions that, while necessary for overall system functioning, are not necessary to make a functional computer.
Is this distinction merely academic? Not when you are designing an embedded control system. Choosing an SBC as your controller allows you to concentrate on engineering the control system, rather than designing your own computer.
“The motivation to opt for an SBC-based solution,” according to Povey, “should come from some combination of space constraints, processor performance requirement, or need for flexibility to, say, allow for future upgrading.”
These attributes are exactly what SBCs can bring to the party.
“The life of the [SBC] is a bit longer,” adds Christine Van De Graaf, product marketing manager for Kontron’s embedded modules division. “You’re looking at a 10 year life cycle as the target, with a minimum of five years. Commercial PC technology life cycles are typically three years at the longest.”
There are two approaches to creating an SBC-based controller: roll-your-own (RYO) or commercial-off-the-shelf (COTS). Both approaches start with a specification list. The list should include:
Size and shape of the “hole” the SBC will have to fit into;
Maximum power available to run the SBC;
Constraints on cooling (Can you use a fan, or are you limited to natural convection, or even just conduction through the case?)
Processing speed requirements;
Real-time and/or less-than-real time response requirements;
Data storage requirements;
Analog and digital I/O channels;
Software requirements; and
Additional requirements special to the application.
“Once you have that list of requirements,” says Pursley, “you can map it to different form factors and make trade offs from there.”
|SBC resources may be augmented with additional circuitry mounted on mezzanine cards and daughter boards.|
In line with the “Don’t re-invent the wheel” philosophy, RYO should be the very last choice. Assuming you’ve already determined that a traditional PLC or PAC solution won’t do the job for you, and a PC-based solution is also out, look for a COTS solution next.
“If doing only a handful [of units] without very high speed data acquisition requirements,” says Joseph Chung, advocate in charge of embedded components at Via Technologies, “what I would do is get a [COTS] board.”
COTS SBCs consist of a microprocessor or microcontroller unit (MCU) mounted on a printed circuit board along with additional chips and components for peripheral devices, such as clocks, additional memory, USB drivers, radio sets for wireless communications, Ethernet chips, and analog output devices.
The number and types of peripheral devices depends on the level of integration in the main processor device. Some microcontrollers integrate entire systems on the MCU chip, while other devices, such as quad-core microprocessors, leverage widely distributed microprocessor chips for high-end processing at low cost, and leave peripherals for additional chips mounted on the board. It’s the difference between an MCU and a standard microprocessor.
“We try to put a lot of embedded control functionality on our chips,” Jennifer Woods, a product marketing manager at MCU vendor Freescale Semiconductor says, “to save board space, increase performance, give more flexibility, and to give as close to of a single chip solution as possible.”
“With the advent of high-end high-performance 32-bit microcontroller platforms,” ARM’s Povey says, “you have the ability to be very deterministic and still run high-end operating systems that can be programmed in higher-level languages.”
To the control engineer specifying a COTS SBC, the identity of the processor and peripheral devices is largely immaterial. The question is: “Does the SBC as a complete system meet the requirements?”
COTS SBCs generally follow one of a limited number of industry standard formats that are managed by an industry consortium or special interest group, such as the PC/104 specification managed by the PC/104 Embedded Consortium or COM Express maintained by the PCI Industrial Computer Manufacturers’ Group (PICMG). The standards specify (at least) board size and shape, and connector pinouts. Board size and shape determines if it will fit in the “hole” in your application. Connector pin-outs are important for interface compatibility. If your application requires many analog inputs, for example, you need sufficient pins available to make the connections.
“PC/104 modules don’t have external connectors directly on the boards,” Van De Graaf points out. “They have pin connectors that attach to ribbon cables to bring external connectors to whichever kind of enclosure you choose. Then you stack, like Legos, a peripheral board on [the processor board]. That stack gets integrated into a metal box and then you have all of your external controls.”
Obviously, when dealing with a space-constrained application, consider only the SBC standards small enough to fit. SBC standards, however, range all the way up to mainframe solutions such as VME, which carries boards up to 9U (15.75 in) in height. (View this article online for more information on “Eurocard standards.”)
COTS solutions range from “naked” circuit boards that leave system integrators with the responsibility to provide physical support and environmental protection for the electronics, to Compact PCI modules that slip into carefully defined standard sub racks that take care of nearly all those issues.
“Several of the entrants to the DARPA Grand Challenge were using Compact PCI because reliability was so important,” recalls Kontron’s Pursley. “They had to deal with much higher shock and vibration than a factory floor environment. A lot of industrial automation users [are now] deciding to use Compact PCI or VME because they are even more rugged than a standard PLC.”
In between naked circuit boards and sub-rack solutions are SBCs mounted in a large number of (mostly proprietary) enclosures with various degrees of environmental integrity. For example, in applications where dust or water intrusion is a concern, a few manufacturers offer hermetically sealed, fanless enclosures where heat is removed via conduction through the case. More commonly, SBC enclosures provide slots or louvers that provide limited protection from splashed fluids, dust and physical intrusion). The need for protection depends, as does so much else, on the application.
One of the advantages of enclosed SBCs is that you need only to drill a few holes to mount the entire enclosure somewhere in the system structure. Standard connectors take care of routing electric power and signals in and out of the unit. Chung points out: “You could put [one] under a desk and screw it in, and then you’re pretty much done.”
Roll your own
“If the [end user] needs at most one or two, or maybe 50 or 100 pieces,” says Chung, “you want to do the least customization possible. The moment you start to customize, complexity becomes very big.”
Customization puts you into the RYO camp, where the control engineer must don a computer-engineer hat as well. RYO becomes advisable when you are developing a commercial product with expected production volume greater than 500-1,000 units, according to Chung, and there is just no COTS way to get the job done.
Several years ago I ran across an application where engineers testing jet engines needed to mount an entire data acquisition (DAQ) system inside a bearing journal. The DAQ electronics included sensors, signal conditioners, analog-to-digital convertors, wireless data communication—and a host computer. The engineers built the whole DAQ system on a half-moon-shaped custom circuit board that mounted to the ball bearing’s inner race. There was no off-the-shelf SBC to fit that available space.
Machine controllers are now available in a range of formats that are both broad and continuous. At one end are traditional PLCs and PACs, which are highly optimized for traditional applications. SBCs form the other end of the range, with choices running from standardized sub racks to completely free-form custom RYO solutions. The best choice comes from carefully assessing your applications needs, then comparing those needs to available choices.
|C.G. Masi is a senior editor with Control Engineering. Contact him at email@example.com|