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Advanced control designs are drowning in data

Posted: 16 Nov 2006 ?? ?Print Version ?Bookmark and Share

Keywords:DAQ requirements? advanced control designs? industrial control? control designer challenges solutions? Peter Varhol?

From complete factories to the individual electromechanical device, real-time DAQ is critical to the control and feedback that lie at the heart of automation. While advanced VLSI technology offers ever higher performance, it also enables increasingly sophisticated control options that are cranking out higher volumes of data. Enabling real-time processing that can handle the data crunch within cramped form factors and mandated budgets has become pivotal to industrial-control design.

While DAQ requirements in industrial monitoring and control are diverse, there are common trends. Data is being collected at increasing levels of granularity. A decade ago, it might have been sufficient to capture the discrete steps of a process, motion or its results. Today, however, every aspect of the process is recorded in detail. The sampling rate for analog processes is high and increasing, although throughput is not as critical as the raw speed of the capture, analysis and feedback.

Cost is paramount. Although cost is an important consideration in manufacturing, many manufacturers have been willing to pay a premium for good DAQ solutions. This trend is changing, though, as the availability of low-cost hardware performing many essential functions makes it more difficult to justify premium pricing for specialized solutions.

DAQ components are getting smaller with good reason. More processes and devices are being subject to control and feedback. They often have space restrictions, limiting the size of hardware for DAQ and control. The solution may have to fit into a space that is itself shrinking as the device being controlled shrinks.

Those trends have implications for DAQ design. The DAQ hardware has to do more and faster within a smaller package. This is often the prescription for an ASIC. Indeed, ASICs are sometimes used, but only when the economics are in line. In many cases, off-the-shelf components are needed to create a solution that meets cost, function and volume goals. In others, integrated SoCs fill the bill.

Engineers building control systems for industrial applications face increasingly limited trade-offs among performance, flexibility and cost. They are counting on vendor support to make those trade-offs without sacrificing important features.

Raw performance continues to drive DAQ needs. Higher rates of sampling make it critical to process inputs even from multiple channels as fast as possible. But there is more to it than thatinputs must be processed and correlated so that the feedback loop can initiate changes earlier. The goal is to make a greater number of smaller adjustments so that responsiveness is rapid and smooth.

DAQ requirements
In some cases, high DAQ performance is also needed to coordinate activity among multiple units. Thanks to modern processor and DSP speeds, it's possible to process data acquired from multiple control units, aggregate that data and feed it back to all the controlled processes or devices to coordinate their activities.

Faster processors are one key to maximizing performance, whether performing DAQ from individual devices or multiple ones. Advanced pipelined processors from the likes of ARM and Motorola are at their best when aggregating long data streams. But A/D conversion must keep up with the abilities of the processor. Because of processor pipelines, lagging data can slow the entire system and make smooth control more difficult.

Thus, analog input must be captured and converted to digital data at a high sampling rate and with high throughput. This requires high-speed peripherals along with A/D components and digital processing, either integrated with the processor or available separately. This means making choices from a variety of individual components or seeking out a specific, highly integrated solution.

Engineers increasingly want flexibility from their control components. The goal is to lower costs and make it possible to leverage engineering skills by focusing on fewer components that are capable of performing multiple tasks.

"Engineers want parts that can work across different control solutions," said Leo McHugh of Analog Devices Inc. (ADI). Using the same design skills across different solutions can also speed time-to-market.

Obtaining such flexibility has the advantage of lowering procurement costs, while ensuring that engineers don't have to learn the details of the different components that perform similar functions. In some cases, however, it will mean that engineers will have to make allowances for the fact that the solution may not be the ideal component for all applications. This might mean extra design time, additional support components and higher real-estate requirements for the solution.

DSP is targeted for industrial control. It integrates peripheral controllers, RAM and flash.

SoC solution
Texas Instruments Inc. (TI) looks to make design trade-offs more palatable through an SoC family of products. The TMS320C2000 family includes on-chip peripherals, a peripheral bus, flash RAM and a 32bit DSP. The integrated approach can reduce cost if the design solution makes use of many of those components. It offers flexibility within the controller family, rather than within individual components.

In all, process control designers evaluate four characteristics in selecting components and integrating them into a solution, said Arefeen Mohammed, an applications engineer for TI: "First, does the processor have the MIPS to handle performance requirements? Second, are there adequate peripherals within the context of the integrated solution? Third, is there an upgrade path as features are added and system requirements increase? Last, are there tools for quickly building, testing and debugging hardware and HW/SW solutions?"

Designing process control solutions with these points in mind also keeps costs under control. Selecting components that meet design goals without overkill in power or integrated peripherals eliminates the complexity and attendant costs of incorporating peripherals or other parts external to the system.

Whether or not an integrated SoC family is the right approach, following Mohammed's protocol will help designers build solutions that meet technical and cost goals.

Many embedded engineers have the opportunity to work with sophisticated tools that assist in layouting, prototyping and simulating hardware designs. Support tools for control system design tend to be comparatively minimal.

Simulation of specific processes, such as A/D conversion, is possible at a high level of abstraction. But in general, designs can vary, such that applying a single set of tools to all possible designs makes it difficult to get the level of visibility needed to tune and debug solutions.

Reference and evaluation boards from vendors such as TI and ADI provide an integrated test bed that can serve as a starting point for new designs. While designers rarely use an evaluation board without making at least some modifications to its components, the boards can often be used for initial proofs of concept.

Test and debug tools are essential in ensuring that the solution is correct and ready for use. Debugging the design is possible with TI's highly integrated single-chip systems.

The company offers a Windows-based debug environment that can be used with the TMS320C2000 digital signal controller family. Because of its on-chip support, the system runs in real-time during debug. This reduces the chances of overlooking race conditions and other speed-sensitive errors.

Software considerations
Most engineers build control applications using high-level languages such as C, an approach with maintenance and reuse advantages over assembly language. Test and debug approaches compatible with high-level languages, such as source code debuggers, are essential in building quality process control systems.

A DAQ solution also requires a great deal of support software: an RTOS for handling interrupts and performing low-level data operations; network stacks for communicating data beyond the point of collection; and applications to store, aggregate and analyze data. Design flexibility is required to ensure that the hardware design is capable of performing as software requirements grow.

The principles of process control applications remain the same as in the past. However, challenges in performance, size, cost and flexibility make the control designer's task more difficult. Often, it is necessary to trade off among the four to produce a working system.

Deciding whether the appropriate design approach is to assemble components separately or to use a family of integrated systems is a choice dependent on the specific application. Both approaches offer advantages and limitations, not only for a single design project, but for future and related projects.

- Peter Varhol
EE Times

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