Embedded Systems Design Europe - November 2007 - (Page 24) RTOS Execution tracing tools reach another level of debugging by offering a visible representation of thread execution, interrupts, preemption, and other events. You can spot troublesome scheduling issues and identify timing problems in your application that cause control instability using these tools. Examples of advanced debugging tools include Green Hills’ EventAnalyzer for Integrity OS, National Instruments’ LabVIEW Execution Trace Toolkit, QNX’s Momentics system profiler for Neutrino, and Wind River’s ScopeTools for VxWorks and Linux. ALGORITHM INTEGRATION The “high-speed” designation not only indicates fast control processing, but also implies advanced control strategies. In fact, the heart of a control system is the algorithm or logic that processes the input signals to produce the appropriate outputs. Depending on the application, the control algorithm can range from simple discrete logic that drives valves and relays to a complex differential equation that determines a motion profile. As the algorithm becomes more complex, it may be necessary to have access to high-level complex math functions such as fast Fourier transforms (FFTs) and proportional-integral-derivative (PID) algorithms. Because most RTOS integrated development environments don’t contain such libraries, you must explore several options. You can develop algorithms yourself; however, that can take significant develop24 ment time and require expertise not readily available. You can find these algorithms as part of an open source library, or you can purchase them from IP vendors. Alternatively, code generation may be an option if you created the control algorithm using a modeling tool that has code generation such as I-Logix’s Statemate or Dynasim’s Dymola. Finally, you may consider high-level development tools that include analysis functions and can target embedded controllers directly, forgoing the need for code generation. EMBEDDED HARDWARE A conversation about high-speed control systems wouldn’t be complete without a discussion of the appropriate hardware processing targets. Often, the available software options are gated by a predetermined hardware platform, or the hardware is limited by the support of a particular software tool chain. You must consider processor performance, cost, and environmental specifications, to name a few important factors. For example, hardware-in-the-loop simulation systems that run intensive control models require high-performance processors and are usually best served by multicore processors. Not all RTOSs have equal support for multicore processors, so the depth of support is an important consideration during your selection process. Industrial machine control systems or field control devices may require processors that can handle extreme temperatures ranging from -40° to +70°C or more in some cases. Battery-powered control devices, of course, require low-power processors. The top performance of processors that meet rough environmental and power restrictions is, by nature, lower than that of general-purpose processors. This means the RTOS has less horsepower to work with, making the performance and footprint even more important. A control system’s connection to the plant is the I/O hardware that interfaces with the physical signals of the system that’s being controlled. The I/O may include any combination of analog voltages, currents, digital signals, pulse-width modulated signals, radio frequency signals, and so on. Look for an RTOS that has wide support for the required I/O types. Whether you’re selecting individual I/O components such as ADCs and DACs or multifunction PCI data acquisition boards with a large number of I/O channels, consider the software development. Expect to develop more low-level code when dealing with single components, as opposed to off-the-shelf hardware for buses such as PCI, PCI Express, CompactPCI, PXI, PXI Express, and VME. Off-the-shelf devices usually have higher-level drivers, but these can vary between vendors. Some offer only board support packages (BSPs) with peek and poke interfaces, while others have full-fledged application programming interfaces (APIs) that include advanced features such as direct memory access (DMA) transfers. I/O boards also vary in critical specs for control systems such as accuracy, isolation, and connectivity to signal conditioning. While the control system connects to the plant through I/O, it cor- NOVEMBER – DECEMBER 2007 | embedded systems design europe | www.embedded.com/europe 020-022-024-025_ESDE.indd 24 6/11/07 16:43:01 http://www.embedded.com/europe
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