Build Your Measurement System on the Right Hardware PlatformWhether your application has two or thousands of I/O points, taking some time to consider the trade-offs among different measurement system options can save you time, money, and frustration in the long run. Most engineers and scientists not only want to start quickly, but they also need a foundation or platform that meets their current needs and offers the flexibility to meet their future needs. There are many parameters to consider, ranging from driver software to application development software to hardware features. Consider the eight measurement hardware platform issues, summarized in the table below, when creating your system.
Sampling Rate and Resolution PerformanceThe signals you acquire determine your sampling rate and resolution requirements. NI offers both PCI and PXI modular instruments with sample rates up to 200 MS/s and resolution up to 24 bits, so either platform is ideal for high-performance test applications. Processor Performance
With Dell, Hewlett-Packard, IBM, and other PC manufacturers racing to offer the latest processors with the fastest clock cycles, off-the-shelf computers deliver the best processor performance. Using PCI measurement devices or a PXI system controlled by a desktop computer with National Instruments MXI-4 technology, take maximum advantage of these fast processors as shown above. For applications requiring a more rugged or compact solution, embedded controllers with PXI offer processing power similar to off-the-shelf desktops. Capacity - Peripheral Slots or Module ExpansionYou also need to consider the number of signals you want to measure. Desktop PCs are an excellent measurement platform for low- to medium-channel-count applications. Most desktop PCs offer two to three PCI slots, and, depending on the measurement module type you need, provide enough space for applications requiring tens of channels. For higher-channel-count applications, PXI chassis have up to 17 slots, and you can daisy-chain multiple chassis for higher channel counts. Triggering and Synchronization Performance
Timing and synchronization are critical, often overlooked aspects of a measurement platform. In manufacturing applications that use a trigger bus, for example, you can measure, move, and inspect simultaneously, which results in dramatic manufacturing throughput improvements. If your application requires tight timing relationships among signals or measurement synchronization tasks, PXI is an ideal platform. With PXI, you have access to eight bused trigger lines that link all PXI slots so that measurement modules can interact, trigger, and control each other, as depicted above. The PXI backplane includes a star trigger line for accurate timing and minimal skew as well as a common 10 MHz clock to synchronize multiple modules. These technologies can dramatically increase test and manufacturing throughput. Many National Instruments PCI boards have a real-time system integration bus so you can synchronize sources, inputs, motion control, and vision. NI LabVIEW greatly simplifies this task by including tight integration with measurement hardware. Increased Reliability or Autonomous Operation
You can increase the reliability of your test system by running it in a real-time operating system (RTOS). The increased reliability of an embedded RTOS is important for applications such as expensive-to-recreate tests. Examples of these applications include tests that destroy prototypes, execute for long periods of time, or require the use of expensive resources like wind tunnels and particle accelerators. Real-time operating systems are increasingly present in mainstream applications. Building on this trend, NI released LabVIEW Real-Time in 1998 to bring real-time capability to a wide range of scientists and engineers. The LabVIEW Real-Time Module is an add-on component for the LabVIEW development system. When installed, this software compiles LabVIEW graphical code and optimizes it for the selected real-time target, such as PXI. With LabVIEW Real-Time, you have the capability to quickly design a custom PXI instrument that has all the flexibility and performance of a PC-based instrument as well as the reliability of an embedded RTOS, as shown above. Using this capability, you can embed LabVIEW Real-Time PXI systems within larger machines or deploy them remotely where they can run autonomously. For example, you can design a LabVIEW Real-Time PXI system in an unmanned submarine to perform remote machine monitoring. Ruggedness and Harsh EnvironmentPXI mechanical aspects are governed by Eurocard specifications (ANSI 310-C, IEC-297, IEEE 1101.1, IEEE 1101.10, and P1101.11), which have a long history of application in industrial environments (for example, VME and VXI). These electronics packaging standards define compact, rugged systems that can withstand harsh industrial environments in rack-mount installations. For example, PXI offers rugged mechanical packaging with filtered-forced-air cooling that provides extended operational temperature ranges up to 55 C. For rugged applications needing high performance, PXI provides superior protection against shock, vibration, and higher temperatures when compared to desktop PCs. Maintenance and UpgradingIf you plan to use the test system for many years, consider a low-maintenance solution. The PXI platform was designed with maintenance in mind. You can access all of the PXI modules and their connectors from the front of the chassis. The handle on the front of each module helps you insert the module correctly into the chassis and remove it easily. Two guides ensure that the module is aligned properly, which prevents damage to the pins on the backplane. You can remove one module from the chassis without affecting the rest of the system. For example, if you want to upgrade to a faster processor, you can remove and replace the controller unit. Because all of the devices are required to work with standard software, you can load a new controller with the software and device drivers and put it back into the chassis. The change should be almost transparent to you. Desktop PC maintenance can be much more time consuming. You must first remove the outer PC cover, which may involve unbolting it from the rack, before you have access to the PCI cards. CostPrice is just one factor that determines ownership cost. In general, using high-volume commercial technologies lowers the cost of system ownership. By taking advantage of Dell, Hewlett-Packard, and other computer manufacturer economics of scale, desktop PCI systems deliver the lowest ownership cost for low- to medium-channel-count applications. And PXI systems combine commercial software, bus, and component technologies to also deliver a low ownership cost for medium- to high-channel-count solutions. |
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