NI PXI Chassis Design Advantages
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Overview
A fundamental component of PXI system is the chassis. In this section learn about specific NI design features that increase chassis cooling performance, improve the acoustical performance, enhance the reliability of the power supply, and allow you to easily configure and monitor your PXI system. Learn what NI does to provide the best possible specifications in each of these areas.
Table of Contents
Cooling
National Instruments chassis are designed and validated to meet or exceed the cooling requirements for the most demanding PXI modules. Before designing a prototype chassis, NI uses Flotherm, a 3D computational fluid dynamics software package that simulates airflow and heat transfer in and around electronic equipment, to determine the best mechanical and cooling configuration prior to producing a physical prototype. Engineers simulate the airflow and heat transfer in a chassis using different design options. For example, they can see the predicted effects of using different fans, mechanical dimensions, turning vanes, and other features within the chassis. This allows them to choose the best design option, which optimizes cooling for the power supply, module slots, controller slot, and backplane portions of the chassis, and start with this design as the first mechanical prototype. Figure 1 shows a Flotherm model speed contour of the NI PXIe-1062Q 8-slot PXI Express chassis prototype before optimizing the design.
Figure 1. NI PXIe-1062Q Speed Contour Preliminary Simulation
Figure 2 shows another Flotherm speed contour of the NI PXIe-1062Q after revisions were made to the chassis design. The main changes were a larger, more powerful cooling fan (see the sections labeled “2” on both figures), different fan mounts made of vibration-dampening material that brought the acoustical noise level introduced by the larger fans lower than the noise level of the original fans, and adding several additional turning vane. These changes resulted in approximately 45 percent more airflow than the initial design concept.
Figure 2. NI PXIe-1062Q Speed Contour Final Simulation
The power specification for PXI Express increased by approximately 20 percent compared to the PXI specification. Each PXI module is allowed to pull 25 Watts (W) of power, while each PXI Express module is permitted to pull 38.25 W. Because of this, the cooling performance of the chassis must also increase. As shown in Figure 2, NI addressed this critical need with a patented design of its PXI Express chassis. The power provided in the NI PXI Express chassis is approximately 20 percent greater than in PXI and the volumetric airflow is almost 50 percent greater. However, it is important to balance the acoustic noise level and airflow. Despite the nearly 100% increase of airflow in an NI PXI Express chassis, NI was able to reduce the acoustic noise by integrating the vibration-dampening fan mounts as discussed previously and using pulse-width modulated fans, which are examined in more detail in the Power Supply section.
NI also provides slot blockers, a plastic modular filler PXI card, to populate unused slots in the chassis. This enhances the airflow in the populated slots by reducing airflow bypass in the empty slots. The use of slot blockers can reduce the temperature rise of electronic components on installed modules by up to 20%.
Many NI PXI chassis have cooling fans in the rear of the chassis. The air is forced through a turning vane and evenly distributed across all module slots. This patented design provides increased cooling and fewer stagnant spots in airflow compared to chassis designs where the fans are located directly underneath the modules. Additionally, if one fan were to fail, the turning vane would evenly distribute the air provided by the other fan(s) to all modules. A chassis with the fans mounted under the modules would provide uneven airflow, and the location directly above the failed fan would have very limited to no airflow. Additionally, not having fans directly beneath modules reduces the amount of electrical noise subjected to the modules from the fans’ motors.
Accoustics
National Instruments takes great care in the design of its chassis to minimize acoustic emissions, despite the powerful cooling they provide. A combination of the fan speed control, the type of fan used, and the method in which the fans are mounted help optimize cooling while minimizing acoustic noise emissions.
Many NI chassis have a fan speed selection switch, permitting the user to set the fan speed to either High or Auto. When the fan speed is set to Auto, the chassis fan speed is controlled proportionally to the ambient air temperature. The ambient air temperature of the system is the temperature at the chassis fan inlet. When the temperature is below 30 °C (86 °F), the chassis operates in an “acoustic performance zone” which minimizes acoustic emissions (see Figure 3). As the ambient temperature rises above 30 °C, the chassis increases the fan speed accordingly.
Figure 3. PXI Chassis Acoustic Emissions Comparison
When the fan speed is set to High, the chassis provides maximum air flow regardless of the ambient temperature. This mode is appropriate for applications where acoustical noise is not a concern and the user wishes to enhance the long term life of the PXI modules in the system by using them at overall lower temperatures.
National Instruments uses pulse-width modulated (PWM) fans in many NI chassis to reduce acoustic emissions further than traditional voltage-controlled fans allow. By controlling a fan via a PWM signal, you can use a wider range of the fan’s RPM, which, in turn, allows the designer to finely tune the acoustic emissions and cooling of the chassis.
In order to meet the cooling requirements of the PXI specification, the fans selected for NI chassis must be powerful. Many of NI’s chassis employ fan mounts manufactured of vibration dampening materials to isolate mechanical vibration in the fans from getting into the chassis frame. The result is a further reduction in acoustic noise. Whenever fans, power supplies or other mechanical moving parts are introduced into a system, electrical noise is also introduced. Some NI chassis have fans located in the rear of the chassis, which limits the amount of electrical noise transmitted to the PXI modules.
National Instruments makes a series of chassis that offer “quieter” operation and are more suitable for use in lab and benchtop applications. These chassis are designated with a “Q” at the end of their product names. For example, the NI PXI-1042Q and PXIe-1062Q chassis are in the family of quiet chassis.
Power Supply
National Instruments has design ownership ofthe instrument grade power supplies in its 8-slot and higher PXI and PXI Express chassis. This means that NI can guarantee long-term availability for these power supplies and fewer design changes to its chassis due to power supply manufacturer changes. PXI vendors who rely on a standard PC power supply have a relatively low bargaining position because the quantities that they purchase are small in comparison to the PC industry. If a component is changed and it worsens the performance on a particular voltage rail, noise, or linearity, there is little that the PXI vendor can do.
The instrument-grade power supplies NI uses are optimized to meet the unique power requirements of PXI as opposed to ATX power supplies, which are designed for use in personal computers. They are custom designed for NI chassis to meet and exceed the PXI minimum power requirements. Table 1 lists the industry’s PXI Hardware Specification, Revision 2.2, power requirements and the minimum power requirements that National Instruments outlines for most of its PXI chassis power supplies (please see the user manual for specific NI model specifications):
Table 1. PXI Hardware Specification, Revision 2.2, Minimum Chassis Power Requirements versus NI PXI Chassis Minimum Power Requirements
The NI goal is for all its PXI systems to support any number of PXI specification-compliant modules up to the maximum number of peripheral slots available in the system. This eases system integration and ensures user success by removing power supply inadequacy risks.
As new applications emerge for PXI-based systems, the temperatures at which these systems can fully operate become important. There is an increasing demand for PXI chassis to operate at high ambient temperatures (up to 55 °C) with minimal power derating. Power derating refers to the loss in available power supplied to the chassis slots when operated at higher temperatures or other extreme specs. Many other vendors’ PXI chassis meet the required PXI specification for available power at lower ambient operating temperatures (20 to 35 °C) but may become unstable or inoperable at higher temperatures (>40 °C). NI instrument-grade power supplies offer the minimum power requirement over the entire specified operating temperature, typically 0 to 50 °C or 55 °C, with no power derating. Again, you can operate a fully loaded chassis at the highest temperature specified on the data sheet. (Refer to product manuals for operating temperature ranges for specific NI chassis.)
The electrical noise generated by chassis fans can degrade the measurement accuracy of PXI and PXI Express peripherals. To help prevent this, National Instruments incorporates a separate, dedicated 12 V power supply in most of its chassis to power the chassis fans, the system slot, and in some cases the power supply fans.
Most NI chassis also feature remote sensing of the output voltage. Remote sensing is used to compensate for voltage drops in the output lines. This design feature is important for PXI and PXI Express chassis in high-power applications. Remote sense provides better regulation at the backplane when there are large load swings.
For systems that require high availability, NI has designed an easy-to-replace power supply and fan shuttle for most of its 8- and higher-slot chassis in the event of a failure. You simply remove the thumb screws that secure the shuttle in place and pull on the two rear handles on the shuttle. After the shuttle easily slides out, you can insert a new one. This design makes the mean time to repair (MTTR) of the power supply to less thanfive minutes. You do not need to remove chassis that are rack-mounted from the rack to replace the power supply and fan shuttle. As long as the back of the chassis is easily accessible, you can replace the power supply and fan shuttle. With these chassis, there is no need to remove the modules or redo any module wiring.
Other NI PXI Design Advantages
For more information about how National Instruments validates our chassis design, what verification tests each manufactured unit undergo, and what software and services are available, please view the other sections of this series:
NI PXI Chassis and Controllers Design Advantages
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