The NI SC Express family offers PXI Express data acquisition (DAQ) modules with integrated signal conditioning for high-performance sensor measurements. National Instruments used the industry-standard PXI platform and implemented unique analog design technologies to provide increased accuracy, high sensitivity and bandwidth, and tight synchronization. When combined with powerful and flexible NI software, the SC Express family provides you with the tools you need to build advanced measurement systems that scales from tens to thousands of channels.
Figure 1. The PXI Express sensor measurement system is powerful and scalable.
1. Achieving High Performance with SC Express
SC Express Design Minimizes Isothermal Error in Thermocouple Measurements
Cold-junction compensation (CJC) error is often the largest error in thermocouple measurement devices. The primary factors in CJC errors are thermistor error and isothermal error. The NI TB-4353 isothermal terminal block uses high-accuracy thermistors that contribute only 0.15 °C of error when used at room temperature, including error from the measurement device. Isothermal error is defined as the temperature difference between the thermistor and the actual temperature of the cold junction. The system environment and design of the measurement device impact this temperature difference. The TB-4353 has a unique design that allows heat to dissipate evenly between the cold junctions and the thermistors. It also features eight thermistors that are distributed on the terminal block and measured by the NI PXIe-4353 to further increase the accuracy of the CJC measurement.
Figure 2. The TB-4353 has eight CJC thermistors to measure any temperature gradients across the terminal block.
SC Express Implements Ratiometric Approach to Wheatstone Bridge-Based Measurements
Many sensors, including strain gages, load cells, pressure sensors, and torque sensors, are based on the concept of a Wheatstone bridge. To accurately measure the ratiometric output of a bridge-based sensor, you must know both the bridge output voltage (VCH) and the excitation voltage. The NI PXIe-433x bridge input modules use circuitry that continuously measures the excitation voltage and applies it as a reference to its analog-to-digital converters (ADC). In this way, variations in the excitation voltage are compensated for, and the modules return data as a ratio of the bridge output voltage and the excitation voltage. This approach removes the measurement’s dependence on the stability and accuracy of the excitation voltage. Because of the ratiometric design, measurements with the NI PXIe-433x bridge input modules are less susceptible to temperature drift and other variations in the excitation voltage.
Figure 3. The output of a Wheatstone bridge is the measurement between the middle nodes of the two voltage dividers.
To view more about Ratiometric Bridge-Based Measurements, Download the Free “Achieving Higher Performance with SC Express” Resource Kit or Read the “Measuring Bridge-Based Sensors with the Ratiometric Approach” White Paper
Anti Aliasing and Low Pass Filters for Superior Signal to Noise Ratio
Aliasing is an undesired effect where the ADC digitizes frequency components of a signal that may be above the Nyquist limit. If an input signal contains several frequency components or harmonics, some of these components might be represented correctly while others contain aliased artifacts. The PXIe-449x series employs both analog and digital lowpass filtering to eliminate components above the Nyquist frequency. Removing the high frequency noise allows the full 24 bits of the ADC to be applied to the signal of interest.
Fiber Optic Sensing Addresses Common Measurement Issues
Extreme electrical conditions, such as potential exposure to high voltage and ambient electromagnetic interference (EMI), are a common source of noise in measurements taken with traditional sensors. The NI PXIe-4844 Optical Sensor Interrogator (OSI) for Fiber Bragg Gratings eliminates these concerns by bringing the EMI immune and non-conductive advantages of Fiber Optic Sensors to the PXI platform. The use of traditional sensors may be problematic in corrosive or explosive environments. However, the chemically inert, electrically passive sensors used by PXIe-4844 OSI module make it immune to the risk of sensor degradation or sparks. With this module, you can read from up to 80 different sensors per module with the best in class sensitivity and accuracy characteristic of the SC Express product family.
To view more about Fiber Optical Sensing, Download the Fiber Bragg Grating Optical Sensing E-Kit
PXI Express Increases Data Throughput
SC Express modules are based on x1 PXI Express, which offers 250 MB/s of bandwidth per device to stream data back to the controller. Because of the added bandwidth provided by PXI Express, the SC Express modules offer simultaneous sampling options using the same channel counts as multiplexed devices. Unlike multiplexed devices that reduce sampling rates as you add more channels, you can use the simultaneous sampling devices to maintain sampling rates as you expand the number of channels. As seen in Figure 4, the SC Express modules can scale to high channel counts at maximum sample rates without exceeding the communication bus bandwidth.
Figure 4. SC Express modules scale to high channel counts without exceeding communication bus bandwidth.
PXI Express Timing and Synchronization
As the latest evolution of the PXI platform, PXI Express has been designed to deliver a higher degree of synchronization to measurement I/O devices while remaining backward-compatible. The 100 MHz differential clock in the PXI Express backplane allows multiple devices to base their sample clocks on the same reference. The backplane trigger lines allow for coordinated triggering of all the devices in the chassis to the same 100 MHz clock edge. As with previous PXI chassis, a timing and synchronization module can replace the backplane clock with a higher-precision time base in a single PXI Express chassis or it can also be used to synchronize measurement devices in multiple PXI Express chassis. If the application requires multiple chassis to be synchronized over large distances, then GPS or IRIG-B PXI timing modules can synchronize multiple chassis by a common clock signal.
To Read More about Synchronization with SC Express, Download the Free “Achieving Higher Performance with SC Express” Resource Kit or Read the “Synchronization Explained” White Paper
2. Software Features and Technologies
The development and operation of Windows-based PXI systems is no different from that of a standard Windows-based PC. As a result, the controller can use industry-standard application programming interfaces, such as NI LabVIEW, LabWindows™/CVI, and Measurement Studio; Visual Basic; and Visual C/C++. As an alternative to Windows-based systems, you can use a real-time software architecture for time-critical applications requiring deterministic loop rates and headless operation (no keyboard, mouse, or monitor). Real-time operating systems help you prioritize tasks so that the most critical task always takes control of the processor when needed.
Figure 5. NI-DAQmx driver software abstracts data acquisition bus technology and provides a consistent API across multiple programming languages.
Easy and Powerful Driver Software
NI-DAQmx driver software goes far beyond a basic DAQ driver to deliver increased productivity and performance and is one of the main reasons National Instruments continues to be a global market leader in PC-based data acquisition. NI-DAQmx driver software controls every aspect of your sensor measurement system from configuration, to programming in LabVIEW, to low-level operating system and device control. The same driver software is used across National Instruments DAQ and signal conditioning hardware. Figure 6 displays benefits included with more than 200 DAQ devices supported by NI-DAQmx to provide ease of use, flexibility, and high performance on multiple programming platforms, including LabVIEW, LabWindows/CVI, C/C++, Visual Basic 6.0, and C#/Visual Basic .NET.
Figure 6. DAQ devices supported by NI-DAQmx provide ease of use, flexibility, and high performance on multiple programming platforms.
While the PXIe-4844 Optical Sensor Interrogator module does not use the NI-DAQmx driver, you still have the ability to communicate with your Fiber Optic Sensors via the NI-OSI driver. You can also use the NI-OSI Explorer for out of the box sensor configuration.
LabVIEW Graphical Programming Language
LabVIEW is an integral part of virtual instrumentation because it provides an easy-to-use application development environment designed specifically with the needs of engineers and scientists in mind. LabVIEW offers powerful features that make it easy to connect to a wide variety of hardware and other software. One of the most powerful features that LabVIEW offers engineers and scientists is its graphical programming environment. With LabVIEW, you can design custom virtual instruments by creating a graphical user interface on the computer screen through which you acquire, analyze, and display your data.
Figure 7. The DAQ Assistant provides a single function for all data acquisition programming.
You can customize front panels with knobs, buttons, dials, and graphs to emulate control panels of traditional instruments, create custom test panels, or visually represent the control and operation of processes. The similarity between standard flow charts and graphical programs shortens the learning curve associated with traditional, text-based languages.
National Instruments PXI sensor measurement systems implement signal conditioning specific to your sensor to ensure the highest accuracy of your measurement. Data acquisition and signal conditioning modules on the PXI platform tightly synchronize with more than 1,500 modules and have different chassis and controller configurations, depending on your application requirements. The power and flexibility of LabVIEW and the NI-DAQmx and OSI device drivers empower you to design your measurement software to meet your diverse and ever-changing analysis and logging needs. Whether you are measuring a small number of the same sensor or more than 1,000 channels of diverse sensor types, the modular PXI platform and NI software will scale to fit your needs.
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