Incorporating Wireless Measurements with Wired Data Acquisition Systems

Wireless technology provides promising possibilities and benefits – reduced cost of cabling; measurements previously prohibited by physical location; distributed measurements; and intelligent, self-healing networks. Wireless is poised to play a significant role in shaping the capabilities of future measurement systems. But exactly what role and how? Will wireless replace wired systems? Will existing investments still be useful in future systems? What important decisions should be made before selecting a wireless data acquisition device? When should wireless not be considered as a possible solution?

There are many questions that need to be answered and there is still time to contemplate the possibilities before your existing measurement systems run the risk of technology obsolescence. This article helps to answer some of these questions by considering several high-level design decisions of wireless measurement systems and how to complement existing wired systems with wireless technologies.

Wireless Measurement Systems

While adoption of wireless technologies in test, measurement, and control applications has been dwarfed by that of consumer electronics, the interest is no less. However, replacing wired systems with wireless is not as simple as unplugging the wires and putting a wireless network in place. Through decades of use, experience, and technological innovation, engineers come to expect certain things out of measurements systems that wireless systems cannot sufficiently answer. Two important uncertainties shared by many considering wireless systems involve security and reliability. To address these concerns, wireless standard organizations, often led by leading silicon manufacturers, continue to implement security and wireless reliability improvements into new iterations of wireless standard protocols, thus giving data acquisition providers the ability to take advantage of the improvement in security and reliability by using a compliant radio and software architecture.		Simple. Secure. Wi-Fi Data Acquisition. Watch the Wi-Fi DAQ Guided Tour  »
Still, there are additional requirements wireless measurement systems currently struggle to fulfill when compared to existing wired systems including data bandwidth and latency, synchronization, I/O selection, and integration within a multivendor system.

Bandwidth and Latency

PC-based measurement systems are often limited within the constraints of the bandwidth and latency specifications of the physical bus they use for communication. Bandwidth equates to the amount of data that can be transferred across the bus in a certain amount of time. Latency roughly sets the rule for how fast the data can get from its starting location to its destination. When comparing the bandwidth and latency specifications of wireless to that of other popular buses used in data acquisition applications today (PCI Express, PXI, USB 2.0), it’s like stepping into a time machine and going back 25 years or more.

Two popular wireless networks being adopted in wireless measurement products include IEEE 802.11 and IEEE 802.15.4. Also referred to as Wi-Fi, IEEE 802.11 is popular for home and office networks. IEEE 802.15.4, the protocol which ZigBee is based upon, is popular for low-power, distributed networking. The theoretical bandwidth of these two buses is comparable to or worse than the ISA bus popularized in the 1980s. When compared to a first generation x1 (by-one) PCI Express link, 802.11n (the most recent iteration of the bus) and 802.15.4 provide 10x and 1000x less bandwidth, respectively.

This inherent limitation in wireless networks implies that wireless cannot replace wired systems for all use cases. High-speed, high-channel-count dynamic measurements continue to benefit from high-bandwidth buses physically connected to PCs. Other low-speed (static) and lower channel count dynamic and sensor measurements that do not strain bandwidth limitations of existing buses can take advantage of new wireless technologies.

Synchronization

An important criterion of most measurement systems is synchronous measurements across multiple channels, devices, and even systems. Synchronization is accomplished through various ways, but in general involves sharing a clock or trigger signal via physical wire, or through a time-based approach where multiple, local time-bases synch their oscillators to a common point in time and then operate at a similar frequency. These synchronization techniques have respective benefits and drawbacks. Signal-based synchronization enables more precise, tighter synchronization between different channels, devices, and systems (ns- or ps-level precision possible), but limits the distance between synchronized systems (maximum of 100 m or less). With time-based synchronization, you can synchronize systems over much longer distances (potentially limitless if using GPS), but the possible precision decreases (ms typical).

Regarding timing and triggering, many current wireless measurements systems operate independently of others providing no way to share a signal or time-based signal for synchronization. For measurements where multiple channels of acquired data and phase relationship of the signal are pertinent to obtaining accurate results, synchronization is of the utmost importance. Many wired measurement systems used today for such systems employ very accurate timebases, phased-locked loop (PLL) circuitry, and impedance-matched signal paths. Based on principle alone, wired systems are required for the most stringent requirements of synchronization. However, wired networks and wireless networks are poised to benefit from emerging standards and additional research as evident by IEEE 1588 and GPS technologies.

I/O Selection and Power Availability

As intriguing as wireless sounds, the technology is still young as far as test, measurement, and control industries are concerned. This limits the number and the capability of available devices. There are hundreds of different sensors, all of which require specialized signal conditioning to provide accurate measurements. For more than 20 years, National Instruments has innovated and shipped PC-based measurement products to enable these measurements totaling more than 50 million measurement channels worldwide. Wireless measurement systems will not replace these existing measurement channels, but provide benefits that complement existing systems where applicable.

Integration within a Multivendor System

Though this article is not inclusive, the last and most important limitation of today’s wireless measurement systems is their ineffectiveness at operating with other measurement and control systems, whether wired or wireless. As might be said in many industries, when a technology is new and full of potential, time-to-market of a solution might be considered more important than its completeness and interoperability. Typically, today’s wireless products focus on making a general measurement (voltage or current) and then transmitting that data securely and reliably while consuming as little power as possible. The focus is on hardware and sometimes proprietary wireless network.

Little attention is spent on the software side of the devices which provides the interoperability to the larger enterprise. As the Process Industries Editor of Control Engineering wrote in his November 2007 article on wireless topologies, “Getting the data out of the device and into the control system usually uses proprietary software that is not interoperable across platforms.” You could say they do their one specific job well. The problem is that measurement products deployed in any industry must be able to speak to other measurement and control products; regardless of where or how that data is acquired. For wireless adoption to grow beyond the hype, interoperability through a higher-level software environment must be addressed.

Hybrid Measurement Systems and Wireless Technologies

Because of the limitations of existing wireless technologies and products, many applications struggle to be solved by a exclusively wireless solution. Most measurement systems have some requirement which forces the integration of a wired system, whether it is bandwidth, synchronization, I/O availability, power requirements, or system integration. You realize the maximum benefit of wireless technologies through use in a hybrid system. Hybrid systems combine components of multiple measurement and control platforms regardless of their location, data transfer method, and vendor. Hybrid systems revolve around a central PC architecture, which may combine stand-alone instruments based on Ethernet or GPIB, PC-based instruments in PXI, portable measurements through USB, and wireless measurements over Wi-Fi or ZigBee. Hybrid systems use an open-software development environment to manage and communicate across the entire measurement and control system.

Figure 1. Hybrid measurement systems employ an open software platform to combine measurement products regardless of communication bus and vendor.

The key to creating and maintaining a hybrid system is implementing a system architecture that transparently accommodates multiple bus technologies and uses an open software platform to communicate across vendor-specific systems. By taking this approach, you benefit from picking the best data acquisition and control hardware for a specific task based on the task specifications. NI LabVIEW can provide the glue necessary to make the entire system work. With LabVIEW, you can reuse existing measurement systems including PC-based data acquisition, modular instrumentation, stand-alone instrumentation, and integration of new wireless products. Some examples of integrating LabVIEW with wireless technologies include:

• Communicating via standard protocols using built-in LabVIEW libraries including TCP/IP
• Deploying LabVIEW with the LabVIEW PDA module to PDAs with Wi-Fi and Bluetooth communication
• Connecting existing Ethernet-based NI programmable automation controllers (PACs), including NI CompactRIO and Compact FieldPoint, to industrial Wi-Fi access points and GPS radios
• Using LabVIEW instrument drivers provided to communicate to a variety of third-party wireless sensor nodes

For additional information on integrating wireless measurements with existing systems, refer to the white paper Develop a Wireless Measurement Systems.

Coexistence of Wired and Wireless Systems

Of the technologies on the near roadmap, wireless is one of the most promising technologies for data acquisition. However, there is a transition period where new technology does not replace old technology, but must interoperate with it. This trend is no less true in test and measurement as new systems involve the combination of new, modular-based instrumentation like PXI along with legacy stand-alone or VXI instrumentation. Through the use of an open software platform such as LabVIEW, you can begin adding wireless measurement capabilities to achieve the benefits of the technology, while reusing the existing measurement system investments.

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