Wireless Sensor Technology for Real-Time Applications

The ability to make real-time decisions based on pressure, temperature, or flow measurements while a process is running can provide significant advantages in a measurement and control system. These advantages can be expressed in different ways such as cost savings through improved resource management, or reduced reliability upon mobile workers. While measurement data can be used to dynamically control a process, it can also be displayed over a network to allow remote monitoring of the process status in real time. Rising pressures, temperatures, or vibration intensity can easily be adjusted if the appropriate personnel are aware of faulty conditions. As data is collected for process control or a SCADA system, it can also be archived for future reference when a review of process trends could provide additional improvements.

While wireless sensing clearly offers many advantages, the adoption phase moves at a slow pace in many industries due to inaccurate perceptions of the technology. One of the main reasons wireless technology is yet to be fully adopted is price. Many organizations have not taken the time to explore the benefits achieved via wireless sensing in comparison to the price of implementing the technology. Similarly, a company may be content with their existing wired system and be reluctant to make the switch.

The second issue stems from concerns with the technology itself – namely, latency. This element is considered to be critical, and can vary depending on which wireless technology is chosen by an organization. However, latency is an inherent trait of all wireless technologies. Unlike a conventional analog signal, there are delays associated with the analog-to-digital conversion process, as well as the RF transmissions. The time it takes from the start of the conversion process when the raw measurement is taken, until the wireless signal from the sensor is received at the gateway or modem, is considered to be the total latency time. The latency for each wireless technology will differ. Having said this, a wireless sensor network will never be as fast or timely as a standard wired sensor network. Determinism that occurs between each layer of the wireless protocol (network, security, application, etc.) each takes a different amount of time, and this is all generally longer than its corresponding layer embedded in the wired versions of the protocols. Many wireless protocols also have to maintain very strict timing structures to make sure data packets are routed and delivered to the proper locations on time. Maintaining this timing will add to the total determinism and latency of the system.

Each wireless protocol will have its advantages. For instance, the ZigBee protocol offers low latency within the millisecond range and the ability to scale a very large network size. Bluetooth will have slightly higher latencies, but significantly higher data rate capabilities, whereas WirelessHART offers channel-hopping and the ability to interface with hundreds of thousands of HART sensors and equipment that may already be located within the field. That being said, a single WirelessHART network has a maximum size of about 100 devices.

Security is another concern when considering the adoption of wireless technology. While it may not be of the utmost concern in the commercial or industrial arenas, it certainly becomes a key issue amongst the military and medical fields. In many of these applications, it would be inexcusable for data to be lost or obtained by the wrong parties. The 128-bit AES encryption is used across the majority of wireless platforms. There are several precautions that can be taken on top of this in order to further protect the wireless data, but it should be noted they do not offer advantages over standard encryption; they simply add a simpler layer of security on top.

For instance, the ZigBee protocol, like any other protocol, has a well-defined packet structure. Portions of this packet structure can be formatted in a very unique fashion by making alterations to the Application Layer of the protocol. Only those who know the exact format of these packets will be able to properly decode the data. It should be noted that the AES encryption, for example, will contribute to the overall latency by making the wireless transmissions slightly longer in duration. This will, in turn, increase the demands of battery consumption.