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Dec
04

Control Using Wireless Transmitters

Over the last few years the process industry has expressed a growing interest in the application of wireless technology for field measurements. The ISA-SP100 Committee was established in early 2005 to set standards and recommended practices for implementing wireless systems in the automation and control environment with a focus on the field level. Also, various industry consortiums have been established to promote the use of wireless technology. For example, the Hart Communication Foundation has adopted the use of IEEE 802.15.4 physical layer for the implementation of wireless HART. At the ISA2006 conference the HART Communication Foundation sponsored a booth in which wireless transmitters from multiple vendors were demonstrated. However, one of the technical challenges that manufacturers face in applying wireless technology to process measurements is how to reduce the power consumption to a level that can be supported for many years without the need for external power.

If the information from a wireless transmitter is only used to monitor slowly changing measurement values e.g. levels in a tank farm then the transmitter power requirements may be minimized by simply slowing down how often a measurement is made and communicated. However, if the measurement is used in control applications that respond in seconds rather than minutes, then simply slowing down how often a measurement is made and communicated will negatively impact control response. To provide best control, it is necessary to reduce the latency in control response to setpoint or load disturbances. In a traditional control system it is possible to minimize latency by over-sampling the control measurement used in control. However, such an approach is not an option if your objective is to minimize wireless transmitter power consumption.

One means of reducing the need for over-sample control measurements is to synchronize the measurement sample with control execution as is done in Foundation Fieldbus device. Using some of the proposed wireless protocols, such as Time Synchronized Mesh Protocol (TSMP), it is possible to synchronize a measurement sample and its associated communication with control execution done in another node. However, the traditional approach of executing control 4-10X faster than the process time constant still will create communication loads that are a barrier in applying wireless devices in faster process applications.

A few years ago we started looking at techniques that could be used to reduce wireless communication load without sacrificing control performance. It turns out that for many applications a 10X reduction in communications load can be achieved by following simple rules in communication and by restructuring the PID control to use non-periodic sample values. Much of this work is documented in a paper that we presented at ISA2005, Similarity-Based Traffic Reduction to Increase Battery Life in Wireless Process Control Network. An overview of this work is provided in the following:

Control Using Wireless Transmitters

If you would like to learn more about the wireless technology, then a good starting point is Protocols and Architectures for Wireless Sensor Networks (Hardcover) by Holzer Karl and Andreas Willig.