Question: What are the top considerations I need to take into account when planning for control over wireless?
Answer: The prospect for control over wireless is one of the manufacturing industry’s hottest topics, but it’s important to realize that it’s not an appropriate solution for every application. Plants should consider three key factors when evaluating whether control over wireless is right for them:
1) Speed — Today, wireless instrumentation is not as fast as wired instrumentation. So ask yourself: Is the speed of the wireless solution appropriate for your control application? The best wireless technology today offers a one-second response time. So if you’re looking for something with a quarter-second or half-second response, that alone rules out wireless. Evaluate the sum of both sample rate and retry attempts. If the signal isn’t received the first time, you want the system to retry multiple times to ensure the operation is completed within your time budget. Some manufacturers specification give the radio update rate (no retries), whereas others specify an update that includes multiple retries. Make sure you know what you are getting!
2) Reliability — The reliability of the wireless system must match the requirement for the application. Wireless for critical applications, such as safety shutdown, is therefore not recommended. The only time wireless is recommended for critical applications today is when it serves as a redundant point alongside a wired solution. This actually improves overall reliability because two mediums are being used to reduce common-mode failures. The latest generation of redundant, high reliability wireless systems are quite appropriate for non-critical closed loop control, man-in-the-loop applications, or for indication and diagnostic purposes.
3) Latency — Latency is the amount of time it takes for a measured value to travel from the transmitter to the control system. A transmitter might have a one-second update rate, but that doesn’t necessarily mean it takes exactly one second for the signal to make it to the control system. There’s likely a lag time — anywhere between a few hundred milliseconds to as much as five times the sample rate — that must be taken into consideration. If a control system is automatically making adjustments to the process based on information that is up to five seconds old, this could potentially cause problems if the system does not factor in this delay. It’s therefore critical to understand the latency behavior of your wireless network. Architectural differences, such as using a high speed backbone versus sensor mesh, have a considerable impact on latency of signals.
In general, wireless technology is just beginning to mature in terms of use for control. While control over wireless is already gaining traction for non-critical applications, we are a long way away from a plant that is completely wireless.
Jeff Becker is director of global wireless business for Honeywell Process Solutions (Phoenix, Ariz.; www.hpsweb.honeywell.com)
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