At the Achema show last month, a friend working on the trade-show floor for one of the major suppliers came to me deeply puzzled. On one of his panels, a DCS is connected to a positioner and is able to switch between WirelessHART and FSK HART. He first showed me using the FSK HART; the DCS sent out a positioning command and got a response in about 5 seconds. However, when he switched to WirelessHART, he suggested we go and grab a cup of coffee. It would take whopping 5 minutes for the device to reply to the DCS!
This may be the most extreme case I have seen so far. More typically, the response time is about 30 seconds, but it is still far from sufficient for effective control applications.
So is this as fast as WirelessHART can get?
Before asking this question, let us look at how the performance is decided.
If you have read my blog (“Is a Smart Device really Smart?”), you would understand that devices do not have “the freedom of speech.” It is the network manager that decides when the devices listen, talk, and idle. So when it comes to the performance issues of the network, devices don’t have much to do with the network performance.
Let us take a more technical view. As shown in the device’s DLL design diagram below, wireless transmission is decided by the link table. It determines the next timeslot to be serviced. An example link table entry would tell the device that the next time slot is a normal slot and transmit on Channel 10, or a normal slot and listen on Channel 6.
A WirelessHART time slot is 10ms long. Imagine if we have a request time slot + process time (usually this can be done in ms) + a response time slot. In this case, 30ms should be enough. A custom project with a major DCS vendor shows we can easily achieve less than 280ms, which is roughly 4 bursts a second. This rate is fast enough for most process control applications including compressor surge control.
Of course, for end-to-end response time, we still need to know the time it takes for the control system to respond – which is beyond the WirelessHART control question.
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