Diagnostics in Device Busses Improve Operations

I caught up the other day with Emerson’s Dan Daugherty, whom you may recall from an earlier post on Foundation fieldbus operational benefits. Dan is a fieldbus consultant and has a lot of experience working with process manufacturers on their digital bus implementation planning.

Dan also shares that geek gene that afflicts me and many other engineers. For those old enough to remember the late sixties movie, The Graduate, the one word of advice offered to Dustin Hoffman by his father’s friend for his future was “plastics”. For Dan and I who came on the engineering scene in the 1980s, the advice given to us would have been “microprocessors”.

These pervasive, smart little pieces of processed sand are what make this ever-growing digital revolution possible. In our world of process automation, it means a host of digital communications protocols has come along including HART, Foundation fieldbus, Profibus (for Carl’s benefit!), DeviceNet, and AS-i bus to name a few. Note that I say digital communications instead of digital busses to avoid the argument that HART communications are not bus-based. As Dan points out, this is technically true with a bus being defined as parallel wiring used to connect multiple devices. It’s also true that digital communications occur over the HART 4-20mA analog signals.

This digital communications revolution has been fostered by ever-increasingly powerful, lower cost, and lower power-consuming microprocessors. Communications went from one-way process variable (PV) communications to bi-directional communications where PV, signal status, diagnostics, and configuration information could also be communicated between smart devices and automation systems.

Dan shared an example of a variable speed drive connected via Profibus DP or DeviceNet to a DeltaV system. These digital busses are typically used for motor control center (MCC) line-ups including drives, switchgear, and motor starters as well as non-MCC devices like on/off valves, discrete I/O blocks, and valve islands.

In the pre-digital bus era, it would take five pairs of wires to handle the basic functions of: start/stop, feedback, speed setpoint, speed feedback, and status. With Profibus DP or DeviceNet, you need two pairs of wires. Even with all these wires, the fundamental missing element of the old way was the lack of diagnostics.

Even more, because it’s a bus, you can get perhaps 30 or more devices on DeviceNet or Profibus DP segment, and that changes your ratio from 5:2 to 150:2. In some cases (e.g. drives), the power wire can come from another source, so it’s potentially 150:1. So, now you can get diagnostics you didn’t have before, some of which come in every cyclic data exchange (and are easily available to operators if you want them to have them, and deeper dive drill-down diagnostics your maintenance staff can access on demand.

Using these diagnostics to avoid a single abnormal situation likely will provide you a greater return on investment than the entire wiring, installation and commissioning savings provide.

Now, Dan will be the first to point out that nothing comes for free. When designing a project to incorporate Profibus DP or DeviceNet, you need to consider the topology of your segment, segment length limitations, devices per segment and power for the devices. Also, there are differences in the two busses even down to the physical layer. DeviceNet provides power and communications in one cable which has its advantages and disadvantages.

A key point is that all busses (or buses as Dan argues with me) have different purposes in life and it’s unlikely that a plant would or should standardize on just one. With systems like the DeltaV system which support multiple busses natively, you can choose your expensive equipment (ala the Construction Industry Institute’s PEpC process) and then the DeltaV system will be able to connect to which ever bus that equipment uses.

The good news is that once you have developed standards around these digital busses by working with Dan and the other fieldbus consultants, future projects become easier. These standards can extend from the wiring and installation considerations into how the diagnostics are used in defined, modular control strategies to help avoid abnormal situations and improve operational performance.

It’s amazing what these microprocessors have made possible.

Posted Friday, September 7th, 2007 under DeviceNet, Foundation Fieldbus, Profibus.