Intrinsically safe networks such as Foundation Fieldbus and Profibus, at least today, aren't physically capable of handling smaller devices such as sample system components and don't have the intrinsic power capability to support multiple devices without costly extensions.
Another critical barrier to automation has been lack of smaller sized devices (commensurate with the size of sample systems) such as actuators. Miniature actuators are scarce in the instrumentation field, so we haven't been able to borrow from that source. Low cost transmitters, now commonly available for instrumentation, aren't compact enough for a sampling system. The relatively smaller and fragmented market and unique technology requirements have kept the process analytical discipline from automating sooner.
The long and torturous learning process needed to develop proper extractive sampling techniques has created a very conservative mindset regarding acceptance of new technology. In addition, slow recognition of process analytical as a true discipline that crosscuts traditional engineering boundaries (instrumentation, piping, electrical) as well as functional boundaries (lab, process control, maintenance, engineering) has hindered acceptance and understanding needed to address special requirements of process analytical systems such as a purpose-built bus and local control.
NeSSI Generation II recognized the need for a bus specific to the needs of sampling in a hazardous environment. Two variants are at hand — Siemens provides one bus, called I2C, while the other comes from CAN in Automation (CiA) (see "Intrinsically Safe NeSSI Nears," www.ChemicalProcessing.com/articles/2008/147.html) and has been adopted by ABB. Both are intrinsically safe, ultra-compact and suitable for operation in Zone 1/Division 1 environments. They also can handle as many as 20 or 30 components. (The trick is to lower the voltage to 9.5 v to allow current loads in the range of 1 amp.)
The first working examples of these buses essentially are modified extensions of on-board digital buses that have been silently operating, without a hiccup, for years inside many of our gas chromatographs. Once these buses and NeSSI-bus enabled sensors and actuators come to the marketplace, we have the tools in hand to automate our sample systems. It sounds easy but the need for different components to play nicely within a specified power budget will pose a challenge. Of course, if the sampling system isn't located in a hazardous area, the NeSSI bus can be used without an intrinsic safe power supply and associated power constraints.
Figure 2. NeSSI Architecture: Modular system includes
Having a suitable bus allows us to move on two other critical issues: How do we unburden the DCS and manage our own signals? And how do we do closed-loop control and execute simple control tasks for process analytical specific requirements? We can use a NeSSI-bus-enabled local controller rated Division 2/Zone 2 (since it can be located outside of a sampling handling enclosure). A physically large controller would defeat miniaturization efforts; we need a "hockey puck" sized programmable device that talks NeSSI-bus on one side and Ethernet or a fieldbus protocol on the other. We call this device a Sensor Actuator Manager (SAM). This SAM functionality to date has been typically embedded within smart analyzers such as gas chromatographs. Some SAMs employ a programmable logic controller (PLC) to control a sample system. Unfortunately the sample system applets developed for these SAMs have been platform-specific and proprietary. At one CPAC workshops, attendees came up with a list of 60 applets that could provide a standard set of functions to allow a technician to set up, monitor and control a sampling system (and microanalytical device) without custom programming.
Figure 2 shows the NeSSI architecture with mechanical and electrical bus rails along with a SAM. The SAM manages bus signals and controls the sample system via programmable applets. It also serves as an interface between a Zone 1/Division 1 NeSSI–bus handling the sample system sensors and actuators, and a higher-level communication bus. This arrangement allows plug-and-play capabilities of devices within a hazardous location. A wireless personal digital assistant (PDA) or personal computer (PC) enables interaction with the SAM and provides a graphical user interface to visualize flow paths. ("O&M" in the figure refers to an operator and maintenance station in a control room or even offsite.)
Breaking Old Habits
Making optimal use of a NeSSI-bus and distributed control embodied by the SAM will require us to rethink what we've been doing over the last 70+ years.
Sample system fabrication techniques. Size and weight do matter with NeSSI. Because our plan is to get the equipment by-line (i.e., next to the sampling point), it's important that a system is small and light so that service can be done on a replacement basis. Today we have the ability to fabricate modular miniature systems that should be able to be assembled Lego-style by an unskilled person. Yet in many cases we build modular systems that haven't been optimized for space or still require custom tubing work. We should aim to tightly integrate the modular system with its enclosure, to reduce size and weight, as well as eliminate custom tubing. Use of graphical indication of the flow paths certainly will overcome reluctance to use a densely populated sampling system.