Industrial network integrity

InTech, Oct 2006 by Verhappen, Ian, Byres, Eric

New-era industrial network communications require fresh skills and tools

If the reliability of the process rides under a veil of question and uncertainty, there is big trouble.

With industrial communications networks playing a critical role in today's control systems, it is vitally important these networks have the highest level of reliability possible.

The increasing use of fieldbus protocols for communication to field devices and the expansion of the control network to the final field elements (i.e. the transmitters and control valves) means if networks do not achieve the expected degrees of reliability, the whole reliability of the process is questionable. That is big trouble indeed.

Reliability, regardless of where it is necessary, requires both high availability and integrity of the system-a system that fails regularly can be as bad for production as a system that sends the wrong data.

To meet this objective, it is necessary to have the right people with the right knowledge and tools to design and maintain a system. Look at where some of these resources reside.

Point-to-point on the outs

As long as there have been signals connecting one control device to another, there have been challenges associated with keeping the control system reliable.

For example, blocked pneumatic airlines, ground loops in 4-20 mA wiring, and noise on RS-232 circuits all reduce the reliability of the system.

Industry wants reliable to mean 100% availability, but we never really can achieve that. The lofty ideal of Six Sigma or 99.9999% availability will still result in about 30 seconds/year of downtime (99.99999% availability results in about 3 seconds/year downtime).

The expense associated with recovering from the unplanned 30-second interruption can be significant For example, a 30-second outage on a fluid coker can result in a 30-40 day shutdown to clean the vessel, with the resultant loss of production.

By looking at some of the common maladies associated with control system signals over the years, one will be able to see how each subsequent generation of industrial communications reduced the impact of an earlier fault but added other concerns that needed attention too.

The oldest (but still widely used) control networks are the pneumatic airlines connecting transmitters to valves with local controllers. In some cases, signal lines also went back to a central control room.

The most common problems faced with pneumatic systems are air leaks, dirt, and varying system pressure. Air leaks result in wasted air but can also mean a depression in the control signal.

Dirt, which can include condensation and corrosion products, will plug up the various small orifices within the system. Besides damaging to the equipment, dirt also affects the operation of the device by effectively changing the size of the control orifice.

From the 1970s onward, pneumatic control gave way to analog (4-20 mA) current loops. This avoided all the issues associated with air, only to have other challenges appear, such as signal noise from other energy sources, ground loops, and calibration mismatches due to either configuration error or device drifting over time.

To overcome some of the limitations about calibration, smart instruments (typically with a HART interface) now have become the norm for analog control.

Smart instruments include device diagnostics to monitor the health of the device and twoway communications that make it possible to verify the information in the control system and the field device match.

However, this generation of instrumentation is still analog at heart, with point-to-point wiring and the limitations associated with only being able to communicate to one device at a time.

The last 10 years have seen the emergence of fieldbuses as the replacement for point-to-point analog or hybrid control systems. These buses incorporate the field devices themselves as part of the control network supporting true bidirectional communications with verification of all the parameters at both ends of the network.

Because of the environment in which field devices must reside, fieldbus systems typically operate at slower dialup-modem speeds.

The next generation of field devices and networks based on Ethernet technology are now deploying. Most experts agree Ethernet is the high-speed communications medium of the future, no matter whether it platforms on copper cable, fiber, or wireless media.

To maintain the expected high degree of reliability, Ethernet protocols intended for deployment in the industrial environment (outside the control room) are typically restricted to 10Mbps or 100Mbps to improve reliability and ease installation in a typically harsh environment.

Number, diversity of devices

What is interesting about the above technologies, whether pneumatic or Ethernet, is they all need proper design from the start to be reliable.

This requires good engineering and the selection of high quality products that are also designed properly to work in the environment they will be used. For example, expecting a home Ethernet switch to provide reliable service on a plant floor is like expecting your garden hose to work as a pneumatic airline-it just is not realistic.