Choosing analog communication protocols
Are all HART-enabled instruments 4-20 mA? Are there other options?
Dear Control Engineering: After reading the article on using HART in an asset management program, I was wondering if all HART-enabled devices use a 4-20 mA analog signal, or if there are other options.
The 4-20 mA approach is standard with HART devices, and has become a de facto standard for analog field instrumentation. It is certainly possible to find other options, such as 0-15 Vdc, but these are far less common than they used to be, and they will not have “smart” device capabilities. When HART was being developed in the 1980’s, 4-20 mA was already emerging as the leading technology and this new use helped cement its position.
Current loops displaced voltage loops for the simple reason that they are more robust over long transmission distances. Voltage is more affected by wire resistance. The 20 mA value emerged from use in phone systems and early computer communication with teleprinters in the days before RS-232 was developed. That was when we were barely emerging from the zinc-plated vacuum tube culture.
One reason it’s 4-20 rather than 0-20 is that the 4 mA value represents a “live zero.” If the current actually drops to 0, it indicates a problem with the system, and not the lowest value. Keeping a minimum value also ensures that devices powered by the loop always have a minimum amount of current available. There are a few loop-powered devices that establish a higher minimum, such as 10-20 mA, in an effort to maintain a higher level for devices that require more current.
Peter Welander, email@example.com
Annual Salary Survey
After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.
The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.