One of the key differentiators of Foundation Fieldbus is that with its PID Function Block, it supports control in the field and, as a result, single loop integrity. This means that with Foundation Fieldbus, it is possible to maintain control at the last set point without a host system.Implementing control in the field enables certain applications to be more efficient than with conventional instruments or control in the host. But, like all things, this also means some items need to be considered during the design process. Let’s start with those enablers first.
As we all know, if we do not have a reliable control system with availability as close as possible to 100 per cent, we quickly lose faith in the system and circumvent this with "jumpers," loops in manual and the like that defeat the purpose. The same is true for a fieldbus system. Heck, even "finicky" analyzers have a minimum acceptable availability of 95 per cent.A key part of any control system is the infrastructure that carries the signals from the field devices to the controllers, because without signals there, will not be data on which to control nor a means to manipulate the final control elements (valves, VFD, VSD, etc.) to adjust the process operating conditions. It therefore amazes me that for the sake of a few dollars, the same people who ask for "redundant everything" to maintain high reliability do not take basic precautions during the design, specification and installation of their fieldbus systems to prevent a single fault from adversely affecting the entire network.
One of the challenges that Foundation Fieldbus has faced to date is the integration of motors and drives as end devices. This has been a significant shortcoming, since variable speed and variable frequency drives are being used more and more often in projects because, in many cases, they are more energy efficient than control valves and offer equal or better flow control. If the user chooses a variable drive, he then must choose some other protocol such as DeviceNet or Profibus DP, resulting in a system for which the user must support multiple protocols. This is not necessarily a bad thing and we are all aware that we must always use the best tool for the task at hand – within reason.
There is lots of talk and coverage of industrial wireless technology in the trade press. Once again there are multiple camps at work; WirelessHART, OneWireless, the two industry standards camps and, of course, the ISA100 standard that is also under development, which both camps say they will fully support once it gets adopted. One good thing about the ISA100 standard is that the group developing it is planning to support the ability for it to effectively "tunnel" other standard protocols through or over the ISA100 protocol. This tunneling means that once ISA100 is available, each of the other fieldbus protocols should be able to operate with little or no change to the user layer of the protocol itself. That means when you implement Fieldbus, for example, over ISA100, all the functions and capabilities will remain the same as if you were running over wire. So other than these "standards," what are the other considerations associated with installing a wireless system?
All of us take standards for granted. Yet without them, much of what we do today would not be possible, including the use of the Internet, telephones, e-mail and the electricity on which we run it all.In Canada, the Standards Council of Canada oversees all the standards used within the country and represents us on the global stage. The council has offices in Ottawa but, like most standards-setting bodies, it relies on experts from various fields to provide the knowledge on which the standards are based.
Businesses are created for the purpose of generating profits. As such, all investments need to have a positive return to be justified. The incentive or driver for that justification is different at each stage of a project's life cycle.
The recent trend in Foundation Fieldbus power supplies and conditioners has been to develop units that have higher voltage and higher current capacities than needed. This is ironic, since the exact opposite trend is happening to the field devices at the other end of the cable. These devices require less power, lower current, and they are able to operate closer to the fieldbus-specified minimum of nine volts. Let's take a look at what might be considered a reasonable current load for a fieldbus system. As a rule of thumb, use a conservative value of 20 mA for each device when designing a fieldbus system. Most installations do not use more than 12 devices on a single network; not because of physical constraints, but rather to manage the risk of losing so many signals in the event of a single point of failure.
One of the difficulties faced by fieldbus technologies is supporting signal types that they were not originally intended to support. For example, although Foundation Fieldbus supports discrete signals with its discrete input and output blocks, there are few devices that actually make effective use of this capability. Several output devices, such as valves, support the on/off feature, but in the case of limit switches and similar "contact" devices, the present H1 direct options require a field-mounted device that reads the incoming contact or coil status.This is similar to a nano-PLC needing a local power supply voltage to wet the terminals, as well as those of the discrete device being measured/controlled. The device is not truly standalone because it requires a local power supply in addition to the communication cable.
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