How one plant cut 44 wires down to 2

Wiring comparisons between hardwired, digital fieldbus, and wireless instrumentation are plenty. However, most make the assumption that one device equals one real-time signal and one pair of signal wires. But this is generally not correct. A simple hardwired temperature transmitter makes only one measurement and only requires a single pair of wires and a single system analog input channel. However, other transmitters and final control elements have more than one signal each, particularly if you consider using the full capability of devices, not just a small subset to minimize wiring. For instance, one electric actuator / Motor Operated Valve (MOV) has 44 screw terminals for all its control and feedback signals. It is not a 1:1 relationship. Moreover, a digital wireless or fieldbus temperature transmitter can support 4 or 8 sensors respectively, taking the place of 4-8 signal wires, and 4-8 transmitters. So in view of the much greater number of signals per device, how much wiring can actually be saved, and how much functionally can be gained by going all-digital? Here are my personal thoughts:

Process automation field device may have on average 3 I/O signals each for real-time measurement, control, and feedback etc. not even counting internal variables. A guided wave radar level transmitter, conductivity, or pH analyzer has 2 measurement signals; a valve positioner will have 2 I/O signals if it has continuous position feedback and 3 signals if it has limit switch feedback. An on-off valve also has 3 I/O signals. A Coriolis mass-flow meter has 3 measurement signals and a magnetic flow meter has 4. More sophisticated devices have more I/O signals. For instance, process gas analyzer has 5 I/O signals, electric actuators / Motor Operated Valve (MOV) have 16 I/O signals, a gas chromatograph may have as many as 25 I/O signals depending on the application. A variable speed drive can have even more I/O.

Since process automation field device may have on average 3 I/O signals each, connecting all these signals would translate into a lot of cable and I/O cards in a hardwired system of thousands of devices. Because of the high cost of wires from device to junction box, the junction box itself, the home run cable from the junction box to the marshalling cabinet, the I/O cards, the backplane, the power supply, and the other installation hardware like conduit and cable trays etc. as well the labor to lay the cable, cut, label, strip, crimp, and screw all the terminals along the way, all the signals were not connected in most plants. Typically only a bare minimum of signals required for each device were connected. This way the I/O count for the system could be dramatically reduced. However, when only part of the I/O signals are connected, only part of the functionality of the device can be utilized. That is, the plant is not fully benefitting from the capability of the device. For instance, for the MOVs only the basic control signals and feedback signals were connected, guided wave radar level transmitter provided either the surface level or the interface level, not both, and so on. If all device I/O signals are taken into account, not just a bare minimum, the savings offered by wireless and fieldbus is even greater.

For digital there is no longer a limitation of only 1 signal per wire. This means that one pair of wires can connect many devices, but it also means that each device can have many signals on that same pair of wires. That is, using digital networking all the MOV signals can be communicated over the same pair of wires which means that an MOV instead of having 44 signal screw terminals only need 2 fieldbus screw terminals, a dramatic reduction of screws terminals, especially when considering all the intermediate connection points that have to be made from the device to the system I/O cards for each of these signals. Keep in mind that it is possible to have multiple devices per bus. All these devices will probably not be MOVs, but there usually is a mix of a few MOV and other devices. Similarly, a popular control system often used for Fieldbus permits the connection of 16 devices on a single pair of wires, and supports 48 I/O signals on that cable. A popular WirelessHART gateway supports 100 devices on the same network with 4 inputs in each device. This means that with digital communication the full functionality of the every device is integrated and utilized for the benefit of the plant. A tank gauging system for multiple storage tanks require only two wires, including power. A 24,000 I/O hardwired project simply becomes a 8,000 fieldbus device project.

The above devices already exist and many plants are already benefitting from them. Many other conceivable multivariable devices are not available yet but systems based on real-time digital fieldbus I/O are ready to accept such devices as and when they become available. They are not here today, but they are inevitable and each device will have multiple I/O. For instance, multivariable Coriolis flow meter will also measure and compensate for pressure without additional process penetration and wiring. A two-wire mass flow metering system will provide signals and power for Coriolis mass flow, pressure, and temperature transmitter which will share the same pair of wires. Multivariable guided wave radar level transmitter will also measure pressure and process temperature. Multivariable vortex flow meter will also measure pressure and temperature. Radar level transmitter will also measure pressure and temperature.

In the digital ecosystem of WirelessHART, Fieldbus, and PROFIBUS it is no longer about counting the number of I/O signals and figure out if they are analog (continues) or discrete in nature, or if they are input or output, it’s just about counting devices. Inputs and outputs can be changed at a click of a button. For now, many sophisticated fieldbus devices will continue to have many screw terminals for some time because the same terminal block is also used for the hardwired version of the device, but eventually a simplified version of the devices just for fieldbus will become available. See further explanation of virtual marshalling in the fieldbus brochure:

https://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf

WirelessHART, Fieldbus, and PROFIBUS devices are the basis of the emerging Industrial Internet of Things (IIoT) and Industrie 4.0. Well, that’s my personal opinion. If you are interested in how the digital ecosystem is transforming process automation click “Follow” by my photo. Let me know what you think by providing your feedback below, and click “Like” if you found this useful.