Signal Conversion is a Key Building Block For Control Systems

Ian Verhappen is an ISA Fellow, Certified Automation Professional and a recognized authority on industrial communications technologies with 25+ years' experience. 

One of the many aspects of control systems often taken for granted is the physical layer that connects the various nodes, field instruments, junction boxes, I/O, controllers, etc., together into a system. As the signal travels between these nodes, the media on which the signal is transmitted frequently changes from wire to fiber and, as it was in the beginning, once again via air. The signal at each of these interfaces must be converted from either voltage/current (copper) to light (plastic or glass fiber) or air (frequency/amplitude for wireless or pneumatic pressure.)

My definition of "signal conversion" is the change of a signal from one physical format (voltage/current, amplitude/frequency, etc.) to another. Consequently, signal conversion is for all intents within OSI Layer 1 (physical layer) and possibly to some extent within Layer 2 (data link layer, DLL).

In the extreme, one could make the case that an amplifier is the most basic form of signal converter because it converts a low-level signal, potentially "cleans" the signal by removing noise, then increases the signal back to its original, full-strength level.

This signal conversion definition compares with the IEC definition in Electropedia, the IEC's online data dictionary, which states that a gateway is a "functional unit that connects two computer networks with different network architectures and protocols." Notice that the difference between a signal converter and gateway is the requirement to change protocols.

As inferred above, both signal converters and gateways play an increasingly important role because integration from end to end of any control system is becoming expected.

Also Read: Signal Conditioning, Conversion and Processing Keep Evolving

Opto 22's groov is one indication of how the Internet of Things (IoT) is making it possible for a "control loop" to start on someone's cell phone (setpoint) and include the transmitter (PV) as well as the output transducer (OP or MV) in a single system. Herman Storey and I prefer to call IoT the Industrial Internet of Things (I2oT) to point out that we have slightly different and additional requirements in industrial settings.

This migration from a SCADA-centric control system was affirmed in a recent conversation I had with Mike Fahrion, director of product management at B&B Electronics. He agrees that they certainly see this happening with, not only a shift in products from RS-232 to RS-485 or USB to serial converters, but also to more of an integrated I2oT approach with web access and a report/publish environment, rather than a traditional polling environment. This migration and change in data access is altering the signal-conversion landscape.

In a SCADA-centric environment, data is continually polled, while in the I2oT environment, data is updated only when there has been a change deemed to be worth reporting. However, regardless of the architecture, signal conversion continues to play a significant role in system communications, and one could argue a larger role in the I2oT system, since the output from the field gateway is an XML-based protocol through to the applications, databases and dashboards.

Process control basically started with air (pneumatic) connections, and today pneumatic controllers remain the workhorse of industry—certainly in the process industry. The addition of P/I and I/P converters was the beginning of an interesting journey that now includes a wide range of signal conversions, including (once again) current-air, but this time the air-based signal is a continuation of the digital world over a wireless network.

This certainly reaffirms that signal converters are not only here to stay and will remain an important, if neglected, part of a modern control system, but they're also becoming more complex with more demands placed on them, and with more variations (wire, fiber, wireless) added to the mix.
 
As systems become increasingly more complex and more reliant on digital communications, signal converters will remain an important control system building block for the foreseeable future.