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Test & measurement


Wired or wireless?


Should you select wired or wireless architecture? Here, Advantech’s Paul O’Shaughnessy explores the best technology for sensor deployment


systems. Some would even argue that defining the points to be measured was the starting point from which systems were defined, and ultimately dictated what the deployed systems could and could not do. This is no less true as IoT technologies are adopted in the transition to Industry 4.0. In fact, the adoption of these new technologies increases the demand for more and more measurement data, as users include an ever- increasing number of factors into the optimisation of their processes and equipment. This hunger to add new data brings its own challenges and some significant differences. The inherent flexibility of IoT systems means that the required data measurements evolve over time, requiring strategies to integrate pre- existing measurement points with new ones, which can often be physically removed from any pre-existing wiring or communication links. In such cases, the cost of adding the new


T


measurement points, or more normally the cost of recovering the data from these points, can often be the deciding factor in whether a desired system enhancement will make economic sense. It is therefore understandable that engineers are interested in the deployment of wireless measurement systems, as these can often be installed


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aking measurements from real world processes has always been fundamental in data acquisition


much more quickly and cost effectively than those involving the laying and routing of additional wired infrastructure. Deploying wireless sensing technology involves compromises, however, and it is the effect of these compromises that ultimately determines if a wired or wireless architecture is selected and, if the latter, which radio technology is most suitable.


THE CASE FOR WIRED INFRASTRUCTURE The traditional way sensors have been connected to data acquisition systems is via wired interfaces, typically 4-20mA current loops, but also in the case of more specialised measurements via higher speed voltage-based systems. These connections can carry power to the sensors, are reliable, accurate, offer faster measurement transmission and update times than wireless systems and, if correctly installed, are both highly secure and insensitive to noise and other interference. These characteristics come at a cost however. The cost of installation can be very high, often involving alterations to buildings or digging trenches in which to lay the cables. In some cases, cabling to a sensor that is geographically remote may not be physically practical at all. It is also usually not possible to use wired interfaces to mobile


equipment, unless the range of movement is limited and can be accurately forecast.


THE EVOLUTION OF REMOTE I/O Telemetry has been used for many years to alleviate some of these limitations. By moving the physical sensor interface closer to the sensor, digitising its data at that point, and then sending the digitised data onwards via a serial or network data connection we can significantly reduce the cost of installation whilst also improving noise susceptibility, especially where multiple sensor interfaces exist at a single point. It also becomes possible to use radio to transmit the digitised data, extending the reach of data acquisition systems to geographically remote sites. The definition of ‘remote I/O’ is very broad.


For the purposes of this discussion, we will apply it to any device which interfaces to sensors and digitises the resulting measurements before onwards transmission. This means we encompass traditional Remote Telemetry Unit (RTU) and remote I/O devices using protocols such as Modbus, but also USB devices aimed at lab measurements and fieldbus-based data acquisition nodes, as well as more modern incarnations, such as IoT edge and gateway devices. The use of any such device starts to introduce compromises that must be considered.


October 2020 Instrumentation Monthly


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