John Immelman, Managing Director of Endress + Hauser Australia offers valuable inputs on self-adjusting instruments.
Switch on any modern projector (beamer) and the keystone correction automatically squares the image on the screen. Modern process instruments incorporate a fair degree of intelligence used to optimise process variable accuracy and reliability, and can even undertake predictive maintenance. But they fail miserably in one critical aspect even when compared with the humble projector. And that is self adjustment and self-healing.
From the modern digital camera that automatically adjusts focus and exposure for maximum reproduction to engine management systems in automobiles, the self adjustment concept has well and truly taken hold.
BMW’s intelligent drive system is a case in point as the car adjusts the gear selection to better suit a person’s driving style. The variable timing of the intake valves and the engine fuel injection are optimised within tolerable limits, improving fuel consumption, and these critical variables even auto-adjust between services.
If the world of process instruments had evolved in similar fashion, self-adjusting flowmeters in which the diameter would vary according to the flowrate to produce the most accurate and reliable flow measurements would have been a reality. Level measuring devices that utilise the time-of-flight principle, such as ultrasonic and radar, would self-adjust according to the properties of the air through which the measurements are made.
A majority of process instrumentation still use the 4-20mA signal transmission (as opposed to digital field bus technology), and therefore every device has to be ranged during the initial commissioning. However, smart sensors and pressure transmitters would self adjust to increase their accuracy within the operating range. And this would be of good benefit to the users.
If for example, a pressure transmitter operates between 0 and 100 bar with an accuracy of 0.01 per cent, one can determine the inaccuracy at any point in the total range. However, if the pressure being measured varies only between 50 and 60 bar, a clever instrument would self adjust and scale itself to operate between 40 and 70 bar. The result would be a far more accurate instrument.
There are other benefits as well. A typical 0 to 100 bar pressure transmitter may be set to alarm if the reading drops below 10 bar or exceeds 90 bars. However, if the process normally operates between 50 and 60 bars, a process malfunction that causes a pressure spike of 70 bars will pass unnoticed. A self adjusting instrument could pick this up right away.
In new plants, this issue is all the more obvious and the ramifications are manifold. In such cases, each new instrument in the plant is calibrated and the measurement range set to improve accuracy. As the process settles down, the optimised bandwidth for every instrument continues to shift.
This requires further calibration and recommissioning of the instruments to maintain optimal accuracy. This is a laborious, time-consuming process and it could take years for the plant to stabilise. But it does not have to be this way. Imagine if the instruments had the ability to store data, analyse it periodically, register the low and high points over a period of time and self adjust to operate in the optimum range.
This will also eliminate the need to shut down plants in order to optimise the instrumentation. The resultant savings in time, human resource and money as well as the impact on product quality and batch consistency would be high.
Let’s consider self-adjusting processes. With the recent acceptance by process industries of field bus technology, both Profibus and Foundation Fieldbus, it is possible to extract more data from the processes than previously. For example, a differential pressure transmitter can provide the dip measurement for level or flow, as well as the process pressure and temperature.
Using this additional information plantwide it should be possible for the control system to process the data and make the necessary process changes, reducing energy costs, saving processing time, without sacrificing quality – why add too much flocculant, only to have to recover it later, why cool pipework only to heat it again for hygienic cleaning (CIP), why keep filling a tank that never empties below half full?
Till now, instruments have been designed around the reactive or planned reactive philosophy. Now there is a technology to move them up to the next level and make them proactive without any major impact on their cost. And after we cross the self-adjusting hurdle, the next issue is self–healing. Car makers are developing tyre systems that can self check and self inflate in the event of minor leaks.