One of the most obvious reasons for poor control utilization is a broken or unreliable transmitter or sensor. As addressed in Chapter 3 of Advanced Control Foundation – Tools, Techniques, and Applications, if the control system is designed to be consistent with the international function block standard, IEC 61804, the status attribute is always communicated with the measurement value as illustrated below and provides a direct indication of measurement condition. The percentage of time the measurement had a status of Bad, Uncertain, or Limited can be used to determine measurement health. Thus, the status of a function block is the basis for evaluating measurement health.
The status attribute that accompanies a measurement value provides an indication of the quality of the measurement. For a measurement input provided by a traditional transmitter, the status attribute is determined by the analog input card. For a digital value provided by the transmitter, the status attribute can be provided by the transmitter. The status attribute value consists of eight bits. The most significant two bits are used to indicate the quality of the measurement classified as: Bad, Uncertain, Good – Control, and Good – Measurement. The last two bits of the status attribute, the lower-priority bits, are used to indicate high or low limit, no limit, or constant. The four middle bits of the status attribute are used to show why the quality is good, bad, or uncertain. The health of a measurement may be assessed based on the percent of time its status was Bad or Uncertain or a Good but an active limit condition.
After a measurement value and status are generated by the analog input block, the other blocks that use this measurement input process the status and use it in some way, and propagate a status to the downstream blocks. If the input block status is Bad, the Actual mode of the block that uses a measurement provided by the input block is unable to achieve an Automatic mode of operation and thus the Normal and Actual mode attributes will indicate that control is not being utilized as designed.
Most problems associated with a measurement can be quickly addressed by calibration or replacement of the transmitter.