OPTIMIZATION OF A SAMPLED-DATA CONTROL SYSTEM FOR A VARIABLE TIME-LAG PROCESS
Abstract
The objective of this investigation was to establish an optimum control system for continuous-flow grain dryers. Automatic controls have the potential to provide more accurate control and reduce the labor required for the operation of a drying plant as compared with manual control. However, the grain drying process has a very long time lag as compared with typical process control applications and the control sensor introduces an unusual requirement for an accurate, reliable and rapid transducer. A primary sensor was developed to measure the moisture content using a commercially available moisture meter and an automatic weighting machine. Digital computer simulation of the drying process and control was used in the optimization of the control system. The performance of the individual control systems was optimized by using a computed performance index as a measure of the system response. The optimization was accomplished by a computer search routine that automatically adjusted the control system parameters in a manner to reduce the performance index. The optimum set of system parameters for a given set of conditions was found to be a function of the initial grain moisture content for the proportional-reset system. However, an optimum system at one level of initial moisture was unstable at higher moisture levels due to a significant increase in process time. This resulted in the possibility of underdried grain being discharged from the dryer. An on-off system was investigated and found to offer an important advantage in the elimination of underdrying. Although a less serious error, significant overdrying could result from this control mode. A combination system incorporated the advantages of the proportional reset and on-off modes. The combination control system functioned as a proportional-reset system when the final moisture content was below 15.5 percent. When the final moisture content exceeded this level the grain discharge was stopped and the grain dried as a batch until the next sampling period. The discharge of underdried grain was reduced by the on-off control action and the overdrying problem was improved by the proportional-reset control action.
Degree
Ph.D.
Subject Area
Agricultural engineering
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