tailieunhanh - Automated Continuous Process Control Part 6

Tham khảo tài liệu 'automated continuous process control part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 7 3 2003 8 28 PM Page 90 90 RATIO OVERRIDE AND SELECTIVE CONTROL When the flow of hot liquid returns to its normal value and the level increases above the set point the level controller increases its output to speed up the pump. Once the output from the level controller increases above the output from the flow controller the low selector selects the flow controller and operation returns to its normal condition. An important consideration in designing an override control system is that of reset windup protection on any controller that has integration. The output of the controller not selected must stop at 100 not at a higher value or at 0 not at a lower value. Even more desirable would be an operation in which if the controller output selected were 75 the nonselected output would be forced to be close to 75 . With their inherent flexibility DCSs provide this very important capability. What is done is that the integration portion of the controller s not selected is are stopped and forced equal to the output of the selector. For example under normal operating conditions the low selector LS50 selects the flow controller and not the level controller. In this case the integration of the level controller is stopped and forced to equal the output of LS50 . 75 . Mathematically we can explain this procedure by looking at the PI equation of the level controller m t Kce t KC Ú e t dt Kce t LS50 75 Furthermore knowing that KC ti j e t dt LS50 75 and the tuning parameters Kc and p the value of the integral Úe t dt is back-calculated continuously. The proportional part of the controller is allowed to continue working. What this accomplishes is that under normal operating conditions the output of the level controller is greater than that of the flow controller because the proportional part is positive and it adds to the integral part which is kept at a value equal to LS50. However at the moment the level in the tank is equal to the set point of the controller the error