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Automated Continuous Process Control Part 3

Tham khảo tài liệu 'automated continuous process control part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | c02.qxd 7 3 2003 8 20 PM Page 30 30 PROCESS CHARACTERISTICS Figure 2-5.1 Process response curve. to t0.632DO t 2-5.3 The units of t and to are the same time units as those used by the control system. Now that t and to have been evaluated we proceed to evaluate K. Following the definition of gain is 3MC -.2 c02.qxd 7 3 2003 8 20 PM Page 31 OBTAINING PROCESS CHARACTERISTICS FROM PROCESS DATA 31 This gain says that at the present operating condition a change of 1 CO results in a change of 1.2 C in outlet process temperature. This gain correctly describes the sensitivity of the outlet process temperature to a change in controller output. However this gain is only a partial process gain and not the total process gain. The total process gain is the one that says how much the process output c t in TO changes per change in process input m t in CO the reader may refer to Fig. 21.1b to understand this point. That is the process output is given by the transmitter output and it is not the temperature. Therefore we are interested in how much the transmitter output changes per change in controller output or Ac change in transmiter s output TO K -------- ---------7 ---------- Am change in controller s output CO 2-5.4 The change in transmitter output is calculated as follows Ac Ê 150CC 100 4 TO or in general Ê APV A Ê change in process variable in engineering units A Ac ----- 100 o O --------------------- ------- --------------- 100 O1O V span V span of transmitter 2-5.5 Therefore the total process gain for this example is 4 TO 0 8 TO 5 CO .8 CO If the process variable had been recorded in percent of transmitter output there would be no need for any extra calculation. We can now write the transfer function for this process as C s 0.8e- s M s t s 1 This transfer function describes the relation between the transmitter output and the controller output. If a transfer function describing the relation between the transmitter output and any other process input other than the controller .