Answer Posted / abhishek
Main difference between DCS and PLC is about the scanning
time. PLC's have a fixed scanning time whereas DCS
execuation order(scanning) can be varied with certain
minimum limit. Generally a PLC can have a scan rate of as
low as 5msec(fixed) to some where around 20msec(fixed). As
DCS handles larger number of inputs/outputs its scanning
rate is 50msec and this value needs to be defined (can be
kept more than 50msec) while writing a program. But latest
DCS can have scanning rate of 20msec.
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Q1: Consider part of a control loop, which excludes the transmitter, consisting of a process, a controller and a control valve which may be represented by two dead times of 0.5 min each and three exponential lags of 0.8 min., 1.0 min. and 1.5 min. respectively. We wish to express this system as an overall first order plus dead time (FOPLD) model ie gain, time constant and process dead time. (We will see later that this is often done, to simplify controller tuning). For this exercise, gain is considered to be 1.0. (A) If the transmitter is a flow transmitter whose behaviour can be described by a dead time of 0.2 min. and an exponential lag of 0.5 min. in terms of the overall dead time and overall first order lag how can the system behaviour be approximated ? Overall dead time = Overall time constant = (B) If the transmitter is a temperature transmitter with a temperature sensor in a protecting well whose behaviour can be described by a dead time of 0.7 min. and an exponential lag of 15 min. how can the overall system behaviour be approximated now? Overall dead time = Overall time constant =
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