The time to steady state is the estimated time, in seconds, for the
process to respond to input changes. Typical estimates are: (3 x First Order
Time Constant + Dead Time) or (4 x Dead Time), whichever is greater for the
slowest responding input/output pair of the MPCPro block. The default is 120
seconds. In general, the Time to Steady State should not exceed the time it
takes for the integrating process variable to reach one half of its limit
value(s) for a normal process input change.
If any process output exhibits integrating response, use the following
guidelines to estimate the Time to Steady State (TSS):
- For integrating responses
with insignificant lag and/or processes where the nonintegrating response is
significantly slower than integrating, limit the TSS to the approximate time it
takes for the integrating process variable to reach one half of its operational
limit value(s) when a normal change is applied to the process input.
- For processes with
significant lag, the recommended TSS is 120 × approximate lag provided the
limiting condition described in step 1 is not violated.
- If the identified response
is still not satisfactory or the slow dynamics of nonintegrating responses
necessitate violating step 1, estimate the response based on the physical (for
example, volume of a tank) and operational process parameters (for example,
flow rates, low/high tank level limits, and so on). Then enter the response
parameters (Gain per second, dead time, and first order lag, if any) in the
corresponding fields of the Step Response Design dialog.