Rockwell Automation 1746-BTM Barrel Temperature Control Module User Manual - Manual (Page 57)

Publication 1746-UM010B-EN-P - April 2001
Control and Autotune a Loop 5-11
The second critical procedure is that of finding the maximum rate of
change of the system for the given excitation. A number of the BTM’s
auto-tune failures are associated with this procedure: ‘temperature will
exceed deadtime’, ‘too much noise in the system’, etc. For any given
step change of excitation for a given system there will ultimately be a
maximum rate of change (max slope) attributable to that excitation.
This information is used to identify system ‘gain’ and ‘time constant’
information.
When trying to find a maximum slope, the system must be able to rise
in temperature sufficiently to guarantee that a maximum slope has
been attained. Thus a minimum temperature differential has been
identified and documented as being necessary and sufficient to a
successful auto-tune. If there is not enough temperature differential
between the starting temperature and the setpoint to achieve
maximum slope an error is generated and auto-tune is aborted.
Another cause of error here is in having sufficient temperature
differential to attain maximum rate of change but not enough
differential for the system to recover from the test successfully. The
system will overshoot setpoint as a result of the test. The module
‘knows’ when this is going to happen as a result of the gain and time
constant information previously learned and flags it as an error if
conditions are appropriate.
After these two tests are completed, an identical procedure is applied
to exercise the cooling control on the system if it is so configured, in
which case the autotune procedure would continue by first stabilizing
at setpoint. This is a module-wide event, meaning that all zones
enabled on the module must be stable before the procedure will
continue.
When all tests have been completed, the module will default to the
mode it was set at prior to the auto tune, auto or manual, and behave
accordingly.