Analysis of the process is divided into 3 stages:
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Noise and stability analysis of the process.
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First analysis of the response to a step function which provides a first identification model. From this first estimate a filter is calculated to be used in the second analysis.
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A second analysis of the response to a second step function, refined by using a data filter. A full process model is thus obtained.
After each of these two analyses, a set of loop controller parameters to be adjusted is calculated. The equations giving these loop controller parameters are based on the gain and the ratio of response time to process delay.
If the difference between these two analyses is too great, the model’s estimate is rejected and autotuning fails. The loop controller output is set back to its value before the start of autotuning.
On a robust level, the algorithm must be capable of supporting the gain and time constant changes in a ratio 2, without losing its stability. The asymmetrical processes are supported as long as they follow these constarints. If this is not the case, an error is signaled by the diagnostics.
Step Function Parametering
The 2 step functions applied to the output during the autotuning process are characterized by 2 parameters:
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The duration of the step function AT_TMAX, which must be greater than 4 s.
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The amplitude of the step function AT_STEP, which must be greater than 1% of the output scale (OUT_INF, OUT_SUP).
The function also checks that the output does not go beyond the output scale limits. This checking is done as the autotuning starts.
Maximum duration of the step function
The maximum value of AT_TMAX is limited by the task cycle time into which the loop is configured. This value, in seconds, can be configured to a maximum of 65.5 x task cycle time (en ms).
For example, if the task MAST = 50 ms, AT_TMAX = 65.5 x 50 = 3275 seconds maximum.
As an indication, the following table gives the parameter values for some classic process control types:
|
Scheme type
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AT_TMAX (s)
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AT_STEP (%)
|
|
Liquid flow or pressure
|
5 - 30
|
10 - 20
|
|
Gas pressure
|
60 - 300
|
10 - 20
|
|
Level
|
120 - 600
|
20
|
|
Steam temperature or pressure
|
600 - 3600
|
30 - 50
|
|
Composition
|
600 - 3600
|
30 - 50
|
The loop controller adjustment can be done according to the performance criterion value AT_PERF.
The AT_PERF parameter varies between 0 and 1, which means it is possible to prioritize stability for AT_PERF close to 0 or to obtain a more dynamic adjustment (and therefore to optimize response time to disruptions) by pushing AT_PERF towards 1.
NOTE: The parameters AT_PERF, AT_TMAX et AT_STEP are unique to each process control channel. There is therefore more than one set of parameters for the 3 single, cascade or auto-selector loop controllers. Consequently, for one process control channel, one autotuning operation can be activated and executed at a given instant.
