EBSILON®Professional Online Documentation
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Line connections |
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1 |
Scheduled value - target value - set point |
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2 |
Actual value - process value |
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3 |
Controlled value |
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4 |
Inlet for value that starts controller |
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5 |
Inlet for threshold value |
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The colour indicates the type of activation of the controller (depending upon the switch FACT):
General User Input Values Characteristic Lines Physics Used Displays Example
Component 69 is a powerful component combining a switch and a controller. The control process can be activated or deactivated depending upon two additional controlling pins. The scheduled value to be set can be entered selectively externally through the pin 1 or internally as the specification value SCV (flag FSCV), or it is possible to hide this
pin (1). This is done in the tab “Ports“ of the Component Properties.
When defining via SCV, the line 1 need not be connected.
The controller can be deactivated in the start and can be activated after passing a set value. Vice versa, a controller activated at the start can be deactivated after passing the set value. It is also possible to activate or deactivate the controller permanently. In this case, the lines 4 and 5 need not be connected.
The controller also outputs a controlled value in the deactivated state, namely the specification value L3OFF. If one leaves the field L3OFF empty, the controller retains the value upon deactivation, which it had in the last iteration step before being deactivated.
An approximate controlling can be implemented with the help of the specification value TOL. The controller is deactivated, when the process value deviates from the scheduled value as less than TOL. This measure accelerates the calculation, if an exact setting of the target value is not required.
Controllers can be dampened even more as. For this, three more damping levels (“very very high”, “even higher”, ”extremely high“) have been introduced.
The sequence of the specification values in the input screen has been sorted. First are the specification values that relate to the reference value, and then the values that relate to the correction variable. Then follow general controller parameters like characteristic, activation, start value specification, and damping or limitation of change.
Decoupling of turn-on/-off feature and delayed start
The flag FACT served both to turn a controller on and off, and to specify a delayed start. In order to be able to better expand the setting options, this feature has been distributed to the two flags FACT and FFU.
The flag FACT is used to specify at which iteration step the controller is to start (at the earliest). FACT=0 means that the controller is to start as early as possible. The previous variants FACT=-1 and FACT=-2 for deactivating the controller with (-1) and without (-2) start value setting respectively have been marked as “deprecated“, but they are still operational for reasons of compatibility. However, it is recommended to use the new flag FFU for this purpose.
The flag FFU offers various variants for activating and deactivating controllers in different load cases. There are the following setting options:
• Controller active
- always: FFU=1
- only in design case: FFU=4 or -4
- only in off-design case: FFU=5 or -5
- never: FFU=0 or -1
• Controller not active (i.e. does not control) but sets its start value
- always: FFU=0
- only in design case: FFU=5
- only in off-design case: FFU=4
• Controller not active, without start value setting
- always: FFU=-1
- only in design case: FFU=-5
- only in off-design case: FFU=-4
Alternative start value in the case of deactivated controller
The start value one wants to set with deactivated controller is not always suitable as start value in the case of activated controller (e.g. 0 kg/s). You then had to set different starting values in the different profiles. There is an alternative start value L2STARTOFF (Component 39) and L3STARTOFF (Components 12 and 69) respectively for this. It is used when the controller is deactivated but is to set its start value. If no value is entered here, L2START and L3START respectively will be used also in the case of deactivated controller.
Compliance with limits
The limits L2MIN / L2MAX (component 39) and L3MIN / L3MAX (component 12, component 69) were not always strictly adhered to, but only caused the controller to be deactivated when the respective limit was exceeded. This mainly served to prevent a drifting off in the course of the iteration. For the final result, however, this behaviour is unsatisfactory as the precise achieved value depends on the behaviour of the iteration.
For this reason, now the limits are strictly complied with. For reasons of compatibility, however, it is possible to switch back to the old behaviour via the flag FLIM. For this, FLIM has to be set to 0 (“Shut off controller after limit was exceeded“). The standard setting is FLIM=1 (“Stop at limit“).
Functionality of the limits
The limits L3MIN/L3MAX (Component 12, Component 69) and L2MIN/L2MAX (Component 39) only become effective if numerical values are specified for them.
If that is the case, the flag FLIM checks according to the specification with
FLIM=0: the controller is switched off after the limit L3MIN/L3MAX is fallen below / transgressed
or with
FLIM=1: the controller stops on the limit L3MIN/L3MAX.
Examples of the functionality of L3MIN, L3MAX and FLIM see Controller component 39 (EBSILON®Professional,Online Help).
Deactivation of warnings
A warning is issued by default when the controller cannot achieve its target value. In some cases, however, this is unnecessary, e.g. when, in the case of an injection, the inlet temperature is already below the set point temperature. In such cases it is possible to shut off the warning with the flag FWARN.
The flag FWARNOFF allows to activate or deactivate warnings for deactivated controllers. Here it is checked if the start value (in the case of component 69 also the off-value L3OFF) is within the range of validity.
Note:
It is a warning issued only if the relative as well as the absolute deviation is larger than the warning level.
Start Value Transfer In controllers (FMODE)
(Components 12, 39, and 69) it is possible to take over the result for the controlled variable as start value for the next calculation. As there is a certain analogy with respect to the transfer of the reference values for off-design calculations here, the flag for this has been named FMODE as well. The following settings exist:
The transferred value is written onto the specification values L3START and L3STARTOFF. However, it only has an impact if FL3START is set to “internal start value specification“. Also, the transfer only takes place if a value was entered into L3START respectively before. If the field has been set to the “empty”, the field will remain empty. When adding new controllers, FMODE=1 will be set by default. This is consistent with the previous behaviour. In a good modeling, the final result should in fact not (or only slightly) depend on the start value, but a change of the start value may lead to convergence problems. An influence on the final result also arises from the interaction of controllers with threshold values and limit values if it depends on the convergence behaviour when a controller is activated and deactivated respectively.
Flag FWARN
In the controllers there is a flag FWARN that allows setting in which situations the controller is to output warnings. For this there is a new setting FWARN=3.
With this setting, a warning is only output if the controller has not reached its target but the control variable has not reached its limit value either.
This setting makes sense if reaching the limit value represents a “normal” condition, like e.g. in the case of an injection where a certain temperature is to be set downstream
of the injection. By the injection, however, only a reduction of the temperature can be effected. If the temperature is below this set point value, nothing needs to be injected.
FWARN=3 allows to set that in this case no warning is to be effected. The control variable of the controller then equals the lower limit value of 0 kg/s, and no warning is output
if the set point value of the temperature has not been reached.
It is possible to output an error message instead of a warning with the setting FWARN=4.
FWARN=5 allows to individually program in a Kernel expression EWARN under which circumstances a comment, a warning or an error message is to be output.
Set Point Specification via Kernel Expressions
For the controllers that have the option for an internal set point specification (Components 39 and 69) a Kernel expression ESCV can be used as an alternative to the specification value SCV. The control is affected via the flag FSCV:
• FSCV=0: the specification value SCV is used as set point value
• FSCV=1: (only for Component 69)
• FSCV=2: the set point value is determined from the Kernel expression ESCV.
Please note: as the set point value is usually a variable with units and – in contrast to the simple specification value – no automatic unit conversion can be carried out in the case of Kernel expressions, the value must be calculated in standard Ebsilon units.
Control Precision
For the solution of an equation system, Ebsilon continues the iteration as long as the changes from one iteration step to the next are smaller than the specified iteration precision.
Here the termination of the iteration is independent of to what extent the controllers have reached their set point value. Only a warning will be output if the deviation between
actual value and set point value is too great.
In the case of strongly damped controllers in particular, the change from one iteration step to the next is relatively small, so that the convergence criterion is already fulfilled although
the control target has not been achieved yet. In these cases it would be desirable if the iteration were continued for another couple of steps in order to get closer to the specified target. Therefore the possibility to prevent a termination of the iteration in the case of too great a deviation from the set point value has been created.
Inversely, there are also cases where the adjustment of an unimportant variable requires very many iteration steps and thus increases the computing time for the entire model. In such
cases it is desirable to be able to carry out the control with a greater fuzziness.
This option is available with the specification value TOL (for all controllers).
The specification value TOL is used for setting the control precision in both cases. Which one of the two cases is desired is set via the flag FTOL:
FTOL=1 (“TOL=lower bound“) serves to accelerate the control by means of a greater fuzziness. In this case, the controller terminates the control if the relative deviation between actual
value and set point value falls below the bound TOL.
FTOL=2 (“TOL=upper bound“) prevents the termination of the iteration as long as the relative deviation between actual value and set point value transgresses the bound TOL. However,
this does not apply if the correction variable has reached its lower or upper bound. As the controller does not continue operating in this case, carrying out further iteration steps does
not make sense.
For analyzing the convergence behaviour, in the case FTOL=2 you can see in the result value ITNOTCONV up to which iteration step the controller has prevented a termination of the
iteration. This allows to systematically find out the controllers that are responsible for a deterioration of the convergence behaviour and to improve their settings if necessary.
FTOL=0 is the default setting.
As in the case of controllers in Ebsilon the change of the actuating variable is effected via a change factor, controllers previously could not be operated in such a way that the actuating variable was able to change its algebraic sign. To enable this, there is now a specification value CZP that serves to shift the zero point of the controller internally. Shifting is effected in positive direction. If e.g. “100“ is entered, -100 will be mapped onto 0 and it will also be possible to control beyond 0 in the range >-100.
With great values of CZP, the internal actuating variable will become great accordingly so that this way, in the case of similar relative changes, the absolute change of the actuating variable will become very great too. This may lead to convergence problems. In this case, it is recommended to decrease the maximum change factor (CHL3 respectively), and that from the beginning ( ITCHL3 = 0 respectively).
Previously, only fixed values could be entered as range limits for the actuating variable. Now it is possible to use a Kernel expression as the limit. To do so, the corresponding flag (FL3MIN and FL3MAX respectively for Components 12 and 69) has to be set to “Kernel expression“, and an EbsScript that calculates the corresponding limit has to be created in EL3MIN and EL3MAX respectively for Components 12 and 69.
This feature was needed for the variation of the steam inlet pressure for a preheater in order to achieve a certain feed water outlet temperature. Without limitation, the controller decreased the pressure so far that the saturated water temperature dropped below the feed water inlet temperature and no condensation was possible anymore. A fixed limit, however, was not possible either as the feed water inlet temperature is not known in advance but only appears in the course of the calculation. For instance, the following Kernel expression allows to set the lower pressure limit to a reasonable value in each iteration step:
function evalexpr:REAL;
begin
evalexpr:=waterSteamTable(1006, Feedwater.T, 0.0);
end;
To a relative humidity of for example, to set 100% or super saturation, a controller is required.
In the previous handling with the function "air humidity (rel.)", It could happen after iteration course that the air was supersaturated, i. Water contained in the liquid phase. The reason for this was that even with supersaturated air, the relative humidity remained at the value of 100% and the controller had thus reached its set point.
In order to allow a regulation to the saturation point (100%) or the setting of a certain super saturation, there is a function "saturation factor".
The saturation factor always refers to the maximum possible proportion of gaseous water. If the water content is higher, you get liquid water XH2OL. Humid air can only be considered approximately as an ideal gas. With increasing water content, the real proportion of gaseous water XH2OG decreases again. In the ideal approximation, the proportion of gaseous water would then simply remain constant, no matter how much liquid water would add to it. In reality, that's probably not the case and therefore with supersaturated air:
XH2OG> X_SAT = f (p, t (air outlet line))
For values up to 100%, the results of the "Saturation factor" function are the same as those of the "Humidity (rel.)" Function.
Definition Saturation factor for "saturated air" 0 - 100%: The result values agree with the results of the "relative humidity" function.
Definition saturation factor for "supersaturated air"> 100% = corresponds to the ratio: total water content (XH2O) / maximum possible gaseous water content
Water vapour saturation concentration x_sat = f (p, t (air, port 2))
Example: Application of saturation factor:
See example "Saturation factor" in the chapter Component 39 "Controller (internal set value)"
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FMODE |
Flag to define the position in calling sequence Like in Parent Profile (Sub Profile option only) Expression =0 : After calculation in design mode =1 : No =-1: After each calculation |
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FL1L2 |
Flag for comparison type between L1 (Target value) and L2 (process value) Expression
=1: Pressure =2: Temperature =3: Enthalpy =4: Mass flow =5: Power/heat flow =6: Enthalpy of boiling liquid for given pressure =7: Enthalpy of saturated vapour for given pressure =8: Enthalpy of boiling liquid for given temperature =9: Enthalpy of saturated vapour for given temperature =10: Pressure of boiling liquid for given temperature
=13: Composition of the mass ratio (use with FSUBST) =14: Net calorific value =15: Composition as mole ratio (use of FSUBST) of all substances =16: Composition as mole ratio (use of FSUBST) of the dry part (without water/steam) =17: Entropy =18: Volume flow =19: Specific volume =20: Degrees sub cooling (with WD-lines this value refers to the boiling point, when humid air to the dew point temperature) =21: Degrees super heating (with WD-lines this value refers to the boiling point, when humid air to the dew point temperature) =22: Relative Pressure =23: Exergy, Note: Regarding the exergy, however, please be aware that the solution may not be unambiguous as the exergy does not rise |
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FSUBST |
Substance to be controlled (in combination with FL1L2 = Composition) Expression
=0: nothing
further material properties No. 41 - No. 2400
Further material values to be regulated for a composition can be found on the surface of the controller - default value "FSUBST"
Obviously, or entering two to three significant letters of the desired material value is sufficient to obtain a targeted selection of material values. |
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FDAMP |
Controller damping Expression =1: Without |
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FL3 |
Flag for corrected value type Expression =1: Pressure =3: Enthalpy =4: Mass flow |
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FACT |
Number of iterations, after which the controller is activated Expression =0: start immediately =20: start after 20 iterations =30: start after 30 iterations =40: start after 40 iterations =50: start after 50 iterations =100: start after 100 iterations =-1: controller deactivated, only start value setting (deprecated) = -2: controller completely deactivated (deprecated) |
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FSEQ |
Flag to define the position in calling sequence Expression =0: Parallel to other components =1: Late, after fluids are recalculated |
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CHL3 |
Maximum change factor of correction value (set as constant 0.5 until ITCHL3 is reached, after reaching ITCHL3 according to the preset between the boundaries: 0 < CHL3 <= 0.5) |
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ITCHL3 |
Number of iterations, after which the change factor becomes active |
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FCHAR |
Flag for controller characteristic Expression =1: Positive (i.e. an increase of the corrected quantity leads to an increase of the actual value) =-1: Negative (i.e. an increase of the corrected quantity leads to a decrease of the actual value) |
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FFU |
Switch for activation / deactivation / Start value set - Controller Expression =0: OFF: Controller not active, but start value set in all load cases Note to -6/ - 7: In both cases, no controlling takes place. Actually the use of a controller is redundant in this case as the same effect could be achieved |
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ADD21 |
Additive term for comparison actual value / scheduled value |
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FL4L5 |
Comparison between L4 and L5 (necessary for FSCC) Expression =1: Pressure =2: Temperature =3: Enthalpy =4: Mass flow =5: Power/heat flow |
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ADD |
Additive value for comparison This value is added to the set value (line 5), before the comparison is done. |
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ITCHECK |
Check of the ON/OFF state Expression =0: Every time =5: Every 5th iteration =10: Every 10th iteration =20: Every 20th iteration =50: Every 50th iteration =100: Every 100th iteration =-1: never |
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FSCC |
Starting criteria for the controller Expression =10: Start with controller off, at S4-(S5+ADD)>=0 on =11: Start with controller on, at S4-(S5+ADD)< 0 off =20: Start with controller off, at S4-(S5+ADD)< 0 on =21: Start with controller on, at S4-(S5+ADD)>=0 off =0: Permanent off (S4, S5 set to 1) =1: Permanent on (S4, S5 set to 1) |
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L3OFF |
Value of line 3, if controller is off In order to retain the value last reached before deactivation, the field L3OFF must be left blank |
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FLIM |
Handling of L3MIN and L3MAX Like in Parent Profile (Sub Profile option only) Expression =0: Shut controller off after limit was exceeded =1: Stop at limit |
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L3MIN |
Minimum value for the correction value: if, during the regulation, the correction value falls below L3MIN, then instead the |
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L3MAX |
Maximum value for the correction value: if during the regulation the correction value exceeds L3MAX, then instead the |
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FL3START |
Flag for the type of start value input Expression =0: internal definition via the specification value L3START =1: external specification via line 3 (Stricter plausibility checks:, (error messages !) if - a value is used as actuating variable that cannot be modified at all but is determined by other components. )
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L3START |
Start value for the controlled value (if FL3START=0) |
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L3STARTOFF |
Alternative Start value if controller is off (optional) |
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ITSTEP |
Controller is active every n-th step |
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FSCV |
Flag for the source of the target value Expression =0: internal definition via the specification value SCV |
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SCV |
Target value |
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ESCV |
Function for target value function evalexpr:REAL; |
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FTOL |
Meaning of TOL Like in Parent Profile (Sub Profile option only) Expression =0: not used |
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TOL |
Controller tolerance If the relative deviation between the actual value and the scheduled value is less than TOL, the controller is deactivated |
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FSTOP |
Flag for Stop behaviour if controller has not started Expression =0: Do not stop iteration before controller has started |
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FWARN |
Warning in case of missed target Expression =0: Not notification |
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EWARN |
Function for notification function evalexpr:REAL; |
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FWARNOFF |
Plausibility check in case of inactive controller Like in Parent Profile (Sub Profile option only) Expression =0: No warning if range (L3MIN to L3MAX) is exceeded |
Generally, all inputs that are visible are required. But, often default values are provided.
For more information on colour of the input fields and their descriptions see Edit Component\Specification values
For more information on design vs. off-design and nominal values see General\Accept Nominal values
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Values for FL1L2 |
Default value |
Actual value |
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measured |
for comparison |
for comparison |
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1 2 3 4 5 |
Pressure Temperature Enthalpy Mass flow Power / heat flow |
Pressure Temperature Enthalpy Mass flow Power / heat flow |
Pressure Temperature Enthalpy Mass flow Power / heat flow |
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6 7 8 9 10 13 14 15 16 17 18 19 20 21 |
Pressure Pressure Temperature Temperature Temperature Mass fraction of a substance Net calorific value Mole fraction of a substance Mole fraction of a substance (dry) Entropy Volume flow Specific volume Degrees sub cooling Degrees super heating |
Enthalpy' (P) Enthalpy'' (P) Enthalpy' (T) Enthalpy'' (T) Pressure' (T) Mass fraction of a substance Net calorific value Mole fraction of a substance Mole fraction of a substance (dry) Entropy Volume flow Specific volume TS=(P) TS=(P)
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Enthalpy Enthalpy Enthalpy Enthalpy Pressure Mass fraction of a substance Net calorific value Mole fraction of a substance Mole fraction of a substance (dry) Entropy Volume flow Specific volume No.20/21: In the case of water-/steam pipes this value refers to the boiling temperature, in the case of humid air to the dew temperature. |
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Values for FDAMP |
Limits for the change gradient GRi |
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lower limit for GRi |
upper limit for GRi |
Damping behaviour |
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1 2 3 4 5 6 7 8 9 10 |
0.3 0.3 0.3 0.75 0.2 0.1 0.05 |
99999.0 9999.0 2.0 1.25 1.0 0.6 0.3 |
without very little little medium medium high high very high very very high even higher extremely high |
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CCHL3 - Correction factor for CHL3
This characteristic enables one to change the maximum change factor CHL3 continuously in the course of the iteration (ITCHL3 enables an abrupt change in the iteration step ITCHL3). Normally, one will be able to narrow down the permissible deviation at the end of the iteration, in order to achieve a faster convergence.
x-value: Iteration step
y-value: Correction factor (CHL3 used = default value CHL3 * y-value)
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relative deviation |
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(Target value - actual value) Si = ----------------------------------------------------- Target value
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relative change of the corrected value f |
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| f(new)-f(old) | Ki= | ------------------------ | | f(old) |
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Change gradient |
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Ki GRi = ----- Si
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During the steps 1 to 10, GRi will be set for the iteration;
from 1 to 5 with GRi = 0.95, and
from 6 to 10 with GRi = 0.90 .
The gradient GRi will be calculated after iteration 11 by the interpreting the previous controlling success.
The maximum and minimum values for GRi can be defined by the user via FDAMP (see the list of input values).
The value f(new) is limited via a sensitivity according to the following instruction.
f(new)=f(old)*(1 +/- CHL3)
The following rules apply for determining CHL3:
The controller has a "self-learning" characteristic i.e. the best possible change of the correction value for the next iteration is taken from the analysis of the controlling success of the last iteration step. For this, the change gradient is used, which is defined as follows: The change gradient is a measure of the relative change of the correction value as a function of the relative deviation. A change gradient of GRi=1.0 results in a relative change of, for instance, 5% to a change of the correction value of this 5% itself. The change gradient set to GRi=0.5 returns, for this change, the correction value of 2.5%.
This gradient calculated from the last two steps is used to define a new value for the correction value as shown in the following equations.
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Correction value |
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Df = f * GRi * Si f(new) = f(old) * (1.0 + Df) * FCHAR
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There are positive and negative characteristics of controlling. It is positive, when and increase of the corrected quantity leads to an increase of the actual value. It is negative, when and increase of the corrected quantity leads to a decrease of the actual value. A negative characteristic leads to instability of the iteration. A remedy for this is creating a negative value for FCHAR.
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Display Option 1 |
Click here >> Component 69 Demo << to load an example.
