Explanation of Solvers Settings
This chapter provides tips and rules to help you select the right solver for your application or your simulation task and to parameterize it accordingly. As simulations in the frequency domain (steadystate simulations) are less demanding, this chapter focuses on the proper settings for simulations in the time domain (transient simulations). The availability of Advanced Solver Settings depends on the selected solver settings and are described in the linked sections below.
Choosing the Best Solver
Hard Rules
Models with Rigid End Stops can only be simulated with BDF or MEBDF methods! Such models allow for a structural switchover (mathematical). This applies to the following list of model types from the SimulationX libraries (selection):
Mechanics.Translation.EndStop
with mode=="Rigid End Stop"Mechanics.Rotation.EndStop
with mode=="Rigid End Stop"Mechanics.Translation.ElasticFriction
orMechanics.Rotation.ElasticFriction
with rigid=="true"Mechanics.Translation.SpringDamper
orMechanics.Rotation.SpringDamper
with kindR<>"without Backlash"Mechanics.MechanicsMBS.Joints.PrismaticJoint
orMechanics.MechanicsMBS.Joints.RevoluteJoint
with kindL<>"No Losses", kindF=="StickSlip" and rigid=="true" All model types within the libraries
PowerTransmission
, especially*.Couplings
,PowerTransmission2D
andPowerTransmissionMBS
Gear
model)
If no BDF or MEBDF method is selected for such models, SimulationX shows the following error message:
Error in Model1: Unsupported model.
Help: CodeGen.ElementNotSupported
The model 'Rigid End Stop' / 'Ideal Diode' (in component "IDENT
") is not supported by this solver. Please select a BDF or MEBDF method or change the model or the parameter settings.
Soft Rules (Recommendations)
The following overview is a set of recommendations to illustrate which solver works best for the task at hand:
No.  Solver  Explanation 

BDF or MEBDF method (NOT based on compiled C code)  
1  BDF  At the modeling stage, where it is common to create small, less complex structures, using the BDF method is recommended, as this solver offers the best debugging capabilities, while the longer computation time (compared to all compiled solvers (incl. C code compilation)) is negligible. 
2  BDF  When you create new element types (Compounds etc.), use the BDF method, as this solver offers the best debugging capabilities. 
3  MEBDF  If conservation of energy is important for the analysis (e.g. in Mechanics where either damping or friction is taken into account), MEBDF should be preferred, since BDF causes a small, processrelated, numeric damping effect (e.g. visible in the calculation of the onemass oscillator over many periods). 
BDF or MEBDF method (based on compiled C code)  
4  The simulation with a BDF or MEBDF method takes a lot of computation time, especially with larger models. The compiled equivalents, in contrast, take way less longer. Since not all special cases can be taken into account, you should check whether the compiled and noncompiled variants return the same results and show the same behavior and how long the compilation itself may take. If it outweighs the benefits of the compiled variants, you can as well keep using BDF (compiled C code) or MEBDF (compiled C code).  
CVODE (based on compiled C code)  
5  CVODE  For complex models generating analysis results in parameter studies, for example, it is recommended you use compiled solvers, especially CVODE. CAUTION: On top of the time needed for the Modelica compilation (GSA), you need to allow for the C code compilation to take time as well. 
6  CVODE  The following solver settings for CVODE are usually recommendable, because they ensure a more accurate calculation from the beginning and hence fewer iterations during the computation of time steps:

7  CVODE  For (automated) variation calculations yielding a plethora of results in a short time, you should use the settings from no. 6. 
8  CVODE  In general, CVODE delivers a better simulation performance in many mechanical or mechatronic applications than BDF methods. 
9  CVODE  The order control strategy with CVODE is less favorable than with BDF, since CVODE may calculate with small time steps or show a wrong slope behavior for eigenvalues near the imaginary axis (i.e. little damping compared to the oscillation). This can lead to a significantly slower calculation than with BDF. 
Fixed Step Solver methods for realtime applications (based on compiled C code)  
10  FixedStep Solver  Models for realtime calculations, e.g. for infinite simulations or applications in a realtime environment (HiL, SiL), can use the FixedStep Solver from the start. Small time constants in the model (e.g. highfrequency vibration excitation) should be avoided though. The calculation step size dtMin should fulfill the sampling theorem. (Integration method can be "ITI standard"). 
11  FixedStep Solver (comp.)  As a general note, it is recommended you use two solvers: one solver with a fixed step size for final calculations on the realtime system and one solver with a variable step size to help determine the necessary step sizes of occurring changes and to evaluate the quality of simplifications (mostly in the light of realtime capabilities). 
12  C Compilers  The GNU compiler is rarely usable with "large models" (consisting of many elements), because the compilation may fail to run sufficiently. Microsoft compilers, however, usually work. 
Automatic solver settings for Modelica models
Under certain conditions, SimulationX does not use the default values for solver parameters and settings. This is usually the case with simulation models that were originally created in other Modelica environments and have not been saved in SimulationX at any point, e.g. example models from the Modelica Standard Library. The reason for this is that the Modelica specification does not mention any solver types and defines only a few common solver parameters which often enough fall short of the solver type requirements of the various Modelica environments. Modelica defines the following solver parameters (names) which are stored in the model's socalled experiment
annotation:
annotation(
experiment(
StartTime=0, // start time of the simulation in [s]
StopTime=40, // stop time of the simulation in [s]
Interval=0.01, // suitable time resolution for the result grid in [s]
Tolerance=1e5) // relative integration tolerance
);
These annotations are also documented in the Modellica Specification, chapter "18.4 Annotations for Simulation Experiments".
The following sections give you an overview of the conditions and rules that apply when SimulationX sets the solver automatically.
Solver parameters (comparison)
Parameters  Parameters in Modelica  Default Value  Explanation 

tStart

StartTime

0's'

Start time of the simulation run 
tStop

StopTime

1's'

Stop time of the simulation run 
dtMin

n/a  1e08's'

Minimum step size of the calculation 
dtMax

n/a  (tStoptStart)/100

Maximum step size of the calculation 
absTol

n/a  1e05

absolute tolerance 
relTol

tolerance

1e05

Relative tolerance 
dtProtMin

interval

0.001's'

Minimum step size of the output or suitable time resolution for the result grid 
dtDetect

n/a  dtMin*1e4

Minimum step size (only for BDF or MEBDF methods) 
Conversion rules and assumptions for the solver parameters
SimulationX follows the below rules to determine unknown solver parameters from defined solver parameters (or from their default values). Note that SimulationX never overwrites annotations set by other tools. SimulationX never replaces the values for tolerance
or interval
when you enter other values for the solver parameters in SimulationX and save them with the model. When a model is loaded, the SimulationX parameter (if available) is always preferred.
No.  Rule  Condition 

1  relTol := Tolerance

relTol has not been modified, while Tolerance has a value. 
2  absTol := relTol

This applies if relTol has been modified, but not absTol . 
3 
orabsTol := absTol/pow(10,(ceil(log10(absTol)/3))) 
This is an assumption if absTol is set to < 1e5 , while relTol has no value. 
Time range of the simulation (general): dt := abs(tStop  tStart) 

4 
or dtMin := dt^2/1000 
This applies if dt is set to > 0 and dt to < 1e2 . 
5 
or dtMin := dtMin*(relTol/1e8*0.1)^2 
This applies if condition 4 is fulfilled and relTol is set to < 1e8 . 
6  dtMin := 1e8*(if dt<1 then dt else 1)

This applies if condition 4 is not fulfilled, while dt is set to > 0 and relTol to < 1e8 . 
7  dtProtMin := interval

This applies when interval > 0 and dt > 0 , while dtProtMin has no value. 
8  dtProtMin := (tStop  tStart)/500

This is an assumption if neither dtProtMin nor Interval has a value. Instead of 500, SimulationX may use a different value for the number of intervals if this value was set in a vendor annotation (e.g. NumberOfIntervals ) by another tool. 
9  dtMax := (tStop  tStart)/100

This is an assumption if there is no value for dtMax . 
10  dtDetect := dtMin*1e4

This is an assumption if there is no value for dtDetect . 
Automatic selection of the CVODE method for Modelica models
For "pure" Modelica models, e.g. the example models from the Modelica Standard Library, SimulationX sets the solver method to CVODE automatically. This automatic conversion ensures that the computational speed and the accuracy of the results are comparable with those from other Modelica environments, since the CVODE method is the closest thing to the solvers used in those tools. Automatic modifications (if not specified by other Modelica solver parameters) include:
 CVODE method (based on compiled C code)
 absTol=1e6
 relTol=1e6
The automatic conversion is subject to the following rules and conditions:
 There is no
experiment
annotation in the model.  There is an
experiment
annotation, but no SimulationX solver is specified.  If the
experiment
annotation has notolerance
parameter, SimulationX uses the aforementioned values for absTol and relTol.  If the
experiment
annotation has atolerance
parameter, SimulationX converts this value into values for absTol and relTol according to the specifications described in the section Conversion rules.  New solver settings are saved with the model.
 SimulationX never changes models automatically if they are saved as ISX project or as ISM file.
Please note that you can change the automatic values at any time or reset them to the default values.
Structural Switchover (mathematical)
The following describe a definition for ODE solvers (Ordinary Differential Equations), such as CVODE.
To solve ODE systems, the algebraic variables and derivatives of the ODE state variables are calculated from the ODE state variables, partly from assignments or, in the case of implicit equations or algebraic loops, in blocks. What happens when the block variables in certain branches cannot be determined from this block is called structural switchover, because the block becomes singular. In one branch, for example, the derivative of an ODE state variable is calculated from the block. In the other branch, the variable is calculated itself, hence the variable is no longer an ODE state variable. This changes also the differentiation index of this variable (e.g. velocity of static friction, acceleration of sliding friction, switching capacitors and coils in electronic applications on and off). If, however, several potential ODE state variables are connected through a constraint (e.g. a pendulum in the xy plane), the ODE state variables change, which is achieved through dummy pivoting (description at the derivative's level) or state selection (description at the ODE state variable's level) and hence does not constitute a structural switchover. The same goes for blocks which become singular without an if
then
else
construct or state machine (when
constructs), e.g. in x*x=1
).