eldo_user

Speed and Accuracy
Time Step Control
Time Step Control
The resolution of the system of equations uses discrete time steps, and errors are unavoidable in
all realistic cases.
Tip
See “Speed and Accuracy in Eldo” on page 1245.
Time step control designates the set of methods used by the simulator to select these time steps,
so that the accuracy of the solution is maintained within predefined tolerances.
Overview of Time Step Control Algorithms
To simplify, the time step can be controlled in three different ways.
Eldo can use either:
• Local Truncation Error control (LTE)
• Rate-of-change control (DVDT)
• Iteration Count control (IC)
Some variants on these schemes are also available and detailed below, but these are the main
three strategies. To select one of these time step control methods, use .OPTION LVLTIM.
Tip
See “.OPTION LVLTIM” in the Eldo Reference Manual.
By default, Eldo controls the local truncation error (LTE) and determines the time steps it can
take based on estimations of this error. When a solution at time t has been accepted, to progress
in time, Eldo will compute the value h of the largest time step it can take while still maintaining
an acceptable LTE. Note that this is just a guess. The solution at time point t+h is predicted
using the previous solutions at the previous time points. This serves as the initial guess. Next,
Eldo tries to achieve convergence at the new point t+h. If convergence cannot be achieved, the
time step is reduced using a smaller time step h’ (h’