Tips
- When you use this requirement to tune a control system, Control System Tuner
attempts to enforce zero feedthrough (D = 0) on the transfer that the requirement
constrains. Zero feedthrough is imposed because the H 2 norm, and therefore the value
of the tuning goal (see “Algorithms” on page 10-126), is infinite for continuous-time
systems with nonzero feedthrough.
Control System Tuner enforces zero feedthrough by fixing to zero all tunable
parameters that contribute to the feedthrough term. Control System Tuner returns an
error when fixing these tunable parameters is insufficient to enforce zero feedthrough.
In such cases, you must modify the requirement or the control structure, or manually
fix some tunable parameters of your system to values that eliminate the feedthrough
term.When the constrained transfer function has several tunable blocks in series, the
software’s approach of zeroing all parameters that contribute to the overall
feedthrough might be conservative. In that case, it is sufficient to zero the feedthrough
term of one of the blocks. If you want to control which block has feedthrough fixed to
zero, you can manually fix the feedthrough of the tuned block of your choice.To fix parameters of tunable blocks to specified values, see “View and Change Block
Parameterization in Control System Tuner” on page 10-26.- This tuning goal also imposes an implicit stability constraint on the weighted closed-
loop transfer function between the specified inputs to outputs, evaluated with loops
opened at the specified loop-opening locations. The dynamics affected by this implicit
constraint are the stabilized dynamics for this tuning goal. The Minimum decay rate
and Maximum natural frequency tuning options control the lower and upper
bounds on these implicitly constrained dynamics. If the optimization fails to meet the
default bounds, or if the default bounds conflict with other requirements, on the
Tuning tab, use Tuning Options to change the defaults.
Algorithms
When you tune a control system, the software converts each tuning goal into a normalized
scalar value f(x). Here, x is the vector of free (tunable) parameters in the control system.
The software then adjusts the parameter values to minimize f(x) or to drive f(x) below 1 if
the tuning goal is a hard constraint.For Weighted Variance Goal, f(x) is given by:10 Control System Tuning