Scale uncertainty of block or system
scales the amount of uncertainty in an uncertain control design block by
blk_scaled = uscale(
factor is a robustness margin
robgain, or a robust performance returned by
blk_scaled is of the same type as
blk, with the amount of uncertainty scaled in normalized units. For
factor is 0.75, the normalized uncertainty of
blk_scaled is 75% of the normalized uncertainty of
Find Tolerable Range of Gain and Phase Variations
Consider a feedback loop with the following open-loop gain.
L = tf(3.5,[1 2 3 0]);
Suppose that the system has gain uncertainty of 1.5 (gain can increase or decrease by a factor of 1.5) and phase uncertainty of ±30°.
DGM = getDGM(1.5,30,'tight'); F = umargin('F',DGM)
F = Uncertain gain/phase "F" with relative gain change in [0.472,1.5] and phase change of ±30 degrees.
Examine the robust stability of the closed-loop system.
T = feedback(L*F,1); SM = robstab(T)
SM = struct with fields: LowerBound: 0.8303 UpperBound: 0.8319 CriticalFrequency: 1.4482
robstab shows that the system can only tolerate 0.83 times the modeled uncertainty before going unstable. Scale the
F by this amount to find the largest gain and phase variation that the system can tolerate.
factor = SM.LowerBound; Fsafe = uscale(F,factor)
Fsafe = Uncertain gain/phase "F" with relative gain change in [0.563,1.42] and phase change of ±24.8 degrees.
The scaled uncertainty has smaller ranges of both gain variation and phase variation. Compare these ranges for the original modeled variation and the maximum tolerable variation.
DGM = F.GainChange; DGMsafe = Fsafe.GainChange; diskmarginplot([DGM;DGMsafe]) legend('original','safe')
Scale All Uncertain Elements in a Model
Consider the uncertain control system of the example "Robust Performance of Closed-Loop System" on the
robgain reference page. That example examines the sensitivity of the closed-loop response at the plant output to disturbances at the plant input.
k = ureal('k',10,'Percent',40); delta = ultidyn('delta',[1 1]); G = tf(18,[1 1.8 k]) * (1 + 0.5*delta); C = pid(2.3,3,0.38,0.001); S = feedback(1,G*C)
S = Uncertain continuous-time state-space model with 1 outputs, 1 inputs, 4 states. The model uncertainty consists of the following blocks: delta: Uncertain 1x1 LTI, peak gain = 1, 1 occurrences k: Uncertain real, nominal = 10, variability = [-40,40]%, 1 occurrences Type "S.NominalValue" to see the nominal value, "get(S)" to see all properties, and "S.Uncertainty" to interact with the uncertain elements.
Suppose that you do not want the peak gain of this sensitivity function to exceed 1.5. Use
robgain to find out how much of the modeled uncertainty the system can tolerate while the peak gain remains below 1.5.
perfmarg = robgain(S,1.5)
perfmarg = struct with fields: LowerBound: 0.7821 UpperBound: 0.7837 CriticalFrequency: 7.8566
With that performance requirement, the system can only tolerate about 78% of the modeled uncertainty. Scale all the uncertain elements in
S to create a model of the closed-loop system with the maximum level of uncertainty that meets the performance requirement.
factor = perfmarg.LowerBound; S_scaled = uscale(S,factor)
S_scaled = Uncertain continuous-time state-space model with 1 outputs, 1 inputs, 4 states. The model uncertainty consists of the following blocks: delta: Uncertain 1x1 LTI, peak gain = 0.782, 1 occurrences k: Uncertain real, nominal = 10, variability = [-31.3,31.3]%, 1 occurrences Type "S_scaled.NominalValue" to see the nominal value, "get(S_scaled)" to see all properties, and "S_scaled.Uncertainty" to interact with the uncertain elements.
The display shows how the uncertain elements in
S_scaled have changed: the peak gain of the
delta is reduced from 1 to 0.78, and the range of variation of the uncertain real parameter
k is reduced from ±40% to ±31.3%.
factor — Scaling factor
Scaling factor, specified as a scalar. This argument is the amount by which
uscale scales the normalized uncertainty of
M. For instance, if
factor = 0.8, then the function reduces the uncertainty to 80%
of its original value, in normalized units. Similarly, if
2, then the function doubles the uncertainty.
factor is a robustness margin returned by
robgain, or a robust performance returned
Thus, you can use
uscale to find the largest range of modeled
uncertainty in a system for which the system has good robust stability or performance.