Will your controller survive a different motor?
Nominal performance describes one model. Robustness asks what remains true across a stated set of plausible models.
First, picture it
A motor built today may have more inertia or friction than the one used to tune the controller. A good controller should not depend on the nominal numbers being exact. But 'works for two motors' is a narrower claim than 'works for every possible motor'.
What the model says
State the uncertain parameters and their ranges before testing. Hold controller, target, disturbance and seed fixed while varying one plant parameter, then combine changes for a harder case. Compare settling, error, applied input, saturation and failures. Gain/phase margins and sensitivity functions are useful linear nominal-loop indicators, but they do not by themselves certify a nonlinear, limited actuator over every parameter combination.
P ∈ 𝒫 ⇒ test specified requirements for each studied PA finite sweep samples 𝒫; it is not a proof for every member of a continuous uncertainty set.
The curves illustrate a possible difference, not a numerical result from a specific Lab preset. Hold the controller fixed when testing robustness.
One variable, then combined stress
The Lab's model-mismatch lesson changes actual motor inertia while the nominal frequency model remains fixed. This separation is deliberate: it shows why a Bode curve can stay put as the time trace changes. Vary friction separately, then try both together. Report exactly which values you tried and which performance requirement failed first.
If you tune again after every parameter change, you are testing a family of retuned controllers, not robustness of one fixed design. Keep the same controller while asking the robustness question.
Make it concrete
Pin nominal inertia 1×, compare actual inertia 2× with unchanged gains, then vary friction. Check whether both still meet your written settling and voltage conditions.
Stress-test one motor designMake a prediction before moving a control.
- PREDICTWhich requirement will fail first if inertia or friction changes?
- CHANGE ONE THINGKeep gains fixed; compare inertia 1× and 2×, then friction separately.
- OBSERVERecord response, applied voltage and any saturation or fault.
- EXPLAINLimit your conclusion to the parameter range actually tested.
Passing a finite parameter sweep is evidence for the tested cases, not a theorem about all untested parameter values, disturbances or manufacturing variation.
Check your understanding+
If the nominal Bode plot does not change when actual inertia doubles, does the actual motor behave identically?
No. The panel still analyzes the fixed design model; the nonlinear time run uses the changed plant parameter.