A smooth trace can be a sampling artifact. Before retuning a controller, check how the trace was measured.
Start with how the trace was acquired
A low-frequency ripple appears in a motor log. Lowering controller gain is tempting, but the ripple might be a high-frequency component folded down by sampling. Write down the sensor sampling rate, any decimation, the analog filter and timestamps before changing the control law. The plotted frame rate is not necessarily the sensor sampling rate.
Two curves, identical measurements
Use the 7 Hz at 10 Hz preset. Predict a 3 Hz apparent frequency and inspect the circles on the original and dashed candidate. They coincide despite different curves between them. Now raise only the sampling rate to 20 Hz: the candidate becomes the original 7 Hz signal. Finally try the Nyquist boundary preset, then move phase from 0° to 90°. An all-zero log can turn into alternating extremes without changing amplitude or frequency.
Repair the measurement path first
A smoother plot or a digital filter cannot establish what the pre-sampled waveform was. Reacquire with a suitable rate and analog filtering, then compare the same physical operating condition. This isolated sine example contains no sensor noise or jitter; real measurements also need timestamp and calibration checks. Do not treat a clean-looking trace as evidence that the plant stopped vibrating.
Delay changes when a value becomes available. Quantization rounds its amplitude. Aliasing makes different continuous frequencies indistinguishable after sampling. This bench isolates sampling and phase: there is no noise, quantizer, delay or anti-alias filter. A digital low-pass filter after sampling cannot generally recover a component already folded into the signal band. Filter unwanted analog frequencies before sampling and choose a rate for both measurement and feedback dynamics.