You are here: Glossary > Encirclement
Encirclement
If the open-loop phase of a frequency response moves through the -180 degree line between two magnitude crossings and the magnitude is larger than 0 dB at the phase cross, the axis will not be stable. This can occur when the plant has a sharp resonance in the structural loop between the motor and feedback. The image that follows shows an example of encirclement. Here the sensitivity peak is below 6 dB but the axis is not stable.
Figure: Encirclement in Frequency Response Plot
One special condition occurs when a positive open-loop magnitude at an open-loop phase crossover frequency is permitted. A low-frequency phase crossover might occur at a frequency lower than the magnitude crossover, where the plant response is mass-like. A positive open-loop magnitude at this frequency is permitted when the absolute value of the gain margin is above 6 dB.
