Розуміння графіка Найквіста (полярного графіка)
A Діаграма Найквіста — widely called a полярний графік in vibration work — is a graph that displays a changing vibration vector on a polar coordinate system. Unlike a Діаграма Боде, which spreads амплітуда і фаза across two separate Cartesian graphs, the Nyquist plot folds both into a single picture. The distance from the origin is the amplitude of the vibration, and the angle around the plot is its phase angle, so every point on the curve is one complete vibration vector.
1. Definition: Amplitude and Phase on One Diagram
The plot traces the path swept by the tip of the 1× (synchronous) vibration vector as the machine’s speed changes, typically through a startup or shutdown. Speed is marked along the curve with changing colours or symbols, so the analyst can read not only the vector’s magnitude and direction but also the RPM at which each point was captured.
- У "The distance from the origin (centre) represents the amplitude of the vibration — the further out, the larger the response.
- У "The кут навколо ділянки represents the фазовий кут of the vibration relative to the тахометр reference.
Because both axes describe a single rotating component, the Nyquist plot is read as the locus of a vector rather than as a pair of trend lines, and that compactness is exactly what makes it so revealing near a resonance.
2. Why is the Nyquist Plot Important?
The Nyquist plot gives a uniquely compact view of a machine’s dynamic response. Its primary purpose, like the Bode plot, is to identify and analyse критичні швидкості - "У нас тут є резонанси of the rotor-bearing system.
Класичним показником критичної швидкості на графіку Найквіста є петля. As speed approaches a власна частота, the amplitude rises and the curve moves away from the origin. As the speed passes through the critical speed, the phase undergoes a 180-degree shift, sweeping the vector tip around to form a circle or loop. The point of maximum amplitude is the top of the loop, and the critical speed itself sits at the 90-degree phase-shift point on the loop — a far more obvious landmark than the gradual phase ramp on a Bode plot.
3. Interpreting a Nyquist Plot
The shape, size and orientation of the loop carry a great deal of diagnostic information about the rotor’s health and dynamic properties.
- Демпфування: the diameter of the loop is inversely related to system демпфування. A large, well-formed circle indicates low damping and high amplification; a small, tight loop indicates a well-damped system.
- Anisotropy (split criticals): if a rotor system has different stiffness in the horizontal and vertical directions, the plot may show two distinct, overlapping loops — a clear “split critical” caused by directional жорсткість.
- Heavy-spot location: the orientation of the loop helps locate the rotor’s heavy spot — the дисбаланс — relative to the phase reference mark on the shaft, which directs where a корекційна вага should go.
- System changes: comparing Nyquist plots over time reveals shifts in machine condition. A change in loop size or shape signals a change in damping or stiffness, which may point to a тріснутий ротор, a loose фундамент, or evolving bearing characteristics.
- Балансування: the plot is used in advanced flexible-rotor balancing. By observing how the loop moves after a пробна вага is added, an analyst derives the коефіцієнти впливу needed to compute the balancing solution.
4. Capturing the Data in the Field
A Nyquist plot is only as good as the synchronised amplitude-and-phase data behind it, and that data depends on a clean once-per-revolution phase reference taken through a speed sweep. In the field, a portable two-channel analyser such as the Балансет-1а measures 1× amplitude and phase against its optical-tachometer pulse during a розбіг або спуск по течії, providing the vector-versus-speed record from which a polar plot is drawn. The same amplitude-and-phase measurement underpins балансування поля, so one instrument both characterises a resonance and corrects the unbalance that excites it. To check that an order line will actually cross a natural frequency within the operating range, an analyst can sketch the interference first with the Калькулятор діаграми Кемпбелла.
5. Nyquist Plot vs Bode Plot
Nyquist and Bode plots display the exact same data — 1× amplitude and phase versus speed — in different formats, and the choice between them often comes down to analyst preference and the feature being emphasised.
- Bode plot: better for reading the precise RPM of the peak amplitude and the exact start and end of the 180-degree phase shift, because its speed axis is linear and easy to scale.
- Nyquist plot: better for taking in the whole dynamic response at once. It excels at showing the amount of damping through loop size and at exposing split criticals as overlapping loops, both more intuitively than a Bode plot can.
A closely related display is the орбіта вала, which plots motion in the bearing plane rather than a single vector versus speed. Most modern аналізатори вібрації can draw all three, and experienced analysts use them together for a comprehensive динаміка ротора diagnosis.
