What is phase locking?

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Definition: Key phase measurement involves placing a groove or protruding key on the shaft being measured, which is referred to as a key phase marker. When this groove or protrusion moves to the probe position, the distance between the probe and the surface being measured changes, and the sensor generates a pulse signal. One pulse signal is produced for each full rotation of the shaft, and the timing of these pulses indicates the position of the shaft within each rotation cycle. Therefore, by counting the pulses, it is possible to measure the rotational speed of the shaft ； By comparing the pulse with the vibration signal of the shaft, the phase angle of the vibration can be determined, which is useful for the dynamic balancing analysis of the shaft as well as for the fault analysis and diagnosis of equipment. Installation notes: The groove or protrusion key must be large enough so that the peak value of the generated pulse signal is not less than 5V (the AP1670 standard requires it to be not less than 7V). Generally, when φ5 or φ8 probes are used, the width of this groove or protrusion should be greater than 7.6 mm, its depth or height should be greater than 1.5 mm (2.5 mm or more is recommended), and its length should be greater than 10.2 mm. The groove or key should be parallel to the axis centerline, and its length should be as large as possible to ensure that the probe remains aligned with the groove or key even when the shaft experiences axial movement. To avoid excessive changes in the gap between the probe and the surface being measured due to axis phase shift, the phase-probe should be installed radially on the shaft, rather than axially. The phasor probe should be installed on the driving part of the unit as much as possible, so that even if the driving part of the unit becomes disconnected from the load, the sensor will still output a phasor signal. When the machine unit operates at different speeds, multiple set of phasor sensor probes are usually required for monitoring, so as to provide effective phasor signals for various parts of the machine unit. 　　The key phase marker can be a groove or a protruding key. When the mark is a groove, the initial installation clearance should be adjusted with the probe aimed at the entire portion of the shaft (it is advisable to position it at the midpoint along the sensor’s length), rather than aiming it at the groove. And when the mark is a convex key, the probe must have its initial installation clearance adjusted to face the top surface of the protrusion (it is advisable to install it at the midpoint along the sensor’s length), rather than being adjusted to face other flat surfaces of the shaft. Otherwise, when the shaft rotates, it may cause the protruding key to collide with the probe and cut it off. The role of phase measurement in condition monitoring: The first role is to measure the rotational speed, and it is mainly used in devices with variable frequency drives ； 　　The second function is that, whether the equipment is starting up or shutting down, it must pass through the critical speed (except for rigid shafts); by using key phase in combination with a shaft vibration probe, it is possible to accurately capture the vibration trends during startup and shutdown ； 　　The third is to detect axial distortion of the rotating shaft during normal use, as such an abnormality can lead to the complete failure of the shaft, although it occurs very rarely! However, since this kind of deformation cannot be detected by a regular shaft vibration probe on its own, this is called phase angle measurement ； A phase sensor cannot detect axial distortion; a set of specialized sensors is required for that purpose ； 　　The fourth function is to, in conjunction with the shaft vibration probe, obtain the real-time axis position of the rotating shaft, in order to analyze bearing disturbances or eddy currents ； The axis position and axis trajectory are also not determined by key phase sensors; the signals measured by two 90° eddy current sensors can be used to plot these values. The axis trajectory diagram is employed to analyze the direction of bearing precession in order to determine whether friction is present ； 　　The fifth function is synchronous sampling; through phase-shift pulses, it is possible to control sampling throughout the entire cycle of synchronization, thereby eliminating the problem of energy leakage. Things like Hanning windows that are required for data processing in handheld instruments are not necessary in this case. There are other benefits as well, such as high frequency resolution ； 　　The sixth function is phase measurement, which allows for determining the relative vibration patterns of various measurement points; it also provides information on the relationship between the key phases and the slots, thereby aiding in the determination of the position of counterweights ； 　　The seventh function is that diagnosis primarily relies on the relative vibration patterns of various measurement points.