Principle of shaft displacement measurement

Please clarify your question. Nowadays, axial displacement is measured using sensors; are you referring to the working principle of these sensors?

Reply to 1# aqshhsq: Taking Bently’s 3300 and 3500 monitoring systems as examples, the probes, extension cables, and preamplifiers used for measuring shaft displacement and shaft vibration are identical; the difference lies in the monitor unit. The output signal generated by the eddy current probe, which is proportional to the instantaneous displacement (the distance between the probe and the surface being measured), consists of both a DC component and an AC component. The DC component corresponds to the arithmetic average of the signal, while the AC component represents the instantaneous value of the vibration displacement. Axis displacement monitors primarily process the DC component for display, whereas axis vibration monitors mainly process the AC component for display.

Reply to 2# watermelon97: Yes, how does the shaft displacement sensor work?

Reply 4# aqshhsq: The Bently 3300 XL 8mm eddy current proximity sensor consists of a platinum wire coil wound around a solid support; it is enclosed in a stainless steel housing along with corrosion-resistant materials. A coaxial cable pigtail is connected to it, and this cable in turn is connected to the preamplifier. According to Maxwell’s theory of electromagnetic fields, when a high-frequency current is passed through the sensor coil, a high-frequency magnetic field is generated around the coil. When this magnetic field passes through the metal surface of the rotating shaft adjacent to it, it induces an electric eddy current within that surface. According to Lenz’s law, this changing eddy current in turn generates an eddy current magnetic field around it, whose direction is exactly opposite to that of the magnetic field of the primary coil. The superposition of these two magnetic fields alters the impedance of the primary coil. The variation in coil impedance is related both to the eddy current effect and to the magnetostatic effects; if parameters such as magnetic permeability, excitation current intensity, and frequency remain constant, the impedance can be regarded as a single-valued function of the gap between the top of the probe and the metal surface, meaning that there is a proportional relationship between them. Finally, the preamplifier detects the change in impedance and converts it into a voltage signal for output to the monitor.

Currently, sensors for monitoring shaft displacement in equipment such as compressors, turbines, and pumps generally use eddy current displacement sensors; I hope the description below will be helpful to you. The eddy current displacement sensor system mainly consists of a probe, (extension cable), a preamplifier, and the object being measured to form the basic operating system. In the preamplifier, the high-frequency oscillating current flows into the probe coil through the extension cable, generating an alternating magnetic field in the coil at the tip of the probe. If no metal material is present within the effective range of this alternating magnetic field, then all of the energy of this magnetic field will be lost ； When a metal object to be tested is brought near this magnetic field, an induced current is generated on the surface of this metal; in electromagnetics, this is known as eddy current. At the same time, this eddy current field generates an alternating magnetic field in a direction opposite to that of the head coil. As a result of this reaction, the amplitude and phase of the high-frequency current in the head coil change (the effective impedance of the coil). These changes are influenced by parameters such as the magnetic permeability and electrical conductivity of the metal body, the geometric shape and size of the coil, the frequency of the current, and the distance between the head coil and the surface of the metal conductor. It is generally assumed that metal conductors are homogeneous in material and exhibit linear, isotropic properties; therefore, the physical characteristics of coil and metal conductor systems can be described by the parameters of the metal conductor’s electrical conductivity σ, magnetic permeability μ, size factor τ, the distance D between the head coil and the surface of the metal conductor, current intensity I, and frequency ω. Then the characteristic impedance of the coil can be expressed by the function Z=F(τ, ξ, б, D, I, ω). Usually, by keeping the parameters τ, ξ, б, I, and ω within certain ranges, the characteristic impedance Z of the coil becomes a function of the distance D. Although this function is nonlinear with an “S”-shaped curve, it is possible to approximate a linear segment of it. Here, through the processing of the preamplifier circuitry, the change in coil impedance Z, that is, the change in the distance D between the head coil and the metal conductor, is converted into a change in voltage or current. The amplitude of the output signal varies depending on the distance between the probe and the surface of the object being measured; it is based on this principle that eddy current sensors are able to measure parameters such as the displacement and vibration of metal objects.

The axial displacement alarm device is used to monitor axial movement of the rotor during the operation of the turbine. When the axial movement of the rotor exceeds the allowable limit, the axial displacement controller activates, issuing audible and visual alarm signals; at the same time, it shuts down the automatic main steam valve, the throttle valve, and the extraction check valve, thereby initiating an emergency shutdown. Prevent sudden increases in axial thrust or the breakdown of the lubricating oil film from causing bearing scorching accidents.