In today's rapidly developing technology, sensors, as key components for obtaining information, are widely used in various fields. Eddy current sensors play an important role in industrial production, scientific research experiments, and other fields due to their unique advantages. So, what are the testing methods for eddy current sensors? Let's unveil its mysterious veil together.
一、 The working principle is the foundation
To understand the testing method of eddy current sensors, first clarify their working principle. According to Faraday's law of electromagnetic induction, when a bulk metal conductor is placed in a changing magnetic field or undergoes cutting magnetic field lines, a vortex like induced current, known as eddy current, is generated inside the conductor, which is the eddy current effect. Eddy current sensors are made based on this effect.
The coil in the sensor probe is subjected to high-frequency alternating current, generating an alternating magnetic field. When the tested metal body approaches the magnetic field, induced current (eddy current) is generated on the metal surface, and at the same time, the eddy current flow field generates an alternating magnetic field opposite to the direction of the probe coil, which reacts on the probe coil and changes the effective impedance of the coil. This change is related to many parameters. If the control parameters remain unchanged, the characteristic impedance of the coil becomes a single value function of distance. By processing the electronic circuit of the preamplifier, the impedance changes of the coil are converted into voltage or current changes, thereby achieving the measurement of relevant parameters of metal objects.
二、 There are methods for static measurement
（1） Static calibration experiment
In the static calibration experiment, install the eddy current sensor and align it with the measured object (such as an iron measuring piece), and connect the eddy current converter, F/V meter and other equipment. After turning on the power supply, adjust the distance between the sensor and the measured body, and record the value of the F/V meter every certain distance (such as 0.10-0.50 mm) from the contact between the two until the linearity deteriorates seriously. Based on experimental data, draw a V-X curve on coordinate paper (V is the output voltage, X is the distance), which can indicate the approximate linear range and calculate the system sensitivity. For example, if in a certain experiment, within a certain linear range, the output voltage changes by 2V for every 1mm change in distance, then the sensitivity is 2V/mm. During this process, it is important to ensure that the tested object is as parallel as possible to the plane of the sensor test probe, and to align the probe with the center of the tested object to reduce eddy current losses.
（2） The influence of the tested material on the test
Different materials of the tested object have an impact on the characteristics of eddy current sensors. During the experiment, first install the sensor and micrometer head, and connect the circuit. Starting from the contact between the sensor and the iron measuring piece, rotate the micrometer head to change the distance and record the F/V meter reading until significant nonlinearity appears. Then replace the aluminum measuring piece and repeat the operation. Based on the obtained results, draw two V-X curves for aluminum and iron on the same coordinate paper, calculate and compare their sensitivity and linear range. Usually, when the measured object is a magnetic material (such as ordinary steel), the sensitivity of the sensor decreases; When using weakly magnetic materials such as copper and aluminum, the sensitivity is relatively high.
三、 Dynamic measurement showcases skills
（1） Radial vibration measurement of shaft
When measuring the radial vibration of a shaft, it is required that the shaft diameter be more than three times the diameter of the probe. Install two sensor probes simultaneously at each measuring point, located on the same plane on both sides of the bearing, separated by 90 °± 5 °. Generally, they are installed at 45 ° on each side of the vertical centerline. From the perspective of the prime mover end, the X probe is on the right side (horizontal direction) and the Y probe is on the left side (vertical direction). The installation position of the probe should be as close as possible to the bearing, and the centerline of the probe should be orthogonal to the axis. The monitoring surface should have no cracks or discontinuities, and the roughness should be between 0.4-0.8 μ m. By measuring the changes in the output signals of two probes, the radial vibration of the shaft can be obtained.
（2） Measurement of axial displacement of the shaft
Measure the axial displacement of the shaft, and the measuring surface should be integrated with the shaft, forming a circular ring with a width of 1.5 times the diameter of the probe and centered on the centerline of the probe. The installation distance of the sensor probe from the thrust flange should not exceed 305mm, otherwise the measurement result will be inaccurate. For example, in the monitoring of a certain steam turbine, by accurately installing eddy current sensors, the axial displacement of the shaft is monitored in real time, ensuring the safe and stable operation of the steam turbine. （3） Keyphasor measurement
Key phase measurement is achieved by setting grooves or convex keys (key phase markers) on the measured axis. When the marker is turned to the probe position, the sensor generates a pulse signal, which is generated for each rotation of the shaft. By counting the pulses, the shaft speed can be measured, and comparing the pulses with the shaft vibration signal can determine the vibration phase angle, which is used for shaft dynamic balance analysis and equipment fault diagnosis. The groove or convex key should be large enough to ensure that the peak to peak value of the pulse signal is not less than 5V, and the key phase probe should be installed radially on the shaft, preferably in the driving part of the unit. When marked as a groove, adjust the initial installation gap of the probe towards the complete part of the shaft; When using a convex key, adjust it towards the top surface of the protrusion.
There are various testing methods for eddy current sensors, covering multiple aspects of static and dynamic measurement. In practical applications, it is necessary to accurately select suitable testing methods based on specific needs and measurement objects, fully utilize the advantages of high precision and non-contact of eddy current sensors, and provide reliable data support for production and research in various fields, helping technological progress and industrial development.