Eddy current sensors are understood to be able to measure the distance between a measured metal conductor and the probe surface in a non-contact, highly linear, and high-resolution manner, both statically and dynamically. It is a non-contact, linearized metrological tool. Eddy current sensors can accurately measure the static and dynamic relative displacement changes between the measured object (which must be a metal conductor) and the probe end face. This article introduces the installation method of eddy current sensors; interested readers can take a look!
Eddy current sensor installation method
1. Probe Installation
① Before inserting the probe into the mounting hole, ensure that there are no foreign objects in the hole and that the probe can rotate freely without getting tangled with the wire.
② To avoid scratching the probe tip or monitoring surface, a non-metallic gap gauge can be used to determine the probe gap.
③ The probe gap can also be adjusted by an electric method that connects the probe wire to the extension cable and the preamplifier.
After adjusting the probe gap appropriately, tighten the lock nut. Note that overtightening may damage the threads. Once the probe is secured, the probe cable should also be securely fastened. The extension cable length should match the required length of the preamplifier. Any lengthening or shortening will result in measurement errors. The specific installation is shown in Figure 1.
When two sensor probes need to be installed at each measuring point, the two probes should be installed on the same plane on both sides of the bearing, 90° ± 5° apart. Since the bearing cover is generally horizontally divided, the two probes are usually installed at 45° on each side of the vertical centerline. Viewed from the prime mover end, they are defined as the X probe (horizontal direction) and the Y probe (vertical direction), with the X direction on the right side of the vertical centerline and the Y direction on the left side of the vertical centerline, as shown in Figure 2.
2. Installation of extension cable
The extension cable, serving as the intermediate link between the probe and the preamplifier, is a crucial component of the eddy current sensor. Therefore, its installation should ensure it is not easily damaged during use, and exposure to high temperatures should be avoided. The connection between the probe and the extension cable should be securely tightened, and the joint should be wrapped with heat-shrink tubing to prevent grounding and loosening. When coiling the extension cable, avoid coiling it too small to prevent cable breakage. Generally, the coiled diameter of the extension cable should not be less than 55mm.
3. Installation of the preamplifier
The preamplifier should be housed in a cast aluminum enclosure to prevent mechanical damage and contamination. Excess cables should not be attached to the enclosure. Multiple preamplifiers are permissible within the same enclosure, provided the probe-to-preamplifier cable length remains unchanged, to reduce installation costs and simplify cabling from the preamplifier to the monitor. Appropriate isolation and shielding grounding should be used to minimize signal interference. The preamplifier is the signal processing component of the entire sensor system and must be installed away from high-temperature environments. Its surroundings should be free of significant steam and water droplets, corrosive gases, and should be dry, with minimal vibration, and the ambient temperature should not differ significantly from room temperature. During installation, the metal parts of the preamplifier housing should not come into contact with the main unit or ground. During installation, it is essential to avoid other interfering signals that could affect the measurement circuitry.
Locking of mounting gap for 4-axis vibration sensor
Connect the probe, extension cable, and preamplifier, and power on the sensor system. Use a high-precision multimeter to monitor the output voltage of the preamplifier. At the same time, adjust the gap between the probe and the surface being measured. When the output voltage of the preamplifier is approximately between 10-11VDC, tighten the two locking nuts on the probe to secure it.
5 Precautions
① Irregular surfaces of the object being measured will introduce additional errors to the actual measurement. Therefore, the surface of the object being measured should be flat and smooth, and should not have defects such as protrusions, holes, scratches, or grooves.
② When the object being measured is a circular shaft and the center line of the probe is perpendicular to the center line of the shaft, the diameter of the shaft being measured is generally required to be more than 3 times the diameter of the probe head.
③ The surface of the object being tested should be free of scratches, pores, and gaps, and surface plating is not permitted. The material of the object being tested should be consistent with the material specified by the probe and preamplifier.
① The probe is fixed to the bearing seat by a bracket. The bracket should have sufficient rigidity to increase its natural frequency and avoid or reduce the excited natural vibration of the bracket when the measured object vibrates.
② Requirements for initial clearance
Eddy current sensors will only have good linearity when the gap voltage (the gap between the top of the sensor and the object being measured, which is usually indicated by voltage on the instrument) is at a certain value. Therefore, the appropriate initial gap must be adjusted when installing the sensor.
After the rotor rotates and the unit is under load, the rotor will displace relative to the sensor. If the sensor is mounted on top of the bearing, its clearance will decrease; if mounted horizontally on the bearing, its clearance depends on the rotor's rotation direction; when the rotation direction is fixed, its clearance depends on whether it is mounted on the right or left side. To obtain a suitable operating clearance value, the journal displacement and direction after the rotor transitions from a static to a rotating state and the unit is under load should be estimated during installation to take into account when adjusting the initial clearance. Based on field experience, the journal rises approximately half the bearing clearance when the rotor transitions from static to operating speed; the horizontal displacement depends on the bearing type, the clearance on both sides of the bearing, and the operating state of the unit's sliding pin system, and is generally 0.05-0.20 mm.
When adjusting the initial sensor clearance, in addition to the factors mentioned above, the maximum vibration value and the original rotor wobble value must also be considered. The initial sensor clearance should be greater than half of the maximum possible amplitude of the shaft and the original shaft wobble value.
