The eddy current sensor adopts non-contact measurement. It has a wide measuring range, good dynamic response, high measurement accuracy, long-term transmission anti-interference ability, no medium influence, simple structure, easy calibration, continuous long-term reliable operation and so on. In recent years, eddy current sensors have been widely used in the measurement of axial displacement, radial vibration, rotation speed, axis trajectory, etc. of high-speed rotating machinery, and have played an increasingly larger role in production. The following is an example of an SDW-5 eddy current sensor manufactured by Wuxi Wensheng Automation Instrumentation Co., Ltd., which is widely used.
1 eddy current sensor system composition
The SDW-5 eddy current sensor is a non-contact sensor system that converts a gap into a voltage value and measures the distance between the probe tip and the surface of the measured object. The signal provided by the sensor can be used for rotating machinery condition monitoring and fault diagnosis. The sensor system is constructed as shown in Figure 1.
The probe is a coil that is precisely wound on a round mandrel. The head is made of polyphenylene sulfide and screwed into the stainless steel housing [1]. According to the diameter of the probe is divided into two kinds of 5mm, 8mm, extension cable with built-in cable and coaxial cable connection system, requires a distance of 5m or 9m from the probe tip to the front device, the length of the probe and cable can be The same type of predistorter can be freely used; the predistorter has two functions, high frequency signal generated by the oscillator circuit, and the detector can adjust the changed frequency signal to an accurate usable signal. In order to implement these functions, it is necessary to provide -24VDC power supply between its VT and COM terminals. The signal output and power supply of the preamplifier and RDZW-2Na monitoring system are completed by a three-core shielded cable. The driver can be placed 1000 feet away from the monitor without gains.
2 How to improve accuracy in use
(1) surface opening size
Improper opening of the probe mounting position, or improper size of the probe mounting opening, can cause the measurement of the probe to be affected by the metal of the housing, resulting in measurement errors.
In Fig. 2, if the distance between the probe and the bracket is too small, eddy current will be generated on the bracket, affecting the accurate measurement of the probe.
Ensure that the d value is at least 1512 mm during installation. In the measurement of Figure 4, the small size of the mounting opening also affects the accuracy of the measurement. For the 3300 Series, D should be twice the diameter of the probe.
(2) Stiffness of the probe holder
If the rigidity is not enough, the bracket will also vibrate when the machine is running, which obviously will affect the measurement accuracy. It is generally required that the natural frequency is at least equal to 10 times the speed of the machine. Pay special attention to the original installation. To measure the key phase with a probe, a notch or a boss must be large enough to produce a pulse with at least 5 V peak. This key phase slot or protrusion is at least 215 mm deep (or high), 716 mm wide, and 10 mm long.
(3) Shielding and grounding of cables
Coaxial cables and extension cables are themselves shielded cables used to shield external interference signals. In order to shield outside interference, pay attention to the following aspects:
1 The entire system must be grounded at one point, and no ground loop can be formed. Otherwise, earth loops may generate additional interference.
2 For the signal, use only shielded cables, and ensure that the shielded cable is properly grounded, that is, one end is grounded and the grounding resistance is ≤ Ω. 4
3 To ensure that all cables and connectors are absolutely installed properly, the coupling of the probe to the coaxial cable must be wrapped in insulated plastic tape.
4 Keep signal cables away from noise sources.
Wuxi Wensheng Automation Instrumentation Co., Ltd.
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