I. Eddy Current Sensor
Eddy current sensors are non-contact sensors based on the principle of electromagnetic induction, and are widely used in various industrial automation, precision measurement and detection fields.
The working principle of eddy current sensors is based on Faraday's law of electromagnetic induction and Lenz's law. When an alternating current flows through a conductor, an alternating magnetic field is generated around the conductor. This alternating magnetic field induces a current called eddy current in the conductor near the coil. The magnitude of the eddy current depends on the frequency of the current in the coil, the conductivity and permeability of the conductor, and the distance between the coil and the conductor.
An eddy current sensor typically consists of an excitation coil and a detection coil. The excitation coil is supplied with an alternating current, generating an alternating magnetic field. When the coil approaches a conductor, eddy currents are induced in the conductor, which in turn generate a magnetic field opposite to the excitation magnetic field. The detection coil senses this change in the magnetic field and converts it into an electrical signal output.
Based on the configuration of the excitation coil and the detection coil, eddy current sensors can be classified into the following types:
1. Single-coil eddy current sensor: It has only one coil, which is used for both excitation and detection.
2. Dual-coil eddy current sensor: The excitation coil and the detection coil are separated, which can reduce interference and improve measurement accuracy.
3. Multi-coil eddy current sensor: Using multiple coils, more complex measurements and analyses can be performed.
II. What are the differences between eddy current sensor and capacitive sensor detection technologies?
(I) Sensor Structure
The first thing to understand is the difference between capacitive and eddy current sensors, and we must first look at their structures. At the center of a capacitive probe is the sensing element. This piece of stainless steel generates an electric field, which is used to detect the distance to the target. A guard ring and the sensing element are separated by an insulating layer, and the guard ring is also made of stainless steel. The guard ring surrounds the sensing element and focuses the electric field onto the target. Several electronic components are connected to the sensing element and the guard ring. All these internal components are surrounded by an insulating layer and encapsulated in a stainless steel housing. The housing connects to the grounding shield of the cable.
The main functional part of an eddy current probe is the induction coil. This is the coil near the end of the probe. Alternating current passes through the coil to generate an alternating magnetic field. This magnetic field is used to detect the distance to the target. The coil is sealed with plastic and epoxy resin and mounted in a stainless steel housing. Because the magnetic field of an eddy current sensor is not as easily focused as that of a capacitive sensor, the epoxy-coated coil extends from the steel housing onto the probe of the eddy current sensor to allow the entire induction magnetic field to engage with the target.
(II) Sensor spot size, target size and measurement range
In non-contact sensor probes, the sensing area engages with the target within a defined region. The size of this region is called the spot size. The target must be larger than the spot size; otherwise, special calibration is required. The spot size is always proportional to the probe diameter. For capacitive and eddy current sensors, the ratio between the probe diameter and the spot size differs significantly. These different spot sizes will result in different minimum target size requirements.
Capacitive sensors use an electric field for detection. This electric field is focused onto the probe through a guard ring, resulting in a spot size 30% larger than the diameter of the sensing element. The typical ratio of detection range to sensing element diameter is 1:8. This means that for each unit of measurement range, the sensing element diameter must be eight times larger. For example, a 500µm detection range requires a 4000µm (4mm) sensing element diameter. This ratio is used for typical calibration. High-resolution and extended-range calibrations will change this ratio.
Eddy current sensors utilize a magnetic field that completely surrounds the end of the probe. This generates a large inductive field, resulting in a spot size approximately three times the diameter of the probe's induction coil. For eddy current sensors, the ratio of detection range to induction coil diameter is 1:3. This means that for each unit of measurement range, the coil diameter must be three times larger. In this case, for the same detection range of 500µm, only a 1500µm (1.5mm) diameter eddy current sensor probe would be needed. When selecting an inspection technique, consider the target size. Smaller targets may require capacitive sensors. If the target size must be smaller than the sensor's spot size, special calibration may be able to compensate for inherent measurement errors.
