Non-contact sensors using capacitive and eddy current technologies each represent a unique combination of advantages and disadvantages in a variety of applications. Comparing the advantages of the two technologies will help you choose the best technology for your application. The following explains the difference in the performance structure of the different detection technologies between these two sensors:
- The structure of the sensor
The first thing to understand is the difference between capacitive and eddy current sensors, we must first look at their construction. The center of a capacitive probe is the detection element. This piece of stainless steel generates an electric field, which is used to detect the distance to the target. The guard ring and detection element are separated by an insulating layer, and the guard ring is also made of stainless steel. A guard ring surrounds the detection element and focuses the electric field on the target. Some electronic components are connected to the detection element and guard ring. All of these internal components are surrounded by insulation and encapsulated in stainless steel housing. The housing is connected to the ground shield of the cable.
The main functional part of the eddy current probe is the induction coil. This is the coil near the end of the probe. An alternating current is passed through the coil to generate an alternating magnetic field. This magnetic field is used to detect the distance to the target. The coils are sealed with plastic and epoxy and housed in stainless steel housing. Because the magnetic field of an eddy current sensor is not as easy to focus as a capacitive sensor. On the probe of the eddy-current sensor, an epoxy-coated coil protrudes from the steel housing to allow the entire induced magnetic field to engage with the target.
- Sensor spot size, target size, and measurement range
The sensing area of the non-contact sensor probe engages the target within a certain area. The size of this area 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, there is a significant difference in the ratio between probe diameter and spot size. These different spot sizes will result in different minimum target size requirements.
Capacitive sensors use an electric field for detection. The electric field is focused on the probe by the guard ring, which results in a spot size 30% larger than the diameter of the detection element. A typical ratio of detection range to detection element diameter is 1:8. This means that the diameter of the sensing element must be eight times larger for each range unit. For example, a 500µm detection range requires a 4000µm (4mm) detection element diameter. This ratio is used for typical calibrations. Calibration of high resolution and extended range will change this ratio.
Eddy current sensors use a magnetic field that completely surrounds the tip of the probe. This produces a larger induction 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 range unit, the coil diameter must be three times larger. In this case, only one eddy current sensor probe with a diameter of 1500µm (1.5mm) is required for the same detection range of 500µm. When choosing 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.
Learn more about the capacitive sensor. https://www.easybom.com/blog/a/capacitive-sensor-a-full-understanding-of-it