Comparing Capacitive and Eddy-Current Sensors

Sensor Building

Understanding the distinction involving capacitive and eddy-current sensors starts by looking at how they may be constructed. In the center of a capacitive probe is definitely the sensing element. This piece of stainless steel generates the electric field which can be made use of to sense the distance for the target. Separated in the sensing element by an insulating layer may be the calibration temperature guard ring, also produced of stainless steel. The guard ring surrounds the sensing element and focuses the electric field toward the target. All of those internal assemblies are surrounded by an insulating layer and encased inside a stainless steel housing. The housing is connected for the grounded shield in the cable.

The main functional piece of an eddy-current probe may be the sensing coil. This can be a coil of wire near the finish in the probe. Alternating current is passed via the coil which creates an alternating magnetic field; this field is utilised to sense the distance towards the target. The coil is encapsulated in plastic and epoxy and installed in a stainless steel housing. Mainly because the magnetic field of an eddy-current sensor is just not as effortlessly focused as the electric field of a capacitive sensor, the epoxy covered coil extends in the steel housing to allow the full sensing field to engage the target.

Spot Size, Target Size, and Variety

Capacitive sensors use an electric field for sensing. This field is focused by a guard ring around the probe resulting inside a spot size about 30% larger than the sensing element diameter. A common ratio of sensing variety for the sensing element diameter is 1:8. This implies that for every unit of range, the sensing element diameter must be eight occasions larger. One example is, a sensing range of 500µm needs a sensing element diameter of 4000µm (4mm). This ratio is for typical calibrations. High-resolution and extended-range calibrations will alter this ratio.The sensing field of a noncontact sensor's probe engages the target over a particular area. The size of this region is called the spot size. The target should be bigger than the spot size or unique calibration might be required.Spot size is often proportional towards the diameter in the probe. The ratio among probe diameter and spot size is substantially diverse for capacitive and eddy-current sensors. These distinctive spot sizes lead to different minimum target sizes.

When selecting a sensing technologies, take into consideration target size. Smaller sized targets might demand capacitive sensing. In case your target must be smaller than the sensor's spot size, unique calibration may be capable to compensate for the inherent measurement errors.Eddy-current sensors use magnetic fields that fully surround the end with the probe. This creates a comparatively large sensing field resulting inside a spot size about 3 occasions the probe's sensing coil diameter. For eddy-current sensors, the ratio with the sensing range to the sensing coil diameter is 1:3. This implies that for every single unit of variety, the coil diameter must be 3 instances bigger. Within this case, exactly the same 500µm sensing range only requires a 1500µm (1.5mm) diameter eddy-current sensor.

Sensing Approach

The two technologies use different methods to determine the position in the target. Capacitive sensors applied for precision displacement measurement use a high-frequency electric field, ordinarily in between 500kHz and 1MHz. The electric field is emitted from the surfaces in the sensing element. To concentrate the sensing field on the target, a guard ring creates a separate but identical electric field which isolates the sensing element's field from all the things however the target. The quantity of current flow inside the electric field is determined in portion by the capacitance among the sensing element plus the target surface. For the reason that the target and sensing element sizes are continual, the capacitance is determined by the distance involving the probe and the target, assuming the material within the gap will not modify. Adjustments in the distance involving the probe and the target modify the capacitance which in turn adjustments the current flow in the sensing element. The sensor electronics make a calibrated output voltage which is proportional for the magnitude of this present flow, resulting in an indication with the target position.Capacitive and eddy-current sensors use distinctive procedures to ascertain the position with the target.