Feature: Sensors
linearity improvement. This linearity can be further enhanced though the manufacturing and calibration process and changes in material; see Figure 3. While some inductive displacement
sensors also have built-in compensation, it is typically limited to ±3-5% of the full-scale output (FSO). Eddy-current sensors provide compensation for the full measurement channel (±0.025% FSO.) It’s also important to note that, for
Figure 1: Inductive displacement sensor
been developed that uses an air-core coil rather than a ferrite coil. While the eddy current sensor’s
operating principles are in line with Faraday’s Laws, it is the eddy-currents’ effect on the impedance of the coil that is measured rather than the oscillator’s output voltage change. The controller calculates the impedance by looking at the change in the amplitude and phase position of the sensor coil. Benefits of eddy-current sensing
include non-contact measurement of displacement or distance, high resolution and temperature stability, broad temperature range (-40°C to 200°C), robust and reliable sensors, resistance to oil, dust and dirt, and more.
High-performance sensing While all these sensors detect targets such as ferromagnetic and non- ferromagnetic metals in harsh, non- contact environments, the eddy-current sensor’s architecture, as well as its advanced electronics, manufacturing and calibration techniques, make it a much higher performing device. Its performance characteristics can
be split into two categories: inherent and resulting from product design, manufacturing and calibration. Of the three inherent features, the most beneficial are: • Very high measurement frequency, to 5kHz;
• High resolution, to 0.5µm; and • High linearity and temperature stability. Since it’s an air-core coil rather than
ferrite, an alternating current to 1MHz can be used, although the electronics in devices such as the Micro-Epsilon eddyNCDT3001 and eddyNCDT3005 provide measurement frequencies of 5kHz – ten times higher than inductive displacement counterparts that typically reach 50Hz (fifty measurements per second); higher-end sensors reach 100kHz. In terms of linearity and
temperature, the air-core coil does not have to deal with the flux losses or compensate for the thermal expansion of a ferrite core, hence the 10x
maximum accuracy, eddy-current sensors can be calibrated to the target material at the factory, which adds to the cost of these devices.
Removing housing limitations Both proximity and inductive displacement sensors can be housed in solid metal. Eddy- current sensors’ operating principle means they must use a non-metallic cap. Despite this, Micro-Epsilon eddy-current sensors are still certified to IP67. In addition, their packaging has been advancing rapidly, with electronics now being integrated into the device. Micro-Epsilon’s eddyNCDT3001
product line is a new class of eddy- current sensors that come in an M12 housing with both integrated controller and signal-conditioning unit. This makes them much more amenable to standard mechanical formats and requirements, and a more attractive replacement for inductive displacement sensors.
Figure 2: Eddy-current sensor
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