Rather than electric fields, eddy-current sensors use magnetic fields to sense the distance to the target. Sensing begins by passing alternating current through the sensing coil. This creates an alternating magnetic field around the coil. When this alternating magnetic field interacts with the conductive target, it induces a current in the target material called an eddy. This current produces its own magnetic field which oppose the sensing coil’s field
The sensor is designed to create a constant magnetic field around the sensing coil. As the eddies in the target oppose the sensing field, the sensor will increase the current to the sensing coil to maintain the original magnetic field. As the target changes its distance from the probe, the amount of current required to maintain the magnetic field also changes. The sensing coil current is processed to create the output voltage which is then an indication of the position of the target relative to the probe.
Eddy-current sensors use changes in a magnetic field to determine the distance to the target; capacitive sensors use changes in capacitance. There are factors other than the distance to the target that can also change a magnetic field or capacitance. These factors represent potential error sources in your application. Fortunately, in most cases these error sources are different for the two technologies. Understanding the presence and magnitude of these error sources in your application will help you choose the best sensing technology.
The remainder of this article will explain these error sources so that you can make the best choice for your application and get the best parksensor possible results.
In some applications, the gap between the sensor and target can become contaminated by dust, liquids such as coolant, and other materials which are not part of the intended measurement. How the sensor reacts to the presence of these contaminants is a critical factor in choosing capacitive or eddy-current sensors.
Because of the sensitivity to the dielectric constant of the material between the sensor and the target, capacitive displacement sensors must be used in a clean environment when measuring target position.Capacitive sensors assume that changes in capacitance between the sensor and the target are a result of a change in distance between them. Another factor that affects capacitance is the dielectric constant (ε) of the material in the gap between the target and sensor. The dielectric constant of air is slightly greater than one; if another material, with a different dielectric constant, enters the sensor/target gap, the capacitance will increase, and the sensor will erroneously indicate that the target has moved closer to the sensor. The higher the dielectric constant of the contaminant, the greater the effect on the sensor. Oil has a dielectric constant between 8 and 12. Water has a very high dielectric constant of 80. The dielectric sensitivity of capacitive sensors can be exploited for use in sensing the thickness or density of nonconductive materials.
Unlike capacitive sensors, eddy-current sensors use magnetic fields for sensing. Magnetic fields are not affected by nonconductive contaminants such as dust, water, and oil. As these contaminants enter the sensing area between an eddy-current sensor and the target, the sensor’s output is not affected.For this reason, an eddy-current sensor is the best choice when the application involves a dirty or hostile environment.
The two technologies have different requirements for target thickness. The electric field of a capacitive sensor engages only the surface of the target with no significant penetration into the material. Because of this, capacitive sensors are not affected by material thickness.