Eddy current sensors:

Eddy current methods, Figure 3.33, conventionally cover the frequency band up to approx.10 MHz.

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Sensors internal calibration is usually done in the factory. The calibration is material dependent. With regard to the recalibration of the sensor, it can be recalibrated in the field provided the type of the material is the same.

3.3.1.2.1 Sensor Construction

Figure 3.34 Eddy-Current Probe Construction

Sensing coil located near the end of the probe is the main functional piece of an eddy-current sensor. Alternating current is passed through the sensing coil that produces an alternating magnetic field, which sense the distance between the probe and the target. The coil resides within a plastic and epoxy capsule, which protrudes out of a housing made of stainless steel. This is because, unlike the capacitive sensors, magnetic field of an eddy-current sensor is not well focused and the protrusion of the epoxy covered coil allow the sensing field of the coil engage the target as shown in Figure 3.34.

3.3.1.2.2 Spot Size, Target Size, and Range

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The magnetic fields of an eddy current sensor produces a large spot size that is three times the diameter of the probe’s sensing coil as shown in Figure 3.35. On the other hand, the ratio of the sensing distance to the sensing coil diameter is 1:3; e.g. for 1mm sensing range, the diameter of the sensing coil should be 3mm.

Figure 3.36 Magnetic field induces eddy current in conductive target Magnetic field around the sensing coil of the eddy sensors is always maintained constant. As such, when the eddy currents produced in the target oppose the magnetic field of the sensing coil, the eddy sensor increases the current supply to its sensing coil for it to maintain its original magnetic field, Figure 3.36. Depending on the distance of the target from the probe, the required current in the sensing coil to maintain constant magnetic field will vary. The sensing coil current is converted to the output voltage, the value of which indicates the position of the target with respect to the probe.

3.3.1.2.3 Target Materials and Rotating Targets

Permeability and resistivity of the target material have strong influence on the strength of the eddy current. Different materials have different permeability and resistivity. Even for the same materials, these two properties can be different if they underwent different processing techniques; e.g. heat treatment, annealing.

Iron and steel are magnetic materials and they have high permeability, which can cause slight error for the eddy current sensor even within the same material. The

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material’s permeability changes somewhat around the microscopic cracks and material variations often present in the same material. High-resolution eddy-current sensors is able to detect these changes in the magnetic materials of very high permeability.

3.3.1.2.4 Environmental Parameters: Temperature

Eddy-current probes can operate in hostile environments within a temperature range of -25 to +125°C. If Teflon FEP cables are used instead of the standard polyurethane cables, eddy-current probes can measure the temperature up to +200°C. Capacitive probes, on the other hand, only have an operating range of +4 to +50 °C. Additionally, they are affected by condensation.

3.3.1.2.5 Probe Mounting

Figure 3.37 Interference of eddy-current probes mounted near each other As the diameter of the magnetic field in eddy current sensor is more than three times that of the probe coil diameter, the magnetic fields will interact when multiple probes are mounted close to each other, as shown in the Figure 3.37. As a result, the sensor outputs will experience errors.

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Figure 3.38 Mounting hardware can interfere with eddy probe magnetic field There is also magnetic field behind the eddy-current probe. This magnetic field is present up to one and a half times the diameter of the coil. Metallic mounting hardware in this area will interact with the field and influence the sensor output as shown on the Figure 3.38. In such cases, sensors should be calibrated along with the mounting hardware.

3.3.1.2.6 Error Sources

Magnetic field in eddy current sensor can be changed by various factors other than the distance between the target and the probe. These factors produce error in the output signal.

Magnetic fields and sensor’s output of eddy current are not influenced by

nonmagnetic and nonconductive contaminants such as dust, sludge, water, and oil trapped between the eddy-current sensor and the target. Because of this, an eddy- current sensor is a suitable choice for a dirty or hostile environment, e.g. cylinder liner and piston ring condition monitoring.

3.3.1.2.7 Linearity:

The deviation of measured output from that of a straight line characteristics is termed as the linearity specification.

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Figure 3.39 Linearity error band - Measured data deviates from the straight line (Data from reference)

Linearity error is calculated against the best fitting straight reference line of calibrated data using least squares fitting method as shown in Figure 3.39. The maximum deviation of the actual and the ideal line of a point is the linearity error. It is expressed in terms of percent of full scale; e,g, highest error of 0.003mm in the full scale calibration of 1mm, the linearity error is 0.3%.

3.3.1.2.8 Effective Sensor Range:

Effective range of an eddy current transducer, in practice, is the stated range offset from the target surface by 20%; e.g. an eddy current transducer of 2.0 mm range will be effective from 0.4 mm to 2.4 mm from the target surface.

The flat surface area of the target must be bigger than the diameter of the probe tip, otherwise the output signal will decrease.

3.3.1.2.9 Cable length:

Decision on cable length for eddy current sensors is very crucial, as cable length affects calibration. Hence, cable length alteration is not allowed once the sensors are ready to be installed for the specified application. Sensors are usually ordered with tailor made cable lengths to suit the needs of the installation.

-10 -8 -6 -4 -2 0 2 4 6 8 10 0 0.5 1 1.5

ACTUAL/IDEAL VOLTS-GAP

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3.3.1.2.10 Frequency range and power consumption:

The bandwidths of eddy-current sensors lie between 10-15kHz. Nevertheless, bandwidths of as high as 80 kHz can be found in some eddy sensors.

An eddy-current probe consumes power between 40µW to 1mW.

Eddy-current sensors have many advantages compared to other non-contact sensors like optical, capacitive and laser.

 Tolerance of contaminants and dirty environments

 Not sensitive to material in the gap between the probe and target  Much smaller than laser interferometers

 Less expensive than laser and capacitive sensors  High frequency response

The disadvantages of Eddy-Current sensors are as follows:  Extremely high resolution is required

 Large gap between sensor and target is needed

 Non linear relationship between distance and impedance of coil  Temperature dependent

 Effective only on conductive material with adequate thickness 3.3.2 Mathematical Model of eddy current scuffing detection:

Eddy sensors measurement of the roughness of the piston ring surface is used for the model. The monitoring of the roughness is then coordinated with the crank angle to determine the position of damage.

∫

where,

Rs = average roughness integrated over a stripe and the piston stroke

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Ls = Piston stroke as a function of crank angle

The roughness is then compared with the reference roughness measured and stored when the piston and liner were in good condition or in any other reference condition.

The software compares the longitudinal average roughness which is the integral of the roughness profile per stroke over the engine stroke. Greater the number of radially placed eddy sensors, greater is the accuracy of the roughness

measurement.

The system allows the determination of deviations of average roughness in relation to the average reference roughness, and hence, allows the definition of alarm levels.

The sensors can give complimentary information such as oil film thickness, blocked piston rings and piston rings surface condition or profiles.

3.4 Theory and modeling of acoustic emission method: