Examining with Eddy Current

October 1, 2006
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Eddy current-based sensors provide precision displacement measurement.

An eddy current sensor consists of a precision wire coil around a ferrite core. Source: MTI Instruments Inc.

Precision measurement has long been a part of manufacturing and process control. Dimensional measurement of parts, monitoring of production equipment and final inspection of assembled systems require process engineers and plant managers to collect and analyze extensive amounts of data. This ensures production lines operate at peak efficiency and minimal disruptions in the process flow. Measurement devices take many forms, from simple gage blocks and go/no-go fixtures to high-resolution, nanometer-accurate measurement systems. For applications requiring high-accuracy resolution and frequency response, noncontact measurement technologies are becoming an obvious choice.

Noncontact measurement systems offer several advantages over contact methods. They can be used on small or fragile parts where contacting the target can cause damage. They have higher accuracy and resolution than traditional contact devices and can be used in closed-loop, highly dynamic environments to provide real-time process information and control without operator intervention. Choosing and deploying the correct measurement technology for a control loop is as critical as the measurement itself.

This is the typical output of a 14-millimeter ranger eddy current probe. Source: MTI Instruments Inc.

Measurement Technology

Selecting the correct measurement technology requires defining the parameters under which the sensor must operate. Each sensor technology's ability to satisfy the measurement parameters is then analyzed to determine which is most appropriate. Noncontact, displacement measurement sensors measure the gap between the probe face and a target. By monitoring changes in this gap, the absolute target location or target motion through a range can be determined. Advanced calculations can then be made to determine vibration amplitude and phase, the thickness and shape of a piece, or the run-out of a rotating shaft.

Each noncontact measurement technology has its own set of advantages and disadvantages. Selecting the correct technology often involves a compromise between sensor performance and the environment in which it must operate.

When selecting a measurement technology, an engineer must define the key parameters of the application. The most important parameter to determine is the target material. If the material is not partially electrically conductive, a capacitance or eddy current probe will not work. This can be overcome by adding a conductive material at the measurement site, but this may not be feasible. A fiber optic or laser-based system is then considered.

Take special care when probe-mounting fixtures to limit the thermal expansion and contraction of measurement system components. Source: MTI Instruments Inc.
The next important parameter to consider is measurement range. What is the maximum distance the target will travel and how far from the target must the sensor be? Often, the larger the measurement range, the less accurate and precise a measurement will be. Laser- and eddy current-based systems offer large measurement ranges and stand off from the target, but at reduced measurement accuracy and resolution. Therefore, the minimum accuracy and resolution of measurement also must be determined when specifying the probe range. Data acquisition systems that correct for nonlinearity of a measurement probe at extended ranges may be considered to help overcome limitations of a given measurement technology.

After the target material, measurement range, accuracy and resolution are specified, the appropriate measurement technology is chosen and the remaining system features determined. Probe geometry is determined and a probe amplifier is selected that suits the required frequency response of the application. Again, this is often a trade-off between system performance and probe size.

In general, the larger the measurement range, the larger the probe size and the lower the absolute system accuracy. It is important not to overspecify the accuracy and resolution required. It may lead to selecting an incorrect system.

While the cost of deploying measurement systems is a concern, it should not carry undue influence when evaluating each measurement technology. Consider the impact of deploying the incorrect solution. The money saved by choosing a given technology can be quickly overshadowed by the cost of incorrect or insufficient measurement.

Probes may be used with other conductive materials, however, the absolute range and linearity may be adversely affected unless the amplifier electronics are modified. Source: MTI Instruments. Inc.

Advantages and Disadvantages

Eddy current-based sensors are a common choice for precision displacement measurement. With a frequency response of 10 kHz or higher, they often are used in rotating shaft applications to measure axial and radial runout, shaft position and vibration amplitude. When used as part of a plantwide data acquisition system, they can monitor multiple pumps, compressors or turbines throughout the plant from a centralized location. Other applications include thickness measurement, presence or proximity detection, and piece alignment.

The coil emits a toroidal magnetic field approximately three times the diameter of the coil, therefore, the target size should be at least three times the probe-tip diameter. Source: MTI Instruments Inc.
Unlike other noncontact sensors, eddy current probes can be used in dirty environments where water, dirt or machining oils are present in the measurement area. Probe construction is robust and can be integrated into system fixtures for measurements over many years without degradation in performance. However, eddy current probes are limited to applications in which the target surface is electrically conductive.

In its simplest form, an eddy current sensor consists of a precision wire coil around a ferrite core. When an AC current flows through the coil, a magnetic field is formed about the coil. When the coil and its magnetic field are placed in proximity to a conductive target, electric currents are established in the target. These currents, traveling in closed loops opposite the direction of current in the coil, are called eddy currents. The eddy currents, in return, generate their own magnetic field. As the distance between the coil and the target-air gap-is changed, the eddy current magnetic field changes the overall electrical impedance of the sensor coil. The change in coil impedance causes a change in the voltage across the coil, which can be converted to a change in output by the probe amplifier.

When measuring shaft radial run-out, the curved surface of the target will distort the magnetic field and limit the full-scale range of the system, For this measurement, shaft diameter must be at least five times the probe-tip diameter. Source: MTI Instruments Inc.
Starting with the probe face offset from the target approximately 10% of total range, the absolute value of the analog voltage output of the drive electronics increases as the air gap increases. Typically, probe amplifiers have an output of -2 to -14 VDC, which can be offset to positive values using signal processing. By using the linear portion of the output, changes in air gap can be determined.

Nonlinearity is typically specified as a percentage of full range. For example, a 14-millimeter-range probe with a specified nonlinearity of ±1% field strength will have a maximum displacement error of ±0.14 millimeter. Higher linearity can be achieved using signal processing techniques such as look-up tables.

Noncontact Technologies (Source: MTI Instruments Inc.)

Sensor Parameters

There are three parameters that affect eddy current sensor readings: environment, target material and target size.

Environment. Nonconductive mat-erials in the sensor-to-target gap do not affect eddy current sensors. This allows their use in dirty environments-dirt, water, oil and machine fluids-where other displacement sensor technologies fail. However, the presence of conductive materials near the sensor face may distort the magnet field generated by the coil and eddy currents. To minimize the effect of nearby conductive materials, a shielded sensor tip should be used.

Large temperature changes in the measurement area will induce measurement errors. The impedance of the coil and target changes as temperature changes.

Target material. Eddy current displacement sensors require that the target be electrically conductive. The magnitude of the coil impedance change is directly influenced by the resistivity and magnetic permeability of the target material. Ideally, the target is a nonferrous, low-resistance material such as copper, aluminum or brass; however, ferrous material targets can be used.

Systems are frequently calibrated at the manufacturer in accordance to API 670, which specifies the target material to be 4140 stainless steel. For maximum range and accuracy, use target materials with similar electrical resistivity and magnetic permeability to the material for calibration.

Target size. Target size and shape must be considered when selecting an eddy current probe. Target thickness must be at least 1 millimeter. The coil emits a toroidal magnetic field that is approximately three times the diameter of the coil, therefore, the target size should be at least three times the probe tip diameter. When measuring shaft radial runout, the curved surface of the target will distort the magnetic field and limit the full-scale range of the system.

Noncontact displacement measurement is an integral part of process control. Selecting and deploying the correct measurement technology is critical to successfully gathering accurate and reliable data. Eddy current sensors offer a low-cost, high-accuracy solution for many common displacement measurements. Understanding the principles of eddy current probe operation and the parameters that affect their performance is the key to successful deployment. NDT

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Charles J. Hellier has been active in the technology of nondestructive testing and related quality and inspection fields since 1957. Here he talks with Quality's managing editor, Michelle Bangert, about the importance of training.
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