Inductive eddy current technology is an extremely versatile non-contact method for measuring an object's position, distance, or vibration. Unaffected by environmental contaminants or target finish characteristics, these sensors can operate in a vacuum or in fluids, so they work well for dirty applications, like those with oil or dust present. To get the most out of eddy current sensors, follow these tips for reducing errors that can affect a measurement's accuracy.

Eddy Current Measurement Basics

Inductive eddy current sensors operate by generating a high frequency electromagnetic field about a sensor coil, which induces eddy currents in a target material. Eddy current sensors require a conductive target (usually some sort of metal), and sensor performance is affected by target material conductivity. Nonconductive material between the sensor and the target is not detected. The sensors do not require a ground connection to the measuring system. Measuring distance is typically 30-50 percent of sensor diameter.

Inductive eddy current sensors have a large spot size compared to other technologies. They also have a higher frequency response, an advantage when measuring something moving very fast. This can make them a better choice than contact technologies like linear variable displacement transducers (LVDTs), which can interfere with the dynamics of the object being measured. Touching something that is moving to make a measurement adds mass, slowing down the system so it is not being measured at the actual speed.

Eddy current performance is affected by temperature changes, but can ignore contamination that would foul up laser triangulation/LED, ultrasonic, or capacitive measurement technologies.

Sources of Error

Care must be taken to avoid common error sources associated with eddy current sensors. If not, users may not get a good measurement, may get more error than can be tolerated for the application – or they may not be able to get any measurement at all.

The main sources of error in eddy current measurement sensors include:

  • Selecting the wrong circuit type

  • Presence of another metallic object near the target

  • Temperature variations or environmental conditions that affect measurement accuracy

  • Multiple sensors mounted in too close a proximity

  • Incorrect mounting

Tips for Reducing Errors:

1. Select the Right Circuit

Eddy currents can be interpreted and processed into useful information in signal conditioning electronic circuits. Kaman uses three popular types of these circuits to process the signal:

  • Colpitts circuit – single channel analog position measuring systems

  • Balanced bridge circuit – single ended and differential analog linear position measuring systems

  • Phase circuit – single/multiple channel analog high precision position systems

Each signal conditioning circuit type has distinct characteristics, so users should look for the one that performs best in a given application. To select the right circuit, begin by looking at the measurement – what kind are you taking? Is it single or differential? Look at the target – is it magnetic or non-magnetic? Knowing this information will go a long way to setting users on the path to reducing error.

For example, when the Colpitts circuit is used as a position measuring device, the sensor coil becomes the inductor in the oscillator circuit. When the sensor coil interacts with a conductive target, the oscillator frequency and amplitude vary in proportion to the target position. This variation is processed into an analog signal proportional to displacement.

Kaman typically recommends Colpitts circuits for low-cost, general purpose measurements where linearity is not required. They can be a good choice for fuel injector testing, valve lift measurements, shaft or cylinder run out and vibration, and machining and grinding.

With a balanced bridge circuit, the target movement causes an impedance change in the sensor coil. This change of impedance in the coil is measured by the demodulator circuit, linearized by a logarithmic amplifier, and then amplified in the final amplifier stage.

In the single ended configuration, the systems are a good choice for both ferrous and non-ferrous targets, including general purpose linear position measurement, laboratory, research and development, testing, metrology, and factory process control.

Differential bridge systems are frequently the best choice for fast steering mirror (FSM) position applications, pointing and tracking in night vision and laser systems, control systems for active vibration monitoring and control systems, and photolithography stage positioning and control.

If a phase circuit is used, the effects of eddy currents are not only amplitude related, but also phase related. This circuit is based on phase detection using pulse width modulation (PWM) techniques. Typical recommended applications include stage positioning in atomic force microscopy (AFM), Z-axis positioning in photolithography equipment, laser optics positioning, precision grinding, and semiconductor wafer transport mechanisms.

2. Adjust for the Presence of Another Metallic Object Near the Target

Measuring one metallic object when another metallic object is too close is a major source of error. This may depend upon the material, the size of target, and the measurement range. For example, it may be especially difficult to measure if the target is conductive but very small and thin and difficult to get close to. The potential for error will also depend upon how much material is required to interact with the eddy current field.

For example, in a racing engine research and development application, a Kaman customer wanted to ensure they were getting the correct engine stroke and were developing a system to test how various conditions would affect the stroke. They were trying to measure a piston moving, but there was another piston in close proximity and the measurement was “seeing” both pistons.

In this case, Kaman recommended mounting one sensor behind the piston and one in front of it. Each sensor was used to measure half the stroke. The end user was able to get closer to the piston with a smaller sensor that did not “see” the other piston.