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Reducing Error When Using Eddy Current Measurement Techniques


By Kevin Conlin, Business Development Manager, Kaman Precision Products


position, distance or vibration. Unaffec - ted by environmental contaminants or target finish characteristics, these sen- sors can operate in a vacuum or in flu- ids, so they work well for dirty applica- tions, like those with oil or dust present. To get the most out of eddy current sen- sors, these tips can reduce errors that can affect a measurement’s accuracy.


I


nductive eddy current technology is an extremely versatile non-contact method for measuring an object’s


Eddy Current Measurement Basics


Inductive eddy current sensors Eddy current interacting with sensor and target.


operate by generating a high-frequen- cy electromagnetic field around a sen- sor coil, which induces eddy currents in a target material. Eddy current sen- sors require a conductive target (usu- ally some sort of metal), and sensor performance is affected by target material conductivity. Nonconductive material be tween


the sensor and the target is not detect- ed. The sensors do not require a ground connection to the measuring system. Measuring distance is typically 30 to 50 percent of sensor diameter. Inductive eddy current sensors


have a large spot size compared with other technologies. They also have a higher frequency response, an advan- tage when measuring something moving very fast. This can make them a better choice than contact technologies like linear variable dis- placement transducers (LVDTs), which can interfere with the dynam- ics of the object being measured. Touching something that is mov-


ing to make a measurement adds mass, slowing down the system so it is not being measured at the actual speed.


Error in Eddy Current Measurements


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 toler- ated 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 type, presence of another metallic object near the target, temperature varia- tions or environmental conditions that affect measurement accuracy, multiple sensors mounted in too close a proximity, and incorrect mounting.


Selecting the Right Circuit Eddy currents can be interpret-


ed and processed into useful informa- tion in signal conditioning electronic circuits. Kaman uses three popular types of these circuits to process the signal: Colpitts circuits, single-chan- nel analog position measuring sys- tems; balanced bridge circuits, single- ended and differential analog linear position measuring systems; and phase circuits, single/multiple chan- nel analog high-precision position systems. Each signal conditioning circuit


type has distinct characteristics, so users should look for the one that per- forms best in each particular applica- tion. Begin by looking at the measure- ment. What kind is it? Is it single or differential? Is the target magnetic or not? Knowing this information will go along way to setting users on the path to reducing error. For example, when the Colpitts


circuit is used as a position measur- ing device, the sensor coil becomes


Continued on page 76


July, 2019


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