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Reducing Error When Using Eddy Current Measurement Techniques
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the inductor in the oscillator circuit. When the sensor coil interacts with a conductive target, the oscillator fre- quency and amplitude vary in pro- portion 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 linear- ity 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 cir- cuit, linearized by a logarithmic amplifier, and then amplified in the final amplifier stage. In the single-ended configura-
tion, the systems are a good choice for both ferrous and non-ferrous targets, including general-purpose linear posi- tion measurement,
laboratory,
research, development, and testing, metrology, and factory process 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 posi- tioning in photolithography equip-
ment, laser optics positioning, preci- sion grinding, and semiconductor wafer transport mechanisms.
Adjust for the presence of anoth- er metallic object near the tar- get. 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 espe- cially difficult to measure if the tar- get is conductive but very small and thin and difficult to get close to.
Adjust and calibrate for the tem- perature and conditions. Be sure to calibrate the system in the envi- ronment in which it will operate. Calibrating in one environment and then placing it in another may induce errors. High and low temperatures
have the potential to change the measurement. A measurement that is excellent at room temperature may change when taken at 500 or at 1,000°F (260 or 538°C). To adjust for this, if measuring across a wide tem- perature range, users should perform calibrations to show what error may exist at different temperatures. For example, one recent cus-
tomer was taking several measure- ments of blades inside a large gas turbine. The system is optimized for the key measurement, which is taken at 800 to 900°F (427 to 482°C), but also takes measurements at room temperature as the turbine is begin- ning operations. The measurement
could be accurate at one or the other of these points — but not both. For this system, Kaman developed a sen- sor that was accurate at the higher temperature, and then used mathe- matical calculations to develop curves providing information on the error to be expected at the room tem- perature measurement. Environmental conditions are
also an issue in cases where there are multiple moving pieces and the sys- tem is trying to read only one. If the sensors pick up another moving metal object, one may be able to place another piece of metal in between to shield it. The fixed object absorbs the signal but one can still see the mov- ing target. In short, the fixed piece simply becomes part of the environ- mental conditions and one can cali- brate around it. For applications that measure on
a rotating target like a shaft, the sur- face velocity of the shaft can also affect the measurement. As it spins faster, it may look like a smaller target. Again, mathematical calculations can be developed to account for the size of the target, the material it is made of and how fast it is moving to ensure that a proper measurement can be made using an eddy current sensor.
Multiple Sensors and Mounting Two or more sensors mounted
in close proximity to each other may result in their electromagnetic fields intermixing, which may cause inter- ference and reduce measurement accuracy. This interference is in the form of “cross talk,” resulting in beat notes whose frequency will be the dif- ference between the frequencies of the oscillator in each unit. The quality of any measure-
ment depends on the mounting fix- ture. The amount of conductive material in or near the mounting fix- ture will have an impact on system performance. Mounting the sensor in a material that absorbs too much of the field results in measurement errors. For example, mounting an eddy current sensor inside a large steel block and recessing the sensor would absorb the field so it would be hard to measure. A sensor is “side-loaded” when
its field interacts with conductive material other than the target. Shielded sensors reduce this effect. For optimum performance, keep con- ductive material out of this field if possible. Perform in-situ calibrations if conductive material other than the target will be in the sensor’s field. To eliminate the possibility of
error, make sure the sensor is mount- ed so it is aimed as squarely at the target as possible. It should not be recessed and there should be nothing in its way to absorb the signal. If pos- sible, mount the sensor inside a plas- tic block. In addition, the target and sensor should be parallel to each other. Some non-parallelism can exist without inducing significant error. A non-parallelism of up to 3° will increase nonlinearity less than 0.5 percent of full scale. Non-parallelism of 10° will increase nonlinearity approximately 4 percent of full scale. One final tip is to accommodate
room for error. If one is looking for a measurement at half-inch, try to ensure the measurement can be made at three quarters of an inch, so
there will be room to work within. Account for temperature errors by performing measurements with sys- tems ahead of time to understand the shifts that may take place at higher temperatures, and be able to adjust for errors that may occur at different temperatures. To ensure the best measurement
with eddy current technology, select the right sensor, calibrate it carefully for the temperature and environmen- tal conditions it will be operating in, and make sure to mount and position the sensors properly. Contact: Kaman Corp., 217
Smith Street, Middletown, CT 06457 % 860-632-4540 E-mail:
kevin.conlin@
kaman.com Web:
www.kaman.com r
ViscoTec Partners with DELO for Adhesive Testing
Kennesaw, GA — Microdispensing in the electronics industry can help to increase the service life of individual components. Glob top potting, for example, protects sensitive compo- nents, usually semiconductor chips, from mechanical stresses, such as vibrations or temperature fluctua- tions. It also protects them from external environmental influences, such as moisture or corrosion. A vis- cous resin matrix is applied, mostly an epoxy resin adhesive.
July, 2019
Glob top potting with DELO MONOPOX GE615 adhesive.
A recent study looked at the
spray application of a filled glob top material: a DELO MONOPOX GE - 6515 adhesive. This highly viscous adhesive is used, for example, in the automotive industry for potting elec- tronic components that are exposed to demanding environmental conditions. The preeflow eco-SPRAY is the
combination of a volumetric dispens- ing system — based on the eco-PEN dispenser series — and a specially developed “low-flow spray chamber.” This makes it possible to spray a pre- cisely defined quantity, with accura- cy to ±1 percent. With the eco-SPRAY spray dis-
penser, an efficient dispensing appli- cation was achieved in the dispens- ing tests, an alternative to needle dispensing. Contact: ViscoTec America, Inc.,
1955 Vaughn Road, Suite 209, Kennesaw, GA 30144 % 770-422-4281 E-mail:
sales@viscotec-america.com Web:
www.viscotec-america.com
See at SEMICON West, Booth 2065
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