1. Basic Theory of Eddy-Current Sensors
Eddy-Current sensors operate with magnetic fields. The driver creates an alternating current in the sensing coil in the end of the probe. This creates an alternating magnetic field with induces small currents in the target material; these currents are called eddy currents. The eddy currents create an opposing magnetic field which resists the field being generated by the probe coil. The interaction of the magnetic fields is dependent on the distance between the probe and the target. As the distance changes, the electronics sense the change in the field interaction and produce a voltage output which is proportional to the change in distance between the probe and target.
2. High-Performance Sensors
Simple inductive sensors, such as those used in inexpensive proximity switches, are simple devices and in their most basic form could be designed in a high school electronics class. Proximity type sensors are tremendously useful in automation applications and many commercially available models are well made, but they are not suited to precision metrology applications.
In contrast, high-performance eddy-current sensors such as those from Lion Precision for use in precision displacement measurement and metrology applications use complex electronic designs to execute complex mathematical algorithms. Unlike inexpensive sensors, these high-performance sensors have outputs which are very linear, stable with temperature, and able to resolve incredibly small changes in target position resulting in high resolution measurements.
3. Eddy-Current Sensor Advantages
Compared to other noncontact sensing technologies such as optical, laser, and capacitive, high-performance eddy-current sensors have some distinct advantages.
- Tolerance of dirty environments
- Not sensitive to material in the gap between the probe and target
- Less expensive and much smaller than laser interferometers
- Less expensive than capacitive sensors
Eddy-Current sensors are not a good choice in these conditions:
- Extremely high resolution
- Large gap between sensor and target is required (optical and laser are better)
4. Applications For Eddy-Current Sensors
Eddy-Current sensors are useful in any application requiring the measurement or monitoring of the position of a conductive target, especially in a dirty environment.
Eddy-Current sensors are basically position measuring devices. Their outputs always indicate the size of the gap between the sensor's probe and the target. When the probe is stationary, any changes in the output are directly interpreted as changes in position of the target. This is useful in:
- Automation requiring precise location
- Machine tool monitoring
- Final assembly of precision equipment such as disk drives
- Precision stage positioning
Measuring the dynamics of a continuously moving target, such as a vibrating element, requires some form of noncontact measurement. Eddy-Current sensors are useful whether the environment is clean or dirty and the motions are relatively small. Lion Precision eddy-current sensors also have high frequency response (up to 80kHz) to accommodate high-speed motion.
- Drive shaft monitoring
- Vibration measurements
The presence or absence of threads in a tapped hole is a major concern in the automotive and other industries. A simple untapped hole stops production lines and costs thousands of dollars an hour. Eddy-Current sensors, when inserted into a metal hole will provide different outputs depending on the presence or absence of threads.
5. Lion Precision cRIO Eddy-Current Module
Lion Precision has created a custom C Series Module using the NI cRIO-9951 CompactRIO Module Development Kit. The ECD140 Eddy-Current Displacement Sensor for CompactRIO simplifies system design for high performance measurements.
Originally Authored By: Greg Crouch, National Instruments