This micro-displacement sensor (micro position sensor) web page discusses the use of capacitive and eddy-current sensors for measuring micro displacement. Micro displacements are position changes of an object in the micron (micrometer) or nanometer range. Other noncontact techniques are available for this measurement such as laser interferometers, but a capacitive or eddy-current micro displacement sensor offers a small installed footprint and resolution at the nanometer level.
Micro displacement measurement is a measurement of position change. Linear high-resolution micro displacement measurement of conductive objects with capacitive and eddy-current non-contact sensors is the topic of this Application Note. Capacitive sensors can also measure non-conductive objects. A discussion of measuring non-conductive objects with capacitive displacement sensors can be found in the Capacitive Sensor Theory of Operation TechNote (LT03-0020).
At the micro and nano level, noncontact micro-displacement sensors are best suited to displacement (change in position) measurements, rather than absolute measurements.
With time, non-contact micro displacement sensor calibration shifts. It is primarily a DC offset in the output of the sensor. Changes in Sensitivity (gain) of the sensor are much smaller. Measuring changes in position only requires a consistent Sensitivity and is not affected by long-term shifts in DC offset of the output. For this reason, non-contact displacement sensors are usually used to measure relative position, not absolute position, especially for micro-displacements in which there is a need for resolution at the submicron or nanometer level.
The typical reason for measuring micro displacement is to determine how an object responds to some changing condition. Micro-Displacement measurement is usually answering the question: How far does this move when something else changes?
Intentional Displacement: An object is intentionally moved by a motion control system to put it in a particular position. The non-contact displacement measurement indicates the accuracy of the intended displacement of the object.
Part Dimension: Measuring a known good “master” part allows for comparative dimensional measurements of test parts. Differences in the dimensions of the test part relative to the master part are indicated as a displacement measurement by the sensors.
Temperature Variation: Measure an object at initial temperature. Monitor changes in the temperature of interest (often occurring naturally as a machine operates) and a micro-displacement measurement indicates the magnitude of the temperature-related position change.
Vibration: A complex measurement detailed in our Vibration Application Note, linear micro-displacement measurements are made in real time using noncontact sensors with an oscilloscope or data acquisition system to indicate the displacements of the object and their related frequencies.
Pressure: Changing pressure affects the operation of air bearings and other fluid bearings. Micro-Displacement measurements of the object at different pressures indicate the actual behavior of the machine compared to its intended operation as the pressure changes.
Wear: As bearings and slides wear, non-contact displacement measurements of the moving parts will indicate increased movement in axes intended to be stationary. Rotary motions will show increasing displacements in the X, Y, and Z axes as the object turns. Linear slides will show increasing displacements in the two axes perpendicular to the direction of travel.
A Non-contact micro displacement sensor is designed for relative measurements and indicates the change of an object’s position from an initial location in one or more linear axes. A separate noncontact micro-displacement sensor channel is required for each axis of linear displacement measurement.
A non-contact displacement sensor is mounted in a fixture so that the object to be measured is within the sensor's measurement range. Sensors that include a zero (offset) adjustment are often adjusted to zero at this location; this makes it easier to interpret linear displacement measurements when the object moves. If zero adjustment if not possible, the initial output of the displacement sensor is recorded to be subtracted from future measurements to indicate the change in position from the initial position.
Non-contcat sensors for measuring displacement have a “sensitivity” specification which specifies the amount of output change relative to a given change in the target position. This value is given in Volts per unit-of-distance or length (e.g. mm, inch etc.) for analog voltage output sensors. For digital output sensors, this value is given in Counts per unit-of-distance. This sensitivity is used to calculate the physical displacement indicated by output change.
Displacement = Output Change / Sensitivity
Output Change = Volts; Sensitivity = Volts/Unit of distance
Output Change = Counts; Sensitivity = Counts/Unit of distance
High-Performance displacement sensors are usually used to measure micro-displacements. When measuring at the micro-displacement level, error sources that are normally inconsequential become a more significant factor.
Set-screw mounting locks the probe along the probe’s axis, but there may still be movement in the other two axes, especially at the micro and nano levels.
A clamp mount is a more stable mount than a set-screw mount. But at the micro and nano levels, form errors can result in only a two-point clamp much like a set-screw mount.
A three-point clamp mount is inherently stable and not effected by small form errors in roundness.
Thermal expansion and contraction of the sensor mounting system will introduce errors into the measurement. Expansion and contraction of the fixture will likely move the sensor toward or away from the target object. While this affects the measurement, the displacement is real and is not a sensor error; it's a mechanical design problem. Micro displacement sensor mounting systems must be robust, stiff, and as thermally stable as possible.
Mechanical stability is more complicated at the micro level. The micro displacement measurement sensors must be held firmly in place by the mounting system. When measuring displacement at the micro level, a simple set-screw type mount may not be sufficiently stable.
Different methods exist for mounting a cylindrical linear displacement sensor. Using a set-screw in a thru-hole mount holds the probe at only two points – the set-screw and the point opposite the set-screw. Probe rotation is possible in the axis 90° from the set-screw axis. Depending on the width of the surface against which the set-screw pushes the probe, the probe may also be able to rotate along its axis as well. Increasing the force on the set-screw will not increase the probe’s stability in these other two axes.
A still imperfect but better micro displacement sensor mounting scheme is a clamp type mount. This mounting system can stabilize the probe in three axes if the mounting hole and probe are perfectly round. However, any eccentricity of either part results in a two-point mounting system similar to the set-screw system.
An optimal mounting system uses a three- or four-point clamp with each point covering some significant length along the axis of the probe. This clamp system begins with a typical clamp mounting configuration but also removes material from the clamping hole at a few points. This arrangement is not affected by eccentricity of the mounting hole or the eccentricity of the non-contact linear micro displacement measurement sensor – it is stable in all three axes.
The electric field “spot-size” of a capacitive micro displacement sensor is about 130% of the sensing area diameter, the field does not extend to the sides. This is one factor that helps make them immune to any surrounding objects. It also allows them to be mounted flush with the mounting bracket surface in virtually all cases; the only exception is calibrations that use an extremely long measurement range relative to the size of the sensing area; this does not apply to any off-the-shelf calibrations available for Lion Precision probes.
Capacitive micro displacement sensors are unaffected by the material type as long as it is conductive. At extremely high resolutions, they can be affected by target-probe angle.
Multiple capacitive probes can be used together provided the excitation signals are synchronized which is true of all Lion Precision multi-channel capacitive sensor systems.
Micro displacement measurements with capacitive sensors must be made in a clean environment. The displacement measurement will be affected by anything (other than air or vacuum) in the measurement space.
Capacitive sensors have some sensitivity to temperature, but the systems are compensated for temperature changes between 20°C and 35°C with a drift of less than 0.04%F.S./°C.
Typical changes in humidity do not have a significant effect on capacitive sensors. Extremes of humidity will affect the output with a worst case being condensation on the probe or target.
An Eddy-Current micro displacement sensor uses a magnetic field that surrounds the probe tip. This means that, the “spot-size” of eddy-current sensors is about 300% of the probe diameter. This means any metallic objects within three probe diameters from the probe will affect the sensor output.
This magnetic field also extends along the probe’s axis. The distance between the sensing face of the probe and the mounting system must be at least 1.5 times the probe diameter. Eddy-Current sensors cannot be mounted flush with the mounting surface unless the mounting area is counterbored.
If interfering objects near the probe is unavoidable, a special calibration to include or compensate the objects will be necessary.
Eddy-current sensors are calibrated to one specific conductive material. The precision of measurements at the micro-diplacement level requires that they only be used with that specified material.
Eddy-Current probes require minimum clearances between the face of the probe and mounting surfaces. Clearnaces are based on probe diameter.
When multiple probes are used with the same target, they must be separated by at least three probe diameters to prevent interference between channels. If this is unavoidable, special factory calibrations are possible to minimize interference.
Micro displacement measurements with eddy-current sensors are unaffected by foreign material in the measurement area. This is the great advantage of eddy-current sensors - they can be used in rather hostile environments. All non-conductive materials are invisible to eddy-current sensors. Even metallic materials like chips from a machining process are too small to interact significantly with the sensors.
Eddy-Current sensors have some sensitivity to temperature, but the systems are compensated for temperature changes between 15°C and 65°C with a drift of less than 0.01%F.S./°C.
Changes in humidity have no effect on eddy-current displacement measurements.
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