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Sensor Components: A Comparison of Non-Contact Displacement Sensors

May 9, 2012
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The technology is typically used in applications where fast displacement changes have to be acquired or where a longer service life of the sensor is required.

Eddy current sensors can measure through plastic. Source: Micro-Epsilon


The measurement module that defines accuracy, displacement and distance measurement sensors is often found at the center of complex measurement systems for geometric dimensions. In this respect, very high requirements are placed on the displacement sensors in terms of performance and reliability. Generally, displacement sensors can be divided into contact and non-contact sensors.

Non-contact sensors are typically used in applications where fast displacement changes have to be acquired, where no forces can be exerted on the measurement object, where highly sensitive surfaces do not allow any contact or where a longer service life of the sensor is required.

Mechanical engineering without displacement sensors is difficult to imagine. These sensors are used to control various movements, to monitor fill levels, check product quality and many other applications. Yet, it is exactly this sector where sensors meet many different and harsh conditions, which must be fulfilled. Sensors often have to operate reliably in the harshest conditions. They are used in oil, hot vapors or changing temperatures. The sensors also are used on strong vibrating parts, in strong electromagnetic fields or have to be located a certain distance from the target. Important application criteria are accuracy and temperature stability, resolution and cut-off frequency. For these reasons different advantages of the various measuring principles must be considered. This means that no general statement about the optimum measuring principle for a given application can be made.

As well as capacitive and confocal sensors, eddy current technology and laser triangulation sensors have also proved themselves in numerous applications. Non-contact sensors are available in many different versions. Therefore, it is often difficult for the user to make the correct choice of sensor. Every measurement principle has characteristic features that can be construed as advantages or disadvantages depending on the application.



For the capacitive measuring principle, the sensor and target function as electrodes of a capacitor. Source: Micro-Epsilon

The Eddy Current Principle

Strictly speaking, the eddy current principle should be classified as the inductive measuring principle. The effect for measuring via eddy current is based on the extraction of energy from an oscillating circuit. This energy is needed for the induction of eddy currents in electrically-conductive materials. Here, a coil is supplied with alternating current whereby a magnetic field forms around the coil. According to Faraday’s Law of induction, if an electrically-conducting object is present in this magnetic field, eddy currents are produced in it. According to the Lenz Rule, the field of these eddy currents is opposed to the field of the coil, which causes a change in the coil impedance. This distance-dependent impedance change can be captured at the controller as a measurable factor by using the change in the amplitude of the sensor coil.

This principle can be used for electrically-conductive materials. As eddy current penetrates insulations, even metal behind an insulating layer can be used as a measuring object. A special coil winding means that very compact sensor designs are possible, which can still be used across high temperature ranges. All eddy current sensors are insensitive to dirt, dust, moisture, oil and pressure.

Nevertheless, eddy current sensors are subject to some limitations in their use. For example, individual linearization and calibration are necessary for each application. Also, the output signal is dependent on the electrical and mechanical characteristics of the measuring object. However, these restrictions help to give the eddyNCDT measuring principle from Micro-Epsilon extremely high resolutions of a few tenths of a nanometer.

A typical application for eddy current sensors is in a fully automatic welding test rig. This test rig measures the quality of weld seams, which have been produced for moving seam flanks. These results are relevant for repairs to, for example, bridges, which are subject to constant movement due to wind and water. In this case, an eddy current sensor has been selected as the sensor of choice because this measurement method and results are not influenced by the strong electromagnetic fields of the welding robot. The sensor measures the seam flank movement here with micrometer precision with a measuring range of 4mm.



The Capacitive Measuring Principle

The sensor and measuring object function as an ideal plate capacitor for the capacitive measuring principle. If an alternating current of constant frequency flows through the sensor capacitor, the amplitude of the alternating voltage on the sensor is proportional to the distance to the target (ground electrode).

In practice, due to the design of the sensors as guard ring capacitors, almost ideal linear characteristics are achieved. However, a continuous dielectric constant between sensor and target is required for consistent measurement results; the system reacts extremely sensitively to dielectric changes in the measuring gap. Capacitive sensors also measure insulated materials, as these are acquired as a changed dielectric. A linear output signal for insulators is also possible by using an electronic circuit.

As thermal conductivity changes have no influence on the measurement, the principle is also reliable for strong temperature fluctuations. According to the manufacturer, the capaNCDT capacitive sensors are amongst the most precise in the world. Resolutions of less than one nanometer are possible.

A suitable example of this measuring principle in mechanical engineering is measuring the deformation of a brake disc under load. In order to obtain accurate data about the deformation during the braking process, these must be tested under extreme conditions. The brake disc moves at a speed of 2,000 rpm at a temperature of 600°C in a test rig. A measuring system for this test is required which delivers a high measuring rate or cut-off frequency and which is not influenced by changes of the magnetic and conductive characteristics of the object (often caused by temperature). The sensor must also offer extremely high resolution, since the deformation takes place for less than 100µm. The most suitable is the capacitive measuring principle, which meets all required conditions. The eddy current principle would not be considered here because the high conductivity and permeability fluctuations caused by gradients cannot be fully compensated for.



Laser triangulation sensors are used most often. Source: Micro-Epsilon

The Laser Triangulation Principle

A laser diode emits a laser beam, which is aimed at the target. The reflected ray of light is imaged via a lens, either on a CCD/CMOS array or on a PSD element. The intensity of the reflected beam depends on the surface of the measuring object. Therefore, the sensitivity is regulated for analogue PSD sensors. For digital CCD sensors, RTSC (Real Time Surface Compensation)1 regulates intensity changes instantaneously.

The distance from the object to the sensor is calculated from the position of the light spot on the receiver element. Depending on the version, the data is evaluated using the external or internal controller and is output via various types of interface.

The base measuring distance is the greatest advantage of the sensor here but is also the greatest drawback of this measuring principle. For hot or fast moving targets, it is invaluable to be able to measure from a large distance. However, in doing so, it must be noted that the reflected laser beam is not shadowed and therefore cannot strike the receiving array. Therefore, this method is only of limited use for bore holes and cavities. The very small spot size of the optoNCDT series that can be realized with laser sensors is often important. This is in the range of a few micrometers and can therefore also be used for targets in this order of magnitude.

In saw mills, for example, the so-called wane is still present on the flanks of the board after slabbing the tree trunk. In order to remove this wane in an automated way with as little loss of wood as possible, it is necessary to measure the profile of the board at several places. Laser triangulation sensors are used for this because of their large base distance. Eddy current sensors are not suitable due to the non-conductive wood target. Capacitive and confocal sensors can indeed measure wood, however, they have too small a base distance whereby the risk of damaging a sensor would be too great. The laser sensors are directly integrated in the trimming system of saw mills and in this way enable automatic adjustment of the cutting width.



The Confocal Chromatic Measuring Principle

Polychromatic light (white light), starting from the light source in the evaluation unit, is transmitted to the sensor via an optical fiber. The lenses here are arranged so that the light in the longitudinal direction of the optical axis is broken down by controlled chromatic aberration into monochromatic wavelengths. This lens focuses the light beam onto the target surface. Depending on the distance, there are different spectral colors in the focus. In the sensor system, this wavelength of light is used for the measurement, which is exactly focused on the target. The light reflected from this point is imaged by an optical arrangement onto a light sensitive sensor element, on which the associated spectral color is detected and evaluated. A defined distance point is assigned to each wavelength by factory calibration.

This principle enables measurement on practically all types of surface. A thickness measurement can even be made for transparent materials with a sensor, whereby the spectrum of the second surface is interpreted. This method provides an extremely high resolution in the nanometer range because the different wavelengths of the light are used as the medium. Therefore, laser safety precautions are not required.

A clean environment, at least in the path of the beam, also is mandatory. This method is limited by the distance between sensor and target.

Confocal sensors are used in a test rig.2 This test rig is required for checking glass plates for solar power generation systems and to check display covers for damage immediately after production. A glass pane is automatically placed on a measuring table for this so that it can be checked absolutely free of vibration. Confocal sensors placed on a measuring beam traverse over the glass and measure thickness, planarity, cracks and other characteristics of the glass plane over several tracks. Any other measuring principle is not suitable for this application. Glass is not electrically-conductive and also not opaque enough for a laser sensor. A capacitive sensor measures too flat to be able to detect any cracks.



Confocal chromatic sensors are used for surface topographies and for use in narrow cavities. Source: Micro-Epsilon

Making the Right Selection

As numerous, different boundary conditions also apply for the majority of applications, there is no sensor which can be said to be suitable for all applications. All methods have different advantages and limitations. Amongst other things, the user must ask the following questions when selecting the correct sensor for a special measurement task:

What level of accuracy is required?

What material is the target made of?

How is the surface designed?

Is the target electrically-conductive?

At which ambient temperature will the sensor be used?

Is the ambient temperature static or does it change continuously?

What exactly should be measured? Displacement, angle and curvature?

In which medium is the sensor used?

Can mechanical loads occur?

Which measuring range is needed?



In order to simplify the decision, the various measuring principles should now be compared with each other. The following tables show in which criteria a measuring principle shows particular advantages and which criteria are assessed as problematic compared to other methods. V&S



References

Patented by Micro-Epsilon.

Developed by Micro-Epsilon.





Tech Tips

Just a few of the questions a user must ask when selecting the correct sensor:

What level of accuracy is required?

What material is the target made of?

How is the surface designed?

Is the target electrically-conductive?

At which ambient temperature will the sensor be used?



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