TRANSDUCERS, TRANSMITTERS & SENSORS FEATURE
THERMAL STABILITY and its effect on accuracy
In non-contact displacement measurement applications, engineers often overlook the effects of temperature variations on the accuracy of the sensor. It is therefore critical to check supplier datasheets, says Chris Jones of Micro-Epsilon
T
he temperature stability or thermal stability of a sensor indicates the
percentage possible error in the measurement per unit (K or °C). This error is due to a number of factors including the physical expansion of built-in components or the effect of temperature fluctuations on the performance of electronic components within the sensor itself. These effects result in a slight deviation of measurement results (thermal drift) per °C. However, when the temperature range the sensor is measuring over becomes greater, errors larger than sensor linearity can occur. Temperature stability is therefore a
critical factor in ensuring measurement accuracy, particularly in industrial applications where large temperature variations can occur. Non-contact displacement sensors can
measure many different parameters, including distance, vibration, gaps, surface profiles, runout and position. These sensors come in a variety of shapes, sizes and measurement principles. As well as eddy current and laser triangulation sensors, capacitive and confocal sensors are now commonplace in many production, quality and inspection areas. When selecting a suitable non-contact
displacement sensor, it is therefore important to check the supplier’s technical datasheet carefully before making a purchasing decision. If you do this, you may find that many suppliers do not state the ‘temperature error’ or ‘thermal stability’ of their sensors. So how do you know the actual measurement error or how to correct measurement results to account for this? In order to ensure that the correct
buying decision is made, you first need to consider the four primary methods of non-contact displacement measurement, i.e. laser triangulation, confocal, capacitive and eddy current and how temperature variations can affect the measurement accuracy in each case. Laser Triangulation Principle - The benefits of laser triangulation sensors include a small beam spot, very long measuring ranges are possible, the sensor operates independent of the target
material, and a high reference distance between sensor and target. However, the method is limited by a relatively large sensor design (compared to confocal, capacitive and eddy current sensors) and a relatively clean optical path is required for the sensor to operate reliably. For low cost laser sensors, in terms of
measurement errors due to thermal stability, these can be as high as 400ppm/K, which can affect the measurement accuracy. Compared to the other three measuring principles, laser triangulation offers the lowest thermal stability. It should be noted that temperature stability can vary considerably from one laser sensor supplier to another. The Eddy Current Principle –The advantages here are that this method can be used on all electrically conductive, ferromagnetic and non-ferromagnetic metals. The size of the sensor is relatively small compared to other technologies and the temperature range is high due to the resistance measurement of the sensor and cable. The technology is high accuracy and is immune to dirt, dust, humidity, oil, high pressures and dielectric materials in the measuring gap. Compared to laser triangulation, eddy
current sensors generally provide a higher thermal stability. In the last few years, Micro-Epsilon has improved thermal stability further with the development of a range of eddy current sensors that use a patented embedded coil technology (ECT), which overcomes the previous limitations of discrete coil windings. As a result of the sensor’s mechanical robustness, it offers longer service intervals and higher temperature stability. The sensors are also suitable for harsh operating environments, including high vibration, impact shocks and high operating temperatures (up to 350˚C). The Confocal Principle – Both diffuse and specular surfaces can be measured. With transparent materials such as glass, a one-sided thickness measurement can be achieved along with the distance measurement. Also, because the emitter and receiver are arranged in one axis, shadowing is avoided.
Temperature stability is a critical factor in ensuring measurement accuracy, particularly in industrial applications where large temperature variations can occur
Confocal sensors are often selected
when laser triangulation or other optical sensors are not accurate or stable enough on the surface being measured. Confocal offers nanometre resolution and operates almost independently of the target material. A very small, constant spot size (typically 10-25 micron) is achieved. In terms of thermal stability, confocal sensors are more stable than laser triangulation or eddy current sensors. The sensor is considered “passive”, as the controller and electronics are housed separately and so can be located further away from the target object or housed separately in a more controlled temperature environment. The Capacitive Principle –Compared
to the other three measuring methods, the technology offers the highest temperature stability, as changes in the conductivity of the target have no effect on the measurement. The technology is sensitive to changes in the dielectric sensor gap and so operates most effectively in clean, dry applications. Capacitive measurement systems from
Micro-Epsilon operate with an active, low noise cable in combination with an active guard ring capacitor. The system has an almost perfect impermeable electrical shield, which ensures precise measurements. In addition, the guard ring electrode provides a protected, completely homogeneous measuring field for high stability and interference-free, accurate measurements. Cylindrical, flat capacitive displacement sensors from Micro-Epsilon typically provide high temperature stabilities down to just 5ppm/K or -60nm/°C across a temperature range of -270˚C to +200°C. Long term stability is typically +/- .002% FSO/month.
Micro-Epsilon
www.micro-epsilon.co.uk
PROCESS & CONTROL | NOVEMBER 2016 17
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