A wide variety of sensors are available in the market for thickness measurement. Largely, the sensors can be classified as contact or non-contact sensors. The benefits of non-contact sensors are that they exert no influence on the measurement object, operate without wear and are still able to function reliably for very fast processes, which is often no longer possible with contact sensors.
Measurement type
The material of the measurement object, the required accuracy, the environment in which the measurement takes place and at which speed the measurements should be made, are critical factors in the selection of the best method for measuring thickness.
In the case of thickness measurement of metals, two-sided thickness measurement is usually preferred. For example, one sensor is placed above and another sensor below the measurement object. In the case of dynamic processes, the measured values must be acquired and output synchronously. If both sensors are not operating synchronously, each movement of the target will result in a measurement error. The sensors must also be mounted exactly opposite each other, otherwise differences can occur. The new CSP2008 Universal Controller from Micro-Epsilon, for example, is available for the synchronous processing of the signals. The CSP2008 Universal Controller handles the processing of up to six sensor signals. Two sensors can be directly connected to the controller. Four other sensors are connected to the controller via separate Ethercat modules. The CSP2008 provides a wide range of possible arithmetic calculations of these sensor signals and is therefore one of the most advanced controllers for digital signals on the market today. Due to the very high data transfer rates using Ethernet, Ethercat and RS422, data output is performed extremely quickly. The Ethercat interface is intended for real-time applications.
Measurement on metal
The measurement principles suitable for thickness measurement of metals must be selected depending on each individual application. Laser sensors are often used for this, because they offer high resolution and measuring rates with large base distances. However, capacitive sensors or eddy current sensors, which provide a higher resolution than laser sensors, can also be used. The advantage of using eddy current sensors is that they only react to metallic objects. For example, if there are liquids or non-metallic foreign bodies in the measuring gap, these do not influence measurements. Capacitive sensors are also ideal for these tasks. They provide nanometre resolution but require a clean environment. When using eddy current sensors, the user must distinguish whether measurements will be performed on ferromagnetic metals or not. There are usually no problems when measuring on non-ferromagnetic metals. Also, for ferromagnetic metals, the object is not moved laterally under the sensor. However, if the measurement object under the sensor is moved laterally as, for example, in the thickness measurement of sheet metal strips, increased measurement error will be expected. This physical effect can be attributed to the inconsistent density of the metal.
Transparent objects
In contrast to this, single-sided thickness measurement is possible for transparent objects. Confocal sensors are frequently used for this as their light beam penetrates the measurement object. The measurement system is configured so that a peak is detected in the controller at both material transitions. Afterwards, the thickness is calculated from the difference between the two peaks. It is important there are no opaque particles in the measurement object, as the light cannot penetrate the object here. Transparent objects can also be measured from two sides. As well as the confocal system, laser sensors are used if an alternating measuring mode is possible. The sensors are programmed so that they generate the measured values alternately and in this way do not influence the opposing laser. The use of eddy current sensors is not possible here.
Thickness of plastics
There are various approaches for measuring the thickness of plastics. For example, single-sided thickness measurement is the standard for blown film. Here, capacitive sensors react to the non-conductive film in front of the sensor. It is critical that the measurement object is a constant distance from the sensor.
If there is metal behind the plastic, a single-sided thickness measurement can also be performed using a combination sensor. For example, an eddy current sensor can be combined with a laser sensor in such a way that both measure the object in one axis. Other types of combination sensors are eddy current sensors combined with optical micrometres or with capacitive sensors.
Safety assured
Thickness measurements are mainly required in process control and quality assurance, e.g. for the control of extrusion systems. Sheet thickness is also a very important factor in the manufacture of steel strip, particularly with respect to material costs with respect to the steel and coatings. If the sheet is not rolled to the required thickness, material is wasted. Using the right measuring system will help manufacturers avoid this wastage and increase productivity.
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