How the Innovative Capacitive Displacement Sensor Can Measure Thin Gel Layers to Micrometer Accuracies

The pharmaceutical industry is well-known for its innovations. However, advancements in the field of research frequently pose challenges for industrial production. Therefore, a renowned pharmaceutical manufacturer was searching for a reliable measurement technology appropriate for measuring thin gel layers to micrometer accuracies. A capacitive displacement sensor supplied with an Ethernet interface offers a suitable solution.

During production, a thin gel layer with a thickness of approximately 200 µm is applied to a stainless steel roll. During this stage, the gel thickness has to be continuously monitored for the purposes of quality assurance. Measuring transparent material is a significant challenge for optical measurement techniques: when using the laser triangulation method, for instance, the laser beam can penetrate into the gel and this would subsequently lead to incorrect measurement results as the sensor may no longer be measuring on the surface. Since the gel contains water and is electrically conductive, electromagnetic measurement methods, such as capacitive sensors or eddy current, can be applied.

The Eddy Current Principle

The eddy current principle must be classified as the inductive measuring principle. The effect for measuring through eddy current is based on the energy extraction from an oscillating circuit. This energy is required for the induction of eddy currents in electrically- conductive materials. A coil, incorporated in a sensor housing, is energized by a high-frequency alternating current. The electromagnetic field of the coil induces eddy currents in the conductive measurement object and causes the resulting alternating current resistance of the coil to change, which causes an electrical signal that is proportional to the distance of the measurement object to the sensor coil. Since the eddy currents cannot spread inside the gel, the eddy current system is not suitable for the thickness measurement of the gel layer.

The Capacitive Measuring Principle

The capacitive measuring principle is mainly based on how an ideal plate-type capacitor functions. If an alternating current of constant frequency flows through sensor capacitor, the amplitude of the alternating voltage on the sensor is proportional to the distance to the target material (ground electrode).

Since the sensors are designed as guard-ring capacitors, almost perfect linear characteristics can be obtained. However, a continuous dielectric constant between the target and sensor is needed for reliable measurement results; the system reacts very sensitively to dielectric changes in the measuring gap. Capacitive sensors can also measure insulated materials, because these materials are acquired as a changed dielectric. By using an electronic circuit, it is possible to obtain a linear output signal for insulators. Since thermal conductivity changes do not have influence on the measurement, the capacitive measuring principle is reliable for strong temperature fluctuations.

The capacitive sensors are amongst the most accurate in the world, say the manufacturer. These sensors measure electrically-conductive measurement objects to nanometer accuracies in clean industrial environments. The modular capaNCDT 6200 system, used by pharmaceutical manufacturers, is a new measurement system that stands out due to its performance ratio and excellent price. The modular design allows the user to connect up to 4 measuring channels. The assessment of the measured values is carried out through Ethernet interface.

Sensor and measurement object function as electrodes of a capacitor

Sensor and measurement object function as electrodes of a capacitor

The capaNCDT 6200 is the first entirely modular multi-channel measurement system. The controller can be extended to 4 channels and is therefore suitable for flexible applications.

The capaNCDT 6200 is the first entirely modular multi-channel measurement system. The controller can be extended to 4 channels and is therefore suitable for flexible applications.

Ethernet as an Industrial Language

The Ethernet protocol employs integrated collision detection of data packets and assures complete and faultless transfer of data. The open connectivity protocol allows unlimited communication flow within networks without using the operating system of the terminal device and the hardware. Quick data rates of up to 10 GBit/s are equally impressive. The standard modules are bulk-produced, making them particularly inexpensive. The measurement data can be assessed independently of the location and remote maintenance can be carried out on a global basis. Both operation and configuration happen in a standard web browser, i.e. there is no requirement for installing any additional software.

For this application, an open Ethernet interface allows easy sensor integration to existing systems with less wiring. The data of up to 4 channels can be transmitted to a PC through an Ethernet cable and the data can be simultaneously shown on the display.

This information has been sourced, reviewed and adapted from materials provided by Micro Epsilon.

For more information on this source, please visit Micro Epsilon.

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