The Dual-Sensor Approach
Many different types of sensors exist for every conceivable measurement principle, and each measurement principle has its own strengths. Similarly, each technique has restrictions regarding its possible applications. For example, a laser sensor cannot be used for distance measurement through rubber, which in contrast can be carried out extremely well by an eddy current sensor.
With this in mind, a new approach has been developed that is conceptually very simple: to combine the strengths of two different measuring principles without allowing the restrictions to accumulate. From this concept, new fields of measurement applications arise.
Solutions applying this concept are being called “dual sensors,” because two existing sensors-each using different measurement principles-are combined and the signal outputs evaluated together.
In-Process MeasurementAs an example, a system can be developed that automatically inspects the thickness of plastic parts in a production mold. In cars, many parts consist of plastic and tight tolerance limits are set, which must be maintained. Therefore, it is important to inspect the thickness of the parts produced. Preferably, this takes place directly during production in order to avoid possible scrap costs.
To produce plastic parts using modern techniques, polyurethane material is sprayed into a mold, where it takes on the shape of the required part. The nozzle for the spray process is located on a robot arm, which moves precisely along the various contours of the mold. It also is on this robot arm that the sensors are located, which measure the thickness of the sprayed part. Directly after the spray process, the robot arm moves to previously defined points where the sensor measures the thickness. If an error occurs anywhere, it can be immediately rectified.
The thickness measurement of the sprayed skin presented developers with a few problems. The texture of the surface, the complicated geometrical shape and the materials used-polyurethane and metal-make exacting demands on the measurement technology. Specialists were able to develop a solution for this measurement application using two sensors based on different measurement principles.
Combining PrinciplesThe fundamental principle underlying the measurement setup is that the distance to both surfaces is measured simultaneously. Through subtraction, the difference of both values gives the thickness of the layer. In order to measure the distance to the metal surface of the mold, an eddy current sensor is used, which is suitable for precise distance measurement to electrically conducting surfaces. Here, a high frequency alternating current flows through a coil cast into the sensor housing. The electromagnetic field produced by the coil induces eddy currents in the conducting measurement target, which, in turn, also produces an electromagnetic field. As a result, the impedance of the coil changes. This change of impedance provides an electrical signal that is proportional to the distance of the mold from the sensor. Because eddy currents can only be induced in ferromagnetic materials, the signal is not influenced by the thickness of the plastic.
The distance to the surface of the plastic foam is measured using a laser triangulation technique. With this measurement method, a laser projects a spot of light, which is monitored by a camera, onto the surface to be measured. If the distance between the sensor and the surface changes, the angle also changes, at which the camera records the light spot. The distance can be determined accurately using a simple trigonometric calculation. The resolution that is possible using this method is down to a few micrometers.
For this application a modified eddy current sensor was used, having two holes for the laser beams. This avoids any axial offset between the two sensors. The resolution of the eddy current sensor is 0.75 millimeter. The laser triangulation sensor used has a resolution of 1.5 micrometers. A charged-coupled device (CCD) array is used as the receiver element in this sensor. The sensor also can cope with difficult surfaces, such as a shiny black or plastic surface.
More MeasurementsAnother solution is the thickness measurement of flat film, which uses another dual sensor. This also comprises an eddy current sensor, but this time used in combination with a through-beam sensor. With the through-beam sensor, the transmitter creates a light curtain and transmits it in the direction of the receiver. The receiver evaluates the received amount of light. If an object is located within the light curtain, the received amount of light is correspondingly reduced and the output signal changes accordingly.
In the flat film application, the film is passed over a metal roll. The eddy current sensor measures the distance to the roll through the film. The through-beam sensor is mounted such that the film and the roll cover part of the light curtain. If both signals are now evaluated together, precise results can be calculated regarding the thickness of the flat film.