Precision Sensors For Real-Time Position Measurement

Precision Sensors For Real-Time Position Measurement

Advances in manufacturing technologies have created the need for higher accuracy and faster processing. Article by Shinrin Takahashi, Control Products Marketing Department, Advanced Automation Company, Azbil Corporation.


Edge detection is a critical component in many manufacturing processes. During manufacturing, various components may have to be aligned within a fraction of a millimeter, all the while not slowing down the production line.


Take liquid crystal display (LCD) manufacturing, for example. Edge detection sensors are essential for precise glass alignment and for prevention of meander by metal foil and film as they are rolled up. In addition to high accuracy (high resolution), high-speed processing that can keep up with the speed of glass transport or film windup is required.


Problems Surrounding Edge Detection

Edge detection sensors literally detect the edge of an object and changes in its position. The simplest method of sensing an edge optically is to project light (such as a collimated laser beam) onto the object and, by using an imaging sensor, to detect the border between the lit area and the shaded area blocked by the object. This presents a couple problems, however.

First, the detection accuracy depends on the how well the sensor can resolve that border so this method requires a sensor with an exceptional pixel pitch (resolution). Second, this method does not work well with transparent objects. There is simply not enough difference in the amount of light (light intensity) between the lit and shaded areas for such objects.

Advances in manufacturing technologies have created the need for higher accuracy and faster processing. To address all of these issues, Azbil adopted a unique edge detection method that utilises a characteristic of light called Fresnel diffraction.


How High-Accuracy Position Sensors Work

Azbil’s high-accuracy position sensor projects a collimated laser beam from the sensor head’s emitter onto an object, with the edges of the object diffracting the laser, even if the object is transparent glass. A receiver’s CMOS linear image sensor measures the amount of Fresnel diffraction and then calculates detailed edge positions using a unique algorithm called Fresnel diffraction approximate correction. Despite the long and complicated name, the result of this algorithm is simple. This proprietary algorithm allows for a high level of accuracy even when a linear image sensor with a lower resolution is used. This also has the added benefit of reducing the amount of required calculations.

These sensors achieve a measurement resolution of ±0.1 μm and measurement cycle of 250 μs, placing them among the highest levels in the industry. The sensor head is only 8 mm thick (for the 7 mm measurement width model) and can be mounted virtually anywhere and on various types of equipment, with minimal impact on the equipment’s footprint. In addition to the aforementioned LCD manufacturing, these high-accuracy position sensors can be used with equipment for processes such as semiconductor wafer alignment, film thickness measurement, measurement of the gap between rollers, detection of foreign matter or intermixed components, inner diameter measurement of pressed material, and detection of breakage and cracks.

On today’s factory floor, there is also a high need for connectivity and communication among the different phases of production. The position sensors are designed to support the open field network standards MECHATROLINK-III and EtherCAT. This allows measurements to be sent to the host control system at high speed, reducing the amount of cables needed and time required for installation and maintenance.

Manufacturers are constantly tasked with improving product quality, which starts with increasing production speed and accuracy, while making sure they do not deliver subpar products. High-accuracy position sensors enable fast and highly accurate measurement and can detect phenomena predictive of problems in order to prevent the production of a defective product. In this highly demanding manufacturing climate, high-accuracy position sensors have the capability to contribute to a wide range of needs.

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