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How to choose the right sensor or gauge for the application

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Quality in Control explains the functioning of various gauges and sensors to help users choose the right one for their application.

Typically a sensor is a device with an electronic output but no display, while a gauge is a combination of sensor and display that shows a physical unit.

Scanning laser gauges

The scanning laser gauge continuously moves a laser beam from top to bottom or left to right to effectively measure the length of the shadow period. This can happen at speeds up to 2400 scans/sec., which means that most product movements do not impact measurement accuracy. Even fast vibration is basically cancelled out over the average of multiple scans.

Scanning laser gauges can measure in one, two or three axis simultaneously and are mainly used to measure cables, wires, tubes, rods and bars. Their high accuracy and resilience can be attributed to the high laser power; as long as it gets a shadow on the receiver end, it will basically produce a valid measurement. The surface or material is irrelevant and to a limited degree they can be used to detect lumps and neck-downs.

These gauges have a defined measuring field with the maximum size object usually the size of the measuring field less 1mm the expected movement of the product. However, depending on the application the product does not have to be entirely in the measuring field. 

If used in the penetration mode, it can be used to measure a shadow entering the measuring field from one or both directions. For instance, it can be used to measure the thickness over a known roller or the gap between two rollers. 

In such applications the position of the gauge relative to the object to be measured becomes important as any movement of the sensor distorts the result.

Laser triangulation

Triangulation laser sensors emit a laser beam onto the surface and use an oblique camera to look at the position of the dot that was created. Based on the position it will determine the distance of that dot from the face of the sensor. Also called distance sensors for this reason, they can be used to measure any sort of mechanical dimension as long as the sensor is applied correctly and its output is displayed accordingly.

Two sensors can be mounted opposite to each other to directly show thickness irrespective of the position of the product. The ‘Range’ indicates the acceptable variation in distance that can be displayed while ‘Stand-off Distance’ is typically the distance from the face of the sensor to the centre of the range.

The measurement surface plays a crucial part in this measurement method; a surface with consistent reflectivity should be guaranteed if high accuracy is required. Additionally, since the camera views from an oblique angle, the orientation of the sensor is critical when sharp steps are to be measured because the edge can shadow the camera and the signal may be lost. 

Highly reflective objects can lead to insufficient diffusion with most of the light being deflected or reflected in the wrong direction. Likewise, very black surfaces that absorb a lot of light may require a higher laser intensity and make automatic laser power control a necessity.

However, many sensors can measure the blackest of black, only struggling with highly porous black material.

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