TPT March 2019

I NSPE C T I ON & T E S T I NG

Assessing measuring rate, averaging and accuracy of measuring devices

the same conditions and, therefore, a characteristic of the measuring value noise of the device itself. The specification of only a numerical value for repeatability is not sufficient. It might be that one supplier indicates the standard deviation of single values, whereas another calculates those based on a sequence of averaged values. The ‘measurement rate’ of a measuring device is the number of measurement values generated per second. This is a further important comparison criterion where more is seen as better. For an objective comparison, however, the knowledge of the interdependence between measurement rate and absolute accuracy and repeatability of a single measurement is crucial. It may be the case that a measuring device with a higher measurement rate but lower single-value precision is less suitable for controlling or characterisation of a process than a device with a lower measuring rate but higher single-value precision. For example, this is the case when a long averaging time is necessary due to a lower single-value precision. There is a risk that actual product variations that occur within this averaging time are levelled out while really being present. In the worst case, the specifications might even be violated without this being signalled by the measuring device. The displayed ‘real value’ results from single measurements taken in ten-minute intervals. Averaging over a period of one hour only smooths the extreme values. When averaging the varying temperature for more than 12 hours, the changes in temperature are displayed lower than they actually are. Furthermore, if the mean value is generated over an entire day, the information about the daily temperature variations is completely lost. A device that needs the latter averaging depth will not be suitable for a process where an alarm has to be raised or an adjustment has to be made depending on the temperature range. A practical example taken from the hose and tube production process is diameter measurement based on the shadow projection method with rotating mirrors. Often high measuring rates are indicated, which result from the rotation

WHEN deciding whether to invest in a measuring device, one of the main factors – besides the cost – is which device is the ‘best’. This is never as simple as it sounds. In digital photography, for instance, the size of the sensors and, thus, of the individual pixel, is more important than the total number of pixels. The pixel count, however, is commonly the relevant sales argument. For that reason it makes sense to question the characteristics of a measuring device. Often further information about the conditions under which these characteristics are valid, such as temperature, position dependency and so on are missing. Specifications usually contain the following characteristics: measuring range; absolute accuracy (also correctness); repeatability (also precision); and measuring rate. ‘Measuring range’ indicates minimum/ maximum object sizes that are measurable. Sometimes the visual range is specified instead, meaning the overall range in which the objects to be measured are allowed to move. Occasionally, information about the minimum and/or maximum measurable size is missing. The colloquial meaning of ‘accuracy’ is the total of all measuringerrors. However, for the evaluation of a measuring device, it has to be differentiated: ‘absolute accuracy’ means the comparison of a mean measuring value with a certified standard value. ‘Repeatability’ is defined as the scattering of the measuring values under

rate multiplied by the number of mirror facets. The specification of accuracy, however, is usually based on mean values of up to one second due to a relatively poor single-value precision. This has various reasons. Each single measurement is done with a different mirror facet. Product movements during measurement increase or decrease the product diameter – depending on the direction of movement – as the measurement of both product edges is not done simultaneously but sequentially. Lastly, the diameter information is only derived from the transition from dark to light and light to dark. The rest of the time, the information content of the measurement signal is zero. In contrast to this, for other measuring techniques line scan cameras are used. On one hand, product edges are record- ed simultaneously, so product movement is not an issue. On the other hand, each single pixel in the diffraction seam out- side the product shadow can be directly linked to the product edges, giving hun- dreds of reference points instead of just two. This leads to a higher single-value precision, and the measuring value has to be averaged nowhere near as long to be used for controlling or characteri- sation of a production process. A mere comparison of measuring rates without considering these circumstances is obvi- ously not sufficient. For an objective comparison of two measuring devices, it is important to define the requirements of the process. The details given by the manufacturer should also be taken into consideration so that the investment in a new device leads to an increase in quality, process optimisation and cost savings.

Sikora AG – Germany Fax: +49 421 48900 90 Email: sales@sikora.net Website: www.sikora.net

Laser Series 6000

87

www.read-tpt.com

MARCH 2019