Micrometric measuring systems

METROTRONICS notes: temperature

Metrology leaves the laboratory and enters the factory

With this article I will delve into one of the aspects that most influences the measurements made on the production line.

I'll give a brief summary for those who haven't read the previous article.

In the manufacturing sector, especially in the automotive sector, there has been a significant increase in the demand for measurements made directly on the production line.

Hence the need to ensure that all products comply with the specifications, thus making statistical sample checks carried out in the laboratory insufficient.

The need to get out of the laboratory involves a series of not indifferent activities, everything that falls under the scope of the adaptation of metrological instruments so that they can operate directly and effectively in an industrial production environment has been identified with the term "Metrotronics", a word composed of metro(logy) e (mecha)tronic.

Among the various aspects of automatic measurement in an uncontrolled environment I now highlight the thermal aspect.

Temperature is definitely one of the most controlled parameters in a laboratory environment, especially if length measurements are performed.

Usually in many metrological laboratories the temperature is kept within a tolerance of ± 1°.

To have a tangible reference of the uncertainty generated by this tolerance I point out that a block of aluminum of 42 mm for each degree centigrade expands by about 1µm.

In a production environment where the temperature can vary between 15 and 35 degrees, the effects of this uncertainty are multiplied tenfold.

For this reason, in order to measure in the laboratory a piece of a few kilos coming from production, it is necessary to wait for it to stabilize for more than an hour (see table fig.1 - stabilization time - mm).

On the production line, however, the practice that is most commonly adopted to reduce this uncertainty is to calibrate the instruments with reference samples with periods appropriate to the uncertainty to be obtained, usually at least once a day.

It is assumed that the dilation of the reference sample and the measured piece are equal and the calibration, or perhaps in this case it is better to say the zeroing of the instrument, allows to compensate for the ambient temperature.

One of the problems with this practice is that the reference specimens are often not made of the same material as the measured specimen, so they have different coefficients of expansion and this adversely affects the uncertainty.

There are also temperature-compensated measuring systems or, more simply, spreadsheets that transform the readout taking into account the ambient temperature taken from a thermometer.

Here we are still in the field of sample measurements, but carried out in a production environment. This procedure is perhaps not correct from a formal point of view, but it is still widely used, first of all by companies that do not have a temperature-controlled laboratory, but also by companies that do and that use this procedure for more frequent checks.

Let's move on to automatic measurement systems. Whether you use LVDT (Linear Variable Displacement Transducer) probes, optical scales, optical vision systems, lasers, or others, the principle does not change, the temperature causes the measured part to expand with all that this entails. Some of these instruments allow the compensation of the temperature, usually the ambient temperature, more rarely that of the analyzed piece.

Measuring the temperature of the piece would seem to be the most logical action, but in practice the speed with which measurements are normally taken by these automatic systems does not allow a reliable reading of the temperature.

In fact, thermal contact probes such as thermocouples or Pt100 are very slow due to the thermal inertia of the materials from which they are made, on the other hand, infrared thermal probes (pyrometers) are affected, especially for metals, by the problem of emissivity, which affects their accuracy.

Also, whatever type of probe you use, you will only measure the outer surface of the part, and in metrotronics this knowledge is not always sufficient.

The pieces measured on line often come from processes that raise the temperature, more rarely from processes that lower it. Even in conditions of presumed stability, i.e. from pieces that come from warehouses or processes that do not affect the thermal aspect, it is often normal to detect differences of a few degrees with respect to the ambient temperature.

Under these circumstances it is easy that the average temperature of the piece does not correspond to the temperature of its surface, let alone the ambient temperature.

However, knowledge of the average temperature of the part is necessary for correct measurement, especially if the required accuracy is less than one thousandth of the measurement.

There are probably different solutions to these problems, but we have examined the problem by focusing on two aspects.

The first is to realize a sufficiently fast and repeatable IR probe that, starting from the morphology of the piece, compensates the problem of emissivity in a more effective way than the classic compensation parameter RTC (reflected temperature compensation).

The second is to apply a method to know the average temperature of an object that is, for example, cooling. To achieve this goal we are helped by the fact that normally the pieces measured in the production environment are always the same and we know their characteristics.

Having as starting data some characteristics of the piece such as volume, morphology and material, it is possible to determine some thermodynamic parameters such as HTC (Heat Transfer Coefficient) and Biot number.

From here, by measuring the surface temperature at different times on the production line, it is possible to estimate the average temperature by means of thermodynamic formulas. The application of Fourier's law for the conduction flow in the solid at the surface and Newton's cooling law for the convective loss at the surface, allow to obtain already discrete results; more complex formulas allow a better reliability.

prototype of fast temperature sensor HCE-TMP-02

The result of these two investigations are the new, particularly compact thermal sensors of the HCE-TMP series, which are specially designed to thermally compensate measuring systems in production.