kpiece and the scale assumed for the simplest case, only the difference in expansion coefficients determines the measured length deviation DL.
On a summer day at 35 C on the production floor, when measuring a 100-millimeter component made of PVDC without correction, an additional measurement error of 208 microns is expected (See Table 1). In practice, slightly different deviations are expected because the scale temperature and workpiece temperature are usually different. This does not change the fact, however, that the deviations are unacceptably large.
Lengthwise expansions of the scale and workpiece can compensate for one another if the scale and workpiece are made of the same material, such as steel-to-steel. In practice, this is usually not the case, so the expansion coefficients are different. One can conclude that an appropriate correction for the thermal influence is needed.
Integrated Temperature CompensationA new series of CMMs provides a solid design base, made of granite with encapsulated bearings. The devices are modular and can therefore be equipped to meet exact operator requirements. The equipment includes image-processing sensors, touch probes and laser probes. The machines allow simple manual measurements as well as CNC-controlled measurement with 2-D and 3-D CAD data.
All CMMs in the new series are designed to meet requirements of production floor use. They can calculate corrections based on expansion coefficients of the device and workpiece, workpiece temperature and associated temperatures of the scales. Sensors measure these temperatures, and thermal expansion of the scales is calculated automatically.
Requirements are the entry of a precise value for expansion coefficients of the workpiece and the maintenance of spatial (DK/m) and time (DK/h) temperature gradients, in accordance with the manufacturer’s specifications. If the expansion coefficient of the workpiece is not precisely known, an estimate of the value will reduce the measured length deviation. This applies especially to large measurement areas.
Results of Temperature CorrectionIf temperature compensation is necessary, one cannot assume there will be no more temperature influences. Despite the correction for thermal influences, there is a remaining measurement uncertainty that is lower when the expansion coefficient of the workpiece is known and the temperature is measured more precisely. (See Table 2.)
Limit deviation is defined as the maximum possible (estimated) deviation from the true value. It is assumed to be distributed rectangularly.
According to international standards, the expanded measurement uncertainty is the standard deviation multiplied by the expansion factor k=2. Therefore, it applies to a confidence interval of 95%.
For the selected measurement example, it is assumed that the temperature can be measured at a limit deviation of 0.5 K. The expansion coefficient of the workpiece is known with a maximum deviation of 5%. A measurement uncertainty of 16 microns thus remains. Here, that is only 7.7% of the original error of 207.8 microns without correction.
Tips for Practical ApplicationTemperature correction is more reliable and effective if the following actions are taken:
- Allow as little temperature change as possible.
- Avoid air drafts.
- Enclose the devices if rapid changes in temperature
- Keep heat sources in constant operation.
- Install devices at least 1 meter from the walls.
- Thermally insulate the floor.
- Do not allow direct sunlight.
- The measured object should be at the ambient
- Avoid touching parts with hands, or wear gloves.
- Ensure constant temperature during a measurement
- Carry out a drift check by reviewing the reference coordinate system, for long measurement cycles.
must sometimes measure at temperatures that deviate greatly from calibration
- For measuring small lengths, a simple linear correction in each
measurement axis can be implemented at a
- Lengthwise expansions of the scale and workpiece can compensate for
one another if the scale and workpiece are made of the same
- New modular CMMs measur