Accurate dimensional measurements in production environment using Dynamic Length Metrology

Giuseppe Dalla Costa

Research output: Book/ReportPh.D. thesis

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Abstract


The work described in this thesis concerns accurate dimensional measurements per-formed in a production environment with particular focus on injection moulded polymer parts. The work has encompassed the development of probing solutions to be implement-ed with the new method of Dynamic Length Metrology (DLM) introduced by DTU Me-chanical Engineering.
A state of the art review on the topic of accurate dimensional measurement outside a temperature and humidity controlled metrology laboratory (directly in production envi-ronment) has been carried out, with particular focus on traceability and measurement uncertainty of length measurement technologies and temperature sensors.
Experimental investigations on non-conventional probing systems have been per-formed with focus on assessing the uncertainty in DLM applications. The uncertainty contribution of the probing force of contact measurements is studied in devices with pas-sive and active force application system. A cutting-edge interferometric system with wide absolute measurement range is investigated as an alternative to contact devices. Infrared thermography is studied as a valid non-contact system for measuring non-uniform tem-perature fields; measurements from a thermal imager are compared with reference meas-urements performed using a RTD contact sensor.
Methods for the concurrent acquisition of data for dimensions, temperature, forces, humidity, etc. over time, dynamically, have been investigated, and probing solutions de-veloped. Probing solutions involve both the measurement operation as well as the data analysis. An innovative solution for elastic deformation compensation is proposed using a measuring device with active force control. The effect of temperature is investigated on polymer parts replicating the thermal conditions after injection moulding. The compensa-tion of thermal effects is implemented through the definition of an apparent coefficient of thermal expansion from the analysis of length and temperature information. A study on the optimal measuring time, regarding starting time and measurement period, is per-formed. A highly non-uniform temperature field on the workpiece allows the study of the influence of the sensor location on the measurement results. The investigations on hygro-scopic swelling involve study on polymer parts in saturated conditions to verify the rela-tionship between moisture absorption, dimensional expansion and ambient relative humidity. Subsequently the focus is moved on the transient state typical of the post-production period of injection moulded parts. A strategy for compensation of hygroscopic swelling is then proposed.
An optical solution using a vision system with integrated 2d reference grid has been proposed to provide fast and traceable length measurement. The integration of the refer-ence artefact in the measuring system provides direct traceability with uncertainties ideal-ly independent from the measured dimension. The methodology involves image pro-cessing with edge detection routines which have been studied and optimized using an artefact with ideal edges. The solution is then validated on an industrial polymer part.
A novel method based on digital image correlation (DIC) has been proposed for pro-ducing transient displacement data. The influence on the accuracy of different computa-tional choices and of the surface appearance of the measured workpiece has been assessed using a industrial ABS part. The benefits of the method applied to DLM measurements have been verified.
Finally an industrial POM part produced by injection moulding has been investigated in an industrial environment using the DLM method. Dimensional instabilities due to temperature, moisture absorption and residual stresses are analysed in a preliminary study. A set of probing strategies to consider the abovementioned factors has been devel-oped and validated with a comparison between DLM prediction and reference measure-ments carried out under controlled conditions in a metrology laboratory.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages224
ISBN (Electronic)978-87-7475-539-5
Publication statusPublished - 2017

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