Contactless Position
Measurement System


Contents

Summary
Description
Development
References

Summary

[08-FEB-24] The particle accelerators and fusion reactors that will be built in the next twenty years make liberal use of precision-machined, highly-reflecting stainless steel components. In the case of the particle accelerators, we must not handle the components during assembly, or else we will contaminate their surfaces and ruin their performance. In the case of fusion reactors, we must stay reduce the time for which we are near the components to a minimum, or we will suffer from radiation sickness. In order to bring these components into contact and line up their bolt holes, we need an instrument that measures their relative positions and moves them together correctly. Traditional front-illuminating machine vision systems cannot locate highly-reflecting components accurately. In this Phase I SBIR project, funded by United States Department of Energy grant number 13557088, we built an accurate and effective Contactless Position Measurement System (CPMS) for highly-reflecting components. We place infrared backlights behind the components and take two silhouette images of the components from two different locations. These stereo images allow us to apply the principles of triangulation, parallax, and scale to deduce the location and orientation of each component. At a range of 50 cm, we are able to measure the relative positions of two stainless steel flanges with an accuracy of 150 μm rms.

Description

[05-FEB-24] Our Contactless Position Measurement System (CPMS) permit automatic alignment of highly-reflecting components that cannot or should not be touched by human hands. During Superconducting Radio Frequency (SRF) string assembly, human contact causes contamination, and cotamination spoils the resonance of the cavities. During maintenance of fusion reactors, the components we remove from the reactor are radioactive. The length of time for which humans can handle the components in order to carry out repairs is limited. We would like to measure the relative positions of such objects so we can bring them into alignment with motorized stages. If possible, we would like to make our measurement without touching the objects. But measuring the location of highly-reflecting objects is challenging.


Figure: Two Contactless Position Measurement Systems Applied to String Assembly. Each CPMS consists of two calibrated, infra-red only Silhouette Cameras (SCAMs), a calibrated reference plate, and a backlight to form the silhouette images.

The CPMS uses an infrared background and stereoscopic cameras to capture infrared silhouette images of two or more components in its field of view. The reflections of ambient light that make it so difficult to locate these objects are subdued by the use of an infrared background in a room in which the only light is provided by fluorescent or solid-state lamps. The CPMS combines the silhouette images with knowledge of the dimensions of the objects to obtain the relative positions of components in its field of view. At a range of 50 cm, and without taking multiple measurements in the same position, the CPMS is accurate to 50 μ rms in the transverse directions and 300 μm in the longitudinal direction. If we take the average of ten measurements, our accuracy to better than 50 μm transverse and 150 μm longitudinal.


Figure: A Half-Meter CPMS. Blue and Black SCAMs view a sphere mounted on a micrometer stage. We use spheres and our coordinate measurement machine (CMM) to calibrate the two SCAMs. For interior of a blue SCAM see here.

The CPMS measures position and orientation, which together we call its pose, with respect to its own calibrated reference plate. Because the components the CPMS will view are machined out of highly-reflecting stainless steel, we know their dimensions to within a ±125 μm (5 mils). The CPMS takes stereo silhouette images of the components, combines the images with our knowledge of the component dimensions, and so deduces the pose pose of each component, provided that some portion of each silhouette is evident in the SCAM images. The CPMS measurement of position and orientation allow us to maneuver the components into contact using motorized stages. Once the components are in contact, the risk of contamination of their interiors is greatly reduced, and the time taken to complete their attachment is greatly reduced.


Figure: Two Flanges with Alignment Pins in Silhouette. The CPMS treats the flanges as two separate bodies and measures the position of each independently. The pins allow us to measure rotation about the cylindrical axis.

An SCAM is similar to the well-established BCAM alignment instrument, but with a larger field of view and correspondingly lower precision. Each SCAM mounts on three balls in exactly the same way as a BCAM. The CPMS plate holds two SCAMs and defines its own coordinate system. We calibrate the plate by moving a sphere around in front of the SCAMs, taking images of the sphere, and at the same time measuring its position with our Coordinate Measuring Machine (CMM). This calibration procedure allows us to calibrate both cameras so that we can project modeled objects accurately from the global CPMS coordinate system one the two SCAM image sensors.


Figure: Superconducting Resonant Cavity String Assembly. All connections with bolts require contactless alignment. The flanges in our mock-up are those of the smallest connection on the lower left of the view.

To measure the location of a component we begin with a silhouette image, guess the approximate location of the component and project a model of the component onto our image. Our purpose is not to measure the size or shape of the component, but rather to determine the pose of a component whose size and shape we already know. We count the pixels in the image where the modelled silhouette and the actual silhouette disagree. We adjust the model pose until we minimize this disagreement. The final pose of the model is our measurement of the pose of the actual component.


Figure: The CPMS Manager Fitting Two Flanges with Alignment Pins. Orange pixels are in the silhouette, but not overlapped by our model. Blue pixels are overlapped by our model, but not contained in the silhouette. All gray-scale pixels show agreement between the model and silhouette.

Development of the CPMS was funded by the United States Department of Energy (DoE) under SBIR Phase I grant number 13557088. With the Phase I grant, we solved all the technical challenges required to build a CPMS. Our application for further funding in Phase II to develope a larger, motorized CPMS received favorable reviews but was nevertheless declined. We now offer custom-made CPMS instruments for sale. Our customer specifies the nominal operating range of the instrument to be anywhere from 50 cm to 200 cm. We can provide the CPMS as a basic measurement-only system with stereo cameras and backlight, or equipped with a motorized translation and rotation stage that will align two components automatically with the help of CPMS measurements.

Development

[08-FEB-24] For a detailed account of the design and testing of the CPMS, see our Development Log.

References

Development Log: Development of the CPMS at OSI starting August 2022.

Interim Reports: Talks, reports, and guides produced during development.

Phase I SBIR Application: Project abstract and summary from our Phase I Small Business Innovation Research (SBIR) application entitled "Contactless Position Measurement for Highly Reflective Components", submitted 26-FEB-22, grant number 13557088, awarded 27-JUN-22.

Phase II SBIR Application: Project summary and narrative from our Phase II Small Business Innovation Research (SBIR) application, submitted 14-APR-23, grant number 13862200, declined 19-JUL-23.

Reviews of Phase II Application: Reviews of our Phase II application, grant number 13862200.

Interim Report, IA: Interim progress report filed April 2023 with DoE.

Press Release, Phase I: Company's public announcement of receipt of award, upon receipt of funding, 31-AUG-22.

Infrared Backlight (A3046): A diffuse source of infrared light that may be flashed for bright, sharp, silhouette images.