Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 88 (2017) 190 – 195
8th International Topical Meeting on Neutron Radiography, Beijing, China, 4-8 September 2016
Design feasibility study for a demultiplexer miniaturized for microtomographic imaging Udo Langa , Pavel Trtikb,∗, Manuela Nottera , Jan Hovindb , Eberhard H. Lehmannb b Neutron
a Lucerne University of Applied Sciences and Arts, CC Mechanical Systems, Technikumstrasse 21, 6048 Horw, Switzerland Imaging and Activation Group, Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
Abstract In this paper a design feasibility study of a miniaturized demultiplexer that allows for simultaneous tomographic imaging on three independent rotational axes. While the general concept has been proven on a larger scale for three centimeter sized samples, the downscaling of this concept is presented herein. The main requirements are firstly, positioning of the samples parallel to the detector screen for stable imaging and secondly, the use of backlash free gears to avoid any angular play. Based on these requirements a concept is shown that consists of three precisely positioned anti-backlash axes within a housing with a spacing of 3.5 mm. c 2017 2016The TheAuthors. Authors. Published by Elsevier © Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ITMNR-8. Peer-review under responsibility of the organizing committee of ITMNR-8
Keywords: multiaxial simultaneous tomography ; neutron imaging ; neutron microscopy
1. Introduction The acquisition times for neutron radiographies are usually on the order of many seconds to minutes and for neutron tomographies it is therefore on the order of hours up to tens of hours (e.g. Masalles et al. (2015)). For the very high resolution neutron radiographies (Trtik et al. (2015)) the acquisition times are – due to low flux of neutron sources in comparison to X-ray sources – rather long (i.e. about 10 minutes). Thus, it is mandatory to use the available detector size as efficiently as possible. Previously, the authors presented successful experiments with a setup consisting of three axes to obtain images of three samples simultaneously with an axes spacing of 50 mm (Trtik et al. (2016)). There are ongoing efforts at the Neutron Imaging and Activation Group of the Paul Scherrer Institut to develop a high resolution neutron imaging testsetup – Neutron Microscope (Masalles et al. (2015)). Recently, the spatial resolution of this instrument was reported to be about 5 micrometers (Trtik and Lehmann (2016)). Due to the relatively limited field of view of this instrument (max. 10 x 10 mm), also a miniaturized mechanical demultiplexer is therefore needed. The main requirements imposed on the miniaturized version of the demultiplexer are as follows: • three parallel sample axes ∗
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1875-3892 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ITMNR-8 doi:10.1016/j.phpro.2017.06.026
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Fig. 1. Functional blocks of miniaturized demultiplexer for the Neutron Microscope. Solutions have to be systematically developed for each one of the functional blocks of the device.
• positioning of demultiplexer axes absolutely in parallel with detector screen to avoid blurring of the tomographical images • avoid any angular play when direction of rotation is reversed • distance between centerlines of axes should not exceed 3.5 mm • all axes should preferably have the same direction of rotation • the use of existing rotary table and control system. In this paper the systematic development of a design that fullfills all of these requirements is presented and an outlook is given on how it will be implemented and used in the future. 2. Demultiplexer design In this section the methodical approach to systematically find a solution and the resulting design will be presented. 2.1. Development of solution Due to the challenging requirements the potential solutions had to be developed and evaluated systematically. Thus, the device was symbolized by functional blocks (see Fig. 1) and separate solutions were found for each block and then finally composed to a complete system. The overall result can be seen in Fig. 2. This figure shows how the functional blocks are integrated. In the following subsections the design of the most important functional blocks is explained in detail. 2.2. Torque Transmission This functional block is the key element of the demultiplexer. Hence it requires detailed consideration. The requirements mentioned in section 1 provide the boundary conditions for the design of the torque transmission. 2.2.1. Anti-backlash This function is the most important one and at the same time the most difficult one to achieve at the miniaturized scales of the demultiplexer device. It was finally decided to use anti-backlash gears and pinions (from Reliance
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Fig. 2. Overview of the realization of the functional blocks.
Precision Ltd, Huddersfield, UK). Such gears consist of two individual gears that are connected through two rotational springs. The resulting rotational tension prevents any angular play as the gears will always be in contact with neighboring gears as shown by O’Neil (2002). 2.2.2. Connection Rotary Table - Axes The existing rotary table Huber EZ0675 (from HUBER Diffraktionstechnik GmbH & Co. KG, Rimsting, Germany) had to be adapted for the use with the new device. Thus, a reliable connection from the drive to the axes had to be found which provided alignment without addition of play or skew. Furthermore, the solution had to ensure the parallel aligment of demultiplexer and detector screen. The solution is an adapter shaft in series with a coupling. Additionally, it was taken advantage of the clearance fit integrated into the rotary table’s connection flange. The adapter shaft thus perfectly fits to this clearance fit. The coupling is an additional element to further balance out any potential misalignments. Fig. 3 shows the detailed concept of the connection between rotary table and adapter axes. It should also be noted that the housing of the device must be held in exact position in order to avoid any misalignments between housing and axes. This goal was achieved by alignment pins between rotary table and fasteners. The alignment pins also ensure the parallel alignment relative to the detector screen as long as the rotary table is correctly set up i.e. its edges are parallel or perpendicular, respectively, to the detector screen. 2.2.3. Torque transmission between axes After the coupling, the torque must be transmitted to three different axes at a distance of 3.5 mm in between, i.e. all mechanical parts needed for transmission and movement must also fit within this distance. The basic idea to achieve this goal is to use two auxiliary axes to first transmit the torque from the central axis hereto and then to the two outer sample axes. For both steps an anti-backlash gear is needed, i.e. four in total. It should also be mentioned that due to the limited space, dowel pins are used for the axes as mechanical production of such small shafts would be
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Fig. 3. (a) overview of the connection between rotary table and adapter shaft; (b) detailed view of the adapter shaft.
complicated and expensive. The final design can be seen in Fig. 4. The individual shaft-hub connections are herein designed as follows: • sample axes: brass gears are glued to axes • auxiliary axes: anti-backlash gears are connected to axes by clamps designed specifically for this purpose and provided by the manufacturer of the anti-backlash gears.
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Fig. 4. (a) placement of sample axes and auxiliary axes within housing; (b) side view of setup; the upper brass gear transmits the torque from the central axis to the auxiliary axes, from there it is then transmitted to the side sample axis by the lower brass gear; (c) top view with indication of flux of torque.
2.2.4. Bearings The bearings also require special attention due to the very limited space. Thus, ball bearings (from myonic GmbH, Leutkirch, Germany) normally used in precision engineering and sleeve bearings (igus GmbH, Koeln, Germany) were chosen. Fig. 5 shows how all axes are held in place exactly by this combination. The ball bearings are the locating bearings transmittung axial and radial loads while the bush bearings are the non-locating bearings and can only transmit radial loads.
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Fig. 5. The ball bearings are located in the lower housing plate while the sleeve bearings are put into the upper housing. By consequently sticking to this order the assembly of the device becomes simpler.
2.3. Assembly Between sleeve bearings and housing there is a tight clearance fit. Thus, the sleeve bearings have first to be pressed into the housing. The same is true for the lower housing plate where the ball bearings have to be pressed in. Afterward, all axes are mounted into the ball bearings and then all gears and pinions onto them. Finally, the upper housing with its sleeve bearings is put on top of the axes. 3. Conclusions and Outlook In this paper, a design for a demultiplexer with three sample axes for an efficient use of neutron beam time and screen size has been presented. The biggest challenge was to integrate all sample axes including all bearings and gears within a distance of 7 mm. This task could be solved by systematic engineering. The device will be manufactured in the near future and its performance tested in realistic conditions of a neutron imaging beamline. References Masalles, A., Lehmann, E., Mannes, D., 2015. Non-destructive Investigation of ”The Violinist” - a Lead Sculpture by Pablo Gargallo, Using the Neutron Imaging Facility NEUTRA in the Paul Scherrer Institute. Physics Procedia 69, 636 – 645. O’Neil, S.J., 2002. Methods to Minimize Gear Backlash. URL: http://machinedesign.com/motion-control/methods-minimize-gear-backlash. Trtik, P., Geiger, F., Hovind, J., Lang, U., Lehmann, E., Vontobel, P., Peetermans, S., 2016. Rotation axis demultiplexer enabling simultaneous computed tomography of multiple samples. MethodsX 3, 320 – 325. Trtik, P., Hovind, J., Gruenzweig, C., Bollhalder, A., Thominet, V., David, C., Kaestner, A., Lehmann, E.H., 2015. Improving the Spatial Resolution of Neutron Imaging at Paul Scherrer Institut - The Neutron Microscope Project. Physics Procedia 69, 169 – 176. Trtik, P., Lehmann, E.H., 2016. Progress in High-Resolution Neutron Imaging at the Paul Scherrer Institut - The Neutron Microscope Project. Journal of Physics: Conference Series 746, 012004.
