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A MECHANICAL DESIGN FLOW USING OPEN SOURCE TOOLS: DEVELOPMENT OF PRESSURE TRANSDUCERS AS A CASE STUDY

D. Rodriguez Sanmartin,a, b A. Lawal,b P. Cooper,b G. Awcock,a S. Busbridge,a

a The University of Brighton, Faculty of Science and Engineering, Lewes Road, Brighton, BN2 4GJ, UK b PSM Instrumentation Ltd, Unit 3, Burrell Road, Haywards Heath, West Sussex, RH16 1TW, UK E-mail: [email protected]

Abstract:

PSM Instrumentation Ltd. (PSM), a UK firm specialising in instrumentation for liquid level measurement for the marine industry, and the University of Brighton are currently collaborating in a 2 year research programme funded by the KTP scheme. Its aims include introducing new-to-company technologies into PSM, such as alternative manufacturing methods and design tools, and to use them to design products which could be taken from concept to commercialisation.

The development of Computer Aided Engineering (CAE) design and modelling tools has transformed how research and development is carried out over the last decade. CAE platforms speed up development of new products by using computer models, generally based on Finite Element Analysis (FEA), which allow for the analysis of different physics such as thermal, stress and electromagnetic. This result in more robust designs whilst also reducing the number of prototype iterations needed, which reduces development costs.

Open source CAE platforms, are now able to offer SME’s sophisticated and competent capabilities at zero capital cost, although indirect cost such as learning how to use the tools would be initially incurred. Therefore, open source tools pave the way to a scalable alternative to commercial packages, such that CAE capabilities could be embedded within an SME as a result of a KTP scheme.

This presentation describes a mechanical design flow using open source tools including:  the creation of 3D models for visualisation purposes;  the finite element analysis (FEA) for the calculation of thermal and mechanical stresses;  the visualisation of FEA results.  the generation of parts manufacturing drawings; The development of a pressure transducer at PSM will be used as a case study to demonstrate how open source tools were used in its development.

Keywords: Small to Medium Enterprises (SMEs), Knowledge Transfer Partnerships (KTP), Computer Aided Engineering (CAE), Finite Element Analysis (FEA), Open Source software.

1 Introduction generally based on Finite Element Analysis (FEA), which allow for the analysis of different The development of Computer Aided physics such as thermal, stress and Engineering (CAE) design and modelling tools electromagnetic. These methods result in more has transformed how research and robust designs which in turn reduce the number development is carried out over the last of prototype iterations needed and thus decade. CAE platforms speed up development reduces development costs. of new products by using computer models, For Small and Medium Enterprises (SME’s) piezoelectric equations, open source tools commercial CAE platforms still represent a are not appropriate compared to costly option, which, at best, may limit access commercially available packages. to a sub-optimal number of licensed seats and/or software modules, at worst, may deter  The third option would be to use a them completely from engaging with these combination of both, where different powerful tools. Open source CAE platforms, software tools would be used at different however, are now able to offer SME’s steps of the design / modelling process. sophisticated and competent capabilities at zero capital cost, though indirect cost such as The Table 1 below shows a strengths, learning how to use the tools would be initially weaknesses opportunities and threats (SWOT) incurred. Thus open source CAE tools could analysis for both commercial and open source pave the way to a scalable alternative to CAE platforms. The decision on whether to commercial packages, such that CAE incorporate a commercial, open source or a capabilities could be embedded within an SME combination of both would depend on the as a result of a KTP scheme. SMEs objectives, both short and long term.

Developments in both commercial and Open In terms of risks or threats, for an SEM Source Platforms leave SME’s with three committing to the procurement of commercial possible scenarios for the incorporation of CAE packages involves the risk of obsolescence of capabilities: the tools, either because the tools themselves  Procuring software licences to access a become obsolete or the organization needs CAE platform which would have upfront change. Also, if these tools are not used to full cost capital expenditure in the region of capacity their procurement would represent a £15-25k (per license) depending on the waste of expensive resources. On the other design and modelling capabilities required; hand, open source tools present the risk for the  Engaging on incorporation Open Source SME of not being able to successfully handover CAE capabilities, which would not require projects unless there is a strategy in place to do an initial capital expenditure, but were other so like would be the case of an SME engaging issues such as difficult access to support or on a KTP project. training, could be an issue. Also, for some specialised applications, such as solving of

Commercial CAE Open Source CAE S trengths W eaknesses S trengths W eaknesses  Support and training from  Licensing costs (£15-25k)  No upfront licensing  Lack of training / support. providers.  Licensing cost (to costs.  Expertise required  Access to off the shelf increase number of  Free software updates. introducing these Tailored capabilities. licences)  Access to capabilities into an SME.  Easier to achieve short  Expensive packages multi-disciplinary software  Tools need more term goals. needed to solve tools. validation (slow). multidisciplinary  Unlimited number of  Risk of under-delivering problems. licenses. on short term goals.  Access to source code. O pportunities T hreats O pportunities T hreats  Leveraging knowledge  Obsolescence (licences  Collaboration with other  Project handover and experience of need to be renewed every open source users.  Incompatibility with commercial companies. 5-8 years).  Developing in house commercial packages.  Achieving short term  Procured packages not tailored tools.  Lack of specialised goals. adapting to future SME  Continuous improvement personnel on these needs. of CAE capabilities. packages.  Waste of expensive  Adaptability to changing resources if not used. SME needs.

Table 1: Strengths, Weaknesses, Opportunities and Threats (SWOT) analysis for commercial and Open Source CAE platforms in the context of an SME.

include the choices of technology and materials The first part of the paper provides the context and the different physical problems involved of the KTP project and describes some of the such as stress-strain analysis and temperature technical challenges involved in the induced stress. development of a pressure transducer. These The second part describes a general design process flow including: Typical operation requirements of pressure  the creation of 3D models for visualisation transducers for tank gauging applications are purposes; shown in Table 2 below. A 1 bar sensor is  the finite element analysis (FEA) for the being used as an example, though in practice calculation of thermal and mechanical different transducers would be used to cover a stresses; wide range of pressures (from 0.1 to 10 Bar).  the visualisation of FEA results. These include the specification of a proof  the generation of parts manufacturing pressure, i.e. the pressure up to which the drawings; transducer can be subject to without compromising its performance. The third part of the presentation explores the scenario in which only open source tools are Another requirement of pressure sensors is that used in the design and development process these should produce a measurement which is and aims to demonstrate how this general independent of the specified temperature o o design process flow can be implemented using range, typically between -40 C and +80 C for open source tools, using the development of a PSM applications. This is achieved by pressure transducer in the KTP project as a designing structures which have very low case study. temperature drifts, which can be achieved by the selection of materials with very similar 2 Technical challenges thermal expansion coefficients. involved in the design of a All these design requirements, make pressure pressure transducer for transducers devices where the coupling of different physics, such as pressure and marine environments temperature, needs to be minimized at the PSM Instrumentation Ltd. (PSM), a UK firm design stage and compensated for during the specialising in instrumentation for liquid level device operation. In addition, the transducers measurement for the marine industry, and the need to remain linear during their specified University of Brighton are currently pressure ranges. collaborating in a 2 year research programme funded by the KTP scheme. Its aims include Parameter Values introducing new-to-company technologies into Pressure range 0-1Bar PSM, such as alternative manufacturing Proof Pressure 3Bar methods and design tools, and to use them to Burst Pressure 10Bar design products which could be taken from Accuracy 0.001Bar (0.1% of the pressure concept to commercialisation. (@20C and 1Bar) range) Operating temperature -30oC to +80oC One of the aims of the KTP project is to range incorporate into PSM CAE capabilities which Legislation compliance ATEX, legislation on explosive would be used for the design of pressure atmospheres sensors. From a modelling perspective, EMC, legislation on pressure sensors represent a challenging electromagnetic compatibility. problem because there are different physics involved in their design such as the fluid Table 2: Typical technical requirements of a 1Bar structure interaction between a diaphragm and pressure transducer for tank gauging a liquid for which the pressure needs to be applications. Pressure ranges for tank gauging measured. The deflection of the diaphragm is vary typically from 0.1 to 10Bar . them transform into other magnitude (such as a change in capacitance, or resistance). In terms of sensor design, these requirements mean, independently of the transducer In addition, devices working in the marine technology used, that two problems need to be industry encounter very harsh environments addressed: when deployed on the field. For example, they  The repeatability of the device output for can be exposed to substances such as crude the pressure range, independently of the oil or seawater. They also need to meet transducer technology used, in order to different legislation requirements to operate in maintain the resolution of the sensor. potentially explosive atmospheres (ATEX  Compensation for temperature induced legislation), and ensure they are immune to strain. electromagnetic interference (EMC). For example, in the case when a flat diaphragm 3 A general design process is used, the diaphragm will have a pressure range for which it would behave linearly. flow However, when the pressure applied is too The previous section described common issues high, the diaphragm will suffer a plastic which need to be addressed in the design of deformation and will not return to its original pressure transducers. position after the pressure is relieved. A general design process is represented in a A transducer technology under evaluation on block diagram form in Figure 1. It consists of the KTP project is the use of strain gauges, three main stages: Concept generation; bonded to the back of a diaphragm. The idea prototype design and manufacture and being that changes in pressure would result on prototype testing. a change in the resistance of the strain gauges.

The equations governing the deformation of an 1. - Device concept ideal diaphragm is a problem well understood 2. - Device Modelling Concept with and has a regular stress/strain distribution 3. – Is it feasible from a Iteration for ideal structure. For example the strain manufacturing perspective? s e s p n

gauge manufacturer has an application note on y o t i t o t this subject. This, together with the desirable a r o r e 4. – Device design t I values of change in resistance in strain gauges P 5. – Prototype can be used to make a first guess of the manufacturing and diaphragm key dimensions (thickness and assembly diameter). 6. – Prototype characterisation Temperature induced strain it is an undesirable 7. – Lessons learned effect in pressure transducers which may occur 8. – Does the device concept due to the strain is generated due to need refining? differences in the coefficients of thermal expansion of different materials constituting the Figure 1: A general design process including concept generation, manufacturing and transducer and/or when there is a mismatch in characterisation. temperatures between different regions of a device. In terms of transducer design it ought to The first stage is the concept generation, which be minimised in order to obtain a predictive drift can be validated using models of the device. At with temperature which can then be corrected this stage manufacturing issues, such as the by the sensor’s electronics. cost of manufacturing part, need to be taken into consideration. In case of not being Common strategies to minimise the successful the concept needs to be rejected or temperature induced strain are: modified and the concept generation process is  The matching of the CTE’s of the materials iterated again. involved in the design. For example, strain gauges are commonly specified by their The second stage is the device design and manufacturers to match different manufacturing. At this stage detailed substrates. manufacturing drawings and specifications of  And the mechanical decoupling of parts of all the elements need to be produced, and the sensor subject to temperature drawing files adapted for the manufacturing gradients, for example corrugated processes used produced (designs of silicon diaphragms are used for this purpose on etching masks, laser or CNC machining files, the design of LVDT based sensors. and device parts and assembly drawings, are examples of manufacturing processes which In the case of real geometries where chamfers may need to be considered). Once all the and/or fillets are used for ease of manufacture, elements of the prototype assembly are the use of three dimensional models, combined manufactured the prototype assemblies need to with FEA models can help to visualise and be carried out. predict the stress distribution. These, together with characterisation data, could help to gain a The final step is the device testing to verify that further understanding of the device during the the expected performance was obtained and to design process. explore how the device could be optimized. After the device characterisation and the analysis of results the device design and manufacturing cycle can be iterated to produce multinational companies, such as EDF which successive generations of prototype devices. are developing open source SALOME platform over the last 10 years incorporating open Advances in CAE over the last decade allow source tools. detailed CAD drawings of geometry or a device, to be meshed and then be analysed A possible choice of open source design tools using Finite Element Analysis (FEA). These that can be used for the design process numerical methods can be used to solve described on the previous section are complex problems with coupled physics and/or highlighted on the diagram below: the complex geometries which would be difficult to solve using analytical methods. FEA tools in turn allow a higher number of iterations at the 1. FreeCAD Concept design concept stage which has the effect of Parametric draw ings Iteration reducing the number of prototype iterations s needed to obtain a device ready for commercialisation thus reducing the

2. Salome development time and costs. e s p n

y Meshing o t i t o t a

r 3. Elmer A number of FEA packages are commercially o r e t I available, and some examples are: ANSYS by P Finite element analysis solver ANSYS Inc. Southpoint, US ; ABAQUS by 4. ParaView SIMULIA, a subsidiary of Dassault Systèmes, Visualization Providence, US ; COMSOL MultiPhysics by 5. NanoCAD COMSOL Ltd. US; and LUSAS, a trade mark of Manufacturing draw ings Finite Element Analysis Inc., Kingston upon Thames, Surrey, UK. Other tools such as 6. – Prototype characterisation SolidWorks (Dassault Systèmes SolidWorks 7. – Lessons learned Corp, Concord, US), which were previously 8. – Does the device concept specialised on design incorporate now FEA need refining? capabilities to their products. Commercial packages tend to be specialized on particular Figure 2: Open source tools that can be used for applications. Two examples are: LUSAS which the design and modelling of a pressure is used for civil engineering projects; and transducer. PZFlex (Wiedlinger Associates Inc, US) which is used for prototyping of ultrasound The design process would consist of: transducers and wave propagation analysis.  Using FreeCAD to create parametric models of the geometry under study which Also, there are Open Source FEA packages can be easily modified. are also capable of solving multi physics  Import models from FreeCAD into the problems are also available. These include Salome platform using the industry ELMER which was developed at the University standard .step file format. of Finland , CodeSaturne for CDF analysis  Define the domains (surfaces, volumes and developed by the EDF group in France , and borders) of interest and create a mesh of OpenFoam an CDD analysis tool developed by the part in Salome. the Open Foam foundation which is now part of  Export the meshed part from Salome into the SGI group, Fremont, US . Elmer using the .unv mesh file format.  Define boundary conditions and solver to 4 Implementing a design use in Elmer, linear elasticity and process flow using open temperature for example, and solve the finite element analysis (FEA) problem. source tools  Export the FEA results to ParaView using The open source movement, originated by the .vtk file format. Richard Stallman in 1983 with the goal of  Analyse the FEA results in ParaView and creating a freely available Unix based operating compare different simulation runs. system, has been developing CAE tools which  Finally, NanoCAD can be used to produce are available under the General public Licence CAD drawings, in the industry standard (GNU). Some of the best examples are Elmer, .dwg file format, for the manufacturing of an FEA package for the solving of coupled parts. physics and OpenFoam CFD. The potential of these tools has caught the attention of After the FEA results would then need to be validated using characterisation data obtain Figure 3 below shows a possible way in which using prototype devices to identify which areas the drawings of a part can be defined in of the device would need further optimization. FreeCAD. By defining constrains on a section of a part, an outline of the sections of the part is These open source packages can be installed produced which can then be revolved along an on a single computer, though a dual-boot axis to obtain a 3D model of the part. Changes technique to install Linux and Windows on the on constrains of the 2D sketch, will result on a same computer would need to be used due to change of the 3D part, which allows for the their operating system compatibilities as shown quick production of design variants. in the Table 3 below: a) Linux Windows Mac Software (Ubuntu) (Vista or 7) (OX) package 32 64 32 64 32 64 FreeCAD (ver. 0.13)       (*) Salome / (ver. 7.2.0)      Elmer (ver. 7.0)       ParaView       (ver. 3.00.0) b) c) NanoCA D       (ver. 4.5) Table 3: Operating system compatibility for FreeCAD, the Salome platform, paraView and NanoCAD for Linux, Windows and Mac. (*) The previous version of Salome was compatible with Windows 32bit systems, but for the latest and current version it will only be available for Linux.

Minimum recommended hardware requirements for the above tools are: 2GB Figure 2: Drawings of a pressure port with a RAM, 4GB of storage, 500MB of video grooved neck made drawn in FreeCAD. a) memory, and Pentium 4 processor or Snapshot of some of constrains used to draw a equivalent, which can be readily accessible on section of the part. b) Section of the part which low cost computers. However, and depending was revolved around the vertical axis to obtain on the application, higher hardware the 3D part. c) Side view of the 3D part. c) requirements are required when dealing with Isometric view of the 3D part. large data sets. One of the features of FreeCAD is that drawings can be exported using step files, a 4.1 Using the FreeCaD common standard for 3D drawings which can parametric modeller for be imported into other design tools. For example, FreeCAD drawings can be imported design evaluation into the Salome platform when saved as step FreeCaD is a parametric modeller which can be files. used to quickly draw 3D models of parts, and be used for visualization purposes. Being a 4.2 Finite element analysis parametric modeller means that the basic components of a part are drawn and then using open source tools constrains are applied to them until the number As described in previous the previous section, it of degrees of freedom of the part is zero. The is important to understand both the pressure advantage of this drawing technique is its induced strain, in order to predict the device flexibility and the ability of produce very quickly performance, and to minimise the temperature several design variations from the same induced strain. drawing, just by changing some of the constrains applied to the part. This would be Finite element analysis can be used to gain a more difficult to do using the GEOM module of further understanding of these characteristics the Salome platform. for complex geometries where the use of ideal models may not be suitable. FEA allows for the pressure or temperature. Meshes produced analysis of different design options using using the Mesh module of the Salome platform models and, combined with the analysis of test can then be exported into Elmer using the .unv results, can reduce the amount of iterations mesh file format. needed in the prototyping phase of the design. Once the geometry is imported into Elmer, the As previously described, Elmer, OpenFoam multi-physics problem under study can be and Code Saturne are open source FEA tools. defined. For example, Elmer allows for the They all have in common the need to generate calculation of electromagnetic fields, a mesh representing the model purpose of the stress/strain in linear elasticity problems and study. temperature induced stress.

The approach proposed on this paper is to use The final step of the FEA is the visualization to import step files from the parametric and analysis of the results. ParaView, which is FreeCad modeller into the Salome Meca also included as one of the modules in the platform. Once the model is opened in the Salome platform. ParaView is a very powerful Salome the model can be defined further, for visualization package developed in the US for example by defining the boundaries which will the visualization of very large data sets which be subject to changes in applied pressure or can be used for analysing FEA data. Results of temperature, and defining the different the FEA in Elmer can be exported into materials in the part. The mesh module of the ParaView using the .vtk file format as shown in Salome Platform can then be used to generate Figure 4 below: the mesh files which can be imported into a FEA modeller, such as Elmer.

Figure 3 below shows how the part designed using FreeCAD was meshed using the mesh module of the Salome Platform.

Figure 4: Vonmises stress on a diaphragm (0.2mm × 24mm diameter), subject to a pressure of 10Bar generated using Elmer and visualised in ParaView. For visualization purposes a clipping through the middle of the structure was used Figure 3: Picture of a mesh produced using the and the displacements in the structure were Salome Platform, where surfaces of interest have exaggerated by a factor of 50. The top picture been defined. The original drawings were created shows the whole of the pressure port and the in FreeCad and then exported to Salome using bottom picture a closed up view of the the .step file format. In practice finer meshes diaphragm and the mesh used for the simulation. would be generated to carry out FEA. The figure depicts the Vonmises stress on a The domains of interest, such as such as the diaphragm (0.2mm × 24mm diameter), subject surfaces of the diaphragm and the volume of to a pressure of 10Bar and how the ParaView the device are also defined at this stage so they can be used as a visualization to interrogate can be identified by the FEA modeller to assign the data. For example, in the figure a clipping materials and boundary conditions such as through the middle of the structure was used and the displacements in the structure were .dwg support, an industry standard interface, exaggerated by a factor of ×50. and an open architecture.

The model could be used to calculate the So far, NanoCAD has 100.000 users in Russia stresses and strain in the structure for different and is now expanding internationally. Nanosoft boundary conditions, such as applied pressure business is based upon gaining revenues by or temperature, and to optimise the geometry of providing support and tailoring NanoCAD for the design such as the thickness of the specific applications, in contrast to the diaphragm. In addition the model could predict traditional approach of gaining revenues for what would happen if a different material were licensing. used by changing the material properties used in Elmer. An example of how NanoCAD was used during the KTP project is shown in Figure 5 below, 4.3 Producing manufacturing depicting the CAD drawings that were used for manufacturing and a picture of the part on the drawings with NanoCAD Eng insert. In particular, this part is being used as a tool for manipulating test pressure ports during NanoCad is an open source computer aided machining and assembly. design application released by the russian CAD vendor Nanosoft in march 2012. It has native

Insert: Picture of the manufactured part

Figure 5: Example of a manufacturing drawing drawn using NanoCad Eng 4.5. In particular these drawings were used to manufacture a fixture to assist with the handling of pressure ports. The insert shows a picture of the manufactured part.

5 Summary and Conclusions adopted the strategy of using a combination of commercial and open source software packages to carry out their finite element CAE platforms have changed design processes analysis. over the last decade, reducing developing time and costs. There are both commercial and A SWOT analysis of both commercial and open Open Source CAE platforms available. In some source packages, which highlights that the right industries, such as car manufacturers for the choice for an SME would depend its modelling of vehicle aerodynamics have organizational objectives. Licensing costs and the risk of obsolescence being is major issue of commercial packages; whereas difficult access to training and support the major disadvantage of open source.

A general design process and how it could be implemented using open source tools has been described. In the proposed design process the parametric modeller FreeCad would be used for design visualization and evaluation. FEA analysis would be carried out using the GEOM and mesh modules of the Salome platform for the mesh generation, Elmer for solving multi- physics problems, and ParaView for the visualization and analysis of the FEA results. For the production of 2D manufacturing drawings in the standard dwg file format, the tool NanoCAD is being used.

These tools can be used to simulate in the effects of different physics such as stress-strain and pressure, which are typical problems encountered on the design of pressure transducers, which can help to optimise devices at the design stage.

Finally, the KTP scheme can provide a platform for the incorporation of open source CAE tools into an SEM, as done in the project between the University of Brighton and PSM instrumentation for the development pressure sensors.

6 Acknowledgements

The authors wish to thank Knowledge Transfer Partnerships UK government scheme for funding the collaborative project between PSM Instrumentation Ltd and The University of Brighton (KTP 7864).

7 References

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