Freedom and constraint topologies

The FACT design approach was created in 2005 by Jonathan Brigham Hopkins while a Master’s student in [https://meche.mit.edu/people/faculty/mculpepp@MIT.EDU#education Professor Martin L. Culpepper]’s Precision Compliant Systems Laboratory at MIT. FACT was first published in a short conference paper in the 2006 proceedings of the 21st Annual Meeting of the American Society for Precision Engineering{{cite web |last1=Hopkins |first1=Jonathan |title=A Quantitative, Constraint-based Design Method for Multi-axis Flexure Stages for Precision Positioning and Equipment, Proc. of the 21st Annual Meeting of the American Society for Precision Engineering (ASPE), Monterey, CA, October 2006 |citeseerx=10.1.1.568.6427 |url=http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.568.6427&rep=rep1&type=pdf}} and was later published in depth in Hopkins’ 2007 Master's thesis.{{cite web |last1=Hopkins |first1=Jonathan |title=Design of Parallel Flexure Systems via Freedom and Constraint Topologies (FACT), M.S. thesis, Massachusetts Institute of Technology |url=https://dspace.mit.edu/handle/1721.1/39879 |publisher=MIT Libraries|hdl=1721.1/39879 }} FACT has been expanded in later works such as Hopkins' 2010 PhD Thesis.

{{Main|Compliant Mechanism#Design Methods}}

Other compliant mechanism design methods include generative design, pseudo-rigid-body analysis,{{cite journal |last1=Jensen |first1=Brian D. |last2=Howell |first2=Larry L. |title=Identification of Compliant Pseudo-Rigid-Body Four-Link Mechanism Configurations Resulting in Bistable Behavior |journal=Journal of Mechanical Design |date=1 December 2003 |volume=125 |issue=4 |pages=701–708 |doi=10.1115/1.1625399}} and other constraint-based and [screw theory]-based design approaches.{{cite journal |last1=Li |first1=Chenglin |last2=Chen |first2=Shih-Chi |title=Design of compliant mechanisms based on compliant building elements. Part I: Principles |journal=Precision Engineering |date=1 May 2023 |volume=81 |pages=207–220 |doi=10.1016/j.precisioneng.2023.01.006}} See the main article for pros and cons of kinematics and structural optimization.

FACT combines principles of screw theory, linear algebra, projective geometry, and exact-constraint design. The methodology employs a library of vector spaces derived from these principles and represented by geometric shapes. These shapes are categorized into freedom spaces, constraint spaces, and actuation spaces, each serving a unique purpose in the design process.

• Freedom Spaces represent the allowed deformations of a system; the system's degrees of freedom (DOF). They are modeled as twist vectors.
• Constraint Spaces guide the arrangement of flexible elements within a system to ensure it deforms only as intended. Each constraint space is complementary to a freedom space. They are modeled as wrench vectors.
• Actuation Spaces guide the arrangement, number, and kind of actuators within a flexible system so that the system deforms as desired under load. Like constraint spaces, they are modeled as wrench vectors{{cite journal |last1=Hopkins |first1=Jonathan |title=A Screw Theory Basis for Quantitative and Graphical Design Tools that Define Layout of Actuators to Minimize Parasitic Errors in Parallel Flexure Systems |journal=Precision Engineering |date=2010 |volume=34 |issue=4 |page=767-776 |doi=10.1016/j.precisioneng.2010.05.004 |url=https://www.sciencedirect.com/science/article/abs/pii/S0141635910000899|url-access=subscription }}

The FACT library allows traversal of the complete solution space of flexible systems for any combination of degrees of freedom. The rules of FACT vary depending on the configuration of the flexible system desired. Here are the basic steps to design a parallel flexure bearing.

1. Determine how the stage should move. What degrees of freedom (DOF) are needed? (Fig 1)
2. Find the matching freedom space in the FACT library (Fig 2)
3. Identify the constraint space matching the required freedom space (Fig 2)
4. Select and arrange flexible elements that satisfy the constraint space. According to Maxwell, the degrees of constraint and degrees of freedom must sum to 6 for the system to be exactly constrained{{cite book |last1=Maxwell |first1=James Clerk |last2=Nivens |first2=W. D. |title=General Considerations Concerning Scientific Apparatus in The Scientific Papers of James Clerk Maxwell |date=1890 |publisher=Dover Press}} (Fig 3)
5. Design the rigid bodies and connect each flexture to each body at their ends. When one body is held fixed, it becomes the "ground". The other body (the "stage") then attains the chosen DOF.

Sometimes it may be desirable to over-constrain the system by adding redundant constraints within the constraint space. This adds stiffness and may be required for symmetry, which can improve thermal stability.

File:An example set of four degrees of freedom (three intersecting and orthogonal rotations and on translation).jpg

File:Example complementary freedom and constraint space (i.e., 4 DOF Type 1 from the FACT library of geometric shapes).jpg

File:Selecting wire flexure elements from within a constraint space such that the resulting topology is exactly-constrained.jpg

File:Two parallel flexure system examples with the same flexible element topology but different rigid body geometries that achieve the same four degrees of freedom (DOFs).jpg

File:Freedom and constraint topologies (FACT) library poster.pdf

All flexible systems can be organized according to three primary configurations – parallel, serial, and hybrid. FACT alone covers parallel, serial, and some hybrid systems.

• Parallel systems consist of two rigid bodies connected directly together by parallel flexible elements.
• Serial systems {{cite web |last1=Hopkins |first1=Jonathan |title=Design of flexure-based motion stages for mechatronic systems via Freedom, Actuation and Constraint Topologies (FACT), Ph.D. thesis, Massachusetts Institute of Technology |url=https://dspace.mit.edu/handle/1721.1/62511 |publisher=MIT Libraries|hdl=1721.1/62511 }}{{cite journal |last1=Hopkins |first1=Jonathan |title=Synthesis of Precision Serial Flexure Systems Using Freedom and Constraint Topologies (FACT) |journal=Precision Engineering |date=October 2011 |volume=35 |issue=4 |page=638-649 |doi=10.1016/j.precisioneng.2011.04.006 |url=https://www.sciencedirect.com/science/article/abs/pii/S0141635911000857|url-access=subscription }} consist of two or more parallel systems stacked or nested in a chain from one rigid body to the next.
• Hybrid systems{{cite journal |last1=Hopkins |first1=Jonathan |title=Designing Hybrid Flexure Systems and Elements Using Freedom and Constraint Topologies |journal=Mechanical Sciences |date=October 1, 2013 |volume=4 |issue=2 |page=319-331 |doi=10.5194/ms-4-319-2013 |doi-access=free |bibcode=2013MecSc...4..319H |url=https://ms.copernicus.org/articles/4/319/2013/}} consist of any other configuration of parallel and serial system combinations.
• Interconnected hybrid systems{{cite journal |last1=Sun |first1=Frederick |title=Mobility and Constraint Analysis of Interconnected Hybrid Flexure Systems via Screw Algebra and Graph Theory |journal=Journal of Mechanisms and Robotics |date=June 2017 |volume=9 |issue=3 |page=031018 |doi=10.1115/1.4035993 |url=https://asmedigitalcollection.asme.org/mechanismsrobotics/article/9/3/031018/472714/Mobility-and-Constraint-Analysis-of-Interconnected }} are a special kind of hybrid configuration where intermediate rigid bodies are also interconnected together by flexible elements, which create internal loops within the system. FACT must be supplemented with Graph theory in order to handle such systems. Mechanical metamaterials fall in this category.{{cite journal |last1=Shaw |first1=Lucas |title=Computationally Efficient Design of Directionally Compliant Metamaterials |journal=Nature Communications |date=January 2019 |volume=10 |issue=1 |page=291 |doi=10.1038/s41467-018-08049-1|pmid=30655524 |pmc=6336888 |bibcode=2019NatCo..10..291S }}

FACT is covered in various educational resources:

• It is taught in a graduate class at UCLA by Dr. Hopkins.{{Cite web |url=https://www.flexible.seas.ucla.edu/ |title=Flexible Research Group |access-date= |website=UCLA |publisher=University of California, Los Angeles}}
• The course is available on YouTube via the channel "The FACTs of Mechanical Design" as a free lecture series.{{Cite web |url=https://www.youtube.com/TheFACTSofMechanicalDesign |title=The FACTs of Mechanical Design |access-date= |website=YouTube}}
• The book "Handbook of Compliant Mechanisms" discusses FACT within the context of compliant mechanism design.{{Cite book |last=Howell |first=Larry |title=Handbook of Compliant Mechanisms |year=2013 |publisher=John Wiley & Sons |isbn=978-1-119-95345-6}}

• Mechanism (engineering)
• Degrees of freedom
• Six degrees of freedom
• Compliant mechanism
• Engineering analysis
• Engineering design
• Kinematics
• General topology
• Axiomatic design
• Overconstrained mechanism

• {{youtube|h=theFACTsofMechanicalDesign|The FACTs of Mechanical Design}}
• [http://www.flexible.seas.ucla.edu Flexible Research Group]
• [https://samueli.ucla.edu/people/jonathan-hopkins/ UCLA: Jonathan Hopkins]

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Category:Mechanical engineering

Category:Kinematics

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