CoMeT Overview
CoMeT is a tool for concept design of compliant
mechanisms. The software is designed to let a user quickly sketch an idea
in a Matlab based GUI, then perform deformation, stress, optimization and
sensitivity analyses of a concept. The tool is targeted to provide a bridge
between rough back of the envelope calculations (short, order of magnitude
analysis) and detailed FEA (time consuming, accurate analysis).
With this
tool, designers can:
- Test feasibility of design concepts (without time needed to build a CAD/FEA
model, mesh, post process, etc...)
- Obtain answers within 5 - 10 % of measured values
- Rapidly converge on an optimum
- Develop their engineering intuition about compliant mechanism
The CoMeT GUI is organized into five modules:
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| Figure 1: CoMeT editor interface (user is
drawing/defining a curved beam) |
The editor is similar to a basic CAD/drawing interface that emulates
sketching by hand on paper. The user creates a CoMeT model by drawing the
compliant mechanism concept by hand (via mouse), setting material properties,
structural attributes (i.e. cross sections and taper angles) and boundary
conditions. Available beam geometries are:
+ Rectangular
+ Round
+ Elliptical
+ Curved (single radius of curvature)
+ Tapered (single or double taper)
Within the editor, the user can also create, save, and retrieve files containing the
mechanism geometry and parameters (material properties, boundary conditions,
cross sections, etc..). The
thumbnail in the corner gives an alternate view at all times, facilitating three
dimensional drawing.
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| Figure 2: CoMeT analyzer interface |
The analyzer plots the deformed response and presents analysis data.
Figure 2 shows the analyzer interface in which a
twisting moment (in the z-direction) is acting on a compliant ring. Moments/forces
can be super imposed
and plotted to view combined loading responses. Probing nodes, beams,
loads, etc. brings up all the data associated with the current object and load
state.
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| Figure 3: CoMeT motion diagnosis -
HexFlex compliant mechanism with loads (red) & fixed nodes (squares) |
The motion diagnosis tool performs analysis that may prove useful
in mechanism design. Rigid mechanisms can be characterized by a transmission ratio, which
determines the displacement reduction and force amplification from input to
output, for a conservative system. In CoMeT, "T" is the
intrinsic transmission ratio and "eta" is an efficiency
factor that quantifies a
system's ability to reject disturbances at the output for a given transmission
ratio.
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| Figure 4: CoMeT motion diagnosis - Stain
energy analysis of HexFlex compliant mechanism |
The energy stored in the elastic components of a mechanism is an indication
of which elements are most "active" and critical to system performance. In the case shown above, the short brown beam
(top, middle) stores two thirds of the strain
energy of the system when the upper beam is pushed to the left. It is also the
first beam to fail and the most critical feature of the
design.
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| Figure 5: CoMeT sensitivity study
interface |
This module can be used to determine how sensitive a design variable is to
mechanism geometry. CoMeT generates the partial derivative of the
design variable with respect to each node coordinate and beam property. Sensitivity
studies are useful in establishing manufacturing tolerances and suggesting conceptual design
changes.
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| Figure 6: CoMeT optimizer tools |
The optimizer is a powerful tool that may be used to find multivariate
optima. Figure 6 shows graphical results of a two-variable optimization study. The surface plot
shows how variable A (userZ = MaxSig or maximum stress in this example) changes with
variables B (userX) and C (userY). Through this interface, designers can graphically rendering
relationships that would be impractical to visualize/study in numerical form.
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