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2 INTRODUCTION TO COMET-LITE ........................................................... - 7 - 2.1 CoMeT-Lite as a design tool – Why is CoMeT-Lite needed?.............................. - 7 -
2.1.1 Human-in-the-loop compliant mechanism synthesis….. ............................... - 7 - 2.1.2 The new approach: CoMeT-Lite ........................................................................ - 7 - 2.1.3 Getting the most out of CoMeT-Lite with Tablet PCs...................................... - 8 -
2.2 CoMeT-Lite as a pedagogical tool ........................................................................ - 8 - 2.3 The nature and accuracy of CoMeT-Lite modeling ............................................. - 9 - 2.4 Organization and layout of this document .......................................................... - 9 -
3.2.1 Set units ............................................................................................................ - 11 - 3.2.2 Set material properties .................................................................................... - 12 -
3.3 Define topology, loads and constraints ............................................................. - 12 -
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1 License, registration, system requirements & contacts CoMeT-Lite (a base installation of CoMeT) was developed by the MIT Precision Systems Design and Manufacturing Laboratory (PSDAM). This user guide is an introduction to the Beta release of CoMeT-Lite 1.0. CoMeT-Lite is furnished under a license (section 1.1) and may be used only in accordance with the terms of this license.
1.1 License agreement Read this license agreement before you use CoMeT-Lite. Downloading CoMeT-Lite software constitutes an agreement on your part to abide by this license agreement. CoMeT-Lite may be used for either academic or evaluation purposes after registration (see next section) and subject to the following terms. CoMeT-Lite may not be used for commercial purposes without explicit written consent from the director of PSDAM. You agree not to reverse engineer, re-create or use any part or whole of the CoMeT-Lite code by itself or in another program, document, or in any other means of information conveyance. CoMeT-Lite comes "as is" with no guarantees, liability or warranties express or implied. PSDAM and MIT will not be liable for damages resulting from the use of CoMeT-Lite or this user guide. The information and software discussed in this document are subject to change without notice. CoMeT-Lite is intended for concept evaluation purposes only. CoMeT-Lite should not be used for development or engineering purposes.
1.2 Registration Registration is required before you use this user guide or CoMeT-Lite. Registered users may access technical support and upgrades. To register provide:
□ Full Name □ Title □ Place of business □ E-mail address
This may be done by emailing the information to: [email protected] .
This information will not be shared beyond PSDAM.
1.3 System requirements Operating system: Windows 2000 or XP recommended Software: Matlab 6, R13 required RAM: Recommended 128 Mb minimum Computer: Tablet PC recommended; CoMeT-Lite will run on laptops and desktops
1.4 Contact information Prof. Martin Culpepper
MIT Precision System Design and Manufacturing Laboratory (PSDAM: http://psdam.mit.edu/)
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2 Introduction to CoMeT-Lite 2.1 CoMeT-Lite as a design tool 2.1.1 Human-in-the-loop compliant mechanism synthesis…..
Students and practicing engineers generally design compliant mechanisms using this synthesis process:
Create geometryin CAD/FEA
t~10s of minutes
Sketch concepton papert~minutes
Define loads& constraints
t~minutes
Mesh model &analyze
t~10s of minutes
Evaluateresults
t~minutes
Finaldesign
Iteratet~10s of minutes to hours
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The manual (human-in-the-loop) and automated components within the iterative loop may take tens of minutes for simple mechanisms and hours or days for complex mechanisms. For inexperienced users, this process is exhausting and holds little pedagogical value or intellectual reward. Likewise, experienced designers would prefer to spend time creating new designs rather than running CAD/FEA. The real issue to be addressed is not the lack of a good synthesis tool which can better pair the strengths of the designer and the computer. CoMeT was created to address this issue.
2.1.2 The new approach: CoMeT-Lite
CoMeT-Lite is equipped with a GUI which allows designers to define a mechanism’s geometry, boundary conditions and loading conditions via hand sketches. This approach provides a more effective way for the designer (creative engine) and computer (analytic engine) to “converse” about the mechanism and analyze the mechanism. This is accomplished via a GUI that automatically converts the designer’s sketch into matrix equations which are “understood” and analyzed by MATLAB. The GUI converts MATLAB’s analysis back to a sketch (flexed mechanism shape) and numbers (stresses, strains and displacements) which are easily understood by the designer. As a result, the synthesis (or exploratory learning process) process can be completed in minutes rather than hours or days.
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2.1.3 Getting the most out of CoMeT-Lite with Tablet PCs
On a Tablet PC a designer may sketch on screen with a “pen”, thus better emulating pen-paper design.
Analysis & iterationDeformation
Optimize
Stress & and strain
Geometry
Sensitivity
Key boardHand sketch
Boundary conditions
Key boardHand sketch
or
or
A: CoMeT-Lite run on a Tablet PC B: CoMeT-Lite analysis procedure
Figure 1: CoMeT-Lite GUI on a Tablet PC and the CoMeT-Lite analysis procedure
2.2 CoMeT-Lite as a pedagogical tool and research tool Use in the classroom:
CoMeT-Lite was funded in-part through the Microsoft-MIT iCampus project. A main focus of this project was to develop software tools that can be used at MIT to improve learning in science and engineering. CoMeT-Lite was designed for use as:
(1) An exploratory learning tool – The sketching and analysis components enable students to rapidly explore many designs and learn about screw axes (3D), virtual centers of rotation, stress, strain and stiffness. Through exploration, students gain a working sense of compliant mechanism fundamentals.
(2) A project design tool – CoMeT-Lite can be used to enhance learning in students’ compliant mechanism projects. Students may spend more time on the project’s learning goals rather than devoting project time to learning a CAD program and/or an FEA program. This can be done without loosing sight of the important fundamentals of the analysis. CoMeT-Lite is used at MIT (2.000: How & Why Machines Work–psdam.mit.edu/2.000/start.html) by students to design compliant robotic nano-manipulators.
Team 1 Team 2 Team 3 A: 2.000 class project: x-y nano-manipulator B: Students prepare for nano-manipulator contest
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Use in compliant mechanism research:
CoMeT-Lite has been used at MIT to synthesize 1st order designs of compliant mechanisms for robotics applications and award winning Nanomanipulation equipment. Future development for educational purposes is ongoing under continued Microsoft funding through the MIT iCampus project. Development for research-based applications (modules: 2D→3D templates, plasticity, dynamics, reconfigurability) is ongoing under National Science Foundation grant 0348242, CAREER-Research and Education Plans for Modeling and Design of Fixtures and Six-Axis Manipulators for Nanomanufacturing, awarded by the NSF Nanomanufacturing program.
2.3 The nature and accuracy of CoMeT-Lite modeling CoMeT-Lite is based on linear, isotropic elasticity theory. This method is generally accurate to within 5 % of experimental results for small-moderate mechanism displacements. This level of accuracy is sufficient to identify promising designs for bench-level studies. CoMeT-Lite does not model geometric non-linearities, stress concentrations, stress stiffening or P-∆ effects. CoMeT advanced addresses such issues.
2.4 Organization and layout of this document This user guide is structured in a different manner than traditional user guides. Following this chapter, we jump into tutorial examples that provide hands-on training with the software. These tutorials are followed by a section on helpful hints and best practices, then sections covering CoMeT-Lite commands. The last section in the user guide describes the installation process for CoMeT-Lite.
Sections 3, 4 and 5 contain tutorial exercises which require that MATLAB and CoMeT-Lite be installed on your computer. Sections 3 and 4 demonstrate how to analyze flexible machine elements (bars and cantilevers). Section 5 demonstrates how to perform an analysis on a precision compliant mechanism.
It is also recommended that you browse section 7 before starting the tutorials. Section 7 covers the basics of the CoMeT GUI, commands and program capabilities.
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3.1 Starting CoMeT-Lite To start CoMeT-Lite, start Matlab, then type “cometlite” at the Matlab prompt. For more details, see section 8.
3.2 Initial settings Before creating a model, users should set the units and materials.
3.2.1 Set units
One can choose a unit system from the following systems:
□ Inches and pounds □ mm-MPa-g □ SI □ CGS □ British
Be sure to follow the shaded commands on the left for all tutorial exercises
Example commands General commands
Start a new CoMeT file In Edit mode, click on File menu and select New.
Start a new CoMeT file
In Edit mode, click on File menu and select New.
Set unit system to SI
Units
1) Click on the Other button.
2) Click on the Units button.
3) Select SI from the drop down menu.
4) Click on the Change + convert button.
Setting the units system
1) While in Edit mode, click on the Other button.
2) Click on the Units button.
3) Select the unit system you’d like to use in the drop down menu.
4) Click Change labels to change labels used in the analysis. Click Change+convert to change labels and to convert the values in the present CoMeT model to the new unit system.
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3.2.2 Set material properties
Users may assign pre-set material properties, such as Young’s Modulus, shear Modulus, Poisson’s ratio, yield stress and density by choosing appropriate material types from a material list. Custom values may be keyed in.
Example commands General commands
Setting material properties to those of steel
Materials
1) Click on the Material button.
2) Select Steel from the drop down menu.
Setting material properties
Using standard materials
1) Click on the Other button.
2) Click on the Material button.
3) Select a material from the drop down menu:
- Aluminum
- Steel
- Copper
- Cu-Be
- Silicon
Properties are shown in the text windows.
Using custom material properties
1) Click on the Other button.
2) Click on the Material button.
3) Type the appropriate values into the text boxes.
3.3 Define topology, loads and constraints In this step, users build a model for analysis. Clicking the Edit button launches the editor. By default, the Edit mode starts automatically when CoMeT-Lite is launched.
3.3.1 Create nodes
We will not be creating nodes in this example. This will be covered in section 5.3.1.
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Node
Loads
Create
MxMyMz
1) Deselect the Create button.
2) Click on the Create button.
3) Select Moment from the drop down.
4) Enter Mx = 0, My = 0, Mz = 1000.
5) To apply the load, click on the rightmost node located at [1,0].
As we are working in SI units, moments are assigned in Newton-
meters.
We now have a fully translatable (so MATLAB can analyze) mechanism sketch:
● Geometry ● Constraints ● Loads
4.3 Solve and query mechanism kinematics and mechanics Click on the Analyze button to run the analysis. This button is located in the bottom right of the CoMeT-Lite GUI, above the message box.
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In addition to the deformation plot, there are other ways to obtain results from the analysis.
4.3.1 Probe nodal displacements Example commands General commands
Checking node displacements
MainProbe node
Node
1) Click on the Main button if it is not already depressed.
2) Click on the Probe node button.
3) Click on the leftmost node to verify all displacements and
rotations are 0.
4) Click on the rightmost node to observe the displacements
and rotations. Note that the z rotation at the node is zero, the
rotations due to Fy (-2000 N) and Mz (1000 N-m) have
canceled each other out.
Checking node displacements
1) Click on the Main button.
2) Click on the Probe node button.
3) Click on a node to see how the mode was displaced.
Stress and reaction forces
Select beam
Main
Probe other
Stress and reaction forces
1) Click on the Probe other button.
2) Click on the middle of the cantilever (not a node) to activate a display which shows the reaction forces on the cantilever and stress in the cantilever.
If a stress value is not displayed, click on the Configure button in Analyze mode and check the Compute stresses check box, then repeat steps 1 and 2.
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5 Example: Four bar compliant mechanism In this example, we show you:
(1) The commands used to complete modeling steps in any analysis (2) The commands used to complete the example modeling steps [shaded in gray]. Be sure to follow the shaded directions
For your reference the following files are located within the Matlabroot\comet\ directory. (1) A file containing the completed model (fourbar-[finished].mat) (2) A video showing the completion of this example in full (fourbar.wmv)
Please see section 8.1 for the location and explanation of the Matlabroot directory. Example: Four bar compliant mechanism
Problem definition A mechanism is needed to support and guide an optical element through a rotation about its center with low parasitic errors 1. A four-bar compliant mechanism, controlled by an actuation Force F, is suggested as a solution. Maintaining a maximum stress of less than 10% of the yield stress is desired to reduce the kinematic errors due to micro-scale dislocation propagation within the steel.
In this example, you will develop the CoMeT-Lite model of a four bar compliant mechanism (see above) and perform a rapid design check to flesh out the approximate sizes and feature locations/orientations needed to meet the design requirements given in the table below.
Optical element
Base
MechanismClamp
Element center
Force
Screw axis
Link 1
Link 2
Link 3
Link 4
zx
y
Rigid plate
Elementcenter
Material Geometry [All dimension in inches] Force Design requirements
1Parasitic errors: Motions in directions other than intended motions. Our parasitic errors are in x, y, z, θx and θy
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Steel E = 30 Mpsi
σy = 100 ksi
Envelope
Instantcenter
xy
F = ±1 lbf Range: ∆θz ± 50 µradians
Parasitic errors: ∆x, ∆y < 20 µinches
5.1 Starting CoMeT-Lite To start CoMeT-Lite, start Matlab, then type “cometlite” at the Matlab prompt. For more details, see section 8.
5.2 Initial settings Before creating a model, users should set the units and materials.
5.2.1 Set units
One can choose a unit system from the following systems:
□ Inches and pounds □ mm-MPa-g □ SI □ CGS □ British
Example commands General commands
Start a new CoMeT file In Edit mode, click on File menu and select New.
Start a new CoMeT file
In Edit mode, click on File menu and select New.
Setting the unit system to inches and pounds
Units
UnitsOther
1) Click on the Other button.
2) Click on the Units button.
3) Select inches and pounds from the drop down menu.
4) Click on the Change + convert button if inches and
Setting the unit system
1) In Edit mode, click on the Other button.
2) Click on the Units button.
3) Select the unit system you’d like to use in the drop down menu.
4) Click Change labels to change labels used in the analysis. Click Change+convert to change labels and to convert the values in the present CoMeT model to the new unit system.
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5.2.2 Set material properties
Users can set material properties, such as Young’s Modulus, shear Modulus, Poisson’s ratio, yield stress and density by choosing appropriate material types from the material lists, or by entering custom values.
Example commands General commands
Setting material properties to those of steel
Steel
100000
OtherMaterial
Using custom material properties
1) Click on the Material button.
2) Select Steel from the drop down menu.
The yield stress (sigmay) displayed for the default steel properties must be changed to match our application.
3) Type 100000 in the sigmay box & hit the Enter key on your
keyboard.
Setting material properties
Setting custom material properties
1) Click on the Other button.
2) Click on the Material button.
3) Type the appropriate values into the respective text boxes.
Setting standard materials
1) Click Other in the object buttons.
2) Click on the Material button.
3) Select a material from the drop down menu:
- Aluminum
- Steel
- Copper
- Cu-Be
- Silicon
4) Properties are shown in the text windows.
5.3 Define topology, loads and constraints In this step, users build a model for analysis. Clicking the Edit button launches the editor. By default, CoMeT-Lite enters the Edit mode when it is launched. You have been setting material and unit system properties in the Edit mode.
5.3.1 Create nodes
One can create, move and delete nodes. Nodes may be used to define:
(1) The start and endpoint of beams
(2) The vertices of rigid plates
(3) As construction geometry
Here we will create nodes which have two functions:
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4) Enter x = 2, y = 2, z = 0.
5) Press the Update View button
Note: this process sets the x-y sketch plane at z = 0 and
defines the view center at x = y = 2.
Create nodes to outline the mechanism envelope and instant center position
Create
Nodes
~ (0,0)
~ (3.15, 2.25)
~ (3.15, 0)
~ (0, 2.25)
~ (1.57,1.61)
1) Click on the Node button.
2) Click Create button.
3) Click near four points to define approximate vertices of the mechanism’s envelope: (0,0) (0, 2.25) (3.15, 2.25) (3.15, 0). Vertices (0, 2.25) and (3.15, 2.25) are only used as guides (e.g. construction geometry) to help outline the size of the device. We will later remove the vertices before the mechanism is analyzed.
4) Click near (1.57,1.61) to place a node near the desired instant center of rotation.
There is no need to be hyper-accurate in locating these nodes as we are seeking a 1st order analysis. You can view node locations and move them if they are off the intended position by say more than ~ 0.1 inch.
1) Click on the Move button.
2) Click on the node to be moved, and then click on the new location.
Create nodes
1) Click on the Node button.
2) Click Create button.
3) Indicate the positions of nodes by clicking desired points in the main plot view.
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5.3.2 Create beam elements
Example commands General commands
Sketch beams
Create
Beams
1st 3rd
Instant center
2nd
1) Click on the Beams button.
2) Click on the Create button.
3) Select Rectangular (0) from the drop down menu.
4) Set theta x =0, w = 0.1, h = 0.5
As we are working in inches-pounds, the w and h values are assigned in inches.
5) Read the entirety of step 5 before proceeding:
Draw the first link/beam by clicking on the node near (0,0) and then click to end the 1st beam as shown in the figure above. This beam should terminate at about y = 0.97 inches (see figure below for 0.97 dimension) and a line along this beam should intersect the desired instant center (see figure above for example).
6) Read the entirety of step 6 before proceeding
Draw the third link/beam by clicking on the node near (3.15,0) and then click to end the beam near the endpoint of the 3rd beam as shown in the figure above. This beam should terminate at about y = 0.97 inches (see figure below for 0.97 dimension) and a line along this beam should intersect the desired instant center (see figure above for example).
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5.3.4 Create plates
Example commands General commands
Adding plates
Create
Plate
Nodes selected
1) Click on the Plates button.
2) Click on the Create button.
3) Select the three nodes that will form the vertices of the rigid plate.
Note: Depending on how you sketched your mechanism, the nodes you select may not be nodes 5, 6 or 7. It is sufficient to select the nodes indicated in the preceding figure.
Adding plates
1) Click on the Plates button.
2) Click on the Create button.
3) Select the three nodes that will form the vertices of the rigid plate. You may remove nodes from the plate by selecting the appropriate button.
5.3.5 Define loads
Example commands General commands
Applying loads
Create
Loads
Forceparameters
1st
2nd
Applying loads
1) Click on the Loads button.
2) Click on the Create button.
3) Select Force or Moment from the
drop down menu. 4) Type the magnitude of the desired load in the text box.
5) Click on the node to which the load should be applied.
The head of the vector representing the load is always represented in the sketch window as resting on the node it is acting upon.
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1) Click on the Loads button.
2) Click on the Create button.
3) Select Force from the drop down menu. 4) Enter Fx = 1, Fy = 0, Fz = 0.
As we are working in inches-pounds units, forces are assigned
in pounds.
5) To apply the load, click on the node between the 1st and 2nd
links/beams.
5.3.6 Add centroids
Centroids are added to the vertex of a plate/rigid body to tell CoMeT to analyze the screw kinematics of the plate/rigid body.
Example commands General commands
Adding centroids
Create
Centroids
1) Click on the Centroids button.
2) Click on the Create button.
3) Click on the apex of the rigid plate, e.g. the instant center.
A pink triangle is added to indicate the location of the centroid.
Adding centroids
1) Click on the Centroids button.
2) Click on the Create button.
3) Click on one or more nodes for
which you want to monitor the screw
kinematics.
A pink triangle is added to indicate the location a new centroid.
5.3.7 Remove construction geometry
Before we can have a translatable (so MATLAB can analyze) mechanism sketch, we must remove the “free floating nodes” that were used to define the top of the mechanism’s envelope. These nodes must be deleted before we analyze the model.
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5.4 Solve and query mechanism kinematics and mechanics Click on the Analyze button to run the analysis. This button is located in the bottom right of the CoMeT-Lite GUI, above the message box.
5.4.1 Probe nodal displacements Example commands General commands
Optional: Scaling the amplification factor You may wish to scale the ampfactor by moving the slider, then clicking on the Plot button to update the display.
SliderMain
Plot
Checking node displacements
Probenode
Main
39.8 µradians
1) Click on the Probe node button.
2) Click on node at the apex of the plate (see figure above).
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Our “first guess” was pretty close, the parasitic errors in x and y
are less than 2 micro-inches. Unfortunately, the range is 10
micro radians too low. To fix this, we will re-enter Edit mode and
change the cross sections of the beams to reduce the rotation
stiffness of the mechanism.
Modify beam parameters 1) Click on the Edit button to re-enter Edit mode.
Modify
Beams
w
UpdateLast 2) Click on the Beams button.
3) Click on the Modify button.
4) Click on the 1st link/beam.
5) Change the width, w, of the beam from 0.1 to 0.09 inches.
6) Click on the Update Last button at the bottom of the screen.
7) Click on the 3rd link/beam.
8) Change the width, w, of the beam from 0.1 to 0.09 inches.
9) Click on the Update Last button at the bottom of the screen.
Modify beam parameters 1) Click on the Edit button to re-enter
Edit mode.
2) Click on the Beams button.
3) Click on the Modify button.
4) Click on the beam you wish to
modify.
5) Change the beam parameters.
6) Click on the Update Last button at
the bottom of the screen.
Re-evaluate node displacements 1) Click on the Analyze button to re-enter the analysis mode. You may wish to scale the ampfactor by moving the slider and then clicking the Plot button to update the display.
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Probenode
Main
54.6 µradians
2) Click on the Probe node button.
3) Click on node at the apex of the plate (see figure above).
From this information, we may make three important
observations:
A. The parasitic errors in x and y are less than 2 micro-inches.
B. The half range is now larger than the 50 µradian requirement.
C. The performance of the mechanism is very sensitive to beam
thickness (we only changed it by 0.010 inches) and achieved
nearly a 25% change in performance. Tolerances on this
dimension will be key.
Checking stresses
Probeother
Main
Stress
1) Click on the Probe other button.
2) Click on the middle of the links/beams (not on a node) to
query the reaction forces on the links/beams and maximum
Stress and reaction forces
1) Click on the Probe other button
2) Click on the middle of the link/beam (not a node) to activate a display which shows the reaction forces on the link/beam and stress in the link/beam.
If a stress value is not displayed, click on the Configure button in Analyze mode and check the Compute stresses check box, then repeat steps 1 and 2.
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stress within the links/beams.
3) The stress in each beam is less than 10% of the yield stress
of 100 000 psi.
Checking screw theory kinematics
Probe otherMain
Position & orientation
1) Make sure the Probe other button is still selected.
2) Click on the screw axis symbol, represented by an .
The x-y position of the screw axis (instant center) is close
enough to the desired position to prove that this mechanism
concept is a reasonable approach to pursue. We could continue
to improve the accuracy of the analysis through iteration.
Checking screw theory kinematics
1) Click on the Probe other button.
2) Click on the screw axis,
represented by an .
The position and orientation of the screw axis are displayed.
Optional: Save your work 1) Click on File menu, select Save as and provide a file name.
Saving your work
1) Click on File menu, select Save as and provide a file name.
- End of example -
This tutorial exercise has shown how CoMeT-Lite can be use to validate mechanism concepts. At this point we end the tutorial, however the reader should realize that the design engineer would likely continue the analysis by improving the node position accuracy, analyzing the stiffness and analyzing the load capacity of the flexure. A reasonable next step would be to re-enter Edit mode and reposition the nodes to be closer to their intended positions, then perform another analysis.
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6 Additional Information 6.1 Node numbering CoMeT-Lite renumbers nodes during analysis, making the grounded nodes the higher number nodes. As a result, Node 1 may no longer be Node 1 after switching into Analyze mode. This renumbering process will be removed from the program in the next release of the software.
6.2 Help functions CoMeT-Lite provides helpful hints and instructions embedded in the GUI. These include:
- Messages posted to the message box in the lower right corner
- Mouse hover messages that appear if the mouse cursor remains over a button, field, or field
label for one second
6.3 Model constraint and topology A few things are not immediately apparent and should be noted below:
- A fully formed or “translatable” model that can be analyzed must contain AT LEAST
a. One beam
b. One grounded node
c. One load
- There are some “topological” requirements:
a. Structure must not be under constrained. Every node must be connected, directly or
indirectly, to ground.
b. Grounded nodes should not be loaded or assigned a centroid.
6.4 Matlab – CoMeT-LITE compatibility CoMeT-Lite has been designed in and tested with Matlab Release 13, version 5.1 (service pack 1). Based upon our experience, it is likely that CoMeT-Lite will work with version 6.5.0.
6.5 Font sizes in CoMeT-Lite In the full version of CoMeT, users may set font sizes and colors. This capability has been restricted in CoMeT-Lite to simplify the installation procedure. If you should find the fonts too small or large, contact Prof. Culpepper to obtain a customized copy of CoMeT-Lite with the proper font sizing for your screen resolution.
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7 The CoMeT-Lite GUI The GUI is broken into the following sections: □ Main menu bar □ Mode options □ Option tools □ Thumb nail view □ Sketch window □ Mode buttons □ Message window
Main menu bar
Mode options
Option tools
Thumb-nail view
Sketch window
Mode buttons
Message window
Figure 3: CoMeT-Lite Graphical User Interface (GUI)
7.1 Menu bar General CoMeT-Lite commands may be issued via menus in the menu bar.
File menu Command Description New Make a new CoMeT file
Load Open a file
Save As Save a file as another file name
Save Save a file
Set Path Set the directory where files are saved by default
Print Print the current window
Exit CoMeT Exit CoMeT-Lite and close the window
View menu Command Description Standard Choose a standard view:
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Auto Scale Adjust the view to match the current mechanism size Done Return to main CoMeT-Lite GUI
Preference Menu Command Description Change Material
Change material properties
Change Units
Change unit system
Color scheme
Set CoMeT-Lite GUI color scheme
Selecting Disabled in CoMeT-Lite Save Current Preferences
Save user preferences to CoMeTPrefs.mat.
Reset Preferences to Default
Reset preferences to the original installed default values
Help Menu Command Description CoMeT help CoMeT-Lite help, not available in beta release
About CoMeT
Display information about CoMeT-Lite
7.2 Message box
The message box contains the last fifty helpful hints, directions and alerts.
7.3 Mode menu
The mode menu is used to switch between the Edit mode and the Analyze mode. Comet Advanced has additional modes which are listed in sections 7.3.3-7.3.5. New modes currently under development are listed in sections 7.3.6-7.3.10.
7.3.1 Edit mode
Edit mode is used to create the geometry of a mechanism, specify loads, specify boundary conditions, assign units and assign material properties.
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Options used to create, modify and delete model components are shown in the following figure.
Figure 4: Commands available in Edit mode
The Mode options and corresponding parameters are summarized in the following table. Note: The Create or Modify buttons must be selected in each mode to access the parameters.
Edit mode options and parameters Option Parameters Description
Beam
CoMeT-Lite models four types of beam elements: - Rectangular cross-section - Circular cross-section - Tapered cross-section and length - Curved Different properties are required according to beam type. The theta x option window sets the beam rotation about its x axis (axis defined along a vector from a beam starting point to an ending point). Beams can be created between nodes or by clicking on a start and end point.
Nodes
Nodes are points in space. In CoMeT-Lite, one can define the position of a node by clicking a mouse or by keying in the position. CoMeT-Lite only allows nodes that are: - Free to move in all 6 DOF (Blue dots) - Coupled between beams (Blue dots between beams) - Grounded to constrain all 6 DOF (Black squares)
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Load
Two load types may be applied in CoMeT-Lite: - Forces [Fx, Fy, Fz ] - Moment [Μx, Μy, Μz] In the Edit mode, one can define the magnitude and direction of loads. Loads may only be applied at nodes.
Centroid
A centroid may be assigned to a node to indicate that the user wants CoMeT-Lite to determine the screw kinematics (e.g. instant center) of this point when it analyzes the mechanism. See section 5.3.6 for demonstration and more explanation.
Plate
Users can create plates by picking three or more nodes in the main plot view. Plates are infinitely rigid bodies contained within a compliant mechanism. See section 5.3.4 for demonstration and more explanation.
7.3.2 Analyze mode
The Analyze mode is used to trigger the MATLAB analysis of the mechanism and to study qualitative and quantitative results.
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Main option tools Parameters Description
Configure
With the Configure tool, one can set the scale of motion/displacement scaling. This scaling, called ampfactor, is useful for mechanisms where the displacement/motion under load is too small to see at a 1:1 scale. For instance, the ampfactor is set at 8800 in the illustration at left. The beam width is used to define the thickness of the beam grphic displayed in the window. This does not change the geometry of the beam, it only changes the graphics display of the beam.
Probe node
Probe nodes by clicking on them. When a node has been selected, you may view how the node has displaced.
Probe other: Beam probe
The following information can be obtained by probing a beam with the Probe other tool: (1) Reaction forces (2) Reaction moments (3) Stress
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Probe other: Centroid probe ( )
The following information can be obtained by probing a centroid with the Probe other tool: (1) Centroid displacements (2) Relate actuator forces to centroid motions
FA = Vector of actuator forces applied to a mechanism XC = Vector of centroid displacements/rotations
AFC FSX = and ACFinv FXS =
(3) Relate actuator displacement inputs to centroid motions XA = Vector of actuator displacements applied to a mechanismXC = Vector of centroid displacements/rotations
AXC XSX = and ACXinv XXS = Note after clicking on a matrix call button (e.g. the SF button in the illustration at left), the matrix appears at the MATLAB command prompt within the MATLAB GUI, not within the CoMeT-Lite GUI. We are working on a more straight forward means to provide this data within the CoMeT-Lite GUI.
Probe other: Screw axis ( )
The following information can be obtained by probing an instant screw axis or instant center with the Probe other tool:
(1) Location of the instant screw axis/instant center
(2) Orientation of the instant screw axis/instant center In 2D, the instant screw axis is seen as an instant center.
7.3.3 Sensitivity mode
This module is available in advanced CoMeT only. While using this module, one may determine sensitivity of mechanism performance to variations in mechanism geometry and loading. This module is designed primarily for use in tolerance analysis.
7.3.4 Optimization mode
This module is available in advanced CoMeT only. The module is used to optimize various mechanism attributes (mass, deflection, stress, etc…) via multiple input/criteria optimization.
7.3.5 Diagnosis mode
This module is available in advanced CoMeT only. This module is designed to analyze the components of a mechanism and provide design feedback which diagnosis which parts of the mechanism are most and least useful in achieving a desired kinematic goal.
7.3.6 Plasticity
This module is presently under development. The module is being designed to analyze the kinematic and mechanics behavior of mechanisms for which parts of the mechanism must be plastically deformed. For instance, this module may be used to design the compliant mechanisms and flexures that are plastically deformed to obtain alignment. Practical examples may be found in plastic deformation of hinged flexures used in fiber optic alignment and laser cavity alignment.
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7.3.7 Folded kinematics of 3D compliant mechanisms
This module is under development. A major focus of PSDAM research is to develop synthesis tools, manufacturing processes and best practices/rules of thumb that enable the formation of monolithic 3D compliant mechanisms from 2D sheets of material. This module will determine the kinematics of the 3D mechanism and the shape of the 2D template which can be formed or folded (ala origami) to make a 3D mechanism.
7.3.8 Thermal expansion
This module is under development. This module is designed for use in precision mechanism design where the thermal expansion of a compliant mechanism may cause unwanted errors in its kinematic behavior.
7.3.9 Dynamic response
This module is under development. The module is designed to predict the natural frequencies and mode shapes for the 1st, 2nd and 3rd modes. The module is target for use with spatial (6 axis) compliant mechanisms.
7.3.10 Reconfigurability
This module is under development. The module is designed to identify components of the mechanism that may be actively actuated/controlled and thereby actively modify the behavior of the mechanism.
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8 CoMeT-Lite installation CoMeT-Lite Beta does not have an automated installation. Fortunately manual setup is simple and requires less than 10 minutes. The reader should complete the directions exactly as they are laid out. We recommend that you close all programs and log on as Administrator. Installation consists of four steps:
1. Download CoMeT-Lite files
2. Unzip CoMeT-Lite files
3. Create a directory which will house the CoMeT-Lite files
4. Copy CoMeT-Lite files into the directory created in step 3
8.1 Installing CoMeT-Lite MATLAB must be installed before you install CoMeT-Lite. MATLAB should be closed down
during the installation.
Step 1: Download CoMeT-Lite files
Open a web browser and go to http://compliantmechanisms.org. The most recent download and upgrades may be found by clicking on the Design Tools link. Download the file CoMeT-Lite_X.Y.zip to your computer (X and Y represent version numbers), making sure to keep note of the location to which you are saving the file.
Step 2: Unzip CoMeT-Lite files
You will need an unzipping program such as Winzip (winzip.com) to unzip the CoMeT-Lite files from the zip file you downloaded. Unzip the files, keeping note of the location to which you extracted the files. You will need to copy these files in the next step.
Step 3: Create a directory to contain CoMeT-Lite files
Create a directory called “comet” within the MATLAB program or Matlabroot directory. For example, if MATLAB is installed in c:\software\matlab\, then the Matlabroot directory is c:\software\matlab.
Step 4: Copy CoMeT-Lite files
Copy all CoMeT-Lite files and folders you unzipped in step 2 to the comet directory created in step 3.
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8.2 Launching CoMeT-Lite 1. Start MATLAB
2. Change the working directory to the directory where the CoMeT-lite files were copied
3. Type “cometlite” at the MATLAB command prompt
A new window showing the CoMeT-Lite GUI will appear. You may then start using CoMeT.
8.3 Troubleshooting We recommend the following after installation:
1) Reboot the computer
2) Log on as Administrator and launch CoMeT-Lite, verify the CoMeT-Lite software runs properly
3) Log off the Administrator account
4) Log on with your user account and verify the CoMeT-Lite works properly
If CoMeT-Lite works when you are logged on as Administrator and does not work properly when you are logged on with another user account, you may need to set the security permissions on the Matlabroot directory and the comet directory to allow your user account to read/write to these folders. See windows help for instructions on how to do this.
Should you have other types of problems with this installation, please erase the installation, reboot the computer and retry the installation. If you are unable to complete the installation, email Prof. Culpepper ([email protected]) with a detailed description of the problem/error messages.