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Advanced Mechanisms
By D Cheshire^
Page 1 of 8
This^ tutorial^ builds^ on^ the^ experience
of^ working^ with^ mechanisms provided^ in^ the^ MECHANISMS^ tutorial.
It^ is^ essential^ that^ you^ have
completed and fully understand that tutorial before you progress on thesemore advanced functions.Two new functional areas are covered here. First the concept of usinggears and drivers to power mechanisms is covered. Secondly a novel usefor mechanism in preparing assembly instructions is demonstrated. Thewhole tutorial is based around the mechanism shown in Figure 1 which isa simple wash/wipe assembly for the rear window of a hatchback car.^ Figure 1 : Wash/Wipe MechanismMechanism Review By way of review we will go through the process
of assembling^ the mechanism.^ This^ uses^ the^ techniques
already^ shown^ in^ the MECHANISMS tutorial and so instructions will be brief. If you need to,refer back to the earlier tutorial.Create a new empty assembly file called
wipermechanism^ and assemble the mechanism in the following order using the joint properties describedin^ this^ table.^ Some^ assembled^ pairs
(marked^ assembly^ only)^ do^ not require a joint and should be assembled in the normal way. The steps witha grey background can be omitted without affecting the function of themechanism if you are short of time.
References Part for assembly^ Joint
Reference 1^ Reference 2 None^ None None Bracket.prt Mate^ face^ surface^ of^ Align washer axis A_3 towasher to face surface ofbracket axis A_8bracket Assembly only Washer.prt Axis^ alignment^ –^ Translation^ –^ base^ gearwheel^ A_2^ withwasher A_3 Pin Joint Gearwheel.prt ofgearwheel with top ofwasher Align gearshaft axis A_2 to bracket axis A_8 Assembly Only Gearshaft.prt Mate^ face^ surface^ of gearshft to face surface of bracket Align motor axisA_2^ to^ bracketaxis A_6 Assembly Only Motor.prt Mate face^ and alignsurface ofmotor axis A_5 tomotor tobracket axis A_8underside ofbracket
Advanced Mechanisms
By D Cheshire^ Axis^ alignment Pin Joint Pinnion.prt
-^ pinion^ Translation – top of pinionA_2 with motor A_2with top of motor shaft Align nut3mm axis A_2 to motor axis A_5 Assembly Only Nut3mm.prt Mate^ base^ surface^ of nut3mm^ to^ underside^ of motor bracket Align^ axle^ axis^ Assembly Only Axle.prt A_2^ to^ Mate base surface of axlebracket axis A_7to top of bracket Align nut7mm axis A_2 to bracket axis A_7 Assembly Only Nut7mm.prt Mate^ base^ surface^ of nut7mm^ to^ underside^ of motor bracket Axis^ alignment^ Pin Joint Arm.prt -^ arm^ Translation – base of axleA_10 with axle A_2with^ top^ of^ step^ on^ axleshaft
Align nut7mm axis A_2 to arm axis A_10 Assembly Only Nut7mm.prt Mate^ base^ surface^ of nut7mm to top of arm Axis alignment – longarmA_5 with gearwheel A_46 Pin Joint Longarm.prt Translation^ –^ bottom^ oflongarm with top of step ongearwheel Axis alignment – longarmA_6 with arm A_6 Create a second joint in the same partusing the button Cylinder Joint Align nut7mm axis A_2 to gearwheel axis A_46. Assembly Only Nut7mm.prt Mate base of nut7mm with step on gearwheel with 2.5 offset. Align nut7mm axis A_2 to arm axis A_6. Assembly Only Nut7mm.prt Mate base of nut7mm with top surface of arm with 2.5 offset.
Advanced Mechanisms
By D Cheshire^ Figure 3 : Servo Motor Dialog To see the motor in operation it is necessary to define an Analysis usingMECHANISM > ANALYSES and creating a New analysis. First define thegraphical display parameters. Enter the values shown in Figure 4. Alsoclick on the Motors tab and press the^ or^ button to add the motordefinition to the analysis. When this is done you should be able to pressthe RUN button at which point the motor will run and the mechanism willbe flexed through its^ full^ range^ of^ movement^ –^ you^
should^ see^ the
movement on the screen.^ Figure 4 : Analysis DefinitionAssembly Simulation One novel use of mechanisms is to simulate the steps undertaken in theprocess of assembling the components together.
To achieve this is a different set of joints would be needed than those used in the mechanismso it is necessary to save the assembly simulation as a different file from
the^ mechanism^ simulation.^ So^ to
start^ open^ the^ wipermechanism assembly and choose FILE > SAVE AS and enter a new name such as wiperassembly. You will notice that the file that is open in front of you nowis still^ wipermechanism^ so choose FILE > CLOSE WINDOW then FILE >OPEN and locate the newly created
wiperassembly^ file. This is of course currently identical to the original assembly file.Now we need to modify this file to simulate the assembly. Basically eachpart in the assembly needs to be defined by a joint which allows the part tomove along the direction it will be assembled.For example the second part in the assembly is the washer. Originally thiswas assembled without a joint at all. It now needs to be assembled with ajoint that allows the washer to move along the axis of the washer – thistype of joint is called a cylinder (you could also use a slider but that alsorestricts rotation which is not necessary). This modification (using EDITDEFINITION) is shown in Figure 5. Step 1. Delete theMATE constraintusing^. Step^ 2.^ Convertconstraint into a jointby^ clicking^ onConnections arrow.^ Figure 5 : Modifying the Washer Other parts, such as the gearwheel already have a joint defined but thejoint is the wrong type and can be changed e.g. the pin joint can bechanged to a cylinder joint).^ Figure 6 : Modifying the Gearwheel
Step 3. Hold CTRLand^ ALT^ and^ drag the^ part^ to^ itsposition^ beforeassembly. Step 1. Change thejoint^ to^ theappropriate type.
Step 2. Hold CTRLand^ ALT^ and^ dragthe^ part^ to^ itsposition^ beforeassembly.
Advanced Mechanisms
By D Cheshire^ So to prepare the current model into an assembly simulation you need togo through each part in turn, right click on the name in the feature tree andchoose EDIT DEFINITION then create or change the joint type. Finally,drag the part to a position away from the assembly – in the position whereit would start the assembly process. Close the assembly dialog and moveon to the next part. The following table describes the joints for each part.Part Name^ Modification^
Result Bracket.prt^ No Change Washer.prt^ Delete the MATE constraint.Convert to CYLINDER joint.Move to starting position. Gearwheel.prt^ Convert PIN joint to CYLINDER.Move to starting position. Gearshaft.prt^ Delete the MATE constraint.^ Convert to CYLINDER joint.^ Move to starting position Motor.prt^ Delete the MATE constraint.Convert to CYLINDER joint.Move to starting position.
Pinion.prt^ Convert PIN joint to CYLINDER.Move to starting position. Nut3mm.prt^ Delete the MATE constraint.^ Convert to CYLINDER joint.^ Move to starting position. Axle.prt^ Delete the MATE constraint.Convert to CYLINDER joint.Move to starting position. Nut7mm.prt^ Delete the MATE constraint.^ Convert to CYLINDER joint.^ Move to starting position. Arm.prt^ Convert PIN joint to CYLINDER.Move to starting position.
Advanced Mechanisms
By D Cheshire^
Page 7 of 8
The assembly should now have all the joints defined and the parts in theirstarting positions. Now we have to create an animation. You did this in theMECHANISM tutorial but we will review and introduce some new functionsnow. Creating an Animation To define an animation choose APPLICATION > ANIMATION. (You mayget a warning message about invalid servo motors because the servomotor defined earlier is no longer valid – this can be ignored) The screenwill change with the important addition of an area below the main graphicswindow. This is where the timeline editor will appear. A timeline defineswhat events happen and at what time they start/stop. But what are theseevents and how do we define them?
In simple terms^ an event^ is a mechanism position. The model can be dragged to different positions anda snapshot taken of the model in that position. Here is how…Choose the^ icon or the ANIMATION > SNAPSHOT command. TheDrag Dialog shown in Figure 6 will appear.^ Figure 6 : The Drag Dialog You can use the camera icon^ at the top of the dialog to take snapshotsof the ‘mechanism’ in its current position. Do this now to save a snapshotof the ‘mechanism’ in its starting position. This will be saved under thename Snapshot1.
We now have to create a snapshot with each component in its assembledposition. You could use the drag icon
to position each part in its assembled position but there are some more accurate functions we canuse.The^ CONSTRAINTS^ tab^ provides functions^ to^ control^ the^ position^
of components. There are functions similar to assembly functions such asMATE and ALIGN but these are temporary constraints only. Choose theMATE icon^ now and pick the top surface of the bracket and the bottomsurface of the washer. The washer should snap to the correct assembledposition. You may find that some of the other parts move as well due tothe internal relationships. You can use the drag icon
to reposition these other parts to their original position at which point you can use the cameraicon^ again to take another snapshot of the ‘mechanism’ after thewasher is assembled. This will be called Snapshot2.This basic procedure needs to be repeated for every part if the assembly.Some of the parts (the Blade) may require MATE
and/or ALIGN functions to correctly position them. Don’t forget after each position isdefined to take a snapshot. When the ‘mechanism’ is fully assembled youshould have 17 snapshots or events defined that need to be turned into ananimation. Close the Drag dialog.^ Figure 7 : Key Frame Editor
Advanced Mechanisms
By D Cheshire^ Press the^ icon or choose ANIMATION > KEY FRAME SEQUENCE.Choose NEW and the Keyframe Sequence Editor dialog appears (seeFigure 7). Type Assembly in the name field. Below Keyframe Snapshot1should be listed and time 0.000. In the dialog press
to add this Snapshot1 to the animation. Then select Snapshot2 from the list andchange the time to 1 and press^
again (The time of 1 sec could be set to the actual time that this part of the assembly takes). Repeat this processfro each of the 17 snapshots defined. Press OK. You should see theAssembly animation appearing in the timeline. You can right click on thetimescale at the bottom of the screen and choose EDIT TIME DOMAIN tochange the overall time (to 16 secs) and frame rate of the animationperiod.Now^ in^ the^ animation^ toolbar^ press
or^ ANIMATION^ >^ START^ to generate^ all^ the^ intermediate^ frames
between^ the^ snapshots.^ Once generated you can press^ or ANIMATION > PLAYBACK to play thesequence^ using^ the^ Animate^ dialog
which^ has^ controls^ like^ a^ video recorder.^ Figure 8 : Animate Dialog In the Animate dialog the CAPTURE option can be used to capture anMPEG movie from the sequence which can be replayed in Media Player.Notice that position of the mechanism and the view that you are looking atthe mechanism from is independent so you can look at the mechanismfrom any viewing position before playing the animation. It is normal tozoom in on the centre of the assembly – in this case the bracket – so thatthe parts ‘appear’ from of the screen as they are assembled.
However you may wish to change the view during the animation – this ispossible using the ANIMATION > VIEW @ TIME or
icon. First though you have to define the views that you want using the VIEW > VIEWORIENTATION > REORIENT command (use the mouse to get the viewyou want then press type in a meaningful name then press SAVE).Create two views now one called CLOSEUP which is zoomed into themotor end of the bracket and one called FAR which shows the wholeassembly. Now you have the views you need choose ANIMATION > VIEW@ TIME or^ icon. Choose CLOSEUP as the name and a time of 0 thenpress the apply button. Choose CLOSEUP again and a time of 12 thenpress the apply button. Choose FAR and a time of 14 then press the applybutton. Close the dialog. In the timeline at the bottom of the screen asecond line should have appeared with the view names. Now in theanimation toolbar press^ or ANIMATION > START to generate all theintermediate frames between the snapshots and then
or ANIMATION >
PLAYBACK to play the sequence. The animation should start with a close-up then after 12 secs zoom out. Review So what should you have learnt?^ •^ How to assemble mechanisms using different joint types.^ •^ How to create gears and motors^ •^ How to simulate the assembly process.^ •^ How to create and save animations.^ •^ How to change the view during an animation.Any problems with these? Then you should go back through the tutorial –perhaps several times – until you can complete it without any help.
