motion simulation solid work

What is motion simulation? What problems  can it solve? How can it benefit the product  design  process? This paper  addresses some of these  issues  and looks at sample  problems  that motion simulation can solve. It also presents real-life applications  of motion simulation used as a CAE design  tool.

introduction

Motion simulation provides com- plete, quantitative information about the kinematics—including position, velocity, and acceleration, and the dynamics—including joint reactions,

inertial forces, and power require- ments, of all the components  of a moving mechanism.

Since the 1980s, when computer-aided  engineering (CAE) methods first became  available in design engineering, finite element analysis (FEA) became the first widely adopted  simulation tool. Over the years, it has helped design engineers  study the structural performance of new products, and replace many time-consuming, costly prototypes with inexpensive computer simulations run on CAD models.

Today, because of the growing complexity of mechanical products and increas- ingly fierce competition to bring new designs to market faster, engineers  feel mounting pressure  to extend the scope of simulation beyond FEA. Along with simulating structural performance with FEA, engineers  also need to determine the kinematics and dynamics of new products before the building of physical prototypes.

Motion simulation—also known as rigid body dynamics—offers a simulation approach to solving those issues. Its use is growing fast, and as it does, design engineers  want to know more about it, asking: What it is? What problems can

it solve? How can it benefit the product design process?

Motion simulation for mechanism analysis and  synthesis

Suppose  an engineer is designing an elliptic trammel meant for tracing differ- ent ellipses. When he has defined mates in the CAD assembly, he can animate the model to review how the components  of the mechanism move (Figure 1). Although assembly animation can show the relative motion of assembly compo- nents, the speed  of motion is irrelevant and timing is arbitrary. To find velocities, accelerations,  joint reactions, power requirements, etc., the designer needs  a more powerful tool. This is where motion simulation comes in.

FigURe  1: VArIoUS  poSITIonS  of  EllIpTIC  TrAMMEl  SIMUlATEd  USIng

CAd  AnIMATor

Motion simulation provides complete, quantitative information about the kinematics—including position, velocity, and acceleration, and the dynamics— including joint reactions, inertial forces, and power requirements, of all the components  of a moving mechanism. Often of great additional importance,

the results of motion simulation can be obtained virtually at no additional time expense, because everything needed  to perform motion simulation has been defined in the CAD assembly model already, and just needs  to be transferred  to the motion simulation program.

The motion simulation program uses material properties from the CAD

parts to define inertial properties

of the mechanism components, and translates CAD assembly mating

conditions into kinematic joints.

In the case  of the elliptic trammel described  above, the designer needs  only to decide the speed  of the motor, the points to be traced, and the motion results he wishes to see. The program does everything else automatically, without the user’s intervention. The motion simulation program uses  material properties

from the CAD parts to define inertial properties of mechanism components, and translates  CAD assembly mating conditions into kinematic joints. It then auto- matically formulates equations that describe the mechanism motion.

Unlike flexible structures  studied with FEA, mechanisms  are represented as assemblies  of rigid components  and have few degrees of freedom. A numeri- cal solver solves the equations of motion very quickly, and results include full information about displacements,  velocities, accelerations,  joint reactions, and inertial loads of all the mechanism components, as well as the power necessary to sustain the motion (Figure 2).

FigURe  2: lInEAr  VEloCITy  And  MoTor  poWEr  rEqUIrEMEnT  CAlCUlATEd by MoTIon  SIMUlATor

A simulation of the motion of the inverted slider mechanism shown in

Figure 3 presents an exercise commonly found in textbooks on the kinematics of machines. Here, the objective is to find the angular speed  and the acceler- ation of the rocking arm, while the crank rotates at a constant  speed. Several analytical methods can solve the problem, and the complex numbers method

is perhaps  the most frequently used by students. However, solving such a problem “by hand” requires intensive calculations, and even with the help of computerized spreadsheets, it may take a few hours to construct velocity

and acceleration plots. Then, if the geometry of the slider changes, the whole thing has to be repeated—making this an interesting assignment  for under- graduate  students  but completely impractical in real life product development. Motion simulation software makes it possible to simulate the motion of the inverted slider practically instantly, using data already present in the CAD assembly model.

Motion simulation conducts inter- ference  checks  in real time, and provides the exact spatial and time positions of all mechanism compo- nents as well as the exact interfer- ing volumes.

FigURe  3: SIMUlATIon  of  An  InVErTEd  SlIdEr  MECHAnISM  To  CAlCUlATE AngUlAr  VEloCITy  of  roCkIng  ArM

Motion simulation also checks  for interferences,  and this is a very different pro- cess  from the interference  checking available with CAD assembly animation. Motion simulation conducts interference  checks  in real time, and provides the exact spatial and time positions of all mechanism components  as well as the exact interfering volumes. Even more, when the geometry changes, as shown in the quick return mechanism in Figure 4, the software updates  all results in sec- onds. Each and every result pertaining to motion may be presented graphically or tabulated in any desired format.

FigURe  4: USErS  CAn  EASIly  dETECT  And  CorrECT  InTErfErEnCE  bETWEEn

SlIdEr  And  drIVEn  lInk.

Engineers can represent  simple mechanisms  such as the elliptic trammel or inverted slider described  above as 2D mechanisms. Although these  are dif- ficult and time-consuming to analyze by hand, they do posses analytical solu- tion methods. However, 3D mechanisms, even simple mechanisms  such as that shown in Figure 5, have no established  method of analytical solution.

But motion simulation can solve the problem easily in seconds, because it is designed  to handle mechanisms  of any and every complexity, both 2D and 3D. The mechanism may contain a large number of rigid links, springs, dampers, and contact pairs with virtually no penalty in solution time. For example, the motions of the front-end suspension of the snowmobile in Figure 6, exercise machine in Figure 7, or CD drive in Figure 8, may be simulated with the same ease  as that of the inverted slider.

In addition to mechanism analysis, product developers can also use

motion simulation for mechanism synthesis by converting trajectories of motion into CAD geometry.

FigURe  5: A SIMplE  3d MECHAnISM  IS  VEry  dIffICUlT  To  AnAlyzE  “by  HAnd”

bUT  prESEnTS  no  problEMS  for  MoTIon  SIMUlATIon.

FigURe  6: A fronT-End  SUSpEnSIon  of  A SnoWMobIlE  ConSISTS  of nUMEroUS  lInkS  InClUdIng  SprIngS  And  dAMpErS.

FigURe  7: An  ExErCISE  MACHInE  dESIgn  bEnEfITS  froM  MoTIon  SIMUlATIon

USEd  To  opTIMIzE  THE STEpS’  MoTIon  TrAjECTorIES  And  CAlCUlATE  THE

poWEr  gEnErATEd  by THE USEr.

FigURe  8: A Cd drIVE  IS  A CoMplEx  MECHAnISM,  yET EASIly  AnAlyzEd  by

MoTIon  SIMUlATIon.

In addition to mechanism analysis, product developers can also use motion simulation for mechanism synthesis by converting trajectories of motion into CAD geometry, and using it to create  a new part geometry. Figure 9 shows a sample problem. This design features  a cam that should move a slider along

a guide rail, and uses  motion simulation to generate a profile of that cam. The user expresses the desired slider position as a function of time and traces  the slider movement on the rotating blank cam (the round plate). Then he converts the trace path into CAD geometry to create  the cam profile shown in Figure 10.

Designers can also use trajectories of motion to verify the motion of an industrial robot.

FigURe  9: A dISplACEMEnT  fUnCTIon  IS ApplIEd  To  MAkE THE  SlIdEr  TrAVEl

Along  THE gUIdE  rAIl.

FigURe 10: TrAVEl of THE SlIdEr IS TrACEd on THE roTATIng roUnd plATE To

CrEATE A CAM profIlE, IllUSTrATEd HErE WITH A grooVE CUT In THE plATE.

Designers can also use trajectories of motion, for example, to verify the motion of an industrial robot, such as that shown in Figure 11, and test the toolpath to obtain information necessary when selecting the size of robot needed, and to establish power requirements—all without the need for any physical tests.

FigURe  11: SIMUlATEd  MoVEMEnT  of  An  IndUSTrIAl  roboT  THroUgH  SEV- ErAl  poSITIonS  MAkES IT poSSIblE  To  CrEATE  A ToolpATH  WITHoUT  Any pHySICAl  TESTS.

What is motion simulation? What problems  can it solve? How can it benefit the product  design  process? This paper  addresses some of these  issues  and looks at sample  problems  that motion simulation can solve. It also presents real-life applications  of motion simulation used as a CAE design  tool.

introduction

Motion simulation provides com- plete, quantitative information about the kinematics—including position, velocity, and acceleration, and the dynamics—including joint reactions,

inertial forces, and power require- ments, of all the components  of a moving mechanism.

Since the 1980s, when computer-aided  engineering (CAE) methods first became  available in design engineering, finite element analysis (FEA) became the first widely adopted  simulation tool. Over the years, it has helped design engineers  study the structural performance of new products, and replace many time-consuming, costly prototypes with inexpensive computer simulations run on CAD models.

Today, because of the growing complexity of mechanical products and increas- ingly fierce competition to bring new designs to market faster, engineers  feel mounting pressure  to extend the scope of simulation beyond FEA. Along with simulating structural performance with FEA, engineers  also need to determine the kinematics and dynamics of new products before the building of physical prototypes.

Motion simulation—also known as rigid body dynamics—offers a simulation approach to solving those issues. Its use is growing fast, and as it does, design engineers  want to know more about it, asking: What it is? What problems can

it solve? How can it benefit the product design process?

Motion simulation for mechanism analysis and  synthesis

Suppose  an engineer is designing an elliptic trammel meant for tracing differ- ent ellipses. When he has defined mates in the CAD assembly, he can animate the model to review how the components  of the mechanism move (Figure 1). Although assembly animation can show the relative motion of assembly compo- nents, the speed  of motion is irrelevant and timing is arbitrary. To find velocities, accelerations,  joint reactions, power requirements, etc., the designer needs  a more powerful tool. This is where motion simulation comes in.

FigURe  1: VArIoUS  poSITIonS  of  EllIpTIC  TrAMMEl  SIMUlATEd  USIng

CAd  AnIMATor

Motion simulation provides complete, quantitative information about the kinematics—including position, velocity, and acceleration, and the dynamics— including joint reactions, inertial forces, and power requirements, of all the components  of a moving mechanism. Often of great additional importance,

the results of motion simulation can be obtained virtually at no additional time expense, because everything needed  to perform motion simulation has been defined in the CAD assembly model already, and just needs  to be transferred  to the motion simulation program.

The motion simulation program uses material properties from the CAD

parts to define inertial properties

of the mechanism components, and translates CAD assembly mating

conditions into kinematic joints.

In the case  of the elliptic trammel described  above, the designer needs  only to decide the speed  of the motor, the points to be traced, and the motion results he wishes to see. The program does everything else automatically, without the user’s intervention. The motion simulation program uses  material properties

from the CAD parts to define inertial properties of mechanism components, and translates  CAD assembly mating conditions into kinematic joints. It then auto- matically formulates equations that describe the mechanism motion.

Unlike flexible structures  studied with FEA, mechanisms  are represented as assemblies  of rigid components  and have few degrees of freedom. A numeri- cal solver solves the equations of motion very quickly, and results include full information about displacements,  velocities, accelerations,  joint reactions, and inertial loads of all the mechanism components, as well as the power necessary to sustain the motion (Figure 2).

FigURe  2: lInEAr  VEloCITy  And  MoTor  poWEr  rEqUIrEMEnT  CAlCUlATEd by MoTIon  SIMUlATor

A simulation of the motion of the inverted slider mechanism shown in

Figure 3 presents an exercise commonly found in textbooks on the kinematics of machines. Here, the objective is to find the angular speed  and the acceler- ation of the rocking arm, while the crank rotates at a constant  speed. Several analytical methods can solve the problem, and the complex numbers method

is perhaps  the most frequently used by students. However, solving such a problem “by hand” requires intensive calculations, and even with the help of computerized spreadsheets, it may take a few hours to construct velocity

and acceleration plots. Then, if the geometry of the slider changes, the whole thing has to be repeated—making this an interesting assignment  for under- graduate  students  but completely impractical in real life product development. Motion simulation software makes it possible to simulate the motion of the inverted slider practically instantly, using data already present in the CAD assembly model.

Motion simulation conducts inter- ference  checks  in real time, and provides the exact spatial and time positions of all mechanism compo- nents as well as the exact interfer- ing volumes.

FigURe  3: SIMUlATIon  of  An  InVErTEd  SlIdEr  MECHAnISM  To  CAlCUlATE AngUlAr  VEloCITy  of  roCkIng  ArM

Motion simulation also checks  for interferences,  and this is a very different pro- cess  from the interference  checking available with CAD assembly animation. Motion simulation conducts interference  checks  in real time, and provides the exact spatial and time positions of all mechanism components  as well as the exact interfering volumes. Even more, when the geometry changes, as shown in the quick return mechanism in Figure 4, the software updates  all results in sec- onds. Each and every result pertaining to motion may be presented graphically or tabulated in any desired format.

FigURe  4: USErS  CAn  EASIly  dETECT  And  CorrECT  InTErfErEnCE  bETWEEn

SlIdEr  And  drIVEn  lInk.

Engineers can represent  simple mechanisms  such as the elliptic trammel or inverted slider described  above as 2D mechanisms. Although these  are dif- ficult and time-consuming to analyze by hand, they do posses analytical solu- tion methods. However, 3D mechanisms, even simple mechanisms  such as that shown in Figure 5, have no established  method of analytical solution.

But motion simulation can solve the problem easily in seconds, because it is designed  to handle mechanisms  of any and every complexity, both 2D and 3D. The mechanism may contain a large number of rigid links, springs, dampers, and contact pairs with virtually no penalty in solution time. For example, the motions of the front-end suspension of the snowmobile in Figure 6, exercise machine in Figure 7, or CD drive in Figure 8, may be simulated with the same ease  as that of the inverted slider.

In addition to mechanism analysis, product developers can also use

motion simulation for mechanism synthesis by converting trajectories of motion into CAD geometry.

FigURe  5: A SIMplE  3d MECHAnISM  IS  VEry  dIffICUlT  To  AnAlyzE  “by  HAnd”

bUT  prESEnTS  no  problEMS  for  MoTIon  SIMUlATIon.

FigURe  6: A fronT-End  SUSpEnSIon  of  A SnoWMobIlE  ConSISTS  of nUMEroUS  lInkS  InClUdIng  SprIngS  And  dAMpErS.

FigURe  7: An  ExErCISE  MACHInE  dESIgn  bEnEfITS  froM  MoTIon  SIMUlATIon

USEd  To  opTIMIzE  THE STEpS’  MoTIon  TrAjECTorIES  And  CAlCUlATE  THE

poWEr  gEnErATEd  by THE USEr.

FigURe  8: A Cd drIVE  IS  A CoMplEx  MECHAnISM,  yET EASIly  AnAlyzEd  by

MoTIon  SIMUlATIon.

In addition to mechanism analysis, product developers can also use motion simulation for mechanism synthesis by converting trajectories of motion into CAD geometry, and using it to create  a new part geometry. Figure 9 shows a sample problem. This design features  a cam that should move a slider along

a guide rail, and uses  motion simulation to generate a profile of that cam. The user expresses the desired slider position as a function of time and traces  the slider movement on the rotating blank cam (the round plate). Then he converts the trace path into CAD geometry to create  the cam profile shown in Figure 10.

Designers can also use trajectories of motion to verify the motion of an industrial robot.

FigURe  9: A dISplACEMEnT  fUnCTIon  IS ApplIEd  To  MAkE THE  SlIdEr  TrAVEl

Along  THE gUIdE  rAIl.

FigURe 10: TrAVEl of THE SlIdEr IS TrACEd on THE roTATIng roUnd plATE To

CrEATE A CAM profIlE, IllUSTrATEd HErE WITH A grooVE CUT In THE plATE.

Designers can also use trajectories of motion, for example, to verify the motion of an industrial robot, such as that shown in Figure 11, and test the toolpath to obtain information necessary when selecting the size of robot needed, and to establish power requirements—all without the need for any physical tests.