1. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Dynamic Simulation: Lagrangian Multipliers Objective  The objective of this module is to introduce Lagrangian multipliers that are used with Lagrange’s equation to find the equations that control the motion of mechanical systems having constraints.  The matrix form of the equations used by computer programs such as Autodesk Inventor’s Dynamic Simulation are also presented.

2. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Basic Problem in Multi-body Dynamics In the previous module (Module 6) we developed Lagrange’s equation and showed how it could be used to determine the equations of simple motion systems. Lagrange’s Equation The examples we considered were for systems in which there were no constraints between the generalized coordinates. The basic problem of multi-body dynamics is to systematically find and solve the equations of motion when there are constraints that bodies in the system must satisfy. Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 2

3. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Non-conservative Forces  The derivation of Lagrange’s equation in the previous module (Module 6) considered only processes that store and release potential energy.  These processes are called conservative because they conserve energy.  Lagrange’s equation must be modified to accommodate non- conservative processes that dissipate energy (i.e. friction, damping, and external forces).  A non-conservative force or moment acting on generalized coordinate q i is denoted as Q i.  The more general form of Lagrange’s equation is Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 3

4. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Simple Pendulum Simple Pendulum c.g. X Y  The pendulum shown in the figure will be used as an example throughout this module.  The position of the pendulum is known at any instance of time if the coordinates of the c.g., X cg,Y cg, and the angle  are known.  X cg,Y cg and  are the generalized coordinates. x y X cg Y cg Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 4

5. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Kinetic and Potential Energies The kinetic energy (T) and potential energy (V) of the pendulum are These equations also give the kinetic and potential energy of the unconstrained body flying through the air. There needs to be a way to include the constraints to differentiate between the two systems. c.g. X Y x y X cg Y cg Unconstrained Body Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 5

6. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Constraint Equations  In addition to satisfying Lagrange’s equations of motion, the pendulum must satisfy the constraints that the displacements at X 1 and Y 1 are zero.  The constraint equations are c.g. X Y x y X cg Y cg X 1,Y 1 Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 6 The c.g. lies on the y - axis halfway along the length.

7. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Lagrangian Multipliers X1X1 Y1Y1 X Y  The kinetic energy is augmented by adding the constraint equations multiplied by parameters called Lagrangian Multipliers.  Note that since the constraint equations are equal to zero, we have not changed the magnitude of the kinetic energy.  The Lagrangian multipliers are treated like unknown generalized coordinates. What are the units of 1 and 2 ? Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 7

8. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Governing Equations Lagrange’s Equation Lagrangian Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 8  In the following slides, Lagrange’s equation will be used in a systematic manner to determine the equations of motion for the pendulum.  The governing equations that will be used are shown here.  There are no non-conservative forces acting on the system ( ).

9. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Equation for 1 st Generalized Coordinate Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 9 Lagrange’s Equation Generalized Coordinates 1 st Equation Mathematical Steps

10. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Equation for 2 nd Generalized Coordinate Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 10 Lagrange’s Equation Generalized Coordinates 2 nd Equation Mathematical Steps

11. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Equation for 3 rd Generalized Coordinate Lagrange’s Equation Generalized Coordinates 3 rd Equation Mathematical Steps Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 11

12. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Equation for 4 th Generalized Coordinate Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 12 Lagrange’s Equation Generalized Coordinates 4 th Equation Mathematical Steps

13. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Equation for 5 th Generalized Coordinate Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 13 Lagrange’s Equation Generalized Coordinates 5 th Equation Mathematical Steps

14. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Summary of Equations Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 14  There are five unknown generalized coordinates including the two Lagrangian Multipliers. There are also five equations.  Three of the equations are differential equations.  Two of the equations are algebraic equations.  Combined, they are a system of differential-algebraic equations (DAE).

15. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Free Body Diagram Approach The application of Lagrange’s equation yields the same equations obtained by drawing a free-body diagram. Free Body Diagram with Inertial Forces Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 15 Summation of Forces in the X-direction Summation of Forces in the Y-direction Summation of Moments about the c.g.

16. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Physical Significance of Lagrangian Multipliers Force required to impose the constraint that X 1 is a constant. Newton’s 2 nd Law in x-direction Lagrangian Multipliers are simply the forces (moments) required to enforce the constraints. In general, the Lagrangian Multipliers are a function of time, because the forces (moments) required to enforce the constraints vary with time (i.e. depend on the position of the pendulum). Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 16

17. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Matrix Format  The computer implementation of Lagrange’s equation is facilitated by writing the equations in matrix format.  Separating the Lagrangian into kinetic and potential energy terms enables Lagrange’s equation to be written as  In this format, the conservative and non-conservative forces are lumped together on the right hand side of the equation. Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 17

18. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Matrix Format The kinetic energy augmented with Lagrangian Multipliers can be written in matrix format as Column array containing generalized coordinate velocities. Column array containing the constraint equations (refer to Module 3 in this section). Column array containing the Lagrangian multipliers. Matrix containing the mass and mass moments of inertia associated with each generalized coordinate. Inertia Matrix Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 18

19. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Matrix Format Lagrange’s equation for a mechanical system becomes Is the constraint equation Jacobian matrix introduced in Module 4 in this section. Column array containing both conservative and non-conservative forces. Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 19

20. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Matrix Format  Another equation for acceleration was obtained in Module 4 based on kinematics and the constraint equations.  Combining this equation with Lagrange’s equation from the previous slide yields: Matrix Form of Equations  This equation can be solved to find the accelerations and constraint forces at an instant in time.  The accelerations must then be integrated to find the velocities and positions. Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 20

21. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Solution of Differential-Algebraic Equations (DAE)  The solution of even the simplest system of DAE requires computer programs that employ predictor-corrector type numerical integrators.  The Adams-Moulton method is an example of the type of numerical method used.  Significant research has led to the development of efficient and robust integrators that are found in commercial computer programs that generate, assemble, and solve these equations.  Autodesk Inventor’s Dynamic Simulation environment is an example of such software. Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 21

22. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk. www.autodesk.com/edcommunity Education Community Module Summary  This module showed how Lagrangian Multipliers are used in conjunction with Lagrange’s equation to obtain the equations that control the motion of mechanical systems.  The method presented provides a systematic method that forms the basis of mechanical simulation programs such as Autodesk Inventor’s Dynamic Simulation environment.  The matrix format of the equations were presented to provide insight into the computations performed by computer software.  The Jacobian and constraint kinematics developed in Module 4 of this section are an important part of the matrix formulation. Section 4 – Dynamic Simulation Module 7 – Lagrangian Multipliers Page 22