Modeling Mechanical Systems

Transcription

1 chp3 1 Modeling Mechanical Systems Dr. Nhut Ho ME584

2 chp3 2 Agenda Idealized Modeling Elements Modeling Method and Examples Lagrange s Equation Case study: Feasibility Study of a Mobile Robot Design Matlab Simulation Example Active learning: Pair-share exercises, case study

3 chp3 3 Idealized Modeling Elements

4 chp3 4 Inductive storage Electrical inductance Translational spring Rotational spring Fluid inertia

5 chp3 5 Capacitive Storage Electrical capacitance Translational mass Rotational mass

6 chp3 6 Energy dissipators Electrical resistance Translational damper Rotational damper

7 chp3 7 Springs - Stiffness Element - Stores potential energy x

8 chp3 8 Actual Spring Behavior

9 chp3 9 Spring Connections Spring in series: K EQ =K 1 K 2 /(K 1 +K 2 ) Spring in parallel: K EQ =K 1 +K 2

10 chp3 10 Dampers and Mass

11 chp3 11 Dampers Connections Dampers in series: B EQ =B 1 B 2 /(B 1 +B 2 ) Dampers in parallel: B EQ =B 1 +B 2

12 chp3 12 Modeling Mechanical Systems

13 chp3 13 Modeling Methods State assumptions and their rationales Establish inertial coordinate system Identify and isolate discrete system elements (springs, dampers, masses) Determine the minimum number of variables needed to uniquely define the configuration of system (subtract constraints from number of equations) Free body diagram for each element Write equations relating loading to deformation in system elements Apply Newton s 2 nd Law: F = ma for translation motion T = Iα for rotational motion

14 chp3 14 Example 1: Automobile Shock Absorber Spring-mass-damper Free-body diagram M 2 d y( t) 2 dt b dy( t) dt ky( t) r( t)

15 chp3 15 Example 2: Mechanical System Draw a free body diagram, showing all forces and their directions Write equation of motion and derive transfer function of response x to input u

16 chp3 16 Example 2: Mechanical System

17 chp3 17 Example 3: Two-Mass System Derive the equation of motion for x 2 as a function of F a. The indicated damping is viscous.

18 chp3 Example 3: Two-Mass System 18

19 chp3 19 Example 4: Three-Mass System Draw the free-body-diagram for each mass and write the differential equations describing the system

20 chp3 20 Example 4: Three-Mass System

21 chp3 21 Example 5: Pair-Share Exercise All springs are identical with constant K Spring forces are zero when x 1 =x 2 =x 3 =0 Draw FBDs and write equations of motion Determine the constant elongation of each spring caused by gravitational forces when the masses are stationary in a position of static equilibrium and when f a (t) = 0.

22 chp3 22 Example 5: Pair-Share Exercise:

23 chp3 23 Example 5: Pair-Share Exercise:

24 chp3 24 Example 6: Pair-Share Exercise Assume that the pulley is ideal No mass and no friction No slippage between cable and surface of cylinder (i.e., both move with same velocity) Cable is in tension but does not stretch Draw FBDs and write equations of motion If pulley is not ideal, discuss modeling modifications

25 chp3 25 Example 6: Pair-Share Exercise Pulley is not ideal Add rotation mass and friction Model the slippage behaviors Add spring to model cable

26 chp3 26 Example 7: Electric Motor An electric motor is attached to a load inertia through a flexible shaft as shown. Develop a model and associated differential equations (in classical and state space forms) describing the motion of the two disks J1 and J2. Torsional stiffness is given in Appendix B

27 chp3 27 Example 7: Electric Motor

28 chp3 28 Example 7: Electric Motor

29 chp3 29 Example 7: Electric Motor

30 chp3 30 Example 8: Pair-Share Exercise: Copy Machine The device from a copying machine is shown. It moves in a horizontal plane. Develop the dynamic model, assuming that mass of bar is negligible compared to attached mass m 2 and angular motions are small. The mass is subjected to a step input F, find an expression for the displacement of point B after the transient motions have died out.

31 chp3 31 Example 8: Pair-Share Exercise: Copy Machine

32 chp3 32 Example 9: Mass-Pulley System A mechanical system with a rotating wheel of mass m w (uniform mass distribution). Springs and dampers are connected to wheel using a flexible cable without skip on wheel. Write all the modeling equations for translational and rotational motion, and derive the translational motion of x as a function of input motion u Find expression for natural frequency and damping ratio

33 chp3 33 Example 9: Mass-Pulley System

34 chp3 34 Example 9: Mass-Pulley System

35 chp3 35 Example 9: Mass-Pulley System

36 chp3 36 Example 10: Pair-Share Exercise: Double Pendulum The disk shown in the figure rolls without slipping on a horizontal plane. Attached to the disk through a frictionless hinge is a massless pendulum of length L that carries another disk. The disk at the bottom of the pendulum cannot rotation relative to the pendulum arm. Draw free-body diagrams and derive equations of motion for this system.

37 chp3 37 Example 10: Pair-Share Exercise: Double Pendulum

38 chp3 38 Example 10: Pair-Share Exercise: Double Pendulum

39 chp3 39 Example 10: Pair-Share Exercise: Double Pendulum