ME451 Kinematics and Dynamics

of Machine Systems

Generalized Forces6.2

October 16, 2013

Radu SerbanUniversity of Wisconsin-Madison

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Before we get started…

Last Time: Discussed Principle of Virtual Work and D’Alembert’s Principle Introduced centroidal reference frames Derived the Newton-Euler EOM for a single rigid body

Today: Inertia properties Generalized forces Constrained variational EOM (variational EOM for a planar mechanism)

Miscellaneous: Matlab 5 and Adams 3 – due tonight, Learn@UW (11:59pm) Homework 7 – (6.1.1, 6.1.2, 6.1.3, 6.1.4) – due Friday, October 18 (12:00pm) Matlab 6 and Adams 4 – due October 23, Learn@UW (11:59pm)

Solutions to Midterm Problems – available on the Learn@UW course page Student feedback responses – available on the SBEL course page

Monday (October 21) lecture – simEngine2D discussion – in EH 2261

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Roadmap: Check Progress

What have we done so far? Derived the variational and differential EOM for a single rigid body

These equations are general but they must include all forces applied on the body

These equations assume their simplest form in a centroidal RF

What is left? Properties of the polar moment of inertia Define a general strategy for including external forces Treatment of constraint forces Derive the variational and differential EOM for systems of constrained bodies

Properties of the Centroid and Polar Moment of InertiaInertial Properties of Composite Bodies

6.1.4, 6.1.5

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Location of the Center of Mass (1/2)

The center of mass is the point on the body where the weighted relative position of the distributed mass sums to zero:

Question: How can we calculate the location of the COM with respect to an LRF ?

where we have defined the total body mass as:

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Location of the Center of Mass (2/2)

For a rigid body, the COM is fixed with respect to the body If the body has constant density, the COM coincides with the

centroid of the body shape If the rigid body has a line of symmetry, then the COM is somewhere

along that axis

Notes:

Here, symmetry axis means that both mass and geometry are symmetric with respect to that axis

If the rigid body has two axes of symmetry, the centroid is on each of them, and therefore is at their intersection

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Polar Moment of InertiaParallel Axis Theorem Also called Mass Moment of Inertia (MMI) The PMI with respect to some LRF is by definition the following integral

Question: Given the value calculated with respect to the centroidal frame, what is the value of this integral with respect to the LRF ?

Parallel Axis Theorem (Steiner’s Theorem)

Jakob Steiner(1796– 1863)

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Masses, centroid locations, and PMI for rigid bodies with constant density and of simple shapes can be easily calculated

Question: how do we calculate these quantities for bodies made up of rigidly attached subcomponents?

Step 1: Calculate the total body mass

Step 2: Compute the centroid location of the composite body

Step 3: For each subcomponent, apply the parallel axis theorem to include the PMI of that subcomponent with respect to the newly computed centroid, to obtain the PMI of the composite body

Note: if holes are present in the composite body,it is ok to add and subtract material (this translatesinto positive and negative mass)

Inertial Properties of Composite Bodies

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Roadmap: Check Progress

What have we done so far? Derived the variational and differential EOM for a single rigid body

These equations are general but they must include all forces applied on the body

These equations assume their simplest form in a centroidal RF Properties of the polar moment of inertia

What is left? Define a general methodology for including external forces, concentrated at a

given point on the body Virtual work and generalized forces

Elaborate on the nature of these concentrated forces. These can be: Models of common force elements (TSDA and RSDA) Reaction (constraint) forces, modeling the interaction with other bodies

Derive the variational and differential EOM for systems of constrained bodies

Virtual Work and Generalized Force6.2

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Including Concentrated Forces (1/3)

Problem: A single rigid body Absolute (Cartesian) generalized coordinates using a centroidal frame A concentrated force acts on the body at point , located by

Question: How do we include the force in the EOM?

Solution: A general methodology is to use D’Alembert’s Principle Write the virtual work done by and include it in the variational form of the

EOM

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Including Concentrated Forces (2/3)

Rearrange the variational EOM as:

and read this as “the virtual work of the sum of the applied (external) forces and the inertial forces is zero for any virtual variations of the generalized coordinates” (D’Alembert’s Principle)

A more compact and convenient form uses matrix-vector notation

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Including Concentrated Forces (3/3)

Nomenclature: generalized accelerations generalized virtual displacements generalized mass matrix generalized forces

Recipe for including a concentrated force in the EOM: Write the virtual work of the given force effect (force or torque) Express this virtual work in terms of the generalized virtual

displacements Identify the generalized force Include the generalized force in the matrix form of the variational

EOM

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Including a Point Force (1/2)

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Including a Point Force (2/2)

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Including a Torque

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Tractor Model[Example 6.1.1]

Derive EOM under the following assumptions: Traction (driving) force generated at rear wheels Small angle assumption (on the pitch angle ) Tire forces depend linearly on tire deflection:

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Setup Compliant connection between points on body and on body In its most general form it can consist of:

A spring with spring coefficient and free length A damper with damping coefficient An actuator (hydraulic, electric, etc.) which applies a force

The distance vector between points and is defined as

and has a length of

(TSDA)Translational Spring-Damper-Actuator (1/2)

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(TSDA)Translational Spring-Damper-Actuator (2/2)

General Strategy Write the virtual work produced by the force element in terms of an appropriate virtual

displacement

where

Express the virtual work in terms of the generalized virtual displacements and

Identify the generalized forces (coefficients of and )

Note: tension defined as positive

Note: positive separates the bodies

Hence the negative sign in the virtual work

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Setup Bodies and connected by a revolute joint at Torsional compliant connection at the common point In its most general form it can consist of:

A torsional spring with spring coefficient and free angle

A torsional damper with damping coefficient An actuator (hydraulic, electric, etc.) which

applies a torque

The angle from to (positive counterclockwise) is

(RSDA)Rotational Spring-Damper-Actuator (1/2)

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(RSDA)Rotational Spring-Damper-Actuator (2/2)

General Strategy Write the virtual work produced by the force element in terms of an appropriate virtual

displacement

where

Express the virtual work in terms of the generalized virtual displacements and

Identify the generalized forces (coefficients of and )

Note: tension defined as positive

Note: positive separates the axes

Hence the negative sign in the virtual work

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Generalized Forces: Summary

Question: How do we specify the terms and in the EOM?

Answer: Recall where these terms come from…

Integral manipulations (use rigid-body assumptions)

Redefine in terms of generalized forces and virtual displacements

Explicitly identify virtual work of generalized forces

Virtual work ofgeneralized

external forces

Virtual work ofgeneralized inertial forces

D’Alembert’s Principle effectively says that, upon including a new external force, the body’s generalized accelerations must change to preserve the balance of virtual work.

As such, to include a new force (or torque), we are interested in the contribution of this force on the virtual work balance.

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Roadmap: Check Progress

What have we done so far? Derived the variational and differential EOM for a single rigid body Defined how to calculate inertial properties Defined a general strategy for including external forces

Concentrated (point) forces Forces from compliant elements (TSDA and RSDA)

What is left? Treatment of constraint forces Derive the variational and differential EOM for systems of constrained

bodies

Variational Equations of Motion for Planar Systems

6.3.1

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Variational and Differential EOMfor a Single Rigid Body

The variational EOM of a rigid body with a centroidal body-fixed reference frame were obtained as:

Since and are arbitrary, using the orthogonality theorem, we get:

Important: The above equations are valid only if all force effects have been accounted for! This includes both applied forces/torques and constraint forces/torques (from interactions with other bodies).

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Matrix Form of the EOM for a Single Body

Generalized Force; includes all forces acting on body :

This includes all applied forces and all

reaction forces

Generalized Virtual Displacement

(arbitrary)

Generalized Mass Matrix

Generalized Accelerations

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[Side Trip]

A Vector-Vector Multiplication Trick

Given two vectors and , each made up of vectors, each of dimension 3

The dot product of and can be expressed as

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Variational EOM for the Entire System (1/2)

Consider a system made up of bodies

Write the EOM (in matrix form) for each individual body

Sum them up

Express this dot product as

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Variational EOM for the Entire System (2/2)

Matrix form of the variational EOM for a system made up of bodies

Generalized Force

Generalized Virtual Displacement

Generalized Mass Matrix

Generalized Accelerations

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A Closer Look at Generalized Forces

Total force acting on a body is sum of applied (external) and constraint (internal to the system) forces:

Goal: get rid of the constraint forces which (at least for now) are unknown

To do this, we need to compromise and give up something…

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Constraint Forces

Constraint Forces Forces that develop in the physical joints present in the system:

(revolute, translational, distance constraint, etc.) They are the forces that ensure the satisfaction of the constraints (they are

such that the motion stays compatible with the kinematic constraints)

KEY OBSERVATION: The net virtual work produced by the constraint forces present in the system as a result of a set of consistent virtual displacements is zero Note that we have to account for the work of all reaction forces present in the

system This is the same observation we used to eliminate the internal interaction

forces when deriving the EOM for a single rigid body

Therefore

provided q is a consistent virtual displacement

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Consistent Virtual Displacements

What does it take for a virtual displacement to be consistent (with the set of constraints) at a given, fixed time ?

Start with a consistent configuration ; i.e., a configuration that satisfies the constraint equations:

A consistent virtual displacement is a virtual displacement which ensures that the configuration is also consistent:

Apply a Taylor series expansion and assume small variations:

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Constrained Variational EOM

We can eliminate the (unknown) constraint forces if we compromise to only consider virtual displacements that are consistent with the constraint equations

Arbitrary Arbitrary Consistent

Constrained Variational Equations of Motion

Condition for consistent virtual displacements