Dynamic Analysis of Mechanical Systems Kinematics
Dynamic Analysis of Mechanical Systems Kinematics
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Dynamic Analysis of Mechanical Systems Kinematics
1
3.3 3D Kinematics of Rigid Bodies
3.3.1 Fixed Axis Rotation
A shown in the figure, let’s consider a rigid body that rotates
with angular velocity about a fixed axis n-n. In the Cartesian
frame O XYZ− , the position of a point P on the body can be
located by a position vector r . As you can see, the path of
point P is an arc in the plane normal to
In a small time interval t , the body swings an small angle
t= . This leads to the change of r
( )
magnitude
direction
sin
sin
sin
t
t t
=
= =
r
r r
r
r
r r
r
According to the definition of derivative, the velocity of point P
can be expressed as
( )
0
lim
t
t
t t →
= = =
rr
v r
(1)
⊥ ⊥v v r,
Taking time derivative of Eq.(1) by the chain rule leads to the
acceleration of P
( )
t n
= = + = +
a a
a r r v r r (2)
where = ---- the angular acceleration of the body.
t = a r ----the tangential component
( )n = a r ----the normal component
Dynamic Analysis of Mechanical Systems Kinematics
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3.3.2 Fixed Point Rotation
As shown in the Figure, rotation of a body about a fixed point can
be regarded as the rotation about an axis called the instantaneous
axis. Thus, α can be decomposed into the following form
⊥= +α α α (3)
where α is aligned with the rotation axis, representing the
change rate of in magnitude, while ⊥α is normal to the
rotation axis, reflecting the change rate of in direction.
3.3.3 General Motion
The general motion of a body can be visualized as a combination
of the translation along with a point (the reference point) on the
body and the rotation about an axis passing through the point.
Let A and B are two points on a body. If we take B as the
reference point, the position vector of A can be represented by
A B AB= +r r r , =AB BAr (4)
Taking time derivative of Eq.(4) gives
A B AB= +v v v
ABv --- the velocity of A relative to B.
For pure translation
A B AB= =v v v 0
For pure rotation with angular velocity about an axis passing
through B
B A AB AB= = = v v v r0
For general motion
A B AB= + v v r (5)
Taking time derivative of Eq.(5) leads to the acceleration of A.
( )A B AB AB= + + a a r r (6)
Dynamic Analysis of Mechanical Systems Kinematics
3
For 2D motion in the x y− plane, because kω = and kα = ,
Eqs.(5) and (6) can be simplified as
( )( )
2
A B AB
A B AB AB
B AB AB
= +
= + +
= + −
v v k r
a a k r k k r
a k r r
(7)
Eq.(7) can also be expressed by using 2D coordinate vector
notation. Let Ar and Br are the 2D coordinate vectors of points A
and B. Then, AB A B= −r r r can be expressed as
cos