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CL-7004 – Robotics and Control
- Robotics Position and
Orientation
Dr.B.Arunadevi
Associate Professor /ECE
Dr.N.G.P Institute of Technology
Coimbatore
Objectives of the Lecture
• Learn to represent position and orientation
• Be able to transform between coordinate
systems.
• Use frames and homogeneous coordinates
• Describe Euler angles
A robot may not injure a human being, or, through inaction, allow a
human being to come to harm.
A robot must obey the orders given it by human beings except where
such orders would conflict with the First Law.
A robot must protect its own existence as long as such protection
Three Laws of Robotics
Description of a Position
• point = position vector
YA
XA
ZA
}A{
A
P










=
z
y
x
A
p
p
p
P
a c o o r d i na t e s y s t e m
Description of an Orientation
• In the example, a
description of {B} with
respect to {A} suffices to
give orientation
• Orientation = System of
Coordinates
• Directions of {B}: XB, YB
and ZB
• In {A} coord. system: A
XB,
A
YB and A
ZB
x0
x1
z0
y0
z1
y1
θ
}A{
}B{
Often a point is not enough:
need orientation
From {A} to {B{
}A{
XB
aX
aY
aZ( ) BAX XX ⋅=αcos
( ) BAY XY ⋅=αcos
( ) BAZ XZ ⋅=αcos
We conclude:










⋅
⋅
⋅
=
BA
BA
BA
B
A
XZ
XY
XX
X
Rotation Matrix
• Stack three unit vectors to form Rotation Matrix
• describes {B} with respect to {A}
• Each vector in can be written as dot product
of pair of unit vectors: cosine matrixcosine matrix
• Rows of : unit vectors of {A} with respect to
{B}
• What is ? What is det( )?
• Position + orientation = Frame
RA
B
[ ]B
A
B
A
B
AA
B ZYXR ˆˆˆ=
RA
B
RA
B
1−
RA
B RA
B
Description of a Frame
• Frame: set of four
vectors giving position +
orientation
• Description of a frame:
position + rotation matrix
• Ex.:
x0
x1
z0
y0
z1
y1
θ
}A{
}B{
{ }BORG
AA
B PRB ,}{ =
• position: frame with
identity as rotation
• orientation: frame with
zero position
Mapping: from frame 2 frame
• If {A} has same orientation as {B}, then {B}
differs from {A} in a translation: A
PBORG
A
P = B
P + A
PBORG
• Mapping: change of description from one frame to
another. The vector A
PBORGdefines the mapping.
XA
ZA
YA
{A}
ZB
YB
{B}
XB
A
P
A
PBORG
B
P
Translated Frames
Rotated Frames
YA
XA
ZA
XB
ZB
YB
B
P
BzByBx
B
ZpYpXpP ++=
B
A
zB
A
yB
A
x
A
ZpYpXpP ++=
[ ]










=
z
y
x
B
A
B
A
B
AA
p
p
p
ZYXP
PRP BA
B
A
=
Description of Rotation = Rotation Matrix
Rotated Frame (cont.)
• The previous expression can be written as
• The rotation mapping changes the
description of a point from one coordinate
system to another
• The point does not change! only its
description
PRP BA
B
A
=
Example (2D rotation)
XA
YA
YB
XB
θ
x0
y0
x1
y1
θθ
θθ
cossin
sincos
001
001
yxy
yxx
+−=
+=
General Frame Mapping
}A{
XB
YB
ZB
B
P
XA
YA
ZA
A
P
A
PBORG
BORG
ABA
B
A
PPRP +=
Replace by the more appealing
equation:












=





110001
PPRP B
BORG
AA
B
A
A “1” added here A row added here
Homogeneous Coords
• Homogeneous coordinates: embed 3D
vectors into 4D by adding a “1”
• More generally, the transformation matrix T
has the form:






=
FactorScalingTrans.Perspect.
VectorTrans.MatrixRot.
T
Operators: Translation, Rotation
and General Transformation
• Translation Operator:
( )1
12
|)|,ˆ( PQQTRANS
QPP
AA
AAA
=
+=
1P
2P
Q
Translation Operator
• Translation Operator:
• Only one coordinate frame, point moves
• Equivalent to mapping point to a 2nd frame
• Point Forward = Frame Backwards
• How does TRANS look in homogeneous
coordinates?
• Rotational Operator
Operators (cont.)
A
P1
A
P2
K
θ
Rotation around axis:
Rotation Operator
• Rotational Operator
The rotation matrix can be seen as rotational
operator
• Takes A
P1 and rotates it to A
P2=R A
P1
• A
P2=ROT(K, q)(A
P2)
• Write ROT for a rotation around K
Operators (Cont.)
• Transformation Operators
* A transformation mapping can be viewed as
a transformation operator: map a point to
any other in the same frame
* Transform that rotates by R and translates
by Q is the same a transforming the frame
by R & Q
Compound Transformation
If {C} is known relative to {B}, and {B} is
known relative to {A}. We want to
transform P from {C} to {A}:
Write down the compound in homog. coords
TTT
PTP
PTP
PTP
B
C
A
B
A
C
BA
B
A
CA
C
A
CB
C
B
=
=
=⇒
=
definehereFrom
More on Rotations
• We saw that a rotation can be represented
by a rotation matrix
• Matrix has 9 variables and 6+ constraints
(which?)
• Rotations are far from intuitive: they do not
commute!
• Rotation matrix can be parameterized in
different manners:
—Roll, pitch and yaw angles
—Euler Angles
—Others
Euler’s Theorem
• Euler’s Theorem: Any two independent
orthonormal coordinate frames can be related by a
sequence of rotations (not more than three) about
coordinate axes, where no two successive
rotations may be about the same axis.
• Not to be confused with Euler angles, Euler
integration, Newton-Euler dynamics, inviscid
Euler equations, Euler characteristic…
• Leonard Euler (1707-1783)
XYZ Fixed
Euler Angles
• This means that we can represent an orientation with 3
numbers
• A sequence of rotations around principle axes is called an
Euler Angle Sequence
• Assuming we limit ourselves to 3 rotations without
successive rotations about the same axis, we could use any
of the following 12 sequences:
XYZ XZY XYX XZX
YXZ YZX YXY YZY
ZXY ZYX ZXZ ZYZ
ZYX Euler
Euler Angles to Matrix
Conversion
• To build a matrix from a set of Euler angles, we
just multiply a sequence of rotation matrices
together:










−+
+−
−
=
yxzxzyxzxzyx
yxzxzyxzxzyx
yzyzy
cccssscsscsc
csccssssccss
ssccc










−⋅









 −
⋅










−
=⋅⋅
100
0
0
0
010
0
0
0
001
zz
zz
yy
yy
xx
xxzyx cs
sc
cs
sc
cs
scRRR
Euler Angle Order
• As matrix multiplication is not commutative, the
order of operations is important
• Rotations are assumed to be relative to fixed world
axes, rather than local to the object
• One can think of them as being local to the object
if the sequence order is reversed
• This gives us 12 redundant ways to store an
orientation using Euler angles
x
y
z
u'
v'
φ
θ
v "
w"
w'=
=u"
v'"
ϕ
u'"
w'"=
Euler Angle I, Animated
Orientation Representation
• Euler Angle I









 −
=










−=









 −
=
100
0cossin
0sincos
,
cossin0
sincos0
001
,
100
0cossin
0sincos
''
'
ϕϕ
ϕϕ
θθ
θθφφ
φφ
ϕ
θφ
w
uz
R
RR
Euler Angle I






















−
+
−
+
−
−
−
=
θθϕθϕ
θφ
θϕφ
ϕφ
θϕφ
ϕφ
θϕ
θϕφ
ϕφ
θϕφ
ϕφ
ϕθφ
cossincossinsin
sincos
coscoscos
sinsin
cossincos
cossin
sinsin
coscossin
sincos
cossinsin
coscos
''' wuz RRRR
Resultant eulerian rotation matrix:
Euler Angle II, Animated
x
y
z
u'
v'
φ
θ =v"
w"
w'=
u"
v"'ϕ
u"'
w"'=
Note the opposite
(clockwise) sense of the
third rotation, φ.
Orientation Representation
• Matrix with Euler Angle II






















−
−+
−
−
+
−
θ
θϕθϕ
θϕ
θϕφ
ϕφ
θϕφ
ϕφ
θφ
θϕφ
ϕφ
θϕφ
ϕφ
cos
sinsinsincos
sinsin
coscossin
coscos
coscossin
sincos
sincos
coscossin
cossin
coscoscos
sinsin
Quiz: How to get this matrix ?
Orientation Representation
• Description of Roll Pitch Yaw
X
Y
Z
ϕ
θ
φ
Quiz: How to get rotation matrix ?
Vehicle Orientation
• Generally, for vehicles, it is most convenient to
rotate in roll (z), pitch (x), and then yaw (y)
• In situations where there
is a definite ground plane,
Euler angles can actually
be an intuitive
representation x
y
z
front of vehicle
Euler Angles - Summary
• Euler angles are used in a lot of applications, but
they tend to require some rather arbitrary
decisions
• They also do not interpolate in a consistent way
(but this isn’t always bad)
• There is no simple way to concatenate rotations
• Conversion to/from a matrix requires several
trigonometry operations
• They are compact (requiring only 3 numbers)

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Robotics position and orientation

  • 1. CL-7004 – Robotics and Control - Robotics Position and Orientation Dr.B.Arunadevi Associate Professor /ECE Dr.N.G.P Institute of Technology Coimbatore
  • 2. Objectives of the Lecture • Learn to represent position and orientation • Be able to transform between coordinate systems. • Use frames and homogeneous coordinates • Describe Euler angles
  • 3. A robot may not injure a human being, or, through inaction, allow a human being to come to harm. A robot must obey the orders given it by human beings except where such orders would conflict with the First Law. A robot must protect its own existence as long as such protection Three Laws of Robotics
  • 4. Description of a Position • point = position vector YA XA ZA }A{ A P           = z y x A p p p P a c o o r d i na t e s y s t e m
  • 5. Description of an Orientation • In the example, a description of {B} with respect to {A} suffices to give orientation • Orientation = System of Coordinates • Directions of {B}: XB, YB and ZB • In {A} coord. system: A XB, A YB and A ZB x0 x1 z0 y0 z1 y1 θ }A{ }B{ Often a point is not enough: need orientation
  • 6. From {A} to {B{ }A{ XB aX aY aZ( ) BAX XX ⋅=αcos ( ) BAY XY ⋅=αcos ( ) BAZ XZ ⋅=αcos We conclude:           ⋅ ⋅ ⋅ = BA BA BA B A XZ XY XX X
  • 7. Rotation Matrix • Stack three unit vectors to form Rotation Matrix • describes {B} with respect to {A} • Each vector in can be written as dot product of pair of unit vectors: cosine matrixcosine matrix • Rows of : unit vectors of {A} with respect to {B} • What is ? What is det( )? • Position + orientation = Frame RA B [ ]B A B A B AA B ZYXR ˆˆˆ= RA B RA B 1− RA B RA B
  • 8. Description of a Frame • Frame: set of four vectors giving position + orientation • Description of a frame: position + rotation matrix • Ex.: x0 x1 z0 y0 z1 y1 θ }A{ }B{ { }BORG AA B PRB ,}{ = • position: frame with identity as rotation • orientation: frame with zero position
  • 9. Mapping: from frame 2 frame • If {A} has same orientation as {B}, then {B} differs from {A} in a translation: A PBORG A P = B P + A PBORG • Mapping: change of description from one frame to another. The vector A PBORGdefines the mapping. XA ZA YA {A} ZB YB {B} XB A P A PBORG B P Translated Frames
  • 10. Rotated Frames YA XA ZA XB ZB YB B P BzByBx B ZpYpXpP ++= B A zB A yB A x A ZpYpXpP ++= [ ]           = z y x B A B A B AA p p p ZYXP PRP BA B A = Description of Rotation = Rotation Matrix
  • 11. Rotated Frame (cont.) • The previous expression can be written as • The rotation mapping changes the description of a point from one coordinate system to another • The point does not change! only its description PRP BA B A =
  • 13. General Frame Mapping }A{ XB YB ZB B P XA YA ZA A P A PBORG BORG ABA B A PPRP += Replace by the more appealing equation:             =      110001 PPRP B BORG AA B A A “1” added here A row added here
  • 14. Homogeneous Coords • Homogeneous coordinates: embed 3D vectors into 4D by adding a “1” • More generally, the transformation matrix T has the form:       = FactorScalingTrans.Perspect. VectorTrans.MatrixRot. T
  • 15. Operators: Translation, Rotation and General Transformation • Translation Operator: ( )1 12 |)|,ˆ( PQQTRANS QPP AA AAA = += 1P 2P Q
  • 16. Translation Operator • Translation Operator: • Only one coordinate frame, point moves • Equivalent to mapping point to a 2nd frame • Point Forward = Frame Backwards • How does TRANS look in homogeneous coordinates?
  • 17. • Rotational Operator Operators (cont.) A P1 A P2 K θ Rotation around axis:
  • 18. Rotation Operator • Rotational Operator The rotation matrix can be seen as rotational operator • Takes A P1 and rotates it to A P2=R A P1 • A P2=ROT(K, q)(A P2) • Write ROT for a rotation around K
  • 19. Operators (Cont.) • Transformation Operators * A transformation mapping can be viewed as a transformation operator: map a point to any other in the same frame * Transform that rotates by R and translates by Q is the same a transforming the frame by R & Q
  • 20. Compound Transformation If {C} is known relative to {B}, and {B} is known relative to {A}. We want to transform P from {C} to {A}: Write down the compound in homog. coords TTT PTP PTP PTP B C A B A C BA B A CA C A CB C B = = =⇒ = definehereFrom
  • 21. More on Rotations • We saw that a rotation can be represented by a rotation matrix • Matrix has 9 variables and 6+ constraints (which?) • Rotations are far from intuitive: they do not commute! • Rotation matrix can be parameterized in different manners: —Roll, pitch and yaw angles —Euler Angles —Others
  • 22. Euler’s Theorem • Euler’s Theorem: Any two independent orthonormal coordinate frames can be related by a sequence of rotations (not more than three) about coordinate axes, where no two successive rotations may be about the same axis. • Not to be confused with Euler angles, Euler integration, Newton-Euler dynamics, inviscid Euler equations, Euler characteristic… • Leonard Euler (1707-1783)
  • 24. Euler Angles • This means that we can represent an orientation with 3 numbers • A sequence of rotations around principle axes is called an Euler Angle Sequence • Assuming we limit ourselves to 3 rotations without successive rotations about the same axis, we could use any of the following 12 sequences: XYZ XZY XYX XZX YXZ YZX YXY YZY ZXY ZYX ZXZ ZYZ
  • 26. Euler Angles to Matrix Conversion • To build a matrix from a set of Euler angles, we just multiply a sequence of rotation matrices together:           −+ +− − = yxzxzyxzxzyx yxzxzyxzxzyx yzyzy cccssscsscsc csccssssccss ssccc           −⋅           − ⋅           − =⋅⋅ 100 0 0 0 010 0 0 0 001 zz zz yy yy xx xxzyx cs sc cs sc cs scRRR
  • 27. Euler Angle Order • As matrix multiplication is not commutative, the order of operations is important • Rotations are assumed to be relative to fixed world axes, rather than local to the object • One can think of them as being local to the object if the sequence order is reversed • This gives us 12 redundant ways to store an orientation using Euler angles
  • 29. Orientation Representation • Euler Angle I           − =           −=           − = 100 0cossin 0sincos , cossin0 sincos0 001 , 100 0cossin 0sincos '' ' ϕϕ ϕϕ θθ θθφφ φφ ϕ θφ w uz R RR
  • 31. Euler Angle II, Animated x y z u' v' φ θ =v" w" w'= u" v"'ϕ u"' w"'= Note the opposite (clockwise) sense of the third rotation, φ.
  • 32. Orientation Representation • Matrix with Euler Angle II                       − −+ − − + − θ θϕθϕ θϕ θϕφ ϕφ θϕφ ϕφ θφ θϕφ ϕφ θϕφ ϕφ cos sinsinsincos sinsin coscossin coscos coscossin sincos sincos coscossin cossin coscoscos sinsin Quiz: How to get this matrix ?
  • 33. Orientation Representation • Description of Roll Pitch Yaw X Y Z ϕ θ φ Quiz: How to get rotation matrix ?
  • 34. Vehicle Orientation • Generally, for vehicles, it is most convenient to rotate in roll (z), pitch (x), and then yaw (y) • In situations where there is a definite ground plane, Euler angles can actually be an intuitive representation x y z front of vehicle
  • 35. Euler Angles - Summary • Euler angles are used in a lot of applications, but they tend to require some rather arbitrary decisions • They also do not interpolate in a consistent way (but this isn’t always bad) • There is no simple way to concatenate rotations • Conversion to/from a matrix requires several trigonometry operations • They are compact (requiring only 3 numbers)