example of Euler angles: Tait Bryan angles

(Example)

Euler Angles: Tait Bryan Angles Examples, Exercises, and Solutions

This entry is the self study companion to Euler Angles: Tait Bryan angles.

The exercises emphasize the six distinct-axis sequences, their passive intrinsic matrix products, the universal Tait Bryan singularity, aerospace 3-2-1 yaw pitch roll, and consistency with passive quaternion representations.

All exercises are stated first. Complete worked solutions follow afterward.

1 Convention summary

PhysicsLibrary uses passive coordinate maps:

Bv = BCA Av.

(1)

For intrinsic i-j-k,

BCA = Ck(γ)Cj (β)Ci(α).

(2)

For Tait Bryan angles,

i ⁄= j, j ⁄= k, i ⁄= k.

(3)

The six intrinsic Tait Bryan sequences are

123, 132, 213, 231, 312, 321.

(4)

Their generic middle-angle singularity is

cosβ = 0.

(5)

For the aerospace intrinsic 3-2-1 sequence,

α = ψ, β = 𝜃, γ = ϕ,

(6)

and

B CA = C1(ϕ)C2 (𝜃)C3(ψ).

(7)

2 Visual reference

Figure. The six intrinsic Tait Bryan sequences.

Figure. The generic Tait Bryan singularity illustrated with intrinsic 3-2-1.

3 Exercises

Recognize a Tait Bryan sequence.
Which of the following are Tait Bryan sequences?

$123, 121, 312, 323, 231, 313.$
Generate the six sequences.
Starting from the statement that each coordinate axis is used exactly once, derive the six possible Tait Bryan axis orders.
Write all six passive products.
Write the passive intrinsic DCM product for each of the six Tait Bryan sequences using generic angles (α,β,γ).
Sequence from a matrix product.
A passive intrinsic DCM is written

$BCA = C2(γ)C1 (β )C3(α ).$
Identify the chronological Tait Bryan sequence.
Another sequence from a matrix product.
A passive intrinsic DCM is written

$BC = C (γ)C (β )C (α ). A 3 2 1$
Identify the sequence.
Single-angle checks for 2-3-1.
For intrinsic 2-3-1,

$BCA = C1(γ)C3 (β )C2(α ).$

Find the remaining matrix when:
1. β = γ = 0;
2. α = γ = 0;
3. α = β = 0.
Universal singularity condition.
Show that every Tait Bryan sequence has the same generic singularity condition

$cosβ = 0.$
What are the singular values on the usual principal branch?
Interpret the singularity geometrically.
For intrinsic 3-2-1, explain what happens to the first and third physical rotation axes at

$π 𝜃 = + -- 2$
and at

$π 𝜃 = − -. 2$
Why the physical orientation remains valid.
Explain why gimbal lock does not mean that the rigid body orientation itself becomes undefined.
Aerospace notation.
For intrinsic 3-2-1, identify which generic angle is yaw, pitch, and roll.
Write the passive DCM product.
Expand the first row of the 3-2-1 DCM.
Starting from

$B CA = C1(ϕ)C2 (𝜃)C3(ψ),$
derive the first row of the matrix.
Extract yaw pitch roll.
Suppose a nonsingular passive 3-2-1 DCM is

$⌊ ⌋ C11 C12 C13 C = ⌈ C21 C22 C23⌉ . C C C 31 32 33$
Write the principal extraction formulas for

$ψ, 𝜃, ϕ.$
Numerical yaw pitch roll matrix.
Compute the passive 3-2-1 DCM for

$ψ = 40 ∘, 𝜃 = − 15∘, ϕ = 25∘.$
Numerical round trip.
Using the matrix from Exercise 13, apply the extraction equations and recover the original principal yaw pitch roll angles.
Equivalent extrinsic description.
Find the extrinsic sequence and chronological angle order equivalent to

$intrinsic 3-2-1(ψ, 𝜃,ϕ).$
Another intrinsic/extrinsic conversion.
Find the extrinsic sequence equivalent to

$intrinsic 1-3-2(α,β, γ).$
Tait Bryan triples are not vectors.
Explain why

$(ψ2 − ψ1,𝜃2 − 𝜃1,ϕ2 − ϕ1)$
is not generally the exact relative attitude between two finite 3-2-1 orientations.
Quaternion counterpart.
Write the passive quaternion product for intrinsic 3-2-1 yaw pitch roll.
Small pitch case.
For intrinsic 3-2-1, suppose

$ϕ = 0, ψ = 0, |𝜃 | ≪ 1.$
Write the first-order DCM approximation.
Convention audit.
A textbook says only:

Yaw pitch roll is represented by R_z(ψ)R_y(𝜃)R_x(ϕ).

List at least five convention questions that must be answered before comparing that formula with PhysicsLibrary.

4 Solutions

Solution 1: recognize a Tait Bryan sequence

A Tait Bryan sequence uses all three axes exactly once.

Therefore

123, 312, 231

are Tait Bryan.

The sequences

121, 323, 313

are proper Euler because the first and third axis labels match.

Solution 2: generate the six sequences

Choose a first axis in three ways.

Then arrange the remaining two axes in two possible orders.

Therefore

3 × 2 = 6.

The six sequences are

123, 132, 213, 231, 312, 321.

(8)

Solution 3: write all six passive products

Using

B CA = Ck(γ)Cj (β)Ci(α),

the six products are

C = C (γ)C (β )C (α), 123 3 2 1

(9)

C = C (γ)C (β )C (α), 132 2 3 1

(10)

C213 = C3 (γ)C1(β )C2(α),

(11)

C231 = C1 (γ)C3(β )C2(α),

(12)

C312 = C2 (γ)C1(β )C3(α),

(13)

and

C321 = C1 (γ)C2(β )C3(α).

(14)

Solution 4: sequence from a matrix product

Compare

C2 (γ)C1(β)C3 (α)

with

Ck (γ )Cj(β)Ci(α ).

Thus

i = 3, j = 1, k = 2.

The chronological intrinsic sequence is

3-1-2.

(15)

Solution 5: another sequence from a matrix product

For

C3(γ )C2 (β)C1 (α ),

we identify

i = 1, j = 2, k = 3.

Therefore the sequence is

1-2-3.

(16)

Solution 6: single-angle checks for 2-3-1

From

B CA = C1(γ)C3 (β )C2(α ),

we obtain

B β = γ = 0 =⇒ CA = C2(α ),

(17)

B α = γ = 0 =⇒ CA = C3(β ),

(18)

and

α = β = 0 =⇒ BCA = C1(γ ).

(19)

Solution 7: universal singularity condition

For every Tait Bryan sequence, the Jacobian from Euler coordinate rates to angular velocity loses rank when the first and third physical rotation axes become collinear.

That occurs when the middle rotation turns through a right angle:

cosβ = 0.

(20)

On the usual principal branch,

π- π- − 2 ≤ β ≤ 2 ,

(21)

so the singular values are

π β = ± 2.

(22)

Solution 8: interpret the singularity geometrically

For intrinsic 3-2-1, the first rotation axis is z_A.

The third rotation is about x₂.

π- 𝜃 = + 2,

the axis x₂ becomes anti-aligned with z_A.

π- 𝜃 = − 2,

the axis x₂ becomes aligned with z_A.

Thus the first and third rotations are about the same physical line, so yaw and roll can no longer be determined independently.

Solution 9: why the physical orientation remains valid

The rigid body still has a perfectly valid orientation in three-dimensional space.

Only the chosen three-coordinate parameterization loses local uniqueness.

A quaternion or DCM remains nonsingular at the same physical attitude.

Therefore gimbal lock is a coordinate singularity, not a disappearance of physical orientation.

Solution 10: aerospace notation

PhysicsLibrary uses

α = ψ = yaw,

β = 𝜃 = pitch,

and

γ = ϕ = roll.

Thus

B CA = C1(ϕ)C2 (𝜃)C3(ψ).

(23)

Solution 11: expand the first row of the 3-2-1 DCM

First multiply

C (𝜃)C (ψ ). 2 3

The first row of that product is

[ ] c𝜃cψ c𝜃sψ − s𝜃 .

Premultiplication by C₁(ϕ) leaves the first row unchanged because the first row of C₁(ϕ) is

[1 0 0].

Therefore the first row of the full DCM is

[ ] c𝜃cψ c𝜃sψ − s𝜃 .

(24)

Solution 12: extract yaw pitch roll

On the nonsingular principal branch,

𝜃 = arcsin(− C13 ),

(25)

ϕ = atan2 (C23, C33),

(26)

and

ψ = atan2 (C12, C11).

(27)

Solution 13: numerical yaw pitch roll matrix

With

∘ ∘ ∘ ψ = 40 , 𝜃 = − 15 , ϕ = 25 ,

the passive DCM is

⌊ ⌋ 0.73994 0.62089 0.25882 BCA ≈ ⌈− 0.66635 0.63247 0.40822⌉ . 0.08906 − 0.46246 0.88211

(28)

Solution 14: numerical round trip

From the matrix,

C13 ≈ 0.25882.

Thus

𝜃 = arcsin(− 0.25882 ) ≈ − 15 ∘.

Next,

ϕ = atan2(0.40822, 0.88211 ) ≈ 25∘,

and

∘ ψ = atan2 (0.62089, 0.73994 ) ≈ 40 .

Therefore the principal round trip recovers

(ψ, 𝜃,ϕ) = (40∘,− 15∘,25∘).

(29)

Solution 15: equivalent extrinsic description

Reverse both axis order and associated angle order:

intrinsic 3-2-1(ψ,𝜃,ϕ ) ≡ extrinsic 1-2- 3 (ϕ, 𝜃,ψ).

(30)

Solution 16: another intrinsic/extrinsic conversion

For intrinsic 1-3-2,

intrinsic 1-3-2(α,β,γ ) ≡ extrinsic 2-3- 1 (γ, β,α ).

(31)

Solution 17: Tait Bryan triples are not vectors

Euler coordinates are nonlinear sequence-dependent coordinates.

The second and third rotations occur about intermediate axes whose directions depend on earlier rotations.

Therefore subtracting coordinate triples does not reproduce exact finite rotation composition.

The exact relative attitude should instead be computed from DCMs or quaternions.

Solution 18: quaternion counterpart

For intrinsic 3-2-1 yaw pitch roll,

BqA = qP1 (ϕ)qP2 (𝜃)qP3 (ψ ).

(32)

The quaternion and DCM must satisfy

BCA = C (BqA ).

(33)

Solution 19: small pitch case

With

ϕ = 0, ψ = 0,

the DCM reduces to

B CA = C2 (𝜃).

For small |𝜃|,

cos𝜃 ≈ 1, sin𝜃 ≈ 𝜃.

Thus

⌊ ⌋ 1 0 − 𝜃 BCA ≈ ⌈0 1 0 ⌉ . 𝜃 0 1

(34)

Solution 20: convention audit

Before comparing the textbook formula with PhysicsLibrary, one should ask:

Is the rotation active or passive?
Is the sequence intrinsic or extrinsic?
What map direction does the matrix represent?
Does the source use right hand positive angles?
Does “yaw pitch roll” mean chronological 3-2-1 intrinsic rotation?
Are column vectors or row vectors being used?
Is the written product chronological order or operator composition order?
Are the frame labels and basis conventions the same?

Without those declarations, identical-looking symbols can represent different transformations.

5 Compact review

Tait Bryan sequences use the three coordinate axes exactly once:

123, 132, 213, 231, 312, 321.

(35)

For intrinsic i-j-k,

BCA = Ck(γ)Cj (β)Ci(α).

(36)

Their generic singularity is

cosβ = 0.

(37)

For aerospace intrinsic 3-2-1,

BCA = C1(ϕ)C2 (𝜃)C3(ψ),

(38)

with

𝜃 = arcsin(− C13 ),

(39)

ϕ = atan2 (C23, C33),

(40)

and

ψ = atan2 (C12, C11).

(41)

6 Sources and exercise provenance

The exercises and solutions in this companion are newly written for PhysicsLibrary to reinforce the Tait Bryan framework developed in Euler angles: Tait Bryan angles.

References

[1] D. M. Henderson, Euler Angles, Quaternions, and Transformation Matrices: Working Relationships, JSC-12960, NASA Johnson Space Center, 1977. NASA Technical Reports Server

[2] J. Diebel, “Representing Attitude: Euler Angles, Unit Quaternions, and Rotation Vectors,” Stanford University, 2006. Online PDF

[3] H. Goldstein, C. Poole, and J. Safko, Classical Mechanics, 3rd ed., Addison Wesley, 2002. Publisher search

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example of Euler angles: Tait Bryan angles

Euler Angles: Tait Bryan Angles Examples, Exercises, and Solutions

1 Convention summary

2 Visual reference

3 Exercises

4 Solutions

Solution 1: recognize a Tait Bryan sequence

Solution 2: generate the six sequences

Solution 3: write all six passive products

Solution 4: sequence from a matrix product

Solution 5: another sequence from a matrix product

Solution 6: single-angle checks for 2-3-1

Solution 7: universal singularity condition

Solution 8: interpret the singularity geometrically

Solution 9: why the physical orientation remains valid

Solution 10: aerospace notation

Solution 11: expand the first row of the 3-2-1 DCM

Solution 12: extract yaw pitch roll

Solution 13: numerical yaw pitch roll matrix

Solution 14: numerical round trip

Solution 15: equivalent extrinsic description

Solution 16: another intrinsic/extrinsic conversion

Solution 17: Tait Bryan triples are not vectors

Solution 18: quaternion counterpart

Solution 19: small pitch case

Solution 20: convention audit

5 Compact review

6 Sources and exercise provenance

References

License

Pending Errata and Addenda

Discussion

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