EN.530.424/624 Dynamics of Robots and Spacecrafts Homework 5
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Dynamics of Robots and Spacecrafts (EN.530.424/624)
Homework 5
1 As in the class, general rotations in space are commonly described with three angles of rotation, called the Euler angles, denoted φ , θ, and ψ . Let us use the following composition of rotations using ZXZ Euler angles:
RZXZ = Rz(φ)Rx(θ)Rz(ψ)
╱ cos ψ cos φ ← cos θ sin φ sin ψ
= (
← sin ψ cos φ ← cos θ sin φ cos ψ
← sin ψ sin φ + cos θ cos φ cos ψ
sin θ cos ψ
、
cos θ . .
Also in homework 2, we showed that for this rotation matrix, R˙ZXZRZXZ(T) = Ω where
Ω = ╱ 、
( ←ω2 ω 1 0 .
and
“ = vect (Ω) = RZXZ“6
where
“ 6 = ╱ 、
( ψ˙ + φ˙ cos θ . .
If six point masses, each with mass m/6, are attached to the axes of the rotating frame shown in Fig. 1,
z’
y’
L 3
L2
L1
L2
Figure 1: Problem 1
(a) compute the kinetic energy of the six particles using the formula:
6
T = 2 6 vi = vi
i–1
where vi is the velocity of the i-th particle measured in a fixed frame, and the vectors in the form x\ , y\ , z\ are described in the fixed coordinate system x, y , z by the rotation matrix RZXZ . To make life easier, only consider the case when ψ = 0 and ψ˙ = 0. While the statement is still true when ψ 0, it is very tedious to show.
(b) Show that this kinetic energy computed above is of the form T = “ 6 = (I6“6 ) = “ 6(T)I6“6 where I6 is a diagonal matrix.
(c) In the class, we learned how to compute the moment of inertia matrix for a system of particles. Compute IR (moment of inertia matrix from a rotating, body-fixed frame) and show that I6 is indeed IR .
2 In the class, we learned about how to describe the motion of a spinning top. In this problem, we want to simulate the motion of the spinning top. Let’s employ the approach with Euler’s equations of motion without generalized coordinates.
In general, we have to solve
I +“ R .(I“R ) = NR
and
dR
= RΩR
dt
(vect(ΩR ) =“R ) together to obtain the trajectory of the center of mass of the spinning top. Recall that xcm(L) = Rxcm(R) where xcm(R) = [0 0 L]T .
Let the total mass of the top m = 1 [kg], L = 0.1 [m], g = 9.8 [m/sec2], I = I = 1, I = 1.5 [kg/m2]. We want to simulate the motion up to tend = 10 [sec]. Plot the trajectory of the center of mass of the top for the following cases. Note that all the angular quantities are expressed in radians.
(a) “ R (0) = [0 0 2.5]T and R(0) = RZXZ(0, 0.95, 0);
(b) “ R (0) = [0 0 3.5]T and R(0) = RZXZ(0, 0.5, 0);
(c) “ R (0) = [0 sin(0.95) 0.45 + cos(0.95)]T and R(0) = RZXZ(0, 0.95, 0).
“Steady precession” is one of the interesting behaviors of a spinning top. It is defined as the motion of a top when θ , ψ˙ , φ˙ are all constant. As a result, the trajectory of the center of mass will follow a circle. Which case behaves close to “steady precession”? Please submit your source code as well. Note that integration of (2) should be done, as discussed in the class, as follows:
R(tk/1) = R(tk) exp (∆t ΩR(tk/1))
where exp denotes the matrix exponential.
3 Let us consider a single rigid body. The general form of the kinetic energy is
T = mvcm = vcm + “ R(T)IR“R
where vcm denotes the (linear) velocity of the center of mass G of the rigid body. Other terms are defined in the class.
(a) Regarding the first term in T, show that it does not change whether you use spatial or body form of the velocity of the center of mass, i.e., vcm(L) and vcm(R) respectively.
(b) Now, let us consider a point, denoted as C, other than the center of mass. This point is not fixed in space, either. In other words, we consider a point in the body that is moving in space, and attach the body fixed frame at C, not G. Can you express the kinetic energy in the form
of
mvC = vC + “R(T)IC“ R
where vC denotes the velocity of the point C, and IC is the body moment of inertia matrix about the point C? Why or why not?
Submission Guideline
● Submission will be done into two parts: analytical and computational parts. The analytic part includes your hand-written answers (calculations, derivations, and so on), Mathematica print- out to pdf format, and necessary plots. Submit this in a single pdf file to “HW5 analytical” on the gradescope.
● Submit all your Mathematica and Matlab codes in a single .zip file that contains codes for each problem (name them by including the problem number). Name your single zip file submission as “YourName HW5.zip” . For example, “JinSeobKim HW5.zip” for a single zip file. Submission will be done through “HW5 computational” on the gradescope.
● Just in case you have related separate files, please make sure to include all the necessary files. If the TA try to run your function and it does not run, then your submission will have a significant points deduction.
● Make as much comments as possible so that the TA can easily read your codes.
2023-01-18