nass-simscape/org/flexible_joints_study.org

1001 lines
34 KiB
Org Mode
Raw Normal View History

2020-05-04 22:11:36 +02:00
#+TITLE: Study of the Flexible Joints
#+SETUPFILE: ./setup/org-setup-file.org
* Introduction :ignore:
In this document is studied the effect of the mechanical behavior of the flexible joints that are located the extremities of each nano-hexapod's legs.
Ideally, we want the x and y rotations to be free and all the translations to be blocked.
However, this is never the case and be have to consider:
2020-05-05 11:27:37 +02:00
- Finite bending stiffnesses (Section [[sec:rot_stiffness]])
- Axial stiffness in the direction of the legs (Section [[sec:trans_stiffness]])
2020-05-04 22:11:36 +02:00
2020-05-05 11:27:37 +02:00
This may impose some limitations, also, the goal is to specify the required joints stiffnesses (summarized in Section [[sec:conclusion]]).
2020-05-04 22:11:36 +02:00
2020-05-05 11:27:37 +02:00
* Bending and Torsional Stiffness
2020-05-04 22:11:36 +02:00
<<sec:rot_stiffness>>
** Introduction :ignore:
In this section, we wish to study the effect of the rotation flexibility of the nano-hexapod joints.
** Matlab Init :noexport:ignore:
#+begin_src matlab :tangle no :exports none :results silent :noweb yes :var current_dir=(file-name-directory buffer-file-name)
<<matlab-dir>>
#+end_src
#+begin_src matlab :exports none :results silent :noweb yes
<<matlab-init>>
#+end_src
#+begin_src matlab :tangle no
simulinkproject('../');
#+end_src
#+begin_src matlab
load('mat/conf_simulink.mat');
open('nass_model.slx')
#+end_src
** Initialization
Let's initialize all the stages with default parameters.
#+begin_src matlab
initializeGround();
initializeGranite();
initializeTy();
initializeRy();
initializeRz();
initializeMicroHexapod();
initializeAxisc();
initializeMirror();
#+end_src
#+begin_src matlab :exports none
initializeSimscapeConfiguration();
initializeDisturbances('enable', false);
initializeLoggingConfiguration('log', 'none');
initializeController('type', 'hac-dvf');
#+end_src
Let's consider the heaviest mass which should we the most problematic with it comes to the flexible joints.
#+begin_src matlab
initializeSample('mass', 50, 'freq', 200*ones(6,1));
initializeReferences('Rz_type', 'rotating-not-filtered', 'Rz_period', 60);
#+end_src
#+begin_src matlab :exports none
K = tf(zeros(6));
Kdvf = tf(zeros(6));
#+end_src
2020-05-05 11:27:37 +02:00
** Realistic Bending Stiffness Values
2020-05-04 22:11:36 +02:00
*** Introduction :ignore:
Let's compare the ideal case (zero stiffness in rotation and infinite stiffness in translation) with a more realistic case:
- $K_{\theta, \phi} = 15\,[Nm/rad]$ stiffness in flexion
- $K_{\psi} = 20\,[Nm/rad]$ stiffness in torsion
#+begin_src matlab
Kf_M = 15*ones(6,1);
Kf_F = 15*ones(6,1);
2020-05-05 10:34:45 +02:00
Kt_M = 20*ones(6,1);
2020-05-04 22:11:36 +02:00
Kt_F = 20*ones(6,1);
#+end_src
The stiffness and damping of the nano-hexapod's legs are:
#+begin_src matlab
2020-05-05 10:34:45 +02:00
k_opt = 1e5; % [N/m]
c_opt = 2e2; % [N/(m/s)]
2020-05-04 22:11:36 +02:00
#+end_src
2020-05-05 10:34:45 +02:00
This corresponds to the optimal identified stiffness.
2020-05-04 22:11:36 +02:00
*** Direct Velocity Feedback
We identify the dynamics from actuators force $\tau_i$ to relative motion sensors $d\mathcal{L}_i$ with and without considering the flexible joint stiffness.
The obtained dynamics are shown in Figure [[fig:flex_joint_rot_dvf]].
It is shown that the adding of stiffness for the flexible joints does increase a little bit the frequencies of the mass suspension modes. It stiffen the structure.
#+begin_src matlab :exports none
%% Name of the Simulink File
mdl = 'nass_model';
%% Input/Output definition
clear io; io_i = 1;
io(io_i) = linio([mdl, '/Controller'], 1, 'openinput'); io_i = io_i + 1; % Actuator Inputs
io(io_i) = linio([mdl, '/Micro-Station'], 3, 'openoutput', [], 'Dnlm'); io_i = io_i + 1; % Force Sensors
#+end_src
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal_p', ...
'type_M', 'spherical_p');
G_dvf_p = linearize(mdl, io);
G_dvf_p.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G_dvf_p.OutputName = {'Dnlm1', 'Dnlm2', 'Dnlm3', 'Dnlm4', 'Dnlm5', 'Dnlm6'};
#+end_src
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal', ...
'type_M', 'spherical', ...
'Kf_M', Kf_M, ...
'Kt_M', Kt_M, ...
'Kf_F', Kf_F, ...
'Kt_F', Kt_F);
G_dvf = linearize(mdl, io);
G_dvf.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G_dvf.OutputName = {'Dnlm1', 'Dnlm2', 'Dnlm3', 'Dnlm4', 'Dnlm5', 'Dnlm6'};
#+end_src
#+begin_src matlab :exports none
freqs = logspace(-1, 3, 1000);
figure;
ax1 = subplot(2, 1, 1);
hold on;
plot(freqs, abs(squeeze(freqresp(G_dvf(1, 1), freqs, 'Hz'))));
plot(freqs, abs(squeeze(freqresp(G_dvf_p(1, 1), freqs, 'Hz'))));
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'log');
ylabel('Amplitude [m/N]'); set(gca, 'XTickLabel',[]);
ax2 = subplot(2, 1, 2);
hold on;
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(G_dvf(1, 1), freqs, 'Hz')))), ...
'DisplayName', 'Flexible Joints');
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(G_dvf_p(1, 1), freqs, 'Hz')))), ...
'DisplayName', 'Perfect Joints');
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'lin');
ylabel('Phase [deg]'); xlabel('Frequency [Hz]');
ylim([-270, 90]);
yticks([-360:90:360]);
legend('location', 'northeast');
linkaxes([ax1,ax2],'x');
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
2020-05-05 10:34:45 +02:00
exportFig('figs/flex_joint_rot_dvf.pdf', 'width', 'full', 'height', 'full');
2020-05-04 22:11:36 +02:00
#+end_src
#+name: fig:flex_joint_rot_dvf
#+caption: Dynamics from actuators force $\tau_i$ to relative motion sensors $d\mathcal{L}_i$ with (blue) and without (red) considering the flexible joint stiffness
#+RESULTS:
[[file:figs/flex_joint_rot_dvf.png]]
*** Primary Plant
#+begin_src matlab :exports none
Kdvf = 5e3*s/(1+s/2/pi/1e3)*eye(6);
#+end_src
Let's now identify the dynamics from $\bm{\tau}^\prime$ to $\bm{\epsilon}_{\mathcal{X}_n}$ (for the primary controller designed in the frame of the legs).
The dynamics is compare with and without the joint flexibility in Figure [[fig:flex_joints_rot_primary_plant_L]].
The plant dynamics is not found to be changing significantly.
#+begin_src matlab :exports none
%% Name of the Simulink File
mdl = 'nass_model';
%% Input/Output definition
clear io; io_i = 1;
io(io_i) = linio([mdl, '/Controller'], 1, 'input'); io_i = io_i + 1; % Actuator Inputs
io(io_i) = linio([mdl, '/Tracking Error'], 1, 'output', [], 'En'); io_i = io_i + 1; % Position Errror
load('mat/stages.mat', 'nano_hexapod');
#+end_src
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal_p', ...
'type_M', 'spherical_p');
2020-05-05 10:34:45 +02:00
G = linearize(mdl, io);
G.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G.OutputName = {'Ex', 'Ey', 'Ez', 'Erx', 'Ery', 'Erz'};
2020-05-04 22:11:36 +02:00
2020-05-05 10:34:45 +02:00
Gl_p = -nano_hexapod.kinematics.J*G;
2020-05-04 22:11:36 +02:00
Gl_p.OutputName = {'E1', 'E2', 'E3', 'E4', 'E5', 'E6'};
#+end_src
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal', ...
'type_M', 'spherical', ...
'Kf_M', Kf_M, ...
'Kt_M', Kt_M, ...
'Kf_F', Kf_F, ...
'Kt_F', Kt_F);
G = linearize(mdl, io);
G.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G.OutputName = {'Ex', 'Ey', 'Ez', 'Erx', 'Ery', 'Erz'};
Gl = -nano_hexapod.kinematics.J*G;
Gl.OutputName = {'E1', 'E2', 'E3', 'E4', 'E5', 'E6'};
#+end_src
#+begin_src matlab :exports none
freqs = logspace(0, 3, 1000);
figure;
ax1 = subplot(2, 1, 1);
hold on;
for j = 1:6
set(gca,'ColorOrderIndex',1);
plot(freqs, abs(squeeze(freqresp(Gl(j, j), freqs, 'Hz'))));
end
for j = 1:6
set(gca,'ColorOrderIndex',2);
2020-05-05 11:27:37 +02:00
plot(freqs, abs(squeeze(freqresp(Gl_p(j, j), freqs, 'Hz'))));
2020-05-04 22:11:36 +02:00
end
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'log');
ylabel('Amplitude [m/N]'); set(gca, 'XTickLabel',[]);
ax2 = subplot(2, 1, 2);
hold on;
for j = 1:6
set(gca,'ColorOrderIndex',1);
2020-05-05 11:27:37 +02:00
if j == 1
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl(j, j), freqs, 'Hz')))), ...
'DisplayName', 'Flexible Joints');
else
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl(j, j), freqs, 'Hz')))), ...
'HandleVisibility', 'off');
end
2020-05-04 22:11:36 +02:00
end
for j = 1:6
set(gca,'ColorOrderIndex',2);
2020-05-05 11:27:37 +02:00
if j == 1
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl_p(j, j), freqs, 'Hz')))), ...
'DisplayName', 'Perfect Joints');
else
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl_p(j, j), freqs, 'Hz')))), ...
'HandleVisibility', 'off');
end
2020-05-04 22:11:36 +02:00
end
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'lin');
ylabel('Phase [deg]'); xlabel('Frequency [Hz]');
ylim([-270, 90]);
yticks([-360:90:360]);
2020-05-05 11:27:37 +02:00
legend('location', 'southwest');
2020-05-04 22:11:36 +02:00
linkaxes([ax1,ax2],'x');
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
exportFig('figs/flex_joints_rot_primary_plant_L.pdf', 'width', 'full', 'height', 'full');
#+end_src
#+name: fig:flex_joints_rot_primary_plant_L
2020-05-05 11:27:37 +02:00
#+caption: Dynamics from $\bm{\tau}^\prime_i$ to $\bm{\epsilon}_{\mathcal{X}_n,i}$ with perfect joints and with flexible joints
2020-05-04 22:11:36 +02:00
#+RESULTS:
[[file:figs/flex_joints_rot_primary_plant_L.png]]
*** Conclusion
#+begin_important
2020-05-05 11:27:37 +02:00
Considering realistic flexible joint bending stiffness for the nano-hexapod does not seems to impose any limitation on the DVF control nor on the primary control.
2020-05-04 22:11:36 +02:00
It only increases a little bit the suspension modes of the sample on top of the nano-hexapod.
#+end_important
** Parametric Study
*** Introduction :ignore:
We wish now to see what is the impact of the rotation stiffness of the flexible joints on the dynamics.
This will help to determine the requirements on the joint's stiffness.
2020-05-05 11:27:37 +02:00
Let's consider the following bending stiffness of the flexible joints:
2020-05-04 22:11:36 +02:00
#+begin_src matlab
2020-05-05 10:34:45 +02:00
Ks = [1, 5, 10, 50, 100]; % [Nm/rad]
2020-05-04 22:11:36 +02:00
#+end_src
We also consider here a nano-hexapod with the identified optimal actuator stiffness.
#+begin_src matlab :exports none
K = tf(zeros(6));
Kdvf = tf(zeros(6));
#+end_src
*** Direct Velocity Feedback
The dynamics from the actuators to the relative displacement sensor in each leg is identified and shown in Figure [[fig:flex_joints_rot_study_dvf]].
The corresponding Root Locus plot is shown in Figure [[fig:flex_joints_rot_study_dvf_root_locus]].
2020-05-05 11:27:37 +02:00
It is shown that the bending stiffness of the flexible joints does indeed change a little the dynamics, but critical damping is stiff achievable with Direct Velocity Feedback.
2020-05-04 22:11:36 +02:00
#+begin_src matlab :exports none
%% Name of the Simulink File
mdl = 'nass_model';
%% Input/Output definition
clear io; io_i = 1;
io(io_i) = linio([mdl, '/Controller'], 1, 'openinput'); io_i = io_i + 1; % Actuator Inputs
2020-05-05 10:34:45 +02:00
io(io_i) = linio([mdl, '/Micro-Station'], 3, 'openoutput', [], 'Dnlm'); io_i = io_i + 1; % Relative Displacement Sensors
2020-05-04 22:11:36 +02:00
2020-05-05 10:34:45 +02:00
G_dvf_s = {zeros(length(Ks), 1)};
2020-05-04 22:11:36 +02:00
for i = 1:length(Ks)
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal', ...
'type_M', 'spherical', ...
'Kf_M', Ks(i), ...
'Kt_M', Ks(i), ...
'Kf_F', Ks(i), ...
2020-05-05 10:34:45 +02:00
'Kt_F', Ks(i));
2020-05-04 22:11:36 +02:00
2020-05-05 10:34:45 +02:00
G = linearize(mdl, io);
G.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G.OutputName = {'Dnlm1', 'Dnlm2', 'Dnlm3', 'Dnlm4', 'Dnlm5', 'Dnlm6'};
2020-05-04 22:11:36 +02:00
2020-05-05 10:34:45 +02:00
G_dvf_s(i) = {G};
2020-05-04 22:11:36 +02:00
end
#+end_src
#+begin_src matlab :exports none
freqs = logspace(-1, 3, 1000);
figure;
ax1 = subplot(2, 1, 1);
hold on;
for i = 1:length(Ks)
2020-05-05 10:34:45 +02:00
plot(freqs, abs(squeeze(freqresp(G_dvf_s{i}(1, 1), freqs, 'Hz'))));
2020-05-04 22:11:36 +02:00
end
plot(freqs, abs(squeeze(freqresp(G_dvf_p(1, 1), freqs, 'Hz'))), 'k--');
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'log');
ylabel('Amplitude [m/N]'); set(gca, 'XTickLabel',[]);
ax2 = subplot(2, 1, 2);
hold on;
for i = 1:length(Ks)
2020-05-05 10:34:45 +02:00
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(G_dvf_s{i}(1, 1), freqs, 'Hz')))), ...
2020-05-05 11:27:37 +02:00
'DisplayName', sprintf('$k = %.0f$ [Nm/rad]', Ks(i)));
2020-05-04 22:11:36 +02:00
end
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(G_dvf_p(1, 1), freqs, 'Hz')))), 'k--', ...
'DisplayName', 'Ideal Joint');
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'lin');
ylabel('Phase [deg]'); xlabel('Frequency [Hz]');
ylim([-270, 90]);
yticks([-360:90:360]);
legend('location', 'southwest');
linkaxes([ax1,ax2],'x');
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
2020-05-05 10:34:45 +02:00
exportFig('figs/flex_joints_rot_study_dvf.pdf', 'width', 'full', 'height', 'full');
2020-05-04 22:11:36 +02:00
#+end_src
#+name: fig:flex_joints_rot_study_dvf
#+caption: Dynamics from $\tau_i$ to $d\mathcal{L}_i$ for all the considered Rotation Stiffnesses
#+RESULTS:
[[file:figs/flex_joints_rot_study_dvf.png]]
#+begin_src matlab :exports none
figure;
gains = logspace(2, 5, 300);
hold on;
for i = 1:length(Ks)
set(gca,'ColorOrderIndex',i);
2020-05-05 10:34:45 +02:00
plot(real(pole(G_dvf_s{i})), imag(pole(G_dvf_s{i})), 'x', ...
2020-05-05 11:27:37 +02:00
'DisplayName', sprintf('$k = %.0f$ [Nm/rad]', Ks(i)));
2020-05-04 22:11:36 +02:00
set(gca,'ColorOrderIndex',i);
2020-05-05 10:34:45 +02:00
plot(real(tzero(G_dvf_s{i})), imag(tzero(G_dvf_s{i})), 'o', ...
2020-05-04 22:11:36 +02:00
'HandleVisibility', 'off');
for k = 1:length(gains)
set(gca,'ColorOrderIndex',i);
2020-05-05 10:34:45 +02:00
cl_poles = pole(feedback(G_dvf_s{i}, (gains(k)*s)*eye(6)));
2020-05-04 22:11:36 +02:00
plot(real(cl_poles), imag(cl_poles), '.', ...
'HandleVisibility', 'off');
end
end
hold off;
axis square;
xlim([-140, 10]); ylim([0, 150]);
xlabel('Real Part'); ylabel('Imaginary Part');
legend('location', 'northwest');
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
2020-05-05 10:34:45 +02:00
exportFig('figs/flex_joints_rot_study_dvf_root_locus.pdf', 'width', 'wide', 'height', 'tall');
2020-05-04 22:11:36 +02:00
#+end_src
#+name: fig:flex_joints_rot_study_dvf_root_locus
#+caption: Root Locus for all the considered Rotation Stiffnesses
#+RESULTS:
[[file:figs/flex_joints_rot_study_dvf_root_locus.png]]
*** Primary Control
2020-05-05 10:34:45 +02:00
The dynamics from $\bm{\tau}^\prime$ to $\bm{\epsilon}_{\mathcal{X}_n}$ (for the primary controller designed in the frame of the legs) is shown in Figure [[fig:flex_joints_rot_study_primary_plant]].
2020-05-05 11:27:37 +02:00
It is shown that the bending stiffness of the flexible joints have very little impact on the dynamics.
2020-05-05 10:34:45 +02:00
#+begin_src matlab :exports none
2020-05-04 22:11:36 +02:00
Kdvf = 5e3*s/(1+s/2/pi/1e3)*eye(6);
#+end_src
#+begin_src matlab :exports none
%% Name of the Simulink File
mdl = 'nass_model';
%% Input/Output definition
clear io; io_i = 1;
io(io_i) = linio([mdl, '/Controller'], 1, 'input'); io_i = io_i + 1; % Actuator Inputs
io(io_i) = linio([mdl, '/Tracking Error'], 1, 'output', [], 'En'); io_i = io_i + 1; % Position Errror
load('mat/stages.mat', 'nano_hexapod');
2020-05-05 10:34:45 +02:00
Gl_s = {zeros(length(Ks), 1)};
2020-05-04 22:11:36 +02:00
for i = 1:length(Ks)
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal', ...
'type_M', 'spherical', ...
'Kf_M', Ks(i), ...
2020-05-05 10:34:45 +02:00
'Kt_M', Ks(i), ...
2020-05-04 22:11:36 +02:00
'Kf_F', Ks(i), ...
2020-05-05 10:34:45 +02:00
'Kt_F', Ks(i));
2020-05-04 22:11:36 +02:00
G = linearize(mdl, io);
G.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G.OutputName = {'Ex', 'Ey', 'Ez', 'Erx', 'Ery', 'Erz'};
2020-05-05 10:34:45 +02:00
Gl = -nano_hexapod.kinematics.J*G;
Gl.OutputName = {'E1', 'E2', 'E3', 'E4', 'E5', 'E6'};
Gl_s(i) = {Gl};
2020-05-04 22:11:36 +02:00
end
#+end_src
#+begin_src matlab :exports none
freqs = logspace(-1, 3, 1000);
figure;
ax1 = subplot(2, 1, 1);
hold on;
for i = 1:length(Ks)
2020-05-05 10:34:45 +02:00
plot(freqs, abs(squeeze(freqresp(Gl_s{i}(1, 1), freqs, 'Hz'))));
2020-05-04 22:11:36 +02:00
end
plot(freqs, abs(squeeze(freqresp(Gl_p(1, 1), freqs, 'Hz'))), 'k--');
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'log');
ylabel('Amplitude [m/N]'); set(gca, 'XTickLabel',[]);
ax2 = subplot(2, 1, 2);
hold on;
for i = 1:length(Ks)
2020-05-05 10:34:45 +02:00
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl_s{i}(1, 1), freqs, 'Hz')))), ...
2020-05-05 11:27:37 +02:00
'DisplayName', sprintf('$k = %.0f$ [Nm/rad]', Ks(i)));
2020-05-04 22:11:36 +02:00
end
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl_p(1, 1), freqs, 'Hz')))), 'k--', ...
'DisplayName', 'Ideal Joint');
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'lin');
ylabel('Phase [deg]'); xlabel('Frequency [Hz]');
ylim([-270, 90]);
yticks([-360:90:360]);
legend('location', 'southwest');
linkaxes([ax1,ax2],'x');
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
exportFig('figs/flex_joints_rot_study_primary_plant.pdf', 'width', 'full', 'height', 'full');
#+end_src
#+name: fig:flex_joints_rot_study_primary_plant
2020-05-05 11:27:37 +02:00
#+caption: Diagonal elements of the transfer function matrix from $\bm{\tau}^\prime$ to $\bm{\epsilon}_{\mathcal{X}_n}$ for the considered bending stiffnesses
2020-05-04 22:11:36 +02:00
#+RESULTS:
[[file:figs/flex_joints_rot_study_primary_plant.png]]
*** Conclusion
#+begin_important
2020-05-05 11:27:37 +02:00
The bending stiffness of the flexible joint does not significantly change the dynamics.
2020-05-04 22:11:36 +02:00
#+end_important
* Translation Stiffness
<<sec:trans_stiffness>>
** Introduction :ignore:
2020-05-05 10:34:45 +02:00
Let's know consider a flexibility in translation of the flexible joint, in the axis of the legs.
2020-05-04 22:11:36 +02:00
** Matlab Init :noexport:ignore:
#+begin_src matlab :tangle no :exports none :results silent :noweb yes :var current_dir=(file-name-directory buffer-file-name)
<<matlab-dir>>
#+end_src
#+begin_src matlab :exports none :results silent :noweb yes
<<matlab-init>>
#+end_src
#+begin_src matlab :tangle no
simulinkproject('../');
#+end_src
#+begin_src matlab
load('mat/conf_simulink.mat');
open('nass_model.slx')
#+end_src
2020-05-05 10:34:45 +02:00
** Realistic Translation Stiffness Values
*** Introduction :ignore:
We choose realistic values of the axial stiffness of the joints:
\[ K_a = 60\,[N/\mu m] \]
#+begin_src matlab
Kz_F = 60e6*ones(6,1); % [N/m]
Kz_M = 60e6*ones(6,1); % [N/m]
Cz_F = 1*ones(6,1); % [N/(m/s)]
Cz_M = 1*ones(6,1); % [N/(m/s)]
#+end_src
*** Initialization
2020-05-04 22:11:36 +02:00
Let's initialize all the stages with default parameters.
#+begin_src matlab
initializeGround();
initializeGranite();
initializeTy();
initializeRy();
initializeRz();
initializeMicroHexapod();
initializeAxisc();
initializeMirror();
#+end_src
#+begin_src matlab :exports none
initializeSimscapeConfiguration();
initializeDisturbances('enable', false);
initializeLoggingConfiguration('log', 'none');
initializeController('type', 'hac-dvf');
#+end_src
Let's consider the heaviest mass which should we the most problematic with it comes to the flexible joints.
#+begin_src matlab
initializeSample('mass', 50, 'freq', 200*ones(6,1));
initializeReferences('Rz_type', 'rotating-not-filtered', 'Rz_period', 60);
#+end_src
#+begin_src matlab :exports none
K = tf(zeros(6));
Kdvf = tf(zeros(6));
#+end_src
2020-05-05 10:34:45 +02:00
*** Direct Velocity Feedback
The dynamics from actuators force $\tau_i$ to relative motion sensors $d\mathcal{L}_i$ with and without considering the flexible joint stiffness are identified.
The obtained dynamics are shown in Figure [[fig:flex_joint_trans_dvf]].
2020-05-04 22:11:36 +02:00
#+begin_src matlab :exports none
%% Name of the Simulink File
mdl = 'nass_model';
%% Input/Output definition
clear io; io_i = 1;
io(io_i) = linio([mdl, '/Controller'], 1, 'openinput'); io_i = io_i + 1; % Actuator Inputs
io(io_i) = linio([mdl, '/Micro-Station'], 3, 'openoutput', [], 'Dnlm'); io_i = io_i + 1; % Force Sensors
#+end_src
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal_3dof', ...
'type_M', 'spherical_3dof', ...
'Kz_F', Kz_F, ...
'Kz_M', Kz_M, ...
'Cz_F', Cz_F, ...
'Cz_M', Cz_M);
G_dvf_3dof = linearize(mdl, io);
G_dvf_3dof.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G_dvf_3dof.OutputName = {'Dnlm1', 'Dnlm2', 'Dnlm3', 'Dnlm4', 'Dnlm5', 'Dnlm6'};
#+end_src
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal', ...
'type_M', 'spherical');
G_dvf = linearize(mdl, io);
G_dvf.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G_dvf.OutputName = {'Dnlm1', 'Dnlm2', 'Dnlm3', 'Dnlm4', 'Dnlm5', 'Dnlm6'};
#+end_src
#+begin_src matlab :exports none
freqs = logspace(-1, 3, 1000);
figure;
ax1 = subplot(2, 1, 1);
hold on;
plot(freqs, abs(squeeze(freqresp(G_dvf(1, 1), freqs, 'Hz'))));
plot(freqs, abs(squeeze(freqresp(G_dvf_3dof(1, 1), freqs, 'Hz'))));
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'log');
ylabel('Amplitude [m/N]'); set(gca, 'XTickLabel',[]);
ax2 = subplot(2, 1, 2);
hold on;
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(G_dvf(1, 1), freqs, 'Hz')))), ...
'DisplayName', '2dof Joints');
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(G_dvf_3dof(1, 1), freqs, 'Hz')))), ...
'DisplayName', '3dof Joints');
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'lin');
ylabel('Phase [deg]'); xlabel('Frequency [Hz]');
ylim([-270, 90]);
yticks([-360:90:360]);
legend('location', 'northeast');
linkaxes([ax1,ax2],'x');
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
2020-05-05 10:34:45 +02:00
exportFig('figs/flex_joint_trans_dvf.pdf', 'width', 'full', 'height', 'full');
2020-05-04 22:11:36 +02:00
#+end_src
#+name: fig:flex_joint_trans_dvf
2020-05-05 10:34:45 +02:00
#+caption: Dynamics from actuators force $\tau_i$ to relative motion sensors $d\mathcal{L}_i$ with (blue) and without (red) considering the flexible joint axis stiffness
2020-05-04 22:11:36 +02:00
#+RESULTS:
[[file:figs/flex_joint_trans_dvf.png]]
2020-05-05 10:34:45 +02:00
*** Primary Plant
2020-05-04 22:11:36 +02:00
#+begin_src matlab
Kdvf = 5e3*s/(1+s/2/pi/1e3)*eye(6);
#+end_src
2020-05-05 10:34:45 +02:00
Let's now identify the dynamics from $\bm{\tau}^\prime$ to $\bm{\epsilon}_{\mathcal{X}_n}$ (for the primary controller designed in the frame of the legs).
The dynamics is compare with and without the joint flexibility in Figure [[fig:flex_joints_trans_primary_plant_L]].
2020-05-04 22:11:36 +02:00
#+begin_src matlab :exports none
%% Name of the Simulink File
mdl = 'nass_model';
%% Input/Output definition
clear io; io_i = 1;
io(io_i) = linio([mdl, '/Controller'], 1, 'input'); io_i = io_i + 1; % Actuator Inputs
io(io_i) = linio([mdl, '/Tracking Error'], 1, 'output', [], 'En'); io_i = io_i + 1; % Position Errror
load('mat/stages.mat', 'nano_hexapod');
#+end_src
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal_3dof', ...
'type_M', 'spherical_3dof', ...
'Kz_F', Kz_F, ...
'Kz_M', Kz_M, ...
'Cz_F', Cz_F, ...
'Cz_M', Cz_M);
2020-05-05 10:34:45 +02:00
G = linearize(mdl, io);
G.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G.OutputName = {'Ex', 'Ey', 'Ez', 'Erx', 'Ery', 'Erz'};
2020-05-04 22:11:36 +02:00
2020-05-05 10:34:45 +02:00
Gl_3dof = -nano_hexapod.kinematics.J*G;
2020-05-04 22:11:36 +02:00
Gl_3dof.OutputName = {'E1', 'E2', 'E3', 'E4', 'E5', 'E6'};
#+end_src
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal', ...
'type_M', 'spherical');
G = linearize(mdl, io);
G.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G.OutputName = {'Ex', 'Ey', 'Ez', 'Erx', 'Ery', 'Erz'};
Gl = -nano_hexapod.kinematics.J*G;
Gl.OutputName = {'E1', 'E2', 'E3', 'E4', 'E5', 'E6'};
#+end_src
#+begin_src matlab :exports none
freqs = logspace(0, 3, 1000);
figure;
ax1 = subplot(2, 1, 1);
hold on;
for j = 1:6
set(gca,'ColorOrderIndex',1);
plot(freqs, abs(squeeze(freqresp(Gl(j, j), freqs, 'Hz'))));
end
for j = 1:6
set(gca,'ColorOrderIndex',2);
plot(freqs, abs(squeeze(freqresp(Gl_3dof(j, j), freqs, 'Hz'))), '--');
end
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'log');
ylabel('Amplitude [m/N]'); set(gca, 'XTickLabel',[]);
title('Diagonal elements of the Plant');
ax2 = subplot(2, 1, 2);
hold on;
for j = 1:6
set(gca,'ColorOrderIndex',1);
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl(j, j), freqs, 'Hz')))));
end
for j = 1:6
set(gca,'ColorOrderIndex',2);
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl_3dof(j, j), freqs, 'Hz')))), '--');
end
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'lin');
ylabel('Phase [deg]'); xlabel('Frequency [Hz]');
ylim([-270, 90]);
yticks([-360:90:360]);
linkaxes([ax1,ax2],'x');
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
2020-05-05 10:34:45 +02:00
exportFig('figs/flex_joints_trans_primary_plant_L.pdf', 'width', 'full', 'height', 'full');
2020-05-04 22:11:36 +02:00
#+end_src
#+name: fig:flex_joints_trans_primary_plant_L
2020-05-05 10:34:45 +02:00
#+caption: Dynamics from $\bm{\tau}^\prime_i$ to $\bm{\epsilon}_{\mathcal{X}_n,i}$ with infinite axis stiffnes (solid) and with realistic axial stiffness (dashed)
2020-05-04 22:11:36 +02:00
#+RESULTS:
[[file:figs/flex_joints_trans_primary_plant_L.png]]
2020-05-05 11:27:37 +02:00
*** Conclusion
#+begin_important
For the realistic value of the flexible joint axial stiffness, the dynamics is not much impact, and this should not be a problem for control.
#+end_important
2020-05-04 22:11:36 +02:00
** Parametric study
*** Introduction :ignore:
2020-05-05 10:34:45 +02:00
We wish now to see what is the impact of the *axial* stiffness of the flexible joints on the dynamics.
Let's consider the following values for the axial stiffness:
2020-05-04 22:11:36 +02:00
#+begin_src matlab
Kzs = [1e4, 1e5, 1e6, 1e7, 1e8, 1e9]; % [N/m]
#+end_src
2020-05-05 10:34:45 +02:00
We also consider here a nano-hexapod with the identified optimal actuator stiffness ($k = 10^5\,[N/m]$).
#+begin_src matlab :exports none
K = tf(zeros(6));
Kdvf = tf(zeros(6));
#+end_src
2020-05-04 22:11:36 +02:00
*** Direct Velocity Feedback
2020-05-05 10:34:45 +02:00
The dynamics from the actuators to the relative displacement sensor in each leg is identified and shown in Figure [[fig:flex_joints_trans_study_dvf]].
It is shown that the axial stiffness of the flexible joints does have a huge impact on the dynamics.
If the axial stiffness of the flexible joints is $K_a > 10^7\,[N/m]$ (here $100$ times higher than the actuator stiffness), then the change of dynamics stays reasonably small.
This is more clear by looking at the root locus (Figures [[fig:flex_joints_trans_study_dvf_root_locus]] and [[fig:flex_joints_trans_study_root_locus_unzoom]]).
2020-05-05 11:27:37 +02:00
It can be seen that very little active damping can be achieve for axial stiffnesses below $10^7\,[N/m]$.
2020-05-05 10:34:45 +02:00
2020-05-04 22:11:36 +02:00
#+begin_src matlab :exports none
%% Name of the Simulink File
mdl = 'nass_model';
%% Input/Output definition
clear io; io_i = 1;
io(io_i) = linio([mdl, '/Controller'], 1, 'openinput'); io_i = io_i + 1; % Actuator Inputs
io(io_i) = linio([mdl, '/Micro-Station'], 3, 'openoutput', [], 'Dnlm'); io_i = io_i + 1; % Force Sensors
2020-05-05 10:34:45 +02:00
G_dvf_3dof_s = {zeros(length(Kzs), 1)};
2020-05-04 22:11:36 +02:00
for i = 1:length(Kzs)
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal_3dof', ...
'type_M', 'spherical_3dof', ...
'Kz_F', Kzs(i), ...
'Kz_M', Kzs(i), ...
2020-05-05 10:34:45 +02:00
'Cz_F', 0.05*sqrt(Kzs(i)*10), ...
'Cz_M', 0.05*sqrt(Kzs(i)*10));
2020-05-04 22:11:36 +02:00
G = linearize(mdl, io);
G.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G.OutputName = {'Dnlm1', 'Dnlm2', 'Dnlm3', 'Dnlm4', 'Dnlm5', 'Dnlm6'};
2020-05-05 10:34:45 +02:00
G_dvf_3dof_s(i) = {G};
2020-05-04 22:11:36 +02:00
end
#+end_src
#+begin_src matlab :exports none
freqs = logspace(-1, 3, 1000);
figure;
ax1 = subplot(2, 1, 1);
hold on;
for i = 1:length(Kzs)
2020-05-05 10:34:45 +02:00
plot(freqs, abs(squeeze(freqresp(G_dvf_3dof_s{i}(1, 1), freqs, 'Hz'))));
2020-05-04 22:11:36 +02:00
end
2020-05-05 10:34:45 +02:00
plot(freqs, abs(squeeze(freqresp(G_dvf(1, 1), freqs, 'Hz'))), 'k--');
2020-05-04 22:11:36 +02:00
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'log');
ylabel('Amplitude [m/N]'); set(gca, 'XTickLabel',[]);
ax2 = subplot(2, 1, 2);
hold on;
for i = 1:length(Kzs)
2020-05-05 10:34:45 +02:00
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(G_dvf_3dof_s{i}(1, 1), freqs, 'Hz')))), ...
'DisplayName', sprintf('$k = %.0g$ [N/m]', Kzs(i)));
2020-05-04 22:11:36 +02:00
end
2020-05-05 10:34:45 +02:00
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(G_dvf(1, 1), freqs, 'Hz')))), 'k--', ...
'DisplayName', 'Perfect Joint');
2020-05-04 22:11:36 +02:00
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'lin');
ylabel('Phase [deg]'); xlabel('Frequency [Hz]');
ylim([-270, 90]);
yticks([-360:90:360]);
2020-05-05 10:34:45 +02:00
legend('location', 'southwest');
2020-05-04 22:11:36 +02:00
linkaxes([ax1,ax2],'x');
#+end_src
2020-05-05 10:34:45 +02:00
#+begin_src matlab :tangle no :exports results :results file replace
exportFig('figs/flex_joints_trans_study_dvf.pdf', 'width', 'full', 'height', 'full');
#+end_src
#+name: fig:flex_joints_trans_study_dvf
#+caption: Dynamics from $\tau_i$ to $d\mathcal{L}_i$ for all the considered axis Stiffnesses
#+RESULTS:
[[file:figs/flex_joints_trans_study_dvf.png]]
#+begin_src matlab :exports none
figure;
gains = logspace(2, 5, 300);
hold on;
for i = 1:length(Kzs)
set(gca,'ColorOrderIndex',i);
plot(real(pole(G_dvf_3dof_s{i})), imag(pole(G_dvf_3dof_s{i})), 'x', ...
'DisplayName', sprintf('$k = %.0g$ [N/m]', Kzs(i)));
set(gca,'ColorOrderIndex',i);
plot(real(tzero(G_dvf_3dof_s{i})), imag(tzero(G_dvf_3dof_s{i})), 'o', ...
'HandleVisibility', 'off');
for k = 1:length(gains)
set(gca,'ColorOrderIndex',i);
cl_poles = pole(feedback(G_dvf_3dof_s{i}, (gains(k)*s)*eye(6)));
plot(real(cl_poles), imag(cl_poles), '.', ...
'HandleVisibility', 'off');
end
end
hold off;
axis square;
xlim([-140, 10]); ylim([0, 150]);
xlabel('Real Part'); ylabel('Imaginary Part');
legend('location', 'northwest');
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
exportFig('figs/flex_joints_trans_study_dvf_root_locus.pdf', 'width', 'wide', 'height', 'tall');
#+end_src
#+name: fig:flex_joints_trans_study_dvf_root_locus
#+caption: Root Locus for all the considered axial Stiffnesses
#+RESULTS:
[[file:figs/flex_joints_trans_study_dvf_root_locus.png]]
2020-05-05 11:27:37 +02:00
#+begin_src matlab :exports none
2020-05-05 10:34:45 +02:00
xlim([-1e3, 0]);
ylim([0, 1e3]);
#+end_src
#+begin_src matlab :tangle no :exports results :results file replace
exportFig('figs/flex_joints_trans_study_root_locus_unzoom.pdf', 'width', 'wide', 'height', 'tall');
#+end_src
#+name: fig:flex_joints_trans_study_root_locus_unzoom
#+caption: Root Locus (unzoom) for all the considered axial Stiffnesses
#+RESULTS:
[[file:figs/flex_joints_trans_study_root_locus_unzoom.png]]
*** Primary Control
The dynamics from $\bm{\tau}^\prime$ to $\bm{\epsilon}_{\mathcal{X}_n}$ (for the primary controller designed in the frame of the legs) is shown in Figure [[fig:flex_joints_trans_study_primary_plant]].
#+begin_src matlab :exports none
2020-05-04 22:11:36 +02:00
Kdvf = 5e3*s/(1+s/2/pi/1e3)*eye(6);
#+end_src
#+begin_src matlab :exports none
%% Name of the Simulink File
mdl = 'nass_model';
%% Input/Output definition
clear io; io_i = 1;
io(io_i) = linio([mdl, '/Controller'], 1, 'input'); io_i = io_i + 1; % Actuator Inputs
io(io_i) = linio([mdl, '/Tracking Error'], 1, 'output', [], 'En'); io_i = io_i + 1; % Position Errror
load('mat/stages.mat', 'nano_hexapod');
Gl_3dof_s = {zeros(length(Kzs), 1)};
for i = 1:length(Kzs)
2020-05-05 10:34:45 +02:00
initializeNanoHexapod('k', k_opt, 'c', c_opt, ...
2020-05-04 22:11:36 +02:00
'type_F', 'universal_3dof', ...
'type_M', 'spherical_3dof', ...
'Kz_F', Kzs(i), ...
'Kz_M', Kzs(i), ...
2020-05-05 10:34:45 +02:00
'Cz_F', 0.05*sqrt(Kzs(i)*10), ...
'Cz_M', 0.05*sqrt(Kzs(i)*10));
2020-05-04 22:11:36 +02:00
G = linearize(mdl, io);
G.InputName = {'Fnl1', 'Fnl2', 'Fnl3', 'Fnl4', 'Fnl5', 'Fnl6'};
G.OutputName = {'Ex', 'Ey', 'Ez', 'Erx', 'Ery', 'Erz'};
Gl_3dof = -nano_hexapod.kinematics.J*G;
Gl_3dof.OutputName = {'E1', 'E2', 'E3', 'E4', 'E5', 'E6'};
Gl_3dof_s(i) = {Gl_3dof};
end
#+end_src
#+begin_src matlab :exports none
freqs = logspace(-1, 3, 1000);
figure;
ax1 = subplot(2, 1, 1);
hold on;
for i = 1:length(Kzs)
plot(freqs, abs(squeeze(freqresp(Gl_3dof_s{i}(1, 1), freqs, 'Hz'))));
end
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'log');
ylabel('Amplitude [m/N]'); set(gca, 'XTickLabel',[]);
ax2 = subplot(2, 1, 2);
hold on;
for i = 1:length(Kzs)
plot(freqs, 180/pi*unwrap(angle(squeeze(freqresp(Gl_3dof_s{i}(1, 1), freqs, 'Hz')))), ...
'DisplayName', sprintf('$k = %.0g$ [N/m]', Kzs(i)));
end
hold off;
set(gca, 'XScale', 'log'); set(gca, 'YScale', 'lin');
ylabel('Phase [deg]'); xlabel('Frequency [Hz]');
ylim([-270, 90]);
yticks([-360:90:360]);
legend('location', 'northeast');
linkaxes([ax1,ax2],'x');
#+end_src
2020-05-05 10:34:45 +02:00
#+begin_src matlab :tangle no :exports results :results file replace
exportFig('figs/flex_joints_trans_study_primary_plant.pdf', 'width', 'full', 'height', 'full');
#+end_src
#+name: fig:flex_joints_trans_study_primary_plant
#+caption: Diagonal elements of the transfer function matrix from $\bm{\tau}^\prime$ to $\bm{\epsilon}_{\mathcal{X}_n}$ for the considered axial stiffnesses
#+RESULTS:
[[file:figs/flex_joints_trans_study_primary_plant.png]]
2020-05-04 22:11:36 +02:00
** Conclusion
#+begin_important
2020-05-05 10:34:45 +02:00
The axial stiffness of the flexible joints should be maximized.
2020-05-05 11:27:37 +02:00
For the considered actuator stiffness $k = 10^5\,[N/m]$, the axial stiffness of the flexible joints should ideally be above $10^7\,[N/m]$.
2020-05-05 10:34:45 +02:00
This is a reasonable stiffness value for such joints.
2020-05-04 22:11:36 +02:00
2020-05-05 10:34:45 +02:00
We may interpolate the results and say that the axial joint stiffness should be 100 times higher than the actuator stiffness, but this should be confirmed with further analysis.
2020-05-04 22:11:36 +02:00
#+end_important
2020-05-05 11:27:37 +02:00
* Conclusion
<<sec:conclusion>>
#+begin_important
For the identified optimal actuator stiffness $k = 10^5\,[N/m]$, the flexible joint should have the following stiffness properties:
- Bending Stiffness: $K_b < 50\,[Nm/rad]$
- Torsion Stiffness: $K_t < 50\,[Nm/rad]$
- Axial Stiffness: $K_a > 10^7\,[N/m]$
#+end_important