nass-simscape/org/compensation_gravity_forces.org

#+TITLE: Compensating the gravity forces to start at steady state
#+SETUPFILE: ./setup/org-setup-file.org

* Introduction                                                        :ignore:
In this file is shown a technique used to compensate the gravity forces at t=0.

The problem is that in presence of gravity, the system does not start at steady state and experience a transient phase (section [[sec:no_compensation]]).

In order to start the simulation at steady state in presence of gravity:
- section [[sec:compute_forces]]: first the stages are initialize in such a way that they are rigid, and the forces/torques applied at the location of their joints is measured
- section [[sec:compensation]]: Then, the equilibrium position of each joint is modified in such a way that at t=0, the forces in each joints exactly compensate the forces due to gravity forces

* Matlab Init                                                :noexport:ignore:
#+BEGIN_SRC matlab
simulinkproject('../');
#+END_SRC

#+BEGIN_SRC matlab
open('nass_model.slx')
#+END_SRC

* Initialization of the Experimental Conditions
We don't inject any perturbations and no reference tracking.
#+BEGIN_SRC matlab
initializeReferences();
initializeDisturbances('enable', false);
initializeController();
#+END_SRC

We include the gravity and log all the signals to display.
#+BEGIN_SRC matlab
initializeSimscapeConfiguration('gravity', true);
initializeLoggingConfiguration('log', 'all');
#+END_SRC

* Without compensation
<<sec:no_compensation>>
Let's simulate the system without any compensation of gravity forces.

#+BEGIN_SRC matlab
initializeGround();
initializeGranite();
initializeTy();
initializeRy();
initializeRz();
initializeMicroHexapod();
initializeAxisc();
initializeMirror();
initializeNanoHexapod();
initializeSample();
#+END_SRC

#+BEGIN_SRC matlab
load('mat/conf_simulink.mat');
set_param(conf_simulink, 'StopTime', '0.5');
#+END_SRC

#+BEGIN_SRC matlab
sim('nass_model');
sim_no_compensation = simout;
#+END_SRC

And we can observe on Figure [[fig:transient_phase_gravity_no_compensation]] that there are some motion in the system.
#+name: fig:transient_phase_gravity_no_compensation
#+caption: Motion of the sample at the start of the simulation in presence of gravity ([[./figs/transient_phase_gravity_no_compensation.png][png]], [[./figs/transient_phase_gravity_no_compensation.pdf][pdf]])
[[file:figs/transient_phase_gravity_no_compensation.png]]

* Simulation to compute the required force in each joint
<<sec:compute_forces>>
We here wish to simulate the system in order to compute the required force in each joint to compensate the gravity forces.

#+BEGIN_SRC matlab
initializeGround();
initializeGranite('type', 'init');
initializeTy('type', 'init');
initializeRy('type', 'init');
initializeRz('type', 'init');
initializeMicroHexapod('type', 'init');
initializeAxisc();
initializeMirror();
initializeNanoHexapod('type', 'init');
initializeSample('type', 'init');
#+END_SRC

We simulate for a short time period (all the bodies are solid, so nothing should move).
#+BEGIN_SRC matlab
load('mat/conf_simulink.mat');
set_param(conf_simulink, 'StopTime', '0.1');
#+END_SRC

#+BEGIN_SRC matlab
sim('nass_model');
#+END_SRC

Verification that nothing is moving by looking at the maximum displacement of the sample:
#+BEGIN_SRC matlab
max(max(simout.Em.En.Data))
#+END_SRC

#+RESULTS:
: 1.0681e-15

We here show the measured total force/torque applied at the location of each joint.
#+BEGIN_SRC matlab
data2orgtable([Fgm 0 0 0; Ftym; Fym; Fsm], {'Granite', 'Translation Stage', 'Tilt Stage', 'Sample'}, {'Fx', 'Fy', 'Fz', 'Mx', 'My', 'Mz'}, ' %.1e ');
#+END_SRC

#+RESULTS:
|                   |       Fx |      Fy |       Fz |       Mx |       My |      Mz |
|-------------------+----------+---------+----------+----------+----------+---------|
| Granite           | -7.6e-12 | 1.2e-11 | -34000.0 |      0.0 |      0.0 |     0.0 |
| Translation Stage | -7.6e-12 | 1.2e-11 | -12000.0 |     31.0 |      2.5 | 6.6e-13 |
| Tilt Stage        | -7.6e-12 | 1.2e-11 |  -8800.0 |     33.0 |    -0.52 | 6.6e-13 |
| Sample            | -5.7e-12 | 1.3e-11 |   -490.0 | -2.5e-12 | -8.1e-13 | 2.7e-13 |

#+BEGIN_SRC matlab
data2orgtable([Fhm; Fnm], {'Micro-Hexapod', 'Nano-Hexapod'}, {'F1', 'F2', 'F3', 'F4', 'F5', 'F6'}, ' %.1e ');
#+END_SRC

#+RESULTS:
|               |     F1 |     F2 |     F3 |     F4 |     F5 |     F6 |
|---------------+--------+--------+--------+--------+--------+--------|
| Micro-Hexapod | -180.0 | -180.0 | -180.0 | -180.0 | -180.0 | -180.0 |
| Nano-Hexapod  | -160.0 | -160.0 | -160.0 | -160.0 | -160.0 | -160.0 |

We save these forces in =Foffset.mat=.
#+BEGIN_SRC matlab
save('mat/Foffset.mat', 'Fgm', 'Ftym', 'Fym', 'Fzm', 'Fhm', 'Fnm', 'Fsm');
#+END_SRC

* New simulation with compensation of gravity forces
<<sec:compensation>>
We now initialize the stages with the option =Foffset=.
#+BEGIN_SRC matlab
initializeGround();
initializeGranite('Foffset', true);
initializeTy('Foffset', true);
initializeRy('Foffset', true);
initializeRz('Foffset', true);
initializeMicroHexapod('Foffset', true);
initializeAxisc();
initializeMirror();
initializeNanoHexapod('Foffset', true);
initializeSample('Foffset', true);
#+END_SRC

And we simulate the system for 0.5 seconds.
#+BEGIN_SRC matlab
load('mat/conf_simulink.mat');
set_param(conf_simulink, 'StopTime', '0.5');
#+END_SRC

#+BEGIN_SRC matlab
sim('nass_model');
sim_compensation = simout;
#+END_SRC

Verification that nothing is moving
#+name: fig:transient_phase_gravity_compensation
#+caption: Motion of the sample at the start of the simulation in presence of gravity when compensating the gravity forces  ([[./figs/transient_phase_gravity_compensation.png][png]], [[./figs/transient_phase_gravity_compensation.pdf][pdf]])
[[file:figs/transient_phase_gravity_compensation.png]]
* Conclusion
#+begin_important
  This initialization technique permits to compute the required forces/torques to be applied in each joint in order to compensate for gravity forces.
  This initialization should be redone for each configuration (change of sample mass, change of tilt angle), but not when changing the stiffness of joints, for instant when changing from lorentz based nano-hexapod or piezo based.
#+end_important