nass-simscape/kinematics/index.org

Kinematics of the station

• Introduction
• Micro Hexapod
  • How the Symetrie Hexapod is controlled on the micro station
  • Control of the Micro-Hexapod using Simscape
    • Introduction
    • Using Bushing Joint
    • Using Inverse Kinematics and Leg Actuators
      • Theory
      • Matlab Implementation

Introduction   ignore

In this document, we discuss the way the motion of each stage is defined.

Micro Hexapod

How the Symetrie Hexapod is controlled on the micro station

For the Micro-Hexapod, the convention for the angles are defined in MAN_A_Software API_4.0.150918_EN.pdf on page 13 (section 2.4 - Rotation Vectors):

The Euler type II convention is used to express the rotation vector. This convention is mainly used in the aeronautics field (standard ISO 1151 concerning flight mechanics).

This convention uses the concepts of rotation of vehicles (ship, car and plane). Generally, we consider that the main movement of the vehicle is following the X-axis and the Z-axis is parallel to the axis of gravity (at the initial position). The roll rotation is around the X-axis, the pitch is around the Y-axis and yaw is the rotation around the Z-axis. The order of rotation is: Rx, Ry and then Rz.

In most case, rotations are related to a reference with fixed axis; thus we say the rotations are around fixed axes. The combination of these three rotations enables to write a rotation matrix. This writing is unique and equal to: \[ \bm{R} = \bm{R}_z(\gamma) \cdot \bm{R}_y(\beta) \cdot \bm{R}_x(\alpha) \]

The Euler type II convention corresponding to the succession of rotations with respect to fixed axes: first around X0, then Y0 and Z0. This is equivalent to the succession of rotations with respect to mobile axes: first around Z0, then Y1' and X2'.

More generally on the Control of the Micro-Hexapod:

Note that for all control modes, the rotation center coincides with Object coordinate system origin. Moreover, the movements are controlled with translation components at first (Tx, Ty, Tz) then rotation components (Rx, Ry, Rz).

Thus, it does the translations and then the rotation around the new translated frame.

Control of the Micro-Hexapod using Simscape

Introduction   ignore

We can think of two main ways to position the Micro-Hexapod using Simscape.

The first one is to use only one Bushing Joint between the base and the mobile platform. The advantage is that it is very easy to impose the wanted displacement, however, we loose the dynamical properties of the Hexapod.

The second way is to specify the wanted length of the legs of the Hexapod in order to have the wanted position of the mobile platform. This require a little bit more of mathematical derivations but this is the chosen solution.

Using Bushing Joint

In the documentation of the Bushing Joint (doc "Bushing Joint") that is used to position the Hexapods, it is mention that the following frame is positioned with respect to the base frame in a way shown in figure fig:bushing_joint_transform.

Joint Transformation Sequence for the Bushing Joint

Basically, it performs the translations, and then the rotation along the X, Y and Z axis of the moving frame. The three rotations that we define thus corresponds to the Euler U-V-W angles.

We should have the same behavior for the Micro-Hexapod on Simscape (same inputs at least). However, the Bushing Joint makes rotations around mobiles axes (X, Y' and then Z'') and not fixed axes (X, Y and Z).

Using Inverse Kinematics and Leg Actuators

Here, we can use the Inverse Kinematic of the Hexapod to determine the length of each leg in order to obtain some defined translation and rotation of the mobile platform.

The advantages are:

• we can position the Hexapod as we want by specifying a rotation matrix
• the hexapod keeps its full flexibility as we don't specify any wanted displacements, only leg's rest position

However:

• even though the rest position of each leg (the position where the stiffness force is zero) is set correctly, the hexapod will we deflected due to gravity

Thus, for this simulation, we remove the gravity.

Theory

For inverse kinematic analysis, it is assumed that the position ${}^A\bm{P}$ and orientation of the moving platform ${}^A\bm{R}_B$ are given and the problem is to obtain the joint variables, namely, $\bm{L} = [l_1, l_2, \dots, l_6]^T$.

From the geometry of the manipulator, the loop closure for each limb, $i = 1, 2, \dots, 6$ can be written as

\begin{align*} l_i {}^A\hat{\bm{s}}_i &= {}^A\bm{A} + {}^A\bm{b}_i - {}^A\bm{a}_i \\ &= {}^A\bm{A} + {}^A\bm{R}_b {}^B\bm{b}_i - {}^A\bm{a}_i \end{align*}

To obtain the length of each actuator and eliminate $\hat{\bm{s}}_i$, it is sufficient to dot multiply each side by itself:

\begin{equation} l_i^2 \left[ {}^A\hat{\bm{s}}_i^T {}^A\hat{\bm{s}}_i \right] = \left[ {}^A\bm{P} + {}^A\bm{R}_B {}^B\bm{b}_i - {}^A\bm{a}_i \right]^T \left[ {}^A\bm{P} + {}^A\bm{R}_B {}^B\bm{b}_i - {}^A\bm{a}_i \right] \end{equation}

Hence, for $i = 1, 2, \dots, 6$, each limb length can be uniquely determined by:

\begin{equation} l_i = \sqrt{{}^A\bm{P}^T {}^A\bm{P} + {}^B\bm{b}_i^T {}^B\bm{b}_i + {}^A\bm{a}_i^T {}^A\bm{a}_i - 2 {}^A\bm{P}^T {}^A\bm{a}_i + 2 {}^A\bm{P}^T \left[{}^A\bm{R}_B {}^B\bm{b}_i\right] - 2 \left[{}^A\bm{R}_B {}^B\bm{b}_i\right]^T {}^A\bm{a}_i} \end{equation}

If the position and orientation of the moving platform lie in the feasible workspace of the manipulator, one unique solution to the limb length is determined by the above equation. Otherwise, when the limbs' lengths derived yield complex numbers, then the position or orientation of the moving platform is not reachable.

Matlab Implementation

We open the Simulink file.

  open 'simscape/hexapod_tests.slx'

We load the configuration and set a small StopTime.

  load('mat/conf_simscape.mat');
  set_param(conf_simscape, 'StopTime', '0.5');

We define the wanted position/orientation of the Hexapod under study.

  tx = 0.1; % [rad]
  ty = 0.2; % [rad]
  tz = 0.05; % [rad]

  Rx = [1 0        0;
        0 cos(tx) -sin(tx);
        0 sin(tx)  cos(tx)];

  Ry = [ cos(ty) 0 sin(ty);
        0        1 0;
        -sin(ty) 0 cos(ty)];

  Rz = [cos(tz) -sin(tz) 0;
        sin(tz)  cos(tz) 0;
        0        0       1];

  ARB = Rz*Ry*Rx;
  AP = [0.01; 0.02; 0.03]; % [m]

  hexapod = initializeMicroHexapod(struct('AP', AP, 'ARB', ARB));

We run the simulation.

  sim('simscape/hexapod_tests.slx')

And we verify that we indeed succeed to go to the wanted position.

  [simout.x.Data(end) ; simout.y.Data(end) ; simout.z.Data(end)] - AP

1.611e-10

-1.3748e-10

8.4879e-11

  simout.R.Data(:, :, end)-ARB

-1.2659e-10	6.5603e-11	6.2183e-10
1.0354e-10	-5.2439e-11	-5.2425e-10
-5.9816e-10	5.532e-10	-1.7737e-10