stewart-simscape/org/control-tracking.org

Stewart Platform - Tracking Control

• Introduction
• Decentralized Control Architecture using Strut Length
  • Control Schematic
  • Initialize the Stewart platform
  • Identification of the plant
  • Plant Analysis
  • Controller Design
  • Simulation
  • Results
  • Conclusion
• Centralized Control Architecture using Pose Measurement
  • Control Schematic
  • Initialize the Stewart platform
  • Identification of the plant
  • Diagonal Control - Leg's Frame
    • Control Architecture
    • Plant Analysis
    • Controller Design
    • Simulation
  • Diagonal Control - Cartesian Frame
    • Control Architecture
    • Plant Analysis
    • Controller Design
    • Simulation
  • Diagonal Control - Steady State Decoupling
    • Control Architecture
    • Plant Analysis
    • Controller Design
  • Comparison
    • Obtained MIMO Controllers
    • Simulation Results
  • Conclusion
• Hybrid Control Architecture - HAC-LAC with relative DVF
  • Control Schematic
• Position Error computation

Introduction   ignore

Let's suppose the control objective is to position $\bm{\mathcal{X}}$ of the mobile platform of the Stewart platform such that it is following some reference position $\bm{r}_\mathcal{X}$.

Depending of the measured quantity, different control architectures can be used:

• If the struts length $\bm{\mathcal{L}}$ is measured, a decentralized control architecture can be used (Section sec:decentralized)
• If the pose of the mobile platform $\bm{\mathcal{X}}$ is directly measured, a centralized control architecture can be used (Section sec:centralized)
• If both $\bm{\mathcal{L}}$ and $\bm{\mathcal{X}}$ are measured, we can use an hybrid control architecture (Section sec:hybrid)

Decentralized Control Architecture using Strut Length

<<sec:decentralized>>

Control Schematic

The control architecture is shown in Figure fig:control_measure_rotating_2dof.

The required leg length $\bm{r}_\mathcal{L}$ is computed from the reference path $\bm{r}_\mathcal{X}$ using the inverse kinematics.

Then, a diagonal (decentralized) controller $\bm{K}_\mathcal{L}$ is used such that each leg lengths stays close to its required length.

  \begin{tikzpicture}
    % Blocs
    \node[block, align=center] (J) at (0, 0) {Inverse\\Kinematics};
    \node[addb={+}{}{}{}{-}, right=1 of J] (subr) {};
    \node[block, right=0.8 of subr] (K) {$\bm{K}_\mathcal{L}$};
    \node[block, right=1 of K] (G) {$\bm{G}_\mathcal{L}$};

    % Connections and labels
    \draw[<-] (J.west)node[above left]{$\bm{r}_{\mathcal{X}}$} -- ++(-1, 0);
    \draw[->] (J.east) -- (subr.west) node[above left]{$\bm{r}_{\mathcal{L}}$};
    \draw[->] (subr.east) -- (K.west) node[above left]{$\bm{\epsilon}_\mathcal{L}$};
    \draw[->] (K.east) -- (G.west) node[above left]{$\bm{\tau}$};
    \draw[->] (G.east) node[above right]{$\bm{\mathcal{L}}$} -| ($(G.east)+(1, -1)$) -| (subr.south);
  \end{tikzpicture}

Decentralized control for reference tracking

Initialize the Stewart platform

  stewart = initializeStewartPlatform();
  stewart = initializeFramesPositions(stewart, 'H', 90e-3, 'MO_B', 45e-3);
  stewart = generateGeneralConfiguration(stewart);
  stewart = computeJointsPose(stewart);
  stewart = initializeStrutDynamics(stewart);
  stewart = initializeJointDynamics(stewart, 'type_F', 'universal_p', 'type_M', 'spherical_p');
  stewart = initializeCylindricalPlatforms(stewart);
  stewart = initializeCylindricalStruts(stewart);
  stewart = computeJacobian(stewart);
  stewart = initializeStewartPose(stewart);
  stewart = initializeInertialSensor(stewart, 'type', 'accelerometer', 'freq', 5e3);

  ground = initializeGround('type', 'rigid');
  payload = initializePayload('type', 'none');
  controller = initializeController('type', 'open-loop');

  disturbances = initializeDisturbances();
  references = initializeReferences(stewart);

Identification of the plant

Let's identify the transfer function from $\bm{\tau}$ to $\bm{L}$.

  %% Name of the Simulink File
  mdl = 'stewart_platform_model';

  %% Input/Output definition
  clear io; io_i = 1;
  io(io_i) = linio([mdl, '/Controller'],        1, 'openinput');  io_i = io_i + 1; % Actuator Force Inputs [N]
  io(io_i) = linio([mdl, '/Stewart Platform'],  1, 'openoutput', [], 'dLm'); io_i = io_i + 1; % Relative Displacement Outputs [m]

  %% Run the linearization
  G = linearize(mdl, io);
  G.InputName  = {'F1', 'F2', 'F3', 'F4', 'F5', 'F6'};
  G.OutputName = {'L1', 'L2', 'L3', 'L4', 'L5', 'L6'};

Plant Analysis

The diagonal terms of the plant is shown in Figure fig:plant_decentralized_diagonal.

All the diagonal terms are equal.

<<plt-matlab>>

Diagonal Elements of the Plant (png, pdf)

The off-diagonal terms are shown in Figure fig:plant_decentralized_off_diagonal.

We see that the plant is decoupled at low frequency which indicate that decentralized control may be a good idea.

<<plt-matlab>>

Diagonal Elements of the Plant (png, pdf)

Controller Design

The controller consists of:

• A pure integrator
• A lead around the crossover frequency to increase the phase margin
• A low pass filter with a cut-off frequency 3 times the crossover to increase the gain margin

The obtained loop gains corresponding to the diagonal elements are shown in Figure fig:loop_gain_decentralized_L.

  wc = 2*pi*30;
  Kl = diag(1./diag(abs(freqresp(G, wc)))) * wc/s * 1/(1 + s/3/wc);

<<plt-matlab>>

Loop Gain of the diagonal elements (png, pdf)

Simulation

  t = linspace(0, 10, 1000);

  r = zeros(6, length(t));

  r(1, :) = 5e-3*sin(2*pi*t);

  references = initializeReferences(stewart, 't', t, 'r', r);

  controller = initializeController('type', 'ref-track-L');

  sim('stewart_platform_model')
  simout_D = simout;

Results

<<plt-matlab>>

Position error $\bm{\epsilon}_\mathcal{X}$ (png, pdf)

<<plt-matlab>>

Position error $\bm{\epsilon}_\mathcal{L}$ (png, pdf)

Conclusion

Such control architecture is easy to implement and give good results. The inverse kinematics is easy to compute.

However, as $\mathcal{X}$ is not directly measured, it is possible that important positioning errors are due to finite stiffness of the joints and other imperfections.

Centralized Control Architecture using Pose Measurement

<<sec:centralized>>

Control Schematic

The centralized controller takes the position error $\bm{\epsilon}_\mathcal{X}$ as an inputs and generate actuator forces $\bm{\tau}$ (see Figure fig:centralized_reference_tracking). The signals are:

• reference path $\bm{r}_\mathcal{X} = \begin{bmatrix} \epsilon_x & \epsilon_y & \epsilon_z & \epsilon_{R_x} & \epsilon_{R_y} & \epsilon_{R_z} \end{bmatrix}$
• tracking error $\bm{\epsilon}_\mathcal{X} = \begin{bmatrix} \epsilon_x & \epsilon_y & \epsilon_z & \epsilon_{R_x} & \epsilon_{R_y} & \epsilon_{R_z} \end{bmatrix}$
• actuator forces $\bm{\tau} = \begin{bmatrix} \tau_1 & \tau_2 & \tau_3 & \tau_4 & \tau_5 & \tau_6 \end{bmatrix}$
• payload pose $\bm{\mathcal{X}) = \begin{bmatrix} x & y & z & R_x & R_y & R_z \end{bmatrix}$

  \begin{tikzpicture}
    % Blocs
    \node[addb={+}{}{}{}{-}] (subr) at (0, 0) {};
    \node[block, right=0.8 of subr] (K) {$K$};
    \node[block, right=1 of K] (G) {$G$};

    % Connections and labels
    \draw[<-] (subr.west)node[above left]{$\bm{r}_{\mathcal{X}}$} -- ++(-1, 0);
    \draw[->] (subr.east) -- (K.west) node[above left]{$\bm{\epsilon}_{\mathcal{X}}$};
    \draw[->] (K.east) -- (G.west) node[above left]{$\bm{\tau}$};
    \draw[->] (G.east) node[above right]{$\bm{\mathcal{X}}$} -| ($(G.east)+(1, -1)$) -| (subr.south);
  \end{tikzpicture}

Centralized Controller

Instead of designing a full MIMO controller $K$, we first try to make the plant more diagonal by pre- or post-multiplying some constant matrix, then we design a diagonal controller.

We can think of two ways to make the plant more diagonal that are described in sections sec:diagonal_control_L and sec:diagonal_control_X.

Note here that the subtraction shown in Figure fig:centralized_reference_tracking is not a real subtraction. It is indeed a more complex computation explained in section sec:compute_pose_error.

Initialize the Stewart platform

  stewart = initializeStewartPlatform();
  stewart = initializeFramesPositions(stewart, 'H', 90e-3, 'MO_B', 45e-3);
  stewart = generateGeneralConfiguration(stewart);
  stewart = computeJointsPose(stewart);
  stewart = initializeStrutDynamics(stewart);
  stewart = initializeJointDynamics(stewart, 'type_F', 'universal_p', 'type_M', 'spherical_p');
  stewart = initializeCylindricalPlatforms(stewart);
  stewart = initializeCylindricalStruts(stewart);
  stewart = computeJacobian(stewart);
  stewart = initializeStewartPose(stewart);
  stewart = initializeInertialSensor(stewart, 'type', 'accelerometer', 'freq', 5e3);

  ground = initializeGround('type', 'rigid');
  payload = initializePayload('type', 'none');
  controller = initializeController('type', 'open-loop');

  disturbances = initializeDisturbances();
  references = initializeReferences(stewart);

Identification of the plant

Let's identify the transfer function from $\bm{\tau}$ to $\bm{L}$.

  %% Name of the Simulink File
  mdl = 'stewart_platform_model';

  %% Input/Output definition
  clear io; io_i = 1;
  io(io_i) = linio([mdl, '/Controller'],        1, 'openinput');  io_i = io_i + 1; % Actuator Force Inputs [N]
  io(io_i) = linio([mdl, '/Relative Motion Sensor'],  1, 'openoutput'); io_i = io_i + 1; % Relative Displacement Outputs [m]

  %% Run the linearization
  G = linearize(mdl, io);
  G.InputName  = {'F1', 'F2', 'F3', 'F4', 'F5', 'F6'};
  G.OutputName = {'Dx', 'Dy', 'Dz', 'Rx', 'Ry', 'Rz'};

Diagonal Control - Leg's Frame

<<sec:diagonal_control_L>>

Control Architecture

The pose error $\bm{\epsilon}_\mathcal{X}$ is first converted in the frame of the leg by using the Jacobian matrix. Then a diagonal controller $\bm{K}_\mathcal{L}$ is designed. The final implemented controller is $\bm{K} = \bm{K}_\mathcal{L} \cdot \bm{J}$ as shown in Figure fig:centralized_reference_tracking_L

Note here that the transformation from the pose error $\bm{\epsilon}_\mathcal{X}$ to the leg's length errors by using the Jacobian matrix is only valid for small errors.

  \begin{tikzpicture}
    % Blocs
    \node[addb={+}{}{}{}{-}] (subr) at (0, 0) {};
    \node[block, right= 1 of subr] (J) {$\bm{J}$};
    \node[block, right=0.8 of J] (K) {$\bm{K}_{\mathcal{L}}$};
    \node[block, right=1 of K] (G) {$\bm{G}$};

    % Connections and labels
    \draw[<-] (subr.west)node[above left]{$\bm{r}_{\mathcal{X}}$} -- ++(-1, 0);
    \draw[->] (subr.east) -- (J.west) node[above left]{$\bm{\epsilon}_{\mathcal{X}}$};
    \draw[->] (J.east) -- (K.west) node[above left]{$\bm{\epsilon}_{\mathcal{L}}$};
    \draw[->] (K.east) -- (G.west) node[above left]{$\bm{\tau}$};
    \draw[->] (G.east) node[above right]{$\bm{\mathcal{X}}$} -| ($(G.east)+(1, -1)$) -| (subr.south);

    \begin{scope}[on background layer]
      \node[fit={(J.south west) (K.north east)}, fill=black!20!white, draw, dashed, inner sep=8pt] (Ktot) {};
      \node[above] at (Ktot.north) {$\bm{K}$};
    \end{scope}
  \end{tikzpicture}

Controller in the frame of the legs

Plant Analysis

We now multiply the plant by the Jacobian matrix as shown in Figure fig:centralized_reference_tracking_L to obtain a more diagonal plant.

  Gl = stewart.kinematics.J*G;
  Gl.OutputName  = {'D1', 'D2', 'D3', 'D4', 'D5', 'D6'};

<<plt-matlab>>

Diagonal Elements of the plant $\bm{J} \bm{G}$ (png, pdf)

All the diagonal elements are identical. This will simplify the design of the controller as all the elements of the diagonal controller can be made identical.

The off-diagonal terms of the controller are shown in Figure fig:plant_centralized_off_diagonal_L.

<<plt-matlab>>

Off Diagonal Elements of the plant $\bm{J} \bm{G}$ (png, pdf)

We can see that this totally decouples the system at low frequency.

This was expected since: \[ \bm{G}(\omega = 0) = \frac{\delta\bm{\mathcal{X}}}{\delta\bm{\tau}}(\omega = 0) = \bm{J}^{-1} \frac{\delta\bm{\mathcal{L}}}{\delta\bm{\tau}}(\omega = 0) = \bm{J}^{-1} \text{diag}(\mathcal{K}_1^{-1} \ \dots \ \mathcal{K}_6^{-1}) \]

Thus $J \cdot G(\omega = 0) = J \cdot \frac{\delta\bm{\mathcal{X}}}{\delta\bm{\tau}}(\omega = 0)$ is a diagonal matrix containing the inverse of the joint's stiffness.

Controller Design

The controller consists of:

• A pure integrator
• A lead around the crossover frequency to increase the phase margin
• A low pass filter with a cut-off frequency 3 times the crossover to increase the gain margin

The obtained loop gains corresponding to the diagonal elements are shown in Figure fig:loop_gain_centralized_L.

  wc = 2*pi*30;
  Kl = diag(1./diag(abs(freqresp(Gl, wc)))) * wc/s * 1/(1 + s/3/wc);

<<plt-matlab>>

Loop Gain of the diagonal elements (png, pdf)

The controller $\bm{K} = \bm{K}_\mathcal{L} \bm{J}$ is computed.

  K = Kl*stewart.kinematics.J;

Simulation

We specify the reference path to follow.

  t = linspace(0, 10, 1000);

  r = zeros(6, length(t));

  r(1, :) = 5e-3*sin(2*pi*t);

  references = initializeReferences(stewart, 't', t, 'r', r);

  controller = initializeController('type', 'ref-track-X');

We run the simulation and we save the results.

  sim('stewart_platform_model')
  simout_L = simout;

Diagonal Control - Cartesian Frame

<<sec:diagonal_control_X>>

Control Architecture

A diagonal controller $\bm{K}_\mathcal{X}$ take the pose error $\bm{\epsilon}_\mathcal{X}$ and generate cartesian forces $\bm{\mathcal{F}}$ that are then converted to actuators forces using the Jacobian as shown in Figure e fig:centralized_reference_tracking_X.

The final implemented controller is $\bm{K} = \bm{J}^{-T} \cdot \bm{K}_\mathcal{X}$.

  \begin{tikzpicture}
    % Blocs
    \node[addb={+}{}{}{}{-}] (subr) at (0, 0) {};
    \node[block, right= of subr] (K) {$\bm{K}_{\mathcal{X}}$};
    \node[block, right= of K] (J) {$\bm{J}^{-T}$};
    \node[block, right= of J] (G) {$\bm{G}$};

    % Connections and labels
    \draw[<-] (subr.west)node[above left]{$\bm{r}_{\mathcal{X}}$} -- ++(-1, 0);
    \draw[->] (subr.east) -- (K.west) node[above left]{$\bm{\epsilon}_{\mathcal{X}}$};
    \draw[->] (K.east) -- (J.west) node[above left]{$\bm{\mathcal{F}}$};
    \draw[->] (J.east) -- (G.west) node[above left]{$\bm{\tau}$};
    \draw[->] (G.east) node[above right]{$\bm{\mathcal{X}}$} -| ($(G.east)+(1, -1)$) -| (subr.south);

    \begin{scope}[on background layer]
      \node[fit={(K.south west) (J.north east)}, fill=black!20!white, draw, dashed, inner sep=8pt] (Ktot) {};
      \node[above] at (Ktot.north) {$\bm{K}$};
    \end{scope}
  \end{tikzpicture}

Controller in the cartesian frame

Plant Analysis

We now multiply the plant by the Jacobian matrix as shown in Figure fig:centralized_reference_tracking_X to obtain a more diagonal plant.

  Gx = G*inv(stewart.kinematics.J');
  Gx.InputName  = {'Fx', 'Fy', 'Fz', 'Mx', 'My', 'Mz'};

<<plt-matlab>>

Diagonal Elements of the plant $\bm{G} \bm{J}^{-T}$ (png, pdf)

The diagonal terms are not the same. The resonances of the system are "decoupled". For instance, the vertical resonance of the system is only present on the diagonal term corresponding to $D_z/\mathcal{F}_z$.

<<plt-matlab>>

Off Diagonal Elements of the plant $\bm{G} \bm{J}^{-T}$ (png, pdf)

Here the system is almost decoupled at all frequencies except for the transfer functions $\frac{R_y}{\mathcal{F}_x}$ and $\frac{R_x}{\mathcal{F}_y}$.

This is due to the fact that the compliance matrix of the Stewart platform is not diagonal.

4.75e-08	-1.9751e-24	7.3536e-25	5.915e-23	3.2093e-07	5.8696e-24
-7.1302e-25	4.75e-08	2.8866e-25	-3.2093e-07	-5.38e-24	-3.2725e-23
7.9012e-26	-6.3991e-25	2.099e-08	1.9073e-23	5.3384e-25	-6.4867e-40
1.3724e-23	-3.2093e-07	1.2799e-23	5.1863e-06	4.9412e-22	-3.8269e-24
3.2093e-07	7.6013e-24	1.2239e-23	6.8886e-22	5.1863e-06	-2.6025e-22
7.337e-24	-3.2395e-23	-1.571e-39	9.927e-23	-3.2531e-22	1.7073e-06

One way to have this compliance matrix diagonal (and thus having a decoupled plant at DC) is to use a cubic architecture with the center of the cube's coincident with frame $\{A\}$.

This control architecture can also give a dynamically decoupled plant if the Center of mass of the payload is also coincident with frame $\{A\}$.

Controller Design

The controller consists of:

• A pure integrator
• A lead around the crossover frequency to increase the phase margin
• A low pass filter with a cut-off frequency 3 times the crossover to increase the gain margin

The obtained loop gains corresponding to the diagonal elements are shown in Figure fig:loop_gain_centralized_X.

  wc = 2*pi*30;
  Kx = diag(1./diag(abs(freqresp(Gx, wc)))) * wc/s * 1/(1 + s/3/wc);

<<plt-matlab>>

Loop Gain of the diagonal elements (png, pdf)

The controller $\bm{K} = \bm{J}^{-T} \bm{K}_\mathcal{X}$ is computed.

  K = inv(stewart.kinematics.J')*Kx;

Simulation

We specify the reference path to follow.

  t = linspace(0, 10, 1000);

  r = zeros(6, length(t));

  r(1, :) = 5e-3*sin(2*pi*t);

  references = initializeReferences(stewart, 't', t, 'r', r);

  controller = initializeController('type', 'ref-track-X');

We run the simulation and we save the results.

  sim('stewart_platform_model')
  simout_X = simout;

Diagonal Control - Steady State Decoupling

<<sec:diagonal_control_SD>>

Control Architecture

The plant $\bm{G}$ is pre-multiply by $\bm{G}^{-1}(\omega = 0)$ such that the "shaped plant" $\bm{G}_0 = \bm{G} \bm{G}^{-1}(\omega = 0)$ is diagonal at low frequency.

Then a diagonal controller $\bm{K}_0$ is designed.

The control architecture is shown in Figure fig:centralized_reference_tracking_S.

  \begin{tikzpicture}
    % Blocs
    \node[addb={+}{}{}{}{-}] (subr) at (0, 0) {};
    \node[block, right=0.8 of subr] (K) {$\bm{K}_{0}$};
    \node[block, right= 1 of K] (J) {$\bm{G}^{-1}(\omega=0)$};
    \node[block, right=1 of J] (G) {$\bm{G}$};

    % Connections and labels
    \draw[<-] (subr.west)node[above left]{$\bm{r}_{\mathcal{X}}$} -- ++(-1, 0);
    \draw[->] (subr.east) -- (K.west) node[above left]{$\bm{\epsilon}_{\mathcal{X}}$};
    \draw[->] (K.east) -- (J.west);
    \draw[->] (J.east) -- (G.west) node[above left]{$\bm{\tau}$};
    \draw[->] (G.east) node[above right]{$\bm{\mathcal{X}}$} -| ($(G.east)+(1, -1)$) -| (subr.south);

    \begin{scope}[on background layer]
      \node[fit={(K.south west) (J.north east)}, fill=black!20!white, draw, dashed, inner sep=8pt] (Ktot) {};
      \node[above] at (Ktot.north) {$\bm{K}$};
    \end{scope}
  \end{tikzpicture}

Static Decoupling of the Plant

Plant Analysis

The plant is pre-multiplied by $\bm{G}^{-1}(\omega = 0)$. The diagonal elements of the shaped plant are shown in Figure fig:centralized_reference_tracking_SD.

  G0 = G*inv(freqresp(G, 0));

<<plt-matlab>>

Diagonal Elements of the plant $\bm{G} \bm{G}^{-1}(\omega = 0)$ (png, pdf)

<<plt-matlab>>

Off Diagonal Elements of the plant $\bm{G} \bm{J}^{-T}$ (png, pdf)

Controller Design

We have that: \[ \bm{G}^{-1}(\omega = 0) = \left(\frac{\delta\bm{\mathcal{X}}}{\delta\bm{\tau}}(\omega = 0)\right)^{-1} = \left( \bm{J}^{-1} \frac{\delta\bm{\mathcal{L}}}{\delta\bm{\tau}}(\omega = 0) \right)^{-1} = \text{diag}(\mathcal{K}_1^{-1} \ \dots \ \mathcal{K}_6^{-1}) \bm{J} \]

And because:

• all the leg stiffness are equal
• the controller equal to a $\bm{K}_0(s) = k(s) \bm{I}_6$

We have that $\bm{K}_0(s)$ commutes with $\bm{G}^{-1}(\omega = 0)$ and thus the overall controller $\bm{K}$ is the same as the one obtain in section sec:diagonal_control_L.

Comparison

Obtained MIMO Controllers

<<plt-matlab>>

Comparison of the MIMO controller $\bm{K}$ for both centralized architectures (png, pdf)

Simulation Results

The position error $\bm{\epsilon}_\mathcal{X}$ for both centralized architecture are shown in Figure fig:centralized_control_comp_Ex. The corresponding leg's length errors $\bm{\epsilon}_\mathcal{L}$ are shown in Figure fig:centralized_control_comp_El.

Based on Figure fig:centralized_control_comp_Ex, we can see that:

• There is some tracking error $\epsilon_x$
• The errors $\epsilon_y$, $\epsilon_{R_x}$ and $\epsilon_{R_z}$ are quite negligible
• There is some error in the vertical position $\epsilon_z$. The frequency of the error $\epsilon_z$ is twice the frequency of the reference path $r_x$.
• There is some error $\epsilon_{R_y}$. This error is much lower when using the diagonal control in the frame of the leg instead of the cartesian frame.

<<plt-matlab>>

Comparison of the position error $\bm{\epsilon}_\mathcal{X}$ for both centralized controllers (png, pdf)

<<plt-matlab>>

Comparison of the leg's length error $\bm{\epsilon}_\mathcal{L}$ for both centralized controllers (png, pdf)

Conclusion

Both control architecture gives similar results even tough the control in the Leg's frame gives slightly better results.

The main differences between the control architectures used in sections sec:diagonal_control_L and sec:diagonal_control_X are summarized in Table tab:centralized_control_comp.

	Leg's Frame	Cartesian Frame	Static Decoupling
Plant Meaning	$\delta\mathcal{L}_i/\tau_i$	$\delta\mathcal{X}_i/\mathcal{F}_i$	No physical meaning
Obtained Decoupling	Decoupled at DC	Dynamical decoupling except few terms	Decoupled at DC
Diagonal Elements	Identical with all the Resonances	Different, resonances are cancel out	No Alternating poles and zeros
Mechanical Architecture	Architecture Independent	Better with Cubic Architecture
Robustness to Uncertainty	Good (only depends on $J$)	Good (only depends on $J$)	Bad (depends on the mass)

Comparison of the two centralized control architectures

These decoupling methods only uses the Jacobian matrix which only depends on the Stewart platform geometry. Thus, this method should be quite robust against parameter variation (e.g. the payload mass).

Hybrid Control Architecture - HAC-LAC with relative DVF

<<sec:hybrid>>

Control Schematic

  \begin{tikzpicture}
    \node[block={3.0cm}{3.0cm}] (G) {$G$};

    % Input and outputs coordinates
    \coordinate[] (outputX) at ($(G.south east)!0.25!(G.north east)$);
    \coordinate[] (outputL) at ($(G.south east)!0.75!(G.north east)$);

    \draw[->] (outputX) -- ++(1.8, 0) node[above left]{$\bm{\mathcal{X}}$};
    \draw[->] (outputL) -- ++(1.8, 0) node[above left]{$\bm{\mathcal{L}}$};

    % Blocs
    \node[addb, left= of G] (addF) {};
    \node[block, left=1.2 of addF] (Kx) {$\bm{K}_\mathcal{X}$};
    \node[addb={+}{}{}{}{-}, left= of Kx] (subx) {};

    \node[block, above= of addF] (Kl) {$\bm{K}_\mathcal{L}$};
    \node[addb={+}{}{}{-}{}, above= of Kl] (subl) {};

    \node[block, align=center, left= of subl] (invK) {Inverse\\Kinematics};

    % Connections and labels
    \draw[<-] (subx.west)node[above left]{$\bm{r}_{\mathcal{X}}$} -- ++(-1.2, 0);
    \draw[->] ($(subx.west) + (-0.8, 0)$)node[branch]{} |- (invK.west);
    \draw[->] (invK.east) -- (subl.west) node[above left]{$\bm{\epsilon}_\mathcal{L}$};
    \draw[->] (subl.south) -- (Kl.north);
    \draw[->] (Kl.south) -- (addF.north);

    \draw[->] (subx.east) -- (Kx.west) node[above left]{$\bm{\epsilon}_\mathcal{X}$};
    \draw[->] (Kx.east) node[above right]{$\bm{\tau}_\mathcal{X}$} -- (addF.west);
    \draw[->] (addF.east) -- (G.west) node[above left]{$\bm{\tau}$};

    \draw[->] ($(outputL.east) + (0.4, 0)$)node[branch](L){} |- (subl.east);
    \draw[->] ($(outputX.east) + (1.2, 0)$)node[branch]{} -- ++(0, -1.6) -| (subx.south);

    \begin{scope}[on background layer]
      \node[fit={(G.south-|Kl.west) (L|-subl.north)}, fill=black!20!white, draw, dashed, inner sep=8pt] (Ktot) {};
    \end{scope}
  \end{tikzpicture}

Centralized Controller

Position Error computation

<<sec:compute_pose_error>> Let's denote:

• $\{W\}$ the initial fixed frame (base in which the measurement is done)
• $\{R\}$ the reference frame corresponding to the wanted pose of the sample
• $\{M\}$ the frame corresponding to the measured pose of the sample

We have then computed:

• ${}^W\bm{T}_R$ which corresponds to the wanted pose of the sample with respect to the granite
• ${}^W\bm{T}_M$ which corresponds to the measured pose of the sample with respect to the granite

Reference Position with respect to fixed frame {W}: ${}^WT_R$

  Dx = 0;
  Dy = 0;
  Dz = 0.1;
  Rx = pi;
  Ry = 0;
  Rz = 0;

  WTr = zeros(4,4);

  R = [cos(Rz) -sin(Rz) 0;
       sin(Rz)  cos(Rz) 0;
       0        0         1] * ...
      [cos(Ry)  0        sin(Ry);
       0        1        0;
       -sin(Ry)  0        cos(Ry)] * ...
      [1        0        0;
       0        cos(Rx) -sin(Rx);
       0        sin(Rx)  cos(Rx)];


  WTr(1:3, 1:3) = R;
  WTr(1:4, 4) = [Dx ; Dy ; Dz; 1];

Measured Position with respect to fixed frame {W}: ${}^WT_M$

  Dx = 0;
  Dy = 0;
  Dz = 0;
  Rx = pi;
  Ry = 0;
  Rz = 0;

  WTm = zeros(4,4);

  R = [cos(Rz) -sin(Rz) 0;
       sin(Rz)  cos(Rz) 0;
       0        0         1] * ...
      [cos(Ry)  0        sin(Ry);
       0        1        0;
       -sin(Ry)  0        cos(Ry)] * ...
      [1        0        0;
       0        cos(Rx) -sin(Rx);
       0        sin(Rx)  cos(Rx)];


  WTm(1:3, 1:3) = R;
  WTm(1:4, 4) = [Dx ; Dy ; Dz; 1];

We would like to compute ${}^M\bm{T}_R$ which corresponds to the wanted pose of the sample expressed in a frame attached to the top platform of the nano-hexapod (frame $\{M\}$).

We have:

\begin{align} {}^M\bm{T}_R &= {}^M\bm{T}_W \cdot {}^W\bm{T}_R \\ &= {}^W{\bm{T}_M}^{-1} \cdot {}^W\bm{T}_R \end{align}

  % Error with respect to the top platform
  MTr = [WTm(1:3,1:3)', -WTm(1:3,1:3)'*WTm(1:3,4) ; 0 0 0 1]*WTr;

  T = MTr;
  Edx = T(1, 4);
  Edy = T(2, 4);
  Edz = T(3, 4);

  % The angles obtained are u-v-w Euler angles (rotations in the moving frame)
  Ery = atan2( T(1, 3),          sqrt(T(1, 1)^2 + T(1, 2)^2));
  Erx = atan2(-T(2, 3)/cos(Ery), T(3, 3)/cos(Ery));
  Erz = atan2(-T(1, 2)/cos(Ery), T(1, 1)/cos(Ery));

  [Edx, Edy, Edz, Erx, Ery, Erz]