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Add recommendation for flexible joints

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Thomas Dehaeze
2025-12-02 16:08:45 +01:00
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delta-robot.tex
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@@ -1,4 +1,4 @@
% Created 2025-12-02 Tue 15:22
% Created 2025-12-02 Tue 16:08
% Intended LaTeX compiler: pdflatex
\documentclass[a4paper, 10pt, DIV=12, parskip=full, bibliography=totoc]{scrreprt}
@@ -22,8 +22,8 @@
\tableofcontents
\clearpage
\label{sec:delta_robot_introduction}
\chapter{Geometry}
\label{sec:delta_robot_kinematics}
\chapter{Studied Geometry}
The Delta Robot geometry is defined as shown in Figure \ref{fig:delta_robot_schematic}.
The geometry is fully defined by three parameters:
@@ -80,6 +80,13 @@ It has a mass of \textasciitilde{}300g
Let's initialize a Delta Robot architecture, and plot the obtained geometry (Figures \ref{fig:delta_robot_architecture} and \ref{fig:delta_robot_architecture_top}).
\begin{minted}[]{matlab}
%% Geometry
d = 50e-3; % Cube's edge length [m]
b = 20e-3; % Distance between cube's vertices and top joints [m]
L = 50e-3; % Length of the struts [m]
\end{minted}
\begin{figure}[htbp]
\centering
\includegraphics[scale=1]{figs/delta_robot_architecture.png}
@@ -134,7 +141,7 @@ Depending on how the YZ plane is oriented (i.e., depending on the Rz angle of th
\begin{figure}[htbp]
\centering
\includegraphics[scale=1]{figs/delta_robot_2d_workspace.png}
\caption{\label{fig:delta_robot_2d_workspace}2D mobility for different orientations}
\caption{\label{fig:delta_robot_2d_workspace}2D mobility for different orientations and worst case}
\end{figure}
\phantomsection
@@ -155,7 +162,7 @@ In the perfect case (flexible joints having no stiffness in bending, and infinit
In order to have some compliance in rotation, the flexible joints need to have some compliance in torsion \textbf{and} in the axial direction.
If only the torsional compliance is considered, or only the axial compliance, the top platform will still not be able to do any rotation.
This is shown below with the Simscape model:
This is shown below with the Simscape model.
Perfect Delta Robot:
\begin{itemize}
@@ -182,20 +189,6 @@ If we consider the torsion of the flexible joints:
We get the same result.
\begin{table}[htbp]
\label{}
\centering
\begin{tabular}{rrrrrr}
1.00000000008084e-06 & 5.61161767954149e-17 & 1.71532908969278e-16 & -5.07840362381474e-20 & 6.66481298362927e-20 & -2.97485996797085e-20\\
9.44550093013109e-17 & 9.99999999981042e-07 & -1.31365181854596e-16 & 2.20736442292089e-20 & -8.27761658859861e-20 & -5.51841106039072e-20\\
1.5172479677791e-16 & 7.57916759433382e-18 & 9.9999999983089e-07 & 2.71965732539802e-20 & -5.0088900526952e-21 & 4.20708731166142e-20\\
3.7990974483635e-15 & -1.49195278352597e-15 & 1.09166051384021e-15 & -4.60992688885629e-28 & 4.46894514064728e-28 & -6.04202812027976e-29\\
1.92223678174906e-15 & 3.62267008376981e-15 & -5.42452104276465e-15 & 3.7737029650938e-28 & -2.99601307180469e-28 & -3.30297132145485e-28\\
3.79407058351761e-16 & -3.46944690994006e-17 & 9.05659407516431e-16 & -1.33615244419949e-28 & 6.31115609464934e-29 & -1.07596265815569e-29\\
\end{tabular}
\end{table}
If we consider the axial of the flexible joints:
\begin{itemize}
\item finite axial stiffness
@@ -205,36 +198,10 @@ If we consider the axial of the flexible joints:
We get the same result.
\begin{table}[htbp]
\label{}
\centering
\begin{tabular}{rrrrrr}
1.0100082922792e-06 & -8.23544084894984e-12 & -1.12982601885747e-11 & -1.30123733161505e-19 & 3.61079565098929e-21 & -2.56479299609534e-20\\
-8.23691147427908e-12 & 1.01002426764947e-06 & 1.16488393394949e-11 & -9.17271935353013e-20 & -1.39093131659514e-20 & -8.92049690205949e-21\\
-1.12984563170605e-11 & 1.1646579416975e-11 & 1.01001628184113e-06 & -1.2624013403036e-19 & -4.41063676650963e-22 & 2.01282903077792e-20\\
3.25566266033832e-16 & 7.02470264849632e-18 & 1.34879235410707e-17 & -4.42814771779206e-29 & 1.06332319737688e-30 & -8.04051214064698e-30\\
-2.34669385520782e-15 & 1.03658500184429e-16 & 2.10026418149746e-15 & 3.04125606965025e-29 & -1.07347740796227e-29 & 1.00530401393662e-28\\
-2.76788049700945e-16 & -3.49174266014038e-17 & 1.36253386200494e-15 & -1.31490052587614e-28 & -1.10493913866777e-30 & 3.45058758900066e-29\\
\end{tabular}
\end{table}
No we consider both finite torsional stiffness and finite axial stiffness.
In that case we get some compliance in rotation.
So it is a combination of axial and torsion stiffness that gives some rotational stiffness of the top platform.
\begin{table}[htbp]
\label{}
\centering
\begin{tabular}{rrrrrr}
1.01000347398415e-06 & 1.08760660507374e-12 & 6.48998902114602e-12 & -1.68758827983914e-12 & 2.72734564017018e-12 & -1.13327901508316e-12\\
-1.7269667100718e-13 & 1.01000152837256e-06 & 4.09018170284123e-13 & -9.80286569772779e-13 & -9.94629340226498e-13 & 8.3468855003368e-13\\
5.40707734744154e-12 & -1.46859567258244e-12 & 1.0099981133678e-06 & 3.34755457607493e-12 & -6.56764113866634e-12 & 1.26237249518737e-12\\
2.67935189951885e-13 & -6.64493555908236e-14 & 7.44661955503977e-13 & 1.59744415094004e-05 & -6.97585818773949e-13 & 1.44845764745933e-13\\
1.91880640934689e-14 & -1.31284160978049e-13 & -5.83827721443037e-13 & 4.97550111979747e-13 & 1.5974440506229e-05 & 8.27388125221276e-14\\
1.63302978397554e-14 & -6.29948151015302e-14 & 1.11798835985125e-13 & -4.74438363934587e-14 & -1.39110944875938e-13 & 3.99840072236264e-06\\
\end{tabular}
\end{table}
Therefore, to model some compliance of the top platform in rotation, both the axial compliance and the torsional compliance of the flexible joints should be considered.
\chapter{Kinematics: Number of modes}
@@ -250,25 +217,23 @@ State-space model with 3 outputs, 3 inputs, and 6 states.
\end{verbatim}
\chapter{Flexible Joint Design}
\label{sec:delta_robot_flexible_joints}
The goal is to extract specifications for the flexible joints of the six struts.
The two most critical flexible joints imperfections are:
\begin{itemize}
\item The axial stiffness, that should be high enough
\item The bending stiffness, that should be low enough
\end{itemize}
The shear and torsional stiffnesses are not foreseen to be very problematic, but their impact will be evaluated.
First, the dynamics of a ``perfect'' Delta-Robot is identified (i.e. with perfect 2DoF rotational joints).
First, in Section \ref{ssec:delta_robot_flexible_joints_geometry}, the dynamics of a ``perfect'' Delta-Robot is identified (i.e. with perfect 2DoF rotational joints).
Then, the impact of the flexible joint's imperfections will be studied.
The goal is to extract specifications for the flexible joints of the six struts, in terms of:
\begin{itemize}
\item bending stiffness (Section \ref{ssec:delta_robot_flexible_joints_bending})
\item axial stiffness (Section \ref{ssec:delta_robot_flexible_joints_axial})
\item torsional stiffness (Section \ref{ssec:delta_robot_flexible_joints_torsion})
\item shear stiffness (Section \ref{ssec:delta_robot_flexible_joints_shear})
\end{itemize}
\section{Studied Geometry}
\label{ssec:delta_robot_flexible_joints_geometry}
The cube's edge length is equal to 50mm, the distance between cube's vertices and top joints is 20mm and the length of the struts (i.e. the distance between the two flexible joints of the same strut) is 50mm.
The actuator stiffness is \(1\,N/\mu m\).
The obtained geometry is shown in Figure \ref{fig:delta_robot_studied_geometry}.
The obtained geometry is shown in Figure [].
\begin{figure}[htbp]
\centering
@@ -286,7 +251,9 @@ The dynamics is shown in Figure \ref{fig:delta_robot_dynamics_perfect}.
\includegraphics[scale=1]{figs/delta_robot_dynamics_perfect.png}
\caption{\label{fig:delta_robot_dynamics_perfect}Dynamics of the delta robot with perfect joints}
\end{figure}
\section{Stiffness seen by the actuator}
\section{Bending Stiffness}
\label{ssec:delta_robot_flexible_joints_bending}
\subsection{Stiffness seen by the actuator, and decrease of the achievable stroke}
Because the flexible joints will have some bending stiffness, the actuator in one direction will ``see'' some stiffness due to the struts in the other directions.
This will limit its effective stroke.
@@ -307,7 +274,7 @@ If we want the parallel stiffness to be much smaller than the stiffness of the a
Therefore, we should aim at \(k_f < 50\,Nm/\text{rad}\).
This should be validated with the final geometry.
\section{Bending Stiffness}
Then, the dynamics is identified for a bending Stiffness of \(50\,Nm/\text{rad}\) and compared with a Delta robot with no bending stiffness in Figure \ref{fig:delta_robot_bending_stiffness_dynamics}.
@@ -320,7 +287,28 @@ It is not critical from a dynamical point of view, it just decreases the achieva
\includegraphics[scale=1]{figs/delta_robot_bending_stiffness_dynamics.png}
\caption{\label{fig:delta_robot_bending_stiffness_dynamics}Effect of the bending stiffness on the dynamics}
\end{figure}
\subsection{Effect on the coupling}
Here, reasonable values for the flexible joints (modelled as a 6DoF joint) stiffness are taken:
\begin{itemize}
\item Torsional stiffness of 500Nm/rad
\item Axial stiffness of 100N/um
\item Shear stiffness of 100N/um
\end{itemize}
And the bending stiffness is varied from low to high values.
The obtained dynamics is shown in Figure \ref{fig:delta_robot_bending_stiffness_couplign}.
It can be seen that the low frequency coupling increases when the bending stiffness increases.
Therefore, the bending stiffness of the flexible joints should be minimized (10Nm/rad could be a reasonable objective).
\begin{figure}[htbp]
\centering
\includegraphics[scale=1]{figs/delta_robot_bending_stiffness_couplign.png}
\caption{\label{fig:delta_robot_bending_stiffness_couplign}Effect of the bending stiffness of the flexible joints on the coupling}
\end{figure}
\section{Axial Stiffness}
\label{ssec:delta_robot_flexible_joints_axial}
Now, the effect of the axial stiffness on the dynamics is studied (Figure \ref{fig:delta_robot_axial_stiffness_dynamics}).
Additional modes can be observed on the plant dynamics, which could limit the achievable bandwidth.
@@ -334,6 +322,8 @@ Therefore, we should aim at \(k_a > 100\,N/\mu m\).
\caption{\label{fig:delta_robot_axial_stiffness_dynamics}Effect of the joint's axial stiffness on the plant dynamics}
\end{figure}
\section{Torsional Stiffness}
\label{ssec:delta_robot_flexible_joints_torsion}
Now the compliance in torsion of the flexible joints is considered.
If we look at the compliance of the delta robot in rotation as a function of the torsional stiffness of the flexible joints (Figure \ref{fig:delta_robot_kt_compliance}), we see almost no effect: the system is not made more stiff by increasing the torsional stiffness of the joints.
@@ -354,6 +344,7 @@ If we have a look at the effect of the torsional stiffness on the plant dynamics
Therefore, the torsional stiffness is not a super important metric for the design of the delta robot.
\section{Shear Stiffness}
\label{ssec:delta_robot_flexible_joints_shear}
As shown in Figure \ref{fig:delta_robot_shear_stiffness_compliance}, the shear stiffness of the flexible joints has some effect on the compliance in translation and almost no effect on the compliance in rotation.
@@ -365,6 +356,22 @@ A value of \(100\,N/\mu m\) seems reasonable.
\includegraphics[scale=1]{figs/delta_robot_shear_stiffness_compliance.png}
\caption{\label{fig:delta_robot_shear_stiffness_compliance}Effect of the shear stiffness of the flexible joints on the Delta Robot compliance}
\end{figure}
\section{Conclusion}
\begin{table}[htbp]
\caption{\label{tab:delta_robot_flexible_joints_recommendations}Recommendations for the flexible joints}
\centering
\begin{tabular}{lll}
Joint's Stiffness & Effect & Recommendation\\
\hline
Bending & Can reduce the stroke, and increase the coupling & Below 50 to 10 Nm/rad\\
Axial & Add modes that can limit the feedback bandwidth & As high as possible, at least 100 Nm/um\\
Torsion & Minor effect & No recommendation\\
Shear & Can limit the stiffness of the system & As high as possible (less important than the axial stiffness), above 100 N/um if possible\\
\end{tabular}
\end{table}
\chapter{Effect of the Geometry}
\label{sec:delta_robot_flexible_geometry}
\section{Effect of cube's size}
Let's choose reasonable values for the flexible joints:
\begin{itemize}
@@ -404,7 +411,7 @@ As shown in Figure \ref{fig:delta_robot_cube_size_compliance_rotation}, the stif
\end{figure}
With a cube size of 50mm, the resonance frequency is already above 1kHz with seems reasonable.
\section{Effect of the strut length ?}
\section{Effect of the strut length}
Let's choose reasonable values for the flexible joints:
\begin{itemize}
\item Bending stiffness of 50Nm/rad
@@ -413,6 +420,22 @@ Let's choose reasonable values for the flexible joints:
\end{itemize}
And we see the effect of changing the strut length.
\subsection{Effect on the compliance}
As shown in Figure \ref{fig:delta_robot_strut_length_compliance_rotation}, the strut length has an effect on the system stiffness in translation (left plot) but almost not in rotation (right plot).
Indeed, the stiffness in rotation is a combination of:
\begin{itemize}
\item The stiffness of the actuator
\item The shear and axial stiffness of the flexible joints
\item The bending and torsional stiffness of the flexible joints, combine with the strut length
\end{itemize}
\begin{figure}[htbp]
\centering
\includegraphics[scale=1]{figs/delta_robot_strut_length_compliance_rotation.png}
\caption{\label{fig:delta_robot_strut_length_compliance_rotation}Effect of the cube's size on the rotational compliance of the top platform}
\end{figure}
\subsection{Effect on the plant dynamics}
As shown in Figure \ref{fig:delta_robot_strut_length_plant_dynamics}, having longer struts:
@@ -420,6 +443,11 @@ As shown in Figure \ref{fig:delta_robot_strut_length_plant_dynamics}, having lon
\item decreases the main resonance frequency: this means that the stiffness in the X,Y and Z directions is decreased when the length of the strut is longer.
This is reasonable as the ``lever'' arm is getting larger, so the bending stiffness and compression of the flexible joints have a larger effect on the top platform compliance.
\item decreases the low frequency coupling: this effect is more difficult to physically understand
Probably: when pushing with one actuator, it induces some rotation of the struts corresponding to the other two actuators.
This rotation is proportional to the strut length.
Then, this rotation, combined with the limited compliance in bending of the flexible joints induces some force applied on the other actuators, hence the coupling.
This is similar to what was observed when varying the bending stiffness of the flexible joints: the coupling was increased with an increased of the bending stiffness (See Figure \ref{fig:delta_robot_bending_stiffness_couplign})
\textbf{So we should also observed a decrease of the coupling when decreasing the bending stiffness of the actuators}
\end{itemize}
But even with relatively short struts (20mm and above), the low frequency decoupling is already around two orders of magnitude, which is enough from a control point of view.
@@ -428,16 +456,7 @@ So, the struts length can be optimized to not decrease too much the stiffness of
\begin{figure}[htbp]
\centering
\includegraphics[scale=1]{figs/delta_robot_strut_length_plant_dynamics.png}
\caption{\label{fig:delta_robot_strut_length_plant_dynamics}Effect of the cube's size on the plant dynamics}
\end{figure}
\subsection{Effect on the compliance}
As shown in Figure \ref{fig:delta_robot_strut_length_compliance_rotation}, the strut length has an effect on the system stiffness in translation (left plot) but almost not in rotation (right plot).
\begin{figure}[htbp]
\centering
\includegraphics[scale=1]{figs/delta_robot_strut_length_compliance_rotation.png}
\caption{\label{fig:delta_robot_strut_length_compliance_rotation}Effect of the cube's size on the rotational compliance of the top platform}
\caption{\label{fig:delta_robot_strut_length_plant_dynamics}Effect of the Strut length on the plant dynamics}
\end{figure}
\section{Having the Center of Mass at the cube's center}