Stéphane on Locomotion - Comments: Zero-tilting moment pointhttps://scaron.info/robotics/zero-tilting-moment-point.html/2023-01-06T16:00:00+01:00Posted by: Stéphane2023-01-06T16:00:00+01:002023-01-06T16:00:00+01:00Stéphanetag:scaron.info,2023-01-06:/robotics/zero-tilting-moment-point.html//comment-reply-external-forces-follow-up<p>Continuing the answer above: in general we can take the ZMP of any wrench, so you could choose to take the ZMP of the wrench "contact forces only", or the ZMP of the wrench "contact forces + stick force". If you are doing balance control the way current methods work (<em>e …</em></p><p>Continuing the answer above: in general we can take the ZMP of any wrench, so you could choose to take the ZMP of the wrench "contact forces only", or the ZMP of the wrench "contact forces + stick force". If you are doing balance control the way current methods work (<em>e.g.</em> by inverse dynamics + torque control, or some flavor of <a class="reference external" href="https://scaron.info/robotics/zero-tilting-moment-point.html/publications/icra-2019.html">whole-body admittance control</a>), it is likely that you will go for the contact wrench only and compute the wrench <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi mathvariant="bold-italic">w</mi><mrow><mi>g</mi><mi>i</mi><mi>e</mi></mrow></msup></mrow><annotation encoding="application/x-tex">\def\bfA{\boldsymbol{A}}
\def\bfB{\boldsymbol{B}}
\def\bfC{\boldsymbol{C}}
\def\bfD{\boldsymbol{D}}
\def\bfE{\boldsymbol{E}}
\def\bfF{\boldsymbol{F}}
\def\bfG{\boldsymbol{G}}
\def\bfH{\boldsymbol{H}}
\def\bfI{\boldsymbol{I}}
\def\bfJ{\boldsymbol{J}}
\def\bfK{\boldsymbol{K}}
\def\bfL{\boldsymbol{L}}
\def\bfM{\boldsymbol{M}}
\def\bfN{\boldsymbol{N}}
\def\bfO{\boldsymbol{O}}
\def\bfP{\boldsymbol{P}}
\def\bfQ{\boldsymbol{Q}}
\def\bfR{\boldsymbol{R}}
\def\bfS{\boldsymbol{S}}
\def\bfT{\boldsymbol{T}}
\def\bfU{\boldsymbol{U}}
\def\bfV{\boldsymbol{V}}
\def\bfW{\boldsymbol{W}}
\def\bfX{\boldsymbol{X}}
\def\bfY{\boldsymbol{Y}}
\def\bfZ{\boldsymbol{Z}}
\def\bfalpha{\boldsymbol{\alpha}}
\def\bfa{\boldsymbol{a}}
\def\bfbeta{\boldsymbol{\beta}}
\def\bfb{\boldsymbol{b}}
\def\bfcd{\dot{\bfc}}
\def\bfchi{\boldsymbol{\chi}}
\def\bfc{\boldsymbol{c}}
\def\bfd{\boldsymbol{d}}
\def\bfe{\boldsymbol{e}}
\def\bff{\boldsymbol{f}}
\def\bfgamma{\boldsymbol{\gamma}}
\def\bfg{\boldsymbol{g}}
\def\bfh{\boldsymbol{h}}
\def\bfi{\boldsymbol{i}}
\def\bfj{\boldsymbol{j}}
\def\bfk{\boldsymbol{k}}
\def\bflambda{\boldsymbol{\lambda}}
\def\bfl{\boldsymbol{l}}
\def\bfm{\boldsymbol{m}}
\def\bfn{\boldsymbol{n}}
\def\bfomega{\boldsymbol{\omega}}
\def\bfone{\boldsymbol{1}}
\def\bfo{\boldsymbol{o}}
\def\bfpdd{\ddot{\bfp}}
\def\bfpd{\dot{\bfp}}
\def\bfphi{\boldsymbol{\phi}}
\def\bfp{\boldsymbol{p}}
\def\bfq{\boldsymbol{q}}
\def\bfr{\boldsymbol{r}}
\def\bfsigma{\boldsymbol{\sigma}}
\def\bfs{\boldsymbol{s}}
\def\bftau{\boldsymbol{\tau}}
\def\bftheta{\boldsymbol{\theta}}
\def\bft{\boldsymbol{t}}
\def\bfu{\boldsymbol{u}}
\def\bfv{\boldsymbol{v}}
\def\bfw{\boldsymbol{w}}
\def\bfxi{\boldsymbol{\xi}}
\def\bfx{\boldsymbol{x}}
\def\bfy{\boldsymbol{y}}
\def\bfzero{\boldsymbol{0}}
\def\bfz{\boldsymbol{z}}
\def\defeq{\stackrel{\mathrm{def}}{=}}
\def\p{\boldsymbol{p}}
\def\qdd{\ddot{\bfq}}
\def\qd{\dot{\bfq}}
\def\q{\boldsymbol{q}}
\def\xd{\dot{x}}
\def\yd{\dot{y}}
\def\zd{\dot{z}}
\bfw^{gie}</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8247em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.02778em;">w</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.8247em;"><span style="top:-3.063em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span><span class="mord mathnormal mtight">i</span><span class="mord mathnormal mtight">e</span></span></span></span></span></span></span></span></span></span></span></span> as above.</p>
<p>With this approach, the robot responds to disturbances with a typical spring-damper-like behavior. You can check this out and build your intuition with the following Docker for instance:</p>
<pre class="code console literal-block">
<span class="gp">$ </span>xhost<span class="w"> </span>+local:docker<span class="w">
</span><span class="gp">$ </span>docker<span class="w"> </span>run<span class="w"> </span>-it<span class="w"> </span>--rm<span class="w"> </span>--user<span class="w"> </span>ayumi<span class="w"> </span>-e<span class="w"> </span><span class="nv">DISPLAY</span><span class="o">=</span><span class="si">${</span><span class="nv">DISPLAY</span><span class="si">}</span><span class="w"> </span>-v<span class="w"> </span>/tmp/.X11-unix:/tmp/.X11-unix:rw<span class="w"> </span>stephanecaron/lipm_walking_controller<span class="w"> </span>lipm_walking<span class="w"> </span>--floor
</pre>
<p>It will start a full-fledged simulation you can play with, looking like this:</p>
<img alt="Simulation and visualization from a LIPM walking Docker container" src="https://scaron.info/images/lipm-walking-docker.png" />
<p>Click and drag the mouse on the robot in the simulator to extert external forces. In the accompanying visualizer (RViz) you will see how this affects the measured and commanded ZMPs from the balance controller. (Be sure to click on <tt class="docutils literal">Walking</tt> → <tt class="docutils literal">Start standing</tt> in the <tt class="docutils literal">mc_rtc_panel</tt> of RViz, as the balance controller is disabled at startup.)</p>
<p>In the figure above, you can see I click-and-dragged a force from the robot's left arm (in the simulator window to the right). Consequently, we see the net contact-wrench ZMP (red arrow in the RViz window to the left) move towards the edge of the left foot sole as the balance controller makes the robot "lean" in the direction of the external force.</p>
Posted by: Stéphane2023-01-06T15:42:00+01:002023-01-06T15:42:00+01:00Stéphanetag:scaron.info,2023-01-06:/robotics/zero-tilting-moment-point.html//comment-reply-external-forces<div class="section" id="zmp-under-external-forces">
<h2>ZMP under external forces<a class="headerlink" href="#zmp-under-external-forces" title="Permalink to this headline">¶</a></h2>
<p>Assuming the robot is subject to external forces, we can derive the zero-tilting moment point (ZMP) again from the Newton-Euler equations of motion:</p>
<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mtable rowspacing="0.25em" columnalign="right left" columnspacing="0em"><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mi>m</mi><msub><mover accent="true"><mi mathvariant="bold-italic">p</mi><mo>¨</mo></mover><mi>G</mi></msub></mrow></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow></mrow><mo>=</mo><mi>m</mi><mi mathvariant="bold-italic">g</mi><mo>+</mo><msup><mi mathvariant="bold-italic">f</mi><mi>c</mi></msup><mo>+</mo><msup><mi mathvariant="bold-italic">f</mi><mrow><mi>e</mi><mi>x</mi><mi>t</mi></mrow></msup></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><msub><mover accent="true"><mi>L</mi><mo>˙</mo></mover><mi>O</mi></msub></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow></mrow><mo>=</mo><mover accent="true"><mrow><mi>O</mi><mi>G</mi></mrow><mo stretchy="true">→</mo></mover><mo>×</mo><mo stretchy="false">(</mo><mi>m</mi><mi mathvariant="bold-italic">g</mi><mo>+</mo><msup><mi mathvariant="bold-italic">f</mi><mrow><mi>e</mi><mi>x</mi><mi>t</mi></mrow></msup><mo stretchy="false">)</mo><mo>+</mo><msubsup><mi mathvariant="bold-italic">τ</mi><mi>G …</mi></msubsup></mrow></mstyle></mtd></mtr></mtable></semantics></math></span></span></span></div><div class="section" id="zmp-under-external-forces">
<h2>ZMP under external forces<a class="headerlink" href="#zmp-under-external-forces" title="Permalink to this headline">¶</a></h2>
<p>Assuming the robot is subject to external forces, we can derive the zero-tilting moment point (ZMP) again from the Newton-Euler equations of motion:</p>
<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mtable rowspacing="0.25em" columnalign="right left" columnspacing="0em"><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mi>m</mi><msub><mover accent="true"><mi mathvariant="bold-italic">p</mi><mo>¨</mo></mover><mi>G</mi></msub></mrow></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow></mrow><mo>=</mo><mi>m</mi><mi mathvariant="bold-italic">g</mi><mo>+</mo><msup><mi mathvariant="bold-italic">f</mi><mi>c</mi></msup><mo>+</mo><msup><mi mathvariant="bold-italic">f</mi><mrow><mi>e</mi><mi>x</mi><mi>t</mi></mrow></msup></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><msub><mover accent="true"><mi>L</mi><mo>˙</mo></mover><mi>O</mi></msub></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow></mrow><mo>=</mo><mover accent="true"><mrow><mi>O</mi><mi>G</mi></mrow><mo stretchy="true">→</mo></mover><mo>×</mo><mo stretchy="false">(</mo><mi>m</mi><mi mathvariant="bold-italic">g</mi><mo>+</mo><msup><mi mathvariant="bold-italic">f</mi><mrow><mi>e</mi><mi>x</mi><mi>t</mi></mrow></msup><mo stretchy="false">)</mo><mo>+</mo><msubsup><mi mathvariant="bold-italic">τ</mi><mi>G</mi><mi>c</mi></msubsup></mrow></mstyle></mtd></mtr></mtable><annotation encoding="application/x-tex">\def\bfA{\boldsymbol{A}}
\def\bfB{\boldsymbol{B}}
\def\bfC{\boldsymbol{C}}
\def\bfD{\boldsymbol{D}}
\def\bfE{\boldsymbol{E}}
\def\bfF{\boldsymbol{F}}
\def\bfG{\boldsymbol{G}}
\def\bfH{\boldsymbol{H}}
\def\bfI{\boldsymbol{I}}
\def\bfJ{\boldsymbol{J}}
\def\bfK{\boldsymbol{K}}
\def\bfL{\boldsymbol{L}}
\def\bfM{\boldsymbol{M}}
\def\bfN{\boldsymbol{N}}
\def\bfO{\boldsymbol{O}}
\def\bfP{\boldsymbol{P}}
\def\bfQ{\boldsymbol{Q}}
\def\bfR{\boldsymbol{R}}
\def\bfS{\boldsymbol{S}}
\def\bfT{\boldsymbol{T}}
\def\bfU{\boldsymbol{U}}
\def\bfV{\boldsymbol{V}}
\def\bfW{\boldsymbol{W}}
\def\bfX{\boldsymbol{X}}
\def\bfY{\boldsymbol{Y}}
\def\bfZ{\boldsymbol{Z}}
\def\bfalpha{\boldsymbol{\alpha}}
\def\bfa{\boldsymbol{a}}
\def\bfbeta{\boldsymbol{\beta}}
\def\bfb{\boldsymbol{b}}
\def\bfcd{\dot{\bfc}}
\def\bfchi{\boldsymbol{\chi}}
\def\bfc{\boldsymbol{c}}
\def\bfd{\boldsymbol{d}}
\def\bfe{\boldsymbol{e}}
\def\bff{\boldsymbol{f}}
\def\bfgamma{\boldsymbol{\gamma}}
\def\bfg{\boldsymbol{g}}
\def\bfh{\boldsymbol{h}}
\def\bfi{\boldsymbol{i}}
\def\bfj{\boldsymbol{j}}
\def\bfk{\boldsymbol{k}}
\def\bflambda{\boldsymbol{\lambda}}
\def\bfl{\boldsymbol{l}}
\def\bfm{\boldsymbol{m}}
\def\bfn{\boldsymbol{n}}
\def\bfomega{\boldsymbol{\omega}}
\def\bfone{\boldsymbol{1}}
\def\bfo{\boldsymbol{o}}
\def\bfpdd{\ddot{\bfp}}
\def\bfpd{\dot{\bfp}}
\def\bfphi{\boldsymbol{\phi}}
\def\bfp{\boldsymbol{p}}
\def\bfq{\boldsymbol{q}}
\def\bfr{\boldsymbol{r}}
\def\bfsigma{\boldsymbol{\sigma}}
\def\bfs{\boldsymbol{s}}
\def\bftau{\boldsymbol{\tau}}
\def\bftheta{\boldsymbol{\theta}}
\def\bft{\boldsymbol{t}}
\def\bfu{\boldsymbol{u}}
\def\bfv{\boldsymbol{v}}
\def\bfw{\boldsymbol{w}}
\def\bfxi{\boldsymbol{\xi}}
\def\bfx{\boldsymbol{x}}
\def\bfy{\boldsymbol{y}}
\def\bfzero{\boldsymbol{0}}
\def\bfz{\boldsymbol{z}}
\def\defeq{\stackrel{\mathrm{def}}{=}}
\def\p{\boldsymbol{p}}
\def\qdd{\ddot{\bfq}}
\def\qd{\dot{\bfq}}
\def\q{\boldsymbol{q}}
\def\xd{\dot{x}}
\def\yd{\dot{y}}
\def\zd{\dot{z}}
\begin{align*}
m \bfpdd_G & = m \bfg + \bff^c + \bff^{ext} \\
\dot{L}_O & = \overrightarrow{OG} \times (m \bfg + \bff^{ext}) + \bftau^{c}_G
\end{align*}</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:3.4032em;vertical-align:-1.4516em;"></span><span class="mord"><span class="mtable"><span class="col-align-r"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:1.9516em;"><span style="top:-4.279em;"><span class="pstrut" style="height:3.2053em;"></span><span class="mord"><span class="mord mathnormal">m</span><span class="mord"><span class="mord accent"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.6813em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span class="mord"><span class="mord"><span class="mord boldsymbol">p</span></span></span></span><span style="top:-3.0134em;"><span class="pstrut" style="height:3em;"></span><span class="accent-body" style="left:-0.25em;"><span class="mord">¨</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.1944em;"><span></span></span></span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.2342em;"><span style="top:-2.4559em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">G</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.2441em;"><span></span></span></span></span></span></span></span></span><span style="top:-2.4137em;"><span class="pstrut" style="height:3.2053em;"></span><span class="mord"><span class="mord"><span class="mord accent"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.9202em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span class="mord mathnormal">L</span></span><span style="top:-3.2523em;"><span class="pstrut" style="height:3em;"></span><span class="accent-body" style="left:-0.1111em;"><span class="mord">˙</span></span></span></span></span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.3283em;"><span style="top:-2.55em;margin-left:0em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">O</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:1.4516em;"><span></span></span></span></span></span><span class="col-align-l"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:1.9516em;"><span style="top:-4.279em;"><span class="pstrut" style="height:3.2053em;"></span><span class="mord"><span class="mord"></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mord mathnormal">m</span><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.03704em;">g</span></span></span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">+</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.11042em;">f</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.7487em;"><span style="top:-3.1473em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">c</span></span></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">+</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.11042em;">f</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.8779em;"><span style="top:-3.1473em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight">e</span><span class="mord mathnormal mtight">x</span><span class="mord mathnormal mtight">t</span></span></span></span></span></span></span></span></span></span></span><span style="top:-2.4137em;"><span class="pstrut" style="height:3.2053em;"></span><span class="mord"><span class="mord"></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mord accent"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:1.2053em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span class="mord"><span class="mord mathnormal">OG</span></span></span><span class="svg-align" style="top:-3.6833em;"><span class="pstrut" style="height:3em;"></span><span class="hide-tail" style="height:0.522em;min-width:0.888em;"><svg xmlns="http://www.w3.org/2000/svg" width='400em' height='0.522em' viewBox='0 0 400000 522' preserveAspectRatio='xMaxYMin slice'><path d='M0 241v40h399891c-47.3 35.3-84 78-110 128
-16.7 32-27.7 63.7-33 95 0 1.3-.2 2.7-.5 4-.3 1.3-.5 2.3-.5 3 0 7.3 6.7 11 20
11 8 0 13.2-.8 15.5-2.5 2.3-1.7 4.2-5.5 5.5-11.5 2-13.3 5.7-27 11-41 14.7-44.7
39-84.5 73-119.5s73.7-60.2 119-75.5c6-2 9-5.7 9-11s-3-9-9-11c-45.3-15.3-85
-40.5-119-75.5s-58.3-74.8-73-119.5c-4.7-14-8.3-27.3-11-40-1.3-6.7-3.2-10.8-5.5
-12.5-2.3-1.7-7.5-2.5-15.5-2.5-14 0-21 3.7-21 11 0 2 2 10.3 6 25 20.7 83.3 67
151.7 139 205zm0 0v40h399900v-40z'/></svg></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">×</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mopen">(</span><span class="mord mathnormal">m</span><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.03704em;">g</span></span></span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">+</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.11042em;">f</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.8779em;"><span style="top:-3.1473em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight">e</span><span class="mord mathnormal mtight">x</span><span class="mord mathnormal mtight">t</span></span></span></span></span></span></span></span></span><span class="mclose">)</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">+</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.13472em;">τ</span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.7144em;"><span style="top:-2.453em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">G</span></span></span><span style="top:-3.113em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight">c</span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.247em;"><span></span></span></span></span></span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:1.4516em;"><span></span></span></span></span></span></span></span></span></span></span></span><p>Here I'm assuming external forces are summed up at the center of mass (COM) and produce no angular momentum. Otherwise we would also add a non-zero moment <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msubsup><mi mathvariant="bold-italic">τ</mi><mi>G</mi><mrow><mi>e</mi><mi>x</mi><mi>t</mi></mrow></msubsup></mrow><annotation encoding="application/x-tex">\def\bfA{\boldsymbol{A}}
\def\bfB{\boldsymbol{B}}
\def\bfC{\boldsymbol{C}}
\def\bfD{\boldsymbol{D}}
\def\bfE{\boldsymbol{E}}
\def\bfF{\boldsymbol{F}}
\def\bfG{\boldsymbol{G}}
\def\bfH{\boldsymbol{H}}
\def\bfI{\boldsymbol{I}}
\def\bfJ{\boldsymbol{J}}
\def\bfK{\boldsymbol{K}}
\def\bfL{\boldsymbol{L}}
\def\bfM{\boldsymbol{M}}
\def\bfN{\boldsymbol{N}}
\def\bfO{\boldsymbol{O}}
\def\bfP{\boldsymbol{P}}
\def\bfQ{\boldsymbol{Q}}
\def\bfR{\boldsymbol{R}}
\def\bfS{\boldsymbol{S}}
\def\bfT{\boldsymbol{T}}
\def\bfU{\boldsymbol{U}}
\def\bfV{\boldsymbol{V}}
\def\bfW{\boldsymbol{W}}
\def\bfX{\boldsymbol{X}}
\def\bfY{\boldsymbol{Y}}
\def\bfZ{\boldsymbol{Z}}
\def\bfalpha{\boldsymbol{\alpha}}
\def\bfa{\boldsymbol{a}}
\def\bfbeta{\boldsymbol{\beta}}
\def\bfb{\boldsymbol{b}}
\def\bfcd{\dot{\bfc}}
\def\bfchi{\boldsymbol{\chi}}
\def\bfc{\boldsymbol{c}}
\def\bfd{\boldsymbol{d}}
\def\bfe{\boldsymbol{e}}
\def\bff{\boldsymbol{f}}
\def\bfgamma{\boldsymbol{\gamma}}
\def\bfg{\boldsymbol{g}}
\def\bfh{\boldsymbol{h}}
\def\bfi{\boldsymbol{i}}
\def\bfj{\boldsymbol{j}}
\def\bfk{\boldsymbol{k}}
\def\bflambda{\boldsymbol{\lambda}}
\def\bfl{\boldsymbol{l}}
\def\bfm{\boldsymbol{m}}
\def\bfn{\boldsymbol{n}}
\def\bfomega{\boldsymbol{\omega}}
\def\bfone{\boldsymbol{1}}
\def\bfo{\boldsymbol{o}}
\def\bfpdd{\ddot{\bfp}}
\def\bfpd{\dot{\bfp}}
\def\bfphi{\boldsymbol{\phi}}
\def\bfp{\boldsymbol{p}}
\def\bfq{\boldsymbol{q}}
\def\bfr{\boldsymbol{r}}
\def\bfsigma{\boldsymbol{\sigma}}
\def\bfs{\boldsymbol{s}}
\def\bftau{\boldsymbol{\tau}}
\def\bftheta{\boldsymbol{\theta}}
\def\bft{\boldsymbol{t}}
\def\bfu{\boldsymbol{u}}
\def\bfv{\boldsymbol{v}}
\def\bfw{\boldsymbol{w}}
\def\bfxi{\boldsymbol{\xi}}
\def\bfx{\boldsymbol{x}}
\def\bfy{\boldsymbol{y}}
\def\bfzero{\boldsymbol{0}}
\def\bfz{\boldsymbol{z}}
\def\defeq{\stackrel{\mathrm{def}}{=}}
\def\p{\boldsymbol{p}}
\def\qdd{\ddot{\bfq}}
\def\qd{\dot{\bfq}}
\def\q{\boldsymbol{q}}
\def\xd{\dot{x}}
\def\yd{\dot{y}}
\def\zd{\dot{z}}
\bftau^{ext}_G</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1.0689em;vertical-align:-0.2753em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.13472em;">τ</span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.7936em;"><span style="top:-2.4247em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">G</span></span></span><span style="top:-3.063em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight">e</span><span class="mord mathnormal mtight">x</span><span class="mord mathnormal mtight">t</span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.2753em;"><span></span></span></span></span></span></span></span></span></span> at the COM.</p>
<p>Then, the gravito-inertial wrench becomes a "gravity-extero-inertial" wrench <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msubsup><mi mathvariant="bold-italic">w</mi><mi>O</mi><mrow><mi>g</mi><mi>i</mi><mi>e</mi></mrow></msubsup><mo>=</mo><mo stretchy="false">[</mo><msup><mi mathvariant="bold-italic">f</mi><mrow><mi>g</mi><mi>i</mi><mi>e</mi></mrow></msup><mtext> </mtext><msubsup><mi mathvariant="bold-italic">τ</mi><mi>O</mi><mrow><mi>g</mi><mi>i</mi><mi>e</mi></mrow></msubsup><mo stretchy="false">]</mo></mrow><annotation encoding="application/x-tex">\def\bfA{\boldsymbol{A}}
\def\bfB{\boldsymbol{B}}
\def\bfC{\boldsymbol{C}}
\def\bfD{\boldsymbol{D}}
\def\bfE{\boldsymbol{E}}
\def\bfF{\boldsymbol{F}}
\def\bfG{\boldsymbol{G}}
\def\bfH{\boldsymbol{H}}
\def\bfI{\boldsymbol{I}}
\def\bfJ{\boldsymbol{J}}
\def\bfK{\boldsymbol{K}}
\def\bfL{\boldsymbol{L}}
\def\bfM{\boldsymbol{M}}
\def\bfN{\boldsymbol{N}}
\def\bfO{\boldsymbol{O}}
\def\bfP{\boldsymbol{P}}
\def\bfQ{\boldsymbol{Q}}
\def\bfR{\boldsymbol{R}}
\def\bfS{\boldsymbol{S}}
\def\bfT{\boldsymbol{T}}
\def\bfU{\boldsymbol{U}}
\def\bfV{\boldsymbol{V}}
\def\bfW{\boldsymbol{W}}
\def\bfX{\boldsymbol{X}}
\def\bfY{\boldsymbol{Y}}
\def\bfZ{\boldsymbol{Z}}
\def\bfalpha{\boldsymbol{\alpha}}
\def\bfa{\boldsymbol{a}}
\def\bfbeta{\boldsymbol{\beta}}
\def\bfb{\boldsymbol{b}}
\def\bfcd{\dot{\bfc}}
\def\bfchi{\boldsymbol{\chi}}
\def\bfc{\boldsymbol{c}}
\def\bfd{\boldsymbol{d}}
\def\bfe{\boldsymbol{e}}
\def\bff{\boldsymbol{f}}
\def\bfgamma{\boldsymbol{\gamma}}
\def\bfg{\boldsymbol{g}}
\def\bfh{\boldsymbol{h}}
\def\bfi{\boldsymbol{i}}
\def\bfj{\boldsymbol{j}}
\def\bfk{\boldsymbol{k}}
\def\bflambda{\boldsymbol{\lambda}}
\def\bfl{\boldsymbol{l}}
\def\bfm{\boldsymbol{m}}
\def\bfn{\boldsymbol{n}}
\def\bfomega{\boldsymbol{\omega}}
\def\bfone{\boldsymbol{1}}
\def\bfo{\boldsymbol{o}}
\def\bfpdd{\ddot{\bfp}}
\def\bfpd{\dot{\bfp}}
\def\bfphi{\boldsymbol{\phi}}
\def\bfp{\boldsymbol{p}}
\def\bfq{\boldsymbol{q}}
\def\bfr{\boldsymbol{r}}
\def\bfsigma{\boldsymbol{\sigma}}
\def\bfs{\boldsymbol{s}}
\def\bftau{\boldsymbol{\tau}}
\def\bftheta{\boldsymbol{\theta}}
\def\bft{\boldsymbol{t}}
\def\bfu{\boldsymbol{u}}
\def\bfv{\boldsymbol{v}}
\def\bfw{\boldsymbol{w}}
\def\bfxi{\boldsymbol{\xi}}
\def\bfx{\boldsymbol{x}}
\def\bfy{\boldsymbol{y}}
\def\bfzero{\boldsymbol{0}}
\def\bfz{\boldsymbol{z}}
\def\defeq{\stackrel{\mathrm{def}}{=}}
\def\p{\boldsymbol{p}}
\def\qdd{\ddot{\bfq}}
\def\qd{\dot{\bfq}}
\def\q{\boldsymbol{q}}
\def\xd{\dot{x}}
\def\yd{\dot{y}}
\def\zd{\dot{z}}
\bfw^{gie}_O = [\bff^{gie}\ \bftau^{gie}_O]</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1.2361em;vertical-align:-0.2935em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.02778em;">w</span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.9426em;"><span style="top:-2.4065em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">O</span></span></span><span style="top:-3.1809em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span><span class="mord mathnormal mtight">i</span><span class="mord mathnormal mtight">e</span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.2935em;"><span></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1.2361em;vertical-align:-0.2935em;"></span><span class="mopen">[</span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.11042em;">f</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.909em;"><span style="top:-3.1473em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span><span class="mord mathnormal mtight">i</span><span class="mord mathnormal mtight">e</span></span></span></span></span></span></span></span></span><span class="mspace"> </span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.13472em;">τ</span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.9426em;"><span style="top:-2.4065em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">O</span></span></span><span style="top:-3.1809em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span><span class="mord mathnormal mtight">i</span><span class="mord mathnormal mtight">e</span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.2935em;"><span></span></span></span></span></span></span><span class="mclose">]</span></span></span></span>:</p>
<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mtable rowspacing="0.25em" columnalign="right left" columnspacing="0em"><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><msup><mi mathvariant="bold-italic">f</mi><mrow><mi>g</mi><mi>i</mi><mi>e</mi></mrow></msup></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow></mrow><mo><mover><mo><mo>=</mo></mo><mrow><mi mathvariant="normal">d</mi><mi mathvariant="normal">e</mi><mi mathvariant="normal">f</mi></mrow></mover></mo><mi>m</mi><mo stretchy="false">(</mo><mi mathvariant="bold-italic">g</mi><mo>−</mo><msub><mover accent="true"><mi mathvariant="bold-italic">p</mi><mo>¨</mo></mover><mi>G</mi></msub><mo stretchy="false">)</mo><mo>+</mo><msup><mi mathvariant="bold-italic">f</mi><mrow><mi>e</mi><mi>x</mi><mi>t</mi></mrow></msup></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><msubsup><mi mathvariant="bold-italic">τ</mi><mi>O</mi><mrow><mi>g</mi><mi>i</mi><mi>e</mi></mrow></msubsup></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow></mrow><mo><mover><mo><mo>=</mo></mo><mrow><mi mathvariant="normal">d</mi><mi mathvariant="normal">e</mi><mi mathvariant="normal">f</mi></mrow></mover></mo><mover accent="true"><mrow><mi>O</mi><mi>G</mi></mrow><mo stretchy="true">→</mo></mover><mo>×</mo><mo stretchy="false">(</mo><mi>m</mi><mi mathvariant="bold-italic">g</mi><mo>+</mo><msup><mi mathvariant="bold-italic">f</mi><mrow><mi>e</mi><mi>x</mi><mi>t</mi></mrow></msup><mo stretchy="false">)</mo><mo>−</mo><msub><mover accent="true"><mi>L</mi><mo>˙</mo></mover><mi>O</mi></msub></mrow></mstyle></mtd></mtr></mtable><annotation encoding="application/x-tex">\def\bfA{\boldsymbol{A}}
\def\bfB{\boldsymbol{B}}
\def\bfC{\boldsymbol{C}}
\def\bfD{\boldsymbol{D}}
\def\bfE{\boldsymbol{E}}
\def\bfF{\boldsymbol{F}}
\def\bfG{\boldsymbol{G}}
\def\bfH{\boldsymbol{H}}
\def\bfI{\boldsymbol{I}}
\def\bfJ{\boldsymbol{J}}
\def\bfK{\boldsymbol{K}}
\def\bfL{\boldsymbol{L}}
\def\bfM{\boldsymbol{M}}
\def\bfN{\boldsymbol{N}}
\def\bfO{\boldsymbol{O}}
\def\bfP{\boldsymbol{P}}
\def\bfQ{\boldsymbol{Q}}
\def\bfR{\boldsymbol{R}}
\def\bfS{\boldsymbol{S}}
\def\bfT{\boldsymbol{T}}
\def\bfU{\boldsymbol{U}}
\def\bfV{\boldsymbol{V}}
\def\bfW{\boldsymbol{W}}
\def\bfX{\boldsymbol{X}}
\def\bfY{\boldsymbol{Y}}
\def\bfZ{\boldsymbol{Z}}
\def\bfalpha{\boldsymbol{\alpha}}
\def\bfa{\boldsymbol{a}}
\def\bfbeta{\boldsymbol{\beta}}
\def\bfb{\boldsymbol{b}}
\def\bfcd{\dot{\bfc}}
\def\bfchi{\boldsymbol{\chi}}
\def\bfc{\boldsymbol{c}}
\def\bfd{\boldsymbol{d}}
\def\bfe{\boldsymbol{e}}
\def\bff{\boldsymbol{f}}
\def\bfgamma{\boldsymbol{\gamma}}
\def\bfg{\boldsymbol{g}}
\def\bfh{\boldsymbol{h}}
\def\bfi{\boldsymbol{i}}
\def\bfj{\boldsymbol{j}}
\def\bfk{\boldsymbol{k}}
\def\bflambda{\boldsymbol{\lambda}}
\def\bfl{\boldsymbol{l}}
\def\bfm{\boldsymbol{m}}
\def\bfn{\boldsymbol{n}}
\def\bfomega{\boldsymbol{\omega}}
\def\bfone{\boldsymbol{1}}
\def\bfo{\boldsymbol{o}}
\def\bfpdd{\ddot{\bfp}}
\def\bfpd{\dot{\bfp}}
\def\bfphi{\boldsymbol{\phi}}
\def\bfp{\boldsymbol{p}}
\def\bfq{\boldsymbol{q}}
\def\bfr{\boldsymbol{r}}
\def\bfsigma{\boldsymbol{\sigma}}
\def\bfs{\boldsymbol{s}}
\def\bftau{\boldsymbol{\tau}}
\def\bftheta{\boldsymbol{\theta}}
\def\bft{\boldsymbol{t}}
\def\bfu{\boldsymbol{u}}
\def\bfv{\boldsymbol{v}}
\def\bfw{\boldsymbol{w}}
\def\bfxi{\boldsymbol{\xi}}
\def\bfx{\boldsymbol{x}}
\def\bfy{\boldsymbol{y}}
\def\bfzero{\boldsymbol{0}}
\def\bfz{\boldsymbol{z}}
\def\defeq{\stackrel{\mathrm{def}}{=}}
\def\p{\boldsymbol{p}}
\def\qdd{\ddot{\bfq}}
\def\qd{\dot{\bfq}}
\def\q{\boldsymbol{q}}
\def\xd{\dot{x}}
\def\yd{\dot{y}}
\def\zd{\dot{z}}
\begin{align*}
\bff^{gie} & \defeq m (\bfg - \bfpdd_G) + \bff^{ext} \\
\bftau^{gie}_O & \defeq \overrightarrow{OG} \times (m \bfg + \bff^{ext}) - \dot{L}_O
\end{align*}</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:3.6783em;vertical-align:-1.5892em;"></span><span class="mord"><span class="mtable"><span class="col-align-r"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:2.0892em;"><span style="top:-4.1415em;"><span class="pstrut" style="height:3.2053em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.11042em;">f</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.909em;"><span style="top:-3.1473em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span><span class="mord mathnormal mtight">i</span><span class="mord mathnormal mtight">e</span></span></span></span></span></span></span></span></span></span></span><span style="top:-2.2762em;"><span class="pstrut" style="height:3.2053em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.13472em;">τ</span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.9426em;"><span style="top:-2.4065em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">O</span></span></span><span style="top:-3.1809em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span><span class="mord mathnormal mtight">i</span><span class="mord mathnormal mtight">e</span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.2935em;"><span></span></span></span></span></span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:1.5892em;"><span></span></span></span></span></span><span class="col-align-l"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:2.0892em;"><span style="top:-4.1415em;"><span class="pstrut" style="height:3.2053em;"></span><span class="mord"><span class="mord"></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel"><span class="mop op-limits"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:1.153em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span><span class="mop">=</span></span></span><span style="top:-3.5669em;margin-left:0em;"><span class="pstrut" style="height:3em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight"><span class="mord mathrm mtight" style="margin-right:0.07778em;">def</span></span></span></span></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mord mathnormal">m</span><span class="mopen">(</span><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.03704em;">g</span></span></span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mord"><span class="mord accent"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.6813em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span class="mord"><span class="mord"><span class="mord boldsymbol">p</span></span></span></span><span style="top:-3.0134em;"><span class="pstrut" style="height:3em;"></span><span class="accent-body" style="left:-0.25em;"><span class="mord">¨</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.1944em;"><span></span></span></span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.2342em;"><span style="top:-2.4559em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">G</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.2441em;"><span></span></span></span></span></span></span><span class="mclose">)</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">+</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.11042em;">f</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.8779em;"><span style="top:-3.1473em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight">e</span><span class="mord mathnormal mtight">x</span><span class="mord mathnormal mtight">t</span></span></span></span></span></span></span></span></span></span></span><span style="top:-2.2762em;"><span class="pstrut" style="height:3.2053em;"></span><span class="mord"><span class="mord"></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel"><span class="mop op-limits"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:1.153em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span><span class="mop">=</span></span></span><span style="top:-3.5669em;margin-left:0em;"><span class="pstrut" style="height:3em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight"><span class="mord mathrm mtight" style="margin-right:0.07778em;">def</span></span></span></span></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mord accent"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:1.2053em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span class="mord"><span class="mord mathnormal">OG</span></span></span><span class="svg-align" style="top:-3.6833em;"><span class="pstrut" style="height:3em;"></span><span class="hide-tail" style="height:0.522em;min-width:0.888em;"><svg xmlns="http://www.w3.org/2000/svg" width='400em' height='0.522em' viewBox='0 0 400000 522' preserveAspectRatio='xMaxYMin slice'><path d='M0 241v40h399891c-47.3 35.3-84 78-110 128
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11 8 0 13.2-.8 15.5-2.5 2.3-1.7 4.2-5.5 5.5-11.5 2-13.3 5.7-27 11-41 14.7-44.7
39-84.5 73-119.5s73.7-60.2 119-75.5c6-2 9-5.7 9-11s-3-9-9-11c-45.3-15.3-85
-40.5-119-75.5s-58.3-74.8-73-119.5c-4.7-14-8.3-27.3-11-40-1.3-6.7-3.2-10.8-5.5
-12.5-2.3-1.7-7.5-2.5-15.5-2.5-14 0-21 3.7-21 11 0 2 2 10.3 6 25 20.7 83.3 67
151.7 139 205zm0 0v40h399900v-40z'/></svg></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">×</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mopen">(</span><span class="mord mathnormal">m</span><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.03704em;">g</span></span></span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">+</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.11042em;">f</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.8779em;"><span style="top:-3.1473em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight">e</span><span class="mord mathnormal mtight">x</span><span class="mord mathnormal mtight">t</span></span></span></span></span></span></span></span></span><span class="mclose">)</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mord"><span class="mord accent"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.9202em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span class="mord mathnormal">L</span></span><span style="top:-3.2523em;"><span class="pstrut" style="height:3em;"></span><span class="accent-body" style="left:-0.1111em;"><span class="mord">˙</span></span></span></span></span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.3283em;"><span style="top:-2.55em;margin-left:0em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">O</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:1.5892em;"><span></span></span></span></span></span></span></span></span></span></span></span><p>And we derive the ZMP as above from <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msubsup><mi mathvariant="bold-italic">τ</mi><mi>Z</mi><mrow><mi>g</mi><mi>i</mi><mi>e</mi></mrow></msubsup><mo>×</mo><mi mathvariant="bold-italic">n</mi><mo>=</mo><mi mathvariant="bold">0</mi></mrow><annotation encoding="application/x-tex">\def\bfA{\boldsymbol{A}}
\def\bfB{\boldsymbol{B}}
\def\bfC{\boldsymbol{C}}
\def\bfD{\boldsymbol{D}}
\def\bfE{\boldsymbol{E}}
\def\bfF{\boldsymbol{F}}
\def\bfG{\boldsymbol{G}}
\def\bfH{\boldsymbol{H}}
\def\bfI{\boldsymbol{I}}
\def\bfJ{\boldsymbol{J}}
\def\bfK{\boldsymbol{K}}
\def\bfL{\boldsymbol{L}}
\def\bfM{\boldsymbol{M}}
\def\bfN{\boldsymbol{N}}
\def\bfO{\boldsymbol{O}}
\def\bfP{\boldsymbol{P}}
\def\bfQ{\boldsymbol{Q}}
\def\bfR{\boldsymbol{R}}
\def\bfS{\boldsymbol{S}}
\def\bfT{\boldsymbol{T}}
\def\bfU{\boldsymbol{U}}
\def\bfV{\boldsymbol{V}}
\def\bfW{\boldsymbol{W}}
\def\bfX{\boldsymbol{X}}
\def\bfY{\boldsymbol{Y}}
\def\bfZ{\boldsymbol{Z}}
\def\bfalpha{\boldsymbol{\alpha}}
\def\bfa{\boldsymbol{a}}
\def\bfbeta{\boldsymbol{\beta}}
\def\bfb{\boldsymbol{b}}
\def\bfcd{\dot{\bfc}}
\def\bfchi{\boldsymbol{\chi}}
\def\bfc{\boldsymbol{c}}
\def\bfd{\boldsymbol{d}}
\def\bfe{\boldsymbol{e}}
\def\bff{\boldsymbol{f}}
\def\bfgamma{\boldsymbol{\gamma}}
\def\bfg{\boldsymbol{g}}
\def\bfh{\boldsymbol{h}}
\def\bfi{\boldsymbol{i}}
\def\bfj{\boldsymbol{j}}
\def\bfk{\boldsymbol{k}}
\def\bflambda{\boldsymbol{\lambda}}
\def\bfl{\boldsymbol{l}}
\def\bfm{\boldsymbol{m}}
\def\bfn{\boldsymbol{n}}
\def\bfomega{\boldsymbol{\omega}}
\def\bfone{\boldsymbol{1}}
\def\bfo{\boldsymbol{o}}
\def\bfpdd{\ddot{\bfp}}
\def\bfpd{\dot{\bfp}}
\def\bfphi{\boldsymbol{\phi}}
\def\bfp{\boldsymbol{p}}
\def\bfq{\boldsymbol{q}}
\def\bfr{\boldsymbol{r}}
\def\bfsigma{\boldsymbol{\sigma}}
\def\bfs{\boldsymbol{s}}
\def\bftau{\boldsymbol{\tau}}
\def\bftheta{\boldsymbol{\theta}}
\def\bft{\boldsymbol{t}}
\def\bfu{\boldsymbol{u}}
\def\bfv{\boldsymbol{v}}
\def\bfw{\boldsymbol{w}}
\def\bfxi{\boldsymbol{\xi}}
\def\bfx{\boldsymbol{x}}
\def\bfy{\boldsymbol{y}}
\def\bfzero{\boldsymbol{0}}
\def\bfz{\boldsymbol{z}}
\def\defeq{\stackrel{\mathrm{def}}{=}}
\def\p{\boldsymbol{p}}
\def\qdd{\ddot{\bfq}}
\def\qd{\dot{\bfq}}
\def\q{\boldsymbol{q}}
\def\xd{\dot{x}}
\def\yd{\dot{y}}
\def\zd{\dot{z}}
\bftau^{gie}_Z \times \bfn = \boldsymbol{0}</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1.2361em;vertical-align:-0.2935em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.13472em;">τ</span></span></span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.9426em;"><span style="top:-2.4065em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.07153em;">Z</span></span></span><span style="top:-3.1809em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span><span class="mord mathnormal mtight">i</span><span class="mord mathnormal mtight">e</span></span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.2935em;"><span></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">×</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.4444em;"></span><span class="mord"><span class="mord"><span class="mord boldsymbol">n</span></span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:0.6444em;"></span><span class="mord"><span class="mord"><span class="mord mathbf">0</span></span></span></span></span></span>.</p>
<p>We thus add this force to the inertial force (mass times gravity) as you were thinking. The rationale for that is that what the robot will be regulating down the line (in <a class="reference external" href="https://scaron.info/robotics/zero-tilting-moment-point.html/robotics/how-do-biped-robots-walk.html">balance control</a>) is the contact wrench <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi mathvariant="bold-italic">w</mi><mi>c</mi></msup></mrow><annotation encoding="application/x-tex">\def\bfA{\boldsymbol{A}}
\def\bfB{\boldsymbol{B}}
\def\bfC{\boldsymbol{C}}
\def\bfD{\boldsymbol{D}}
\def\bfE{\boldsymbol{E}}
\def\bfF{\boldsymbol{F}}
\def\bfG{\boldsymbol{G}}
\def\bfH{\boldsymbol{H}}
\def\bfI{\boldsymbol{I}}
\def\bfJ{\boldsymbol{J}}
\def\bfK{\boldsymbol{K}}
\def\bfL{\boldsymbol{L}}
\def\bfM{\boldsymbol{M}}
\def\bfN{\boldsymbol{N}}
\def\bfO{\boldsymbol{O}}
\def\bfP{\boldsymbol{P}}
\def\bfQ{\boldsymbol{Q}}
\def\bfR{\boldsymbol{R}}
\def\bfS{\boldsymbol{S}}
\def\bfT{\boldsymbol{T}}
\def\bfU{\boldsymbol{U}}
\def\bfV{\boldsymbol{V}}
\def\bfW{\boldsymbol{W}}
\def\bfX{\boldsymbol{X}}
\def\bfY{\boldsymbol{Y}}
\def\bfZ{\boldsymbol{Z}}
\def\bfalpha{\boldsymbol{\alpha}}
\def\bfa{\boldsymbol{a}}
\def\bfbeta{\boldsymbol{\beta}}
\def\bfb{\boldsymbol{b}}
\def\bfcd{\dot{\bfc}}
\def\bfchi{\boldsymbol{\chi}}
\def\bfc{\boldsymbol{c}}
\def\bfd{\boldsymbol{d}}
\def\bfe{\boldsymbol{e}}
\def\bff{\boldsymbol{f}}
\def\bfgamma{\boldsymbol{\gamma}}
\def\bfg{\boldsymbol{g}}
\def\bfh{\boldsymbol{h}}
\def\bfi{\boldsymbol{i}}
\def\bfj{\boldsymbol{j}}
\def\bfk{\boldsymbol{k}}
\def\bflambda{\boldsymbol{\lambda}}
\def\bfl{\boldsymbol{l}}
\def\bfm{\boldsymbol{m}}
\def\bfn{\boldsymbol{n}}
\def\bfomega{\boldsymbol{\omega}}
\def\bfone{\boldsymbol{1}}
\def\bfo{\boldsymbol{o}}
\def\bfpdd{\ddot{\bfp}}
\def\bfpd{\dot{\bfp}}
\def\bfphi{\boldsymbol{\phi}}
\def\bfp{\boldsymbol{p}}
\def\bfq{\boldsymbol{q}}
\def\bfr{\boldsymbol{r}}
\def\bfsigma{\boldsymbol{\sigma}}
\def\bfs{\boldsymbol{s}}
\def\bftau{\boldsymbol{\tau}}
\def\bftheta{\boldsymbol{\theta}}
\def\bft{\boldsymbol{t}}
\def\bfu{\boldsymbol{u}}
\def\bfv{\boldsymbol{v}}
\def\bfw{\boldsymbol{w}}
\def\bfxi{\boldsymbol{\xi}}
\def\bfx{\boldsymbol{x}}
\def\bfy{\boldsymbol{y}}
\def\bfzero{\boldsymbol{0}}
\def\bfz{\boldsymbol{z}}
\def\defeq{\stackrel{\mathrm{def}}{=}}
\def\p{\boldsymbol{p}}
\def\qdd{\ddot{\bfq}}
\def\qd{\dot{\bfq}}
\def\q{\boldsymbol{q}}
\def\xd{\dot{x}}
\def\yd{\dot{y}}
\def\zd{\dot{z}}
\bfw^c</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6644em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.02778em;">w</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.6644em;"><span style="top:-3.063em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">c</span></span></span></span></span></span></span></span></span></span></span>, and by Newton-Euler that wrench is opposite <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi mathvariant="bold-italic">w</mi><mrow><mi>g</mi><mi>i</mi><mi>e</mi></mrow></msup></mrow><annotation encoding="application/x-tex">\def\bfA{\boldsymbol{A}}
\def\bfB{\boldsymbol{B}}
\def\bfC{\boldsymbol{C}}
\def\bfD{\boldsymbol{D}}
\def\bfE{\boldsymbol{E}}
\def\bfF{\boldsymbol{F}}
\def\bfG{\boldsymbol{G}}
\def\bfH{\boldsymbol{H}}
\def\bfI{\boldsymbol{I}}
\def\bfJ{\boldsymbol{J}}
\def\bfK{\boldsymbol{K}}
\def\bfL{\boldsymbol{L}}
\def\bfM{\boldsymbol{M}}
\def\bfN{\boldsymbol{N}}
\def\bfO{\boldsymbol{O}}
\def\bfP{\boldsymbol{P}}
\def\bfQ{\boldsymbol{Q}}
\def\bfR{\boldsymbol{R}}
\def\bfS{\boldsymbol{S}}
\def\bfT{\boldsymbol{T}}
\def\bfU{\boldsymbol{U}}
\def\bfV{\boldsymbol{V}}
\def\bfW{\boldsymbol{W}}
\def\bfX{\boldsymbol{X}}
\def\bfY{\boldsymbol{Y}}
\def\bfZ{\boldsymbol{Z}}
\def\bfalpha{\boldsymbol{\alpha}}
\def\bfa{\boldsymbol{a}}
\def\bfbeta{\boldsymbol{\beta}}
\def\bfb{\boldsymbol{b}}
\def\bfcd{\dot{\bfc}}
\def\bfchi{\boldsymbol{\chi}}
\def\bfc{\boldsymbol{c}}
\def\bfd{\boldsymbol{d}}
\def\bfe{\boldsymbol{e}}
\def\bff{\boldsymbol{f}}
\def\bfgamma{\boldsymbol{\gamma}}
\def\bfg{\boldsymbol{g}}
\def\bfh{\boldsymbol{h}}
\def\bfi{\boldsymbol{i}}
\def\bfj{\boldsymbol{j}}
\def\bfk{\boldsymbol{k}}
\def\bflambda{\boldsymbol{\lambda}}
\def\bfl{\boldsymbol{l}}
\def\bfm{\boldsymbol{m}}
\def\bfn{\boldsymbol{n}}
\def\bfomega{\boldsymbol{\omega}}
\def\bfone{\boldsymbol{1}}
\def\bfo{\boldsymbol{o}}
\def\bfpdd{\ddot{\bfp}}
\def\bfpd{\dot{\bfp}}
\def\bfphi{\boldsymbol{\phi}}
\def\bfp{\boldsymbol{p}}
\def\bfq{\boldsymbol{q}}
\def\bfr{\boldsymbol{r}}
\def\bfsigma{\boldsymbol{\sigma}}
\def\bfs{\boldsymbol{s}}
\def\bftau{\boldsymbol{\tau}}
\def\bftheta{\boldsymbol{\theta}}
\def\bft{\boldsymbol{t}}
\def\bfu{\boldsymbol{u}}
\def\bfv{\boldsymbol{v}}
\def\bfw{\boldsymbol{w}}
\def\bfxi{\boldsymbol{\xi}}
\def\bfx{\boldsymbol{x}}
\def\bfy{\boldsymbol{y}}
\def\bfzero{\boldsymbol{0}}
\def\bfz{\boldsymbol{z}}
\def\defeq{\stackrel{\mathrm{def}}{=}}
\def\p{\boldsymbol{p}}
\def\qdd{\ddot{\bfq}}
\def\qd{\dot{\bfq}}
\def\q{\boldsymbol{q}}
\def\xd{\dot{x}}
\def\yd{\dot{y}}
\def\zd{\dot{z}}
\bfw^{gie}</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8247em;"></span><span class="mord"><span class="mord"><span class="mord"><span class="mord boldsymbol" style="margin-right:0.02778em;">w</span></span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.8247em;"><span style="top:-3.063em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span><span class="mord mathnormal mtight">i</span><span class="mord mathnormal mtight">e</span></span></span></span></span></span></span></span></span></span></span></span>.</p>
</div>
<div class="section" id="zmp-of-a-wrench">
<h2>ZMP of a wrench<a class="headerlink" href="#zmp-of-a-wrench" title="Permalink to this headline">¶</a></h2>
<p>Note how, in general, the ZMP is a characteristic point of any wrench. It is only in the particular case of balancing limbed robots that we consider the ZMP of the contact (equivalently "gravito-extero-inertial") wrench.</p>
</div>
Posted by: Vyankatesh (Venky)2022-12-28T10:59:00+01:002022-12-28T10:59:00+01:00Vyankatesh (Venky)tag:scaron.info,2022-12-28:/robotics/zero-tilting-moment-point.html//comment-external-forces-follow-up<p>Follow-up question: if a humanoid is being pushed by a (say force measuring) stick, do we count it as contact force? How do we proceed with the ZMP computations then?</p>Posted by: Vyankatesh (Venky)2022-12-28T10:57:00+01:002022-12-28T10:57:00+01:00Vyankatesh (Venky)tag:scaron.info,2022-12-28:/robotics/zero-tilting-moment-point.html//comment-external-forces<p>How do the ZMP computations change when external forces are applied to the robot? For example, say a humanoid torso has a magnet embedded in it, and another one is repelling the same. Do we add this force to the gravitational field force?</p>
<p>If a humanoid is being pushed by …</p><p>How do the ZMP computations change when external forces are applied to the robot? For example, say a humanoid torso has a magnet embedded in it, and another one is repelling the same. Do we add this force to the gravitational field force?</p>
<p>If a humanoid is being pushed by a (say force measuring) stick, do we count it as contact force? How do we proceed with the ZMP computations then?</p>