Human Multi-Robot Physical Interaction: a Distributed Framework


The objective of this paper is to devise a general framework to allow a human operator to physically interact with an object manipulated by a multi-manipulator system in a distributed setting. A two layer solution is devised. In detail, at the top layer an arbitrary virtual dynamics is considered for the object with the virtual input chosen as the solution of an optimal Linear Quadratic Tracking (LQT) problem. In this formulation, both the human and robots’ intentions are taken into account, being the former online estimated by Recursive Least Squares (RLS) technique. The output of this layer is a desired trajectory of the object which is the input of the bottom layer and from which desired trajectories for the robot end effectors are computed based on the closed-chain constraints. Each robot, then, implements a time-varying gain adaptive control law so as to take into account model uncertainty and internal wrenches that inevitably raise due to synchronization errors and dynamic and kinematic uncertainties. Remarkably, the overall solution is devised in a distributed setting by resorting to a leader-follower approach and distributed observers. Simulations with three 6-DOFs serial chain manipulators mounted on mobile platforms corroborate the theoretical findings.

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Correspondence to Martina Lippi.

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This work is supported by Dipartimento di Eccellenza granted to DIEI Department, University of Cassino and Southern Lazio and by H2020-ICT project CANOPIES-A Collaborative Paradigm for Human Workers and Multi-Robot Teams in Precision Agriculture Systems (Grant Agreement N. 101016906).



To prove Theorem 1, let us first derive the closed loop dynamics or robot i. By folding (45) in (1), it holds

$$ {\boldsymbol{M}}_{\!i}\dot{\boldsymbol{s}}_{i} = -{\boldsymbol{C}}_{i}\boldsymbol{s}_{i} - \boldsymbol{K}_{s}\boldsymbol{s}_{i} - {\varDelta} \boldsymbol{u}_{i} + \boldsymbol{h}_{i} + {\boldsymbol{Y}}_{\!\!i}(\boldsymbol{x}_{i}, \dot{\boldsymbol{x}}_{i},\boldsymbol{\rho}_{i}, \dot{\boldsymbol{\rho}}_{i} )\tilde{\boldsymbol{\pi}}_{i}\!\!\!\! $$

Now, let us consider the following Lyapunov function

$$ V=\frac{1}{2}\sum\limits_{i=1}^{N}\left( \boldsymbol{s}_{i}^{\mathrm{T}}{\boldsymbol{M}}_{i}\boldsymbol{s}_{i}+\frac{1}{k_{f}}\hat{\varDelta} \boldsymbol{u}_{f,i}^{\mathrm{T}}\hat{\varDelta} \boldsymbol{u}_{f,i}+\tilde{\boldsymbol{\pi}}_{i}^{\mathrm{T}}\boldsymbol{K}_{\pi}\tilde{\boldsymbol{\pi}}_{i}\right) $$

By virtue of (48) and Property 1, the time derivative of V is

$$ \begin{aligned} {}\dot{V} \! =& \! \sum\limits_{i=1}^{N}\!\Big(\boldsymbol{s}_{i}^{\mathrm{T}}{\boldsymbol{M}}_{i}\dot{\boldsymbol{s}}_{i} + \frac{1}{2}\boldsymbol{s}_{i}^{\mathrm{T}}\dot{{\boldsymbol{M}}}_{i}\boldsymbol{s}_{i} + \hat{\varDelta} \boldsymbol{u}_{f,i}^{\mathrm{T}}\tilde{\boldsymbol{h}}_{int,i} - \tilde{\boldsymbol{\pi}}_{i}^{\mathrm{T}}\boldsymbol{K}_{\pi}{\dot{\hat{\boldsymbol{\pi}}}}_{i}\Big)\\ =&\! \sum\limits_{i=1}^{N}\Big(-\boldsymbol{s}_{i}^{\mathrm{T}}\boldsymbol{K}_{s}\boldsymbol{s}_{i}+\frac{1}{2}(\dot{\boldsymbol{M}}_{i}-2{\boldsymbol{C}}_{i})\boldsymbol{s}_{i} +\boldsymbol{s}_{i}^{\mathrm{T}}(\boldsymbol{h}_{i}-{\varDelta} \boldsymbol{u}_{i})\\ &+\hat{\varDelta} \boldsymbol{u}_{f,i}^{\mathrm{T}}\tilde{\boldsymbol{h}}_{int,i} -\tilde{\boldsymbol{\pi}}_{i}^{\mathrm{T}}(\boldsymbol{K}_{\pi}{\dot{\hat{\boldsymbol{\pi}}}}_{i}-{\boldsymbol{Y}}_{i}^{\mathrm{T}}(\boldsymbol{x}_{i}, \dot{\boldsymbol{x}}_{i}, \boldsymbol{\rho}_{i}, \dot{\boldsymbol{\rho}}_{i} )\boldsymbol{s}_{i})\Big)\\ =&\!\sum\limits_{i=1}^{N}\!\Big(\! - \boldsymbol{s}_{i}^{\mathrm{T}}{\kern-.5pt}\boldsymbol{K}_{s}\boldsymbol{s}_{i} + \hat{\varDelta} \boldsymbol{u}_{f,i}^{\mathrm{T}}\tilde{\boldsymbol{h}}_{int,i} - \boldsymbol{s}_{i}^{\mathrm{T}}{\kern-.5pt}({\kern-.5pt}\boldsymbol{e}_{int,i} + k_{f}\!\hat{\varDelta} \boldsymbol{u}_{f,i}\! +\! \frac{1}{N}{~}^{i}\tilde{\boldsymbol{h}}_{h}{\kern-.5pt})\\ & -\kappa_{i}(t)\boldsymbol{s}_{i}^{\mathrm{T}}\boldsymbol{s}_{i} -\tilde{\boldsymbol{\pi}}_{i}^{\mathrm{T}}(\boldsymbol{K}_{\pi}{\dot{\hat{\boldsymbol{\pi}}}}_{i}-{\boldsymbol{Y}}_{i}^{\mathrm{T}}(\boldsymbol{x}_{i}, \dot{\boldsymbol{x}}_{i}, \boldsymbol{\rho}_{i}, \dot{\boldsymbol{\rho}}_{i} )\boldsymbol{s}_{i}) \Big)\\ \end{aligned} $$

Given the parameters update law in Eq. (47), (50) simplifies to

$$ \begin{aligned} \dot{V} =&\sum\limits_{i=1}^{N}\Big(-\boldsymbol{s}_{i}^{\mathrm{T}}\boldsymbol{K}_{s}\boldsymbol{s}_{i}+\hat{\varDelta} \boldsymbol{u}_{f,i}^{\mathrm{T}}\tilde{\boldsymbol{h}}_{int,i}-\kappa_{i}(t)\|\boldsymbol{s}_{i}\|^{2}\\ &-(\tilde{\boldsymbol{\zeta}}_{i}+\hat{\varDelta} \boldsymbol{u}_{f,i})^{\mathrm{T}}(\tilde{\boldsymbol{h}}_{int,i}+\hat{\varDelta} \boldsymbol{u}_{f,i})+\frac{1}{N}\boldsymbol{s}_{i}^{\mathrm{T}}{~}^{i}\tilde{\boldsymbol{h}}_{h} \Big)\\ \end{aligned} $$

Thus, by choosing

$$\kappa_{i}(t)>\frac{\|\tilde{\boldsymbol{\zeta}}_{i}\|}{\|\boldsymbol{s}_{i}\|^{2}} \|\tilde{\boldsymbol{h}}_{int,i}+\hat{\varDelta} \boldsymbol{u}_{f,i}\|$$

it holds

$$ \dot{V}\leq \sum\limits_{i=1}^{N}\big(-\boldsymbol{s}_{i}^{\mathrm{T}}\boldsymbol{K}_{s}\boldsymbol{s}_{i}-\hat{\varDelta} \boldsymbol{u}_{f,i}^{\mathrm{T}}\hat{\varDelta} \boldsymbol{u}_{f,i}+\frac{1}{N}\|\boldsymbol{s}_{i}^{\mathrm{T}}\|\|{}^{i}\tilde{\boldsymbol{h}}_{h}\|\big) $$

From Lemma 2, \(\|{}^{i}\tilde {\boldsymbol {h}}_{h}\|\) converges to the origin after a finite time Th; then after this time it holds

$$ \dot{V}\leq \sum\limits_{i=1}^{N}\big(-\boldsymbol{s}_{i}^{\mathrm{T}}\boldsymbol{K}_{s}\boldsymbol{s}_{i}-{~}^{i}\hat{\varDelta} \boldsymbol{u}_{f,i}^{\mathrm{T}}{~}^{i}\hat{\varDelta} \boldsymbol{u}_{f,i}\big) $$

which implies that \(\dot {V}\) is semi-negative definite and, consequently, that V is bounded. By leveraging the boundedness of V and, then, of si, \(\hat {\varDelta } \boldsymbol {u}_{f,i}\) and \({\tilde {\boldsymbol {\pi }}}_{i}\), it can be easily shown the \(\ddot V\) is bounded as well. Thus, by virtue of Barbalat’s lemma, \(\dot {V}\) is uniformly continuous and converges to the origin, as well as si and \({\hat {\varDelta } \boldsymbol {u}_{f,i}=k_{f}{\int \limits }_{t_{0}}^{t}{\tilde {\boldsymbol {h}}_{int,i}} d\tau }\) (and, therefore, by definition \(\tilde {\boldsymbol {h}}_{int,i}\)). The main implication of the latter is that, since because of Lemma 2, \(\tilde {\boldsymbol {h}}_{int,i}\) converges to the origin in finite time (that is the internal wrenches estimated via observer converges to the real one), then also eint, i, ∀i, converges to the origin.

In view of the expression of si in (43) and since \(\hat {\varDelta } \boldsymbol {u}_{f,i}\) converges to the origin, it follows

$$ \dot{\boldsymbol{e}}_{x,i}+k_{p}\boldsymbol{e}_{x,i} =-\hat{\varDelta} \boldsymbol{u}_{f,i} $$

which represents an asymptotically stable system (in the state variable ex, i) with vanishing input \(-\hat {\varDelta } \boldsymbol {u}_{f,i}\). Therefore, ex, i asymptotically converges to the origin. Based on the expression of ex, i in (42), it asymptotically holds

$$ ({~}^{i} \hat{\boldsymbol{x}}_{v}-\boldsymbol{x}_{i})+k_{c}{\int}_{t_{0}}^{t}\sum\limits_{j\in\mathcal{N}_{i}}(\boldsymbol{x}_{j}-\boldsymbol{x}_{i})d\tau\rightarrow\boldsymbol{0}_{p} $$

Let us now introduce the object trajectory estimate error \({~}^{i}\tilde {\boldsymbol {x}}_{v} = {\boldsymbol {x}}_{v} - {~}^{i} \hat {\boldsymbol {x}}_{v}\in \Re ^{p}\) and the object trajectory tracking error ev, i = xvxiRp. From (52), it asymptotically holds

$$ \begin{aligned} ({\boldsymbol{x}}_{v}-\boldsymbol{x}_{i})+k_{c}{\int}_{t_{0}}^{t}\sum\limits_{j\in\mathcal{N}_{i}}(\boldsymbol{x}_{j}-{\boldsymbol{x}}_{v}-\boldsymbol{x}_{i}+{\boldsymbol{x}}_{v})d\tau&=-{~}^{i}\tilde{\boldsymbol{x}}_{v}\\ \end{aligned} $$

which can be rewritten as

$$ \begin{aligned} \boldsymbol{e}_{v,i}+k_{c}{\int}_{t_{0}}^{t}\sum\limits_{j\in\mathcal{N}_{i}}(-\boldsymbol{e}_{v,j}+\boldsymbol{e}_{v,i})d\tau&=-{~}^{i}\tilde{\boldsymbol{x}}_{v}\\ \end{aligned} $$

By denoting with \(\tilde {\boldsymbol {x}}_{v}\in \Re ^{Np}\) and evRNp the stacked vectors of the errors \({~}^{i}\tilde {\boldsymbol {x}}_{v}\) and ev, i, respectively, (6) leads to

$$ \boldsymbol{e}_{v}(t)=-k_{c}(\boldsymbol{L}\otimes \boldsymbol{I}_{p}){\int}_{t_{0}}^{t}\boldsymbol{e}_{v} d\tau-\tilde{\boldsymbol{x}}_{v} $$

in which, from (40), \(\tilde {\boldsymbol {x}}_{v}\) converges to the origin in finite-time. Finally, since the communication graph is connected, the immediate consequence of (54) is that \({\int \limits }_{t_{0}}^{t}\boldsymbol {e}_{v,i}={\int \limits }_{t_{0}}^{t}\boldsymbol {e}_{v,j}\), ∀i, j which, based on (42) and (52), implies that ev, i = 0pi. This completes the proof.

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Lippi, M., Marino, A. Human Multi-Robot Physical Interaction: a Distributed Framework. J Intell Robot Syst 101, 35 (2021).

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  • Human-robot interaction
  • Shared control
  • Distributed control