Torque Control of a Poly-articulated Mobile Robot During Obstacle Clearance
This paper describes a control algorithm that optimises the distribution of joint torques of a polyarticulted robot while performing obstacle clearance of a large step. In this work, a special class of polyarticulted locomotion systems known as hybrid whe
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Introduction
In this paper, we address the motion control of poly-articulated mobile robots during obstacle clearance. We are focusing on a special class of mobile systems, often called hybrid wheel-legged robots, which are designed in order to increase both obstacle crossing and terrain adaptation capabilities. Systems like HyLoS (Grand et al. (2002)) and the Workpartner (Halme et al. (2003)) are examples of such hybrid locomotion systems. They are composed of 4 wheel-legs, each wheel-leg being a multi-dof serial chain ended by a driven and steerable wheel. Such robots have the ability to change the position of their center of mass (CoM) and to modify the distribution of their contact forces. Furthermore, they are often redundantly actuated systems exhibiting internal forces that should be optimized. The proposed motion controller is based on a torque control at joint level that addresses the combined optimization of the internal forces and the CoM position, in order to maximize the contact stability (increasing traction and avoiding tip-over).
V. Padois, P. Bidaud, O. Khatib (Eds.), Romansy 19 – Robot Design, Dynamics and Control, CISM International Centre for Mechanical Sciences, DOI 10.1007/978-3-7091-1379-0_36, © CISM, Udine 2013
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P. Jarrault, C. Grand and P. Bidaud
Figure 1. Hylos prototype
First, this paper introduces a stability criterion based on the contact forces slippage. Then, we present the formulation of the optimization of forces distribution. Lastly, simulation results are presented exhibiting the clearance capabilities over a step-like obstacle which height is greater than wheel radius.
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System modelling Equations of forces distribution
We consider a system supported by n legs (Fig. 2). The ith leg is in a frictional contact with the ground at point Pi which coordinates pi are expressed in the frame p = (G, xp , yp , zp ), attached to the chassis of the robot and located at CoM. The contact force of the ground on each leg is t where fxi , fyi and fzi are the components denoted fi = fxi fyi fzi of the force along the contact frame’s axis ci = (Pi , xi , yi , zi ), such that zi is the contact normal and xi , yi are the tangential directions. The equations describing the equilibrium of the system are given by: Gf=F
(1)
t where f = f1 t ... fn t is a [3n × 1] vector containing all contact forces and F is the set of external and inertial wrench applied on the platform. G is a [6 × 3n] matrix giving the equivalent wrench to the contact forces at the center of mass in the frame p (see Grand et al. (2010) for more details). The contact forces must respect constraints related to actuator saturation and Coulomb friction law.
Torque Control of a Poly-articulated Mobile Robot…
Figure 2. Model of the system
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Figure 3. Residual force on a contact
The actuators limits are defined as follow: JT f < τ max −JT f < τ max
(2)
where J = blockdiag(Ji ), Ji being the Jacobian matrix of the ith leg and τ max is the torque limit vector. The contact constraints defined by Coul
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