A pinboard by
Brandon DeHart

PhD Candidate, University of Waterloo


Figuring out how to make robots walk and work with us without tripping or falling over.

A big part of effectively controlling legged robots is trying to figure out what to do when something happens that the robot wasn't expecting. This is especially true of robots which are standing on two legs (or balancing on one, for that matter).

The main focus of this research is to figure out what a bipedal robot should do when it is pushed, bumped, trips, steps on a rock, or any other unexpected event occurs. These unexpected events are generally called "disturbances", and their strength and direction will dictate what the robot's response should be.

WIth a small disturbance, the robot can typically adjust how and where it is applying force by "flexing" its joints. This is usually done by pushing against the ground with its feet or against a wall or railing with its hands. Imagine what you might do with your feet and ankles while standing on a boat that is rocking, shifting your weight around the soles of your feet without really moving your arms and legs.

If the disturbance is stronger, "leaning" and/or changing the robot's posture is likely to be the best approach. This is often enough to compensate for any change in expected behaviour that the robot is sensing. Think of this like bending your knees or swinging your arms to deal with getting pushed or trying to balance on a narrow beam.

Finally, for a strong enough disturbance, the only real option left is to take a step. This changes how and where the robot can apply forces to the ground, but it comes at a cost. The robot must lift a foot, swing its leg, and then reliably plant its swinging foot back on the ground. Although this seems like an easy task for us, it is still a fairly difficult problem for robots. Figuring out where to step, how quickly to do so, whether to also "flex" or "lean", and how soft/slippery the new surface will be are all challenging questions for a robot.

There has been great progress on this topic in recent years, with dramatic improvements made. There are now many different bipedal robots capable of walking around in the lab, and a few are even able to walk around outside. However, the application and integration of the sensing, planning, control, and human interaction aspects of this research are still open problems waiting for reliable and consistent solutions.


Dynamic balance preservation and prevention of sliding for humanoid robots in the presence of multiple spatial contacts

Abstract: The main indicator of dynamic balance is the \(\mathit{ZMP}\) . Its original notion assumes that both feet of the robot are in contact with the flat horizontal surface (all contacts are in the same plane) and that the friction is high enough so that sliding does not occur. With increasing capabilities of humanoid robots and the higher complexity of the motion that needs to be performed, these assumptions might not hold. Having in mind that the system is dynamically balanced if there is no rotation about the edges of the feet and if the feet do not slide, we propose a novel approach for testing the dynamic balance of bipedal robots, by using linear contact wrench conditions compiled in a single matrix (Dynamic Balance Matrix). The proposed approach has wide applicability since it can be used to check the stability of different kinds of contacts (including point, line, and surface) with arbitrary perimeter shapes. Motion feasibility conditions are derived on the basis of the conditions which the wrench of each contact has to satisfy. The approach was tested by simulation in two scenarios: biped climbing up and walking sideways on the inclined flat surface which is too steep for a regular walk without additional support. The whole-body motion was synthesized and performed using a generalized task prioritization framework.

Pub.: 01 Feb '18, Pinned: 06 Mar '18

Whole-body multi-contact motion in humans and humanoids: Advances of the CoDyCo European project

Abstract: Traditional industrial applications involve robots with limited mobility. Consequently, interaction (e.g. manipulation) was treated separately from whole-body posture (e.g. balancing), assuming the robot firmly connected to the ground. Foreseen applications involve robots with augmented autonomy and physical mobility. Within this novel context, physical interaction influences stability and balance. To allow robots to surpass barriers between interaction and posture control, forthcoming robotic research needs to investigate the principles governing whole-body motion and coordination with contact dynamics. There is a need to investigate the principles of motion and coordination of physical interaction, including the aspects related to unpredictability. Recent developments in compliant actuation and touch sensing allow safe and robust physical interaction from unexpected contact including humans. The next advancement for cognitive robots, however, is the ability not only to cope with unpredictable contact, but also to exploit predictable contact in ways that will assist in goal achievement. Last but not least, theoretical results needs to be validated in real-world scenarios with humanoid robots engaged in whole-body goal-directed tasks. Robots should be capable of exploiting rigid supportive contacts, learning to compensate for compliant contacts, and utilising assistive physical interaction from humans. The work presented in this paper presents state-of-the-art in these domains as well as some recent advances made within the framework of the CoDyCo European project.

Pub.: 18 Oct '16, Pinned: 03 Nov '17

Balancing While Executing Competing Reaching Tasks: An Attractor-Based Whole-Body Motion Control System Using Gravitational Stiffness

Abstract: International Journal of Humanoid Robotics, Volume 13, Issue 01, March 2016. Whole-body control (WBC) systems represent a wide range of complex movement skills in the form of low-dimensional task descriptors which are projected on to the robot’s actuator space. Using these methods allow to exploit the full capabilities of the entire body of redundant, floating-base robots in compliant multi-contact interaction with the environment, to execute any single task and simultaneous multiple tasks. This paper presents an attractor-based whole-body motion control (WBMC) system, developed for torque-control of floating-base robots. The attractors are defined as atomic control modules that work in parallel to, and independently from the other attractors, generating joint torques that aim to modify the state of the robot so that the error in a target condition is minimized. Balance of the robot is guaranteed by the simultaneous activation of an attractor to the minimum effort configuration, and of an attractor to a zero joint momentum. A novel formulation of the minimum effort is proposed based on the assumption that whenever the gravitational stiffness is maximized, the effort is consequently minimized. The effectiveness of the WBMC was experimentally demonstrated with the COMAN humanoid robot in a physical simulation, in scenarios where multiple conflicting tasks had to be accomplished simultaneously.

Pub.: 01 Apr '16, Pinned: 06 Sep '17

Centroidal dynamics of a humanoid robot

Abstract: The center of mass (CoM) of a humanoid robot occupies a special place in its dynamics. As the location of its effective total mass, and consequently, the point of resultant action of gravity, the CoM is also the point where the robot’s aggregate linear momentum and angular momentum are naturally defined. The overarching purpose of this paper is to refocus our attention to centroidal dynamics: the dynamics of a humanoid robot projected at its CoM. In this paper we specifically study the properties, structure and computation schemes for the centroidal momentum matrix (CMM), which projects the generalized velocities of a humanoid robot to its spatial centroidal momentum. Through a transformation diagram we graphically show the relationship between this matrix and the well-known joint-space inertia matrix. We also introduce the new concept of “average spatial velocity” of the humanoid that encompasses both linear and angular components and results in a novel decomposition of the kinetic energy. Further, we develop a very efficient \(O(N)\) algorithm, expressed in a compact form using spatial notation, for computing the CMM, centroidal momentum, centroidal inertia, and average spatial velocity. Finally, as a practical use of centroidal dynamics we show that a momentum-based balance controller that directly employs the CMM can significantly reduce unnecessary trunk bending during balance maintenance against external disturbance.

Pub.: 19 Jun '13, Pinned: 06 Sep '17

Dynamic and Reactive Walking for Humanoid Robots based on Foot Placement Control

Abstract: International Journal of Humanoid Robotics, Ahead of Print. This paper presents a novel online walking control that replans the gait pattern based on our proposed foot placement control using the actual center of mass (COM) state feedback. The analytic solution of foot placement is formulated based on the linear inverted pendulum model (LIPM) to recover the walking velocity and to reject external disturbances. The foot placement control predicts where and when to place the foothold in order to modulate the gait given the desired gait parameters. The zero moment point (ZMP) references and foot trajectories are replanned online according to the updated foothold prediction. Hence, only desired gait parameters are required instead of predefined or fixed gait patterns. Given the new ZMP references, the extended prediction self-adaptive control (EPSAC) approach to model predictive control (MPC) is used to minimize the ZMP response errors considering the acceleration constraints. Furthermore, to ensure smooth gait transitions, the conditions for the gait initiation and termination are also presented. The effectiveness of the presented gait control is validated by extensive disturbance rejection studies ranging from single mass simulation to a full body humanoid robot COMAN in a physics based simulator. The versatility is demonstrated by the control of reactive gaits as well as reactive stepping from standing posture. We present the data of the applied disturbances, the prediction of sagittal/lateral foot placements, the replanning of the foot/ZMP trajectories, and the COM responses.

Pub.: 11 Nov '15, Pinned: 06 Sep '17