Matthew M. Williamson

The cog project: building a humanoid robot

To explore issues of developmental structure, physical embodiment, integration of multiple sensory and motor systems, and social interaction, we have constructed an upper-torso humanoid robot called Cog. The robot has twenty-one degrees of freedom and a variety of sensory systems, including visual, auditory, vestibular, kinesthetic, and tactile senses. This chapter gives a background on the methodology that we have used in our investigations, highlights the research issues that have been raised during this project, and provides a summary of both the current state of the project and our long-term goals. We report on a variety of implemented visual-motor routines (smooth-pursuit tracking, saccades, binocular vergence, and vestibular-ocular and opto-kinetic reflexes), orientation behaviors, motor control techniques, and social behaviors (pointing to a visual target, recognizing joint attention through face and eye finding, imitation of head nods, and regulating interaction through expressive feedback). We further outline a number of areas for future research that will be necessary to build a complete embodied system.

Stiffness Isn't Everything

Most robot designers make the mechanical interface between an actuator and its load as stiff as possible[9][10]. This makes sense in traditional position-controlled systems, because high interface stiffness maximizes bandwidth and, for non-collocated control, reduces instability. However, lower interface stiffness has advantages as well, including greater shock tolerance, lower reflected inertia, more accurate and stable force control, less damage during inadvertent contact, and the potential for energy storage. The ability of series elasticity (usually in the form of a compliant coating on an end-effector) to stabilize force control during intermittent contact with hard surfaces is well known. This paper proposes that for natural tasks where small-motion bandwidth is not of paramount concern, actuator to load interfaces should be significantly less stiff than in most present designs. Furthermore, by purposefully placing the majority of interface elasticity inside of an actuator package, a new type of actuator is created with performance characteristics more suited to the natural world. Despite common intuition, such a series-elastic actuator is not difficult to control.

Making Complex Articulated Agents Dance

We discuss the tradeoffs involved in control of complex articulated agents, and present three implemented controllers for a complex task: a physically-based humanoid torso dancing the Macarena. The three controllers are drawn from animation, biological models, and robotics, and illustrate the issues of joint-space vs. Cartesian space task specification and implementation. We evaluate the controllers along several qualitative and quantitative dimensions, considering naturalness of movement and controller flexibility. Finally, we propose a general combination approach to control, aimed at utilizing the strengths of each alternative within a general framework for addressing complex motor control of articulated agents.

Designing rhythmic motions using neural oscillators

Neural oscillators offer simple and robust solutions to problems such as locomotion and dynamic manipulation. However, the parameters of these systems are notoriously difficult to tune. This paper presents an analysis technique which alleviates the difficulty of tuning. The method is based on describing function analysis, and can predict the steady state motion of the system, analyze stability, and be used to determine robustness to system changes. The method is illustrated using a number of design examples including an implementation of juggling on a real robot.

Bipedal locomotion control with rhythmic neural oscillators

A biologically inspired locomotion control design approach is presented which is based on a mutually inhibited neural oscillator model. The entrainment between the dynamics of neural oscillators and the natural dynamics of the plant is very important for neural oscillator driven rhythmic control. A systematic design approach for rhythmic control is studied in the paper. First, the global system dynamics is divided into two separate parts, namely, the dynamics of neural oscillators and the natural dynamics of the controlled plant. Second, a compensator block is proposed to shape the natural dynamics of the plant so that the global dynamic entrainment and the desired plant motion can be achieved more easily. Furthermore, a design guideline for global dynamic entrainment is given. Finally, the design approach is applied to bipedal locomotion control of a simulated walking robot. The simulation results are also presented in the paper.