Team CHARLI

Jaekweon Han, Robert Nguyen, Dennis Hong

RoMeLa; Department of Mechanical Engineering, Virginia Tech, USA

www.romela.org/robocup

Abstract This paper details the hardware, software, and electrical design of the humanoid robot CHARLI (Cognitive Humanoid Autonomous Robot with Learning Intelligence). CHARLI may be the first and only US entry into the humanoid adult size division of RoboCup 2010.

1 Introduction

CHARLI (Cognitive Humanoid Autonomous Robot with Learning Intelligence) is a 1.35 m tall, 12.5 kg, autonomous humanoid robot, which will be the first of its kind in the United States. This humanoid robot will be used as a platform for research, education, outreach, and publicity for Virginia Tech. The completed humanoid robot will be able to autonomously navigate the hallways of campus buildings walking on two feet, and perform human-like complex motions such as giving tours indoors.

2 Research

CHARLI serves as a research platform used for studying dynamic gaits and walking control algorithms. With few exceptions (i.e. the Honda ASIMO, the Sony QRIO, and the KAIST HUBO [1–5]), most legged robots today walk using what is called the static stability criterion. The static stability criterion is an approach to prevent the robot from falling down by keeping the center of mass of its body over the support polygon by adjusting the position of its links and pose of its body very slowly to minimize dynamic effects [3]. Thus at any given instant in the walk, the robot could "pause" and not fall over. Static stability walking is generally energy inefficient since the robot must constantly adjust its pose to keep the center of mass of the robot over its support polygon, which generally requires large torques at the joint actuators (similar to a human standing still with one foot off the ground). Humans naturally walk dynamically with the center of mass rarely inside the support polygon. Thus human walking can be considered as a cycle of continuously falling and catching its fall: a cycle of exchanging potential energy and kinetic energy of the system like the motion of an inverted pendulum. Humans fall forward and catch themselves with the swinging foot while continuously progressing forward. This falling motion allows the center of mass to continually move forward, minimizing the energy that would reduce the momentum. The lowered potential energy from this forward motion is then increased again by the lifting motion of the supporting leg.

One natural question that arises when examining dynamic walking is how to classify the stability of the gait. Dynamic stability is commonly measured using the Zero Moment Point (ZMP), which is defined as the point where the influence of all forces acting on the mechanism can be replaced by one single force without a moment term [6]. If this point remains in the support polygon, then the robot can have some control over the motion of itself by applying force and/or torque to the ground. Once the ZMP moves to the edge of the foot, the robot is on the verge of stability and can do nothing to recover without extending the support polygon (planting another foot or arm). Parameterized gaits can be optimized using the ZMP as a stability criterion or stable hyperbolic gaits can be generated by solving the ZMP equation for a path of the center of mass. Additionally, the ZMP can be measured directly or estimated during walking to give the robot feedback to correct and control its walking. CHARLI is developed and being used for research on such dynamic gaits and control strategies for stability [3, 7].

3 Hardware

CHARLI has 20 degrees of freedom (four in each leg, four in each arm, and two in the head). The robot's links are fabricated out of aluminum. The robot uses Robotis' Dynamixel EX-106, RX-64, and RX-28 motors for the joints [8]. The motors operate on a serial RS485 network, allowing the motors to be daisy chained together. Each motor has its own built-in potentiometer (with the exception of the EX-106 which has an optical encoder) and position feedback controller, creating distributed control. The computers, sensors, electronics, and computer ports are distributed about CHARLI's upper torso.

CHARLI's legs are designed using a parallel four-bar linkage which forces the foot to always be parallel to the hip as well as allowing the motors to arranged in a parallel configuration instead of a serial configuration. The benefit of arranging actuators in parallel instead of in series is that this allows loads to be distributed across multiple actuators, which reduces the performance demands of each actuator.

4 Electronics

CHARLI's electronic system will be very similiar to the one used on DARwIn IV but will be modified to account for the increased power required to operate CHARLI. It provides power distribution, communication buses, computing platforms, and sensing schemes aimed at making sense of a salient environment. CHARLI's power is provided by three 18.8V (nominal), 2.2 Ah, lithium polymer batteries. Each battery provides power to a separate isolated area: upper body, left half of the lower body, and right half of the lower body.

Computing tasks are performed on a Compulabs fit-PC2 computing system that runs the Intel Atom Z530 CPU with 1GB of onboard RAM and built-in WiFi. The PC runs off the main battery supply, and consumes 8W at full CPU usage. In addition, the computer connects to both a Philips SPC1300 camera and an Arduino microcontroller board.

The Arduino board, featuring the ATmega1280, acts as the communication relay between the Dynamixel motors and the fitPC. The microcontroller also provides sensor acquisition and processing. Switches, a 6 degree of freedom Inertial Measurement Unit (IMU), and foot sensors will aid in correcting gait cycles in the face of perturbations. A block diagram that outlines the computing relationship is shown in Fig. 2.

5 Software

The software architecture for the robots is shown in Fig. 3. This architecture is novel in that it uses MATLAB as a common development platform. Since many of the students do not have strong programming backgrounds, this development platform allows them to participate more fully on the team. Low-level interfaces to the hardware level are implemented as compiled Mex routines callable from MATLAB. These routines provide access to the camera and the microcontroller which inturn communicates with the servos and the IMU, and allow the higherlevel routines to modify joint angles and stiffnesses.

Additionally, by changing a simple PATH variable, a set of simulated interfaces can be swapped in for onboard development and testing. This allows for easy debugging on logged data even without access to the robotics hardware. The MATLAB routines consist of a variety of modules, layered hierarchically:

• Sensor Module that is responsible for reading joint encoders, IMU, foot sensors, battery status, and button presses on the robot.
• Camera Interface to the video camera system, including setting parameters, switching cameras, and reading the raw YUYV images.
• Effector Module to set and vary motor joints and parameters, as well as body and face LED's.
• Vision Uses acquired camera images to deduce presence and relative location of the ball, goals, lines, and other robots.
• World Models world state of the robot, including pose and altered ball location.
• Game StateMch Game state machine to respond to Robocup game controller and referee button pushes.
• Head StateMch Head state machine to implement ball tracking, searching, and lookaround behaviors.
• Body StateMch Body state machine to switch between chasing, ball approach, dribbling, and kicking behaviors.
• Keyframe Keyframe motion generator used for scripted motions such as getup and kick motions.
• Walk Omnidirectional locomotion module.

In order to simplify development, all interprocess communications are performed by passing MATLAB structures between the various modules, as well as between robots. [9]

6 Vision

In each new setting, we may encounter different field conditions such as a change in lighting or the actual color hue of the field objects. In order to account for this, we log a series of images that are then used to train a lookup table . A MATLAB tool enables us to define the YCbCr values that correspond to green, yellow, white, etc. Once these specific values are selected and defined, the distribution of the points in the color space are spread out and generalized to account for a greater variation. This is done with a Gaussian mixture model that analyzes the probability density function of each of the previously defined pixel values. The boundaries of the color classes are then expanded according to Bayes Theorem. We can then process the individual pixels of the new images by matching their YCbCr values to the broadened definition of the values in the lookup table.

After the image is segmented into its corresponding color classes using the look-up table, the segmentation is bitwise OR-ed in 4x4 blocks. The initial object hypotheses for the ball and goal posts are found by finding connected components in the smaller, bit OR-ed, image, and then using the original image we calculated the statistics of each region. Processing the bit OR-ed image first allowed us to greatly speed up the computation of the system. The bit OR-ed image also produced the set of points that are used in our line detection algorithm.

We then check the segmented components for certain attributes like size, shape, and position in order to classify objects, such as the ball and the goal posts. We also compute statistics for the position of detected objects in the world coordinate system using the inverse kinematics of the robot, the centroid, and the bounding box to further filter the object hypotheses. Using these we are able to track the ball and identify the existence and size of goal posts and consequently localize our position on the field. [9]

7 Conclusion

Building on previous research and RoboCup experience as well as using the technology from the DARwIn family of robots, we hope to design and create CHARLI to be the first adult sized humanoid robot in the US that can compete in RoboCup.

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