03-05-2011, 10:46 AM
Abstract
This article presents AmphiBot II, an am-phibious snake robot designed for both serpentine loco-motion (crawling) and swimming. It is controlled by anon-board central pattern generator (CPG) inspired bythose found in vertebrates. The CPG is modelled as achain of coupled nonlinear oscillators, and is designed toproduce travelling waves. Its parameters can be modi-¯ed on the °y. We present the hardware of the robot andthe structure of the CPG, then the systematic parametertests done in simulation and with the real robot to char-acterize how the speed of locomotion depends on theparameters determining the frequency, amplitude andwavelength of the body undulation.
Keywords| AmphiBot II, amphibious robot, snakerobot, central pattern generator, swimming, crawling
I. Introduction
Snake-like robots have been studied for many years,particularly for their ability to deal with di±cult envi-ronments, in which other types of robots often fail. Oneof ¯rst snake-like robot, named an active cord mecha-nism [1], has been constructed in 1972. Since then avariety of di®erent snake robots have been designed,see [2], [3], [4], [5], [6], some of which are currently usedfor the inspection of pipes [7], for example. A review ofsnake robots can be found in [8] and [9]. Most of theserobots have been designed for locomotion on ground,and only a few working examples of swimming snakerobots currently exist. The most interesting ones arethe eel robot REEL II [10], the lamprey robot builtat Northeastern University [11] and the spirochete-likeHELIX-I [12].The goals of this project are two-fold: (1) to buildan amphibious snake-like robot that can both crawland swim for outdoor robotics tasks, taking inspirationfrom snakes and elongate ¯shes such as lampreys, and(2) to demonstrate the use of central pattern genera-tors (CPGs) as a powerful method for online trajectorygeneration for crawling and swimming in a real robot.To the best of our knowledge, AmphiBot II is one ofthe ¯rst amphibious snake robots controlled online bya central pattern generator. As will be presented inthis article, CPGs can be implemented as systems ofcoupled nonlinear oscillators which produce coordinatedrhythmic patterns (in this case travelling waves). Thesepatterns can be designed to be stable, i.e. to exhibitlimit cycle behavior, which makes them robust againstexternal perturbations. In addition, they can easily bemodulated online by the change of a few control param-eters. This makes them ideally suited for online trajec-tory generation with a human in the loop: the user canFig. 1. The AmphiBot II robot.modify CPG parameters to adjust the speed and direc-tion of locomotion without dealing with the complexityof individually controlling each degree of freedom.The robot presented in this paper, AmphiBot II, isthe new version of AmphiBot I [13], [14]. Comparedto its predecessor, it features a signi¯cant number ofimprovements:² A better mechanical design: the construction of therobot is greatly simpli¯ed, and all pieces can be assem-bled without soldering.² More powerful motors: the maximal torque has beenincreased by a factor of 3.5.² Wireless communication capabilities: the robot hasnow an internal transceiver, which can be used to con-trol it remotely without any wires.² Improved waterproo¯ng.² Onboard CPG: the motor commands are now gen-erated online, directly in the robot, by a central pat-tern generator running on a microcontroller, thereforeremoving the need of running the controller on an ex-ternal computer.In the rest of the article, we will ¯rst describe thehardware of the robot (Section II). We then presentour Central Pattern Generator model (Section III) anda simulator of the robot that we used for initial char-acterization (Section IV). Systematic characterizationswith the simulated and real robots of the speed of lo-comotion as a function of di®erent control parametersof the CPG are presented in Section V. We ¯nish thearticle with two short sections on future work and con-clusions.II. Hardware descriptionThe AmphiBot II robot is designed to be modular: itis constructed out of several identical segments, namedelements. The robot described in this paper has 7 ac-tuated elements (i.e. 7 degrees of freedom) and a head(which is externally identical to the other elements);however with the current electronics a robot with upto 127 segments can theoretically be built by simplyadding other elements to the chain. The external cas-Proceedings of the 9th International Conference on Climbing and Walking RobotsBrussels, Belgium - September 200619ing of each element consists of two symmetrical parts,which are ¯xed together with screws. The elements areconnected (both mechanically and electrically) using acompliant connection piece ¯xed to the output axis,which contains 6 wires. All parts of the robot bodyare molded using polyurethane resin lighted with glassmicroballs; the connection pieces are also molded withpolyurethane. All the output axes of the elements arealigned, therefore producing planar locomotion. To en-sure the waterproo¯ng of the robot, a custom O-ringis used. An element has an external length of 9.4 cm,and a section of 5.5 by 3.7 cm. The total length ofthe robot presented in this paper is 77.2 cm, consider-ing that the connection piece introduces a distance of0.25 cm between two adjacent elements. The asymmet-ric friction with the ground, required for the robot tocorrectly crawl on the ground, is obtained by ¯xing acouple of passive wheels to each element with double-face adhesive tape. Currently the wheels are removedfor swimming. The density of the robot is slightly lowerthan 1 kg/m3, so that the robot °oats under the surfacewhen in water. The battery is placed at the bottom ofthe elements to have the center of mass below the ver-tical center, therefore ensuring the vertical stability ofthe robot during both swimming and crawling. Duringswimming, the robot is connected to a small aquariumpump through a highly °exible PVC tube: maintain-ing a little overpressure inside the elements avoids anypossible leakage.A. Actuated elementsEach element contains three printed circuits (a powerboard, a PD motor controller and a small water detec-tor) connected with a °at cable, a DC motor with anintegrated incremental encoder, a set of gears (whichuses two additional printed circuits as mechanical sup-port) and a rechargeable Li-Ion battery. The elementsare thus completely independent from each other (bothelectrically and mechanically). In this description, forsimplicity, we will not distinguish on which of theprinted circuits each component is located.The power circuit generates the voltage required bymost of the electronics (5 V) using a capacitive charge-pump step-up converter (LTC3200-5).The motor controller is based on a PIC16F876A mi-crocontroller, which runs a PD motor controller devel-oped at the Autonomous Systems Laboratory of theEPFL. It is connected to the I2C bus of the robotthrough a simple bidirectional repeater (built using twoBSS138 MOS transistors), which is very useful to pro-tect the microcontroller internal drivers. The motor hasan integrated magnetic incremental encoder, which gen-erates 512 pulses for every complete rotation of the mo-tor axis. The encoder is connected to a LS7084 quadra-ture detector that ¯lters and decodes the signals comingfrom the encoder, generating a direction °ag and a clocksignal, which are connected to the microcontroller.The motor coil is powered through three SI9986bu®ered H-bridges connected in parallel (each of whichhas a maximum current of 1 A; the maximal currentthat can be drawn by the motor is thus 3 A). These H-bridges are driven by the microcontroller with a Pulse-Width Modulation (PWM) signal, allowing the speedof the motor to be changed by modifying the duty cycleof the control signal.To measure the current used by the motor (and then,indirectly, its torque), a couple of 0.2 resistors in par-allel are inserted between the output of the H-bridgesand the motor. The voltage drop obtained on these re-sistors is ampli¯ed by a INA146 operational ampli¯erand sent to an analog input of the microcontroller.All the electronics can be either powered by the in-ternal Li-Ion battery, or by an external power source(connected to the last element and distributed inter-nally to all elements). When no external power source isconnected, the battery (connected to the rest of the cir-cuit through a DS2764 battery monitoring/protectioncircuit that controls two IRF7410 power MOSFETs)directly powers the motor. When an external powersource is connected, an inductive step-down converter(LT1977) generates a voltage of approximately 4.6 V,which can both replace the battery voltage (to powerthe motor and the step-up converter) and power theLTC1733 battery charger. The circuit accepts up to30 V (to reduce as much as possible the current onthe internal wires, which have a limited section). Theswitch between the internally generated 4.6 V and thebattery is realized with a LTC4411 \ideal diode" and aSS34 Schottky diode. The used battery has a capacityof 600 mAh, and can power an element for approxi-mately two hours of continuous use in normal condi-tions. When empty, the battery can be recharged inapproximately one hour. The battery protection circuitdisconnects the battery when its voltage drop below acritical threshold, thus preserving it from the often ir-reversible complete discharging.A signal coming from a reed contact (currently placedin the head element) allows to switch o® the robot byplacing a magnet on it. This solution was found to besimpler than using a big waterproof switch. This signalis connected to the enable pin of the aforementionedLTC4411 (no current is drawn, the signal can thereforebe directly generated using one of the batteries).The water detector circuit, used internally to detectand localize any leakage, is placed at the bottom ofthe element. It has a sensitive surface of about 1 cm2,consisting of several parallel tracks, half of which areconnected to the power source through a resistor. Whenwater (or a big amount of moisture) is on this surface,it acts like a resistor between the power source and thebase of an NPN transistor, which begins to conduce.When water is detected, the circuit blinks a LED ¯xedthrough the top of the element, therefore allowing theuser to immediately detect the leakage.
Download full report
http://citeseerx.ist.psu.edu/viewdoc/dow...1&type=pdf