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NXTBee to be very reliable. CMU alsoworked closely with Dick Swan, the inven-tor of ROBOTC, to integrate libraries intoROBOTC to make multi-robot communica-tions easy for students to implement. Thisstory describes the types of lessons thatCMU has developed and where you canfind them to test them out.

CURRICULUM DEVELOPMENTOur goal is to develop a series of lessonsto teach the concepts of multi-robot com-munication. The target audiences for theselessons are students with a basic under-standing of programming single robots.Our initial lessons focus on robot setupand basic communications. This next sec-tion describes CMU’s initial labs which arecapable of being implemented usingLEGO, VEX, or Arduino robots.

Setup. This lab provides a set of instruc-tions that show people how to setup theXbee radio? Plug it in, check for communi-cations between the radio and the homerobot, check to see if there is communica-tions between the Xbee and another Xbee.

Basic Communications. These labs consistof several sample programs that can beloaded onto the robot and include instruc-tions on how to modify the code. This set oflessons passes messages back and forth anddisplays them on the robots LCD screen.

Sending and Interpreting Instructions.This lab has one robot send informationand another robot interpret the data anddo something based on the message. (i.e.move 360 encoder counts)

Interpreting and Formatting Data. Thisnext set of labs requires the student todevelop functions that turn parameterslike 90 into a 90 degree turning behavior,or a parameter like 1 or .5 into a behaviorthat allows the robot to travel one meter orone half meter.

In order to accomplish the introductorylabs, we begin by teaching students a how

to share messages between robots usingstrings. In computer programming astring is traditionally a sequence of charac-ters, either as a literal constant or as somekind of variable. With each lesson we pro-

vide example code in ROBOTC thatbroadcasts a “string” out to the network –that is, it sends a “message” to all otherrobots that are listening. Next, we provid-ed sample code on how to receive a string.

A pioneering group forges a path toaffordable multi-agent robotics

Robotic technologies are ubiqui-tous and are integrated intomany modern devices yet most

people do not recognize these devices asrobots. The best example of a roboticsystem that millions of people have butthey do not referee to as a robot is a cell-phone. Today’s cellphones have numer-ous sensors such as accelerometers,gyroscopes, cameras, multi-touchscreens, Bluetooth, wireless modemsallowing internet access and GlobalPositioning Systems integrated intothem (all robotic technologies).

These sensors, when integrated into asystem, can be used to provide a veryaccurate description of the location ofyour phone. In order to accomplish thislocation discovery, your phone is pre-programmed to use the correct combina-tion of data from sensors, the Internet anda system of cellular networks to pin-pointits location. Once the system has its loca-tion, it uses other pre-programmed algo-rithms and data to provide you with a setof directions to restaurants, gas stationsor to a person’s house. Programming androbotic technologies are finding their wayinto everyday devices and many peoplebelieve that the skill sets that studentslearn through CS and Robotics are “newbasic skills” for future innovators. TheDefense Advanced Research ProjectsAgency (DARPA) is providing support toresearchers at Carnegie Mellon to devel-op inexpensive systems and trainingmaterials that will teach students thebasics of multi-system communications.The developed materials from theresearch are being made available for freethrough CMU’s Computer ScienceStudent Network (CS2N).

Over the years, DARPA has workedto increase American innovation byfunding research and technology devel-opment that not only has improved ourmilitary capabilities but also has signifi-cantly improved our quality of life (pro-

jects such as the Internet and GPS).Recently, DARPA is investing in com-puter science and robotic innovationwith programs like the DARPA GrandChallenge, the DARPA UrbanChallenge, and the CS2N. The currentstate of educational robotics is beingoutpaced by robotics innovation;schools are teaching students how toprogram single robots, yet tomorrow’srobotic systems will require multi-agentcommunications. In the book “Outliers”,Malcolm Gladwell repeatedly talksabout the “10,000-Hour Rule”, claimingthat the key to success in any field is, toa large extent, a matter of practicing aspecific task for a total of around 10,000hours. If we want American’s studentsto lead the world in innovation, then weneed to develop an affordable systemthat will allow them to begin to put theirfirst 10,000 hours in on thinking aboutmulti-agent communications.

HARDWARE TESTING ANDDEVELOPMENT

Our goal is to develop an inexpensivecommunications system that will not costthe end-user more than $150 per set; the

cost assumes that the user already has apair of robot controllers. We testedArduino, LEGO, and VEX robot con-trollers using the low cost 1mW XBeeRadio and an assortment of hardwarefrom www.sparkfun.com. We were able

to build a homebrew solution for lessthan $100 for a pair of robots. The home-brew solution requires the user to have amodest electronics background and toknow what to purchase; a list of partsand building instructions can be found atour site. This summer, Dexter Industries,www.dexterindustries.com, developed asolution for the MINDSTORMS NXTcalled the NXTBee which costs $99 perpair. We used the Dexter solution allsummer for training and found the

“A string is traditionallya sequence of

characters, either as aliteral constant or assome kind of variable.”

This article was coordinated byRobin Shoop, Director of the

Carnegie Mellon Robotics Academy.Contributors to this piece, in addi-

tion to Robin, included:Dr. Manuela Veloso,

Computer Science Faculty CMUDr. Howie Choset,

Robotics Faculty CMURobert Avanzato, Penn StateAbington Engineering Faculty

Tim Friez, Robotics Academy Staff Somchaya Liemhetcharat,

Graduate Student Ben Morse, Graduate Student

Programming and Multi-RobotCommunications

ROBOTC Code Example In the slide below you will see some of the reserve words that are incorporated into ROBOTC.

InitRS485();• A Function to initializethe Xbee radio

SendString(message);• Sends a string to everyother robot in range

ReceiveString(message);• Receives a string thathas been sent from another robot

• Delays the program until the message is received

In the ROBOTC Multi-Robot Communication Libraries there are three key functions thatare used. First is an initialization command which is a command specific for the NXTthat is used to setup the NXT’s sensor port 4 to communicate with the Xbee WirelessRadio. The Xbee Wireless Radio requires a serial (RS-232 or RS-485) connection tocommunicate from the robot controller. The NXT’s sensor port 4 can be used as ahigh-speed connection which supports RS-485, so the command is named“InitRS485()”. On the VEX platform, this command will setup one of the user controlledserial ports to communicate with the wireless radio.

The second command, “SendString”, is used to send a string of data using the wirelessradio to any other device that may be configured to be listening on the same network.This SendString command is used to broadcast out to everyone, rather than to a specificrobot. Future implementations of SendString will have the ability to send to a specificrobot or to groups of robots. Because ROBOTC uses standard C-Style commands, you willbe able to send strings with more data (such as numbers and parameters) using variousstring manipulation commands found in the C-Language. The third command,“ReceiveString”, is used to receive a string of data using the wireless radio from any otherdevice that is configured to be listening on the same network. The ReceiveString com-mand will loop internally until it receives data, although a timeout can be specified to haveit continue on in the program if data isn’t received within a certain period of time.

Strings are the basic communication toolsused to pass messages.

VEX Cortex controllerwith XBee.

Arduino MEGA controller and XBee.

LEGO NXT; with XBee.

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Together, these two snippets of codeprovide the basic framework for multi-robot communication. Also, it is com-mon in multi-robot communication forone robot to wait until a specific messageis received, and we provided a codesample of a function that does it. At theend of the introductory lessons, we cre-ated a list of programming challengeddesigned to give students practice hav-ing robots share and interpret messages.

MULTI-ROBOT LESSONS TESTEDWITH HIGH-SCHOOL TEACHERS

The initial Multi-Robot lesson develop-ment occurred during spring of 2011. Thedevelopment team worked on the curricu-lar materials over a three month periodinternally and then piloted the materialswith a group of CS UndergraduateWomen who visited CMU as part of an“Opportunities for UndergraduateResearch in Computer Science”program. This section highlightsa robotic challenge given to over160 teachers who attended CMURobotics Academy’s one-weeksummer classes. The multi-robotprogramming portion of trainingstarted the fourth day of a fiveday program. The fourth dayproved to be ideal because bythen teachers had learned basicprogramming concepts like wait-states, loops, conditional state-ments, variables, functions, andhow to pass parameters. Theseconcepts are foundational and

necessary to implement multi-robot com-munications concepts.

The robotic challenge followed aseries of lessons designed to explain thevarious parts of multi-robot communica-tions. Each lesson consisted of severalshort exercises (send and receive mes-sages) designed to provide the newlearners with the confidence they wouldneed to complete the overall challenge.The teachers were grouped in pairs tosolve the problems; each pair had twoXbee enabled robots to allow messaging.The labs were the same labs that wedeveloped during the spring: Setup,Basic Communications, Sending andInterpreting Instructions, andInterpreting and Formatting Data.

On Friday, the teachers participated ina class-wide exercise that involved all theteacher groups and showcased “broad-cast communications” between 7 robots.

The instructors programmed the primaryrobot which acted as the primary way-point, and all of the other robots had totranslate that information and decide on alocation to “surround” the primary robot.In order to facilitate this, the first lessonthat we taught was the concept of operat-ing in a common world. All 7 robots wereplaced in a grid environment, whereblack lines were used to demarcate thegrid of the world. Using a grid system,we could emulate a Cartesian coordinatesystem of the robot’s world to form thebasis of the common language betweenthe robots. The grid-based world allowedthe robots to “know where they were”based on their initial start positions andorientations, we used wheel encoderfeedback to localize their position on theboard at any given time.

The instructors’ robot autonomouslymoved from the initial starting position toanother randomly chosen location andcommunicated its coordinates to the other

robots. Thanks to the common language(variable names and functions) created byall the teachers as a group, every robotunderstood the message and knew wherethe instructors’ robot was located.

Another multi-robot concept that wastaught was “turn-taking” among a teamof robots. Because there are 6 robots thatneed to navigate from their starting posi-tion to their final destination, the teach-ers needed to create a predeterminedorder so that robots would know whento move. Once the instructors’ robot sentits location to the other robots, each

robot would store the position and thenthe first robot in the predeterminedorder would start moving. Once thatrobot reached its destination, it wouldsend a message to the rest of the teaminforming them that its movement wascomplete and where it was located. Thenext robot would then start its turn andmove to its position; each robot neededto store the message, and based on thespace left determine where it was to go.

The remaining robots programmedby the teachers then moved to theirrespective positions around the instruc-tors’ robot, one after another by takingturns to move and communicate.

When the last robot reached itsdesired position, it also announced thatits turn was over, and the robots nowutilized another concept of the lessons –coordinated actions. In this case, theteacher’s robots were programmed to doa dance, all at the same time.

In the end, this multi-robot communi-cations activity proved to teach manybasic multi-robot communications con-cepts. Teachers started with a code basethat allowed them to send and store loca-tions; this was necessary due to the timeconstraints. Each team had to create theirown encoder-based movement and path

planning code and to apply multi-robotcommunications code that allowed themto send and receive messages, take turns,store variables, and autonomously pathplan based on prior robots messages. Weconducted pre and post surveys of teach-ers involved in the program. The sur-vey’s goal was to help us to learn moreabout the appropriateness of the level ofthe challenge, the curriculum’s ability towork in the teacher’s classroom, and theteacher’s perception of the importance ofteaching multi-robot communication in arobotics classroom. The survey resultswere encouraging, Darlene Cook, an ele-mentary school teacher from AustinTexas said “I thought the multi-robotchallenge and technology was awesome.It was a week of intense information. Alittle overwhelming at times for a tech-nology challenged person. However, Igot it! It was fast pace. At times when weachieved the goal or challenge, there wasanother challenge immediately follow-ing. There wasn’t enough time for theprevious challenge to soak in and/or toreflect and think about how we achievedit. However, the info was superb!”

MULTI-ROBOT CURRICULUMEXTENSIONS

In the spring of 2011 Carnegie Mellonreceived a request from Robert Avanzato,an engineering faculty member at PennState Abington near Philadelphia.Avanzato was looking to join our researchstaff for the fall term as part of his sabbati-cal. CMU was pleased to accommodateAvanzato who has a long history of help-ing to organize and coordinate middle

school through college level robotics com-petitions at the Penn State Abington cam-pus. Avanzato believes, “These types ofevents are especially important in urbanareas such as the Philadelphia region topromote interest in computer science andSTEM.” Avanzato is leading the develop-ment of formalizing a set of laboratoryexercises that build on and provide scaf-folding for the initial lessons developed byCMU. He plans to integrate the laborato-ries and projects into his undergraduateintroductory robotics course at Penn StateAbington in the spring of 2012. Hebelieves that the combination of theROBOTC development platform and theinexpensive LEGO and VEX robots can beused to teach introductory concepts at alllevels. The technology offers accessibilityto middle and high school students, butalso offers enough sophistication to beuseful in a college level introductoryrobotics classroom.

In this next section we will describe aseries of multi-robot challenges thatAvanzato is collaboratively developingwith staff here at the Robotics Academy.All lessons will be posted for free access atour website. The lessons assume that theuser has a basic understanding ofROBOTC and are designed to developmulti-robot communications skills andtechniques starting from the beginnerslevel. When completed the hope is that thelessons are scaffolded in ways and includeenough information so that a motivatedstudent can teach themselves, or they canbe used by a practicing teacher for use intheir classroom. Each lesson includes: aproject description, the hardware and soft-

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E D U B O T Sware required, fully commented workingcode, teacher notes, the lesson setup, pic-tures and videos of working solutions,and multiple extension activities for stu-dents looking for harder challenges.

Mirror Robot Lab. In this lab, two robotsare placed in position with the samestarting point facing forward. A ball ispositioned on a black line at anunknown distance in front of bothrobots. The leader robot has a light sen-sor and the follower robot has only agripper (no sensors). The leader robotwill drive forward and detect the line

then transmit this distance to the follow-er robot. Using the data received fromthe leader robot, the follower robot willthen be able to drive up and collect theball using its gripper.

Measurement Lab. Two robots areequipped with a light sensor for line fol-lowing, encoders to measure distance,and a touch sensor to detect a bump.

The robots start at opposite ends of thepath and use line-following to navigatearound a closed path. The robots useencoders to measure the distance theyhave traveled. When the robots collide,they transmit their measured distances(exchange data) and calculate and dis-play the total perimeter of the path. Thismulti-robot project demonstrates howrobots can share a task and combineresults to improve efficiency and com-plete a task more quickly.

The Leader Follower Lab. This projectdemonstrates communication and coor-

dination between two NXT robotsalthough the lab can be accomplishedusing either VEX or the Arduino plat-form. The NXT leader robot uses a sonarsensor to navigate an obstacle course.While the robot is moving through themaze it is also storing data that it willtransmit to the follower robot. Once theleader robot finishes the course it sends amessage back to the NXT follower robot.

The follower robot rep-resents a specializedrobot such as a payload-carrying robot. The fol-lower robot is requiredto navigate the mazebased on the communi-cation from the leader,scout robot. This multi-robot project demon-strates how one robotcan share sensor datawith another robot toaccomplish navigation.

Proximity Lab. In anoth-er lab, three NXT robotsare each equipped withIR sensors and are facingan IR emitting ball. The

three robots are positioned at differentdistances relative to the ball and the chal-lenge is to have the robot team decidewhich robot is closest and have that robotclosest approach the ball. In this activity,

the team leader robot first queries eachrobot to wirelessly report its distancefrom the ball. The team leader robot thendetermines the minimum distance, andtransmits a command to have the appro-priate robot move towards the ball. Thislab demonstrates intelligent decisionmaking based on feedback.

MOVING FORWARDThis project has made significantprogress. Our goal was to develop aninexpensive system for education thatcan be used to teach basic multi-robotcommunications concepts. It is criticalthat we give students practice thinkingabout multi-agent communication in thenetworked world that they grow up in.To learn more about the project log intothe Computer Science Student Networkat www.cs2n.org where all of the lessonsare posted for free.

LinksCarnegie Mellon Robotics Academy,www.education.rec.ri.cmu.edu, (412) 681-7160

Computer Science Student Network,www.cs2n.org

For more information, please see our sourceguide on page 89.