Capivara Team description for LARC/CBR 2018

Alexandre Calerio de Oliveira, Andre Correa Harada, Anna Caroline Bozzi, Carlos Eduardo Magrin,Darci Luiz Tomasi Junior, Fabio Iareke, Felipe Gustavo Bombardelli, Henrique Augusto Richter,

Laura Silva Lopes, Matheus de Campos Horstmann, Matheus Orlando Fernandes Volanin

Abstract— This paper shows an overview of the Capivarateam preparation for the XVII Latin American RoboticsCompetition (LARC) and XVI Brazilian Robotics Competition(CBR) in 2018 on the IEEE Very Small Size Soccer (VSSS)league. With this, the team expects to contribute with futurenew players on the processes of making a robot team for thecompetition. The main idea is to explain the team’s choicesduring the assembling process, so some parts of the projectcan be replicated by others.

I. INTRODUCTION

This Team Description Paper (TDP) aims to describethe process of elaboration and construction of the Capivarateam. The team will compete on XVII Latin AmericanRobotics Competition (LARC) and XVI Brazilian RoboticsCompetition (CBR) for IEEE Very Small Size Soccer (VSSS)league. The team is not completed yet, the robots still needsome adjusts on mechanics and electronics and the visionpart is already being discussed, although the team has notfigured out some strategies, the intent is to use physics topredict the movement for the decision taken. This paper willpresent all the things that have been discussed until now,about mechanics and structure, electronics, vision and thepossible strategies.

II. MECHANICS AND STRUCTURE

The mechanics must be solid, balanced and resistant, sothe robots can run in the field and hit each other withoutbreaking or damaging the electronics.

A. Chassis

The robot chassis has length of 75x75x75mm and wasbuilt by a 3D printer, so for the maintenance it is easy to bedisassembled. The structure is shown on figures 1 and 3.

Compared to our last year robot, an interesting modifica-tion was the balance. As the robot only have two wheels, itkeeps dragging the back on the floor, generating instabilityand more friction, the idea is to use some kind of tinnyspherical wheels.

Fig. 1. Robot Chassis

Fig. 2. Prototype of Robot Chassis

B. Motors and Wheels

As robot is really small with maximum length of75x75x75mm, fit everything inside is a real challenge and thefirst thing not to fit was a encoder, which the team considersimportant. So to solve this, one of the wheels attached to

a motor was shifted parallel as shown on figure 3 For themotor, the team choose to use a Pololu Gearmotor medium-power with gears that gives 300rpm maximum speed at 6Vcc.The Pololu magnetic encoder used provides a 12 counts perrevolution of the motor shaft and the two wheels are madewith plastic and silicone tires as shown on figure 4

Fig. 3. Motors and Wheels

Fig. 4. Wheels

III. ELECTRONICS

The electronics embedded in a VSS robot is intendedto be very simple, so, the board design was conceived tocontain only the strict necessary components and modules.In the Figure 5 can be seen a prototype for the robot’selectronics. It consists in an base board with the voltageregulators, some auxiliary capacitors, resistors, a dual dipswitch, an on/off switch and connectors for the battery andthe motors. Attached to the main board lies the functionalmodules which are: Two optical barrier sensors, one ArduinoPro Mini, one H bridge board and one radio module. Theselast three modules are connected to the main board throughconnectors with the intention of to keep the circuit modularand to facilitate some eventual maintenance. The dip switchis intended to provide a radio individual identity to the robotinside it’s wireless network.

Fig. 5. Robot’s electronics prototype board

The decisions of choices for the main functional electroniccomponents, modules e batteries are discussed below.

A. Microcontroller

The chosen microcontroller is an ATMega328P [1] embed-ded in Arduino Pro Mini board. This board has only 33mm x17mm and contains all the main external components neededfor the microcontroller, besides, provides 20 I/O ports plusa TTL UART interface as can be seen in Figure 6. Also,the ATmega has embedded a SPI hardware wich is used tocommunicate to the radio’s module.

Fig. 6. Arduino Pro Mini board

Once the board don’t have an USB interface, an USB toUART adapter based in CP2102 [2] integrated circuit is usedin order to transfer the software to microcontroller’s internalflash memory via Arduino IDE.

B. Communication

Based in the analysis of the experience of the participantteams in previous editions [3] came up two main candi-dates to the communication modules: XBee and nRF24L01.Although both work in ISM Band [9] XBee is limited tothe frequency range 2.4 - 2.4835 Ghz [10] [8] improvingthe chances of get interference from IEEE 802.11 networks[7] as WiFi and Bluetooth devices, very common in many

ambients, while the nRF24L01 [4] can work in frequenciesup to 2.525Ghz wich can be useful to avoid such kind ofRF interference. Moreover a XBee module is far expensive,requires more physical space in the circuit board and morepower consumption as compared to the nRF24L01 module.In face of that aspects, the nRF24L01 module, seen in Figure7, was the first choice.

Fig. 7. nRF24L01+ radio module board

In order to validate the nRF24L01 radio module, was builtthree radio prototypes to simulate the robot’s team and onemore attached to the PC’s USB port to act as a controller. Inthe Figure 8 can bee seen the controller module prototype atthe left and one of the three robot’s radio testing prototypeat the right.

Fig. 8. Controller and robot radio testing prototypes

To perform radio’s tests, the prototypes were configuredin a star topology network which was able to exchange upto one thousand messages per second reliably. In face of alltechnical aspects and the network tests results, the nRF24L01module was definitely added to the robot’s circuit project.

C. H BridgeThe H bridge is based in the TB6612FNG [5] integrated

circuit which is suitable to control up to two DC motors ata constant current of 1.2A (3.2A peak). The two motors (Aand B) can be separately controlled, the speed of each motoris controlled via a PWM input signal with a frequency up to100kHz. Also, this integrated circuit is easy to find embeddedin a small 21mm x 17mm board as suitable to incorporatepin’s connectors, as can be seen in Figure 9. That improvesmodularity and keeps the maintenance easy to perform.

Fig. 9. H Bridge board based in the IC TB6612FNG

D. Sensors

The decision to incorporate sensors to the robot’s circuittook account the analysis of the experience of the participantteams in previous editions [3] as was done with the radiomodule. Seven teams had reported to use a variety of sensorsas gyroscope, magnetometer and accelerometer but noneof them have reported some improvement of performancerelated to those sensors. In team’s discussion about thesensors the only useful candidate that came to light wasthe encoder. As integrant part of the PID controller, theencoder can bring to the project real benefits as it helpsin the reliability of the system responsible to the robot’sspeed and movement control. In face of that, was decidedthat the encoders would be the only sensors to be present inthe robot’s electronic circuit.Due robot’s physical space constraints, the choice of use mo-tors on the same axis and encoders attached to these motor’saxis was discarded in order to keep the mechanical projectas simple as possible. As alternative, one of the motorswas shifted parallel without to compromise the maximumdimension allowed to the robot’s category.

E. Batteries

The discussion about the adequate battery technologyto be used in the robot considered some main aspects ascharge supply lasting, weight, robot’s inner space constraints,voltage and current needed for the circuit board and motors.All these aspects leaded to the Li-Po technology which hasa high density charge and nowadays is available in manyshapes and sizes for a reasonable price. The robot’s batteriesconfiguration was defined to use two single cells performinga 7.4V nominal voltage pack and can be able to supply upto 720mAh. A single Li-Po cell can be seen in Figure 10.

Fig. 10. Single cell Li-Po robot’s battery

IV. GAME FIELD

The Figure 11 shows the field constructed by the team.The field is constructed using a 1,34m for 1.54m of MDFwood and painted with PVC ink. As part of the structure,four aluminum brackets are used to hold the camera at 2meters from floor. The field is not completed yet, missing toadd the sides responsible to not let the robots to leave thefield and the camera is not defined yet.

Fig. 11. Game Field

V. VISION

To recognize the robots and the ball in the game field, it isused the OpenCV library[6] in C++ programming language.The OpenCV is used to read each frame of the camera. Therobot’s uniform is composed with two colors: yellow or blueto identify the team and red, as shows the figure 12. Therobots are distinguished by the size of the red box. The colorred is used to identify the ball and the direction of the robottoo. For each color, it is created a vector of clusters in whichcluster represents an element of that color in the game field.To calculate this clusters, the program go through the frameof the camera and for each pixel, calculates it’s chromaticity

in each RGB channel. This values are used to verify if a pixelis inside of a pre-determined interval in each RGB channel.

Fig. 12. Team Uniform

When a pixel is inside the interval, it is accounted for acluster. Clusters too large or too small are discarded. Thenext step is to use the euclidean distance to calculates thedistances of clusters yellow and blue from the red clusters.The closer clusters are assumed to form the uniform of thesame robot. With the clusters it is possible to determine theposition of the robots, allies and enemies, and the ball.

The figure 13 shows the vision algorithm recognizing theball and robots through the uniforms. The figure shows eachcluster founded and how this information is used to identifythe robots and ball.

Fig. 13. Vision algorithm recognizing the robots and the ball

VI. STRATEGIES

The team haven’t got to strategies yet, but the idea is touse physics to predict the ball movement and decide whichaction each robot will take, trying to intercept the ball in thebest position.

ACKNOWLEDGMENT

The team would like to thanks all the students involved inthe project, for their dedication and helping with all hardwareand software development. We also would like to thanks theprofessor, who gave us the opportunity of making part of

this project and, all sponsors who believed and contributedto the success of the project, in particular Darci Luiz TomasiJunior from DN Tomasi Ltda.

REFERENCES

[1] Atmel. Datasheet MCU ATmega328.http://www.atmel.com/Images/Atmel-42735-8-bit-AVR-Microcontroller-ATmega328-328P_datasheet.pdf. Online; accessed in June 2017.

[2] Silicon Labs. Datasheet CI CP2102. https://www.silabs.com/Support%20Documents/TechnicalDocs/CP2102-9.pdf. Online; accessed in June 2017.

[3] Laura Praca Lacerda, Alexandre Calerio de Oliveira, and EduardoTodt. Survey on ieee very small league robots (submited). 14rd LatinAmerican Robotics Symposium, 2017.

[4] Nordic Semiconductors. Datasheet CI nRF24L01. http://www.nordicsemi.com/eng/Products/2.4GHz-RF/nRF24L01.Online; accessed in June 2017.

[5] Toshiba Semiconductors. Datasheet CI TB6612FNG. https://toshiba.semicon-storage.com/info/docget.jsp?did=10660&prodName=TB6612FNG. Online; accessed in June2017.

[6] OpenCV team. OpenCV. http://opencv.org/. Online; accessedin June 2017.

[7] Wikipedia. IEEE 802.11 standards. https://en.wikipedia.org/wiki/IEEE_802.11. Online; accessed in June 2017.

[8] Wikipedia. IEEE 802.15.4 standards. https://en.wikipedia.org/wiki/IEEE_802.15.4. Online; accessed in June 2017.

[9] Wikipedia. ISM Band. https://en.wikipedia.org/wiki/ISM_band. Online; accessed in June 2017.

[10] Wikipedia. ZigBee. https://pt.wikipedia.org/wiki/ZigBee. Online; accessed in June 2017.