Control Algorithm for a Biped Robot Based onServo-Motors Controlled by an Android Application

Chavez Ariel, Fernandez Andrea, Machado Luis, Revelo Jefferson

July 10th, 2015

Abstract

This paper presents the con-trol of a bipedal robotic platform.The design presented is capableof forward movement, the robot isable to walk repetitively, also de-tects and dodges obstacles. Alsopresents a control algorithm thatactivate the kick secuence. Therobot can be controlled in twoways: manually or automaticallywith an Android application. Toobtain these movements has beenimplemented a basic control sys-tem, determined by a generatorof movement patterns. The mainobjective to be pursued with thedesign of this robot is to get a ro-bust electronic platform.

1. INTRODUCTION

In recent years it has been noted howrobotics has begun to cease to belong ex-clusively to the industrial world, push outinto the daily life of people. The roboticstarts to open a large number of possibil-ities, such as virtual pets, micro-cleaningrobots, etc.; limited only by the human ca-pacity to carry them out. One possibility,which for decades man has imagined, butcould not carry out until a few years ago,has been performing a robot with move-ments like a human, with the same or verysimilar motion characteristics. Everyone

knows the enormous complexity of design-ing a robotic system, as may become a robotarm, which has 6 DOF. In the case of a hu-manoid robots that fact go even further, asthere are systems with more than 30 DOF,which gives the system a high mobility butthis require a high computational load inthe control system. This implies a high costin most bipedal robotic systems designeduntil these days.

In general, a bipedal locomotion systemconsists of several members that are inter-connected with actuated joints. In essence,a man walking robot is nothing more than arobotic manipulator with a detachable andmoving base. The design of bipedal robotshas been largely influenced by the most so-phisticated and versatile biped known toman, the man himself. Therefore, mostof the models/machines developed bear astrong resemblance to the human body. Al-most any model or machine can be charac-terized as having two lower limbs that areconnected through a central member.

Although the complexity of the systemdepends on the number of degrees of free-dom, the existence of feet structures,upperlimbs, etc., it is widely known that evenextremely simple unactuated systems cangenerate ambulatory motion.

Thus, this paper presents a design of

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the lower extremities of a biped robot witha robotic system of low cost and high inter-connection capacity.

2. BIPED ROBOT DESIGN

Bipedal robotics platform used wascalled by its manufacturer as ”BRAT”meaning Bipedal Robotic ArticulatingTransport, it has six degrees of frees (6DOF); two ankles: right and left, two onhis knees: right and left and two on itships: right and left.

Fig.1: Degrees of freedom of the biped robot

This bipedal platform has a basic sim-ilarity to the human structure, it is con-structed of anodized aluminum. Thebipedal platform has the ability to makeseveral moves: move forward, turn left orright as basic moves, being able to performseveral routines movements. It presents agreat stability because the robot supportsurface has a large area, compared to thelength of his legs. This robot can detectobstacles using an ultrasonic sensor.

2.1. Degrees of Freedom(DOF)

The degrees of freedom that owns theplatform are generated by six servomotors(HS-422) to move each of the joints: ankle,knee, hip, both left and right.

The robot has the ability to mobilizetheir ankles, knees and hips, right and left,controlling the motors of each ankle.

Fig.2: Ankle’s move

The robot moves its knees sideways, andhave big feets to give better stability whileit makes the walking routines.

Fig.3: Knee’s move

Fig.4: Hip’s move

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Materials and devices used will be shownin the next chapter.

3. HARDWARE

Bipedal platform called BRAT has twooperating modes: manual mode, where therobot is controlled by an Android appli-cation, and automatic mode, in which therobot is not led by any user.

ATMEGA8 microprocessor is used asthe robot’s brain. This microprocessor han-dles all the peripherals that owns the plat-form such as servo motors, ultrasonic sensorand also perform Bluetooth communicationwith an Android device.

Fig.5: ATMEGA8

ATtiny 13A microcontroller was chosenfor the task of sensing the distance with theultrasonic sensor, and to uncouple the mo-tion control program and the obstacle de-tection program.

Fig.6: ATtiny 13A

For articular movements six actuatorsare used, which allow different movementroutines.

Fig.7: Servo-motor HITEC HS-244

To detect objects at a distance under 20cm, the robot uses an ultrasonic sensor.

Fig.8: Ultrasonic sensor

To communicate the bipedal platformwith the Android application a Bluetoothmodule was used.

Fig.9: HC-05 Bluetooth

4. DEVELOPMENT OF CONTROLPROGRAM

Both, the principal microcon-troller(Atmega8), that was used for controlthe biped; and microcontroller ATtiny, usedfor control the ultrasonic sensor; were pro-grammed in ATMEL STUDIO 6.2, while

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the application for wireless communicationwas developed in App Inventor.

The schematic diagram of the controlcircuit shown in the picture below, showsthe connection of motors, ultrasonic sensorand bluetooth.

Fig.10: Control circuit diagram

The control program is performed with4 subroutines contained in tables. There isa table to walk, another to turn right, oneto turn left and the last one allows kicking.

When ultrasonic sensor detects an obsta-cle at a distance of 20 cm, sends a 1L signalto the main microcontroller, this makes therobot interrupted its walk and turn.

The interface developed for robot con-trol allows to select two modes of operation,manual and automatic; in manual mode hasoptions like walking, turn right, turn leftand kick. The microcontroller receives acommand to select any of these options andreacts to the order received.

Android application is shown in figurebelow

Fig.11: Android application

To understand the control program,main loop program flow chart are shown be-low. This show the different funtions thathave the biped robot, like serial comunica-tion, detection of obstacles, motors controland the main program

Fig.12: Main progam loop

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First , we created a table of sequences thatallow the robot to perform different actionsafter that we configure the timers, the US-ART and interruptions . We check if theuser wishes to work in manual or automaticmode , and depending on this, the ordersare executed.

Fig.13: Timer interrupt routine

The timer interrupt routine has a counterwhich is incresed and the pulse width of eachPWM is verified, when the pulse width isgreater than the pulse width set the pin isput in 0L.

Fig.14: Obstacle detection routine

Fig.15: Serial comunication routine

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Fig.16: Ultrasonic sensor flowchart

5. RESULTS

5.1. Biped walking

Fig.17: Biped walking

5.2. Biped turning

Fig.18: Biped turning

5.3. Biped kicking

Fig.19: Biped kicking.

6. CONCLUSIONS

The robot varies his rotation time de-pending of the surface on which the robotis walking, in rough surfaces rotates fasterthan on smooth surfaces , in the last onesthe robot starts to skid , for this reason ittakes longer to turn. Likewise, the walkingis much better on a roughened surface thanin a smooth surface .

It was possible to get the target set todevelop an automatic mode with motiondetection and obstacle avoidance.

An influential factor was the develop-ment of the motion sequences including theweight of the batteries, as these cause thecenter of mass of the robot varies differently.

The power consumption of the actuatorsis considerable, so it was decided to use anindependent source for the microcontroller,in order to avoid brownouts that reset thecontrol system.

BIOGRAPHY

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Chavez Ariel:He was born in

Banos - Ecuador. He

completed his sec-

ondary education at

the San Alfonso high

school. He studies

at the National Poly-

technic School career

in Electronics and

Control Engineering.

He qualified Adequacy CEC. Areas of interest:

robotics, computer science, micro controllers,

industrial automation and control.

FernandezAndrea: she was

born in Tabacundo -

Ecuador. She com-

pleted her secondary

education at the In-

stituto Tecnologico

Superior Nelson Tor-

res. She studies at

the National Poly-

technic School ca-

reer in Electronics

and Control Engineering. She qualified Ade-

quacy CEC. Areas of interest: computer sci-

ence, microcontrollers, industrial automation

and control.

MachadoLuis: He was

born in Guaranda

- Ecuador . He

completed his sec-

ondary education at

the Colegio Cente-

nario Nacional Pedro

Carbo. He qualified

Adequacy CEC. He

studies Electronic

and Control engi-

neering at EPN. Ar-

eas of interest: In-

dustries systems, domotic, robotics and micro-

controlled sistems

Revelo Jeffer-son: He was born

in Ibarra - Ecuador.

He completed his

secondary education

at Technic San Jose

hihg school.He stud-

ies at EPN Elec-

tronic and Control

engineering. Areas

of interest: Indus-

tries systems and se-

curity systems.

References

[1] Atmega8 Datasheet

[2] Herrera Marco,’Ensamblaje y Controlde una plataforma Bıpeda mediante unPC’,Quito, 2009

[3] Candelas Francisco,’Servomotores’,Universidad de Alicante, 2007

[4] Seungmoon Song,Joohyung Kim, andKatsu Yamane,’Development of aBipedal Robot that Walks Like anAnimation Character’

[5] Announced Specification of HS-422Standard Deluxe Servo

[6] Announced Specification of HS-311Standard Deluxe Servo

[7] Vaidyanathan.V.T and Sivaramakr-ishnan.R, ’Design, Fabricationand Analysis of Bipedal WalkingRobot’,Department of ProductionTechnology, Madras Institute ofTechnology, Anna University, INDIA

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