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Project 2-RobCom: A Combat Robot

Introduction

If you have dreamed of creating a combat robot that

can destroy or knock out the enemies, rivals, space

in vaders, or oth er supernatural crea tures, you must

take a look at this project. RobCol11 is the answer for

your aspirat ions-your dream coming into rca lity.

Using inexpensive anel C0111111 011 parts, even inexperi­

enced readers who may not yet be prepared to build

complex circuits wi th microprocessors can build the

RobCom.

The evil genius will be ab le to bui ld the RobCom

and challenge his or her friends to a real combat of robots. You can also invite your colleagues to bui ld

combat robots. Ask them to put into the project all

thei r imagination. to create weapons. and to build

defenses so they won 't be knocked out in the first

encounter.

RobCom was one of the proj ects proposed by the

author to his pupils of the mechalronics course at

Colegio M ater Amabilis in Guarulhos, Brazi l. Figure

3.2.1 shows some of the projects made by the stu­

dents. You can see how these evil geniuses con­

structed their destruction machines.

What Is RobCom?

RobCom is a remote-controlled robot buill with

common part s. In the basic version, it is bui lt to calTY

a rubber ba lloon th at it is protecting and three nee­

dles as weapons. The reader can add other weapons,

of course, depending on the rules o f combat agreed

upon by the other competi tors.

To make the project easier, the remote control

uses a cable. Some advantages ex isllo using a cable

in place of other remote conlrolmeans,such as

infra red ( IR) or radio/reqllency (RF). Beyond the

simplicity of the ca ble method (no spccial circui ts are

requ ired), the problems of interferences and noise,

common in the places where the combat takes place •

don't exist.

RobCom has two small DC motors di rectly driv­

ing two rear wheels, which arc made of standard CD s.

The single front wheel is able to turn freely in any

directiol1. 1l1e recommended fron t wheel is one that

can be found in old office chairs or other furniture.

Figure 3.2.2 shows the wheel used in the robol.

Figure 3.2.2 From whee/used in fil e RobCol1l.

The control unit , which is placed at the end of a

cable, is simply a small box with two special switches

Figure 3.2.1 Close-lip sho lving (1 1'0 robo(s in combat.

,. and a joysLick. 1l1c switches can control two circuits (two poles) at the same lime, and each switch has three positions. When the j oystick is in the central

posi tion. the con trolled ci rcuit is disa bled; no power is

releascd to the motors. Certain positions of the

switches con trol certain fu nctions of the 1110 tor, as

shown here:

22 Mechatronics for the Evil Genius

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Position Motor

FOlward Molor fllns forward

Released r ... IOIor off

Backward Molor runs backward

Combining the three posit ions of the switches,

the robot can run in any direction. as the following

table shows.l11e arrows indica te the direction of

movement.

Robot Switch R Switch B Symbol Movement

Released Released Stalled

Pressed Pressed t R un fo rward in forward forwa rd st raight linc

Pressed Pressed

• Ru n backward in

backward backward stra ighlline

Pressed r-fo rward Released Turn right forward

Pressed L. backward Re leased Turn right backward

Pressed ~ Released forward Turn left forward

Pressed .J Released backward Turn left backward

RobCol11 is powered by four AA cells placed in

the contro lun it.lllis placement reduces the weight of

the mobile unit, increases the mobi lity, and is very

important in combat.

Objectives-The Combat

The basic idea of robot combat is to put two Rob­

Coms in an arena formed by four pieces of wood , as shown in Figure 3.2.3, and let them try to pop the

other's ba lloon using needles as weapons.

Cont rolling the forward and backward movement

of the robot, the player can find the best position for

an attack without exposing his or her own balloon to

the attack of the enemy. Strategies must be created

by the RobCom evil geni us to wi n the combat.

The winner is the fi rst robot to pop the enemy's

balloon.llle balloons ca n have a tiny amount of flour

or talc injected in them, adding a realistic effect for

Section Three

Figure 3.2.3 The arena IIsed in the combat is a sqllare made by fOllr pieces of Ivood.

the explosion. A cloud of smoke wi ll then announce

that the balloon is popped and the combat is over.

A normal combat session usuall y lasts from 1 to 5

minutes (see Figure 3.2.3). [n an organized competi­

tion, a simple elimination process can be used.

Rules are very important in order to avoid major

differences among the competitors and to level the

playing fi eld. Rules include specifications for maxi­

mum d imensions of the robots, the size of the rubber

ba lloon, the use of defense screens, the length of the needles, thc typcs of motors used, the power supply

voltage, and so on.

Rules for the actual combat are necessary, too. At the end of this project is a list of rules that we have

found are usefu l in our combat.

Figure 3.2.LJ Scene of combat IVhere IIVO RobComs (es( (heir pOIve/:

Th e Pro j ects

Winner

2 3

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() o & P.I rt

~ o tr o rt

u

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I

The Proj ect

The RobCom is a robot that runs on three wheels.

The front two are used to turn the robot in the

desired direction, and the third is a free wheel.

Two small DC motors are couplcd directly to the wheels, which are made with CDs or some other

material of the reader's choice.

The motors are controlled by a remotc cont rol,

wired to the robot by a 3-meter cable. As indicated

previously, the use of a cable makes the project easy

to bui ld and very inexpensive, as no special materials

are needed. TIle remote control also houses the cells that power the motors. Two switches allow the robot

to move backward and forward and to change direc­

lion.

The chassis can be made using common materials

such as cardboard, CD boxes, plastic, wood, and so

on. It is up to the reader to use his or her imagination

to create his or her own version while fo llowing the

rules of the competi tion.

When creating your RobCol11, it is important to

reduce the weight, making it as fast and well balanced

as possible .

Building the RobCom

TIle basic RobCom is the one described in the intro­

duction. Of course, the evil genius can change the

RobCom by adding new weapons or defenses

according to the combat rules.

The Electric Circuit

Begin your project by building the electric circuit. Thc simple schematic diagram for the RobCom is

shown in Figure 3.2.5.

As we can see in the fi gure, the single power sup­

ply is formed by four AA cell s. TIlis supply powers

two small , 6-vol1 DC motors via SI and S2. SI and S2

are a special arra ngement o[ two switches with three

positions each. In the middle position, the switches

are off.

61 + 6V -

S1

Figure 3 .2 .5 Schemmic diagram fo r the electric circuit of the RobCom.

S1 and S2 determine the direction of the motors.

The switches send the power to the motors by a spe­cial four-wire cable. TIle recommended length of the

4 X 26 Americon wire gauge (AWG) cable is 3

meters.

The switches are pl aced in a small box (plast ic or

other material) form ing the joystick. TIle different

colors of the cable wires are important and will help

the reader know where each must be soldered. Figure 3.2.6 shows the electric circuit assembled.

Figure 3.2.6 The electric circllit ready to be installed in rhe RobCom.

Parts List-The Electric Circuit

S1, S2 2 poles x 3 through switches (see text)

B1 6 volls of power (4 AA cells with hOlder)

MI , M2 6-voll DC motors

24 Me c hat r onics for the Evil Geniu s

3 meters o f four-wire cable (4 x 26 AWG)

Plastic box

Solder

The Mechanical Part

Figure 3.2.7 shows the basic moun ting of the robot ,

deta iling how the motor is coupled to the wheel. Fig­

ure 3.2.8 shows a rea r view of the robot.

Cable CD Cardboard ·V·

Figure 3.2.7 Side vie.v oflhe RobCom shGlving hall' Ihe 1I1010r is COliI' led 10 the CD.

Rubber Band

Glue / CDBOX

~~lt==;±:i, ~TubeOf Spherograph Pen

End 01 Axis Protection

CD Box

_ CD

Figure 3.2.8 Th e molars are gilled 10 the CD box. They are kepI ill contact lVith Ihe CD by Ihe force of a rubber balld. This makes it possible 10 trallsmil all the pO lva (0 the robol,

Putting the Pieces Together

The next figures show the seq uence of operat ions to

mou nt the robot. Figure 3.2.9 shows the free wheel

fixed to the CD box used as a chassis and how to

Section Three

Figure 3.2 .9 InserriJlg a metal sheet between/he CD boxes before gluing thell1 logelher.

insert a metal sheet between them to keep the struc­

ture rigid.

Figure 3.2.10 shows how to add an isolation sheet

to the CDs to increase the adherence. Rubber bands

glued to the CD ca n a lso be used to accomplish this. TIle reader is free to create the best way to increase

the adherence, making the robot faster and agile.

CD

.-- Insulation Tape

or Friction Tape

Figure 3.2 .10 Placing all isolalion sllee/to the CD to increase Ille adherence.

The plast ic wheel, taken from a toy, is glued to the

CD. Small plastic cars and other toys are good

sources for this wheel. I prefer the type with metallic

ax les. Figure 3.2.11 shows how to glue the wheel.

Pieces of ca rdboard can be used to make supports

for the wheels. The spi ndles of the wheels are

inserted into the drinking straws. At the end of the

spindle, a small piece of a plastic cover connects the spindle to the cardboard support , as shown in Figure

3.2.12. This cover can be a small piece of the tube

from a ballpoint pen or even the casing from an elec­

tric wire.

111e motors are gl ued to the box. 8 e sure that the

motors' sha ft s will be al igned with the wheels (the

The Projects 25

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Figure 3.2.11 Gluing the wheel to the CD.

cr:::> - -

Drinking Straw

t Glove Wheel

Figure 3.2.12 The Ivheel is coll llected to a cardboard support, which is glued 10 the chassis.

Motor

Rubber Band

\ 0 Hole

Figure 3.2.13 Motors in place.

co

CDs). Figure 3.2.13 shows the motors glued and con-

~ tacting the CDs. The motors are forced to maintain

r contact with the CDs by a rubber band holding them

Q) in place.

'r To increase the transmission power of the motor to the CD, the spindle is covered with a small glove.

As mentioned previously, the glove can be made

llsing a piece of plastic tube taken (rom a ballpoint pen or even a plastic cover from an electric wire.

T11e arms of the robot are made wi th needles, placed in a piece of ca rdboard as shown in Figure

3.2.14. Figure 3.2.15 shows the RobCom ready for

combat.

Finally, you can attach the rubber balloon to the

robot using a rubber band.

~..-__ Rubber Balloon

~>L--r;:.-._ Cardboard

Figure 3.2.14 Th e a/'lliS are placed ill the frolll of the RobCom. This piece of cardboard also serves as ({ screen, protecting againstlhe atfacks of lit e enemies.

Figure 3.2.15 RobCom ready for combat.

Testing the Combat Robot

Insert the batteries in the cell holder. Pressing the

switches in the control unit , the motors should be

26 Mechat r onics for the Ev il Genius

•

t

acti vated. If not, check the solder and the cable. If

one or both motors run in the opposite directions

(e.g., forwa rd when you press backward) , invert the wire of the motor.

With the robot o n the ground, test to see if the

robot will move freely in a ll directions when you

press the controls. If the motors have difficul ties in

moving the robol, verify that they arc pressing

against the CDs with the necessa ry fo rce. If all the

movements are sa tisfactory, your RobCom is ready

and the competit ion. The robot characteristics should be as follows:

• The length of the robot must be be tween 15 and 25 centimeters.

• The maximum number of needles used as arms sho uld be three.

• TIle maximum length of the need les should be 20 centimeters (includ ing the support ).

• -n,e screen in front of the robot should be lim-for combat. it ed to 10 X 15 centi meters.

The Combat

The reader is certain ly free to create his or her own

ru les for competit ion. H owever, we call offer some

suggestions based 0 11 the experience of many combat

sessions organized in the school where the author teaches. Figure 3.2.16 shows several RobComs in an

arena, wait ing for the beginning of the contest.

Combat Rules and Specifications for the Robots

To avoid major differences among the robots, it is

important to establish some rules regarding the robot

Figure 3 .2 .16 Corr/bat of RobCollls at Co /egio Mtller AlI1abilis in Gaara/has, Brazil .

Section Three

• All the robots should use the sa me type of motor.

• The power supply must be fo ur AA ce lls fo r a ll robots.

• The rubber balloons must be equal in size.

• 0 other weapon is allowed (or otherwise combi ned) .

• The arena is formed by four pieces of wood with the d imensions between 3 X 3 and 4 X 4 meters.

Combat Rules

• T he competi tors cannot enter the arena.

• The competi to rs ca nnot pu ll the robots by the remote-control cable.

• T he competitors must begin at opposite cor­ners of the arena.

• The combat begins when the referee gives the order.

• TIle combat ends when the balloon of one competitor is popped.

• If the two ba lloons are exploded at the same time, the robots must compete against each

other in a second round to determine the wi nner.

The Projects 27

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Exploring the Project

Many changes can be made to the original proj ect specificat ions. 1l1c fol lowing are simple examples of

the changes that can be made:

• Wood rods can be used to substitute fo r thc plastic chassis.

• A plastic chassis with different fo rmats call be used.

'Tnc same project ca n also be lIsed with vehicles other than combat robots. ~1l1 e reader can mount his

or her own walking robot lIsing the control system described here. Figure 3.2. 17 shows such a robot.

t Cells and Switches

o

Gear BOl(

• • Figure 3.2 .17 A common robollfsillg the remote control described in this project.

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Cross Themes

TIle control of movement is a theme that translates

easily in to the physics curriculum. Teachers who want

to explore this as a cross theme can invite the pupils

to research the next items:

•

•

•

Analysis of movcmcnt : Describe the type of movement the robot makes during combat and how fast changes in directi on or speed can alter the equilibrium.

Friction: Analyze the effect of fr icti on in the robot mObility.

Transmission: Find the best way 10 transmit the power from the 1110tor to the wheels. Think about the use o f gea rs.

Additional Circuits and Ideas

l1le basic circuillised in the project is very simple. It lIses no electronic pieces and no complex clements.

TIle reader who is experienced in electronics can upgrade the circuit and create some interesting proj­

ects.

Using a Joystick

Figure 3.2.18 shows how a common joystick, such as

the ones found in video games or pes, can be used to

control the two motors in the RobCom.

TIle circuit' uses four relays to controllhe two

motors. ll1e fou r switches o f the joystick arc used to power the motors on and off or to reverse the current

across them.ll1e position of the joystick affects what

happens to the motor:

Joystick Position

Center

Up

Down

Righi

Left

Upper ri ght

Upper left

Lower right

Lower left

Left Motor

Locked

Forward

l3ackward

Forward

Backward

Forward

Locked

Locked

Backw<lrd

Parts List- Using a Joystick

Right Motor

Locked

Forward

l3ackward

l3ackward

Forward

Locked

Forward

Backward

Locked

01 to 0 8 1 N914 or equivalent si licon diodes

Kl to K4 6 V to 12 X 50 mA reversible relays (select voltage according the motor)

28 Mechatronics for the Evil Genius

D3

K1 D1

Ri9ht~ K2 D2

D5

Down

Left Motor

o

D4 ....

~fO, -~

D6 ,-

1 ,~

o

D7

D8

+ 6/12V

I~ -K3 t

K4 I ~ -t ~

Right Motor

-- -- - -Figure 3.2.18 Controlling the combnt robot Ivith a game joystick.

Adding a Pulse Width Modulation (PWM) Control

A PWM control for the motor wi ll allow the reader

to change the speed of the RobCom. The PWM con­trol is described in detail in Project 3.3.

Figure 3.2.19 shows how to add a I'WM block to

the RobCom. Notice that only one I'WM control is necessary to change the speed of both motors. Or if

the reader wants, he or she can use one PWM control

for each motor.

81

+ 81

Figure 3.2.19 A PWM control is added to the RobCol11 10 control the speed.

Adding Weapons

The reader can put his or her imagin ation to work creating new weapons for the RobCom. Of course, it

is important to be sure the weapons meet combat

rules.

Figure 3.2.20 shows how a small DC motor can be

used to add movement to the needle, making it much

more dangerous as a tool aga inst the enem y_

Another idea is to couple a rotary ball wi th nee­

dles, as shown in Figure 3.2.21. In this case, the reader

must take care that the ball does not pop his or her own ba lloon.

Drinking Straw Solid Wire

t I ~r:::::) /~\ ~·<ltit=Tt ~ - _ _ Needle

Motor

I( •

Figure 3.2.20 The mOlOr coupled 10 the needle can be activated by diodes when the RobCom runs fo rward.

3~------------------------------------------------______________________________________ _ Section Three The Pro jects 29

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ro 0 rt

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.... --- Molor

------.~ Figure 3.2.21 A rOlary weapon fo r Ihe combal robol.

Rdding a Death Circuit

An interesti ng improvement fo r the project is a dealh

circuit. This circuit is formed with two reed switches

and a magne t, placed as shown in Figure 3.2.22.

As we call see, the current flows across the motors

and passes through the reed switches. 111e magnet is

attached with a ru bber band inside the balloon. If the

o balloon is full , the magnet touches the reed switches,

U and the motors are powered.

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If the balloon pops, the magne t fa lls and the reed

switches open. TIle motors are no longer powered,

and the robot stall s.

SWl

SW2

Magnet

Saloon

Figure 3.2.22 Placing the magnet in the balloon.

Rdding Sound

Figure 3.2.23 shows a simple sound-effect circui t for

the RobCom .

If a 47 oF capacitor is used, the circuit generates

sounds like a siren produces when the motors are

activated. If a 10 f.LF capacitor is lIsed, the circuit gen­

erales pulses imitating a machine gun.

From Switches

C2 47 nF

113 To

"",---<~_+-_ Ml or 214 M2

SPKR 4/Sn

4X IN4148

Figure 3.2.23 SOllnd-effect cirCllit for RobCom.

Using an H-Bridge

111e digital control o f the RobCom can be imple­

mented by usi ng an H-bridge. Th is idea is based on

the fact that fOllr transistors can be used to control

the current across a motor in the sa me way a double­

pIIII/dollble-thrOlv (DPDT) switch cont rols the cur­

rent. 111e circu it proposed in Figure 3.2.24 is a full

bridge or H-bridge using fou r D a rli ngton transistors.

Using an H -bridge for the control has two advan­

tages. First, the curre nt across the cable is reduced.

Second , logic signals can be used in the control. In

th is case, even a computer can be used to control the

RobCom.

'1l1e circui t works as follows: When the forwa rd

(FWR) input is high, Ql a nd Q 4 are on and the cur­

rent flows across the motor in the direction indicated

by a rrow 1. When the rewind (REW) input is high,

Q2 and Q3 are on and the current flows as indicated by arrow 2.

Notice that Ql and Q3 can 't be turned on at the

same ti me, as can Q2 a nd Q4, because that would

30 Mechatronics for the Evil Genius

o + V

FW

J o-A ......--'IMr+-l:

B

J RW

Figure 3.2 .2lj The circuir for an f-1- bridge.

mean a short circuit [or the current be l ween + 12

volts and ground . It is a fo rbidden state that can

cause the transistor to burn. The circuit can be con­

tro lled in the fo ll owing ways:

+v

Rt A 4

10kfi

Input R Input B Motor

Low Low Stalled

High Low Run forward

Low H igh Run backward

High High Forbidden

Figure 3.2.25 shows how to add a fifth transistor

a nd a logic system to avoid this forbidden state.

Parts List-Using an H-Bridge

Qt TI P122

Q3

lCl 4011 -4 NAND gates (CMOS inte­grated circuits [ICs])

Q l to Q5 TIP122 negative-positive-negative (N PN) Darlington tra nsisto rs

R l to R5 10 kfl X 1/8-watt resistor (brown, black, orange)

M D C motor (up to 500 m Al

+v o

M

Q2 TIP122

Q4 TIP122

R3 10 kfi

R4 10kn

TIP122

R3 10 kG

o o-----JtN~-_+K QS

Figure 3.2.25 I-I-bridge wilh logic.

Section Three The Projects 31

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:;tJ o 0-o rt

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The bridge works in the following manner:

Low

x (don' t care)

High

H igh

Low

Run forward

Ru n backward

Stalled

This same bridge can be implemented using the

same bipolar transistors used in the BD13S (500 rnA) or TI P31 (2 A) , as shown in Figure 3.2.26.

Rl lkn

A o---+J\N\r-+K

R2 lkn

B e>-<t-+J\N\r-+K

Figure 3.2.26 H-bridge using com l11OI7 bipolar transistors.

Using Gearboxes

The mechanics of the RobCom can also be improved

with the use of gea rboxes. As shown in Figure 3.2.27,

Common Plastic

Gear Boxes Wheel

I

~\ ~ Motors

Figure 3.2.27 Gearboxes can be used to pOlVer the robot. The photo sholVs a gearbox suitable for th is project.

the RobCom can be powered by small gearboxes, increasing the effic iency and a llowing the robot to be

built in a more compact manner. The gearbox can be

used to power the CD wheels or plastic wheels.

Remote Control

Starting with the ideas in this project, the reader who knows a lot about electronics call easily install a wire­

less remote control in the combat robot. Small trans­

mitterlreceiver modules, such as the ones shown in

Figure 3.2.28, are idea l for this task .

When using wireless remote controls, care should

be taken by the builders to choose different freq uen­cies for the systems. If two robots use the sa me fre­

quency, interference problems will put the combat in

jeopardy.

Figure 3.2.28 Common hybrid modll ies fo r transmitters and receivers .

Technology Today

Combat robots are rea l these days. Mili tary robots ca n be used to fight, to carry resources, on rescue mis­

sions, or to go where it is too dangerous for a human

soldier.

32 Mechatronics for the Evil Genius