We decided to go for a tank like drive system where two wheels are mounted on either side of the robot which can be driven independently forward or backwards.  By driving one wheel forwards and the other one backwards the robot can be made to spin on the spot.  This means it can change direction very quickly and easily.  Both wheels are driven full forwards or full backwards for straight line motion.  If the motors are proportionally controllable then the robot can turn at any rate to face or follows its opponent in the arena.
 
Our entire budget for the project was £250 pounds plus anything else we could muster from our fluff filled pockets.   This was helped though by having access to the Shadow Work Shop where we could use tools and machines and some material resources for free.  After doing a lot of research on the on internet, looking at all the different existing designs we could find, we decided to go for battery drills as a drive motors.  As we didn't have access to machines or money capable of letting us design and build our own gear boxes and they provided a quick and reliable alternative.  Drills are great.  They have a decent motor, proper gearing at two different speeds, a nice handy chuck for attaching to axels, and easily rechargeable and changeable batteries.

After a lot of letter writing and groveling, Bosch agreed to send us a couple of battery drills.   We were very surprised and pleased that they responded and as it turned out we couldn't have done it without them.  Bosch are now our favorite company in the whole wide world, and they didn't even want us to put their logo on the side of the robot!  Anyway they sent us two GSR 9.6 VES-2 battery drills with four batteries and fast chargers worth about 260 pounds each.  Although we wanted  12 volt drills the 9.6 volts seemed to deliver enough torque.

    Radio Control:
 

Controlling the robot turned out to be a complete  nightmare.  None of us had ever fiddled with radio control stuff past simple Tamia R/C cars using two channel AM sets and mechanical speed controllers. If anyone is thinking of making a robot for the first time it is a really good idea to have someone experienced with R/C gear and motor control in the team.
 

Speed Controllers:

Speed Controllers are units which go in-between the motor and the battery to control the power being fed to the motor.  They are controlled by a remote radio control set.  As far as I know two types exist.  Mechanical and Electronic.
 

• Mechanical:
 
Mechanical speed controllers normally consist of an actuated arm moved by a servo which runs up and down a resistive coil or electrodes of varying resistance.  As the arm moves it effectively puts different resistances in series with the motor and battery.  The higher the resistance the slower the motor turns.  This system is great in some ways. Mainly because it is cheap and you can see what's going on.  It's a simply mechanism and if it stops working you can normally find out what's broken with a multimeter and some common sense.  The disadvantage is that they get very hot if you start drawing a lot of current through them to drive the motors, something not uncommon in Robot Wars.  That is if you stall the motor, for example when you are trying to pin another robot to the wall, the resistors in the speed controller get very hot and risk burning out or melting something else.  They also tend to be fairly course as only discrete resistance's available and so the motor only has a certain range of speeds.
 
 
• Electronic:

The electronic type also has advantages.  They work by pulse width modulating the current supplied to the motor.  In effect all they do is turn the power to the motor on and off very quickly at different rates.  That is if you want the motor to go slow, you make the off part of the cycle longer.  The controller increases the speed of the motor by increasing the amount of time the current  is turned on, so over a given time period more power is delivered to the motor and it speeds up. This gives the electronic type two massive advantages.  Firstly, because the unit is not acting as a resistor its self it doesn't get any where near as hot as a mechanical one (they still get hot though due to the resistance of components).  The second is that the power delivered is almost infinitely variably from 0 to a maximum.  The degree of variation depends on the quality and speed of the electronics.   Electronic controllers can also offer special functions such as normal and ABS breaking.  The disadvantages are expense and debugging.  We also found that with the model we used only about 80% of the power available from the battery was getting  through to the motor even at full throttle.
 

After finding all this out we decided to buy electronic controllers, more expensive but offering the control needed in Robot Wars.  But it wasn't any where near as easy as that.

The first step we took was to go to our local model shop,  Hobby Stores in Camden Town. We walked in and told them what we wanted to control which was, two 9.6 volt drill motors drawing a maximum of about 9 amps stalled,  in forward and reverse.  They gave us two Teamline  'Piranha Scale' speed controllers (one for each drill) at £50 each rated in the catalog at 12 V  and 120 Amps continuous in forward and reverse.  We thought this was great and rushed home to try them out.

The first problem we had was that we wanted to use two speed controllers with one 2 channel receiver (at that time all we had was a crappy 2 Channel AM set).  We read that the speed controllers contained B.E.C.'s, or battery eliminator circuits.  These circuits allow you to power the receiver unit directly from the main power source being used to power the drive motors instead of having to add a separate battery pack.  The circuits are necessary as the voltage that the receiver runs at is often drastically different from the voltage you are using to drive the vehicle.  If you powered it without them, you would blow up the receiver :(    Our problem was that we didn't know what would happen if you plugged two B.E.C. outputs into one receiver at the same time.  The risk is that you will feed in twice as much voltage than the receiver is built for and fry its guts.   What we wanted to do, to be safe, was to disconnect the B.E.C.'s and use a separate battery pack to run the receiver.    Our first attempt resulted in one of the speed controllers bellowing smoke and strange smells, which was weird because they both worked for about 10 seconds before failing even without any load on the motors.

We took it back to the shop.

They replaced it.  We brought it home and then the other one failed. Noooo!!  By this point we thought it was probably our fault and wondered if it was worth trying to con the model shop into replacing another one.  So to strengthen our case we took all the gear we had to the shop and showed them what we were doing.  The man behind the counter took yet another Piranha controller out of the cabinet and connected it up how he though it should go.  Again after about 10 seconds it blew up.  The guy went a little redder, tried not to look stupid and scratched his head for a bit.  He decided that they must be faulty (this was after 3 failures) and said he would take them away and have them repaired for us,  but for now he didn't have any of the same type to replace the ones that we had broken.  Instead he gave us a 'Piranha Speed' controller saying that it was exactly the same as the 'Piranha Scale'  we believed him and ran home all excited again.  Then we found out that a Piranha Speed didn't have any reverse function, which wasn't really very useful for a robot that's meant to go in all directions.

We took it back to the shop.

He offered to mail us one when he had it.  A week later it arrived and we tried again.  Yet again they blew.

Guess what?? Then we took them back to the shop.

By this time the people in the model shop were beginning to get majorly pissed off with us .  We demanded our money back, and got it.  £100 back plus 4 blown speed controllers.  Later we found out why.

Apparently Piranha controllers are meant to be used in racing boats only  (hence the name Piranha, eh?).  In electric boat racing there is virtually no way of stalling the motor as its driving a propeller in free water.  There is also very little momentum in the system once its running at full speed.   Also in boat racing it is very unlikely that you will ever want to go backwards for any length of time.  What we were doing was running a fairly large motor with a wheel attached onto one end at full speed forward and then slamming it into reverse, something that that speed controller was never supposed to cope with. 

So we had found the problem.  By this point we were pretty pissed off.  The holidays were slipping away and with every extra part we had to order another week in delivery time was lost.  Also by this time Mentorn, the company organizing the UK robots wars, had made a deal with Ripmax, a model bits distributor, which meant we could get 20% off speed controllers and Radio gear from them!!   Which was great.  We got the catalog and found a better set of speed controllers.  We decided on  a couple of Sung-Ji  RF 10's rated at 12V 800 Amp peak, 200 Amp continuous (yeah right!) for about £45 each.  So we ordered them the same day and sat back waiting for the delivery.  Another week went by.  We rang Ripmax hundreds of times.  Apparently they were out of stock.  They arrived, and not so surprisingly they had sent the wrong one's.  We were VERY pissed off indeed.  But all we could do was send them back and wait another week for the proper ones.

Once they arrived they worked fine.  What a relief.  By that time it was only about 4 weeks before the actual competition.  The two speed controllers were set up through a mixer unit from S.M. model supplies, which allowed the tank drive system to be driven using conventional car controls, that is when the stick is full left it automatically mixes the single channel into two, running the left one full back and the right full forwards, causing the robot to rotate to the left.  It mixes all the in-between positions too to allow for uni directional travel including forwards and backwards.
 

This picture shows all the electronics packed into an aluminum protective box.  You can see the purple heat sinks of the two speed controllers and the white servo arm used to switch the solinoids which fired the pick axe. 

 
 
We also had to upgrade the radio set to a 6 channel 40 MHz FM set as the interference on the AM set in the Television studio was massive.  We wanted a 4 channel one (2 for drive and one for weapons)  but Ripmax had run out, so we got a 6 channel instead.  We bought a Futaba Skysport 6A for around £ 140 (price reduction included).
 
 
 

We decided to make the chassis out of wood as it meant we could kind of design it as we went along (cutting edge stuff I know)  by cutting holes where we wanted and screwing things in easily using wood screws.  If we had chosen a metal chassis I think too much time would have been spent drilling and working the metal, which is a lot less forgiving and much more expensive to use.  I much prefer to work with wood than metal.  Its much more 'friendly'  and when you haven't got too much time you don't want to spend hours drilling 0.1 accurate holes in precise positions when whacking in a wood screw would do the job in 3 seconds!  As it tuned out the chassis was easily strong enough to support me standing on it even with the amour on top.  We used 1/4 inch ply.
 

We looked at many different types of weapons, and ways of powering them.  Saws, hammers, spikes and jaws all had a look in but when working in Shadow you normally end up using what you've got.  Since we already had a nice little pneumatic cylinder and a few can's of compressed gas (the kind used for mini air brush kits) we decided to use them as the actuator.  A couple of old 12 V solenoid valves provided the switching mechanism combined with a servo and two micro switches.  A couple and pulley mechanism was also designed to allow the travel of the piston to be converted into a rotary motion to move a hammer through 180 degrees.  The cylinder fortunately already had a return spring fitted inside it so the hammer was retracted by simple tuning off the gas and letting it vent out as the spring expanded.

   Armor:
 

Hugo designed the shape of the armor.  We knew that the weapon wasn't going to be a major part of our design and so its shell had to be designed with an offensive feature.  A wedge shape in other words, (cut to the chase).

But we didn't want another boring wedge shape box on wheels, so we put a curve on the back and angled sides, it didn't fool anyone.



 

Check out some of the other slightly madder idea's Hugo came up with (click on the thumb nails).  Click 'BACK' to return to this page.