Simplifying

Parts have been taking much longer to arrive than expected so I have been having a look at my design and found a neat way to simplify it and remove two sets of bevel gears. This is because when I had been taking into account the size of the motors I had forgotten that most of the motor will be overhanging it's mounting position, meaning that in fact I could mount them in line with each other reducing the need for bevel gears.

This has also meant that I can reduce the length of the overall bot by about and inch, which has been quite needed as the length was starting to be too large, almost 9 inches. This would've meant that the bot could of taken up most of the arena.

Also, I have added mounts on the base for the bodywork to attach on to.

Newer simplified design

Lego Bushes

So after ordering all my Lego, I realised I had forgotten one main component, the lego bushings. These are used to keep axles in place. Because I only needed a couple and the minimum order spend on most brickowl stores is £2, I chose to try and 3d print them myself. As well as making a useful component, 3d printing something so small and precise would give me lots of experience in using the 3d printing process.

At first I downloaded a model from the 3d warehouse and adapted it so it could be printed easier (less overhanging edges). With this done I tried printing out my first model.

I found that this was too small. I went back to the design and started changing it by making the hole wider. After being printed out this also didn't fit. Then I had the idea of printing a row of the bushes but each one scaled up by a mm. I did that and found the one that gave me the best interference fit and the file.

With this success I now have the ability to print of as many lego bushings as I so desire. The final result is as shown below.

First Few Parts

A few parts have come thorugh! My lego bits and bobs have arrived from brickowl and the wheels arrived from eBay. The wheels seem really good and grippy which seems a relief as that is the one thing you cannot tell from a photo! I assembled the bits I had and found a couple of problems.

The first one being that the tolerances in the 3d printer and the recorded measurement of the wheel meant that there is a slight rubbing between the wheel and one of the parts of the printed middle section. This has been momentarily solved by filing away the offending part. However, I have also changed the 3d design so this won't be a problem again.

The next, slightly larger, problem I encountered was that the mounts for the rear wheels didn't seem strong enough and had a lot of play in them (meaning the wheel could move up and down). This is partly because I had to drill out the holes as I got the size wrong in the model. I have solved this by making the mounts larger. Also, the reason that it didn't seem strong I now realise is that when I was printing the middle section, to save time knowing that it was a prototype, I made the infill only 20%. This means that only 20% of the inside of the object is filled; and is surprisingly strong despite this. When I print the finial piece it will have a 100% infill which should make it much stronger

3d Printing

So I've got a fairly decent design going on sketchup so I thought for my initial prototype I'd print of the middle section using the university's 3d printers. However, I did come across a problem. The 3d printer software requires a .stl type file. However, Sketchup has no export to stl option. This meant I had to go through the long winded process of exporting the file as a .dxf or .dwg to open it in autoCAD. However, when opened in autoCAD the export to .stl option failed. I found that this was true even for simple objects exported from sketchup such as a cube. This was a real problem as it meant I would then have to design my entire again on autoCAD. Although not the end of the world this would really slow down my progress.

After much fiddling about with the two software packages, I found a solution. Google Sketchup, as well as a 3d warehouse, also has an extension warehouse where you can download add-ons for sketchup made by other people or companies. A quick search revealed there was an extension for importing and exporting .stl files. I tried it out and found it worked fine!

I went to the workshop, shown how to operate the printer and then printed out the middle section! It took a couple of hours but when done added the mdf claws to see how to fit it together. The end result is shown below.

Designing away

Parts are taking a while to arrive so I am taking this time to make my design as good as possible. It seems everytime I go back to look at the design I find something slightly wrong with it, for example some of the mounting points were at different heights meaning that the bevel gears would not mesh correctly. This is stuff that is not immediately obvious but would be when assembling the bot. I am hoping out to iron out as many problems at this stage as possible before assembling the design.

As you can see, bevel gears are being used everywhere to help keep the narrow scorpion shape. The hammer is on a sliding mount meaning I can adjust it to make sure the gears mesh well (The blue section represents the battery).

Lego!

So a couple of days after I ordered my lego wheels I discovered a website called brickowl. It enables you to buy cheap specific lego parts off different stores. I found this very useful as I can now order the bevel gears for a low price, something that was looking to be very expensive on site such as eBay. I've ordered a few different parts to test out hopefully they should be ok

More Designs

With parts ordered and knowing their dimensions from data sheets, I am now in the process of designing the complete model on Google Sketchup. I have been having a few issues with fitting the motors in sideways as it has been making the robot too wide if I wanted to keep the same kind of shape (wide at the front then getting narrower). I was then told about bevel gears. These transmit rotary motion 90 degrees. This has meant I can place my rear motors in line with the front of the robot and help keep it thinner. The bevel gears I have used lego gears for ease, although these can be changed to metal at a later date

The claw has also been redesigned to be more scorpion like and scarier.

The Hammer

The hammer component has been a real stumbling block on the design of the robot. The aim was to design a hammer that, most importantly, looks really cool and works well. I had two options, I could either use a pneumatic ram or a motorised system.

To start off with I looked at using a pneumatic ram. I found this post on a forum about a very successful antweight robot. The author mentions in the alternative parts section that lego pneumatics could be used for the ram. I started researching the lego and came across some interesting things.

Lego Ram

For a lego pnuematic system you need these following items:

• Ram
• Valve
• Air Tank
• Pump

I found a kit for all of these things for £50 here. However, extra parts would have to be purchased as the lego does not support electrical switching (all done by hand). This would mean an electric vacuum pump and solenoid valve would have to be found that interfaces with the lego parts. This would look like the below design if made:

Another, much simpler way, to make the hammer would be to use some kind of motor, such as a stepper motor or a servo.

After much thought, I chose to use a servo. This is because, despite the fact that the pneumatic stuff would be cool, it would be way too complicated. Also, the power given by the pneumatic ram would be over the top as I don't really want to cause too much damage to the other robots. However, the pneumatic stuff could be added in if a second version of this bot was ever made.

Parts

So I ordered most the parts I need a couple of days ago and I thought I'd list what they are.

Motors
I have ordered 4 of these micrometal gearmotors. I have chosen the gear ratio to be 100:1. This is slightly slower than most other antweights but has much more torque (320rpm with no load and 2.2kg-cm at stall). This is what is needed for a house robot. Using the specs given, this gear ratio would give a speed of about 2mph with a wheel size ratio on 1".

I also got a standard motor mount for each motor.



Wheels
Wheels were a surprisingly tricky part to find. I needed four small and grippy wheels. I started looking at the pololu wheels but found they were often too large. Eventually I stumbled across lego wheels. I seemed to be able to find the correct size of wheels but I had no way of telling how grippy they were from the photos. Eventually I took the plunge and ordered 4 wheels intended for a crane kit from eBay. They had a 60mm diameter and a 20mm width.


Battery

I needed a large battery, much larger than any of the other antweights. This is because, not only does it have more "stuff," it also has to fight in every single round of the robot wars. A quick bit of research found that the best type of batteries to use would be the lithium polymer (Lipoly) type of battery. These batteries have a very high capacity to weight ratio and are the go to battery for RC enthusiasts. The one I chose, here, has a very large capacity but still fairly small so should be able to fit in well inside the robot.

Servo
For the servo used to control the claw, it was important that I chose a servo with a very high torque. This is so that it could easily pick up other bots. There is also no need for it to be super past either. At first I started looked at retract servos. These are servos traditionally used for landing gear on RC aircraft so they can only hold two positions which are 180 degrees apart. Because of this it is possible to make them with lots and lots of torque. However, if I were to use a retract servo it would mean that I would have to design my mechanism perfectly with no room for error and also it could only be open and close, with no position in the middle. With that in mind I had to find just a normal, but high torque servo.

Eventually I found a good servo that gave me 11kg-cm. This is much higher than any others at this price (just under £10) as often to get a really high torque servo you can pay upwards of £30-40.

Initial Designs

With the claw design sorted, I can now move on to thinking about how the whole thing is going to fit together. Then parts could be chosen and ordered. The very first thing that was decided upon would be that the house robot would be 4 wheel drive. Because of the weight limit, most antweights can only have 2wd. However, because I don't have a weight limit it would be a good idea to have 4wd as I would have more grip meaning I could easily move other robots without them moving me.

Pololu gearmotor

Speaking to previous years competitors and looking at bots from the antweight world series, I was shown to a range of micrometal gearmotors which could give out a very high amount of torque for the size of the motors. I decided to use one of these motors however the exact gear ratio would be decided upon later.

With this in mind, I made my first design on Sketchup. Google Sketchup conveniently has a 3-d warehouse where you can download models made by other people. These are often to scale so really helped save time when making this initial design. Both a servo and the gearmotors had been modelled before by other people so I could download them and add them to the design.

With that in mind I had my first initial design:

After I made this I showed it to a few of the previous years competitors and they gave me some feedback including enclosing the wheels so they are armoured for enemy bots and flipping the claw upside down to be able to get a better grip on enemy robots and also because it looks cooler.

This lead me on to my next design, which also incorporates a pneumatic hammer, which I will discuss in my next blog post.

The Claw

The first part I decided to focus on was the claw mechanism. After some quick research I found a claw that I could buy that was servo operated and looked really cool! However, I quickly realised that this would not be a good choice as it could only open to just under 4". Seeing as most the robots are 4" wide, it would mean that the claw would struggle to pick any of them up. However, I liked the metallic look of it and the fact it was controlled by a servo and decided that any claw I had needed to be metal (plastic would be easily broken by spinners) and controlled by a servo.

The next one I found looked much better, had an opening width of 4.20" and was controlled by a servo. However, it was made of plastic. As mentioned above, I need the gripper part to be made of metal as plastic is very easily broken by spinner robots. Seeing as it was such a simple mechanism, I decided that I would have a go at making one myself. This would be cheaper and the final product would be more suited to my need.

I am somewhat of a novice when it comes to CAD, having only really used a tiny bit of SolidWorks before for a GCSE project a few years ago. With the software installed on the university computers I had two choices, AutoCAD or Google Sketchup. I started of by having a bit of a play with AutoCAD, knowing that it is a very useful and powerful tool in the right hands. However, in my hands I found it difficult to make even the simplest of shapes and I could see myself spending a very long time learning how to use the package. I decided that, because all my shapes are fairly simple, to use Google Sketchup, safe in the knowledge that if anything I needed to do that required a more complex tool I could export from Sketchup to AutoCAD.

After a while of playing about on Sketchup, a fair few mistakes and start overs, I had my first claw design. This, I hoped, would work just like the above claw but I now had the ability to make it out of whatever material I so desired. I chose at first to print it out on the laser cutter in MDF to check the mechanism was working properly.

This being my first time using a laser cutter it took a couple of attempts to get it right (my first attempt ended up with a miniature claw!) but eventually I had it printed out fine. I assembled it (as shown below) and surprisingly, everything worked fine! The claw now opens to about 5.2" and can now be made out of whatever material I want.

So it begins...

My project this summer entails building a house robot for the University of Bristol's Robot Wars. This, much like the original TV show, which we all know and love, involves pitting two robots against each other until one robot is either immobilised or thrown out of the arena.

However, this robot is to compete in the antweight series of Robot Wars. There are a few differences between the antweight series and the televised one (heavyweight). The main one being (obviously) the weight. Heavyweight robots have a weight limit of 100kg whereas the antweights have a limit of only 150g and have to be able to fit into a 4" cube. Also, antweight robot arenas have two sides with no barriers for the robots to be pushed off.

This video  shows the last robot wars at Bristol.

From this you can see what kind of robot I am aiming to build. However, there is one main difference between my robot and all the others. Mine is going to be a house robot.

A house robot is an obstacle in the main arena that takes part in every single battle. House robot's are bigger (no weight or size limit) but are restricted to certain zones and only take part in the action when a players robot enters that zone and once the player leaves that zone, it cannot chase.

House Robot's for the original Robot Wars TV series

House robots are an important part in the robot wars TV series success. They provided a recurring character that the audience could latch on to and would still be there if a popular competitor was knocked out at an early stage.With that in mind, it is important that the whole design of the house robot is to look seriously "cool" and threatening. All the other robots in the original series have "characters" to play and were all very unique so it would be best if I decided on a theme to go with first.

After talking to some of the previous years competitors for inspiration, it was decided that I would make a scorpion type robot with claws and a hammer. The idea being that it would not destroy robots, only pick them up and move them to break up the play, and with a hammer that looks really cool but doesn't do loads of damage With this decided the next stage would be to start designing an initial model on CAD and choosing parts.