Constructing Robot Bases – Part 1

Overview

This article outlines the things I have learnt when constructing a base for a robot.  Hopefully, this will help you to avoid some of the pitfalls and problems along the way.

I plan to write further articles following on from this, detailing particular construction technique with a fully worked example.

The robots I have in mind are the typical desktop or indoor floor rover, along with those of a similar size such as mini-sumo and micromouse.  I will not be covering larger combat robots or outdoor rovers.  These both require robust construction techniques, powerful drives and batteries and inevitably a fair amount of money.

What to consider

It is very tempting when making a robot to launch in and attach a couple of motors (because they were cheap) to a pair of wheels (because they look good) to a sheet of metal (because that’s what you make robots from).  Whilst many robots have been built like this, and you will learn a great deal building it and debugging it a little thought and planning will go a long way to making a much more successful and reliable robot.

So what do you want your robot to do? Is it to be a mini-sumo, a carpet-vac, or a sentry with laser eyes and a death ray to deter intruders?  Even if you answer is a test bed or to learn about robots good planning can help.  A test bed robot with ample brackets to mount sensors will be a joy to work with.  The frustration of an erratic robot that misbehaves because the Selotape holding the sensor comes undone has to be experienced, especially if you don’t notice and keep trying to correct it in software

Size

First off, are there any size constraints? If the robot is for a competition there may be prescribed sizes that must not be exceeded, or a micromouse must be able to fit within the maze.  For a desktop robot something around 200mm (8”) square should be considered the upper limit.  For a general floor rover around 300mm (12”) is about right.

Remember as your robot gets bigger you will have to make the base more rigid, which invariably involves making it heavier.  Heavier bases require bigger motors and batteries which require stronger, heavier bases……

Also look at your chosen material (see below) you may find that it comes in standard sizes and it is cost effective to work within these constraints, rather than going for a larger sheet and wasting a large section.

If your robot must be a specific size remember to take into account all the components you will need.  Sensors protrude, those perfect wheels may have a nut that extends beyond the rim or the strain relief on your modified servos holds them just too far apart.  Whatever construction technique you intend to use it is worthwhile making a mock-up in paper or card to check how it all fits together.  Remember to allow space for the electronics and also think about access.  You will undoubtedly need to get at the batteries and the processor or programming socket.  Make sure you can get at these without having to dismantle everything.

Style

By style I mean type of locomotion, although cosmetic styling can be a nice to have and it certainly helps to win approval from non-roboteers.

By far the most popular type of robot has two drive motors, one on either side, each individually controlled to give ‘tank’ or ‘skid’ style steering.  To drive the robot many forms of locomotion can be used, wheels, tracks or legs to name the most common.  If you are new to robots stick with wheels.  They are simple to use and frankly they work.

Most people want to build a tracked robot, my advice is unless you’ve got some experience under your belt don’t do it.  You can hack a motorised model tank or the like, but the results are often disappointing.  Unless very well built a tracked vehicle will perform much worse than a wheeled one over even the simplest obstacle.  They are prone to toppling unless they have suspension and unless you get an expensive set up you will become very good at re-fitting the tracks.

With any tank steered robot the turning circle is affected by its centre of rotation.  With tracks this is automatically in the centre of the vehicle, but with wheels their position can dramatically change the effective turning radius.

From the diagram above it would seem that centrally placed wheels are the answer as the turning radius (shown in red)is the smallest.  However, this design can be less stable as the robot is balanced on the two drive wheels, during braking or acceleration inertia will tilt the body, sometimes toppling you robot over.  Also by placing the wheels centrally ground clearance is reduced, this can be a problem on rough terrain or even clearing a door threshold.

This particular problem can be overcome by placing the driven wheels at one end with a third floating wheel or skid to support the robot.  The downside is the increased effective turning radius as shown above.

Whilst the above few paragraphs may appear to be contradicting each other the point is that you need to decide how your robot should behave before building it.  A micromouse, for instance can be at an advantage if it can spin through 180º in a maze.  This would be best achieved by centrally placed wheels (I know many tricycle style mice have been very successful but none that I have seen can turn on the spot).  On the other hand a tricycle style base will negotiate obstacles far more successfully.

One final thought on the tricycle layout do you put the drive wheels at the front or back?  Front drive wheels will tend to clear obstacles better as they pull the robot up and over.  Rear mounted wheels can lead to a more stable platform.

Materials

Robots can be made from just about anything.  It is important that you make your base using techniques and materials you are able to work.  It’s no good designing a titanium framed robot with Kevlar shell if the only tools you have at your disposal are a pair of scissors and a ruler.

Outlined below are some common materials that can be used for your robot, it is not an exhaustive list but should give some useful pointers.

Card or Paper

Extremely useful for mock ups very successful bases can be made using card.  To get strength and rigidity try a few simple tricks:

Laminate

Glue several layers together using pva (wood glue) or even wallpaper paste, weigh it down with a few books and let it dry, this can take several days!  I put the card in a polythene bag so it doesn’t stick to the books. 

Use gummed tape

When joining card try using the brown paper gummed or ‘licky’ tape, often sold for picture framing.  This can make an extremely strong and rigid structure.

Resin

You can make your paper or card structure into a strong and rigid structure using polyester resin (the resin part of fibreglass) often sold in car repair shops.  Because the paper/card is absorbent you do not need the glass fibre.

Plastics

There is a huge array of plastics to choose from, most are light, strong rigid and available in a multitude of colours.  Two readily available sheet products are acrylic sheet and foamed pvc (or sintra in the states).  Both can be cut with hand tools and bent or shaped using heat (but that’s another article).  One word of caution plastic robot + nylon carpet = static.  If this could be a problem consider a different material.

Metal

The two most common metals used are aluminium and steel, both are relatively easy to work and are available in a huge range of sizes and thicknesses.  A common misconception is that aluminium will make the lightest robot.  This is not always the case, steel is much stronger than aluminium so can be thinner and hence lighter.

Wood

With the exception of balsa and plane (available from model stores) solid wood is generally unsuitable for small robots.  Man made sheet material however can be ideal, especially useful is ply I’ve had great success using model ply, which is light, easy to cut with a knife and surprisingly rigid.

PCB Board

Several commercial robots (take a look at the Tab Build Your Own Robot books) and some highly competitive robots (some of the MITEE micromice for instance) have been built with the pcb forming the body of the robot.  If you are good at working with pcb’s this can be a successful solution.  Bear in mind though that pcb boards are not designed to take high loads and could crack or de-laminate rendering your entire robot useless.

I hope the above have proved useful, I intend – time permitting - to expand on various techniques and illustrate them with full plans and instructions for building a robot base.