A while back I started writing a series of articles on building robot bases. I did't get as far as the larger outside ones but have copied what I can find here in case it is of any help. Sorry it tails off towards the end as the publishers decided against going ahead (they did pay me so that was good). I also can't find the illustrations but hopefully the text is self explanatory.
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 your 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.
<Diagram – show turning radiuses>
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.
<Diagram – show mid wheels off ground>
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.
1. Constructing Robot Bases – Part 2
This second article outlines some more things to consider when constructing your robot base.
In part one, I briefly covered the outline design and choice of materials. In the concluding article I will cover choices of motors, forms of locomotion and power sources. The advice given is based on my experience and should be treated as such.
Motors
Invariably any robot will need some kind of drive motor(s) to move it around. There is a bewildering choice to suit every need and pocket. For the purposes of this article I will assume that like me you do not have endless pots of money for your robot project. Also that you do not have access to a fully equipped machine shop.
1.1.1. Electric Motors
Invariably all electric motors will be too fast and are not powerful enough to drive a robot directly (yes I know there are exceptions but they are way out of the budget of this article). In order to drive a robot at a reasonable rate the speed of the motor must be reduced. The most common way of achieving this is with a gearbox, others include drive belts and pulleys or chains and sprockets. For the sake of this article I will refer to all mechanical methods of reducing the motor output speed as gearing.
As the speed is reduced by gearing the torque is increased. Torque is the ability to do work, or turning power. In a perfect world halving the speed would double the torque, unfortunately there are many losses due to friction etc so this does not hold quite true. It is however a useful way to think of things.
For the average robot maker constructing a gearbox is a daunting prospect, requiring precision engineering. Fortunately ready made gearboxes are available from a number of sources. Listed below are some readily available sources.
Toys
An obvious source for small motors is motorised toys. Whilst these can prove useful a few words of caution are in order.
Most toys are built down to a budget. The electric motors are of indifferent quality at best, they are power hungry (for their size), inefficient and most importantly they are electrically noisy. This means that they generate a lot of electrical noise which can be picked up and affect other parts of the robots circuitry. Also it is rare for the motor and gearbox to be self contained. It is far more common to find the gears are supported by the body of the toy. Unless you can make use of the toy base as is they are best avoided.
There are always exceptions and special cases. Many people have successfully modified all kinds of toys to make robots.
