IAN LANG ELECTRONICS
So on the right we see a highly practical solution. This is a bicycular (two-wheeled) chassis and each wheel is driven independently of the other by a motor and gearbox of the same specification. The two small blobs at either end are either castors or free-spinning trailing wheels, akin to the sort you'd find on a shopping trolley. In the diagram I've shown the driving wheels at centre but it's also possible to put them at the end. If you have the driving wheels exactly at the centre the robot will turn around on its own axis; that is the centre of the robot will be where it was before even though the ends of the robot are facing different directions. The drawback is you need two trailing wheels for support. If you put them at the end, the robot will need a space twice its own length to turn in. So, how does this work. Consider the diagram as below:
Steering a robot is more difficult than is generally appreciated. Mechanics is the key here rather than electronics though electronics do play their part. Let's start with a four-wheeled robot. Here's a few diagrams of what happens.
In the diagram on the left the chassis of the robot is on four wheels. The two uppermost are fixed and the lowermost are steering. When the steering wheels are straight, the robot can move linearly in the directions shown by the red arrows. There's no problem here, all the wheels are moving in the same way.
Where's the drive? You could have it at the front and let the steering wheels do the driving too, with the other pair being idler wheels, or at the back with either one or both doing the driving. You'll get a lot more traction if both are, but this presents problems too.
In practice, in cars the front wheels generally do both the driving and the steering. There are exceptions, BMW cars are rear-wheel driven and the problem with rear-wheel drive is that you lose a lot of traction in the snow and ice as the car tries to push forward against a non-frictive surface and the front wheels are not moving. The force tends to push the back end out into trees, people, ditches etc. which is quite funny to watch if somebody you don't like is attempting to set off in his shiny expensive vehicle and crumps the back on a gate post. Schadenfreude, nothing quite like it.
On the right we see the same chassis with the wheels turned and consequently the motion of the chassis when driven. Some problems are manifest here because the wheels on the ouside of the turn (on the right in this diagram) will have to describe a larger arc in the same time than the wheels on the inside of the turn, meaning in theory that they will have to turn faster; however the driven wheels, if both on the same axle, can't do that, and the result is that on a less frictive surface such as a hard wooden floor that's been polished the bot will have a tendency to go forward as well as turn, meaning a very wide turning circle. One way is a mechanically difficult way to release power from the inside wheel when turning, another is to have only one drive wheel to begin with and let the other three idle.
The other drawback here is that there is a limit to how much you can turn the steering wheels. If you have them at 90 degrees the wheels will drag in the same direction as the driving wheels and the bot will propel itself forward rather than turn. In fact you begin to experience that at more than about 60 degrees. A four wheel bot cannot describe a tight turning circle, and you need plenty of space on the chassis to allow the wheels to turn.
A three-wheeled chassis as seen on the left could theoretically be driven by the third wheel which does the steering as well. This is a popular solution as the motor and wheel can be slung into a movable assembly but the drawback is the need of the space for the motor which will restrict the arc of wheel in at least one direction. On tricycular (three-wheeled) devices you will usually find that the third wheel is smaller than the other two and the drawback again is the drag factor if the wheel is at 90 degrees, although you can get it a lot further round than 60 degrees.
The major drawback is the lack of stability. There is a significant area to the left and right of the third wheel that is outside the wheelbase and on which not much weight can be balanced. If the wheel is at an extreme and the bot is turning at speed the bot will tip over at the base. Reliant cars could do this, which is why corners were taken slowly.
So, there are two wheels, A and B, and quite arbitrarily we will consider the eye perspective as being at wheel A. There are two directions, 1 and 2, and equally arbitrarily we can consider 1 as forward and 2 as reverse. If wheel A is spinning anti-clockwise as we look, and wheel B is spinning in the same direction at the same speed, the robot will move forwards (direction 1). Similarly if wheel A is spinning in the clockwise direction, and wheel B follows suit the robot will reverse. Now, remember these are two independently powered motors working through their own gearboxes. At no point does wheel A reference wheel B. So if we set wheel A going clockwise but wheel B going in the other direction, the robot chassis will move in a turn on its own axis, and the way it will turn is forward at wheel B and backward at wheel A. Wheel B will at some point be where direction 1 is now, and if we continue the turn wheel B will end up where where wheel A is now and we will have a full 180 degree turn- and although the robot has moved in the sense that if faces the other direction, the centre of the chassis will still be in the same position relative to the floor because the wheels are at chassis centre.
Similarly by reversing the spin of both wheels from the above the robot can be made to turn the other way. This is the simplest possible mechanical construction and you will find it on most toy robots and some higher-grade ones too.
On the right there is a popular type of double gearbox, the Tamiya
type made in this case by Pololu and available in the UK from
Proto-pic for £11:89 and clicking on the picture takes you to
their webpage for it. As you can see the motors and the gear
trains sit independently from each other and an output shaft is
provided onto which you may attach wheels directly or via a wheel
mount to get a bit more torque. You might want to elongate those
shafts but if you do you'll need not only an in-line coupler but
a bearing if the extension is very long to avoid bending the extension
shaft. Looking at the photograph it is immediately apparent what the
drawback is; there's no guarantee that the motors are going to spin at
exactly the same speed and if dirt gets in the gearbox of one it will slow
it down slightly causing a drift to one side in the robot. Correcting the voltage in one case and keeping both boxes scrupulously clean is of course the answer.
Here's a rather bizarre concept that you'll probably never see, which is a unicycular bot. The motor, driver and gearbox assembly are affixed to the wheel, and because the wheel can go backwards and forwards it just needs to turn through 180 degrees to be able to move the chassis and anything mounted to it in any direction desired. The chassis is supported at all four corners by a castor or a trailer wheel. A system like this is not common to robots but it is to reach-type fork lift trucks which need to be able to turn in very constricted spaces. There is no reason why you could not use it to move a robot but the mechanics are quite difficult; the whole assembly must move with the wheel which calls for some linkages or a very clever wiring system. Generally the two-wheeled system is preferable.
Wheels have a certain diameter. If they meet a trench that's wide and deep enough, they can get stuck in that trench. So to combat that, vehicles and bots can be fitted with caterpillar tracks. If you happen to have a light engineering workshop and/or a rubber-working factory at your disposal then it is very easy to make a chassis that runs on caterpillar tracks. If not, the alternative is to buy a pre-made one, and one of the best is the Dagu Rover 5.
If you are going to get one of these, and I strongly suggest that you do, the best place in the UK is Robotbits.co.uk and if you click on the picture to the left it takes you to their webpage for it. It's a big beast, and has two independent drives, you can see where the motor and gearbox assembly is towards the rear. the two wheels at the front are idlers. Moreover the tension on the rubber tracks is such that you can alter the ground clearance of the wheels which you do by unscrewing the locking plates, moving the wheel arms to the desired position and rescrewing down the locking plates. It does not move terribly fast but it does have a lot of torque and can go over quite large obstacles. You can dispense with the tracks and add wheels if you wish.
This is a very strong plastic chassis and is priced at Robotbits to appeal to the student, hobbyist, beginner and experimenter. I dare say that the chassis is of such good quality that you could make a marketable device from it if you wished and certainly it's high enough quality for A level D&T or electronics projects and even suitable for a Technical School/ University experiment. Bargain!
How do we steer it? It's exactly the same principle as the two-wheeled robot because in essence that is what it is. spinning the drive motors the same way makes the tracks go the same way and the chassis moves in a straight line, either backwards or forwards. Spinning them in opposite directions makes the tracks go in opposite directions, pulling the chassis round. This is the second most common technique for propelling and steering bots and ROVs and is the only real solution for a ROV or bot that has to deal with outdoor work. If you read the short piece about Wheelbarrow, you'll see it has tracks. So do bulldozers, tanks, large excavators and a host of other machines that have to propel themselves through a terrain where it cannot be taken for granted that there will be a smooth and even surface. Tracks can go where wheels can't, but wheels are more easily replaceable than tracks.
The five methods above are the possibilities for steering your devices but we have not talked much about wheelbases in this article. A wheelbase is the extemities, both across and along the chassis, of the wheel. You measure across from the outside of the wheels, and along between axle centres. You'll see why this is important in the next article which deals with combined centres of gravity (CCOG) in addition.
Ian Lang, August 2013