Although it is tempting to use legs for a robot, it is clear that in this case most of the work will involve walking rather than anything else. This is especially true for robots with two legs. It is far from easy to build a two legged robot that can do anything more than walk, and even that is almost impossible. Yes, there are several biped robots around, but most of them do absolutely nothing apart from walking, and they do not do that very well either (at least not when they are off camera). Walking gets simpler if there are many legs, but many of the problems remain. Since my goal with this project is to build a robot where locomotion is just one of several abilities, I decided against using legs. This leaves a choice between wheels and treads.
Most mobile robots use a circular body with a differential drive with one wheel at each side of the robot. This has may advantages. One is that it is easy to control the movements of the robot by simply varying the speeds of the two wheels. Another is that the robot cannot get stuck while turing since it will always be turning around the center of its body. This is a really useful property for a robot that does not know exactly what its surroundings look like. As a consequence, this is the design of many mobile robots ranging from the small Khepera and e-puck to larger robot such as the Pioneer. If stepper motors or some form of encoders are used to measure the motion of the wheels, this design also results in fairly accurate odometry.
But there are also problems with this design. One is that differential drive depends on one or several caster wheels. These have a tendency to get stuck and usually require that the robot only runs on a fairly smooth surface. Another problem is that the success of this design depends on the distribution of the weight of the robot. Ideally, the center of mass should be on top of the driving wheels, but it is very easy to end up with a robot where this is not the case and this will reduce its performance considerably. One way to make a wheeled robot handle uneven terrain is to place the two driving wheels in the front. Although this works well, the robot will no longer turn around its center and it is very likely that the back of the robot will collide with obstacles when it is turning unless it has a good model of its environment.
The figure below show the difference between the turning movement of a robot with the wheels in the middle and one with the wheels in the front. The black spot marks the center of rotation. The wheel arrangement to the right could make the robot hit an object to the left while turning. In my experience, this happens just about every time the robot tries to avoid an obstacle.
Some of the disadvantages of a standard differential drive can be overcome by using several wheels on each side of the robot. A typical design will use three wheels on each side. This allows the robot to move over uneven terrain. There are two problems however. The first is that it is necessary to let the motor drive all the wheels. Otherwise it is possible for the driving wheel to lose contact with the ground which will make the robot stuck. This can be accomplished either by using one motor for each wheel (like the Predator Sumo) or with some form of power transmission from a single motor to all the wheels. In either case, it adds to the complexity of the robot. Another problem with this design that it shares with treads is that it depends on the wheels ability to slip on the round while turning. If there are too much friction, the robot will not move smoothly. This is in fact a problem with several commercial mobile robots when they are used indoors (for example the Koala). An obvious solution would be to design a robot with four wheels and to steer it like a car. The only problem with it is that it will look like a car rather than a robot.
A particularly interesting wheel design is to use a so called holonomic steering. This a design used for example by the Robotino and the PPRK. Typically, this consists of three wheels in 60 degrees angles to each other around the robot. These wheels are designed so that they can rotate in one direction and easily slide in the orthogonal direction. By controlling the different speeds of the three wheels, the robot can be made to move in any direction and even turn. In practice, this design requires a very smooth surface which is a problem if the robot is supposed to move over a rough ground.
Finally, it is possible to use treads. The main advantage of treads compared to wheels is that its is easier to produce traction on a larger area which ideally should be better in rough terrain and for climbing obstacles. The main disadvantage is the same as for six-wheeled robots: turning depends on sliding over the ground which is not always easily done. On the other hand, treads look incredibly cool which is why I decided to try a threaded design. The only problem is to get hold of some treads which is not easily done. More about that later.
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