Ground unit construction and structure ideas: implementing omniwheels
|March 30, 2011||Posted by Guillaume under Building, Ground robots, Ideas, Refining the project, Testing||
Thinking about the project in itself, we focused on the ground units in the flock and their movements. In order to follow the commands it has been given, a ground unit must turn on itself and then move forward an indicated direction (and eventually make a last rotation to be well oriented with the leader). This sequence of three different steps can obviously be executed in one single move; but without considering the obstacle avoidance part, the split way seems to be a reliable first approach. However, we wanted to suppress one of those constraint so as to make the flock units movements much more easier to establish. This way we tried to get rid of the “rotation” part in order to gain time: thus we came out with the idea of using omniwheels. It’s something we already thought about for our last project and agreed that it would be worthy to implement for this one.
Now that you’re aware of the reasons that led us over here, it’s maybe about time to tell you some more about the omniwheels (the name in itself can give you clues about its function but as this isn’t a very common system, we’ll spend some time on giving explanations). Before giving any theory, a visual contact with the device should give you some hints and satisfy your curiosity.
|Omniwheel design on www.holonomicwheel.com||TETRIX 3″ Omni Wheel found on www.legoeducation.us||Excentric design of omniwheel found in the LEGO Lab|
So, as you have noticed, an omniwheel (or polywheel or even holonomic wheel) is a wheel composed of little wheels all around the circumference of the big one. The axis of those little wheels are tangent to the circumference of the big one and they are all included in the same plan (thus, they should all be orthogonal to the axis of the big wheel). This architecture confers “normal wheel properties” to the omniwheel but on top of that, the omniwheel can slide along its axis of rotation (or along the projection of this axis on the surface the wheel is used on).
Nevertheless, they are some different architectures according to the creativity of the builders as you can see on the last picture on right above (the axis of the “little wheels” are not orthogonal with the axis of the omniwheel). A omniwheel isn’t omnidirectional in itself but using several of them can turn a robot into a omnidirectional car. And this was the point of our work…
The omniwheel above is the one we came out with. We only used LEGO pieces, it is composed of 16 little wheels and as you can see the structure isn’t that massive. After that we tackled the robot construction, knowing that we wanted to use 3 of those omniwheels. The two main problems were to work with the angles so as to respect the LEGO construction conventions (the axis of the omniwheels must cross in one point and their center must describe an equilateral triangle) and have a robust structure for the robot.
Those two video show how the robot is moving and it was our first tests so from this point, we had to find out what could and should be upgraded. Namely, if you watch closely when the robot stops, it tends to balance and doesn’t here but could fall in another situations (different speed, different slope, etc). Another notable point is that the robot “bends itself” when moving or turning: this is because the fixations and the structure are not strong enough (and this was the most annoying problem). So from this point, we tried to lower the robot and widen the motors so as to provide a better stability; and tried to reinforce the structure without overweighting it (despite of the fact that we can’t neglect the stability even if it’s at the expense of the robot’s weight).
This way, we crafted a new robot much more stable (and quite heavier too) but the balance and the solidity were no longer problems. Something that need to be mentioned is that we inclined the omniwheels instead of installing them perpendicular to the floor (according to our experiments, this difference doesn’t seem to alter the robot’s behaviour and movements from any other “regular omnidirectionnal” robot).