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Working out the omnidirectional behavior of the kiwi drive

It’s been a while we’ve been working on this omnidirectional robot. We spent a lot of time on the construction (so as to provide a solid and reliable structure), trying to figure out how to move it (just for pure experiments, regardless to the omnidirectional purposes) and now, the last step was to implement the kiwi drive ( i.e. how to move the robot in any direction without rotating it). In order to make it easy to understand and picture the movements of the robot, we’ll refer to the image below.

 

Omnidirectional robot with axis and conventions

 

The motor A | B | C is along [OA) | [OB) | [OC) and can move the whole robot according to the black arrow if activated (in the opposite direction if we apply a “negative” power) and every motor is at 120° from each other. So this was basically all the information we had about the structure of our robot and we had to implement the kiwi drive with no knowledge about its working. We’ll explain step by step the way we programmed it.

First, we tried to make the robot go in “basic” directions. Indeed, if A gets the power |p| and B gets -|p|, the robot will move along [ON), translating itself. So if we input two opposite powers to two motors, the robot will translate along one of the blue axes. This way, we knew how to go towards six directions in omnidirectional mode using the parameters given in the table below.

 

Repartition of the power for the motors according to angle

 

From this point, we wanted to check other values: we started with 30°. Our first strategy was to take the values of 0° and 60° and make the average for every motor. We ended up with 0.5, -1 and 0.5. The result was better than expected because it actually worked… So we tried to use the same strategy for any angle and thus we applied a linear interpolation (we came out with the diagram above). In order to make it easy to control and to appreciate playing with it (that’s one of the big advantages when you make a master thesis in robotics), we struggled to implement the XBox 360 Controller in our system and make the control very smooth. We deal directly with the controller in our code, we don’t use software like XPadder1 that we used for instance in another projects; so the control is very light and fast. –we used the class XBoxController2 instead

You can see and enjoy the results on the video below… (notice that the robot is fully omnidirectional and we used the second joystick in order to make it turn on itself). Nevertheless we’re still aware that the robot makes some errors, especially at low speed: it makes it turn a bit. We’re working on a solution right now (trying for the moment to integrate a compass sensor) and we’re looking forward to lower the number of messages the computer sends to the brick so as not to flood the robot with useless and numerous commands.

 

 
References

  1. http://www.xpadder.com/ []
  2. everything that you need to run in on Windows is available on this website http://www.aplu.ch/home/apluhomex.jsp?site=36 []

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