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I thought I would be clever and use TWO shopping-cart-style casters (more stable) instead of the ONE usual omnidirectional ball caster. Now that I am running it I am almost positive that the kinematics are totally different and in fact very hard to model.

Can someone confirm or deny this belief, and give any context (links etc.) so I can dig deeper?

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  • $\begingroup$ Can you expand on what you think is different and why? Also, what is the size / scale of this robot? Is there any suspension? $\endgroup$
    – Ben
    Oct 25, 2021 at 18:32
  • $\begingroup$ It's been a while. I had a bug in my code. But I can also say that the transition from turning to straight is challenging because the wheels are turned and take a certain amount of time to align themselves with straight travel. So, during that interval, the kinematics predict straight travel (because both wheels are turning at the same speed) but the robot persist on turning and throws off the odometer totally. $\endgroup$
    – pitosalas
    Oct 26, 2021 at 19:27

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Two things:

  1. There is a reason most robots only have a single caster: three points define a plane. If your robot has 4 wheels, and no suspension, due to manufacturing tolerances it is extremely unlikely that you will be able to get all four wheels to be touching the ground at the same time. Most likely, one of the wheels will be slightly off the ground. And hopefully, it isn't one of your drive wheels! (Maybe this is why there is always one shaky wheel of shopping carts?) And the problem actually gets worse in uneven terrain. Imagine if the robot went over a bump such that it lifted one of the drive wheels off the ground. That is obviously not good. With three wheels, this won't happen.

  2. The kinematics of how this un-powered caster wheel affects the robot motion is usually not modeled. Typically, the robot or caster is light enough that it is not severely affected. And differences between predicted motion and real-life motion are why we use sensors anyway.

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You might take a look at this paper dealing with a powered wheelchair, which has two caster wheels. https://hal.inria.fr/hal-02124781/document

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