Dynamixel large array in sync (wheel mode)

Hi all,

I am considering using Dynamixel servos (either the XL-320 or the AX-12A) for an installation that requires 100 units to rotate in sync.

I would need them to rotate more than 360 degres, so it would have to be in Wheel mode rather than Joint mode.

Will the units remain in sync, if they all receive the same instructions? Or will they slowly drift, and I need to regularly sync them by using Joint Mode and use Goal Position to move them to the 0 position? Would this be an absolute position for all units or would I need an external sensor and some logic?

Thank you for your help

It looks like that you have a very challenging project:

  1. Do all your 100 XL-320s or AX-12As have to hold something in a fixed location? i.e., they need to support something big or of decent weight? If yes, then you cannot use the DXLs in Wheel Mode, as this mode cannot support any load on them, friction will help some to keep the “wheels” in place once they are stopped - but I don’t know if that is enough to suit your needs.

  2. If you need more than 360 degrees of turn/goal position, then the XL-320s and AX-12A won’t make it, as their position encoders can only monitor 300 degrees. You need to think about using XL-330-M288-T and using Extended Position Control Mode (there is not much cost difference between XL-320s and XL-330s - and also not much cost difference between AX-12A and XL-430-W450).

With XL-330s or XL-450s, you will have many CONTROL Options, from Broadcast ID = 254, to SHADOW ID, to Synch-Read and Synch-Write.

  1. The XL-330s and XL-450s used TTL communications so they are restricted to short wires between themselves. So, with 100 DXLs you probably have some DXLs that are located “far away” from the controller. Thus, you will need to think about using a Communication Bridge also to boost the control signal to get to those “far away” DXLs.
  1. The Extended Position Control Mode also has a “side-effect” at Power Off/On for the Present Position Parameter that you will have to monitor somehow, requiring some extra sensors too (see screen capture below).

Possibly, you may have to design/use some 100 little gears to keep the Actual Goal Position degree-change to less than 360 degrees on the actual DXLs?