### Author Topic: A question about the parallel axes of a gantry robot  (Read 1681 times)

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#### ManMan88

• Beginner
• Posts: 2
##### A question about the parallel axes of a gantry robot
« on: June 07, 2014, 09:05:29 AM »
Hi all,

My first post in this forum, I'm happy I found it

I'm looking into building a gantry robot, which has two parallel motor driven ballscrews for the main axis.
I consulted with a company about it, and they sugested using only one driven ballscrew and on the other side just using a rail.
They clame that using two parallel ballscrews can easily cause problems (if they dont move precisely togather) and that they need to be perfectly controled.
However, I thought that it would be pretty easy to send the same command through one driver (or the same command for 2 drivers) so the two motors and axes will perform exactly the same.
Am I wrong here? (For the sake of this argument lets say that one motor is strong enough to move the axis.)

Plus, do you know what are the pros and cons for each of the designs that I just mentioned?
(What I guess is that only one ballscrew would be cheaper, but it seems to me like it could cause problems because the force on the moving part acts only on one side causing it to "rotate" around its center of mass. Ofcourse the rotation is being oposed by the other rail, but it seems to me like it could insert alot of inaccuracy to the system).

Thanks for the help,
Ron

By the way I'm not a native English speaker, so sorry for mistakes

#### jwatte

• Supreme Robot
• Posts: 1,345
##### Re: A question about the parallel axes of a gantry robot
« Reply #1 on: June 07, 2014, 12:27:45 PM »
Two screws (one on each side) could mean a more rigid gantry, because they will not allow any diagonal skew.

However, stepper motors may miss steps, which will mis-align the two screws, which will be a problem. If you have two motors, you also need to absolute encoders on the outputs, so that you can make sure they are always in sync.

Another option might be to put the screw in the middle, and use rails on both edges.