Re: Bridgeport Series 1 CNC Questions
Originally Posted by
nbman
Yes, I see it now.
The Acorn is Open Loop, and the Allin1 is Closed Loop...Pro/Cons? As a hobbyist, is it something I should even be thinking about?
Closed loop is better and no reason that a hobbiest shouldn't be using it. There are a few low cost, hobby class options.
There are a couple of ways to do this, without getting in to the really high end systems with dual loops and all digital controls. You can close the loop at the drive, or at the controller. The Clearpath motors close the loop at the drive with no easy possibility of closing the loop at the controller. This also true of the hybrid stepper systems you find on EBay. The DMM Tech servos close the loop at the drive or allow closing the loop at the controller.
The most economical method is to close the loop at the drive, that way you can pretty much have the accuracy of a higher end system with the simplicity of an open loop system. Any control system that supports step & direction open loop can operate steppers, Clearpath, or DMM Tech. Machine accuracy is totally dependant on the condition of the mechanical systems and the resolution of the motor encoder. Normally the motor encoder resolution far exceeds the possible mechanical resolution of the system.
The next step up is to close the loop at the controller, this is really no more accurate than closing the loop at the drive when using the motor encoder or an encoder attached to the ball screw. Typically the drives are controlled by a +/- 10V analog signal from the controller, but it is possible to close the loop using step & direction. The controllers are more expensive, and the wiring is more complex. And you are still at the mercy of any inaccuracies in the mechanical system. You do have a bit more control over the system, but servo tuning is a bit more involved.
To achieve the best performance you can put the encoder on the load (machine table) using a linear encoder and close the loop at the controller. For this you can use glass scales or (my prefered device) magnetic scales. Doing this takes much of the mechanical system out of the equation and feeds the actual table position back to the controller.
Dynomotion and Mesa controllers are worth a look also.
I hope this helps.
Jim Dawson
Sandy, Oregon, USA