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Stopping Distance
« on: April 01, 2011, 12:42:06 AM »
I am trying to ascertain the stopping distance I should allow for two conditions on the router I am constructing.
Currently it is not up and running yet so I cannot do exact tests.
The basic configuration consist of a 1604 ball screw with a size 42 stepper. I have a 1:2 reduction timing pulley
between stepper and ball screw. I would like to ask users based on their experience. If the axis is running at
a high  speed (whatever that may be) and the 1) Mach soft limit or 2)Electrical Over travel limit are triggered
independently how much mechanical gap should I allow to prevent the gantry smashing into the end of its stroke?
In a nut shell how fast will the axis stop moving and how far would the gantry travel when the stepper is stopped in the two conditions?
Does the momentum play a part? Thank you for your valuable input. Regards Daryn ???

Offline Tweakie.CNC

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Re: Stopping Distance
« Reply #1 on: April 01, 2011, 02:46:40 AM »
Hi Daryn,

The configuration of your system plays a major part here. It is preferable to configure your 'enable' signal to remain active, irrespective of an e-stop or fault condition and in this way the stepper acts as a brake to the axis travel (a heavy Z axis driven with ball screws would almost certainly continue its movement until the tool contacts the work or fixture if it's drive power was completely removed by a fault condition or e-stop).
On my system it would be totally impractical to mount the limit switches in such a position that they would prevent the mechanical stop being reached when the axis is being driven at it's full G0 speed (the switches would reduce my operating area far too much) but they have been positioned to prevent the mechanical stop being reached when the axis is driven at my maximum G1 cutting feedrate. Unfortunately everything has to be a compromise.


Offline RICH

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Re: Stopping Distance
« Reply #2 on: April 01, 2011, 06:42:11 AM »
It should be able to stop as fast as it can accelerate to max velocity. So one could find when the motor skips when accelerating ...........would be trial and error.
There are analylsis programs that can calculate it. Those calcs are used to find the proper motor for an application.  I will remark that if the pulse stream is removed the stepper will instantly stop turning,but, if the force is greater than the resisting force of the axis you will get additional movement. So there are a number of things to take into consideration if calculated.

Re: Stopping Distance
« Reply #3 on: April 03, 2011, 03:53:57 PM »
Thank you Tweakie and RICH, guess I'll have to do some testing first.
Regards Daryn