Re: Low cost limit switch suggestions

Hello Bob,

The reason for having limits "the computer can see" is to allow
toolpath file restarts... (such limits are called "soft" limit
switches).

The most complete systems have 5 switches (and two mechanical stops).
These are:

1)one pair "hard" limits. Usually located just inside the mechanical
stops for each axis. These will drop power to try to prevent the kind
of damage from runaway servos mentioned in your original post.
Position is non-recoverable (a total restart is needed to continue
using the machine).

2) One pair "soft" limits. These are located just inside of the hard
limits and far enough from them to allow an axis to decelerate to a
full stop from rapid speed without tripping the hard limit switches.
As mentioned above, these DO preserve coordinate position and allow
restarting the file without restarting the machine. On modern
controls, these "soft" limits have been incorporated into the control
software in many cases. Sometimes they are called "stroke" limits and
parameters to set them are entered into the control by the factory at
manufacture. When a "soft" limit is encountered, most machines will
allow motion only AWAY from the switch. This avoids the possibility
of accidentally triggering the hard limit and requiring a machine
restart.

3) One "home" switch. Most commercial machines use a hall effect or
microswitch for this function and wire it in series(electrical AND)
with the index pulse from the encoder attached to the axis. This
means the switch is only setting the "rough home" postion, as the
index pulse will set the final home spot. On our simpler home-based
systems we will need to do something similar, or specify a higher
grade microswitch for "home" duty. An applications engineer at a
large switch company will be very helpful to find an appropriate
switch.

If the machine in question is full size, you WILL want to have BOTH
hard and soft limits, IMO. (Especially with the performance of the
Gecko drives; all that machine mass is a scary thing if it gets out
of control!!) Wire the geckos to the outer hard limits, which can
be "run-of-the-mill" microswitches, and also take this to the
emergency stop input of CNCPRO.

Then set up an inner limit loop for soft limits and feed these to the
limit input(s) of CNCPRO. Setup will determine which of these is
treated as "home". Yes, you "could" just use one switch in the "home"
position and again, wire it to the limit or home input(s) of CNCPRO.

Hope this helps.

Ballendo


--- In CAD_CAM_EDM_DRO@y..., Robert Allen & Marsha Camp <mcamp@t...>
wrote:

> Hi,
> I'm putting servos w/gekko 320s on the x and y axes of a
> older ball screw equipped mill and am going to use CNC PRO.
>
> In talking to Mariss of gekkodrives he recommended that the
> limits kill the power to the drives because if a drive fails
> there is a good chance that the output transistors will
> short and the motor will travel at top speed until it blows
> the fuse or hits the end of the ballscrew (not good).
>
> This means I used up the original limits that are on the
> machine.
>
> So here are the questions, does any one have a suggestion
> for limit switches that are inexpensive, rugged, accurate,
> and relatively small (I want it all). Also it would be nice
> if I could only have to install one per axis which brings up
> the second question,
>
> If I already have max. travel limits do I need any thing but
> a home switch?
>
> What are the reasons for having limits the computer can see?
>
> Bob KB1FRW